LTC4270/LTC4271
1
42701fd
For more information www.linear.com/LTC4270
n PoE PSE Switches/Routers
n PoE PSE Midspans
APPLICATIONS
TYPICAL APPLICATION
FEATURES DESCRIPTION
12-Port PoE/PoE+/LTPoE++
PSE Controller
The LT C
®
4270/LTC4271 chipset is a 12-port power sourc-
ing equipment (PSE) controller designed for use in IEEE
802.3at Type 1 and Type 2 (high power) compliant Power
over Ethernet (PoE) systems. Transformer-isolated com-
munication protocol replaces expensive opto-couplers
and complex isolated 3.3V supply resulting in significant
BOM cost savings. The LTC4270/LTC4271 chipset delivers
lowest-in-industry heat dissipation by utilizing low-RDS(ON)
external MOSFETs and 0.25Ω sense resistors.
Advanced power management features include per-port
12-bit current monitoring ADCs, DAC-programmable cur-
rent limit, and versatile fast shut-down of preselected ports.
Advanced power management host software is available
under a no-cost license. PD discovery uses a proprietary
dual-mode 4-point detection mechanism ensuring excel-
lent immunity from false PD detection. Midspan PSEs
are supported with 2-event classification and a 2 second
backoff timer. The LTC4270/LTC4271 includes an I2C
serial interface operable up to 1MHz.
The LTC4270/LTC4271 is available in multiple power
grades allowing delivered PD power up to 90W.
n 12 Independent PSE Channels
n Compliant with IEEE 802.3at Type 1 and 2
n Chipset Provides Electrical Isolation
n Reduced BOM Cost
n Eliminates up to 6 High Speed Opto-Couplers
n Eliminates Isolated 3.3V Power Supply
n Low Power Dissipation
n 0.25Ω Sense Resistance Per Channel
n Very High Reliability 4-Point PD Detection
n 2-Point Forced Voltage
n 2-Point Forced Current
n VEE and VPORT Monitoring
n 1 Second Rolling IPORT Averaging
n Supports 2-Pair and 4-Pair Output Power
n 1MHz I2C Compatible Serial Control Interface
n Available In Three Power Grades
n A-Grade – LTPoE++™ 38.7W to 90W
n B-Grade – PoE+ 25.5W
n C-Grade – PoE 13W
n Available In 52-Lead 7mm × 8mm (LTC4270) and
24-Lead 4mm × 4mm (LTC4271) QFN Packages
LTC4270/LTC4271 FAMILY
LTC4270
GRADE ISOLATION LTPoE++ PoE+PoE
MAX
DELIVERED
POWER
ATransformer l l l 90W
BTransformer l l 25.5W
CTransformer l13W
L, LT , LT C , LT M , Burst Mode, Linear Technology and the Linear logo are registered trademarks
and LTPoE++ is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
••
3.3V
3.3V 0.1µF
–54V
100Ω
100Ω
100Ω
100Ω
••
3.3V
1µF 0.1µF
–54V
100Ω
100Ω
100Ω
100Ω
GP0
GP1
MID
RESET
MSD
INT
AUTO
SCL
AD0
AD1
AD2
AD3
AD6 DGND CAP1 CAP2
DND
DPD
CND
CPD
DNA SENSE1
GATE1
OUT1
LTC4270
DPA
CNA
CPA
XIO0 XIO1 0.22µF
100V
0.22µF
100V
S1B
S1B
–54V
–54V
42701 TA01a
–54V
S1B
S1B
PORTn
PORT1
SDAIN
SDAOUT
LTC4271
NO ISOLATION
REQUIRED ON
I2C INTERFACE
VDD33
2nF 2kV
VEE
SENSEn
GATEn
OUTn
AGND VSSK
0.25Ω
0.25Ω
–54V
>47µF
SYSTEM
BULK CAP
+
1µF
10Ω
LTC4270/LTC4271
2
42701fd
For more information www.linear.com/LTC4270
ABSOLUTE MAXIMUM RATINGS LTC4270
Supply Voltages
AGND – VEE ........................................... –0.3V to 80V
VSSK (Note 7) ..................... VEE – 0.3V to VEE + 0.3V
Digital Pins
XIOn ................................. VEE – 0.3V to CAP2 + 0.3V
Analog Pins
SENSEn, GATEn, OUTn ........ VEE – 0.3V to VEE + 80V
CAP2 (Note 13) ....................... VEE –0.3V to VEE + 5V
CPA, CNA, DPA, DNA ..............VEE – 0.3V to VEE + 0.3
(Note 1, Note 4)
Operating Ambient Temperature Range
LTC4270I .............................................–40°C to 85°C
Junction Temperature (Note 2) ............................ 125°C
Storage Temperature Range ......................–65 to 150°C
ABSOLUTE MAXIMUM RATINGS LTC4271
Supply Voltages
VDD – DGND ......................................... –0.3V to 3.6V
Digital Pins
SCL, SDAIN, SDAOUT, INT, RESET, MSD, ADn, AUTO,
MID, GPn ........................DGND – 0.3V to VDD + 0.3V
Operating Ambient Temperature Range
LTC4271I..............................................–40°C to 85°C
(Note 1)
Analog Pins
CAP1 (Note 13) ...........................–0.3V to DGND + 2V
CPD, CND, DPD, DND ..........DGND – 0.3 to VDD + 0.3
Junction Temperature (Note 2) ............................ 125°C
Storage Temperature Range ......................–65 to 150°C
LTC4270/LTC4271
3
42701fd
For more information www.linear.com/LTC4270
PIN CONFIGURATION
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION MAX PWR TEMPERATURE RANGE
LTC4271IUF#PBF LTC4271IUF#TRPBF 4271 24-Lead (4mm × 4mm) Plastic QFN –40°C to 85°C
LTC4270AIUKG#PBF LTC4270AIUKG#TRPBF LTC4270AUKG 52-Lead (7mm × 8mm) Plastic QFN 90W –40°C to 85°C
LTC4270BIUKG#PBF LTC4270BIUKG#TRPBF LTC4270BUKG 52-Lead (7mm × 8mm) Plastic QFN 25.5W –40°C to 85°C
LTC4270CIUKG#PBF LTC4270CIUKG#TRPBF LTC4270CUKG 52-Lead (7mm × 8mm) Plastic QFN 13W –40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
LTC4270 LTC4271
1615 17 18 19
TOP VIEW
53
VSSK
UKG PACKAGE
52-LEAD (7mm × 8mm) PLASTIC QFN
20 21 22 23 24 25 26
5152 50 49 48 47 46 45 44 43 42 41
33
34
35
36
37
38
39
40
8
7
6
5
4
3
2
1SENSE1
GATE1
OUT1
SENSE2
GATE2
OUT2
CAP2
SENSE3
GATE3
OUT3
SENSE4
GATE4
OUT4
XIO0
SENSE12
GATE12
OUT12
SENSE11
GATE11
OUT11
AGND
SENSE10
GATE10
OUT10
SENSE9
GATE9
OUT9
XIO1
VEE
VEE
CPA
CNA
DPA
DNA
NC
NC
NC
NC
NC
VEE
SENSE5
GATE5
OUT5
SENSE6
GATE6
OUT6
SENSE7
GATE7
OUT7
SENSE8
GATE8
OUT8
32
31
30
29
28
27
9
10
11
12
13
14
TJMAX = 125°C, θJA = 40°C/W
EXPOSED PAD (PIN 53) IS VSSK, MUST BE SOLDERED TO PCB
24 23 22 21 20 19
789
TOP VIEW
UF PACKAGE
24-LEAD (4mm × 4mm) PLASTIC QFN
10 11 12
6
5
4
3
2
1
13
14
15
16
17
18
AD0
AD1
AD2
AD3
AD6
MID
SCL
SDAIN
SDAOUT
INT
RESET
DNC
MSD
GP0
GP1
AUTO
VDD33
CAP1
NC
CPD
CND
DPD
DND
VDD33
25
DGND
TJMAX = 125°C, θJA = 37°C/W
EXPOSED PAD (PIN 25) IS DGND, MUST BE SOLDERED TO PCB
LTC4270/LTC4271
4
42701fd
For more information www.linear.com/LTC4270
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VEE Main PoE Supply Voltage AGND – VEE
For IEEE Type 1 Compliant Output
For IEEE Type 2 Compliant Output
For LTPoE++ Compliant Output
l
l
l
45
51
54.75
57
57
57
V
V
V
Undervoltage Lock-Out AGND – VEE l20 25 30 V
VDD VDD Supply Voltage VDD – DGND l3.0 3.3 3.6 V
Undervoltage Lock-Out VDD – DGND 2.7 V
VCAP1 Internal Regulator Supply Voltage VCAP1 – DGND 1.84 V
VCAP2 Internal Regulator Supply Voltage VCAP2 – VEE 4.3 V
IEE VEE Supply Current (AGND – VEE) = 55V l9 15 mA
REE VEE Supply Resistance VEE < 15V l12 kΩ
IDD VDD Supply Current (VDD – DGND) = 3.3V l10 15 mA
Detection
Detection Current – Forced Current First Point, AGND – VOUTn = 9V
Second Point, AGND – VOUTn = 3.5V
l
l
220
143
240
160
260
180
µA
µA
Detection Voltage – Forced Voltage AGND – VOUTn, 5µA ≤ IOUTn ≤ 500µA
First Point
Second Point
l
l
7
3
8
4
9
5
V
V
Detection Current Compliance AGND – VOUTn = 0V l0.8 0.9 mA
VOC Detection Voltage Compliance AGND – VOUTn, Open Port l10.4 12 V
Detection Voltage Slew Rate AGND – VOUTn, CPORT = 0.15µF l0.01 V/µs
Min. Valid Signature Resistance l15.5 17 18.5 kΩ
Max. Valid Signature Resistance l27.5 29.7 32 kΩ
Classification
VCLASS Classification Voltage AGND – VOUTn, 0mA ≤ IOUTn ≤ 50mA l16.0 20.5 V
Classification Current Compliance VOUTn = AGND l53 61 67 mA
Classification Threshold Current Class 0-1
Class 1-2
Class 2-3
Class 3-4
Class 4-Overcurrent
l
l
l
l
l
5.5
13.5
21.5
31.5
45.2
6.5
14.5
23
33
48
7.5
15.5
24.5
34.9
50.8
mA
mA
mA
mA
mA
VMARK Classification Mark State Voltage AGND – VOUTn, 0.1mA ≤ ICLASS ≤ 5mA l7.5 9 10 V
Mark State Current Compliance VOUTn = AGND l53 61 67 mA
Gate Driver
GATE Pin Pull-Down Current Port Off, VGATEn = VEE + 5V
Port Off, VGATEn = VEE + 1V
l
l
0.4
0.08
0.12
mA
mA
GATE Pin Fast Pull-Down Current VGATEn = VEE + 5V 30 mA
GATE Pin On Voltage VGATEn – VEE, IGATEn = 1µA l8 12 14 V
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. AGND – VEE = 54V and VDD – DGND = 3.3V unless otherwise noted.
(Notes 3 & 4)
LTC4270/LTC4271
5
42701fd
For more information www.linear.com/LTC4270
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. AGND – VEE = 54V and VDD – DGND = 3.3V unless otherwise noted.
(Notes 3 & 4)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Output Voltage Sense
VPG Power Good Threshold Voltage VOUTn – VEE l2 2.4 2.8 V
OUT Pin Pull-Up Resistance to
AGND
0V ≤ (AGND – VOUT) ≤ 5V l300 500 700 kΩ
VCUT Overcurrent Sense Voltage VSENSEn – VEE,
hpen = 0Fh, cutn = D4h
hpen = 0Fh, cutn = E2h (Note 12)
l
l
89
152
94
159
99
168
mV
mV
Overcurrent Sense in AUTO Pin
Mode
Class 0, Class 3
Class 1
Class 2
Class 4
l
l
l
l
89
26
49
152
94
28
52
159
99
30
55
168
mV
mV
mV
mV
VLIM Active Current Limit in 802.3af
Compliant Mode
VSENSEn – VEE, hpen = 0Fh, limn = 80h, (AGND–VEE) = 55V
VEE < VOUT < AGND – 29V
AGND – VOUT = 0V (Note 12)
l
l
102
25
106
112
50
mV
mV
Active Current Limit in High Power
Mode
hpen = 0Fh, limn = C0h, AGND–VEE = 55V
VOUT – VEE = 0 – 10V
VEE + 23V < VOUT < AGND – 29V
AGND – VOUT = 0V (Note 12)
l
l
l
204
102
25
212
106
225
115
50
mV
mV
mV
Active Current Limit in AUTO Pin
Mode
VEE < VOUT < AGND – 10V, AGND–VEE = 55V
Class 0 to Class 3
Class 4
l
l
102
204
106
212
112
225
mV
mV
VMIN DC Disconnect Sense Voltage VSENSE – VEE, rdis Bit = 0
VSENSE – VEE, rdis Bit = 1 (Note 12)
l
l
2.6
1.3
3.8
1.9
4.9
2.45
mV
mV
VSC Short-Circuit Sense VSENSEn – VEE – VLIM (Note 12)
rdis Bit = 0
rdis bit = 1
l
l
125
70
200
100
255
125
mV
mV
Port Current Readback
Resolution No Missing Codes, Reported as 14-Bits 12 Bits
LSB Weight VSENSEn – VEE 30.518 µV/LSB
Conversion Period 25.1 ms/
Convert
Port Voltage Readback
Resolution No Missing Codes, Reported as 14-Bits 12 Bits
LSB Weight VSENSEn – VEE 5.8350 mV/LSB
Digital Interface
VILD Digital Input Low Voltage ADn, RESET, MSD, GPn, AUTO, MID (Note 6) l0.8 V
I2C Input Low Voltage SCL, SDAIN (Note 6) l1.0 V
VIHD Digital Input High Voltage (Note 6) l2.2 V
Digital Output Voltage Low ISDAOUT = 3mA, IINT = 3mA
ISDAOUT = 5mA, IINT = 5mA
l
l
0.4
0.7
V
V
Internal Pull Up to VDD ADn, RESET, MSD, GPn 50 kΩ
Internal Pull Down To DGND AUTO, MID 50 kΩ
LTC4270/LTC4271
6
42701fd
For more information www.linear.com/LTC4270
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
XIO
VOLX XIO Digital Output Low VXIOn – VEE, IXIOn = 5mA l0.7 V
VOHX XIO Digital Output High VXIOn – VEE, IXIOn = 100µA l3.5 V
XIO Digital Input Low Voltage VXIOn – VEE l0.8 V
XIO Digital Input High Voltage VXIOn – VEE l3.4 V
Internal Pull Up to CAP2 XIO0, XIO1 50 kΩ
PSE Timing Characteristics
tDET Detection Time Beginning To End of Detection (Note 7) 220 ms
tCLE Class Event Duration (Note 7) 12 ms
tCLEON Class Event Turn On Duration CPORT = 0.6µF (Note 7) l0.1 ms
tME Mark Event Duration (Note 7, Note 11) 8.6 ms
tMEL Last Mark Event Duration (Note 7, Note 11) l16 22 ms
tPON Power On Delay in AUTO Pin Mode From End of Valid Detect to Application of Power to Port
(Note 7)
l60 ms
Turn-On Rise Time (AGND – VOUT): 10% to 90% of (AGND - VEE) CPORT =
0.15µF (Note 7)
l15 24 µs
Turn-On Ramp Rate CPORT = 0.15µF (Note 7) l10 V/µs
tTOCL Turn-On Class Transition CPORT = 0.15µF (Note 7) l0.1 ms
tED Fault Delay From ICUT or ILIM Fault to Next Detect
(Note 7)
l1.0 1.1 s
Midspan Mode Detection Backoff RPORT = 15.5kΩ (Note 7) l2.3 2.5 2.7 s
Power Removal Detection Delay From Power Removal After tDIS to Next Detect (Note 7) l1.0 1.3 2.5 s
tSTART Maximum Current Limit Duration
During Port Start-Up
(Note 7) l52 59 66 ms
tCUT Maximum Overcurrent Duration
After Port Start-Up
(Note 7) l52 59 66 ms
Maximum Overcurrent Duty Cycle (Note 7) l5.8 6.3 6.7 %
tLIM Maximum Current Limit Duration
After Port Start-Up – tLIM Enabled
tLIM = 1 (Note 7, Note 12) l10 12 14 ms
Maximum Current Limit Duration
After Port Start-Up – tLIM as tCUT
tLIM = 0 (Note 7, Note 12) l52 59 66 ms
tMPS Maintain Power Signature (MPS)
Pulse Width Sensitivity
Current Pulse Width to Reset Disconnect Timer (Note 7,
Note 8)
l1.6 3.6 ms
tDIS Maintain Power Signature (MPS)
Dropout Time
(Note 7, Note 5) l320 350 380 ms
tMSD Masked Shut Down Delay (Note 7) 6.5 µs
I2C Watchdog Timer Duration (Note 7) l1.5 2 3 s
Minimum Pulse Width for Masked
Shut Down
(Note 7) l3 µs
Minimum Pulse Width for RESET (Note 7) l4.5 µs
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. AGND – VEE = 54V and VDD – DGND = 3.3V unless otherwise noted.
(Notes 3 & 4)
LTC4270/LTC4271
7
42701fd
For more information www.linear.com/LTC4270
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I2C Timing
fSCLK Clock Frequency (Note 7) l1 MHz
t1Bus Free Time Figure 5 (Notes 7, 9) l480 ns
t2Start Hold Time Figure 5 (Notes 7, 9) l240 ns
t3SCL Low Time Figure 5 (Notes 7, 9) l480 ns
t4SCL High Time Figure 5 (Notes 7, 9) l240 ns
t5SDAIN Data Hold Time Figure 5 (Notes 7, 9) l60 ns
t5Data Clock to SDAOUT Valid Figure 5 (Notes 7, 9) l130 ns
t6Data Set-Up Time Figure 5 (Notes 7, 9) l80 ns
t7Start Set-Up Time Figure 5 (Notes 7, 9) l240 ns
t8Stop Set-Up Time Figure 5 (Notes 7, 9) l240 ns
trSCL, SDAIN Rise Time Figure 5 (Notes 7, 9) l120 ns
tfSCL, SDAIN Fall Time Figure 5 (Notes 7, 9) l60 ns
Fault Present to INT Pin Low (Notes 7, 9, 10) l150 ns
Stop Condition to INT Pin Low (Notes 7, 9, 10) l1.5 µs
ARA to INT Pin High Time (Notes 7, 9) l1.5 µs
SCL Fall to ACK Low (Notes 7, 9) l130 ns
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. AGND – VEE = 54V and VDD – DGND = 3.3V unless otherwise noted.
(Notes 3 & 4)
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. With the exception of (VDD –
DGND), exposure to any Absolute Maximum Rating condition for extended
periods may affect device reliability and lifetime.
Note 2: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 140ºC when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3: All currents into device pins are positive; all currents out of device
pins are negative.
Note 4: The LTC4270 operates with a negative supply voltage (with
respect to AGND). To avoid confusion, voltages in this data sheet are
referred to in terms of absolute magnitude.
Note 5: tDIS is the same as tMPDO defined by IEEE 802.3at
Note 6: The LTC4271 digital interface operates with respect to DGND. All
logic levels are measured with respect to DGND.
Note 7: Guaranteed by design, not subject to test.
Note 8: The IEEE 802.3af specification allows a PD to present its
Maintain Power Signature (MPS) on an intermittent basis without being
disconnected. In order to stay powered, the PD must present the MPS for
tMPS within any tMPDO time window.
Note 9: Values Measured at VILD and VIHD
Note 10: If a fault condition occurs during an I2C transaction, the INT pin
will not be pulled down until a stop condition is present on the I2C bus.
Note 11: Load characteristics of the LTC4270 during Mark: 7V < (AGND –
VOUTn) < 10V or IOUT < 50µA.
Note 12: See the LTC4271 Software Programming documentation for
information on serial bus usage and device configuration and status
registers.
Note 13: Do not source or sink current from CAP1 and CAP2.
LTC4270/LTC4271
8
42701fd
For more information www.linear.com/LTC4270
Classification Transient Response
to 40mA Load Step
TYPICAL PERFORMANCE CHARACTERISTICS
Powering Up into a 180µF Load Classification Current Compliance
VDD Supply Current vs Voltage VEE Supply Current vs Voltage
802.3af Power On Sequence in
AUTO Pin Mode
802.3at Power On Sequence in
AUTO Pin Mode
Power On Sequence with 10VPP
60Hz Noise
FORCED CURRENT
DETECTION
VEE = –55V
CLASS 3 PD
VEE
50ms/DIV
–60
PORT VOLTAGE (V)
–10
–20
–30
–40
–50
0
42701 G01
FORCED VOLTAGE
DETECTION
802.3af
CLASSIFICATION
POWER ON
AGND
FORCED CURRENT
DETECTION
VEE = –55V
CLASS 4 PD
VEE
50ms/DIV
–60
PORT VOLTAGE (V)
–10
–20
–30
–40
–50
0
42701 G02
FORCED VOLTAGE
DETECTION
802.3at
CLASSIFICATION
POWER ON
AGND
AGND
50ms/DIV
–25
PORT VOLTAGE (V)
0
–5
–10
–15
–20
5
42701 G03
FORCED VOLTAGE
DETECTION
802.3af
CLASSIFICATION
POWER ON
PORT OFF
FORCED CURRENT
DETECTION
NORMAL DETECT
DETECT WITH 60Hz NOISE
5ms/DIV
42701 G04
FET ON
FOLDBACK
VEE = –54V
AGND
VEE
VEE
0mA
PORT VOLTAGE
20V/DIV
PORT CURRENT
200mA/DIV
GATE VOLTAGE
10V/DIV
425mA
CURRENT LIMIT
LOAD FULLY CHARGED
CLASSIFICATION CURRENT (mA)
42701 G06
0 2010 30 40 50 60 70
0
–2
–4
–6
–8
–10
–12
–14
–16
–18
–20
CLASSIFICATION VOLTAGE (V)
50µs/DIV
40mA
0mA
42701 G05
–20V
PORT
VOLTAGE
1V/DIV
PORT
CURRENT
20mA/DIV
VDD = 3.3V
VEE = –54V
VDD SUPPLY VOLTAGE (V)
42701 G07
2.7 2.92.8 3.0 3.1 3.2 3.3 3.4 3.63.5
15.0
12.0
9.0
6.0
3.0
0.0
IDD SUPPLY CURRENT (mA)
–40
25°C
85°C
VEE SUPPLY VOLTAGE (V)
42701 G08
–60 –50 –40 –30 –20
9.0
8.5
8.0
7.0
7.5
6.5
6.0
IEE SUPPLY CURRENT (mA)
–40
25°C
85°C
LTC4270/LTC4271
9
42701fd
For more information www.linear.com/LTC4270
802.3at ILIM Threshold
vs Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
DC Disconnect Threshold
vs Temperature 802.3at Current Limit Foldback
INT and SDAOUT Pull Down
Voltage vs Load Current
MOSFET Gate Drive With Fast
Pull Down
802.3at ICUT Threshold
vs Temperature
TEMPERATURE (°C)
–40
204
VLIM (mV)
ILIM (mA)
216
212
208
220
816
864
848
832
880
40 60 80 100 120–20 0
42701 G09
20
PORT 1
REG 48h = C0h
RSENSE = 0.25Ω
TEMPERATURE (°C)
–40
152
VCUT (mV)
ICUT (mA)
162
164
160
158
156
154
166
608
648
640
632
624
616
664
656
40 60 80 100 120–20 0
42701 G10
20
PORT 1
REG 47h = E2h
RSENSE = 0.25Ω
TEMPERATURE (°C)
–40
1.25
VMIN (mV)
IMIN (mA)
2.00
2.25
1.75
1.50
2.50
5
8
7
6
10
9
40 60 80 100 120–20 0
42701 G11
20
PORT 1
REG 47h = E2h
RSENSE = 0.25Ω
VOUTn (V)
–54
0
ILIM (mA)
VLIM (mV)
600
500
800
700
400
300
200
100
900
0
125
100
75
50
25
225
200
175
150
–36 –27 –18 –9 0–45
42701 G12
PORT 1
REG 48h = C0h
RSENSE = 0.25Ω
100µs/DIV
GND
0mA
42701 G14
VEE
VEE
PORT
CURRENT
500mA/DIV
GATE
VOLTAGE
10V/DIV
PORT
VOLTAGE
20V/DIV
CURRENT LIMIT
50Ω FAULT REMOVED
50Ω
FAULT
APPLIED
VDD = 3.3V
VEE = –54V
FAST PULL DOWN
LOAD CURRENT (mA)
42701 G13
0 2010 30 40 50 60
3.0
2.5
2.0
1.5
1.0
0.5
0.0
PULLDOWN VOLTAGE (V)
LTC4270/LTC4271
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TEST TIMING DIAGRAMS
VPORTn
INT
VOC
VEE
tDET
tME
tMEL
VMARK
VCLASS
15.5V
20.5V
tCLE
tCLE
tCLEON
PD
CONNECTED
0V
42701 F01
FORCED-CURRENT
CLASSIFICATION
tPON
FORCED-
VOLTAGE
Figure 1. Detect, Class and Turn-On Timing in AUTO Pin or Semi-auto Modes
VLIM VCUT
0V
VSENSEn TO VEE
INT
42701 F02
tSTART, tCUT
VMIN
VSENSEn
TO VEE
INT
tDIS
tMPS 42701 F03
Figure 2. Current Limit Timing
Figure 3. DC Disconnect Timing
LTC4270/LTC4271
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I2C TIMING DIAGRAMS
SCL
SDA
42701 F06
AD6 01 AD3 AD2 AD1 AD0 A7 A6 A5 A4 A3 A2 A1 A0
R/W ACK D7 D6 D5 D4 D3 D2 D1 D0
ACK ACK
START BY
MASTER
ACK BY
SLAVE
ACK BY
SLAVE
ACK BY
SLAVE
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
REGISTER ADDRESS BYTE
FRAME 3
DATA BYTE
STOP BY
MASTER
SCL
SDA AD6 01 AD3 AD2 AD1 AD0 A7 A6 A5 A4 A3 A2 A1 A0
R/W ACK ACK 01 AD3 AD2 AD1 AD0 D7 D6 D5 D4 D3 D2 D1 D0
R/W ACK ACK
START BY
MASTER
ACK BY
SLAVE
ACK BY
SLAVE
42701 F07
STOP BY
MASTER
REPEATED
START BY
MASTER
ACK BY
SLAVE
NO ACK BY
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
REGISTER ADDRESS BYTE
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
DATA BYTE
AD6
SCL
SDA
42701 F08
AD6 1 0AD3 AD2 AD1 AD0 D7 D6 D5 D4 D3 D2 D1 D0
R/W ACK ACK
START BY
MASTER
ACK BY
SLAVE
NO ACK BY
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
DATA BYTE
STOP BY
MASTER
SCL
SDA
42701 F09
0 0 110AD30AD600 1 AD2 AD1 AD0
R/W ACK ACK1
START BY
MASTER
ACK BY
SLAVE
NO ACK BY
MASTER
FRAME 1
ALERT RESPONSE ADDRESS BYTE
FRAME 2
SERIAL BUS ADDRESS BYTE
STOP BY
MASTER
Figure 6. Writing to a Register
Figure 7. Reading from a Register
Figure 8. Reading the Interrupt Register (Short Form)
Figure 9. Reading from Alert Response Address
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PIN FUNCTIONS
LTC4270
SENSEn (Pins 1, 4, 8, 11, 15, 18, 21, 24, 30, 33, 37,
40): Port n Current Sense Input. SENSEn monitors the
external MOSFET current via a 0.5Ω or 0.25Ω sense
resistor between SENSEn and VEE. Whenever the voltage
across the sense resistor exceeds the overcurrent detection
threshold VCUT, the current limit fault timer counts up. If
the voltage across the sense resistor reaches the current
limit threshold VLIM, the GATEn pin voltage is lowered to
maintain constant current in the external MOSFET. See
Applications Information for further details. If the port is
unused, the SENSEn pin must be tied to VEE.
GATEn (Pins 2, 5, 9, 12, 16, 19, 22, 25, 29, 32, 36, 39):
Port n Gate Drive. GATEn should be connected to the gate
of the external MOSFET for port n. When the MOSFET is
turned on, the gate voltage is driven to 12V (typ) above
VEE. During a current limit condition, the voltage at GATEn
will be reduced to maintain constant current through the
external MOSFET. If the fault timer expires, GATEn is pulled
down, turning the MOSFET off and recording a port fault
event. If the port is unused, float the GATEn pin.
OUTn (Pins 3, 6, 10, 13, 17, 20, 23, 26, 28, 31, 35, 38):
Port n Output Voltage Monitor. OUTn should be connected
to the output port. A current limit foldback circuit limits
the power dissipation in the external MOSFET by reducing
the current limit threshold when the drain-to-source volt-
age exceeds 10V. The port n Power Good bit is set when
the voltage from OUTn to VEE drops below 2.4V (typ). A
500k resistor is connected internally from OUTn to AGND
when the port is idle. If the port is unused, the OUTn pin
must be floated.
CAP2 (Pin 7): Analog Internal 4.3V Power Supply Bypass
Capacitor. Connect 0.1µF ceramic cap to VEE.
XIO0 (Pin 14): General Purpose Digital Input Output. Logic
signal between VEE and VEE + 4.3V. Internal pull up.
XIO1 (Pin 27): General Purpose Digital Input Output. Logic
signal between VEE and VEE + 4.3V. Internal pull up.
AGND (Pin 34): Analog Ground. Connect AGND to the
return for the VEE supply.
VEE (Pins 41, 51, 52): Main PoE Supply Input. Connect to
a –45V to –57V supply, relative to AGND. Voltage depends
on PSE type (Type 1, Type 2 or LTPoE++.)
DNA (Pin 47): Data Transceiver Negative Input Output
(Analog). Connect to DND through a data transformer.
DPA (Pin 48): Data Transceiver Positive Input Output
(Analog). Connect to DPD through a data transformer.
CNA (Pin 49): Clock Transceiver Negative Input Output
(Analog). Connect to CND through a data transformer.
CPA (Pin 50): Clock Transceiver Positive Input Output
(Analog). Connect to CPD through a data transformer.
VSSK (Exposed Pad Pin 53): Kelvin Sense to VEE. Connect
to sense resistor common node. Do not connect directly
to VEE plane. See Layout Guide.
Common Pins
NC, DNC (LTC4271 Pins 7,13; LTC4270 Pins 42, 43, 44,
45, 46): All pins identified with “NC” or “DNC” must be
left unconnected.
LTC4271
AD0 (Pin 1): Address Bit 0. Tie the address pins high or low
to set the starting I2C serial address to which the LTC4271
responds. The chip will respond to this address plus the
next two incremental addresses. The base address of the
first four ports will be (A610A3A2A1A0)b. The second and
third groups of four ports will respond at the next two
logical addresses. Internally pulled up to VDD.
AD1 (Pin 2): Address Bit 1. See AD0.
AD2 (Pin 3): Address Bit 2. See AD0.
AD3 (Pin 4): Address Bit 3. See AD0.
AD6 (Pin 5): Address Bit 6. See AD0.
MID (Pin 6): Midspan Mode Input. When high, the LTC4271
acts as a midspan device. Internally pulled down to DGND.
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CPD (Pin 8): Clock Transceiver Positive Input Output
(Digital). Connect to CPA through a data transformer.
CND (Pin 9): Clock Transceiver Negative Input Output
(Digital). Connect to CNA through a data transformer.
DPD (Pin 10): Data Transceiver Positive Input Output
(Digital). Connect to DPA through a data transformer.
DND (Pin 11): Data Transceiver Negative Input Output
(Digital). Connect to DNA through a data transformer.
VDD33 (Pins 12, 20): VDD IO Power Supply. Connect to
a 3.3V power supply relative to DGND. VDD33 must be
bypassed to DGND near the LTC4271 with at least a 0.1μF
capacitor.
RESET (Pin 14): Reset Input, Active Low. When the RESET
pin is low, the LTC4270/LTC4271 is held inactive with all
ports off and all internal registers reset to their power-up
states. When RESET is pulled high, the LTC4271 begins
normal operation. RESET can be connected to an external
capacitor or RC network to provide a power turn-on delay.
Internal filtering of the RESET pin prevents glitches less
than 1μs wide from resetting the LTC4270/LTC4271.
Internally pulled up to VDD.
INT (Pin 15): Interrupt Output, Open Drain. INT will pull
low when any one of several events occur in the LTC4271.
It will return to a high impedance state when bits 6 or 7
are set in the Reset PB register (1Ah). The INT signal can
be used to generate an interrupt to the host processor,
eliminating the need for continuous software polling. In-
dividual INT events can be disabled using the INT Mask
register (01h). See LTC4271 Software Programming
documentation for more information. The INT pin is only
updated between I2C transactions.
SDAOUT (Pin 16): Serial Data Output, Open Drain Data
Output for the I2C Serial Interface Bus. The LTC4271 uses
two pins to implement the bidirectional SDA function to
simplify optoisolation of the I2C bus. To implement a stan-
dard bidirectional SDA pin, tie SDAOUT and SDAIN together.
See Applications Information for more information.
SDAIN (Pin 17): Serial Data Input. High impedance data
input for the I2C serial interface bus. The LTC4271 uses two
pins to implement the bidirectional SDA function to simplify
optoisolation of the I2C bus. To implement a standard
bidirectional SDA pin, tie SDAOUT and SDAIN together.
See Applications Information for more information.
SCL (Pin 18): Serial Clock Input. High impedance clock
input for the I2C serial interface bus. The SCL pin should
be connected directly to the I2C SCL bus line. SCL must
be tied high if the I2C serial interface bus is not used.
CAP1 (Pin 19): Core Power Supply Bypass Capacitor. Con-
nect a 1µF Bypass capacitance to DGND for the internal
1.8V regulator. Do not use other capacitor values.
AUTO (Pin 21): AUTO Pin Mode Input. AUTO pin mode
allows the LTC4271 to detect and power up a PD even if
there is no host controller present on the I2C bus. The
AUTO pin determines the state of the internal registers
when the LTC4271 is reset or comes out of VDD UVLO
(see LTC4271 Software Programming documentation). The
states of these register bits can subsequently be changed
via the I2C interface. Internally pulled down to DGND. Must
be tied locally to either VDD or DGND.
GP1 (Pin 22): General Purpose Digital Input Output for
customer applications. Referenced to DGND.
GP0 (Pin 23): General Purpose Digital Input Output for
customer applications. Referenced to DGND.
MSD (Pin 24): Maskable Shutdown Input. Active low.
When pulled low, all ports that have their corresponding
mask bit set in the mconfig register (17h) will be reset.
Internal filtering of the MSD pin prevents glitches less
than 1μs wide from resetting ports. The MSD Pin Mode
register can configure the MSD pin polarity. Internally
pulled up to VDD.
DGND (Exposed Pad Pin 25): Digital Ground. DGND should
be connected to the return from the VDD supply.
PIN FUNCTIONS
LTC4270/LTC4271
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OVERVIEW
Power over Ethernet, or PoE, is a standard protocol for send-
ing DC power over copper Ethernet data wiring. The IEEE
group that administers the 802.3 Ethernet data standards
added PoE powering capability in 2003. This original PoE
spec, known as 802.3af, allowed for 48V DC power at up
to 13W. This initial specification was widely popular, but
13W was not adequate for some requirements. In 2009,
the IEEE released a new standard, known as 802.3at or
PoE+, increasing the voltage and current requirements to
provide 25W of power.
The IEEE standard also defines PoE terminology. A device
that provides power to the network is known as a PSE, or
power sourcing equipment, while a device that draws power
from the network is known as a PD, or powered device.
PSEs come in two types: Endpoints (typically network
switches or routers), which provide data and power; and
Midspans, which provide power but pass through data.
Midspans are typically used to add PoE capability to existing
non-PoE networks. PDs are typically IP phones, wireless
access points, security cameras, and similar devices.
PoE++ Evolution
Even during the process of creating the IEEE PoE+ 25.5W
specification it became clear that there was a significant
and increasing need for more than 25.5W of delivered
power. The A-grade LTC4270/LTC4271 chipset responds
to this market by allowing a reliable means of providing up
to 90W of delivered power to a LTPoE++ PD. The LTPoE++
specification provides reliable detection and classification
extensions to the existing IEEE PoE protocols that are
backward compatible and interoperable with existing Type
1 and Type 2 PDs. Unlike other proprietary PoE++ solutions
Linear’s LTPoE++ provides mutual identification between
the PSE and PD. This ensures the LTPoE++ PD knows it
may use the requested power at start-up because it has
detected a LTPoE++ PSE. LTPoE++ PSEs can differentiate
between a LTPoE++ PD and all other types of IEEE compli-
ant PDs allowing LTPoE++ PSEs to remain compliant and
interoperable with existing equipment.
APPLICATIONS INFORMATION
LTC4270/LTC4271 Product Family
The LTC4270/LTC4271 family is a fourth generation
12-port PSE controller that implements 12 PSE ports in
either an endpoint or midspan design. Virtually all neces-
sary circuitry is included to implement an IEEE 802.3at
compliant PSE design, requiring only an external power
MOSFET and sense resistor per channel; these minimize
power loss compared to alternative designs with onboard
MOSFETs and increase system reliability in the event a
single channel fails.
All grades of the LTC4270/LTC4271 family offer advanced
fourth generation PSE features, including per-port cur-
rent monitoring, VEE monitoring, port current policing,
one second current averaging and four general purpose
input/output pins.
The LTC4270/LTC4271 chipset implements a proprietary
isolation scheme for inter-chip communication. This
architecture dramatically reduces BOM cost by replacing
expensive opto-isolators and isolated power supplies with
a single low-cost transformer.
The LTC4270/LTC4271 comes in three grades which sup-
port different PD power levels.
The A-grade LTC4270/LTC4271 chipset extends PoE
power delivery capabilities to LTPoE++ levels. LTPoE++
is a Linear Technology proprietary specification allowing
for the delivery of up to 90W to LTPoE++ compliant PDs.
The LTPoE++ architecture extends the IEEE physical power
negotiation to include 38.7W, 52.7W, 70W and 90W power
levels. The A-grade LTC4270/LTC4271 also incorporates
all B- and C-grade features.
The B-grade LTC4270/LTC4271 is a fully IEEE-compliant
Type 2 PSE supporting autonomous detection, classifica-
tion and powering of Type 1 and Type 2 PDs. The B-grade
LTC4270/LTC4271 also incorporates all C-grade features.
The C-grade LTC4270/LTC4271 is a fully autonomous
802.3af Type 1 PSE solution. Intended for use only with
the AUTO pin tied high, the C-grade chipset autonomously
supports detection, classification and powering of Type 1
PDs. As a Type 1 PSE, two event classification is prohibited
and Class 4 PDs are automatically treated as Class 0 PDs.
LTC4270/LTC4271
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PoE BASICS
Common Ethernet data connections consist of two or four
twisted pairs of copper wire (commonly known as CAT-5
cable), transformer-coupled at each end to avoid ground
loops. PoE systems take advantage of this coupling ar-
rangement by applying voltage between the center-taps
of the data transformers to transmit power from the PSE
to the PD without affecting data transmission. Figure 10
shows a high level PoE system schematic.
To avoid damaging legacy data equipment that does not
expect to see DC voltage, the PoE spec defines a protocol
that determines when the PSE may apply and remove
power. Valid PDs are required to have a specific 25k
common-mode resistance at their input. When such a PD
is connected to the cable, the PSE detects this signature
resistance and turns on the power. When the PD is later
disconnected, the PSE senses the open circuit and turns
power off. The PSE also turns off power in the event of a
current fault or short circuit.
When a PD is detected, the PSE optionally looks for a
classification signature that tells the PSE the maximum
power the PD will draw. The PSE can use this information
to allocate power among several ports, to police the current
consumption of the PD, or to reject a PD that will draw more
power than the PSE has available. The classification step
APPLICATIONS INFORMATION
is optional; if a PSE chooses not to classify a PD, it must
assume that the PD is a 13W (full 802.3af power) device.
New in 802.3at
The newer 802.3at standard supersedes 802.3af and brings
several new features:
• A PD may draw as much as 25.5W. Such PDs (and the
PSEs that support them) are known as Type 2. Older
13W 802.3af equipment is classified as Type 1. Type 1
PDs will work with all PSEs; Type 2 PDs may require
Type 2 PSEs to work properly. The LTC4270/LTC4271
is designed to work in both Type 1 and Type 2 PSE de-
signs, and also supports non-standard configurations
at higher power levels.
• The Classification protocol is expanded to allow Type
2 PSEs to detect Type 2 PDs, and to allow Type 2 PDs
to determine if they are connected to a Type 2 PSE.
Two versions of the new Classification protocol are
available: an expanded version of the 802.3af Class
Pulse protocol, and an alternate method integrated
with the existing LLDP protocol (using the Ethernet
data path). The LTC4270/LTC4271 fully supports the
new Class Pulse protocol and is also compatible with
the LLDP protocol (which is implemented in the data
communications layer, not in the PoE circuitry).
Figure 10. Power Over Ethernet System Diagram
42701 F10
Tx
Rx
Rx
Tx
DATA PAIR
DATA PAIR
VEE GATE
SPARE PAIR
SPARE PAIR
1/12
LTC4270/LTC4271
AGND
I2C
–54V
CAT 5
20Ω MAX
ROUNDTRIP
0.05µF MAX
RJ45
4
5
4
5
1
2
1
2
3
6
3
6
7
8
7
8
RJ45
PSE PD
PWRGD
–54VOUT
LTC4265
GND
DC/DC
CONVERTER +
–
VOUT
GND
–54VIN
LTC4270/LTC4271
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• Fault protection current levels and timing are adjusted
to reduce peak power in the MOSFET during a fault;
this allows the new 25.5W power levels to be reached
using the same MOSFETs as older 13W designs.
Extended Power LTPoE++
A-grade LTC4270/LTC4271 parts add the capability to
autonomously deliver up to 90W of power to the PD.
LTPoE++ PDs may forgo 802.3 LLDP support and rely
solely on the LTPoE++ Physical Classification to negoti-
ate power with LTPoE++ PSEs; this greatly simplifies
high-power PD implementations.
LTPoE++ may be optionally enabled for A-grade LTC4270/
LTC4271s by setting both the High Power Enable and
LTPoE++ Enable bits.
The higher levels of LTPoE++ delivery impose additional
layout and component selection constraints. LTC4270 pin
selects allow the AUTO pin mode LTC4271 to autonomously
power up to supported power levels. If the AUTO pin is
high, the XIO1 and XIO0 pins are sampled at reset to de-
termine the maximum deliverable power. PDs requesting
more than the available power limits are not powered.
Table 1. LTPoE++ AUTO Pin Mode Maximum Delivered Power
Capabilities
POWER XIO1 XIO0
38.7W 0 0
52.7W 0 1
70W 1 0
90W 1 1
BACKWARDS COMPATIBILITY
The LTC4270/LTC4271 chipset is designed to be backward
compatible with the LTC4266, operating in Type 2 mode,
without software changes; only minor layout changes
are required to implement a fully compliant IEEE 802.3at
design.
Some LTC4266 registers have been obsoleted in the
LTC4270/LTC4271 chipset. The obsoleted registers are not
required for 802.3at compliant PSE operation. For more
details about software differences between the LTC4266
and LTC4270/LTC4271, refer to the LTC4271 Software
Programming document.
APPLICATIONS INFORMATION
Operation with high power mode disabled is obsoleted in
the LTC4270/LTC4271 chipset. All operations previously
available in low power mode are fully implemented as a
subset of the high power mode capabilities.
OPERATING MODES
The LTC4270/LTC4271 includes 12 independent ports,
each of which can operate in one of four modes: manual,
semi-auto, AUTO pin, or shutdown.
Table 2. Operating Modes
MODE
AUTO
PIN OPMD
DETECT/
CLASS POWER-UP
AUTOMATIC
ICUT/ILIM
ASSIGNMENT
AUTO Pin 1 11b Enabled
at Reset
Automatically Yes
Reserved 0 11b N/A N/A N/A
Semi-auto 0 10b Host
Enabled
Upon
Request
No
Manual 0 01b Once
Upon
Request
Upon
Request
No
Shutdown 0 00b Disabled Disabled No
In manual mode, the port waits for instructions from the
host system before taking any action. It runs a single
detection or classification cycle when commanded to by
the host, and reports the result in its Port Status register.
The host system can command the port to turn on or off
the power at any time.
In semi-auto mode, the port repeatedly attempts to detect
and classify any PD attached to it. It reports the status of
these attempts back to the host, and waits for a command
from the host before turning on power to the port. The
host must enable detection (and optionally classification)
for the port before detection will start.
AUTO pin mode operates the same as semi-auto mode
except it will automatically turn on the power to the port if
detection is successful. AUTO pin mode will autonomously
set the ICUT and ILIM values based on the class result. This
operational mode is only valid if the AUTO pin is high at
reset or power-up and remains high during operation.
In shutdown mode, the port is disabled and will not detect
or power a PD.
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Regardless of which mode it is in, the LTC4270/LTC4271
will remove power automatically from any port that gener-
ates a current limit fault. It will also automatically remove
power from any port that generates a disconnect event if
disconnect detection is enabled. The host controller may
also command the port to remove power at any time.
Reset and the AUTO/MID Pins
The initial LTC4270/LTC4271 configuration depends on
the state of the AUTO and MID pins during reset. Reset
occurs at power-up, or whenever the RESET pin is pulled
low or the global Reset All bit is set. Changing the state of
AUTO or MID after power-up will not properly change the
port behavior of the LTC4270/LTC4271 until a reset occurs.
Although typically used with a host controller, the LTC4270/
LTC4271 can also be used in a standalone mode with no
connection to the serial interface. If there is no host pres-
ent, the AUTO pin must be tied high so that, at reset, all
ports will be configured to operate automatically. Each port
will detect and classify repeatedly until a PD is discovered,
set ICUT and ILIM according to the classification results,
apply power to valid PDs, and remove power when a PD
is disconnected.
Table 3 shows the ICUT and ILIM values that will be auto-
matically set in standalone (AUTO pin) mode, based on
the discovered class.
Table 3. ICUT and ILIM Values in Standalone Mode
CLASS ICUT ILIM
Class 1 112mA 425mA
Class 2 206mA 425mA
Class 3 or 0 375mA 425mA
Class 4 638mA 850mA
The automatic setting of ICUT and ILIM values only occurs
if the LTC4270/LTC4271 is reset with the AUTO pin high.
If the standalone application is a midspan, the MID pin must
be tied high to enable correct midspan detection timing.
APPLICATIONS INFORMATION
RESISTANCE
PD
PSE
0Ω 10k
15k
42701 F11
19k 26.5k
26.25k23.75k
150Ω (NIC)
20k 30k
33k
Figure 11. IEEE 802.3af Signature Resistance Ranges
DETECTION
Detection Overview
To avoid damaging network devices that were not designed
to tolerate DC voltage, a PSE must determine whether the
connected device is a real PD before applying power. The
IEEE specification requires that a valid PD have a common-
mode resistance of 25k ±5% at any port voltage below
10V. The PSE must accept resistances that fall between
19k and 26.5k, and it must reject resistances above 33k
or below 15k (shaded regions in Figure 11). The PSE may
choose to accept or reject resistances in the undefined
areas between the must-accept and must-reject ranges. In
particular, the PSE must reject standard computer network
ports, many of which have 150Ω common-mode termina-
tion resistors that will be damaged if power is applied to
them (the black region at the left of Figure 11).
4-Point Detection
The LTC4270/LTC4271 uses a 4-point detection method to
discover PDs. False-positive detections are minimized by
checking for signature resistance with both forced-current
and forced-voltage measurements.
Initially, two test currents are forced onto the port (via the
OUTn pin) and the resulting voltages are measured. The
detection circuitry subtracts the two V-I points to determine
the resistive slope while removing offset caused by series
diodes or leakage at the port (see Figure 12). If the forced-
current detection yields a valid signature resistance, two
test voltages are then forced onto the port and the result-
ing currents are measured and subtracted. Both methods
must report valid resistances for the port to report a valid
detection. PD signature resistances between 17k and 29k
(typically) are detected as valid and reported as Detect
Good in the corresponding Port Status register. Values
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APPLICATIONS INFORMATION
FIRST
DETECTION
POINT
SECOND
DETECTION
POINT
VALID PD
25kΩ SLOPE
275
165
CURRENT (µA)
0V-2V
OFFSET VOLTAGE
42701 F12
Figure 12. PD Detection
outside this range, including open and short circuits, are
also reported. If the port measures less than 1V at the
first forced-current test, the detection cycle will abort and
Short Circuit will be reported. Table 4 shows the possible
detection results.
Table 4. Detection Status
MEASURED PD SIGNATURE DETECTION RESULT
Incomplete or Not Yet Tested Detect Status Unknown
< 2.4k Short Circuit
Capacitance > 2.7µF CPD too High
2.4k < RPD < 17k RSIG too Low
17k < RPD < 29k Detect Good
> 29k RSIG too High
> 50k Open Circuit
Voltage > 10V Port Voltage Outside Detect Range
More on Operating Modes
The port’s operating mode determines when the LTC4270/
LTC4271 runs a detection cycle. In manual mode, the port
will idle until the host orders a detect cycle. It will then
run detection, report the results, and return to idle to wait
for another command.
In semi-auto mode, the LTC4270/LTC4271 autonomously
polls a port for PDs, but it will not apply power until com-
manded to do so by the host. The Port Status register is
updated at the end of each detection cycle.
If a valid signature resistance is detected and classification
is enabled, the port will classify the PD and report that
result as well. The port will then wait for at least 100ms (or
2 seconds if midspan mode is enabled), and will repeat the
detection cycle to ensure that the data in the Port Status
register is up-to-date.
If the port is in semi-auto mode and high power opera-
tion is enabled, the port will not turn on in response to
a power-on command unless the current detect result is
detect good. Any other detect result will generate a tSTART
fault if a power-on command is received. In high power
mode the port must be placed in manual mode to force a
port on regardless of detect outcome.
Behavior in AUTO pin mode is similar to semi-auto; how-
ever, after detect good is reported and the port is classified
(if classification is enabled), it is automatically powered
on without further intervention. In standalone (AUTO pin)
mode, the ICUT and ILIM thresholds are automatically set;
see the Reset and the AUTO/MID Pins section for more
information.
The signature detection circuitry is disabled when the
port is initially powered up with the AUTO pin low, in
shutdown mode, or when the corresponding Detect En-
able bit is cleared.
Detection of Legacy PDs
Proprietary PDs that predate the original IEEE 802.3af stan-
dard are commonly referred to today as legacy devices. One
type of legacy PD uses a large common-mode capacitance
(>10μF) as the detection signature. Note that PDs in this
range of capacitance are defined as invalid, so a PSE that
detects legacy PDs is technically noncompliant with the
IEEE spec. The LTC4270/LTC4271 can be configured to
detect this type of legacy PD. Legacy detection is disabled
by default, but can be manually enabled on a per-port basis.
When enabled, the port will report Detect Good when it
sees either a valid IEEE PD or a high-capacitance legacy
PD. With legacy mode disabled, only valid IEEE PDs will
be recognized.
CLASSIFICATION
802.3af Classification
A PD may optionally present a classification signature to
the PSE to indicate the maximum power it will draw while
LTC4270/LTC4271
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APPLICATIONS INFORMATION
operating. The IEEE specification defines this signature as
a constant current draw when the PSE port voltage is in the
VCLASS range (between 15.5V and 20.5V), with the current
level indicating one of 5 possible PD classes. Figure13
shows a typical PD load line, starting with the slope of
the 25k signature resistor below 10V, then transitioning to
the classification signature current (in this case, Class 3)
in the VCLASS range. Table 5 shows the possible clas-
sification values.
Table 5. 802.3af and 802.3at Classification Values
CLASS RESULT
Class 0 No Class Signature Present; Treat Like Class 3
Class 1 3W
Class 2 7W
Class 3 13W
Class 4 25.5W (Type 2)
If classification is enabled, the port will classify the PD
immediately after a successful detection cycle in semi-auto
or AUTO pin modes, or when commanded to in manual
mode. It measures the PD classification signature by ap-
plying 18V for 12ms (both values typical) to the port via
the OUTn pin and measuring the resulting current; it then
reports the discovered class in the Port Status register.
If the LTC4270/LTC4271 is in AUTO pin mode, it will ad-
ditionally use the classification result to set the ICUT and
VOLTAGE (VCLASS)
0
CURRENT (mA)
60
50
40
30
20
10
05 10 15 20
42701 F13
25
TYPICAL
CLASS 3
PD LOAD
LINE
48mA
33mA
PSE LOAD LINE
23mA
14.5mA
6.5mA
CLASS 4
CLASS 2
CLASS 1
CLASS 0
CLASS 3
OVER
CURRENT
Figure 13. PD Classification
ILIM thresholds. See the Reset and the AUTO/MID Pin
section for more information.
The classification circuitry is disabled when the port is
initially powered up with the AUTO pin low, in shutdown
mode, or when the corresponding Class Enable bit is cleared.
802.3at 2-Event Classification
The 802.3at specification defines two methods of classify-
ing a Type 2 PD. A-grade and B-grade LTC4270/LTC4271
parts support 802.3at 2-event classification.
One method adds extra fields to the Ethernet LLDP data
protocol; although the LTC4270/LTC4271 is compatible
with this classification method, it cannot perform clas-
sification directly since it doesn’t have access to the data
path. LLDP classification requires the PSE to power the
PD as a standard 802.3af (Type 1) device. It then waits for
the host to perform LLDP communication with the PD and
update the PSE port data. The LTC4270/LTC4271 supports
changing the ILIM and ICUT levels on the fly, allowing the
host to complete LLDP classification.
The second 802.3at classification method, known as
2-event classification or ping-pong, is supported by
the LTC4270/LTC4271. A Type 2 PD that is request-
ing more than 13W will indicate Class 4 during normal
802.3af classification. If the LTC4270/LTC4271 sees
Class 4, it forces the port to a specified lower voltage
(called the mark voltage, typically 9V), pauses briefly, and
then re-runs classification to verify the Class 4 reading
(Figure 1). It also sets a bit in the High Power Status
register to indicate that it ran the second classification
cycle. The second cycle alerts the PD that it is connected
to a Type2 PSE which can supply Type 2 power levels.
2-event ping-pong classification is enabled by setting a bit
in the port’s High Power Mode register. Note that a ping-
pong enabled port only runs the second classification cycle
when it detects a Class 4 device; if the first cycle returns
Class 0 to 3, the port determines it is connected to a Type 1
PD and does not run the second classification cycle.
Invalid Type 2 Class Combinations
The 802.3at specification defines a Type 2 PD class
signature as two consecutive Class 4 results; a Class 4
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followed by a Class 0-3 is not a valid signature. In AUTO
pin mode, the LTC4270/LTC4271 will power a detected
PD regardless of the classification results, with one excep-
tion: if the PD presents an invalid Type 2 signature (Class
4 followed by Class 0 to 3), the LTC4270/LTC4271 will
not provide power and will restart the detection process.
To aid in diagnosis, the Port Status register will always
report the results of the last class pulse, so an invalid
Class 4–Class 2 combination would report a second class
pulse was run in the High Power Status register (which
implies that the first cycle found class 4), and Class 2 in
the Port Status register.
POWER CONTROL
The primary function of the LTC4270/LTC4271 is to
control the delivery of power to the PSE port. It does this
by controlling the gate drive voltage of an external power
MOSFET while monitoring the current via an external sense
resistor and the output voltage at the OUT pin. This circuitry
serves to couple the raw VEE input supply to the port in
a controlled manner that satisfies the PDs power needs
while minimizing both power dissipation in the MOSFET
and disturbances on the VEE backplane.
Inrush Control
Once the command has been given to turn on a port, the
LTC4270/LTC4271 ramps up the GATE pin of that port’s
external MOSFET in a controlled manner. Under normal
power-up circumstances, the MOSFET gate will rise until
the port current reaches the inrush current limit level
(typically 425mA), at which point the GATE pin will be
servoed to maintain the specified IINRUSH current. During
this inrush period, a timer (tSTART) runs. When output
charging is complete, the port current will fall and the GATE
pin will be allowed to continue rising to fully enhance the
MOSFET and minimize its on-resistance. The final VGS is
nominally 12V. The inrush period is maintained until the
tSTART timer expires. At this time if the inrush current limit
level is still exceeded, the port will be turned back off and
a tSTART fault reported.
APPLICATIONS INFORMATION
Current Limit
Each LTC4270/LTC4271 port includes two current limit-
ing thresholds (ICUT and ILIM), each with a corresponding
timer (tCUT and tLIM). Setting the ICUT and ILIM thresholds
depends on several factors: the class of the PD, the volt-
age of the main supply (VEE), the type of PSE (Type 1 or
Type 2), the sense resistor (0.5Ω or 0.25Ω), the SOA of
the MOSFET, and whether or not the system is required
to enforce class current levels.
Per the IEEE specification, the LTC4270/LTC4271 will al-
low the port current to exceed ICUT for a limited period of
time before removing power from the port, whereas it will
actively control the MOSFET gate drive to keep the port
current below ILIM. The port does not take any action to
limit the current when only the ICUT threshold is exceeded,
but does start the tCUT timer. If the current drops below
the ICUT current threshold before its timer expires, the
tCUT timer counts back down, but at 1/16 the rate that it
counts up. If the tCUT timer reaches 60ms (typical) the
port is turned off and the port tCUT fault is set. This allows
the current limit circuitry to tolerate intermittent overload
signals with duty cycles below about 6%; longer duty cycle
overloads will turn the port off.
The ILIM current limiting circuit is always enabled and ac-
tively limiting port current. The tLIM timer is enabled only
when the tLIM Enable bit is set. This allows tLIM to be set
to a shorter value than tCUT to provide more aggressive
MOSFET protection and turn off a port before MOSFET
damage can occur. The tLIM timer starts when the ILIM
threshold is exceeded. When the tLIM timer reaches 12ms
(typical) the port is turned off and the port tLIM fault is
set. When the tLIM Enable bit is disabled tLIM behaviors
are tracked by the tCUT timer, which counts up during both
ILIM and ICUT events.
ICUT is typically set to a lower value than ILIM to allow the
port to tolerate minor faults without current limiting.
Per the IEEE specification, the LTC4270/LTC4271 will
automatically set ILIM to 425mA (shown in bold in Table 6)
during inrush at port turn-on, and then switch to the
LTC4270/LTC4271
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APPLICATIONS INFORMATION
programmed ILIM setting once inrush has completed. To
maintain IEEE compliance, ILIM should be kept at 425mA
for all Type 1 PDs, and 850mA if a Type 2 PD is detected.
ILIM is automatically reset to 425mA when a port turns off.
Table 6. Example Current Limit Settings
ILIM (mA)
INTERNAL REGISTER SETTING (hex)
RSENSE = 0.5Ω RSENSE = 0.25Ω
53 88
106 08 88
159 89
213 80 08
266 8A
319 09 89
372 8B
425 00 80
478 8E
531 92 8A
584 CB
638 10 90
744 D2 9A
850 40 C0
956 4A CA
1063 50 D0
1169 5A DA
1275 60 E0
1488 52 49
1700 40
1913 4A
2125 50
2338 5A
2550 60
2975 52
ILIM Foldback
The LTC4270/LTC4271 features a two-stage foldback circuit
that reduces the port current if the port voltage falls below
the normal operating voltage. This keeps MOSFET power
dissipation at safe levels for typical 802.3af MOSFETs,
even at extended 802.3at power levels. Current limit and
foldback behavior are programmable on a per-port basis.
Table 6 gives examples of recommended ILIM register
settings.
The LTC4270/LTC4271 will support current levels well
beyond the maximum values in the 802.3at specification.
The shaded areas in Table 6 indicate settings that may
require a larger external MOSFET, additional heat sinking,
or setting tLIM Enable.
MOSFET Fault Detection
LTC4270/LTC4271 PSE ports are designed to tolerate
significant levels of abuse, but in extreme cases it is pos-
sible for the external MOSFET to be damaged. A failed
MOSFET may short source to drain, which will make the
port appear to be on when it should be off; this condition
may also cause the sense resistor to fuse open, turning
off the port but causing the LTC4270 SENSE pin to rise
to an abnormally high voltage. A failed MOSFET may also
short from gate to drain, causing the LTC4270 GATE pin
to rise to an abnormally high voltage. The LTC4270 OUT,
SENSE and GATE pins are designed to tolerate up to 80V
faults without damage.
If the LTC4270/LTC4271 sees any of these conditions for
more than 180μs, it disables all port functionality, reduces
the gate drive pull-down current for the port and reports
a FET Bad fault. This is typically a permanent fault, but
the host can attempt to recover by resetting the port, or
by resetting the entire chip if a port reset fails to clear the
fault. If the MOSFET is in fact bad, the fault will quickly
return, and the port will disable itself again. The remaining
ports of the LTC4270/LTC4271 are unaffected.
An open or missing MOSFET will not trigger a FET Bad
fault, but will cause a tSTART fault if the LTC4270/LTC4271
attempts to turn on the port.
Port Current Readback
The LTC4270/LTC4271 measures the current at each port
with an internal A/D converter. Port data is only valid when
the port power is on and reads zero at all other times. The
converter has two modes:
• 100ms mode: Samples are taken continuously and the
measured value is updated every 100ms
LTC4270/LTC4271
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• 1s mode: Samples are taken continuously; a moving
1second average is updated every 100ms
Port Current Policing
The LTC4270/LTC4271 can augment tCUT current moni-
toring with a policing function to track the one second
current averages. A port violating the user-specified Port
Police Threshold will be shut off with both a tCUT and
Police event recorded. A port current Police event can be
differentiated from a port tCUT violation by reading both
events bits; both bits are set for a Police violation while
only the tCUT bit is set for tCUT timer violations.
Port Voltage Readback
The LTC4270/LTC4271 measures the output voltage at
each port with an internal A/D converter. Port data is
only valid when the port power is on and reads zero at
all other times.
Disconnect
The LTC4270/LTC4271 monitors powered ports to ensure
the PD continues to draw the minimum specified current. A
disconnect timer counts up whenever port current is below
7.5mA (typ), indicating that the PD has been disconnected.
If the tDIS timer expires, the port will be turned off and the
disconnect bit in the fault event register will be set. If the
current returns before the tDIS timer runs out, the timer
resets. As long as the PD exceeds the minimum current
level more often than tDIS, it will remain powered.
Although not recommended, the DC disconnect feature
can be disabled by clearing the corresponding enable bits.
Note that this defeats the protection mechanisms built
into the IEEE specification, since a powered port will stay
powered after the PD is removed. If the still-powered port
is subsequently connected to a non-PoE data device, the
device may be damaged.
The LTC4270/LTC4271 does not include AC disconnect
circuitry, but includes AC Disconnect Enable bits to main-
tain compatibility with the LTC4259A. If the AC Disconnect
Enable bits are set, DC disconnect will be used.
APPLICATIONS INFORMATION
Masked Shutdown
The LTC4270/LTC4271 provides a low latency port shed-
ding feature to quickly reduce the system load when
required. By allowing a pre-determined set of ports to
be turned off, the current on an overloaded main power
supply can be reduced rapidly while keeping high priority
devices powered. Each port can be configured to high or
low priority; all low-priority ports will shut down within
6.5μs after the MSD pin is pulled low, high priority ports
will remain powered. If a port is turned off via MSD, the
corresponding Detection and Classification Enable bits are
cleared, so the port will remain off until the host explicitly
re-enables detection.
In the LTC4270/LTC4271 chipset the active level of MSD
is register configurable as active high or low. The default
is LTC4266-compatible active low behavior.
VEE Readback
The LTC4270/LTC4271 measures the VEE voltage with an
internal 12-bit A/D converter.
General Purpose IO
Two sets of general purpose IO pins are available in the
LTC4270/LTC4271 chipset. The first set of general purpose
IO are GP1 and GP0. These fully bidirectional IO are 3.3V
CMOS IO on the LTC4271 chip.
The second set of general purpose IO pins are XIO1 and
XIO0. These fully bidirectional IO are 4.3V CMOS IO on
the LTC4270 chip.
Code Download
LTC4271 firmware is field-upgradable by downloading
and executing RAM images. RAM images are volatile
and must be re-downloaded after each VDD power cycle,
but will remain valid during reset and VEE power events.
Contact Linear Technology for code download procedures
and RAM images.
LTC4270/LTC4271
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APPLICATIONS INFORMATION
SERIAL DIGITAL INTERFACE
Overview
The LTC4270/LTC4271 communicates with the host us-
ing a standard SMBus/I2C 2-wire interface. The LTC4270/
LTC4271 is a slave-only device, and communicates with
the host master using the standard SMBus protocols.
Interrupts are signaled to the host via the INT pin. The
Timing Diagrams (Figures 5 through 9) show typical
communication waveforms and their timing relationships.
More information about the SMBus data protocols can be
found at www.smbus.org.
The LTC4270/LTC4271 requires both the VDD and VEE sup-
ply rails to be present for the serial interface to function.
Bus Addressing
The LTC4270/LTC4271’s primary 7-bit serial bus address
is A610A3A2A1A0b, with bit 6 controlled by AD6 and the
lower four bits set by the AD3-AD0 pins; this allows up
to 10 LTC4270/LTC4271s, on a single bus. Ten LTC4270/
LTC4271 are equivalent to 30 quad PSEs or 120 ports. All
LTC4270/LTC4271s also respond to the broadcast address
0110000b, allowing the host to write the same command
(typically configuration commands) to multiple LTC4270/
LTC4271s in a single transaction. If the LTC4270/LTC4271
is asserting the INT pin, it will also respond to the alert
response address (0001100b) per the SMBus specification.
Each LTC4270/LTC4271 is logically composed of three
quads of four ports each. Each quad occupies separate,
contiguous I2C addresses. The AD6, AD3-0 pins set the
address of the base quad while the remaining quads are
consecutively numbered. I2C addresses outside of the
x10xxxxb range are considered illegal and will not respond.
Each internal quad is independent of the other quads, with
the exception of writes to the Chip Reset, MSD Inversion
and General Purpose Input Output registers. These registers
are global in nature and will affect all quads.
Figure 14. Example I2C Bus Addressing
Interrupts and SMBAlert
Most LTC4270/LTC4271 port events can be configured
to trigger an interrupt, asserting the INT pin and alerting
the host to the event. This removes the need for the host
to poll the LTC4270/LTC4271, minimizing serial bus traf-
fic and conserving host CPU cycles. Multiple LTC4270/
LTC4271s can share a common INT line, with the host
using the SMBAlert protocol (ARA) to determine which
LTC4270/LTC4271 caused an interrupt.
Register Description
For information on serial bus usage and device configura-
tion and status, refer to the LTC4271 Software Program-
ming documentation.
ISOLATION REQUIREMENTS
IEEE 802.3 Ethernet specifications require that network
segments (including PoE circuitry) be electrically isolated
from the chassis ground of each network interface de-
vice. However, network segments are not required to be
isolated from each other, provided that the segments are
connected to devices residing within a single building on
a single power distribution system.
QUAD 0QUAD 1QUAD 2
LTC4271
AD0
AD1
AD2
AD3
AD6
SCL
SDAIN
SDAOUT
SCL
SDA
AD0
3.3V
AD1
AD2
AD3
AD6
SCL
SDAIN
SDAOUT
I2C ADDRESS
0100000
0100001
0100010
QUAD 0QUAD 1QUAD 2
LTC4271
I2C ADDRESS
0100111
0101000
0101001
42701 F14
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APPLICATIONS INFORMATION
For simple devices such as small PoE switches, the isola-
tion requirement can be met by using an isolated main
power supply for the entire device. This strategy can be
used if the device has no electrically conducting ports
other than twisted-pair Ethernet. In this case, the SDAIN
and SDAOUT pins can be tied together and will act as a
standard I2C/SMBus SDA pin.
If the device is part of a larger system, contains additional
external non-Ethernet ports, or must be referenced to
protective ground for some other reason, the Power over
Ethernet subsystem must be electrically isolated from the
rest of the system.
The LTC4270/LTC4271 chipset simplifies PSE isolation by
allowing the LTC4271 chip to reside on the non-isolated
side. There it can receive power from the main logic sup-
ply and connect directly to the I2C/SMBus bus. Isolation
between the LTC4271 and LTC4270 is implemented using
a proprietary transformer-based communication protocol.
Additional details are provided in the Serial Bus Isolation
section of this data sheet.
EXTERNAL COMPONENT SELECTION
Power Supplies and Bypassing
The LTC4270/LTC4271 requires two supply voltages to
operate. VDD requires 3.3V (nominally) relative to DGND.
VEE requires a negative voltage of between –45V and
–57V for Type 1 PSEs, –51V to –57V for Type 2 PSEs,
or –54.75V to –57V for LTPoE++ PSEs, relative to AGND.
Digital Power Supply
VDD provides digital power for the LTC4271 processor,
and draws a maximum of 15mA. A ceramic decoupling
cap of at least 0.1μF should be placed from VDD to DGND,
as close as practical to each LTC4271 chip. A 1.8V core
voltage supply is generated internally and requires a 1µF
ceramic decoupling cap between the CAP1 pin and DGND.
In the LTC4270/LTC4271, VDD should be delivered by the
host controller’s non-isolated 3.3V supply. To maintain
required isolation AGND and DGND must not be con-
nected in any way.
Main PoE Power Supply
VEE is the main isolated PoE supply that provides power
to the PDs. Because it supplies a relatively large amount
of power and is subject to significant current transients,
it requires more design care than a simple logic supply.
For minimum IR loss and best system efficiency, set VEE
near maximum amplitude (57V), leaving enough margin
to account for transient over or undershoot, temperature
drift, and the line regulation specifications of the particular
power supply used.
Bypass capacitance between AGND and VEE is very im-
portant for reliable operation. If a short circuit occurs at
one of the output ports it can take as long as 1μs for the
LTC4270 to begin regulating the current. During this time
the current is limited only by the small impedances in the
circuit and a high current spike typically occurs, causing a
voltage transient on the VEE supply and possibly causing
the LTC4270/LTC4271 to reset due to a UVLO fault. A 1μF,
100V X7R capacitor placed near the VEE pin along with an
electrolytic bulk capacitor of at least 47µF is recommended
to minimize spurious resets.
Serial Bus Isolation
The LTC4270/LTC4271 chipset uses transformers to
isolate the LTC4271 from the LTC4270. In this case, the
SDAIN and SDAOUT pins can be shorted to each other
and tied directly to the I2C/SMBus bus. The transformers
should be 10BASE-T or 10/100BASE-T with a 1:1 turns
ratio. It is important that the selected transformers do not
have common-mode chokes. These transformers typically
provide 1500V of isolation between the LTC4271 and the
LTC4270. For proper operation strict layout guidelines
must be met.
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APPLICATIONS INFORMATION
External MOSFET
Careful selection of the power MOSFET is critical to system
reliability. LTC recommends either Fairchild IRFM120A,
FDT3612, FDMC3612 or Philips PHT6NQ10T for their
proven reliability in Type 1 and Type 2 PSE applications.
SOA curves are not a reliable specification for MOSFET
selection. Contact LTC Applications before using a MOSFET
other than one of these recommended parts.
Sense Resistor
The LTC4270/LTC4271 is designed to use 0.25Ω current
sense resistors to reduce power dissipation. Four com-
monly available 1Ω resistors (sized according to power
dissipation) can be used in parallel in place of a single
Figure 15. LTC4270/LTC4271 Proprietary Isolation
0.25Ω resistor. In order to meet the ICUT and ILIM accuracy
required by the IEEE specification, the sense resistors
should have ±1% tolerance or better, and no more than
±200ppm/°C temperature coefficient. In addition, the sense
resistors must meet strict layout guidelines.
Port Output Cap
Each port requires a 0.22μF cap across its outputs to keep
the LTC4270 stable while in current limit during startup
or overload. Common ceramic capacitors often have sig-
nificant voltage coefficients; this means the capacitance is
reduced as the applied voltage increases. To minimize this
problem, X7R ceramic capacitors rated for at least 100V
are recommended and must be located close to the PSE.
••
3.3V
3.3V 0.1µF
–54V
100Ω
100Ω
100Ω
100Ω
T1
••
3.3V –54V
100Ω
100Ω
100Ω
100Ω
T2
GP0
GP1
MID
RESET
MSD
INT
AUTO
SCL
AD0
AD1
AD2
AD3
AD6 DGND CAP1 CAP2
DND
DPD
CND
CPD
DNA
SENSE1
GATE1
OUT1
LTC4270
DPA
CNA
CPA
XIO0 XIO1 0.22µF
100V
0.22µF
100V
S1B
S1B
–54V
–54V
42701 F15
–54V
S1B
S1B
PORTn
PORT1
SDAIN
SDAOUT
LTC4271
NO ISOLATION
REQUIRED ON
I2C INTERFACE
VDD33
VEE
SENSEn
GATEn
OUTn
AGND VSSK
0.25Ω
0.25Ω
>47µF
SYSTEM
BULK CAP
+
1µF 0.1µF
–54V
2nF 2kV 1µF
10Ω
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APPLICATIONS INFORMATION
Figure 16. LTC4270 Surge Protection
Surge Protection
Ethernet ports can be subject to significant cable surge
events. To keep PoE voltages below a safe level and protect
the application against damage, protection components,
as shown in Figure 16, are required at the main supply,
at the LTC4270 supply pins and at each port.
Bulk transient voltage suppression (TVSBULK) and bulk
capacitance (CBULK) are required across the main PoE
supply and should be sized to accommodate system level
surge requirements.
Each LTC4270 requires a 10Ω, 0805 resistor (R1) in series
from supply AGND to the LTC4270 AGND pin. Across the
LTC4270 AGND pin and VEE pin are an SMAJ58A, 58V
TVS (D1) and a 1µF, 100V bypass capacitor (C1). These
components must be placed close to the LTC4270 pins.
Finally, each port requires a pair of S1B clamp diodes:
one from OUTn to supply AGND and one from OUTn to
supply VEE. The diodes at the ports steer harmful surges
into the supply rails where they are absorbed by the surge
suppressors and the VEE bypass capacitance. The layout
of these paths must be low impedance.
LAYOUT GUIDELINES
Strict adherence to board layout, parts placement and
routing guidelines is critical for optimal current reading
accuracy, IEEE compliance, system robustness, and
thermal dissipation. Refer to the DC1682A Demo Board
as a layout reference. Contact LTC Applications to obtain
a full set of layout guidelines, example layouts and BOMs.
42701 F16
AGND
VEE
SENSEnVSSK GATEn OUTn
OUTn
TO
PORT
S1B
S1B
C1
1µF
100V LTC4270
D1
SMAJ58A
R1
10Ω
+
CBULK
TVSBULK
VEE
VEE
Cn
0.22µF
X7R
100V
Qn
RSENSEn
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TYPICAL APPLICATION
0.01µF
200V
0.01µF
200V
1
2
3
4
5
6
7
8
RJ45
CONNECTOR
••
3.3V –54V
100Ω
100Ω
T2
T3
••
3.3V –54V
ISOLATED
GND
–54V
100Ω
100Ω
GP0
GP1
MID
RESET
MSD
INT
AUTO
SCL
AD0
AD1
AD2
AD3
AD6
DGND VEE VSSK
CAP1 CAP2
DND
DPD
CND
CPD
DNA
AGND
DPA
CNA
CPA OUT12
OUT1
GATE1
SENSE1
GATE12
SENSE12
XIO0 XIO1
42701 F16
SDAIN
SDAOUT
LTC4271 LTC4270
VDD33
3.3V 0.1µF 1µF
1µF
SMCJ58A
+
>47µF
SYSTEM
BULK CAP
S1B
1
11
12
2
T1
0.25Ω, 1%
RS
0.22µF
100V
X7R
S1B
FDMC3612
•
•
••
•
•
•
•
••
•
•
0.01µF
200V
0.01µF
200V
0.01µF
200V
0.01µF
200V
75Ω75Ω
75Ω75Ω
1
2
3
4
5
6
7
8
RJ45
CONNECTOR
PHY
(NETWORK
PHYSICAL
LAYER
CHIP)
SMAJ58A
1µF 0.1µF
–54V
ISOLATED
2nF 2kV
1µF
100V
X7R
10Ω
LTC4270/LTC4271
29
42701fd
For more information www.linear.com/LTC4270
PACKAGE DESCRIPTION
4.00 0.10
(4 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION
(WGGD-X)—TO BE APPROVED
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, IF PRESENT
PIN 1
TOP MARK
(NOTE 6)
0.40 0.10
2423
1
2
BOTTOM VIEW—EXPOSED PAD
2.45 0.10
(4-SIDES)
0.75 0.05 R = 0.115
TYP
0.25 0.05
0.50 BSC
0.200 REF
0.00 – 0.05
(UF24) QFN 0105
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.70 0.05
0.25 0.05
0.50 BSC
2.45 0.05
(4 SIDES)
3.10 0.05
4.50 0.05
PACKAGE OUTLINE
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45° CHAMFER
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
UF Package
24-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1697)
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LTC4270/LTC4271
30
42701fd
For more information www.linear.com/LTC4270
PACKAGE DESCRIPTION
UKG Package
52-Lead Plastic QFN (7mm × 8mm)
(Reference LTC DWG # 05-08-1729 Rev Ø)
7.00 0.10
(2 SIDES)
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
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 0.20mm ON ANY SIDE, IF PRESENT
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
(SEE NOTE 6)
PIN 1 NOTCH
R = 0.30 TYP OR
0.35 × 45°C
CHAMFER
0.40 0.10
5251
1
2
BOTTOM VIEW—EXPOSED PAD
TOP VIEW
SIDE VIEW
6.50 REF
(2 SIDES)
8.00 0.10
(2 SIDES)
5.50 REF
(2 SIDES)
0.75 0.05
0.75 0.05
R = 0.115
TYP
R = 0.10
TYP 0.25 0.05
0.50 BSC
0.200 REF
0.00 – 0.05
6.45 0.10
5.41 0.10
0.00 – 0.05
(UKG52) QFN REV Ø 0306
5.50 REF
(2 SIDES)
5.41 0.05
6.45 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.70 0.05
6.10 0.05
7.50 0.05
6.50 REF
(2 SIDES) 7.10 0.05 8.50 0.05
0.25 0.05
0.50 BSC
PACKAGE OUTLINE
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LTC4270/LTC4271
31
42701fd
For more information www.linear.com/LTC4270
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 9/11 Updated Absolute Maximum Ratings.
Updated Electrical Characteristics.
Updated GATEn pin description.
Updated Inrush Control section.
Updated Power Supplies and Bypassing section.
Updated Typical Applications.
2
4, 5, 6, 7
13
21
25
28, 32
B 3/12 Removed temperature readback feature.
Changed LTPoE++ power levels from 35W, 45W to 38.7W, 52.7W respectively.
Removed Lead Temperature.
Updated Electrical Characteristics (removed temperature dots).
Clarified AUTO Pin mode relationship to Reset Pin.
Modified Related Parts.
1, 5, 15, 23
1, 16, 17
2
5, 6
17-18
32
C 8/12 Removed temperature dot for VDD Undervoltage Lock-Out
Added 0.1µF label to capacitor on CAP2 pin
4
28, 32
D 06/15 Updated surge protection recommendations
Simplified Power over Ethernet system diagram
1, 26, 27, 28, 32
16
LTC4270/LTC4271
32
42701fd
For more information www.linear.com/LTC4270
LINEAR TECHNOLOGY CORPORATION 2011
LT 0615 REV D • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC4270
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LTC4257-1 IEEE 802.3af PD Interface Controller Internal 100V, 400mA Switch, Dual Current Limit, Programmable Class
LTC4263 Single IEEE 802.3af PSE Controller Internal FET Switch
LTC4265 IEEE 802.3at PD Interface Controller Internal 100V, 1A Switch, 2-Event Classification Recognition
LTC4266 Quad IEEE 802.3at PoE PSE Controller With Programmable ICUT/ILIM, 2-Event Classification, and Port Current and Voltage Monitoring
LTC4266A Quad LTPoE++ PSE Controller Provides Up to 90W. Backwards Compatible with IEEE 802.3af and IEEE 802.3at PDs. With
Programmable ICUT/ILIM, 2-Event Classification, and Port Current and Voltage Monitoring
LTC4266C Quad IEEE 802.3af PSE Controller With Programmable ICUT/ILIM, 1-Event Classification, and Port Current and Voltage Monitoring
LTC4267 IEEE 802.3af PD Interface With Integrated
Switching Regulator
Internal 100V, 400mA Switch, Dual Inrush Current, Programmable Class
LTC4267-1 IEEE 802.3af PD Interface With Integrated
Switching Regulator
Internal 100V, 400mA Switch, Programmable Class, 200kHz Constant Frequency PWM
LTC4267-3 IEEE 802.3af PD Interface With Integrated
Switching Regulator
Internal 100V, 400mA Switch, Programmable Class, 300kHz Constant Frequency PWM
LTC4269-1 IEEE 802.3af PD Interface With Integrated
Flyback Switching Regulator
2-Event Classification, Programmable Class, Synchronous No-Opto Flyback Controller,
50kHz to 250kHz, Aux Support
LTC4269-2 IEEE 802.3af PD Interface With Integrated
Forward Switching Regulator
2-Event Classification, Programmable Class, Synchronous Forward Controller, 100kHz to
500kHz, Aux Support
LTC4278 IEEE 802.3af PD Interface With Integrated
Flyback Switching Regulator
2-Event Classification, Programmable Class, Synchronous No-Opto Flyback Controller,
50kHz to 250kHz, 12V Aux Support
LTC4274 Single IEEE 802.3at PoE PSE Controller With Programmable ICUT/ILIM, 2-Event Classification, and Port Current and Voltage Monitoring
LTC4274A Single LTPoE++ PSE Controller Provides Up to 90W. Backwards Compatible with IEEE 802.3af and IEEE 802.3at PDs. With
Programmable ICUT/ILIM, 2-Event Classification, and Port Current and Voltage Monitoring
LTC4274C Single IEEE 802.3af PSE Controller With Programmable ICUT/ILIM, 1-Event Classification, and Port Current and Voltage Monitoring
LTC4311 SMBus/ I2C Accelerator Improved I2C Rise Time, Ensures Data Integrity
TYPICAL APPLICATION
0.01µF
200V
0.01µF
200V
1
2
3
4
5
6
7
8
RJ45
CONNECTOR
••
3.3V –54V
100Ω
100Ω
T2
T3
••
3.3V –54V
ISOLATED
GND
–54V
100Ω
100Ω
GP0
GP1
MID
RESET
MSD
INT
AUTO
SCL
AD0
AD1
AD2
AD3
AD6
DGND VEE VSSK
CAP1 CAP2
DND
DPD
CND
CPD
DNA
AGND
DPA
CNA
CPA OUT12
OUT1
GATE1
SENSE1
GATE12
SENSE12
XIO0 XIO1
42701 F16
SDAIN
SDAOUT
LTC4271 LTC4270
VDD33
3.3V 0.1µF 1µF
1µF
SMCJ58A
+
>47µF
SYSTEM
BULK CAP
S1B
1
11
12
2
T1
0.25Ω, 1%
RS
0.22µF
100V
X7R
S1B
FDMC3612
•
•
••
•
•
•
•
••
•
•
0.01µF
200V
0.01µF
200V
0.01µF
200V
0.01µF
200V
75Ω75Ω
75Ω75Ω
1
2
3
4
5
6
7
8
RJ45
CONNECTOR
PHY
(NETWORK
PHYSICAL
LAYER
CHIP)
SMAJ58A
1µF 0.1µF
–54V
ISOLATED
2nF 2kV
1µF
100V
X7R
10Ω
