LT8610AC/LT8610AC-1
1
8610acfa
For more information www.linear.com/LT8610AC
Typical applicaTion
FeaTures DescripTion
42V, 3.5A Synchronous
Step-Down Regulator with
2.5µA Quiescent Current
The LT
®
8610AC/LT8610AC-1 is a compact, high efficiency,
high speed synchronous monolithic step-down switching
regulator that consumes only 2.5µA of quiescent current.
Compared to the LT8610AB, the LT8610AC/LT8610AC-1
has a lower feedback voltage of 800mV and a lower mini-
mum VIN of 3V. The LT8610AC/LT8610AC-1 has the same
additional features of the LT8610AB as compared to the
LT8610: higher maximum output current of 3.5A, faster
minimum on time of 30ns, and higher light load efficiency.
The LT8610AC/LT8610AC-1 has a SYNC pin for syn-
chronization to an external clock. A capacitor on the TR/
SS pin programs the output voltage ramp rate during
startup. The PG flag signals when VOUT is within ±8% of
the programmed output voltage as well as fault conditions.
The LT8610AC-1 has internal compensation optimized for
high switching frequencies resulting in improved transient
response compared to the LT8610AC. The LT8610AC-1
also has a feature which will soft-start the part when
exiting dropout or brownout conditions reducing output
voltage overshoot.
FEEDBACK
VOLTAGE MIN VIN
OUTPUT
CURRENT
MIN ON
TIME
IMPROVED
EMI
LT8610AC 0.80V 3.0V 3.5A 30ns Yes
LT8610AB* 0.97V 3.4V 3.5A 30ns Yes
LT8610A* 0.97V 3.4V 3.5A 30ns Yes
LT8610* 0.97V 3.4V 2.5A 50ns –
*See LT8610A/LT8610AB or LT8610 data sheet.
5V 3.5A Step-Down Converter
applicaTions
n LT8610 Feature Set, Plus:
3V Minimum Input Voltage
800mV Feedback Voltage
3.5A Maximum Output Current
Fast 30ns Minimum Switch-On Time
Improved Burst Mode Efficiency
Improved EMI
n Wide Input Voltage Range: 3V to 42V
n Ultralow Quiescent Current Burst Mode
®
Operation:
2.5μA IQ Regulating 12VIN to 3.3VOUT
n High Efficiency Synchronous Operation:
95% Efficiency at 1A, 5VOUT from 12VIN
93% Efficiency at 1A, 3.3VOUT from 12VIN
n Low Dropout Under All Conditions: 200mV at 1A
n Safely Tolerates Inductor Saturation in Overload
n Adjustable and Synchronizable Frequency:
LT8610AC: 200kHz to 2.2MHz
LT8610AC-1: 1.5MHz to 2.2MHz
n Accurate 1.015V Enable Pin Threshold
n Output Soft-Start and Tracking
n Small Thermally Enhanced 16-Lead MSOP Package
n Automotive and Industrial Supplies
n GSM Power Supplies
L, LT , LT C , LT M , Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective owners.
Efficiency at 5VOUT
BSTVIN
EN/UV
PG
SYNC
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA01a
FB
0.1µF
V
OUT
5V
3.5A
4.7µF
V
IN
5.5V TO 42V
1µF
10nF
10pF
4.7µH
1M
191k
f
SW
= 700kHz
60.4k
47µF
×2
0.1 1 100010010
LOAD CURRENT (mA)
EFFICIENCY (%)
80
90
100
8610ac1 TA01b
70
50
60
40
30
VIN = 12V
VIN = 24V
VIN = 36V
LT8610AC/LT8610AC-1
2
8610acfa
For more information www.linear.com/LT8610AC
pin conFiguraTionabsoluTe MaxiMuM raTings
VIN, EN/UV, PG ..........................................................42V
BIAS .......................................................................... 30V
BST Pin Above SW Pin................................................4V
FB, TR/SS, RT, INTVCC . .............................................. 4V
SYNC Voltage . ............................................................6V
Operating Junction Temperature Range (Note 2)
LT8610ACE/LT8610ACE-1 ...................... –40 to 125°C
LT8610ACI/LT8610ACI-1 ........................ –40 to 125°C
LT8610ACH/LT8610ACH-1 ..................... –40 to 150°C
Storage Temperature Range ......................–65 to 150°C
(Note 1)
1
2
3
4
5
6
7
8
SYNC
TR/SS
RT
EN/UV
VIN
VIN
NC
GND
16
15
14
13
12
11
10
9
FB
PG
BIAS
INTV
CC
BST
SW
SW
SW
TOP VIEW
17
GND
MSE PACKAGE
16-LEAD PLASTIC MSOP
θJA = 40°C/W, θJC(PAD) = 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
elecTrical characTerisTics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
orDer inForMaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT8610ACEMSE#PBF LT8610ACEMSE#TRPBF 8610AC 16-Lead Plastic MSOP –40°C to 125°C
LT8610ACIMSE#PBF LT8610ACIMSE#TRPBF 8610AC 16-Lead Plastic MSOP –40°C to 125°C
LT8610ACHMSE#PBF LT8610ACHMSE#TRPBF 8610AC 16-Lead Plastic MSOP –40°C to 150°C
LT8610ACEMSE-1#PBF LT8610ACEMSE-1#TRPBF 610AC1 16-Lead Plastic MSOP –40°C to 125°C
LT8610ACIMSE-1#PBF LT8610ACIMSE-1#TRPBF 610AC1 16-Lead Plastic MSOP –40°C to 125°C
LT8610ACHMSE-1#PBF LT8610ACHMSE-1#TRPBF 610AC1 16-Lead Plastic MSOP –40°C to 150°C
Consult LT C Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping
container.Consult LT C 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/
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage l2.6 3.0 V
VIN Quiescent Current VEN/UV = 0V
l
1.0
1.0
3
8
µA
µA
VEN/UV = 2V, Not Switching, VSYNC = 0V
l
1.7
1.7
4
10
µA
µA
VEN/UV = 2V, Not Switching, VSYNC = 2V 0.28 0.5 mA
VIN Current in Regulation VOUT = 0.8V, VIN = 6V, Output Load = 100µA
VOUT = 0.8V, VIN = 6V, Output Load = 1mA
l
l
30
225
60
500
µA
µA
Feedback Reference Voltage VIN = 6V, ILOAD = 0.5A
VIN = 6V, ILOAD = 0.5A
l
0.794
0.788
0.800
0.800
0.806
0.812
V
V
Feedback Voltage Line Regulation VIN = 4.0V to 42V, ILOAD = 1A l0.004 0.02 %/V
Feedback Pin Input Current VFB = 1V –20 20 nA
LT8610AC/LT8610AC-1
3
8610acfa
For more information www.linear.com/LT8610AC
elecTrical characTerisTics
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: The LT8610ACE/LT8610ACE-1 is guaranteed to meet performance
specifications from 0°C to 125°C junction temperature. Specifications over
the –40°C to 125°C operating junction temperature range are assured by
design, characterization, and correlation with statistical process controls.
The LT8610ACI/LT8610ACI-1 is guaranteed over the full –40°C to 125°C
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER CONDITIONS MIN TYP MAX UNITS
INTVCC Voltage ILOAD = 0mA, VBIAS = 0V
ILOAD = 0mA, VBIAS = 3.3V
3.23
3.25
3.4
3.29
3.57
3.35
V
V
INTVCC Undervoltage Lockout 2.37 2.47 2.57 V
BIAS Pin Current Consumption VBIAS = 3.3V, ILOAD = 1A, 2MHz 9.2 mA
Minimum On-Time ILOAD = 1A, SYNC = 0V
ILOAD = 1A, SYNC = 3.3V
l
l
15
15
30
30
45
45
ns
ns
Minimum Off-Time 95 125 ns
Oscillator Frequency RT = 221k, ILOAD = 1A (LT8610AC Only)
RT = 60.4k, ILOAD = 1A (LT8610AC Only)
RT = 18.2k, ILOAD = 1A (LT8610AC/LT8610AC-1)
l
l
l
180
665
1.85
210
700
2.00
240
735
2.15
kHz
kHz
MHz
Top Power NMOS On-Resistance ISW = 1A 120 mΩ
Top Power NMOS Current Limit l5 6.7 8 A
Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A 65 mΩ
Bottom Power NMOS Current Limit VINTVCC = 3.4V 3.4 4.3 5.4 A
SW Leakage Current VIN = 42V, VSW = 0V, 42V –1.5 1.5 µA
EN/UV Pin Threshold EN/UV Rising l0.955 1.015 1.075 V
EN/UV Pin Hysteresis 50 mV
EN/UV Pin Current VEN/UV = 2V –20 20 nA
PG Upper Threshold Offset from VFB VFB Falling l5 8.0 11 %
PG Lower Threshold Offset from VFB VFB Rising l–11 –8.0 –5 %
PG Hysteresis 0.4 %
PG Leakage VPG = 3.3V –40 40 nA
PG Pull-Down Resistance VPG = 0.1V l680 2000 Ω
SYNC Threshold SYNC Falling
SYNC Rising
0.8
1.6
1.1
2.0
1.4
2.4
V
V
TR/SS Source Current l1.0 2.0 3.2 µA
TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V 230 Ω
operating junction temperature range. The LT8610ACH/LT8610ACH-1 is
guaranteed over the full –40°C to 150°C operating junction temperature
range. High junction temperatures degrade operating lifetimes. Operating
lifetime is derated at junction temperatures greater than 125°C.
Note 3: This IC includes overtemperature protection that is intended to
protect the device during overload conditions. Junction temperature will
exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
will reduce lifetime.
LT8610AC/LT8610AC-1
4
8610acfa
For more information www.linear.com/LT8610AC
Typical perForMance characTerisTics
Efficiency at 3.3VOUT Efficiency vs Frequency at 1A Reference Voltage
Efficiency at 5VOUT Efficiency at 5VOUT Efficiency at 3.3VOUT
LOAD CURRENT (A)
0
EFFICIENCY (%)
80
90
100
2 3
8610ac1 G01
70
60
75
85
95
65
55
50 0.5 11.5
3.5
2.5
fSW = 700kHz (LT8610AC)
L = IHLP-2525EZ-01, 4.7µH
VIN = 12V
VIN = 24V
VIN = 36V
LOAD CURRENT (mA)
EFFICIENCY (%)
0.01 101 1000100
8610ac1 G02
0.1
80
90
100
70
50
60
40
20
30 VIN = 12V
VIN = 24V
VIN = 36V
fSW = 700kHz (LT8610AC)
L = IHLP-2525EZ-01, 4.7µH
0 2 3
0.5 11.5
3.5
2.5
LOAD CURRENT (A)
EFFICIENCY (%)
80
90
100
8610ac1 G03
70
60
75
85
95
65
55
50
VIN = 12V
VIN = 24V
VIN = 36V
fSW = 700kHz (LT8610AC)
L = IHLP-2525EZ-01, 4.7µH
0.01 101 10001000.1
VIN = 12V
VIN = 24V
VIN = 36V
LOAD CURRENT (mA)
30
EFFICIENCY (%)
90
100
80
50
70
60
40
8610ac1 G04
20
fSW = 700kHz (LT8610AC)
L = IHLP-2525EZ-01, 4.7µH
SWITCHING FREQUENCY (MHz)
0.25
80
85
100
1.75
8610ac1 G05
75
70
0.75 1.25
2.25
1.50
0.50 1.00 2.00
65
60
90
95
EFFICIENCY (%)
VIN = 12V
VIN = 24V
VOUT = 3.3V
L = IHLP-2525EZ-01, 4.7µH
(LT8610AC)
TEMPERATURE (°C)
–55
FB PIN VOLTAGE (V)
35
8610ac1 G06
0.800
–25 5 65
0.792
0.796
0.788
0.784
0.816
0.812
0.804
0.808
95 125
155
TEMPERATURE (°C)
–55
35
1.00
–25 5 65
0.96
0.98
0.94
0.92
1.02
95 125
EN RISING
EN FALLING
Line Regulation
Load Regulation
LOAD CURRENT (A)
0
–0.25
CHANGE IN V
OUT
(%)
–0.15
–0.05
0.05
0.5 11.5 2
8610ac1 G08
2.5
0.15
0.25
–0.20
–0.10
0
0.10
0.20
3.5
3
VOUT = 3.3V
VIN = 12V
INPUT VOLTAGE (V)
0
CHANGE IN V
OUT
(%)
0.03
0.09
0.15
40
8610ac1 G09
–0.03
–0.09
0
0.06
0.12
–0.06
–0.12
–0.15 105 2015 30 35
45
25
VOUT = 3.3V
ILOAD = 0.5A
TA = 25°C, unless otherwise noted.
EN Pin Thresholds
LT8610AC/LT8610AC-1
5
8610acfa
For more information www.linear.com/LT8610AC
Typical perForMance characTerisTics
No Load Supply Current No Load Supply Current Top FET Current Limit vs Duty Cycle
Top FET Current Limit Bottom FET Current Limit
INPUT VOLTAGE (V)
0
0
INPUT CURRENT (µA)
0.5
1.5
2.0
2.5
5.0
3.5
10 20 25
45
8610ac1 G10
1.0
4.0
4.5
3.0
5 15 30 35 40
VOUT = 3.3V
IN REGULATION
TEMPERATURE (°C)
–55 –25
0
INPUT CURRENT (µA)
10
25
565 95
8610ac1 G11
5
20
15
35 125
155
VOUT = 3.3V
VIN = 12V
IN REGULATION
DUTY CYCLE
0
CURRENT LIMIT (A)
3.5
4.0
4.5
0.6
1.0
8610ac1 G12
3.0 0.2 0.4 0.8
5.0
5.5
8.0
7.0
7.5
6.5
6.0
TEMPERATURE (°C)
–55
4.50
CURRENT LIMIT (A)
4.75
5.00
5.25
5.50
5.75
6.00
6.25
6.50
6.75
7.00
–25 5 35 65
8610ac1 G13
95
125
30% DC
TEMPERATURE (°C)
–55
3.00
CURRENT LIMIT (A)
3.25
3.50
3.75
4.00
5.50
–25 5 35 65
8610ac1 G14
95
155
125
4.25
4.50
4.75
5.00
5.25
Switch Drop
Switch Drop
TEMPERATURE (°C)
–55 –25
0
SWITCH DROP (mV)
100
250
565 95
8610ac1 G15
50
200
150
35 125 155
TOP SW
BOT SW
SWITCH CURRENT = 1A
SWITCH CURRENT (A)
0
0
SWITCH DROP (mV)
50
150
200
250
2
450
8610ac1 G16
100
1
0.5 2.5
1.5
3
300
350
400
TOP SW
BOT SW
Minimum On-Time Minimum Off-Time
TEMPERATURE (°C)
–55
25
27
29
31
MINIMUM ON-TIME (ns)
33
35
37
39
45
5 65 95 125
8610ac1 G17
43
41
–25 35
155
VOUT = 3.3V
VOUT = 0.8V
ILOAD = 1.5A
VSYNC = 5V
TEMPERATURE (°C)
–50
MINIMUM OFF-TIME (ns)
95
35
8610ac1 G18
80
–25 5 65
75
125
120
115
110
105
100
90
85
95 125
155
VOUT = 3.3V
ILOAD = 0.5A
TA = 25°C, unless otherwise noted.
LT8610AC/LT8610AC-1
6
8610acfa
For more information www.linear.com/LT8610AC
Typical perForMance characTerisTics
Minimum Load to Full Frequency
Burst Frequency
Frequency Foldback
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
60
70
80
15 25
40
8610ac1 G22
40
20
0
50
30
10
5 10 20 30 35
VOUT = 3.3V
fSW = 700kHz
PULSE-SKIPPING MODE
(LT8610AC)
FB VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
300
400
500
0.6
0.8
8610ac1 G23
200
100
00.2 0.4 0.50.1 0.3 0.7
600
700
800
VOUT = 3.3V
VIN = 12V
VSYNC = 0V
RT = 60.4k
(LT8610AC)
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
400
500
600
800
8610ac1 G21
300
200
0200 400 600 700100 300 500
100
800
VIN = 12V
VOUT = 3.3V
L = 4.7µH
(LT8610AC)
700
Soft-Start Tracking
Soft-Start Current PG High Thresholds PG Low Thresholds
TR/SS VOLTAGE (V)
0
FB VOLTAGE (V)
0.8
1.0
0.6 1.0
8610ac1 G24
0.6
0.4
0.2 0.4 0.8
1.2
0.2
0
TEMPERATURE (°C)
–50
SS PIN CURRENT (µA)
2.3
35
8610ac1 G25
2.0
1.8
–25 5 65
1.7
1.6
2.4
2.2
2.1
1.9
95 125
155
VSS = 0.5V
TEMPERATURE (°C)
–55
PG THRESHOLD OFFSET FROM V
REF
(%)
35
8610ac1 G26
8.0
–25 5 65
7.0
7.5
6.5
6.0
10.0
9.5
8.5
9.0
95 125
155
FB RISING
FB FALLING
TEMPERATURE (°C)
–55
PG THRESHOLD OFFSET FROM V
REF
(%)
35
8610ac1 G27
–8.0
–25 5 65
–7.0
–7.5
–6.5
–10.0
–6.0
–9.5
–8.5
–9.0
95 125
155
FB RISING
FB FALLING
LOAD CURRENT (A)
DROPOUT VOLTAGE (mV)
400
8610ac1 G19
200
0
600
800
300
100
500
700
0 0.5 11.5 2 2.5
3.5
3
Dropout Voltage Switching Frequency
TEMPERATURE (°C)
–55
SWITCHING FREQUENCY (kHz)
730
35
8610ac1 G20
700
680
–25 5 65
670
660
740
RT = 60.4k
(LT8610AC)
720
710
690
95 125
155
TA = 25°C, unless otherwise noted.
LT8610AC/LT8610AC-1
7
8610acfa
For more information www.linear.com/LT8610AC
RT Programmed Switching
Frequency Minimum Input Voltage
Bias Pin Current Bias Pin Current
SWITCHING FREQUENCY (MHz)
0.2
RT PIN RESISTOR (kΩ)
150
200
250
1.8
8610ac1 G28
100
50
125
175
225
75
25
00.6 11.4
2.2
(LT8610AC)
TEMPERATURE (°C)
–55
MINIMUM INPUT VOLTAGE (V)
2.9
35
8610ac1 G29
2.8
2.4
–25 5 65
2.2
2.3
2.1
2.0
3.0
2.7
2.5
2.6
95 125
155
INPUT VOLTAGE (V)
5
BIAS PIN CURRENT (mA)
4.5
5.5
45
8610ac1 G30
3.5
2.5 15 25 35
10 20 30 40
6.5
4.0
5.0
3.0
6.0
VBIAS = 5V
VOUT = 5V
ILOAD = 1A
fSW = 700kHz
(LT8610AC)
SWITCHING FREQUENCY (MHz)
0
0
BIAS PIN CURRENT (mA)
2
4
6
8
10
12
0.5 1 1.5 2
8610ac1 G31
2.5
VBIAS = 5V
VOUT = 5V
VIN = 12V
ILOAD = 1A
(LT8610AC)
Typical perForMance characTerisTics
Switching Waveforms Burst Mode Operation Switching Waveforms
IL
1A/DIV
VSW
5V/DIV
500ns/DIV
12VIN TO 5VOUT AT 1A
8610ac1
G32
IL
500mA/DIV
VSW
5V/DIV
VOUT
20mV/DIV
20µs/DIV
VSYNC = 0V
COUT = 47µF
L = 4.7µH
8610ac1
G33
IL
1A/DIV
VSW
10V/DIV
500ns/DIV
36VIN TO 5VOUT AT 1A
8610ac1
G34
TA = 25°C, unless otherwise noted.
LT8610AC/LT8610AC-1
8
8610acfa
For more information www.linear.com/LT8610AC
LT8610AC Transient Response
Start-Up Dropout Performance
LT8610AC-1 Transient Response LT8610AC-1 Transient Response
LT8610AC Transient Response
Start-Up Dropout Performance
IL
2A/DIV
VOUT
200mV/DIV
50µs/DIV
1.5A TO 3.5A TRANSIENT
12VIN, 3.3VOUT
C
OUT
= 47µF × 2
8610ac1
G35
VOUT
1V/DIV
VIN
1V/DIV
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
8610ac1
G37
VIN VOUT
IL
2A/DIV
VOUT
200mV/DIV
50µs/DIV
200mA TO 2A TRANSIENT
12VIN, 3.3VOUT
COUT = 47µF × 2
8610ac1
G36
VOUT
1V/DIV
VIN
1V/DIV
100ms/DIV
20Ω LOAD
(250mA IN REGULATION)
8610ac1
G38
VIN VOUT
Typical perForMance characTerisTics
TA = 25°C, unless otherwise noted.
IOUT
2A/DIV
VOUT
200mV/DIV
50µs/DIV
1.5A TO 3.5A TRANSIENT
12VIN, 3.3VOUT
COUT = 47µF × 2
8610ac1 G39
IOUT
2A/DIV
VOUT
200mV/DIV
50µs/DIV
100mA TO 2A TRANSIENT
12VIN, 3.3VOUT
COUT = 47µF × 2
8610ac1 G40
LT8610AC/LT8610AC-1
9
8610acfa
For more information www.linear.com/LT8610AC
pin FuncTions
SYNC (Pin 1): External Clock Synchronization Input.
Ground this pin for low ripple Burst Mode operation at low
output loads. Tie to a clock source for synchronization to
an external frequency. Apply a DC voltage of 3V or higher
or tie to INTVCC for pulse-skipping mode. When in pulse-
skipping mode, the IQ will increase to several hundred µA.
Do not float this pin.
TR/SS (Pin 2): Output Tracking and Soft-Start Pin. This
pin allows user control of output voltage ramp rate during
start-up. A TR/SS voltage below 0.8V forces the LT8610AC/
LT8610AC-1 to regulate the FB pin to equal the TR/SS pin
voltage. When TR/SS is above 0.8V, the tracking function
is disabled and the internal reference resumes control of
the error amplifier. An internal 2.2μA pull-up current from
INTVCC on this pin allows a capacitor to program output
voltage slew rate. This pin is pulled to ground with an
internal 230Ω MOSFET during shutdown and fault condi-
tions; use a series resistor if driving from a low impedance
output. This pin may be left floating in the LT8610AC if
the tracking function is not needed. A minimum of 100pF
of external capacitance must be used on the LT8610AC-1.
RT (Pin 3): A resistor is tied between RT and ground to
set the switching frequency.
EN/UV (Pin 4): The LT8610AC/LT8610AC-1 is shut down
when this pin is low and active when this pin is high.
The hysteretic threshold voltage is 1.015V going up and
0.97V going down. Tie to VIN if the shutdown feature is
not used. An external resistor divider from VIN can be used
to program a VIN threshold below which the LT8610AC/
LT8610AC-1 will shut down.
VIN (Pins 5, 6): The VIN pins supply current to the
LT8610AC/LT8610AC-1 internal circuitry and to the internal
topside power switch. These pins must be tied together
and be locally bypassed. Be sure to place the positive
terminal of the input capacitor as close as possible to the
VIN pins, and the negative capacitor terminal as close as
possible to the GND pins.
NC (Pin 7): No Connect. This pin is not connected to
internal circuitry.
SW (Pins 9, 10, 11): The SW pins are the outputs of the
internal power switches. Tie these pins together and con-
nect them to the inductor and boost capacitor. This node
should be kept small on the PCB for good performance.
BST (Pin 12): This pin is used to provide a drive voltage,
higher than the input voltage, to the topside power switch.
Place a 0.1µF boost capacitor as close as possible to the IC.
INTVCC (Pin 13): Internal 3.4V Regulator Bypass Pin.
The internal power drivers and control circuits are pow-
ered from this voltage. INTVCC maximum output cur-
rent is 20mA. Do not load the INTVCC pin with external
circuitry. INTVCC current will be supplied from BIAS if
VBIAS > 3.1V, otherwise current will be drawn from VIN.
Voltage on INTVCC will vary between 2.8V and 3.4V when
VBIAS is between 3.0V and 3.6V. Decouple this pin to power
ground with at least a 1μF low ESR ceramic capacitor
placed close to the IC.
BIAS (Pin 14): The internal regulator will draw current from
BIAS instead of VIN when BIAS is tied to a voltage higher
than 3.1V. For output voltages of 3.3V and above this pin
should be tied to VOUT. If this pin is tied to a supply other
than VOUT use a 1µF local bypass capacitor on this pin.
PG (Pin 15): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within ±8% of the final regulation voltage, and there are
no fault conditions. PG is valid when VIN is above 3.0V,
regardless of EN/UV pin state.
FB (Pin 16): The LT8610AC/LT8610AC-1 regulates the FB
pin to 0.800V. Connect the feedback resistor divider tap
to this pin. Also, connect a phase lead capacitor between
FB and VOUT. Typically, this capacitor is 4.7pF to 10pF.
GND (Pin 8, Exposed Pad Pin 17): Ground. These pins
are the return path of the internal bottom-side switch and
must be tied together. Place the negative terminal of the
input capacitor as close to the GND pin and exposed pad
as possible. The exposed pad must be soldered to the PCB
in order to lower the thermal resistance.
LT8610AC/LT8610AC-1
10
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block DiagraM
+
+
–
+
–
SLOPE COMP
INTERNAL 0.80V REF
OSCILLATOR
BURST
DETECT
3.4V
REG
M1
M2
CBST
COUT
V
OUT
8610ac1 BD
SW L
BST
9-11
SWITCH
LOGIC
AND
ANTI-
SHOOT
THROUGH
ERROR
AMP
SHDN
±8%
VC
SHDN
TSD
INTVCC UVLO
VIN UVLO
SHDN
TSD
VIN UVLO
EN/UV
1V +
–
4
12
17
GND
INTVCC 13
BIAS 14
GND
8
PG
15
FB
R1C1
R3
OPT
R4
OPT
R2
RT
CSS
OPT
VOUT
16
TR/SS
2.2µA
2
RT
3
SYNC
1
VIN
V
IN
CIN
CVCC
5, 6
LT8610AC/LT8610AC-1
11
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operaTion
The LT8610AC/LT8610AC-1 is a monolithic, constant
frequency, current mode step-down DC/DC converter. An
oscillator, with frequency set using a resistor on the RT pin,
turns on the internal top power switch at the beginning of
each clock cycle. Current in the inductor then increases
until the top switch current comparator trips and turns off
the top power switch. The peak inductor current at which
the top switch turns off is controlled by the voltage on the
internal VC node. The error amplifier servos the VC node
by comparing the voltage on the VFB pin with an internal
0.8V reference. When the load current increases it causes
a reduction in the feedback voltage relative to the reference
leading the error amplifier to raise the VC voltage until the
average inductor current matches the new load current.
When the top power switch turns off, the synchronous
power switch turns on until the next clock cycle begins or
inductor current falls to zero. If overload conditions result
in more than 4.3A flowing through the bottom switch, the
next clock cycle will be delayed until switch current returns
to a safe level.
If the EN/UV pin is low, the LT8610AC/LT8610AC-1 is shut
down and draws 1µA from the input. When the EN/UV pin
is above 1.015V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8610AC/
LT8610AC-1 operates in Burst Mode operation in light
load situations. Between bursts, all circuitry associated
with controlling the output switch is shut down, reducing
the input supply current to 1.7μA. In a typical application,
2.5μA will be consumed from the input supply when
regulating with no load. The SYNC pin is tied low to use
Burst Mode operation and can be tied to a logic high to use
pulse-skipping mode. If a clock is applied to the SYNC pin
the part will synchronize to an external clock frequency and
operate in pulse-skipping mode. While in pulse-skipping
mode the oscillator operates continuously and positive
SW transitions are aligned to the clock. During light loads,
switch pulses are skipped to regulate the output and the
quiescent current will be several hundred µA.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will draw
current from VIN. The BIAS pin should be connected to
VOUT if the LT8610AC/LT8610AC-1 output is programmed
at 3.3V or above.
Comparators monitoring the FB pin voltage will pull the
PG pin low if the output voltage varies more than ±8%
(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8610AC/LT8610AC-1’s oper-
ating frequency when the voltage at the FB pin is low. This
frequency foldback helps to control the inductor current
when the output voltage is lower than the programmed
value which occurs during start-up or overcurrent condi-
tions. When a clock is applied to the SYNC pin or the SYNC
pin is held DC high, the frequency foldback is disabled
and the switching frequency will slow down only during
overcurrent conditions.
The LT8610AC-1 differs from the LT8610AC in that the
internal compensation is optimized for higher switching
frequency operation and a TR/SS pin discharge circuit is
added to reduce output voltage overshoot when exiting
dropout or brownout conditions. The internal compensa-
tion change increases the bandwidth of the control loop,
which improves transient response. As a consequence
the minimum programmable switching frequency of the
LT8610AC-1 is increased to 1.5MHz. The SS discharge
circuit regulates the TR/SS pin to the same voltage as the
FB pin during dropout or brownout. This feature provides
some soft-starting when the part exits dropout or brownout
which reduces output voltage overshoot. Both of these
changes are intended to reduce output voltage overshoot
in 2MHz switching frequency applications.
LT8610AC/LT8610AC-1
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cal converters. By maximizing the time between pulses,
the converter quiescent current approaches 2.5µA for a
typical application when there is no output load. Therefore,
to optimize the quiescent current performance at light
loads, the current in the feedback resistor divider must
be minimized as it appears to the output as load current.
While in Burst Mode operation the current limit of the
top switch is approximately 1.3A for the LT8610AC/
LT8610AC-1 resulting in output voltage ripple shown in
Figure 2. An increase in output capacitance proportionally
decreases the output voltage ripple (Table 1). As load ramps
upward from zero the switching frequency will increase
but only up to the switching frequency programmed by the
resistor at the RT pin as shown in Figure 1a. The output
load at which the LT8610AC/LT8610AC-1 reaches the
Figure 1. SW Frequency vs Load Information in Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
Figure 2. Burst Mode Operation
Minimum Load to Full FrequencyBurst Frequency
(1a) (1b)
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
400
500
600
800
8610ac1 F01a
300
200
0200 400 600 700100 300 500
100
800
VIN = 12V
VOUT = 3.3V
L = 4.7µH
(LT8610AC)
700
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
60
70
80
15 25
40
8610ac1 F01b
40
20
0
50
30
10
5 10 20 30 35
VOUT = 3.3V
fSW = 700kHz
PULSE-SKIPPING MODE
(LT8610AC)
IL
500mA/DIV
VOUT
20mV/DIV
VSW
5V/DIV
20µs/DIVVSYNC = 0V
COUT = 47µF
L = 4.7µH
8610ac1 F02
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8610AC/
LT8610AC-1 operates in low ripple Burst Mode operation,
which keeps the output capacitor charged to the desired
output voltage while minimizing the input quiescent cur-
rent and minimizing output voltage ripple. In Burst Mode
operation the LT8610AC/LT8610AC-1 delivers single
pulses of current to the output capacitor followed by sleep
periods where the output power is supplied by the output
capacitor. While in sleep mode the LT8610AC/LT8610AC-1
consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1a)
and the percentage of
time the LT8610AC/LT8610AC-1 is in sleep mode increases,
resulting in much higher light load efficiency than for typi-
LT8610AC/LT8610AC-1
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programmed frequency varies based on input voltage,
output voltage, and inductor choice.
Inductor value has a very strong effect on Burst Mode ef-
ficiency. Larger value inductors allow more charge to be
transferred to the output per pulse, which increases both
efficiency and output voltage ripple. If higher efficiency is
needed in a Burst Mode application, increasing inductor
value can be a quick solution.
Table 1. Output Voltage Ripple vs Output Capacitance for
LT8610AC when VIN = 12V, VOUT = 3.3V, and L = 4.7µH
OUTPUT CAPACITANCE OUTPUT RIPPLE
47µF 40mV
47µF ×220mV
47µF ×410mV
For some applications it is desirable for the LT8610AC/
LT8610AC-1 to operate in pulse-skipping mode, offering
two major differences from Burst Mode operation. First is
the clock stays awake at all times and all switching cycles
are aligned to the clock. In this mode much of the internal
circuitry is awake at all times, increasing quiescent cur-
rent to several hundred µA. Second is that full switching
frequency is reached at lower output load than in Burst
Mode operation (see Figure 1b). To enable pulse-skipping
mode, the SYNC pin is tied high either to a logic output
or to the INTVCC pin. When a clock is applied to the SYNC
pin the LT8610AC/LT8610AC-1 will also operate in pulse-
skipping mode.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1=R2 VOUT
0.80V –1
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to the
converter, which is approximately:
IQ=1.7µA +VOUT
R1+R2
VOUT
VIN
1
n
(2)
where 1.7µA is the quiescent current of the LT8610AC/
LT8610AC-1 and the second term is the current in the
feedback divider reflected to the input of the buck operating
at its light load efficiency n. For a 3.3V application with R1
= 1M and R2 = 316k, the feedback divider draws 2.5µA.
With VIN = 12V and n = 80%, this adds 0.8µA to the 1.7µA
quiescent current resulting in 2.5µA no-load current from
the 12V supply. Note that this equation implies that the
no-load current is a function of VIN; this is plotted in the
Typical Performance Characteristics section.
When using large FB resistors, a 4.7pF to 10pF phase-lead
capacitor should be connected from VOUT to FB.
Setting the Switching Frequency
The LT8610AC uses a constant frequency PWM architec-
ture that can be programmed to switch from 200kHz to
2.2MHz by using a resistor tied from the RT pin to ground.
A table showing the necessary RT value for a desired
switching frequency is in Table 2A. The LT8610AC-1 can
be programmed to switch from 1.5MHz to 2.2MHz. The
minimum allowed programmable switching frequency is
higher for the LT8610AC-1 compared to the LT8610AC
because the LT8610AC-1 has internal compensation which
is optimized for higher switching frequencies to improve
transient response by increasing control loop bandwidth.
A table showing the necessary RT value for a desired
switching frequency is shown in Table 2B.
The RT resistor required for a desired switching frequency
can be calculated using:
RT=
46.5
fSW
– 5.2
(3)
where RT is in kΩ and fSW is the desired switching fre-
quency in MHz.
LT8610AC/LT8610AC-1
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Table 2A. LT8610AC SW Frequency vs RT Value
fSW (MHz) RT (kΩ)
0.2 232
0.3 150
0.4 110
0.5 88.7
0.6 71.5
0.7 60.4
0.8 52.3
1.0 41.2
1.2 33.2
1.4 28.0
1.6 23.7
1.8 20.5
2.0 18.2
2.2 15.8
Table 2B. LT8610AC-1 SW Frequency vs RT Value
fSW (MHz) RT (kΩ)
1.4 28.0
1.6 23.7
1.8 20.5
2.0 18.2
2.2 15.8
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller induc-
tor and capacitor values may be used. The disadvantages
are lower efficiency and a smaller input voltage range.
The highest switching frequency (fSW(MAX)) for a given
application can be calculated as follows:
fSW(MAX) =
V
OUT
+V
SW(BOT)
tON(MIN) VIN – VSW(TOP) +VSW(BOT)
( )
(4)
where VIN is the typical input voltage, VOUT is the output
voltage, VSW(TOP) and VSW(BOT) are the internal switch
drops (~0.42V, ~0.21V, respectively at maximum load)
and tON(MIN) is the minimum top switch on-time (see the
Electrical Characteristics). This equation shows that a
slower switching frequency is necessary to accommodate
a high VIN/VOUT ratio.
For transient operation, VIN may go as high as the abso-
lute maximum rating of 42V regardless of the RT value,
however the LT8610AC/LT8610AC-1 will reduce switching
frequency as necessary to maintain control of inductor
current to assure safe operation.
The LT8610AC/LT8610AC-1 is capable of a maximum duty
cycle of greater than 99%, and the VIN-to-VOUT dropout
is limited by the RDS(ON) of the top switch. In this mode
the LT8610AC/LT8610AC-1 skips switch cycles, resulting
in a lower switching frequency than programmed by RT.
For applications that cannot allow deviation from the pro-
grammed switching frequency at low VIN/VOUT ratios use
the following formula to set switching frequency:
VIN(MIN) =
V
OUT
+V
SW(BOT)
1– fSW •tOFF(MIN)
– VSW(BOT) +VSW(TOP)
(5)
where VIN(MIN) is the minimum input voltage without
skipped cycles, VOUT is the output voltage, VSW(TOP) and
VSW(BOT) are the internal switch drops (~0.42V, ~0.21V,
respectively at maximum load), fSW is the switching fre-
quency (set by RT), and tOFF(MIN) is the minimum switch
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8610AC/LT8610AC-1 is designed to minimize so-
lution size by allowing the inductor to be chosen based
on the output load requirements of the application. Dur-
ing overload or short-circuit conditions the LT8610AC/
LT8610AC-1 safely tolerates operation with a saturated
inductor through the use of a high speed peak-current
mode architecture.
A good first choice for the inductor value is:
L=
V
OUT
+V
SW(BOT)
fSW
(6)
where fSW is the switching frequency in MHz, VOUT is
the output voltage, VSW(BOT) is the bottom switch drop
(~0.21V) and L is the inductor value in μH.
LT8610AC/LT8610AC-1
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To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
addition, the saturation current (typically labeled ISAT)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
IL(PEAK) =ILOAD(MAX) +
1
2
∆IL
(7)
where ∆IL is the inductor ripple current as calculated in
Equation 9 and ILOAD(MAX) is the maximum output load
for a given application.
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater than
1A and an ISAT of greater than 1.3A. During long duration
overload or short-circuit conditions, the inductor RMS
rating requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 0.04Ω, and the core material
should be intended for high frequency applications.
The LT8610AC/LT8610AC-1 limits the peak switch cur-
rent in order to protect the switches and the system from
overload faults. The top switch current limit (ILIM) is at
least 6A at low duty cycles and decreases linearly to 5A
at DC = 0.8. The inductor value must then be sufficient to
supply the desired maximum output current (IOUT(MAX)),
which is a function of the switch current limit (ILIM) and
the ripple current.
IOUT(MAX) =ILIM –
∆I
L
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
∆IL=VOUT
L•fSW
•1– VOUT
VIN(MAX)
(9)
where fSW is the switching frequency of the LT8610AC/
LT8610AC-1, and L is the value of the inductor. There-
fore, the maximum output current that the LT8610AC/
LT8610AC-1 will deliver depends on the switch current
limit, the inductor value, and the input and output voltages.
The inductor value may have to be increased if the inductor
ripple current does not allow sufficient maximum output
current (IOUT(MAX)) given the switching frequency, and
maximum input voltage used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. For applications
requiring smaller load currents, the value of the inductor
may be lower and the LT8610AC/LT8610AC-1 may operate
with higher ripple current. This allows use of a physically
smaller inductor, or one with a lower DCR resulting in
higher efficiency. Be aware that low inductance may result
in discontinuous mode operation, which further reduces
maximum load current.
Inductor value has a very strong effect on Burst Mode ef-
ficiency. Larger value inductors allow more charge to be
transferred to the output per pulse, which increases both
efficiency and output voltage ripple. If higher efficiency is
needed in a Burst Mode application, increasing inductor
value can be a quick solution.
For more information about maximum output current
and discontinuous operation, see Linear Technology’s
Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitor
Bypass the input of the LT8610AC/LT8610AC-1 circuit with
a ceramic capacitor of X7R or X5R type placed as close as
possible to the VIN and PGND pins. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 4.7μF to 10μF ceramic capacitor is
adequate to bypass the LT8610AC/LT8610AC-1 and will
easily handle the ripple current. Note that larger input
capacitance is required when a lower switching frequency
is used. If the input power source has high impedance, or
there is significant inductance due to long wires or cables,
additional bulk capacitance may be necessary. This can
be provided with a low performance electrolytic capacitor.
LT8610AC/LT8610AC-1
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Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT8610AC/LT8610AC-1 and to force this very high
frequency switching current into a tight local loop, mini-
mizing EMI. A 4.7μF capacitor is capable of this task, but
only if it is placed close to the LT8610AC/LT8610AC-1 (see
the PCB Layout section). A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT8610AC/LT8610AC-1. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT8610AC/LT8610AC-1 circuit is plugged into a live sup-
ply, the input voltage can ring to twice its nominal value,
possibly exceeding the LT8610AC/LT8610AC-1’s voltage
rating. This situation is easily avoided (see Linear Technol-
ogy Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT8610AC/LT8610AC-1 to produce the DC output. In
this role it determines the output ripple, thus low imped-
ance at the switching frequency is important. The second
function is to store energy in order to satisfy transient loads
and stabilize the LT8610AC/LT8610AC-1’s control loop.
Ceramic capacitors have very low equivalent series resis-
tance (ESR) and provide the best ripple performance. For
good starting values, see the Typical Applications section.
Use X5R or X7R types. This choice will provide low output
ripple and good transient response. Transient performance
can be improved with a higher value output capacitor and
the addition of a feedforward capacitor placed between
VOUT and FB. Increasing the output capacitance will also
decrease the output voltage ripple. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer and may cause loop instability. See
the Typical Applications in this data sheet for suggested
capacitor values.
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capacitance
under the relevant operating conditions of voltage bias and
temperature. A physically larger capacitor or one with a
higher voltage rating may be required.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8610AC/LT8610AC-1 due to their
piezoelectric nature. When in Burst Mode operation, the
LT8610AC/LT8610AC-1’s switching frequency depends
on the load current, and at very light loads the LT8610AC/
LT8610AC-1 can excite the ceramic capacitor at audio fre-
quencies, generating audible noise. Since the LT8610AC/
LT8610AC-1 operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet to a ca-
sual ear. If this is unacceptable, use a high performance
tantalum or electrolytic capacitor at the output. Low noise
ceramic capacitors are also available.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8610AC/
LT8610AC-1. As previously mentioned, a ceramic input
capacitor combined with trace or cable inductance forms a
high quality (underdamped) tank circuit. If the LT8610AC/
LT8610AC-1 circuit is plugged into a live supply, the input
voltage can ring to twice its nominal value, possibly exceed-
ing the LT8610AC/LT8610AC-1’s rating. This situation is
easily avoided (see Linear Technology Application Note 88).
Enable Pin
The LT8610AC/LT8610AC-1 is in shutdown when the
EN pin is low and active when the pin is high. The rising
threshold of the EN comparator is 1.015V, with 45mV of
hysteresis. The EN pin can be tied to VIN if the shutdown
feature is not used, or tied to a logic level if shutdown
control is required.
Adding a resistor divider from VIN to EN programs the
LT8610AC/LT8610AC-1 to regulate the output only when
VIN is above a desired voltage (see the Block Diagram).
Typically, this threshold, VIN(EN), is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions. The
VIN(EN) threshold prevents the regulator from operating
at source voltages where the problems might occur. This
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threshold can be adjusted by setting the values R3 and
R4 such that they satisfy the following equation:
VIN(EN) =
R3
R4 +1
•1.015V
(10)
where the LT8610AC/LT8610AC-1 will remain off until VIN is
above VIN(EN). Due to the comparator’s hysteresis, switch-
ing will not stop until the input falls slightly below VIN(EN).
When operating in Burst Mode operation for light load
currents, the current through the VIN(EN) resistor network
can easily be greater than the supply current consumed
by the LT8610AC/LT8610AC-1. Therefore, the VIN(EN)
resistors should be large to minimize their effect on ef-
ficiency at low loads.
INTVCC Regulator
An internal low dropout (LDO) regulator produces the 3.4V
supply from VIN that powers the drivers and the internal
bias circuitry. The INTVCC can supply enough current for the
LT8610AC/LT8610AC-1’s circuitry and must be bypassed
to ground with a minimum of 1μF ceramic capacitor.
Good bypassing is necessary to supply the high transient
currents required by the power MOSFET gate drivers. To
improve efficiency the internal LDO can also draw cur-
rent from the BIAS pin when the BIAS pin is at 3.1V or
higher. Typically the BIAS pin can be tied to the output of
the LT8610AC/LT8610AC-1, or can be tied to an external
supply of 3.3V or above. If BIAS is connected to a supply
other than VOUT, be sure to bypass with a local ceramic
capacitor. If the BIAS pin is below 3.0V, the internal LDO
will consume current from VIN. Applications with high input
voltage and high switching frequency where the internal
LDO pulls current from VIN will increase die temperature
because of the higher power dissipation across the LDO.
Do not connect an external load to the INTVCC pin.
Output Voltage Tracking and Soft-Start
T
he LT8610AC/LT8610AC-1 allows the user to program its
output voltage ramp rate by means of the TR/SS pin. An inter-
nal 2.2μA pulls up the TR/SS pin to INTVCC. Putting an external
capacitor on TR/SS enables soft starting the output to pre-
vent current surge on the input supply.
During the soft-start
ramp the output voltage will proportionally track the TR/SS
pin voltage. For output tracking applications, TR/SS can
be externally driven by another voltage source. From 0V
to 0.8V, the TR/SS voltage will override the internal 0.8V
reference input to the error amplifier, thus regulating the
FB pin voltage to that of TR/SS pin. When TR/SS is above
0.8V, tracking is disabled and the feedback voltage will
regulate to the internal reference voltage.
The LT8610AC-1 has an additional TR/SS pin feature not
included in the LT8610AC. During dropout or brownout,
when the FB pin is below the regulated value, the LT8610AC-1
regulates the TR/SS pin to the same voltage as the FB pin.
When the dropout or brownout condition goes away, the
LT8610AC-1 output voltage will slowly ramp-up with the
soft-start voltage preventing output voltage overshoot.
Scope shots of the LT8610AC-1 recovering from dropout
and short-circuit are show in Fig. 3.
In the LT8610AC, the TR/SS pin may be left floating if the
function is not needed. In the LT8610AC-1, the TR/SS pin
must have at least 100pF of external capacitance.
VOUT
1V/DIV
VSS
1V/DIV
VIN
5V/DIV
20ms/DIV
12VIN → 3.3VIN → 12VIN
WITH 3.3VOUT AT 1A
8610ac1 F03a
VOUT
1V/DIV
IOUT
2A/DIV
VSS
1V/DIV
10ms/DIV
5.5A BROWNOUT WITH
12VIN, 3.3VOUT AT 1A LOAD
8610ac1 F03b
Figure 3. LT8610AC-1 Soft Starting to Eliminate Output Voltage
Overshoot When Exiting Dropout (3a) and Brownout (3b)
(3b)
(3a)
LT8610AC/LT8610AC-1
18
8610acfa
For more information www.linear.com/LT8610AC
applicaTions inForMaTion
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, VIN voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8610AC/LT8610AC-1’s output voltage is
within the ±8% window of the regulation point, which is
a VFB voltage in the range of 0.736V to 0.864V (typical),
the output voltage is considered good and the open-drain
PG pin goes high impedance and is typically pulled high
with an external resistor. Otherwise, the internal pull-down
device will pull the PG pin low. To prevent glitching both
the upper and lower thresholds include 0.4% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1.015V, INTVCC has fallen
too low, VIN is too low, or thermal shutdown.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.4V (this can be ground or a logic low output).
To synchronize the LT8610AC/LT8610AC-1 oscillator to
an external frequency connect a square wave (with 20%
to 80% duty cycle) to the SYNC pin. The square wave
amplitude should have valleys that are below 0.4V and
peaks above 2.4V (up to 6V).
The LT8610AC/LT8610AC-1 will not enter Burst Mode
operation at low output loads while synchronized to an
external clock, but instead will pulse skip to maintain
regulation. The LT8610AC may be synchronized over
a 200kHz to 2.2MHz range, while the LT8610AC-1 can
only be synchronized over a 1.5MHz to 2.2MHz range.
The RT resistor should be chosen to set the LT8610AC/
LT8610AC-1 switching frequency equal to or below the
lowest synchronization input. For example, if the synchro-
nization signal will be 500kHz and higher, the RT should
be selected for 500kHz. The slope compensation is set
by the RT value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
by the inductor size, input voltage, and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by RT, then the slope compensation will be
sufficient for all synchronization frequencies.
For some applications it is desirable for the LT8610AC/
LT8610AC-1 to operate in pulse-skipping mode, offering
two major differences from Burst Mode operation. First is
the clock stays awake at all times and all switching cycles
are aligned to the clock. Second is that full switching fre-
quency is reached at lower output load than in Burst Mode
operation. These two differences come at the expense of
increased quiescent current. To enable pulse-skipping
mode, the SYNC pin is tied high either to a logic output
or to the INTVCC pin.
The LT8610AC/LT8610AC-1 does not operate in forced
continuous mode regardless of SYNC signal. Never leave
the SYNC pin floating.
Shorted and Reversed Input Protection
The LT8610AC/LT8610AC-1 will tolerate a shorted output.
Several features are used for protection during output
short-circuit and brownout conditions. The first is the
switching frequency will be folded back while the output
is lower than the set point to maintain inductor current
control. Second, the bottom switch current is monitored
such that if inductor current is beyond safe levels switch-
ing of the top switch will be delayed until such time as the
inductor current falls to safe levels.
The LT8610AC-1 has a feature where the TR/SS pin voltage
will be regulated to the same voltage as the FB pin. This
means the part will soft-start out of an output short-circuit
or brownout condition preventing output voltage overshoot.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the pro-
grammed level. If the SYNC pin is connected to a clock
source or tied high, the LT8610AC/LT8610AC-1 will stay
at the programmed frequency without foldback and only
slow switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where the
output will be held high when the input to the LT8610AC/
LT8610AC-1 is absent. This may occur in battery charg-
LT8610AC/LT8610AC-1
19
8610acfa
For more information www.linear.com/LT8610AC
Figure 4. Reverse VIN Protection
VIN
V
IN
D1
LT8610AC
EN/UV
8610ac1
F04
GND
Figure 5. Recommended PCB Layout for the LT8610AC
VOUT
8610ac1
F05
OUTLINE OF LOCAL
GROUND PLANE
SW
BST
BIAS
INTVCC
GND
9
10
11
12
13
14
15 PG
FB
GND
VOUT
16
SYNC
TR/SS
RT
EN/UV
VIN
1
2
3
4
5
6
7
8
VOUT LINE TO BIAS VIAS TO GROUND PLANE
ing applications or in battery-backup systems where
a battery or some other supply is diode ORed with the
LT8610AC/LT8610AC-1’s output. If the VIN pin is allowed
to float and the EN pin is held high (either by a logic
signal or because it is tied to VIN), then the LT8610AC/
LT8610AC-1’s internal circuitry will pull its quiescent cur-
rent through its SW pin. This is acceptable if the system can
tolerate several μA in this state. If the EN pin is grounded the
SW pin current will drop to near 1µA. However, if the VIN pin
is grounded while the output is held high, regardless of EN,
parasitic body diodes inside the LT8610AC/LT8610AC-1
can pull current from the output through the SW pin and
the VIN pin. Figure 4 shows a connection of the VIN and
EN/UV pins that will allow the LT8610AC/LT8610AC-1 to
run only when the input voltage is present and that protects
against a shorted or reversed input.
applicaTions inForMaTion
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 5 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT8610AC/LT8610AC-1’s VIN pins, GND pins,
and the input capacitor (C1). The loop formed by the input
capacitor should be as small as possible by placing the
capacitor adjacent to the VIN and GND pins. When using
a physically large input capacitor the resulting loop may
become too large in which case using a small case/value
capacitor placed close to the VIN and GND pins plus a larger
capacitor further away is preferred. These components,
along with the inductor and output capacitor, should be
placed on the same side of the circuit board, and their
connections should be made on that layer. Place a local,
unbroken ground plane under the application circuit on
the layer closest to the surface layer. The SW and BOOST
nodes should be as small as possible. Finally, keep the FB
and RT nodes small so that the ground traces will shield
them from the SW and BOOST nodes. The exposed pad on
the bottom of the package must be soldered to ground so
that the pad is connected to ground electrically and also
acts as a heat sink thermally. To keep thermal resistance
low, extend the ground plane as much as possible, and add
thermal vias under and near the LT8610AC/LT8610AC-1
to additional ground planes within the circuit board and
on the bottom side.
Unlike the LT8610, the LT8610AC/LT8610AC-1 has pin 7 as
an NC (no connect) pin. This pin can be soldered to GND to
have an LT8610 compatible PCB layout. Alternatively, pin 7
can be left unconnected to help meet PCB clearance and
creepage requirements between the VIN and GND traces.
LT8610AC/LT8610AC-1
20
8610acfa
For more information www.linear.com/LT8610AC
Typical applicaTions
BSTVIN
EN/UV
SYNC
INTVCC
TR/SS
RT
SW
LT8610AC-1
GND
BIAS
8610ac1 TA02
PG
FB
0.1µF
V
OUT
5V
3.5A
4.7µF
V
IN
5.5V TO 42V
1µF
10nF
10pF
2.2µH
1M
191k
fSW = 2MHz
L: COILCRAFT XAL 5030
18.2k
47µF*
1210
X5R
POWER GOOD
100k
5V 2MHz Step-Down Converter 12V Step-Down Converter
BSTVIN
EN/UV
SYNC
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA09
PG
FB
0.1µF
V
OUT
12V
3.5A
4.7µF
V
IN
12.5V TO 42V
1µF
10nF
10pF
10µH
1M
71.5k
fSW = 1MHz
41.2k
47µF*
1210
X5R
POWER GOOD
100k
L: VISHAY IHLP-2525CZ-01
*Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation.
Figure 6. LT8610AC Case Temperature Rise
INPUT VOLTAGE (V)
CASE TEMPERATURE RISE (°C)
120
140
16 24
36
8610ac1 F06
80
20
40
0
100
60
8 12 20 28 32
fSW = 2MHz
ILOAD = 2.5A
fSW = 2MHz
ILOAD = 3.5A
TA = 25°C
fSW = 1MHz
ILOAD = 3.5A
(LT8610AC)
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8610AC/LT8610AC-1. The exposed pad on the bottom
of the package must be soldered to a ground plane. This
ground should be tied to large copper layers below with
thermal vias; these layers will spread heat dissipated by
the LT8610AC/LT8610AC-1. Placing additional vias can
reduce thermal resistance further. The maximum load
current should be derated as the ambient temperature
approaches the maximum junction rating. Power dissipa-
tion within the LT8610AC/LT8610AC-1 can be estimated
by calculating the total power loss from an efficiency
measurement and subtracting the inductor loss. The die
temperature is calculated by multiplying the LT8610AC/
LT8610AC-1 power dissipation by the thermal resistance
from junction to ambient. The LT8610AC/LT8610AC-1
will stop switching and indicate a fault condition if safe
junction temperature is exceeded.
Temperature rise of the LT8610AC/LT8610AC-1 is worst
when operating at high load, high VIN, and high switching
applicaTions inForMaTion
frequency. If the case temperature is too high for a given
application, then either VIN, switching frequency, or load
current can be decreased to reduce the temperature to an
acceptable level. Figure 6 shows an example of how case
temperature can be managed by reducing VIN, switching
frequency, or load.
LT8610AC/LT8610AC-1
21
8610acfa
For more information www.linear.com/LT8610AC
1.8V 2MHz Step-Down Converter
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA06
FB
0.1µF
V
OUT
1.8V
3.5A
4.7µF
V
IN
3.0V TO 15V
(42V TRANSIENT)
1µF
10nF
4.7pF
1µH
1M
806k
fSW = 2MHz
18.2k
100µF*
1210
X5R
L: VISHAY IHLP-2020BZ-01
5V Step-Down Converter
BSTVIN
EN/UV
SYNC
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA03
PG
FB
0.1µF
V
OUT
5V
3.5A
4.7µF
V
IN
5.5V TO 42V
1µF
10nF
10pF
10µH
1M
191k
fSW = 400kHz
110k
100µF*
1210
X5R
POWER GOOD
100k
L: VISHAY IHLP-2525CZ-01
Typical applicaTions
*Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation.
LT8610AC/LT8610AC-1
22
8610acfa
For more information www.linear.com/LT8610AC
Typical applicaTions
*Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation.
3.3V 2MHz Step-Down Converter
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8610AC-1
GND
BIAS
8610ac1 TA04
FB
0.1µF
V
OUT
3.3V
3.5A
4.7µF
V
IN
3.8V TO 27V
(42V TRANSIENT)
1µF
10nF
4.7pF
2.2µH
1M
316k
fSW = 2MHz
18.2k
47µF*
1210
X5R
L: COILCRAFT XAL 5030
1.8V Step-Down Converter
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA07
FB
0.1µF
V
OUT
1.8V
3.5A
4.7µF
V
IN
3.0V TO 42V
1µF
10nF
4.7pF
4.7µH
1M
806k
fSW = 400kHz
110k
47µF*
×3
1210
X5R
L: VISHAY IHLP-2020BZ-01
LT8610AC/LT8610AC-1
23
8610acfa
For more information www.linear.com/LT8610AC
Typical applicaTions
3.3V Step-Down Converter
Ultralow EMI 5V 2.5A Step-Down Converter
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA05
FB
0.1µF
V
OUT
3.3V
3.5A
4.7µF
V
IN
3.8V TO 42V
1µF
10nF
4.7pF
8.2µH
1M
316k
fSW = 400kHz
110k
100µF*
1210
X5R
L: VISHAY IHLP-2525BD-01
BSTVIN
EN/UV
PG
SYNC
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA11
FB
0.1µF
V
OUT
5V
3.5A
4.7µF
VIN
5.5V TO 42V
1µF
10nF
10pF
4.7µH
4.7µH
1M
FB1
BEAD
191k
fSW = 800kHz
52.3k
4.7µF4.7µF
47µF*
1210
X5R
FB1: TDK MPZ2012S101A
L: VISHAY IHLP-2020BZ-01
*Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation.
LT8610AC/LT8610AC-1
24
8610acfa
For more information www.linear.com/LT8610AC
package DescripTion
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSOP (MSE16) 0213 REV F
0.53 ±0.152
(.021 ±.006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
TYP
0.86
(.034)
REF
0.50
(.0197)
BSC
16
16151413 121110
12345678
9
9
18
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
0.254
(.010) 0° – 6° TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.10
(.201)
MIN
3.20 – 3.45
(.126 – .136)
0.889 ±0.127
(.035 ±.005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 ±0.038
(.0120
±.0015)
TYP
0.50
(.0197)
BSC
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
2.845 ±0.102
(.112 ±.004)
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102
(.065 ±.004)
0.1016 ±0.0508
(.004 ±.002)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0.280 ±0.076
(.011 ±.003)
REF
4.90 ±0.152
(.193 ±.006)
DETAIL “B”
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.12 REF
0.35
REF
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)
LT8610AC/LT8610AC-1
25
8610acfa
For more information www.linear.com/LT8610AC
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 05/15 Added the LT8610AC-1 version
Added the LT8610AC-1 version and Description
Added the LT8610AC-1 version to Order Information
Added the LT8610AC-1 version to Electrical Characteristics and Note 2
Added the LT8610AC-1 version to Pin Functions
Added the LT8610AC-1 version to Operation section
Added the LT8610AC-1 version to Applications section
Added the LT8610AC-1 Typical Application
all
1
2
3
7
11
12 to 22
21, 22, 26
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.
LT8610AC/LT8610AC-1
26
8610acfa
For more information www.linear.com/LT8610AC
LINEAR TECHNOLOGY CORPORATION 2014
LT 0515 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT8610AC
relaTeD parTs
Typical applicaTion
3.3V and 1.8V with Ratio Tracking Ultralow IQ 2.5V, 3.3V Step-Down with LDO
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
FB
0.1µF
V
OUT1
3.3V
3.5A
4.7µF
V
IN
3.8V TO 42V
1µF
10nF
4.7pF
5.6µH
255k
80.6k
fSW = 500kHz
88.7k
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8610AC
GND
BIAS
8610ac1 TA08
FB
0.1µF
V
OUT2
1.8V
3.5A
4.7µF
1µF
4.7pF
3.3µH
100k
30.1k
80.6k
fSW = 500kHz
88.7k
10k
100µF*
1210
X5R
47µF*
1210
X5R
L: VISHAY IHLP-2020CZ-01, 5.6µH
L: VISHAY IHLP-2020CZ-01, 3.3µH
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8610AC-1
GND
BIAS
8610ac1 TA10
FB
0.1µF
VOUT1
3.3V
3.5A
4.7µF
V
IN
3.8V TO 27V
1µF
10nF
4.7pF
2.2µH
1M
316k 2.2µF
V
OUT2
2.5V
20mA
fSW = 2MHz
18.2k
47µF*
1210
X5R
IN
LT3008-2.5
SHDN
OUT
SENSE
L: VISHAY IHLP-2020BZ-01
PART NUMBER DESCRIPTION COMMENTS
LT8610A/
LT8610AB
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA,
MSOP-16E Package
LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA,
MSOP-16E Package
LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA and Input/Output Current Limit/Monitor
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA,
3mm × 5mm QFN-24 Package
LT8620 65V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 65V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA,
MSOP-16E and 3mm × 5mm QFN-24 Packages
LT8614 42V, 4A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA,
3mm × 4mm QFN Package
LT8612 42V, 6A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 3µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 3µA, ISD < 1µA,
3mm × 6mm QFN Package
LT3975 4.3V, 2.5A, 2.2MHz Micropower Step-Down DC/DC Converter with
IQ = 2.7µA
VIN: 4.3V to 42V, VOUT(MIN) = 1.2V, IQ = 2.7µA, ISD < 1µA,
MSOP-16E Package
LT3976 40V, 5A, 2.2MHz Micropower Step-Down DC/DC Converter with
IQ = 3.3µA
VIN: 4.3V to 40V, VOUT(MIN) = 1.2V, IQ = 3.3µA, ISD < 1µA,
MSOP-16E and 3mm × 5mm QFN-24 Packages
*Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation.