LTC4281
1
Rev. A
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TYPICAL APPLICATION
FEATURES DESCRIPTION
Hot Swap Controller with I2C
Compatible Monitoring
The LT C
®
4281 hot swap controller allows a board to
be safely inserted and removed from a live backplane.
Using an external N-channel pass transistor, board supply
voltage and inrush current are ramped up at an adjust-
able rate. An I
2
C interface and onboard ADC allows for
monitoring of board current, voltage, power, energy and
faultstatus.
The device features analog foldback current limiting to
limit the MOSFET power to a constant value. A wide input
voltage operating range comfortably allows applications
from 2.9V to 33V.
The LTC4281 is well suited to high power applications
because the precise monitoring capability and accurate
current limiting reduce the extremes in which both loads
and power supplies must safely operate. Non-volatile con-
figuration allows for flexibility in the autonomous genera-
tion of alerts and response to faults.
12V, 65A Plug-In Board Resident Application
APPLICATIONS
n Allows Safe Board Insertion Into Live Backplane
n 12-/16-Bit ADC with ±0.7% Total Unadjusted Error
n Monitors Current, Voltage, Power and Energy
n Internal EEPROM for Nonvolatile Configuration
n Wide Operating Voltage Range: 2.9V to 33V
n I2C/SMBus Digital Interface (Coexists with PMBus
Devices)
n 12V Gate Drive for Lower MOSFET RDS(ON)
n Programmable Current Limit with 2% Accuracy
n MOSFET Power Limiting with Current Foldback
n Continuously Monitors MOSFET Health
n Stores Minimum and Maximum Measurements
n Alerts When Alarm Thresholds Exceeded
n Reboots on I2C Command
n Input Overvoltage and Undervoltage Protection
n Three General Purpose Input/Outputs
n Internal ±5% or External Timebases
n 28-Pin 4mm × 5mm QFN Package
n Enterprise Servers and Data Storage Systems
n Network Routers and Switches
n Base Stations
n Platform Management
All registered trademarks and trademarks are the property of their respective owners.
Start-Up Waveforms
+
VDD
SMCJ15CA
GATE
10nF
TIMER
LTC4281
INTVCC
CONNECTOR 1
CONNECTOR 2
PLUG-IN
BOARD
NC
NC
NC NC
4281 TA01a
GND
12V
SDA
SCL
ALERT
10Ω
V
OUT
12V
65A
0.5mΩ
SOURCE
FB
GPIO2
GPIO3
GP
GP
UV
OV
SDAI
SDAO
SCL
ALERT
ADR0
ADR1
ADR2
ON
SENSE+SENSE–
ADC+ADC–
WP GNDCLKOUT
NC
CLKIN
4.7µF
100k
12V
BACKPLANE
POWER
GOOD
GPIO1
20ms/DIV
∆V
GATE
10V/DIV
GPIO1(PG)
10V/DIV
V
DD
10V/DIV
V
10V/DIV
50ms DE-BOUNCE
4281 TA01b
CONTACT BOUNCE
LTC4281
2
Rev. A
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TABLE OF CONTENTS
Features ............................................................................................................................ 1
Applications ....................................................................................................................... 1
Typical Application ............................................................................................................... 1
Description......................................................................................................................... 1
Absolute Maximum Ratings ..................................................................................................... 3
Order Information ................................................................................................................. 3
Pin Configuration ................................................................................................................. 3
Electrical Characteristics ........................................................................................................ 4
Typical Performance Characteristics .......................................................................................... 7
Pin Functions ...................................................................................................................... 9
Functional Diagram .............................................................................................................11
Operation..........................................................................................................................12
Applications Information .......................................................................................................13
Register Set ......................................................................................................................31
Detailed I2C Command Register Descriptions ..............................................................................32
Typical Applications .............................................................................................................43
Package Description ............................................................................................................44
Revision History .................................................................................................................45
Typical Application ..............................................................................................................46
Related Parts .....................................................................................................................46
LTC4281
3
Rev. A
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PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
Supply Voltage VDD .................................... –0.3V to 45V
Input Voltages
GATE – SOURCE (Note 3) ...................... –0.3V to 10V
SENSE+, ADC+, SENSE
– ...... VDD – 4.5V to VDD + 0.3V
ADC–. .......................................... –0.3V to VDD + 0.3V
SOURCE ................................................. –0.3V to 45V
ADR0-2, TIMER .....................–0.3V to INTVCC + 0.3V
CLKIN. .................................................. –0.3V to 5.5V
UV, OV, FB, WP, ON, GPIO1-3,
SCL, SDAI ............................................. .–0.3V to 45V
Output Voltages
GATE, GPIO1-3, ALERT, SDAO ................ –0.3V to 45V
CLKOUT ................................... –0.3 to INTVCC + 0.3V
Output Current INTVCC (VDD > 4V) ........................25mA
Operating Ambient Temperature Range
LTC4281C ................................................ 0°C to 70°C
LTC4281I..............................................–40°C to 85°C
Storage Temperature Range .................. –65°C to 125°C
(Notes 1, 2)
9 10
TOP VIEW
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
T
JMAX
= 125°C, θ
JA
= 43°C/W
EXPOSED PAD (PIN 29) PCB CONNECTION OPTIONAL
29
11 12 13
28 27 26 25 24
14
23
6
5
4
3
2
1
ON
OV
GND
WP
INTVCC
TIMER
CLKOUT
CLKIN
GATE
SOURCE
FB
GND
GPIO1
GPIO2
GPIO3
ALERT
UV
VDD
ADC+
SENSE+
SENSE–
ADC–
ADR0
ADR1
ADR2
SDAI
SDAO
SCL
7
17
18
19
20
21
22
16
815
ORDER INFORMATION
TUBE TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC4281CUFD#PBF LTC4281CUFD#TRPBF 4281 28-Lead (4mm × 5mm) Plastic QFN 0°C to 70°C
LTC4281IUFD#PBF LTC4281IUFD#TRPBF 4281 28-Lead (4mm × 5mm) Plastic QFN –40°C to 85°C
Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
LTC4281
4
Rev. A
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The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VDD = 12V unless otherwise noted.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Supplies
VDD Input Supply Range l2.9 33 V
IDD Input Supply Current l3.5 8 mA
VDD(UVL) Input Supply Undervoltage Lockout VDD Rising l2.65 2.7 2.75 V
VDD(HYST) Input Supply Undervoltage Lockout Hysteresis l15 40 75 mV
INTVCC Internal Regulator Voltage l3.1 3.3 3.5 V
INTVCC(UVL) INTVCC Undervoltage Lockout INTVCC Rising l2.45 2.6 2.7 V
INTVCC(HYST) INTVCC Undervoltage Lockout Hysteresis l50 110 175 mV
Current Limit
∆VSENSE Current Limit Voltage DAC Zero-Scale VFB = 1.3V, ILIM = 000
VFB = 0V, ILIM = 000
l
l
12.25
3.4
12.5
3.75
12.75
4.1
mV
mV
Current Limit Voltage DAC Full-Scale VFB = 1.3V, ILIM = 111
VFB = 0V, ILIM = 111
l
l
32.88
8.81
34.37
10.31
35.87
11.81
mV
mV
Current Limit Voltage DAC INL l–0.05 0 0.05 LSB
Fast Current Limit Comparator Offset l0 ±15 mV
ISENSE– SENSE– Pin Input Current VSENSE– = 12V l0 ±1 µA
ISENSE+ SENSE+ Pin Input Current VSENSE+ = 12V l0 90 130 µA
Gate Drive
∆VGATE Gate Drive (VGATE – VSOURCE) (Note 3) VDD = 2.9V to 33V, IGATE = –1µA l10 12.5 13.5 V
IGATE Gate Pull-Up Current Gate On, VGATE = 0V l–15 –20 –30 µA
Gate Pull-Down Current Gate Off, VGATE = 10V l0.5 1.3 3 mA
Gate Fast Pull-Down Current ∆VSENSE =100mV, ∆VGATE = 10V l0.3 0.9 3 A
tPHL(FAST) Overcurrent to GATE Low ∆VSENSE =0mV Step to 100mV, C = 10nF l0.5 1 µs
VGATE ∆VGATE FET Off Threshold l5 8 10 V
Comparator Inputs
IIN UV, OV, FB, ON WP Input Current V = 1.2V l0 ±1 µA
VTH-R VDD, SOURCE Rising Threshold Voltages for
UV, Power Good (Note 6)
5%
10%
15%
l
l
l
–5
–10
–15
–7.5
–12.5
–17.5
–10
–15
–20
%
%
%
VTH-F VDD, SOURCE Falling Threshold Voltages for
UV, Power Good (Note 6)
5%
10%
15%
l
l
l
–10
–15
–20
–12.5
–17.5
–22.5
–15
–20
–25
%
%
%
VTH-R VDD Rising Threshold Voltages of OV (Note 6) 5%
10%
15%
l
l
l
10
15
20
12.5
17.5
22.5
15
20
25
%
%
%
VTH-F VDD Falling Threshold Voltages of OV (Note 6) 5%
10%
15%
l
l
l
5
10
15
7.5
12.5
17.5
10
15
20
%
%
%
VTH UV, OV, FB, ON Rising Threshold l1.26 1.28 1.3 V
VHYST UV, OV, FB, ON Hysteresis l23 43 63 mV
VTH FET-Bad FAULT VDS Threshold l150 200 270 mV
VTH WP Pin Threshold Voltage Falling l1.26 1.28 1.3 V
VHYST WP Pin Hysteresis l2 20 35 mV
tPHL Turn-Off Propagation Delay ON, UV, OV Turn-Off l10 25 45 µs
LTC4281
5
Rev. A
For more information www.analog.com
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VDD = 12V unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
tPHL Fast Turn On Propagation Delay ON Pin Turn On l10 25 45 µs
tDDebounced Turn On Propagation Delay UV, OV Turn On l45 50 55 ms
Crystal Oscillator
VTH CLKIN Pin Rising Threshold l0.4 1 2 V
fMAX Maximum CLKIN Pin Input Frequency l25 MHz
ICLKIN CLKIN Pin Input Current V = 0V to 3.3V l–10 10 µA
ICLKOUT CLKOUT Pin Output Current V = 0V to 3.3V l–150 150 µA
GPIO Pin Functions
VTH GPIO, ALERT Threshold Falling l1.26 1.28 1.31 V
VHYST GPIO, ALERT Hysteresis l2 20 35 mV
VOL GPIO, ALERT Output Low Voltage I = 3mA l0.3 0.4 V
IOH GPIO, ALERT Leakage Current V = 33V l0 ±1 µA
tPHL Stress Condition to GPIO2 Low Propagation
Delay
GATE Low or VDS = 1V l5 13 35 µs
TIMER Pin Functions
VTH TIMER Low Threshold Falling l0.11 0.15 0.19 V
TIMER High Threshold Rising l1.25 1.28 1.31 V
ITIMER TIMER Pull-Up Current V = 0V l–18 –20 –22 µA
TIMER Pull-Down Current V = 1.3V l3 5 7 µA
Doc Overcurrent Auto-Retry Duty Cycle l0.045 0.08 0.11 %
SOURCE, ADC Pin Currents
ISOURCE SOURCE Input Current V = 12V l70 180 350 µA
IADC– ADC– Input Current VADC– = 33V l0 ±1 µA
IADC+ ADC+ Input Current VADC+ = 33V l25 110 µA
ADC
RESOLUTION ADC Resolution (No Missing Codes) l12/16 Bits
VOS ADC Offset Error, Percent of Full-Scale l±0.25 %
TUE ADC Total Unadjusted Error (Note 5) ∆VADC, SOURCE, VDD, GPIO
POWER
ENERGY (Internal Timebase)
ENERGY (Crystal/External Timebase)
l
l
l
l
±0.7
±1.0
±5.1
±1.0
%
%
%
%
FSE ADC Full-Scale Error ∆VADC, SOURCE, VDD, GPIO
POWER
ENERGY (Internal Timebase)
ENERGY (Crystal/External Timebase)
l
l
l
l
±0.7
±1.0
±5.1
±1.0
%
%
%
%
VFS ADC Full-Scale Range ∆VADC = ADC+ – ADC–
SOURCE/VDD = 24V Range
SOURCE/VDD = 12V Range
SOURCE/VDD = 5V Range
SOURCE/VDD = 3.3V Range
GPIO
40
33.28
16.64
8.32
5.547
1.28
mV
V
V
V
V
V
INL ADC Integral Nonlinearity, 12-Bit Mode l0.2 ±5 LSB
LTC4281
6
Rev. A
For more information www.analog.com
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VDD = 12V unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VFS Alarm Threshold Full-Scale Range
(256 • VLSB)
∆VADC
SOURCE/VDD = 24V
SOURCE/VDD = 12V
SOURCE/VDD = 5V
SOURCE/VDD = 3.3V
GPIO
40
33.28
16.64
8.32
5.547
1.28
mV
V
V
V
V
V
RGPIO GPIO ADC Sampling Resistance V = 1.28V l1 2 MΩ
fCONV Conversion Rate 12-Bit Mode, Internal Clock
16-Bit Mode, Internal Clock
l
l
14.5
0.906
15.26
0.954
16
1
Hz
Hz
I2C Interface
VADR(H) ADRn Input High Threshold lINTVCC
– 0.8
INTVCC
– 0.5
INTVCC
– 0.2
V
IADR(IN,Z) ADRn Allowable Leakage in Open State l±3 µA
VADR(L) ADRn Input Low Threshold l0.2 0.5 0.8 V
IADR(IN) ADRn Input Current ADR = 0V, ADR = INTVCC l±80 µA
VSDA,SCL(TH) SDAI, SCL Input Threshold l1.5 1.7 2.0 V
ISDA,SCL(OH) SDAI, SCL Input Current SCL, SDA = 5.0V l±1 µA
VSDAO(OL) SDAO, Output Low Voltage I = 3mA l0.3 0.4 V
ISDAO(OH) SDAO, Pin Input Leakage Current VSDAO = 33V l0 ±1 µA
I2C Interface Timing
fSCL(MAX) Maximum SCL Clock Frequency l400 1000 kHz
tBUF(MIN) Bus Free Time Between STOP/START Condition l0.12 1.3 µs
tHD,STA(MIN) Hold Time After (Repeated) START Condition l30 600 us
tSU,STA(MIN) Repeated START Condition Set-Up Time l30 600 ns
tSU,STO(MIN) STOP Condition Set-Up Time l140 600 ns
tHD,DATI(MIN) Data Hold Time (Input) l30 100 ns
tHD,DATO Data Hold Time (Output) l300 500 900 ns
tSU,DAT(MIN) Data Set-Up Time l30 600 ns
tSP(MAX) Maximum Suppressed Spike Pulse Width l50 110 250 ns
CXSCL, SDA Input Capacitance (Note 4) l10 pF
tD(STUCK) I2C Stuck Bus Timeout l25 30 35 ms
EEPROM Characteristics
Endurance (Notes 7, 8) l10,000 Cycles
Retention (Notes 7, 8) l20 Years
tWRITE Write Operation Time l1 2.2 4 ms
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All currents into pins are positive. All voltages are referenced to
GND unless otherwise specified.
Note 3: An internal clamp limits the GATE pin to a minimum of 11V above
SOURCE. Driving this pin to voltages beyond the clamp may damage the
device.
Note 4: Guaranteed by design and not subject to test.
Note 5: TUE is the maximum ADC error for any code, given as a
percentage of full scale.
Note 6: UV, OV and FB internal thresholds are given as a percent difference
from the configured operating voltage.
Note 7: EEPROM endurance and retention are guaranteed by design,
characterization and correlation with statistical process controls.
Note 8: EEPROM endurance and retention will be degraded when TJ > 85°C.
LTC4281
7
Rev. A
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TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, VDD = 12V unless otherwise noted.
Current Limit Foldback Profile MOSFET Power Limit
Current Limit Threshold
vs Temperature
Current Limit Propagation Delay
vs Overdrive
Supply Current vs Voltage 3.3V Output Supply vs Voltage
3.3V Output Supply vs Load
Current for VDD = 12V
External MOSFET Gate Drive
vs Leakage Current
External MOSFET Gate Drive
vs Temperature
V
DD
(V)
0
5
10
15
20
25
30
35
3.00
3.25
3.50
3.75
4.00
4.25
4.50
I
DD
(mA)
4281 G01
V
DD
(V)
2.50
3
3.50
4
4.50
5
2.50
2.75
3.00
3.25
3.50
INTV
CC
(V)
4281 G02
I
LOAD
(mA)
0
4
8
12
16
20
3.0
3.1
3.2
3.3
3.4
3.5
INTV
CC
(V)
4281 G03
V
OUT
(V)
0
2
4
6
8
10
12
0
5
10
15
20
25
30
V
SENSE
(mV)
4281 G04
R
SENSE
= 1mΩ
V
DD
= 12V
V
OUT
(V)
0
2
4
6
8
10
12
14
0
10
20
30
40
50
60
70
80
90
100
POWER (W)
4281 G05
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
22
23
24
25
26
27
CIRCUIT BREAKER THRESHOLD (mV)
4281 G06
FAST PULL–DOWN
V
ILIM
= 25mV
V
SENSE
- V
ILIM
(mV)
0
20
40
60
80
100
0.1
1
10
100
1k
tPHL(GATE) (µs)
4281 G07
V
DD
= 12V
V
DD
= 5V
V
DD
= 3.3V
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
12.0
12.2
12.4
12.6
12.8
13.0
13.2
∆V
GATE
(V)
4281 G08
V
DD
= 12V
V
DD
= 5V
V
DD
= 3.3V
I
GATE
(LEAKAGE) (µA)
0
4
8
12
16
20
24
0
2
4
6
8
10
12
14
∆V
GATE
(V)
4281 G09
LTC4281
8
Rev. A
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
GPIO Pin Output Low Voltage
vs Load (VOL(GPIO) vs IGPIO)
ADC Total Unadjusted Error
vs Code (TUE vs Code)
ADC Integral Non-Linearity
vs Code (INL vs Code)
ADC Differential Non-Linearity
vs Code (DNL vs Code)
ADC Full-Scale Error
vs Temperature (VFSE vs Temp.)
External MOSFET Gate Drive
Current vs Temperature
(IGATE Current vs Temperature)
16-Bit GPIO ADC Noise Histogram 16-Bit Current ADC Noise Histogram 12-Bit GPIO ADC Noise Histogram
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
–18
–20
–22
–24
–26
I
GATE
(µA)
4281 G10
85°C
25°C
–40°C
I
GPIO
(mA)
0
2
4
6
8
10
0
0.2
0.4
0.6
0.8
1.0
V
OL(GPIO)
(V)
4281 G11
CODE
0
1024
2048
3071
4095
–0.025
–0.020
–0.015
–0.010
–0.005
0.000
ERROR (%)
4281 G12
CODE
0
1024
2048
3072
4096
–1.0
–0.8
–0.6
–0.4
–0.2
–0.0
0.2
0.4
0.6
0.8
1.0
INL (LSB)
4281 G13
Right Click In Graph Area for Menu
Double Click In Graph Area for Data Setup
CODE
1
1024
2048
3072
4095
–1.0
–0.8
–0.6
–0.4
–0.2
–0.0
0.2
0.4
0.6
0.8
1.0
DNL (LSB)
4281 G14
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
–0.20
–0.15
–0.10
–0.05
0.00
0.05
0.10
FULL SCALE ERROR (%)
4281 G15
V
IN
= 1.000V
RESOLUTION = 16b
V
LSB
= 19.5µV
CODE VARIATION (LSB)
–4
–3
–2
–1
0
1
2
3
4
0
1000
2000
3000
4000
5000
NUMBER OF READINGS
4281 G16
∆V
ADC
= 20mV
RESOLUTION = 16b
V
LSB
= 610nV
CODE VARIATION (LSB)
–4
–3
–2
–1
0
1
2
3
4
0
1000
2000
3000
4000
5000
NUMBER OF READINGS
4281 G17
V
GPIO
= 1.000V
RESOLUTION = 12b
V
LSB
= 312.5µV
CODE VARIATION (LSB)
–3
–2
–1
0
1
2
3
0
1000
2000
3000
4000
5000
6000
7000
NUMBER OF READINGS
4281 G18
LTC4281
9
Rev. A
For more information www.analog.com
PIN FUNCTIONS
ADC+: Positive Kelvin ADC Current Sense Input. Connect
this pin to the input side of the current sense resistor.
Must be connected to the same trace as V
DD
or a resistive
averaging network which adds up to 1Ω to VDD.
ADC–: Negative Kelvin ADC Current Sense Input. Connect
this pin to the output of the current sense resistor or a
resistive averaging network.
ADR0-ADR2: Serial Bus Address Inputs. Tying these pins
to ground (L), open (NC), or INTVCC (H) configures one
of 27 possible addresses. See Table1 in the Applications
Information section.
ALERT: I2C Bus ALERT Output or General Purpose Input/
Output. Configurable to ALERT output, general purpose
output or logic input. Tie to ground if unused.
CLKIN: Clock Input. Connect to an optional external crys-
tal oscillator circuit or drive with an external clock. Tie to
ground if unused.
CLKOUT: Clock Output. Connect to an optional external
crystal oscillator circuit. Can be configured in non-volatile
memory to output the internal clock or a low pulse when
the ADC finishes a conversion. Float if unused.
FB: Foldback Current Limit and Power Good Input. A
resistive divider from the output is tied to this pin. When
the voltage at this pin drops below 1.28V, power is not
considered good. The power bad condition may result
in the GPIO1 pin pulling low or going high impedance
depending on the configuration of GPIO_CONFIG register
0x07 bits 4 and 5, also a power bad fault is logged in this
condition if the GATE pin is high. The start-up current
limit folds back to 30% as the FB pin voltage drops from
1.3V to 0V.
GATE: Gate Drive for External N-Channel MOSFET. An
internal 20µA current source charges the gate of the
MOSFET. No compensation capacitor is required on the
GATE pin, but a resistor and capacitor network from this
pin to ground may be used to set the output voltage slew
rate. During turn-off there is a 1mA pull-down current.
During a short-circuit or undervoltage lockout (VDD or
INTVCC), a 900mA pull-down between GATE and SOURCE
is activated.
GND: Device Ground.
GPIO1: General Purpose Input/Open-Drain Output.
Configurable to general purpose output, logic input, and
power good or power bad signal. Tie to ground if unused.
GPIO2: General Purpose Input/Open-Drain Output.
Configurable to general purpose output, logic input,
MOSFET stress output, or data converter input. Tie to
ground if unused.
GPIO3: General Purpose Input/ Open-Drain Output.
Configurable to general purpose output, logic input, or
data converter input. Tie to ground if unused.
INTVCC: 3.3V Supply Decoupling Output. Connect a 1µF
capacitor from this pin to ground. To ensure fault log-
ging after power is lost a 4.7μF capacitor should be used.
25mA may be drawn from this pin to power 3.3V applica-
tion circuitry. Increase capacitance by 1µF/mA external
load if fault logging is used. This pin should not be driven
and is not current limited.
ON: On Control Input. Used to monitor a connection sense
pin on the backplane connector. The default polarity is
high = on, but may be reconfigured to low = on by setting
CONTROL1 register 0x00 bit 5 low. A on-to-off transition
on this pin clears the fault register if CONTROL1 register
0x00 bit 7 is set high. The ON pin has a precise 1.28V
threshold, allowing it to double as a supply monitor.
OV: Overvoltage Input Pin. An overvoltage condition is
present when this pin is above the configured threshold.
Connect a resistive divider when the internal divider is
disabled, otherwise leave open.
SCL: Serial Bus Clock Input. Data at the SDA pin is shifted
in or out on rising edges of SCL. This is a high impedance
pin that is generally driven by an open-drain output from
a master controller. An external pull-up resistor or current
source is required.
SDAI: Serial Bus Data Input. A high impedance
input for shifting in address, command or data bits.
Normally tied to SDAO to form the SDA line.
LTC4281
10
Rev. A
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PIN FUNCTIONS
SDAO: Serial Bus Data Output. Open-drain output for
sending data back to the master controller or acknowledg-
ing a write operation. Normally tied to SDAI to form the
SDA line. An external pull-up resistor or current source
is required.
SENSE+: Positive Kelvin Current Sense Input. Connect
this pin to the input of the current sense resistor or an
averaging network in the case of multiple sense resistors.
The parallel resistance of an averaging network should not
exceed 1Ω. Must operate at the same potential as VDD.
SENSE–: Negative Kelvin Current Sense Input. Connect
this pin to the output of the current sense resistor. The
current limit circuit controls the GATE pin to limit the
sense voltage between the SENSE+ and SENSE– pins to
the value selected in the ILIM register or less.
SOURCE: N-Channel MOSFET Source and ADC Input.
Connect to the source of the external N-channel MOSFET.
This pin provides a return for the gate pull-down cir-
cuit and also serves as the ADC input to monitor the
outputvoltage.
TIMER: Current Limit and Retry Timer Input. Connect a
capacitor between this pin and ground to set a 64ms/µF
duration for current limit, after which an overcurrent fault
is logged and GATE is pulled low. The duration of the off
time is 73s/µF when overcurrent auto-retry is enabled,
resulting in a 0.08% duty cycle.
UV: Undervoltage Input. An undervoltage condition is
present whenever this pin is below the configured thresh-
old. Connect a resistive divider when the internal divider
is disabled. A capacitor may be placed on this pin to filter
brief UV glitches on the input supply.
VDD: Supply Voltage Input and UV/OV Input. This pin has
an undervoltage lockout threshold of 2.7V. The UV and
OV thresholds are also measured at this pin, and the ADC
may be configured to read the voltage at this pin.
WP: EEPROM Write Protect. All writes to the EEPROM
except fault logging are blocked when WP is high.
LTC4281
11
Rev. A
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FUNCTIONAL DIAGRAM
+–
+
–
+
–
+
–
+
–
+
–
ADR2
32
1
∆VSENSE
UVLO1
OV
OV
UV
OV
2.8V
1.280V
5, 10, OR 15%
1.280V
–5, 10, OR 15%
1.280V
5, 10,
OR 15%
VDD(UVLO)
11
2
ADC–
ADC+
23
CLKOUT
CLK
7
CLKIN
8 26
28
ADR1 10
ADR0 9
ALERT
15
SCL 14
SDAO
13
SDAI 12
TIMER
INTVCC
6
GP
1.280V
GPIO1 18
GND 3
I2C
ACC2
TIME
4281 BD
48
16
12
ACC1
MULT
MIN
MAX
LOG
ENERGY
16
12
12
POWER
A/D
CONVERTER 1
A/D
CONVERTER 2
GPIO2
GPIO3
SOURCE
VDD
+
–
+
–
24V
12V
5V
3.3V
10k
10k
28k
25k
164k
SOURCE
200mV
1.280V
FET BAD
WP
VDD
+
–
UV
UV
27
WP
4
+
–
1.280V
ON ON
PG
ADJ
ON
1
+
–
FB
20
TM1 0.2V
20µA
5µA
1.280V
+
–
TM2
ILIM
ADJUST
3.3V
LDO
OSC
+
–
GP
1.280V
GPIO2 17
+
–
GP
LOGIC
1.280V
GPIO3 16
+
–
UVLO2 2.64V
VDD
INTVCC 5
+–
+–
+
–
+
–
+
–
SOURCE
8V
13.5V
CHARGE
PUMP AND
GATE DRIVER 21
22
GATE
FAST CL
SENSE+
SENSE–
SLOW CL
FET ON
GATE UP
75mV25mV
+–
24 25
24V
12V
5V
3.3V
1V
FB 0.3V
SOURCE
10k
10k
28k
25k
164k
LTC4281
12
Rev. A
For more information www.analog.com
OPERATION
The LTC4281 is designed to turn a board’s supply voltage
on and off in a controlled manner, allowing the board to be
safely inserted or removed from a live backplane. During
normal operation, the gate driver turns on an external
N-channel MOSFET to pass power to the load. The gate
driver uses a charge pump that derives its power from
the VDD pin. Also included in the gate driver is 12.5V
GATE-to-SOURCE clamp to protect the oxide of the exter-
nal MOSFET. During start-up the inrush current is tightly
controlled by using current limit foldback.
The current limit (CL) amplifier monitors the load cur-
rent with a current sense resistor connected between the
SENSE
+
and SENSE
–
pins. The CL amplifier limits the cur-
rent in the load by pulling back on the GATE-to-SOURCE
voltage in an active control loop when the sense voltage
exceeds the commanded value.
An overcurrent fault at the output may result in excessive
MOSFET power dissipation during active current limiting.
To limit this power, the CL amplifier regulates the voltage
between the SENSE+ and SENSE– pins at the value set in
the ILIM register. When the output (SOURCE pin) is low,
power dissipation is further reduced by folding back the
current limit to 30% of nominal.
The TIMER pin ramps up with 20μA when the current
limit circuit is active. The LTC4281 turns off the GATE and
registers a fault when the TIMER pin reaches its 1.28V
threshold. At this point the TIMER pin ramps down using
a 5μA current source until the voltage drops below 0.2V
(comparator TM1). The TIMER pin will then ramp up and
down 256 times with 20µA/5µA before indicating that
the external MOSFET has cooled and it is safe to turn on
again, provided overcurrent auto-retry is enabled.
The output voltage is monitored using the SOURCE pin
and the power good (PG) comparator to determine if the
power is available for the load. The power good condi-
tion can be signaled by the GPIO1 pin. The GPIO1 pin
may also be configured to signal power bad, as a general
purpose input (GP comparator), or a general purpose
open-drainoutput.
GPIO2 and GPIO3 may also be configured as general
purpose inputs or general purpose open-drain outputs.
Additionally, the ADC measures these pins with a 1.28V
full-scale. GPIO2 may be configured to pull low to indicate
that the external MOSFET is in a state of stress when the
MOSFET is commanded to be on and either the gate volt-
age is lower than it should be or the DRAIN-to-SOURCE
voltage exceeds 200mV.
The Functional Diagram shows the monitoring blocks of
the LTC4281. The group of comparators on the left side
includes the undervoltage (UV), overvoltage (OV), and
(ON) comparators. These comparators determine if the
external conditions are valid prior to turning on the GATE.
But first the two undervoltage lockout circuits, UVLO1
and UVLO2, validate the input supply and the internally
generated 3.3V supply, INTVCC. UVLO2 also generates
the power-up initialization to the logic circuits and copies
the contents of the EEPROM to operating memory after
INTVCC crosses this rising threshold.
Included in the LTC4281 is a pair of 12 to 16-bit A/D
converters. One data converter continuously monitors the
ADC+ to ADC– voltage, sampling every 16µs and produc-
ing a 12-bit result of the average current sense voltage
every 65ms. The other data converter is synchronized to
the first one and measures the GPIO voltage and SOURCE
voltage during the same time period. Every time the ADCs
finish taking a measurement, the current sense voltage
is multiplied by the measurement of the SOURCE pin to
provide a power measurement. Every time power is mea-
sured, it is added to an energy accumulator which keeps
track of how much energy has been transmitted to the
load. The energy accumulator can generate an optional
alert upon overflow, and can be preset to allow it to over-
flow after a given amount of energy has been transmit-
ted. A time accumulator also keeps track of how many
times the power meter has been incremented; dividing
the results of the energy accumulator by the time accu-
mulator gives the average system power. The minimum
and maximum measurements of GPIO, SOURCE, ADC+
to ADC– and POWER are stored, and optional alerts may
be generated if a measurement is above or below user
configurable 8-bit thresholds.
An internal EEPROM provides nonvolatile configuration
of the LTC4281’s behavior, records fault information and
provides four bytes of uncommitted memory for general
purpose storage.
LTC4281
13
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
A typical LTC4281 application is a high availability system
in which a positive voltage supply is distributed to power
individual hot-swapped cards. The device measures card
voltages and currents and records past and present fault
conditions. The LTC4281 stores min and max ADC mea-
surements, calculates power and energy, and can be con-
figured to generate alerts based on measurement results,
avoiding the need for the system to poll the device on a
regular basis. The LTC4281 is configured with nonvolatile
EEPROM memory, allowing it to be configured during
board level testing and avoid having to configure the Hot
Swap controller at every insertion.
A basic LTC4281 application circuit is shown in Figure1.
The following sections cover turn-on, turn-off and various
faults that the LTC4281 detects and acts upon. External
component selection is discussed in detail in the Design
Example section.
Turn-On Sequence
The power supply on a board is controlled by using an
N-channel pass transistor, Q1, placed in the power path.
Resistor R
S
senses current through Q1. Resistors R1, R2
and R3 define undervoltage and overvoltage levels. R4
prevents high frequency self-oscillations in Q1, capacitors
C4 and C5 form a resonator network with crystal Y1 to
provide an accurate time base.
Several conditions must be present before the external
MOSFET turns on. First the external supply, V
DD
, must
exceed its 2.7V undervoltage lockout level. Next the
internally generated supply, INTVCC, must cross its 2.6V
undervoltage threshold. This generates a 1ms power-on-
reset pulse. During reset the fault registers are cleared
and the control registers are loaded with the data held in
the corresponding EEPROM registers.
Figure1. Typical Application
+
VDD
Z1
SMCJI5C
×2
GATE
CTIMER
15nF
TIMER
LTC4281
INTVCC
CONNECTOR 1
CONNECTOR 2
PLUG-IN
CARD
NC
4281 F01
GND
SDA
SCL
ALERT
R4
10Ω
Q1
PSMN2R0-30YLE × 2
CL
VOUT
12V
65A ADJUSTABLE
R
S
0.5mΩ
SOURCE
FB
GPIO1
GPIO2
GPIO3
POWER GOOD
GP
GP
UV
OV
SDAI
SDAO
SCL
ALERT
ADR0
ADR1
ADR2
ON
SENSE+SENSE–
ADC+ADC–
WP GNDCLKOUTCLKIN
Y1
4MHz
ABLS-4.000MHZ-B4-T
C3
4.7µF
100k
C4
36pF
C5
36pF
12V
BACKPLANE
12V
R2
1.18k
1%
R3
34.0k
1%
R1
3.4k
1%
CF
0.1µF
25V
R7
28.7k
1%
R8
3.57k
1%
CG
10nF
RG 60k
DG (OPT)
OPERATION
An I2C interface is provided to read the A/D data registers.
It also allows the host to poll the device and determine if
faults have occurred. If the ALERT pin is configured as an
ALERT interrupt, the host is enabled to respond to faults
in real time. The I2C device address is decoded using the
ADR0-ADR2 pins. These inputs have three states each
that decode into a total of 27 device addresses, as shown
in Table1.
LTC4281
14
Rev. A
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APPLICATIONS INFORMATION
After a power-on-reset pulse, the UV and OV pins verify
that input power is within the acceptable range. The state
of the UV and OV comparators is indicated by STATUS
register 0x1E bits 1 and 2 and must be stable for at least
50ms to qualify for turn-on. The ON pin is checked to see
that a connection sense (“short”) pin has asserted to the
correct state. By default the ON pin has no delay, but a
50ms debounce delay may be added by setting CONTROL
register 0x00 bit 6 high. When these conditions are satis-
fied, turn-on is initiated. Figure4 shows connection sense
configurations for both high- and low-going short pins.
The ON pin has a precise 1.28V threshold, allowing it to
also monitor a voltage through the short pin, such as a
housekeeping or auxiliary supply delivered by the back-
plane. Use of the UV/OV divider for short pin detection in
high current applications is not recommended, as voltage
drops in the connector and fuse will impair the accuracy
of the intended function.
The MOSFET is then turned on by charging up the GATE
pin with a 20μA current source. When the GATE pin volt-
age reaches the MOSFET threshold voltage, the MOSFET
begins to turn on and the SOURCE voltage then follows
the GATE voltage as it increases.
VDD + 12V
VSENSE
100%
NORMALIZED
MOSFET POWER
100%
4281 F02
30%
VGATE
VOUT
POWER GOOD
(GPIO1)
ILOAD • RS
VDD + 8V
CURRENT
LIMITED
FB
LIMITED POWER
VDD
VGS = 8V
Figure2. Power-Up Waveforms
While the MOSFET is turning on, the power dissipation in
the MOSFET is limited to a fixed value by the current limit
foldback profile as shown in Figure2. As the SOURCE
voltage rises, the FB pin follows as set by R7 and R8. Once
the GATE pin crosses its 8V ∆VGATE threshold and the FB
pin has exceeded its 1.28V threshold, the GPIO1 pin (in
its power good configuration) releases high to indicate
power is good and the load may be activated.
At the minimum input supply voltage of 2.9V, the mini-
mum GATE-to-SOURCE drive voltage is 10V. The GATE-
to-SOURCE voltage is clamped below 13.5V to protect
the gates of 20V N-channel MOSFETs. A curve of GATE-
to-SOURCE drive (∆VGATE) versus VDD is shown in the
Typical Performance Characteristics.
Turn-Off Sequence
A normal turn-off sequence is initiated by card with-
drawal when the backplane connector short pin opens,
causing the ON pin to change state. Turn-off may be also
initiated by writing a 0 to CONTROL register 0x00 bit 3.
Additionally, several fault conditions turn off the GATE
pin. These include an input overvoltage, input undervolt-
age, overcurrent or FET-BAD fault. Setting high any of the
LTC4281
15
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
UV, OV , OC or FET-BAD fault bits (bits 0-2 and 6 of the
FAULT_LOG register 0x04, also latches off the GATE pin
if the associated auto-retry bits are set low.
The MOSFET is turned off with a 1mA current pulling
down the GATE pin to ground. With the MOSFET turned
off, the SOURCE and FB voltages drop as the load capaci-
tance discharges. When the FB voltage crosses below
its threshold, GPIO1 pulls low to indicate that the output
power is no longer good if configured to indicate power
good. If the VDD pin falls below 2.66V for greater than
2µs or INTVCC drops below 2.49V for greater than 2µs, a
fast shut down of the MOSFET is initiated. The GATE pin
is then pulled down with a 900mA current to the SOURCE
pin.
Current Limit Adjustment
The current limit sense voltage of the LTC4281 is adjust-
able between 12.5mV and 34.4mV in 3.1mV steps via the
I2C interface with bits 7-5 of the ILIM_ADJUST register
0x11. Default values are stored in the onboard EEPROM.
This can be used to adjust the sense voltage to achieve a
given current limit using the limited selection of standard
sense resistor values available around 1mΩ. It also allows
the LTC4281 to reduce available current for light loads
or increase it in anticipation of a surge. This feature also
enables the use of board trace as a sense resistors by
trimming the sense voltage to match measured copper
resistance during final test. The measured copper resis-
tance may be written to the undedicated scratch pad area
of the EEPROM so that it is available to scale ADC current
measurements.
Constant Current Start-Up Using a GATE R-C Network
An optional series resistor and capacitor network from
GATE to GROUND (RG and CG in Figure1) provides an
inrush current less than the current limit by limiting the
slew rate of the GATE pin, which pulls up with 20µA. The
current limit timer will not run since the current limit is not
engaged during startup so a small timer capacitor may be
used, which allows the use of MOSFETs with smaller safe
operating area. Power good will not signal until the FB pin
crosses its threshold and the GATE-to-SOURCE voltage
crosses the 8V threshold which indicates the MOSFET is
fully enhanced. When both those conditions are met, the
output voltage is suitable for the load to be turned on and
the impedance back to the supply through the MOSFET
is low. Power good is then asserted with the GPIO1 pin
or read via the interface, signaling that it is safe to turn
on downstream loads. A power-bad fault is not generated
when starting up in this manner because the FB pin will
cross its threshold before the GATE-to-SOURCE threshold
is crossed. RG should be chosen such that IGATE • RG is
less than the threshold of the MOSFET to avoid a current
spike at the beginning of startup. Reducing RG degrades
the stability of the current limit circuit (see Current Limit
Stability below). If a value of R
G
is not found that pro-
duces a voltage less than the MOSFET threshold when the
20µA IGATE current flows through it, while also producing
a stable current limit servo loop, CG may be charged with
a diode (DG in Figure1) during startup in parallel with a
large RG, such as 500kΩ, to discharge it when the part
turns off.
Current Limit Stability
For most applications the LTC4281 current limit loop is
stable without additional components. However there are
certain conditions where additional components may be
needed to improve stability. The dominant pole of the
current limit circuit is set by the capacitance at the gate of
the external MOSFET, and larger gate capacitance makes
the current limit loop more stable. Usually a total of 8nF
GATE-to-SOURCE capacitance is sufficient for stability
and is provided by inherent MOSFET CGS. The stability
of the loop is degraded by reducing the size of the resis-
tor on a gate RC network if one is used to limit startup
current as in Figure1, which may necessitate additional
GATE-to-SOURCE capacitance. Board level short-circuit
testing is highly recommended as board layout can also
affect transient performance, the worst-case condition for
current limit stability occurs when the output is shorted
to ground after a normal start-up.
Parasitic MOSFET Oscillations
Not all circuit oscillations can be ascribed to the current
limit loop. Some higher frequency oscillations can arise
from the MOSFET itself. There are two possible para-
sitic oscillation mechanisms. The first type of oscillation
LTC4281
16
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
occurs at high frequencies, typically above 1MHz. This
high frequency oscillation is easily damped with gate
resistor R4 as shown in Figure1. In some applications,
one may find that this resistor helps in short-circuit tran-
sient recovery as well. However, too large of a resistor will
slow down the turn-off time. The recommended R4 range
is between 5Ω and 500Ω. 10Ω provides stability without
affecting turn-off time. This resistor must be located at the
MOSFET package with no other components connected
to the MOSFET gate pin.
A second type of parasitic oscillation occurs at frequen-
cies between 200kHz and 800kHz when the MOSFET
source is loaded with less than 10µF, and the drain is fed
with an inductive impedance such as contributed by wir-
ing inductance. To prevent this second type of oscillation
load the source with more than 10µF and bypass the input
supply with a 10Ω, 100nF snubber to ground.
Overcurrent Fault
The LTC4281 features an adjustable current limit with
foldback that protects the MOSFET from excessive load
current. To protect the MOSFET during active current
limit, the available current is reduced as a function of the
output voltage sensed by the FB pin such that the power
dissipated by the MOSFET is constant. A graph in the
Typical Performance Characteristics shows the current
limit and power versus FB voltage.
An overcurrent fault occurs when the current limit cir-
cuitry has been engaged for the MOSFET for longer than
the time-out delay set by the TIMER capacitor. Current
limiting begins when the current sense voltage between
the SENSE+ and SENSE– pins reaches the current limit
level (which depends on foldback and the current limit
configuration). The GATE pin is then pulled down and
regulated in order to limit the current sense voltage to
the current limit value. When the GATE pin regulator is
in current limit, the circuit breaker time delay starts by
charging the external timer capacitor from the TIMER pin
with a 20µA pull-up current. If the TIMER pin reaches
its 1.28V threshold, the external switch turns off with a
1mA current from GATE to ground. If the GATE pin stops
current limiting before the TIMER pin reaches the 1.28V
threshold, the TIMER pin will discharge with 5μA. For a
given circuit breaker time delay, tCB, the equation for set-
ting the timing capacitor’s value is as follows:
CT = tCB • 0.016[μF/ms]
If an overcurrent fault is detected the MOSFET is turned
off and the TIMER pin begins discharging with a 5µA pull-
down current. When the TIMER pin reaches its 0.15V
threshold, it will cycle up and down with 20µA and 5µA
256 times to allow the MOSFET time to cool down. When
automatically retrying, the resulting overcurrent duty
cycle is 0.08%. The final time the TIMER pin falls below
its 0.15V lower threshold the switches are allowed to turn
on again if the overcurrent auto-retry bit is set or the
overcurrent fault bit has been reset by the I2C interface.
The waveform in Figure3 shows how the output turns off
following a short circuit.
Overvoltage Fault
An overvoltage fault occurs when the OV pin rises above
the OV threshold for longer than 15µs. This shuts off
the GATE pin with a 1mA current to ground and sets the
overvoltage present and overvoltage fault bits (Bit 0) in
STATUS and FAULT_LOG registers 0x1E and 0x04. If the
voltage subsequently falls back below the threshold for
50ms, the GATE pin is allowed to turn on again unless
overvoltage auto-retry has been disabled by clearing the
OV auto-retry bit (Bit 0) in CONTROL register 0x00. If
an external resistive divider is used, the OV threshold is
1.28V on the OV pin. When using the internal dividers the
OV threshold is referenced to the VDD pin.
Figure3. Short-Circuit Waveform
TIMER EXPIRES
200µs/DIV
TIMER
2V/DIV
GATE
10V/DIV
SOURCE
10V/DIV
Current
50A/DIV
4281 F03
LTC4281
17
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
Undervoltage Fault
An undervoltage fault occurs when the UV pin falls below
its 1.28V threshold for longer than 15µs. This shuts off
the GATE pin with a 1mA current to ground and sets the
undervoltage present and undervoltage fault bits (Bit 0)
in STATUS and FAULT_LOG registers 0x1E and 0x04. If
the voltage subsequently rises back above the threshold
for 50ms, the GATE pin is allowed to turn on again unless
undervoltage auto-retry has been disabled by clearing
the UV auto-retry bit in CONTROL register 0x00. For the
internal thresholds, the UV and OV signals may be filtered
by placing a capacitor on the UV pin.
ON/OFF Control
The ON pin can be configured active high or active low
with CONTROL register 0x00 bit 5 (1 for active high).
In the active high configuration it is a true ON input,
in the active low configuration it can be used as an
ENABLE input to detect card insertion with a grounded
short pin. The delay from the ON pin commanding the
part to turn on until the GATE pin begins to rise is set
by CONTROL registers 0x00 bit 6. If this bit is low the
GATE pin turns on immediately, and if it is high it turns
on after a 50ms debounce delay. Whenever the ON pin
toggles, bit 4 in FAULT_LOG register 0x04 is set to indi-
cate a change of state and the other bits in FAULT reg-
ister 0x04 are reset unless the ON_FAULT_MASK bit 7
in CONTROL register 0x00 is set.
The FET_ON bit, bit 3 of CONTROL register 0x00, is set
or reset by the rising and falling edges of the ON pin and
by I2C write commands. When the LTC4281 comes out of
UVLO the default state for bit 3 is read out of the EEPROM.
If it is a 0, the part is configured to stay off after power-up
and ignore the state of the ON pin. If it is a 1 the condition
of the ON pin will be latched to bit 3 after the debounce
period and the part will turn the GATE on if the ON pin is
in the ON state.
If the system shuts down due to a fault, it may be desirable
to restart the system simply by removing and reinserting
a load card. In cases where the LTC4281 and the switch
reside on a backplane or midplane and the load resides on
a plug-in card, the ON pin detects when the plug-in card
is removed. Figure4 shows an example where the ON pin
4281 F04a
ON
LTC4281
12V
CON 10k
4281 F04b
12V
CON 10k
LTC4281
ON
4281 F04c
12V MAIN
AUX 3.3V
CON
10k
13k
LTC4281
ON
Figure4. Connection Sense Configurations with the ON Pin
(a) ON Configured Active High (Default)
CONTROL Register 0x00 Bit 5=1
(b) ON Configured Active Low CONTROL
Register 0x00 Bit 5=0
(c) ON Pin Sensing of AUX Supply ON
Pin Configured Active High (Default)
LTC4281
18
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
is used to detect insertion. Once the plug-in card is rein-
serted the FAULT_LOG register 0x04 is cleared (except
for bit 5, which indicates the ON pin changed state). After
the ON pin turn-on delay, the system is allowed to start
up again.
If a connection sense on the plug-in card is driving the ON
pin, insertion or removal of the card may cause the pin
voltage to bounce. This results in clearing the FAULT_LOG
register when the card is removed. The pin may be de-
bounced using a filter capacitor, CON, on the ON pin as
shown in Figure4. Note that the polarity of the ON pin is
inverted with CONTROL Register 0x00 bit 5 set to 0.
FET-Bad Fault
In a Hot Swap application several things can prevent the
MOSFET from turning on and reaching a low impedance
state. A damaged MOSFET may have leakage from gate
to drain or have degraded RDS(ON). Debris on the board
may also produce leakage or a short from the GATE pin
to the SOURCE pin, the MOSFET drain, or to ground. In
these conditions the LTC4281 may not be able to pull the
GATE pin high enough to fully enhance the MOSFET, or
the MOSFET may not reach the intended RDS(ON) when
the GATE pin is fully enhanced. This can put the MOSFET
in a condition where the power in the MOSFET is higher
than its continuous power capability, even though the cur-
rent is below the current limit. The LTC4281 monitors the
integrity of the MOSFET in two ways, and acts on both of
them in the same manner.
First, the LTC4281 monitors the voltage between the
MOSFET VDD and SOURCE pins. The LTC4281 has a
comparator that detects a bad DRAIN-to-SOURCE volt-
age (VDS) whenever the VDS is greater than 200mV.
Second, the LTC4281 monitors the GATE voltage. The
GATE voltage may not fully enhance with a damaged
MOSFET, and a severely damaged MOSFET most often
has GATE, DRAIN and SOURCE all shorted together. If the
LTC4281 is in the ON state, but the GATE pin does not
come up to its 8V threshold above SOURCE, a FET-bad
condition is detected.
When either FET-bad condition is present while the
MOSFET is commanded on, an internal FET-bad fault
timer starts. When the timer reaches the threshold set in
FET_BAD_FAULT_TIME register 0x06 (1ms per LSB for a
maximum of 255ms), a FET-bad fault condition is set, the
part turns off, and the GATE pin is pulled low with a 1mA
current. In the case of a GAIN-to-DRAIN short, it may be
impossible for the LTC4281 to turn off the MOSFET. In
this case the LTC4281 can be configured to signal power-
bad to the load so the load goes into a low current state
and send a FET-bad fault alert to the controller that may
be able to shut down upstream supplies and/or flag the
card for service.
The LTC4281 treats a FET-bad fault similar to an overcur-
rent fault, and will auto-retry after 256 timer cycles if the
overcurrent auto-retry bit is set. Note that during start-
up, the FET-bad condition is present because the voltage
from DRAIN to SOURCE is greater than 200mV and the
GATE pin is not fully enhanced, thus the FET-bad timeout
must be long enough to allow for the largest allowable
load to start up. FET-bad faults are disabled by setting the
FET_BAD_FAULT_TIMER value to 0x00.
FET Short Fault
A FET short fault is reported if the data converter mea-
sures a current sense voltage greater than or equal to
0.25mV while the GATE pin is turned off. This condition
sets the FET_SHORT bit 5 in status register 0x1E, and
FET_SHORT_FAULT bit 5 in fault register 0x04.
Power-Bad Fault
The POWER_GOOD status bit, bit 3 in STATUS register
0x1E, is set when the FB pin voltage rises above its 1.28V
threshold. To indicate POWER_GOOD on the GPIO1 pin,
the GATE pin must first exceed the 8V VGS thresholds after
start-up, this requirement prevents POWER_GOOD from
asserting during start-up when the FB pin first crosses
its threshold. After start-up the GPIO1 pin will output the
value of the FB comparator so that POWER_GOOD stays
high even in cases such as an input voltage step that
causes the GATE pins to briefly dip below 8V VGS. See
Figure5.
LTC4281
19
Rev. A
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APPLICATIONS INFORMATION
A power bad fault is generated when the FB pin is low and
the GATE pin is high, preventing power-bad faults during
power-up or power-down.
Fault Alerts
A fault condition sets the corresponding fault bit in
FAULT_LOG register 0x04, ADC_ALERT_LOG register
0x05, and TIMER_OVERFLOW_PRESENT (Bit 1) and
METER_OVERFLOW_PRESENT (Bit 2) in the STATUS
register 0x1F. Fault bits are reset by writing a 0 and the
overflow status bits are reset by resetting the energy
meter by setting and resetting ADC_CONTROL register
0x1D bit 6. A fault condition can also generate an alert
(ALERT asserts low) by setting the corresponding bit in
the alert mask registers: ALERT registers 0x02 and 0x03,
and GPIO_CONFIG register bit 0. A low on ALERT may be
generated upon completion of an ADC measurement by
setting bit 2 in the GPIO_CONFIG register 0x07. This con-
dition does not have a corresponding fault bit. Faults with
enabled alerts set bit 7 in the ALERT_CONTROL register
0x1C, which controls the state of the ALERT pin. Clearing
this bit will cause the ALERT pin to go high and setting this
bit causes it to go low. Alert masking stored in EEPROM
is transferred into registers at power up.
After the bus master controller broadcasts the Alert
Response Address, the LTC4281 responds with its
address on the SDA line and releases ALERT as shown in
Figure16. If there is a collision between two LTC4281s
responding with their addresses simultaneously, then
the device with the lower address wins arbitration and
releases its ALERT pin. The devices that lost arbitration
will still hold the ALERT pin low and will respond with their
addresses and release ALERT as the I2C master broad-
casts additional Alert Response protocols until ALERT is
release by all devices. The ALERT pin can also be released
by clearing ALERT_CONTROL bit 7 in register 0x1C with
the I2C interface.
The ALERT pin can also be used as a GPIO pin, which
pulls low by setting ALERT_CONTROL bit 6 in register
0x1C. The ALERT pin input status is located in STATUS
register 0x1F bit 4.
Once the ALERT signal has been released from a fault, it
will pull low again if the corresponding fault reoccurs, but
not if the fault remains continuously present.
Resetting Faults in FAULT_LOG
The faults in FAULT_LOG register 0x04 may cause the
part to latch off if their corresponding auto-retry bits are
not set. In backplane resident applications it is desirable to
latch off if a card has produced a failure and start up nor-
mally if the card is replaced. To allow this function the ON
pin must be used as a connection sense input. When the
ON_FAULT_MASK bit (CONTROL bit 7) in register 0x00 is
not set, a turn-off signal from the ON pin (card removed)
will clear the FAULT_LOG register except for bit 4 (ON
changed state). The entire FAULT_LOG register is also
cleared when the INTV
CC
pin falls below it’s 2.49V thresh-
old (UVLO), and individual bits may be cleared manually
via the I2C interface. Note that faults that are still present,
as indicated in STATUS register 0x1E, cannot be cleared.
Figure5. POWER_GOOD Logic
POWER_BAD_FAULT PRESENT
POWER_GOOD(GPIO)
POWER_GOOD
STATUS
GATE_HIGH
FET_ON S
R4281 F05
Q
LTC4281
20
Rev. A
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APPLICATIONS INFORMATION
When the ON_FAULT_MASK bit (CONTROL bit 7 in reg-
ister 0x00) is set, a turn-off signal from the ON pin will
not clear the FAULT_LOG register. Additionally, when
the corresponding ON_FAULT_MASK bit is set in the
EEPROM, the FAULT_LOG register 0x04 is loaded from
the EEPROM (0x24) at boot. In this case, stored faults in
the EEPROM are loaded into the FAULT_LOG register. This
may be used in conjunction with disabling fault autoretry
to configure a card to not attempt to turn on again after a
fault has been logged, even after a power cycle, until the
system controller has interrogated the card and cleared
the fault or flagged the card for service. For applications
where the system controller is downstream of the hot
swap, all autoretries should be enabled when the ON_
FAULT_MASK bit in the EEPROM is set and fault logging
is enabled so that logged faults do not permanently latch
the hot swap off.
The FAULT_LOG register is not cleared when auto-retry-
ing. When auto-retry is disabled the existence of a logged
fault keeps the MOSFET off. As soon as the FAULT_LOG
is cleared, the MOSFET turns on. If auto-retry is enabled,
then a high status bit keeps the MOSFET off and the
FAULT_LOG bit is ignored. Subsequently, when the
FAULT_LOG bit is cleared by removal of the fault condi-
tion, the MOSFET is allowed to turn on again even though
the fault bit remains set as a record of the previous fault
condition.
Reboot
The LTC4281 features a reboot command bit, located in
bit7 of ADC_CONTROL register 0x1D. Setting this bit will
cause the LTC4281 to reset and copy the contents of the
EEPROM to operating memory the same as after initial
power-up. The 50ms debounce before the part restarts
is lengthened to 3.2s for reboot in order to allow load
capacitance to discharge and reset before the LTC4281
turns back on. On systems where the Hot Swap controller
supplies power to the I2C master, this allows the master
to issue a command that power cycles the entire board,
including itself.
Data Converters
The LTC4281 incorporates a pair of sigma-delta A/D con-
verters that are configurable to 12 or 16 bits. One converter
continuously samples the current sense voltage, while the
other monitors the input/output voltage and the voltage
on a GPIO input. The sigma-delta architecture inherently
averages signal noise during the measurementperiod.
The data converters may be run in a 12-bit or 16-bit mode,
as selected by bit 1 in ILIM_ADJUST register 0x11. The
second data converter may be configured to measure VIN
at the VDD pin or VOUT at the SOURCE pin by setting bit 3,
and can select between measuring GPIO2 or GPIO3 with
bit 2. The data converter full-scale is 40mV for the cur-
rent sense voltage, a choice of 33.28V, 16.64V, 8.32V or
5.547V for VDD and VSOURCE, and 1.28V for GPIO.
The ADC+ and ADC– pins allow the ADC to measure the
voltage across the sense resistor. Some applications may
use two or more sense resistors in parallel to limit the
power in each resistor or achieve a specific parallel resis-
tance or tolerance unavailable in a single sense resistor.
In this case averaging resistors can be used to accurately
measure the current by choosing averaging resistors with
the same ratio as the sense resistors they connect to. See
Figure6. In this case the effective ADC sense resistor is
RS in parallel with k • RS for the current limit. Scaling
the averaging resistors, RA, by the same scaling factor,
k, allows the ADC to measure the correct sense voltage
for this effective sense resistor. The smallest averaging
resistor should not exceed 1Ω.
Figure6.
RSENSE2
k • RS
4281 F06
RSENSE1
RS
k•RA
ADC+
SENSE+
ADC–
SENSE–
k•RA
RA
RA
Weighted Averaging Sense Voltages
LTC4281
21
Rev. A
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APPLICATIONS INFORMATION
The two data converters are synchronized, and after each
current measurement conversion, the measured current is
multiplied by the measured VDD or VSOURCE to yield input
or output power. After each conversion the measurement
results and power are compared to the recorded min and
max values. If the measurement is a new min or max, then
those registers are updated. The measurements are also
compared to the min/max alarm thresholds in registers
0x08 to 0x0F and will set the corresponding ADC alert bit
in ADC_ALERT_LOG register 0x05 and generate an alert
if configured to do so in ALERT register 0x03.
After each measurement, calculated power is added to
an accumulator that meters energy. Since the current is
continuously monitored by a dedicated ADC, the current
is sampled every 16µs, ensuring that the energy meter
will accurately meter noisy loads up to 62.5kHz noise
frequency. The 6-byte energy meter is capable of accu-
mulating 20 days of power at full scale, which is several
months at a nominal power level. An optional alert may
be generated when the meter overflows. To measure cou
-
lombs, the energy meter may be configured to accumu-
late current rather than power by setting CLK_DIVIDER
register 0x10 bit 7.
A time counter keeps track of how many times power has
been added into the energy meter. Dividing the energy by
the number in the counter will yield the average power
over the accumulated interval. When metering coulombs
dividing the metered charge by the counter produces
the average current over the accumulation interval. The
4-byte time counter will keep count for 10 years in the
12-bit mode before overflowing, and can generate an alert
at full scale to indicate that the counter is about to roll
over. Multiplying the value in the counter by tCONV yields
the time that the energy meter has been accumulating.
Both the energy accumulator and time counter are writ-
able, allowing them to be pre-loaded with a given energy
and/or time before overflow so that the LTC4281 will gen
-
erate an overflow alert after either a specified amount of
energy has been delivered or time has passed.
The following formulas are used to convert the values in
the ADC result registers into physical units. The data in
the 12-bit mode is left justified, so the same equations
apply to the 12-bit mode and the 16-bit mode.
To calculate GPIO voltage:
V=
CODE(word) • 1.280
2
16
−1
To calculate input/output voltage:
V=CODE(word) • VFS(OUT)
2
16
−1
where VFS(OUT) is 33.28V, 16.64V, 8.32V or 5.547V
depending on the part being in 24V, 12V, 5V or 3.3V
mode, respectively.
To calculate current in amperes:
I=CODE(word) • 0.040V
216 −1
( )
•RSENSE
To calculate power in watts:
P=CODE(word) • 0.040V • VFS(OUT) • 2
16
216 −1
( )
2•RSENSE
To calculate energy in joules:
E=CODE(48 bits) • 0.040V • VFS(OUT) • tCONV • 28
216 −1
( )
2•RSENSE
To calculate coulombs:
C=CODE(48 Bits) • 0.040V • tCONV
(216 −1) • R
SENSE
where tCON = (1/fCONV) is 0.065535s for 12-bit mode and
1.0486s for 16-bit mode.
To calculate average power over the energy accumulation
period:
P(AVG)=
E
t
CONV
• CODE(COUNTER)
I(AVG) =C
t
CONV
• CODE(COUNTER)
LTC4281
22
Rev. A
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APPLICATIONS INFORMATION
To calculate GPIO voltage alarm thresholds:
V=
CODE(byte) • 1.280
255
To calculate input/output voltage alarm thresholds:
VALARM =CODE(byte) • VFS(OUT)
255
where VFS(OUT) is 33.28V, 16.64V, 8.32V or 5.5467V
depending on the part being in 24V, 12V, 5V or 3.3V
mode, respectively.
To calculate current alarm thresholds in amperes:
I=CODE(byte) • 0.040V
255 •R
SENSE
To calculate power alarm threshold in watts:
P=CODE(byte) • 0.040V • VFS(OUT) • 2
8
R
SENSE
• 255 • 255
Note that falling alarm thresholds use CODE(byte)+1 in
the above equations since they trip at the top edge of the
code, which is 1LSB higher than the rising threshold.
CLKIN, CLKOUT: Crystal Oscillator/External Clock
Accurately measuring energy by integrating power
requires a precise integration period. The on-chip clock
of the LTC4281 is trimmed to 1.5% and specified over
temperature to 5% and is invoked by grounding CLKIN.
For increased accuracy a crystal oscillator or external pre-
cision clock may be used on the CLKIN and CLKOUT pins.
A 4MHz crystal oscillator or resonator may be connected
to the two CLK pins as shown in Figure1.
Crystal oscillators are sensitive to noise and parasitic
capacitance. Care should be taken in layout to minimize
trace length between the LTC4281 and the crystal. Keep
noisy traces away from the crystal traces, or shield the
crystal traces with a ground trace.
Alternatively, an external clock may be applied to CLKIN
with CLKOUT left unconnected. The LTC4281 can accept
an external clock between 250kHz and 15.5MHz, with
clocks faster than 250kHz reduced to 250kHz by a pro-
grammable divider, the clock frequency is divided by twice
the value in CLK_DIVIDER register 0x10 bits 0-4. Code
00000 passes the clock through without division while
code 01000 divides a 4MHz clock down to 250kHz. The
divided external clock may differ from 250kHz by 5%
without affecting other specifications.
Configuring the GPIO Pins
The LTC4281 has three GPIO pins and an ALERT pin, all of
which can be used as general purpose input/output pins.
The GPIO1 pin is configured using the GPIO_CONFIG
register 0x07 bits 5-4. GPIO2 will pull low to indicate
MOSFET stress if GPIO_CONFIG bit 1 is set and pulls low
if bit 6 is low. GPIO3 pulls low if GPIO_CONFIG bit 7 is set
and is otherwise high impedance. The ALERT pin can be
used as a GPIO pin by setting all the alert enable bits to 0
to disable alerts, then setting bit 6 in ALERT_CONTROL
register 0x1C. Bit 7 in ALERT_CONTROL can also be set
to pull the ALERT pin low, but bit 7 will cause the part to
respond to the alert response protocol, while bit 6 will not.
GPIO1-GPIO3 and ALERT all have comparators monitor-
ing the voltage on these pins with a threshold of 1.28V
when the pins are serving as outputs. The results may be
read from the second byte of the STATUS register, 0x1F,
bits 4-7.
Supply Transients
In card-resident applications, output short circuits work-
ing against the inductive nature of the supply can easily
cause the input voltage to dip below the UV threshold.
In severe cases where the supply inductance is 500nH
or more, the input can dip below the VDD undervoltage
lockout threshold of 2.66V. Because the current passing
through the sense resistor changes no faster than a rate
of V
SUPPLY
/L
SUPPLY
, such as 12V/500nH = 24A/µs, it is
possible for the UV comparator and in particular, the VDD
UVLO circuit to respond before the current reaches the
current limit threshold. The VDD UVLO circuit responds
after a 2µs filter delay, pulling the GATE pin to SOURCE
LTC4281
23
Rev. A
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APPLICATIONS INFORMATION
with 900mA. Once the MOSFET turns off, VDD will return
to its nominal voltage and the part initiates a new startup
sequence. The UV comparator responds after a 15µs fil-
ter delay, making it less likely that this path will engage
before current limiting commences; adding a 100nF filter
capacitor to the UV pin ensures this. The fast current limit
amplifier engages at 3x the current limit threshold, and
has a propagation delay of 500ns. If the supply inductance
is less than 500nH in a 12V application, it is unlikely that
the VDD UVLO threshold will be breached and the fast di/
dt rate allows the current to rise to the 3x level long before
the UV pin responds.
Once the fast current limit amplifier begins to arrest the
short-circuit current, the input voltage rapidly recovers
and even overshoots its DC value. The LTC4281 is safe
from damage up to 45V. To minimize spikes in backplane-
resident applications, bypass the LTC4281 input supply
with an electrolytic capacitor between VDD and GND. In
card-resident applications clamp the VDD pin with a surge
suppressor Z1, as shown in Figure7.
Supply Transient Protection
The worst-case Z1 current is that which triggers the fast
current limit circuit. Several 1500W surge suppressors
may be required to clamp this current for high power
applications. Many 20V to 30V MOSFETs enter avalanche
breakdown before 45V. In those cases the MOSFET can
act as a surge suppressor and protect the Hot Swap con-
troller from inductive input voltage surges. In applications
where a high current ground is not available to connect
the surge suppressor, the surge suppressor may be con-
nected from input to output, allowing the output capaci-
tance to absorb spikes.
Design Example
As a design example, consider the following specifica-
tions: VIN = 12V, IMAX = 50A, CL = 3300μF, VUV(ON) =
10.75V, VOV(OFF) = 14.0V, VPWRGD(UP) = 11.6V, and I2C
address = 1010011, with overcurrent threshold set to
25mV. This completed design is shown in Figure7.
Figure7. Design Example
+
VDD
Z1
SMCJ15CA
× 2
GATE
CTIMER
47nF
TIMER
LTC4281
INTVCC
CONNECTOR 1
CONNECTOR 2
PLUG-IN
BOARD
NC
4281 F07
GND
SDA
SCL
ALERT
R4
10Ω
Q1
PSMN2R0-30YLE × 2
CL
3300µF
V
OUT
12V
50A
R
S
0.5mΩ
SOURCE
FB
GPIO1
GPIO2
GPIO3
POWER GOOD
GP
GP
UV
OV
SDAI
SDAO
SCL
ALERT
ADR0
ADR1
ADR2
ON
SENSE+SENSE–
ADC+ADC–
WP GNDCLKOUTCLKIN
Y1
4MHz
ABLS-4.000MHZ-B4-T
C3
4.7µF
100k
C4
36pF
C5
36pF
12V
BACKPLANE
12V
R2
1.18k
1%
R3
34.0k
1%
R1
3.4k
1%
CF
0.1µF
25V
R7
28.7k
1%
R8
3.57k
1%
LTC4281
24
Rev. A
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APPLICATIONS INFORMATION
Selection of the sense resistor, RS, is set by the current
limit threshold of 25mV:
RS=
25mV
I
MAX
=0.5mΩ
The MOSFET is sized to handle the power dissipation dur-
ing inrush when output capacitor COUT is being charged.
A method to determine power dissipation during inrush
is based on the principle that:
Energy in CL = Energy in Q1
where:
Energy in CL=1
2
CV2=1
2
3.3mF
( )
12
( )
2=0.24J
During inrush, current limit foldback will limit the power
dissipation in the MOSFET to:
PDISS =7.5mV • 12V
R
S
=180W
Calculate the time it takes to charge up COUT:
tSTARTUP =Energy in CL
P
DISS
=0.24J
180W =1.33ms
The SOA (safe operating area) curves of candidate
MOSFETs must be evaluated to ensure that the heat
capacity of the package tolerates 180W for 1.33ms. The
SOA curve of the NXP PSMN2R0-30YLE shows 200W for
80ms, satisfying this requirement. Additional MOSFETs
in parallel may be required to keep the MOSFET tem-
perature or power dissipation within limits at maximum
load current. This depends on board layout, airflow and
efficiency requirements. To get the maximum DC dissipa-
tion below 2W per MOSFET, a pair of PSMN2RO-30YLE
is required for Q1. Since the PSMN2R0-30YLE has 10nF
of gate capacitance it is likely to be stable, but the short-
circuit stability of the current limit should be checked and
improved by adding capacitors from GATE to SOURCE if
needed.
For a start-up time of 1.33ms with a 2x safety margin we
choose:
CTIMER =2 •
t
STARTUP
64ms/µF =2 •
1.33ms
64ms/µF ≅47nF
In the event that the circuit attempts to start up into a
short circuit the current will be 30% of 50A, 15A, and
the voltage across the MOSFET will be 12V which the
MOSFET will carry for 1.33ms. This is within the SOA of
the PSMN2R0-30YLE, so the application will safely sur-
vive this fault condition.
The UV and OV resistor string values can be solved in the
following method. To keep the error due to 1µA of leak-
age to less than 1% choose a divider current of at least
200µA. R1 < 1.28V/200µA = 6.4kΩ. Then calculate the
following equations:
R2 =VOV(OFF)
VUV(ON)
•R1• VTH(UV)
VTH(OV) – VHYST OV
( )
– R1
R3 =VUV(ON) • R1+R2
( )
VTH(UV)
– R1– R2
In our case we choose R1 to be 3.4kΩ to give a resistor
string current greater than 200μA. Then solving the equa
-
tions results in R2 = 1.18kΩ and R3 = 34.0kΩ.
The FB divider is solved by picking R8 and solving for R7,
choosing 3.57kΩ for R8 we get:
R7 =
V
PWRGD(UP)
•R8
VTH(FB)
– R8
Resulting in R7 = 28.7kΩ.
Since this application uses external resistive dividers
for UV, OV and FB, and the operating voltage is 12V, the
CONTROL register 0x01 is set to 0x02 to disable the inter-
nal thresholds and set the ADC to the 12V range. The
EEPROM CONTROL register 0x21 is also set to 0x02 so
the part will boot in the proper configuration.
LTC4281
25
Rev. A
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Since the start-up time is 1.33ms, the FET_BAD_FAULT_
TIME is set to 3ms for a ≥ 2x safety margin by writing
0x03 to the FET_BAD_FAULT_TIME register 0x06.
A 0.1μF capacitor, CF, is placed on the UV pin to prevent
supply glitches from turning off the GATE via UV or OV.
The address is set with the help of Table1, which indicates
binary address 1010011 (0xA6). Address 0xA6 is set by
setting ADR2 high, ADR1 open and ADR0 high.
Next the value of R4 is chosen to be the default value of
10Ω as discussed in the Current Limit Stability section.
A 4MHz crystal is placed between the CLKIN and CLKOUT
pins. The specified part requires 18pF load capacitance
which is provided by C4 and C5. To generate an internal
clock of 250kHz, 1000b is written to the CLOCK_DIVIDER
register 0x10 to divide the 4MHz crystal frequency by 16.
Since the fast pull-down is engaged at 150A, the input TVS
needs to be capable of clamping a 150A surge at a voltage
above the OV threshold but below the 45V absolute maxi-
mum rating of the LTC4281 for about 1µs. The SMCJ15CA
clamps 61.5A at 24V for 8.3ms, and can dissipate 30kW
for 1µs. One SMCJ15CA will meet these requirements.
In addition a 4.7μF ceramic bypass capacitor is placed
on the INTVCC pin. No bypass capacitor is required on
the VDD pin.
Layout Considerations
To achieve accurate current sensing, Kelvin connections
are required. The minimum trace width for 1oz copper
foil is 0.02" per amp to make sure the trace stays at a
reasonable temperature. Using 0.03" per amp or wider
is recommended. Note that 1oz copper exhibits a sheet
resistance of about 530μΩ/£. Small resistances add up
quickly in high current applications.
To improve noise immunity, put the resistive dividers to
the UV, OV and FB pins close to the device and keep traces
to VDD and GND short. It is also important to put the
bypass capacitor C3 as close as possible between INTV
CC
and GND. A 0.1μF capacitor from the UV pin (and OV pin
through resistor R2) to GND also helps reject supply
noise. Figure 8 shows a layout that addresses these
issues. Note that a surge suppressor, Z1, is placed
between supply and ground using wide traces.
It is ill advised to place the ground plane under the power
MOSFETs. If they fail and overheat that could result in a
catastrophic failure as the input gets shorted to ground
when the insulation between them fails.
Digital Interface
The LTC4281 communicates with a bus master using a
2-wire interface compatible with I2C Bus and SMBus, an
I2C extension for low power devices. The LTC4281 is a
read-write slave device and supports SMBus Read Byte,
Write Byte, Read Word and Write Word commands, as
well as I2C continuous read and continuous write com-
mands. Data formats for these commands are shown in
Figure9 through Figure16.
Figure8. Recommended Layout
R1
R3
Z1
R2
CF
CT
4281 F08
C3
LTC4281
26
Rev. A
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ADDRESS
1 0 a4:a0
FROM MASTER TO SLAVE A: ACKNOWLEDGE (LOW)
A: NOT ACKNOWLEDGE (HIGH)
R: READ BIT (HIGH)
W: WRITE BIT (LOW)
S: START CONDITION
P: STOP CONDITION
FROM SLAVE TO MASTER
b7:b0 b7:b0 4281 F10
COMMAND DATAS
0 0 0 0
A A A PW
ADDRESS
1 0 a4:a0 b7:b0 b7:b0
4281 F11
COMMAND DATAS
0 0 0 0
A A A
b7:b0
DATA
0
A PW
ADDRESS
1 0 a4:a0 b7:b0 b7:b0
4281 F12
COMMAND DATAS
0 0 0 0
A A A
b7:b0
DATA
0
A PW• • •
b7:b0
DATA
0
A
ADDRESS
1 0 a4:a0
ADDRESS
1 0 a4:a0b7:b0 b7:b0
4281 F13
COMMAND DATAS S R
0 0 0 1
A A A
01
A PW
Figure10. LTC4281 Serial Bus SDA Write Byte Protocol
Figure11. LTC4281 Serial Bus SDA Write Word Protocol
Figure12. LTC4281 Serial Bus SDA Continuous Write Protocol
Figure13. LTC4281 Serial Bus SDA Read Byte Protocol
APPLICATIONS INFORMATION
Figure9. Data Transfer Over I2C or SMBus
a6 – a0 b7 – b0 b7 – b0
9
ACK
4281 F09
ACKACKADDRESSSTART
CONDITION
STOP
CONDITION
R/WDATADATA
89898
S
SDA
SCL 1 – 71 – 71 – 7
P
LTC4281
27
Rev. A
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APPLICATIONS INFORMATION
ALERT
RESPONSE
ADDRESS
DEVICE
ADDRESS
0 0 0 1 1 0 0
S R
1 1
4281 F16
A PA
0 1 0 a4:a0 0
Figure16. LTC4281 Serial Bus SDA Alert Response Protocol
ADDRESS
1 0 a4:a0
ADDRESS
1 0 a4:a0b7:b0 b7:b0
4281 F15
COMMAND DATA
b7:b0
DATAS S R
0 0 0 1
A A A
01
A
0
A
b7:b0
DATA
0
A PW
• • •
Figure15. LTC4281 Serial Bus SDA Continuous Read Protocol
ADDRESS
1 0 a4:a0
ADDRESS
1 0 a4:a0b7:b0 b7:b0
4281 F14
COMMAND DATAS S R
0 0 0 1
A A A
01
A PW
b7:b0
DATA
0
A
Figure14. LTC4281 Serial Bus SDA Read Word Protocol
START and STOP Conditions
When the bus is idle, both SCL and SDA are high. A bus
master signals the beginning of a transmission with a start
condition by transitioning SDA from high to low while SCL
is high, as shown in Figure10. When the master has
finished communicating with the slave, it issues a STOP
condition by transitioning SDA from low to high while SCL
is high. The bus is then free for another transmission.
I2C Device Addressing
Twenty-seven distinct bus addresses are available using
three 3-state address pins, ADR0-ADR2. Table1 shows
the correspondence between pin states and addresses.
Note that address bits 7 and 6 are internally configured
to 10. In addition, the LTC4281 responds to two spe-
cial addresses. Address 0xBE is a mass write address
that writes to all LTC4281s, regardless of their individual
address settings. Mass write can be disabled by setting bit
4 in CONTROL register 0x00 to zero. Address (0x19) is the
SMBus Alert Response Address. If the LTC4281 is pulling
low on the ALERT pin, it acknowledges this address by
broadcasting its address and releasing the ALERT pin.
Acknowledge
The acknowledge signal is used in handshaking between
transmitter and receiver to indicate that the last byte of
data was received. The transmitter always releases the
SDA line during the acknowledge clock pulse. When the
slave is the receiver, it pulls down the SDA line so that it
remains LOW during this pulse to acknowledge receipt
of the data. If the slave fails to acknowledge by leaving
SDA high, then the master may abort the transmission by
generating a STOP condition. When the master is receiv-
ing data from the slave, the master pulls down the SDA
line during the clock pulse to indicate receipt of the data.
After the last byte has been received the master leaves
the SDA line HIGH (not acknowledge) and issues a stop
condition to terminate the transmission.
LTC4281
28
Rev. A
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APPLICATIONS INFORMATION
Write Protocol
The master begins communication with a START condi-
tion followed by the seven bit slave address and the R/W
bit set to zero, as shown in Figure12. The addressed
LTC4281 acknowledges this and then the master sends a
command byte indicating which internal register the mas-
ter wishes to write. The LTC4281 acknowledges this and
then latches the command byte into its internal Register
Address pointer. The master then delivers the data byte
and the LTC4281 acknowledges once more and writes the
data to the destination register specified by the Register
Address pointer, then the pointer is incremented. If the
Master sends additional bytes, they are written sequen-
tially to the registers in order of their binary addresses.
The transmission is ended when the master sends a STOP
condition.
Read Protocol
The master begins a read operation with a START condi-
tion followed by the seven bit slave address and the R/W
bit set to zero, as shown in Figure15. The addressed
LTC4281 acknowledges this and then the master sends
a command byte which indicates which internal register
the master wishes to read. The LTC4281 acknowledges
this and then latches the command byte into its inter-
nal Register Address pointer. The master then sends a
repeated START condition followed by the same seven bit
address with the R/W bit now set to one. The LTC4281
acknowledges and sends the contents of the requested
register. As long as the master acknowledges the trans-
mitted data byte the internal register address pointer
is incremented and the next register byte is sent. The
transmission is ended when the master sends a STOP
condition.
Data Synchronization
The ADC measurements and subsequent computed val-
ues are 16-48 bits wide, but must be read over the I2C in
8-bit segments. To ensure that the words are not updated
in the middle of reading them, the LTC4281 latches these
results while the I2C interface is busy. As long as the ADC
data is read out in a single transaction, all the data will
be synchronized. A STOP condition frees the LTC4281 to
update the ADC result registers. Status and fault registers
are updated in real time.
Alert Response Protocol
When any of the fault bits in FAULT_LOG register 0x04
are set, an optional bus alert is generated if the appropri-
ate bit in the ALERT register 0x02 is also set. If an alert
is enabled, the corresponding fault causes the ALERT pin
to pull low. After the bus master controller broadcasts the
Alert Response Address, the LTC4281 responds with its
address on the SDA line and then releases ALERT when
it has successfully completed transmitting its address as
shown in Figure16.
The ALERT signal is not pulled low again until the FAULT
register 0x04 indicates a different fault as occurred or
the original fault is cleared and it occurs again. Note that
this means repeated or continuing faults do not generate
alerts until the associated FAULT_LOG register bit has
been cleared.
EEPROM
The LTC4281 has an onboard EEPROM to allow nonvola-
tile configuration and fault logging. The EEPROM regis-
ters are denoted by ‘EE’ in the first column of register
Table2. The EEPROM registers may be read and writ-
ten like any other register except that the EEPROM takes
about 2ms to write data. While the EEPROM is writing, the
EEPROM_BUSY bit, bit 2 in STATUS register 0x1F is set to
1. While the EEPROM is busy the I2C interface will NACK
commands to read or write to EEPROM registers, but
other registers may be accessed during this time. When
the EEPROM finishes writing, the EEPROM_BUSY bit will
reset and the EEPROM_DONE bit, bit 7 in FAULT_LOG
register 0x04 will be set. If configured to generate an alert
on EEPROM_DONE, Bit 7 in ALERT register 0x02), the
ALERT pin will pull low to alert the host that the EEPROM
write has finished and the LTC4281 EEPROM is ready to
receive another byte.
LTC4281
29
Rev. A
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APPLICATIONS INFORMATION
When the LTC4281 comes out of UVLO or receives a
REBOOT command the contents of the EEPROM are cop-
ied to the corresponding operating registers, which are
offset from the EEPROM register addresses by 0x20. The
SCRATCH_PAD registers, 0x4C-0x4F, are free for general
purpose use, such as storing fault history, serial numbers
or calibration data. The factory default EEPROM contents
make the LTC4281 behave similar to the LTC4215 to ease
design migration and provide a useful design starting
point.
The ADC_ALERT_LOG register (0x05) is not loaded from
the EEPROM at boot and the FAULT_LOG register (0x04)
is only loaded at boot if the ON_FAULT_MASK bit is set
in the EEPROM (bit 7, register 0x20). The register data
is copied into the EEPROM when any of the bits in the
log registers transition high and fault logging is enabled
in ADC_CONTROL register 0x1D. Fault logging is dis-
abled by default after boot so that logged faults are not
inadvertently cleared by powering up with a fault condition
and overwriting the EEPROM. A 4.7µF capacitor on the
INTVCC pin allows the LTC4281 to operate and log faults
to the EEPROM if input power is lost. A 1uF capacitor may
be used in applications that do not require fault logging.
The WP pin prevents I2C writes to the EEPROM when
high. Attempts to write to the EEPROM while WP is high
will result in a NACK and no action. Usually the WP pin
is tied high through a resistor with a probe pad to allow
it to be pulled low manually, it may also be tied low to
enable writes all the time or connected to a GPIO pin or
other logic-level signal to allow software control of WP.
The EEPROM may still be read when WP is high. The
FAULT_LOG registers of the EEPROM will still log faults
when the WP pin is high. Analog Devices can provide
programmed parts that have WP locked in a high state to
make it impossible to change the default configuration by
any means. Please contact the factory.
LTC4281
30
Rev. A
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APPLICATIONS INFORMATION
Table1. LTC4281 Addressing
DESCRIPTION
DEVICE
ADDRESS*
7-BIT
DEVICE
ADDRESS BINARY DEVICE ADDRESS LTC4281 ADDRESS PINS
h a6 a5 a4 a3 a2 a1 a0 R/WADR2 ADR1 ADR0
Mass Write 0xBE 0x5F 1 0 1 1 1 1 1 0 X X X
Alert Response 0x19 0x0C 0 0 0 1 1 0 0 1 X X X
0x80 0x40 1 0 0 0 0 0 0 X L NC L
0x82 0x41 1 0 0 0 0 0 1 X L H NC
0x84 0x42 1 0 0 0 0 1 0 X L NC NC
0x86 0x43 1 0 0 0 0 1 1 X L NC H
0x88 0x44 1 0 0 0 1 0 0 X L L L
0x8A 0x45 1 0 0 0 1 0 1 X L H H
0x8C 0x46 1 0 0 0 1 1 0 X L L NC
0x8E 0x47 1 0 0 0 1 1 1 X L L H
0x90 0x48 1 0 0 1 0 0 0 X NC NC L
0x92 0x49 1 0 0 1 0 0 1 X NC H NC
0x94 0x4A 1 0 0 1 0 1 0 X NC NC NC
0x96 0x4B 1 0 0 1 0 1 1 X NC NC H
0x98 0x4C 1 0 0 1 1 0 0 X NC L L
0x9A 0x4D 1 0 0 1 1 0 1 X NC H H
0x9C 0x4E 1 0 0 1 1 1 0 X NC L NC
0x9E 0x4F 1 0 0 1 1 1 1 X NC L H
0xA0 0x50 1 0 1 0 0 0 0 X H NC L
0xA2 0x51 1 0 1 0 0 0 1 X H H NC
0xA4 0x52 1 0 1 0 0 1 0 X H NC NC
0xA6 0x53 1 0 1 0 0 1 1 X H NC H
0xA8 0x54 1 0 1 0 1 0 0 X H L L
0xAA 0x55 1 0 1 0 1 0 1 X H H H
0xAC 0x56 1 0 1 0 1 1 0 X H L NC
0xAE 0x57 1 0 1 0 1 1 1 X H L H
0xB0 0x58 1 0 1 1 0 0 0 X L H L
0xB2 0x59 1 0 1 1 0 0 1 X NC H L
0xB4 0x5A 1 0 1 1 0 1 0 X H H L
* 8-bit hexadecimal address with LSB R/W bit = 0.
LTC4281
31
Rev. A
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REGISTER SET
Table2.
REGISTER NAME COMMAND BYTE DESCRIPTION
READ/
WRITE
DATA
LENGTH DEFAULT
CONTROL 0x00-0x01 Configures On/Off Behavior RW 16 Bits 0xBB02
ALERT 0x02-0x03 Enables Alerts RW 16 Bits 0x0000
FAULT_LOG 0x04 Logs Faults RW 8 Bits 0x00
ADC_ALERT_LOG 0x05 Logs ADC Alerts RW 8 Bits 0x00
FET_BAD_FAULT_TIME 0x06 Selects FET-BAD Fault Timeout RW 8 Bits 0xFF
GPIO_CONFIG 0x07 Configures GPIO Outputs RW 8 Bits 0x00
VGPIO_ALARM_MIN 0x08 Threshold For Min Alarm on VGPIO RW 8 Bits 0x00
VGPIO_ALARM_MAX 0x09 Threshold for Max Alarm on VGPIO RW 8 Bits 0xFF
VSOURCE_ALARM_MIN 0x0A Threshold for Min Alarm on VSOURCE RW 8 Bits 0x00
VSOURCE_ALARM_MAX 0x0B Threshold for Max Alarm on VSOURCE RW 8 Bits 0xFF
VSENSE_ALARM_MIN 0x0C Threshold for Min Alarm on VSENSE RW 8 Bits 0x00
VSENSE_ALARM_MAX 0x0D Threshold for Max Alarm on VSENSE RW 8 Bits 0xFF
POWER_ALARM_MIN 0x0E Threshold for Min Alarm on POWER RW 8 Bits 0x00
POWER_ALARM_MAX 0x0F Threshold for Max Alarm on POWER RW 8 Bits 0xFF
CLOCK_DIVIDER 0x10 Division Factor for External Clock RW 8 Bits 0x08
ILIM_ADJUST 0x11 Adjusts Current Limit Value RW 8 Bits 0x96
ENERGY 0x12-0x17 Meters Energy Delivered to Load RW 48 Bits 0x000000
TIME_COUNTER 0x18-0x1B Counts Power Delivery Time RW 32 Bits 0x0000
ALERT_CONTROL 0x1C Clear Alerts, Force ALERT Pin Low RW 8 Bits 0x00
ADC_CONTROL 0x1D Control ADC, Energy Meter RW 8 Bits 0x00
STATUS 0x1E-0x1F Fault and Pin Status R 16 Bits N/A
EE_CONTROL 0x20-0x21 EEPROM Default RW 16 Bits 0xBB02
EE_ALERT 0x22-0x23 EEPROM Default RW 16 Bits 0x0000
EE_FAULT_LOG 0x24 EEPROM Default RW 8 Bits 0x00
EE_ADC_ALERT_LOG 0x25 EEPROM Default RW 8 Bits 0x00
EE_FET_BAD_FAULT_TIME 0x26 EEPROM Default RW 8 Bits 0xFF
EE_GPIO_CONFIG 0x27 EEPROM Default RW 8 Bits 0x00
EE_VGPIO_ALARM_MIN 0x28 EEPROM Default RW 8 Bits 0x00
EE_VGPIO_ALARM_MAX 0x29 EEPROM Default RW 8 Bits 0xFF
EE_VSOURCE_ALARM_MIN 0x2A EEPROM Default RW 8 Bits 0x00
EE_VSOURCE_ALARM_MAX 0x2B EEPROM Default RW 8 Bits 0xFF
EE_VSENSE_ALARM_MIN 0x2C EEPROM Default RW 8 Bits 0x00
EE_VSENSE_ALARM_MAX 0x2D EEPROM Default RW 8 Bits 0xFF
EE_POWER_ALARM_MIN 0x2E EEPROM Default RW 8 Bits 0x00
EE_POWER_ALARM_MAX 0x2F EEPROM Default RW 8 Bits 0xFF
EE_CLOCK_DIVIDER 0x30 EEPROM Default RW 8 Bits 0x08
EE_ILIM_ADJUST 0x31 EEPROM Default RW 8 Bits 0x96
RESERVED 0x32-0x33 Reserved for Future Expansion, Do Not Write N/A
VGPIO 0x34-0x35 Most Recent ADC Result for VGPIO RW 16 Bits N/A
VGPIO_MIN 0x36-0x37 Min ADC Result for VGPIO RW 16 Bits N/A
LTC4281
32
Rev. A
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REGISTER SET
DETAILED I2C COMMAND REGISTER DESCRIPTIONS
CONTROL Registers (0x00–0x01) (R/W)
Byte 1 (0x00)
BIT(S) NAME DEFAULT OPERATION
B[7] ON_FAULT_MASK 1 If 1, blocks the ON pin from clearing the FAULT_LOG register to prevent repeated logged faults
and alerts.
B[6] ON_DELAY 0 If 1, a 50ms debounce is applied to the ON pin commanding the part to turn on, if 0 the part turns
on immediately.
B[5] ON/ENB 1 The ON pin is active high when this bit is a 1 and active low when this bit is a 0.
B[4] MASS_WRITE_ENABLE 1 Writing a 1 enables MASS_WRITE to all LTC4281s on the I2C bus.
B[3] FET_ON 1 Writing a 1 to this register turns the part on, writing a 0 turns off, overriding the ON pin.
B[2] OC_AUTORETRY 0 Writing a 1 enables the part to auto-retry 256 timer cycles after an OC fault.
B[1] UV_AUTORETRY 1 Writing a 1 enables the part to auto-retry 50ms after an UV fault.
B[0] OV_AUTORETRY 1 Writing a 1 enables the part to auto-retry 50ms after an OV fault.
Byte 2 (0x01)
B[7-6] FB_MODE 00 Selects threshold for POWER_GOOD, 00 = external, 01 = 5%, 10 = 10%, 11 = 15%.
B[5-4] UV_MODE 00 Selects threshold for UV faults, 00 = external, 01 = 5%, 10 = 10%, 11 = 15%.
B[3-2] OV_MODE 00 Selects threshold for OV faults, 00 = external, 01 = 5%, 10 = 10%, 11 = 15%.
B[1-0] VIN_MODE 10 Selects operating range for UV/OV/FB and ADC: 00 = 3.3V, 01 = 5V, 10 = 12V, 11 = 24V.
Table2.
REGISTER NAME COMMAND BYTE DESCRIPTION
READ/
WRITE
DATA
LENGTH DEFAULT
VGPIO_MAX 0x38-0x39 Max ADC Result for VGPIO RW 16 Bits N/A
VSOURCE 0x3A-0x3B Most Recent ADC Result for VSOURCE RW 16 Bits N/A
VSOURCE_MIN 0x3C-0x3D Min ADC Result for VSOURCE RW 16 Bits N/A
VSOURCE_MAX 0x3E-0x3F Max ADC Result for VSOURCE RW 16 Bits N/A
VSENSE 0x40-0x41 Most Recent ADC Result for VSENSE RW 16 Bits N/A
VSENSE_MIN 0x42-0x43 Min ADC Result for VSENSE RW 16 Bits N/A
VSENSE_MAX 0x44-0x45 Max ADC Result for VSENSE RW 16 Bits N/A
POWER 0x46-0x47 Most Recent ADC Result for POWER RW 16 Bits N/A
POWER_MIN 0x48-0x49 Min ADC Result for POWER RW 16 Bits N/A
POWER_MAX 0x4A-0x4B Max ADC Result for POWER RW 16 Bits N/A
EE_SCRATCH_PAD 0x4C-0x4F Spare EEPROM memory RW 32 Bits 0x00000000
RESERVED 0x50-0xFF Reserved for Future Expansion, Do Not Write N/A
LTC4281
33
Rev. A
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DETAILED I2C COMMAND REGISTER DESCRIPTIONS
ALERT Registers (0x02–0x03) (R/W)
Byte 1 (0x02)
BIT(S) NAME DEFAULT OPERATION
B[7] EEPROM_DONE_ALERT 0 Writing a 1 generates alerts when the EEPROM finishes writing.
B[6] FET_BAD_FAULT_ALERT 0 Writing a 1 generates alerts when FET-BAD faults are produced.
B[5] FET_SHORT_ALERT 0 Writing a 1 generates alerts when the ADC detects FET-short faults.
B[4] ON_ALERT 0 Writing a 1 generates alerts when the ON pin changes state.
B[3] PB_ALERT 0 Writing a 1 generates alerts when power-bad faults are produced.
B[2] OC_ALERT 0 Writing a 1 generates alerts when overcurrent faults are produced.
B[1] UV_ALERT 0 Writing a 1 generates alerts when undervoltage faults are produced.
B[0] OV_ALERT 0 Writing a 1 generates alerts when overvoltage faults are produced.
Byte 2 (0x03)
B[7] POWER_ALERT_HIGH 0 Writing a 1 generates alerts when the ADC result is above the POWER_ALARM_MAX threshold.
B[6] POWER_ALERT_LOW 0 Writing a 1 generates alerts when the ADC result is below the POWER_ALARM_MIN threshold.
B[5] VSENSE_ALERT_HIGH 0 Writing a 1 generates alerts when the ADC result is above the VSENSE_ALARM_MAX threshold.
B[4] VSENSE_ALERT_LOW 0 Writing a 1 generates alerts when the ADC result is below the VSENSE_ALARM_MIN threshold.
B[3] VSOURCE_ALERT_HIGH 0 Writing a 1 generates alerts when the ADC result is above the VSOURCE_ALARM_MAX threshold.
B[2] VSOURCE_ALERT_LOW 0 Writing a 1 generates alerts when the ADC result is below the VSOURCE_ALARM_MIN threshold.
B[1] VGPIO_ALERT_HIGH 0 Writing a 1 generates alerts when the ADC result is above the VGPIO_ALARM_MAX threshold.
B[0] VGPIO_ALERT_LOW 0 Writing a 1 generates alerts when the ADC result is below the VGPIO_ALARM_MIN threshold.
FAULT_LOG Register (0x04) (R/W)
Byte 1 (0x04)
BIT(S) NAME DEFAULT OPERATION
B[7] EEPROM_DONE 0 Set to 1 when the EEPROM finishes a write.
B[6] FET_BAD_FAULT 0 Set to 1 when a FET-BAD fault occurs.
B[5] FET_SHORT_FAULT 0 Set to 1 when the ADC detects a FET-short fault.
B[4] ON_FAULT 0 Set to 1 by the ON pin changing state.
B[3] POWER_BAD_FAULT 0 Set to 1 by a power-bad fault occurring.
B[2] OC_FAULT 0 Set to 1 by an overcurrent fault occurring.
B[1] UV_FAULT 0 Set to 1 by an undervoltage fault occurring.
B[0] OV_FAULT 0 Set to 1 by an overvoltage fault occurring.
ADC_ALERT_LOG Register (0x05) (R/W)
Byte 1 (0x05)
BIT(S) NAME DEFAULT OPERATION
B[7] POWER_ALARM_HIGH 0 Set to 1when the ADC makes a measurement above the POWER_ALARM_MAX threshold.
B[6] POWER_ALARM_LOW 0 Set to 1when the ADC makes a measurement below the POWER_ALARM_MIN threshold.
B[5] VSENSE_ALARM_HIGH 0 Set to 1when the ADC makes a measurement above the VSENSE_ALARM_MAX threshold.
B[4] VSENSE_ALARM_LOW 0 Set to 1when the ADC makes a measurement below the VSENSE_ALARM_MIN threshold.
B[3] VSOURCE_ALARM_HIGH 0 Set to 1when the ADC makes a measurement above the VSOURCE_ALARM_MAX threshold.
B[2] VSOURCE_ALARM_LOW 0 Set to 1when the ADC makes a measurement below the VSOURCE_ALARM_MIN threshold.
B[1] GPIO_ALARM_HIGH 0 Set to 1when the ADC makes a measurement above the VGPIO_ALARM_MAX threshold.
B[0] GPIO_ALARM_LOW 0 Set to 1when the ADC makes a measurement below the VGPIO_ALARM_MIN threshold.
LTC4281
34
Rev. A
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DETAILED I2C COMMAND REGISTER DESCRIPTIONS
FET_BAD_FAULT_TIME Register (0x06) (R/W)
Byte 1 (0x06)
BIT(S) NAME DEFAULT OPERATION
B[7-0] FET_BAD_FAULT_TIMEOUT 255 Selects the wait time for a FET-bad fault as a binary integer in ms. 0x00 disables.
GPIO_CONFIG Register (0x07) (R/W)
Byte 1 (0x07)
BIT(S) NAME DEFAULT OPERATION
B[7] GPIO3_PD 0 A 1 in this value will make the GPIO3 pin pull low, a 0 will make the pin high impedance
B[6] GPIO2_PD 0 A 1 in this value will make the GPIO2 pin pull low, a 0 will make the pin high impedance
B[5-4] GPIO1_CONFIG 00 FUNCTION B[5] B[4] GPIO1 PIN
Power Good 0 0 GPIO1 = Power Good
Power Bad 1 0 GPIO1 = Power Bad
General Purpose Output 0 1 GPIO1 = B[3]
General Purpose Input 1 1 GPIO1 = High-Z
B[3] GPIO1_OUTPUT 0 Output data bit to GPIO1 pin when configured as output (1 = high impedance, 0 = pull low)
B[2] ADC_CONV_ALERT 0 Writing a 1 generates alert when the ADC finishes making a measurement
B[1] STRESS_TO_GPIO2 0 Writing a 1 generates alert GPIO2 to pull low when the MOSFET is dissipating power (stress)
B[0] METER_OVERFLOW_ALERT 0 Writing a 1 generates alert when the energy meter accumulator or time counter overflows
VGPIO_ALARM_MIN Register (0x08) (R/W)
Byte 1 (0x08)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VGPIO_ALARM_MIN 0x00 Selects the maximum ADC measurement value that generates a VGPIO_MIN_ALARM
VGPIO_ALARM_MAX Register (0x09) (R/W)
Byte 1 (0x09)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VGPIO_ALARM_MAX 0xFF Selects the minimum ADC measurement value that generates a VGPIO_MAX_ALARM
VSOURCE_ALARM_MIN Register (0x0A) (R/W)
Byte 1 (0x0A)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VSOURCE_ALARM_MIN 0x00 Selects the maximum ADC measurement value that generates a VSOURCE_MIN_ALARM
VSOURCE_ALARM_MAX Register (0x0B) (R/W)
Byte 1 (0x0B)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VSOURCE_ALARM_MAX 0xFF Selects the minimum ADC measurement value that generates a VSOURCE_MAX_ALARM
LTC4281
35
Rev. A
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DETAILED I2C COMMAND REGISTER DESCRIPTIONS
VSENSE_ALARM_MIN Register (0x0C) (R/W)
Byte 1 (0x0C)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VSENSE_ALARM_MIN 0x00 Selects the maximum ADC measurement value that generates a VSENSE_MIN_ALARM
VSENSE_ALARM_MAX Register (0x0D) (R/W)
Byte 1 (0x0D)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VSENSE_ALARM_MAX 0xFF Selects the minimum ADC measurement value that generates a VSENSE_MAX_ALARM
POWER_ALARM_MAX Register (0x0F) (R/W)
Byte 1 (0x0F)
BIT(S) NAME DEFAULT OPERATION
B[7-0] POWER_ALARM_MAX 0xFF Selects the minimum ADC measurement value that generates a POWER_MAX_ALARM
POWER_ALARM_MIN Register (0x0E) (R/W)
Byte 1 (0x0E)
BIT(S) NAME DEFAULT OPERATION
B[7-0] POWER_ALARM_MIN 0x00 Selects the maximum ADC measurement value that generates a POWER_MIN_ALARM
CLOCK_DIVIDER Register (0x10) (R/W)
Byte 1 (0x10)
BIT(S) NAME DEFAULT OPERATION
B[7] COULOMB_METER 0 Setting this bit to a 1 configures the Energy meter to accumulate current instead of power,
making it a Coulomb meter
B[6] TICK_OUT 0 Configures the CLKOUT pin to output the internal time count (conversion time) as an open-drain
output
B[5] INT_CLK_OUT 0 Configures the CLKOUT pin to output the internal system clock as an open-drain output
B[4-0] CLOCK_DIVIDER 01000 The clock frequency input on the CLKIN pin gets divided by twice this integer to produce the
system clock at the target frequency of 250kHz. Code 00000 passes the clock without division.
LTC4281
36
Rev. A
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DETAILED I2C COMMAND REGISTER DESCRIPTIONS
ILIM_ADJUST Register (0x11) (R/W)
Byte 1 (0x11)
BIT(s) NAME Default Operation
B[7-5] ILIM_ADJUST 100 Selects the current limit values [mV]
B[7] B[6] B[5] FB = LOW FB = HIGH FAST COMPARATOR
0 0 0 3.75 12.5 38
0 0 1 4.6875 15.625 47
0 1 0 5.625 18.75 56
0 1 1 6.5625 21.875 66
1 0 0 7.5 25 75
1 0 1 8.4375 28.125 84
1 1 0 9.375 31.25 94
1 1 1 10.3125 34.375 103
B[4-3] FOLDBACK_MODE 10 Selects the voltage range for the current limit foldback profile: 00 = 3.3V, 01 = 5V, 10 = 12V,
11 = 24V
B[2] VSOURCE/VDD 1 Setting this bit to a 1 makes the ADC monitor the SOURCE voltage, 0 for VDD
B[1] GPIO_MODE 1 Setting this bit to a 1 makes the ADC monitor GPIO2, 0 for GPIO3
B[0] 16_BIT 0 Setting this bit to a 1 will make the ADC operate in 16-bit mode, 0 will make the ADC operate in
12-bit mode
ENERGY Register (0x12–0x17) (R/W)
Byte 1-6 (0x12-0x17)
BIT(S) NAME DEFAULT OPERATION
B[48-0] ENERGY_METER 0x000000 Metered energy value
TIME_COUNTER Register (0x18–0x1B) (R/W)
Byte 1-4 (0x18-0x1B)
BIT(S) NAME DEFAULT OPERATION
B[32-0] TIME_COUNTER 0x0000 Counts the number of conversion cycles that power measurements have been accumulated in
the energy meter
ALERT_CONTROL Register (0x1C) (R/W)
Byte 1 (0x1C)
BIT(S) NAME DEFAULT OPERATION
B[7] ALERT_GENERATED 0 This bit is set to 1 when an alert is generated. It must be manually cleared by writing a 0 to it via
I2C. This bit can be set via I2C to simulate an alert
B[6] ALERT_PD 0 When this bit is set to 1 the ALERT pin pulls low as a general purpose output low
B[5-0] RESERVED 000000 Always read as 0
LTC4281
37
Rev. A
For more information www.analog.com
DETAILED I2C COMMAND REGISTER DESCRIPTIONS
ADC_CONTROL Register (0x1D) (R/W)
Byte 1 (0x1D)
BIT(S) NAME DEFAULT OPERATION
B[7] REBOOT 0 Writing a 1 to this bit will cause the LTC4281 to turn off and reboot to the EEPROM default
configuration and restart, if configured to do so, after 3.2s.
B[6] METER_RESET 0 Writing a 1 to this bit resets the energy meter accumulator and time counter and holds them
reset until this bit is cleared.
B[5] METER_HALT 0 Writing a 1 to this bit stops the energy meter and time counter.
B[4-3] RESERVED 00 Always read as 0.
B[2] FAULT_LOG_ENABLE 0 Setting this bit to 1 enables registers 0x04 and 0x05 to be written to the EEPROM when a fault
bit transitions high.
B[1] GATELOW GATELOW Gives the status of the GATE pin 0 if the GATE pin is higher than 8V (Read Only)
B[0] ADC_HALT 0 Single shot mode, writing to this register again with HALT = 1 will allow the ADCs to make a
single conversion and then stop, clearing this bit allows the ADCs to run continuously
STATUS Register(0x1E–0x1F) (R)
Byte 1 (0x1E)
BIT(S) NAME OPERATION
B[7] ON_STATUS A 1 indicates if the MOSFETs are commanded to turn on
B[6] FET_BAD_COOLDOWN_STATUS A 1 indicates that an FET-BAD fault has occurred and the part is going through a
cool-down cycle
B[5] FET_SHORT_PRESENT A 1 indicates that the ADCs have detected a shorted MOSFET
B[4] ON_PIN_STATUS A 1 indicates the status of the ON pin, 1 = high
B[3] POWER_GOOD_STATUS A 1 indicates if the output voltage is greater than the power good threshold
B[2] OC_COOLDOWN_STATUS A 1 indicates that an overcurrent fault has occurred and the part is going through a
cool-down cycle.
B[1] UV_STATUS A 1 indicates that the input voltage is below the undervoltage threshold
B[0] OV_STATUS A 1 indicates that the input voltage is above the overvoltage threshold
Byte 2 (0x1F)
B[7] GPIO3_STATUS A 1 indicates that the GPIO3 pin is above its input threshold
B[6] GPIO2_STATUS A 1 indicates that the GPIO2 pin is above its input threshold
B[5] GPIO1_STATUS A 1 indicates that the GPIO1 pin is above its input threshold
B[4] ALERT_STATUS A 1 indicates that the ALERT pin is above its input threshold
B[3] EEPROM_BUSY This bit is high whenever the EEPROM is writing, and indicates that the EEPROM is not available
until the write is complete
B[2] ADC_IDLE This bit indicates that the ADC is idle. It is always read as 0 when the ADCs are free running, and
will read a 1 when the ADC is idle in single shot mode
B[1] TICKER_OVERFLOW_PRESENT A 1 indicates that the tick counter has overflowed
B[0] METER_OVERFLOW_PRESENT A 1 indicates that the energy meter accumulator has overflowed
LTC4281
38
Rev. A
For more information www.analog.com
DETAILED I2C COMMAND REGISTER DESCRIPTIONS
EE_CONTROL Non-Volatile Register (0x20–0x21) (R/W)
Byte 1 (0x20)
BIT(S) NAME DEFAULT OPERATION
B[7] Same as CONTROL 0x00 1 Stores default state for ON_FAULT_MASK bit. A 1 also allows the contents of EE_FAULT_LOG
register 0x24 to be copied to FAULT_LOG 0x04 at boot.
B[6-4] Same as CONTROL 0x00 011 Stores default state for CONTROL byte 1 (0x00) in nonvolatile memory
B[3] Same as CONTROL 0x00 1 Sets the default ON state. 0 = OFF, 1 = ON-pin state.
B[2-0] Same as CONTROL 0x00 011 Sets the default auto-retry behavior
Byte 2 (0x21)
B[7-0] Same as CONTROL 0x01 0x02 Stores default state for CONTROL byte 2 (0x01) in nonvolatile memory
EE_ALERT Non-Volatile Register (0x22–0x23) (R/W)
Byte 1 (0x22)
BIT(S) NAME DEFAULT OPERATION
B[7-0] Same as ALERT 0x02 0x00 Stores default state for ALERT byte 1 (0x02) in nonvolatile memory
Byte 2 (0x23)
B[7-0] Same as ALERT 0x03 0x00 Stores default state for ALERT byte 2 (0x03) in nonvolatile memory
EE_FAULT_LOG Non-Volatile Register (0x24) (R/W)
Byte 1 (0x24)
BIT(S) NAME DEFAULT OPERATION
B[7-0] Same as FAULT_LOG 0x00 When a new fault occurs, the contents of FAULT_LOG register (0x04) are copied to this
nonvolatile memory location
EE_ADC_ALERT_LOG Non-Volatile Register (0x25) (R/W)
Byte 1 (0x25)
BIT(S) NAME DEFAULT OPERATION
B[7-0] Same as ADC_ALERT_LOG 0x00 When a new ADC Alert is generated, the contents of ADC_ALERT_LOG register (0x05) are
copied to this nonvolatile memory location
EE_FET_BAD_FAULT_TIME Non-Volatile Register (0x26) (R/W)
Byte 1 (0x26)
BIT(S) NAME DEFAULT OPERATION
B[7-0] Same as FET_BAD_FAULT_TIME 0xFF Stores default state for the FET_BAD_FAULT_TIME register (0x06) in nonvolatile memory
EE_GPIO_CONFIG Non-Volatile Register (0x27) (R/W)
Byte 1 (0x27)
BIT(S) NAME DEFAULT OPERATION
B[7-0] Same as GPIO_CONFIG 0x00 Stores default state for GPIO_CONFIG register (0x07) in nonvolatile memory
EE_VGPIO_ALARM_MIN Non-Volatile Register (0x28) (R/W)
Byte 1 (0x28)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VGPIO_ALARM_MIN 0x00 Stores default state for VGPIO_ALARM_MIN register (0x08) in nonvolatile memory
LTC4281
39
Rev. A
For more information www.analog.com
DETAILED I2C COMMAND REGISTER DESCRIPTIONS
EE_VGPIO_ALARM_MAX Non-Volatile Register (0x29) (R/W)
Byte 1 (0x29)
BIT(s) NAME Default Operation
B[7-0] VGPIO_ALARM_MAX 0xFF Stores default state for VGPIO_ALARM_MAX register (0x09) in nonvolatile memory
EE_VSOURCE_ALARM_MIN Non-Volatile Register (0x2A) (R/W)
Byte 1 (0x2A)
BIT(s) NAME Default Operation
B[7-0] VSOURCE_ALARM_MIN 0x00 Stores default state for VSOURCE_ALARM_MIN register (0x0A) in nonvolatile memory
EE_VSOURCE_ALARM_MAX Non-Volatile Register (0x2B) (R/W)
Byte 1 (0x2B)
BIT(s) NAME Default Operation
B[7-0] VSOURCE_ALARM_MAX 0xFF Stores default state for VSOURCE_ALARM_MAX register (0x0B) in nonvolatile memory
EE_VSENSE_ALARM_MIN Non-Volatile Register (0x2C) (R/W)
Byte 1 (0x2C)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VSENSE_ALARM_MIN 0x00 Stores default state for VSENSE_ALARM_MIN register (0x0C) in nonvolatile memory
EE_POWER_ALARM_MIN Non-Volatile Register (0x2E) (R/W)
Byte 1 (0x2E)
BIT(S) NAME DEFAULT OPERATION
B[7-0] POWER_ALARM_MIN 0x00 Stores default state for POWER_ALARM_MIN register (0x0E) in nonvolatile memory
EE_POWER_ALARM_MAX Non-Volatile Register (0x2F) (R/W)
Byte 1 (0x2F)
BIT(S) NAME DEFAULT OPERATION
B[7-0] POWER_ALARM_MAX 0xFF Stores default state for POWER_ALARM_MAX register (0x0F) in nonvolatile memory
EE_CLOCK_DIVIDER Non-Volatile Register (0x30) (R/W)
Byte 1 (0x30)
BIT(S) NAME DEFAULT OPERATION
B[7-0] Same as CLOCK_DIVIDER 0x08 Stores default state for CLOCK_DIVIDER register (0x10) in nonvolatile memory
EE_VSENSE_ALARM_MAX Non-Volatile Register (0x2D) (R/W)
Byte 1 (0x2D)
BIT(S) NAME DEFAULT OPERATION
B[7-0] VSENSE_ALARM_MAX 0xFF Stores default state for VSENSE_ALARM_MAX register (0x0D) in nonvolatile memory
LTC4281
40
Rev. A
For more information www.analog.com
DETAILED I2C COMMAND REGISTER DESCRIPTIONS
EE_ILIM_ADJUST Non-Volatile Register (0x31) (R/W)
Byte 1 (0x31)
BIT(S) NAME DEFAULT OPERATION
B[7-0] Same as ILIM_ADJUST 0x96 Stores default state for ILIM_ADJUST register (0x11) in nonvolatile memory
Reserved (0x32–0x33)
Byte 1 (0x32)
BIT(S) NAME OPERATION
B[7-0] Reserved Always read as 0x00
Byte 2 (0x33)
B[7-0] Reserved Always read as 0x00
VGPIO Register (0x34–0x35)
Byte 1 (0x34)
BIT(S) NAME OPERATION
B[7-0] VGPIO_MSB Stores the MSBs for the most recent VGPIO measurement result
Byte 2 (0x35)
B[7-0] VGPIO_LSB Stores the LSBs for the most recent VGPIO measurement result
VGPIO_MIN Register (0x36–0x37)
Byte 1 (0x36)
BIT(S) NAME OPERATION
B[7-0] VGPIO_MIN_MSB Stores the MSBs for the smallest VGPIO measurement result
Byte 2 (0x37)
B[7-0] VGPIO_MIN_LSB Stores the LSBs for the smallest VGPIO measurement result
VGPIO_MAX Register (0x38–0x39)
Byte 1 (0x38)
BIT(S) NAME OPERATION
B[7-0] VGPIO_MAX_MSB Stores the MSBs for the largest VGPIO measurement result
Byte 2 (0x39)
B[7-0] VGPIO_MAX_LSB Stores the LSBs for the largest VGPIO measurement result
VSOURCE Register (0x3A–0x3B)
Byte 1 (0x3A)
BIT(S) NAME OPERATION
B[7-0] VSOURCE_MSB Stores the MSBs for the most recent VSOURCE measurement result
Byte 2 (0x3B)
B[7-0] VSOURCE_LSB Stores the LSBs for the most recent VSOURCE measurement result
LTC4281
41
Rev. A
For more information www.analog.com
DETAILED I2C COMMAND REGISTER DESCRIPTIONS
VSOURCE_MIN Register (0x3C–0x3D) (R/W)
Byte 1 (0x3C)
BIT(S) NAME OPERATION
B[7-0] VSOURCE_MIN_MSB Stores the MSBs for the smallest VSOURCE measurement result
Byte 2 (0x3D)
B[7-0] VSOURCE_MIN_LSB Stores the LSBs for the smallest VSOURCE measurement result
VSOURCE_MAX Register (0x3E–0x3F) (R/W)
Byte 1 (0x3E)
BIT(S) NAME OPERATION
B[7-0] VSOURCE_MAX_MSB Stores the MSBs for the largest VSOURCE measurement result
Byte 2 (0x3F)
B[7-0] VSOURCE_MAX_LSB Stores the LSBs for the largest VSOURCE measurement result
VSENSE Register (0x40–0x41) (R/W)
Byte 1 (0x40)
BIT(S) NAME OPERATION
B[7-0] VSENSE_MSB Stores the MSBs for the most recent VSENSE measurement result
Byte 2 (0x41)
B[7-0] VSENSE_LSB Stores the LSBs for the most recent VSENSE measurement result
VSENSE_MIN Register (0x42–Ox43) (R/W)
Byte 1 (0x42)
BIT(S) NAME OPERATION
B[7-0] VSENSE_MIN_MSB Stores the MSBs for the smallest VSENSE measurement result
Byte 2 (0x43)
B[7-0] VSENSE_MIN_LSB Stores the LSBs for the smallest VSENSE measurement result
VSENSE_MAX Register (0x44–0x45) (R/W)
Byte 1 (0x44)
BIT(S) NAME OPERATION
B[7-0] VSENSE_MAX_MSB Stores the MSBs for the largest VSENSE measurement result
Byte 2 (0x45)
B[7-0] VSENSE_MAX_LSB Stores the LSBs for the largest VSENSE measurement result
POWER Register (0x46–0x47) (R/W)
Byte 1 (0x46)
BIT(S) NAME OPERATION
B[7-0] POWER_MSB Stores the MSBs for the most recent POWER measurement result
Byte 2 (0x47)
B[7-0] POWER_LSB Stores the LSBs for the most recent POWER measurement result
LTC4281
42
Rev. A
For more information www.analog.com
DETAILED I2C COMMAND REGISTER DESCRIPTIONS
POWER_MIN Register (0x48–0x49) (R/W)
Byte 1 (0x48)
BIT(S) NAME OPERATION
B[7-0] POWER_MIN_MSB Stores the MSBs for the smallest POWER measurement result
Byte 2 (0x49)
B[7-0] POWER_MIN_LSB Stores the LSBs for the smallest POWER measurement result
POWER_MAX Register (0x4A–0x4B) (R/W)
Byte 1 (0x4A)
BIT(S) NAME OPERATION
B[7-0] POWER_MAX_MSB Stores the MSBs for the largest POWER measurement result
Byte 2 (0x4B)
B[7-0] POWER_MAX_LSB Stores the LSBs for the largest POWER measurement result
EE_SCRATCH_PAD Non-Volatile Register (0x4C–0x4F) (R/W)
Byte 1 (0x4C)
BIT(S) NAME DEFAULT OPERATION
B[7-0] SCRATCH_PAD_1 0x00 Uncommitted nonvolatile memory
Byte 2 (0x4D)
B[7-0] SCRATCH_PAD_2 0x00 Uncommitted nonvolatile memory
Byte 3 (0x4E)
B[7-0] SCRATCH_PAD_3 0x00 Uncommitted nonvolatile memory
Byte 4 (0x4F)
B[7-0] SCRATCH_PAD_4 0x00 Uncommitted nonvolatile memory
LTC4281
43
Rev. A
For more information www.analog.com
TYPICAL APPLICATIONS
12V, 50A Backplane Resident Application
+
VDD GATE
CTIMER
15nF
TIMER
LTC4281
Y1
4MHz
ABLS-4.000MHZ-B4-T
INTVCC
CONNECTOR 1
CONNECTOR 2
PLUG-IN
BOARD
NC
R1
34.0k
1%
R4
10Ω
R
S
0.5mΩ
SOURCE
FB
GPIO1
GPIO2
GPIO3
SDAI
SDAO
SCL
ALERT
ON
UV
OV
ADR0
ADR1
ADR2
SENSE1+SENSE1–
ADC+ADC–
WP GND
12V
CLKOUTCLKIN
C3
1µF
C4
33pF
C5
33pF
BACKPLANE
R2
1.18k
1%
CF
0.1µF
25V
12V
GND
CS
150µF
R3
3.4k
1%
R7
28.7k
1%
R8
3.57k
1%
GP
GP
R9
10k
5%
R10
10k
5%
+
SDA
SCL
ALERT
POWER GOOD
4281 TA02
CL
VOUT
12V
50A ADJUSTABLE
Q2
PSMN2R0-30YLE × 2
LTC4281
44
Rev. A
For more information www.analog.com
PACKAGE DESCRIPTION
4.00 ±0.10
(2 SIDES)
2.50 REF
5.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGHD-3).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1
TOP MARK
(NOTE 6)
0.40 ±0.10
27 28
1
2
BOTTOM VIEW—EXPOSED PAD
3.50 REF
0.75 ±0.05 R = 0.115
TYP
R = 0.05
TYP
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
0.25 ±0.05
0.50 BSC
0.200 REF
0.00 – 0.05
(UFD28) QFN 0816 REV C
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.70 ±0.05
0.25 ±0.05
0.50 BSC
2.50 REF
3.50 REF
4.10 ±0.05
5.50 ±0.05
2.65 ±0.05
3.10 ±0.05
4.50 ±0.05
PACKAGE OUTLINE
2.65 ±0.10
3.65 ±0.10
3.65 ±0.05
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev C)
LTC4281
45
Rev. A
For more information www.analog.com
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 11/18 Updated graph for MOSFET Power Limit (G05)
Corrected pin numbers in Functional Diagram
Added RG, CG, DG network in Figure1
Added a section on constant current startup
Updated sections: Resetting Faults in FAULT_LOG, Layout Considerations, EEPROM, EE_CONTROL register
Updated equations to calculate R2, R3, and R7
6
10
13
15
20, 25, 29, 38
23
LTC4281
46
Rev. A
For more information www.analog.com
ANALOG DEVICES, INC. 2015-2018
D17179-0-11/18(A)
www.analog.com
RELATED PARTS
TYPICAL APPLICATION
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PART NUMBER DESCRIPTION COMMENTS
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ADC and I2C
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–12V to –100V
LTC4280 Hot Swap Controller with I2C Internal 8-Bit ADC, Adjustable Short-Circuit Filter Time
LTC4282 Hot Swap Controller with I2C and EEPROM Internal 12-Bit ADC, Power Monitoring, Dual Paths for SOA Sharing
+
VDD
6
5
2 8
5V
R10
3.3k
3 HCPL-0300
INTVCC
Z1
SMCJ15CA
GATE
CTIMER
15nF
TIMER
LTC4281
INTVCC
CONNECTOR 1
CONNECTOR 2
PLUG-IN
BOARD
NC
NC
NC
NC NC
4281 TA02
GND
SCL
SDA
10Ω
CL
RS1
0.5mΩ
Q1
PSMN2R0-30YLE × 2
PRF18BE471QB5RB
SOURCE
FB
ON
GPIO1
GPIO2
GPIO3
CLKIN
UV
OV
SDAI
SDAO
SCL
ALERT
ADR0
ADR1
ADR2
SENSE+SENSE–
ADC+ADC–
WP GND
C3
4.7µF
12V
BACKPLANE
5V
6
5
2 8
5V
R13
3.3k
R12
10k
R9
10k
3 HCPL-0300
INTVCC
2
3
6 8
5 HCPL-0300
INTVCC
4.7kΩ
VOUT
12V
65A ADJUSTABLE