LT8620

65V, 2A Synchronous
Step-Down Regulator
with 2.5µA Quiescent
Current
FEATURES DESCRIPTION
nn Wide Input Voltage Range: 3.4V to 65V The LT®8620 is a compact, high efficiency, high speed
nn Ultralow Quiescent Current Burst Mode® Operation: synchronous monolithic step-down switching regulator
nn 2.5μA I Regulating 12V to 3.3V that accepts a wide input voltage range up to 65V, and
Q IN OUT
nn Output Ripple < 10mV consumes only 2.5µA of quiescent current. Top and bottom
P-P
nn High Efficiency Synchronous Operation: power switches are included with all necessary circuitry
nn 94% Efficiency at 1A, 12V to 5V to minimize the need for external components. Low ripple
IN OUT
nn 92% Efficiency at 1A, 12V to 3.3V Burst Mode operation enables high efficiency down to
IN OUT
nn Fast 30ns Minimum Switch-On Time very low output currents while keeping the output ripple
nn Low Dropout Under All Conditions: 250mV at 1A below 10mVP-P. A SYNC pin allows synchronization to an
nn Safely Tolerates Inductor Saturation in Overload external clock. Internal compensation with peak current
nn Low EMI mode topology allows the use of small inductors and
nn Adjustable and Synchronizable: 200kHz to 2.2MHz results in fast transient response and good loop stability.
nn Accurate 1V Enable Pin Threshold The EN/UV pin has an accurate 1V threshold and can be
nn Internal Compensation used to program VIN undervoltage lockout or to shut down
nn Output Soft-Start and Tracking the LT8620 reducing the input supply current to 1µA. A
nn Small Thermally Enhanced 16-Lead MSOP and capacitor on the TR/SS pin programs the output voltage
24-Lead 3mm × 5mm QFN Packages ramp rate during start-up. The PG flag signals when VOUT
is within ±9% of the programmed output voltage as well as
APPLICATIONS fault conditions. The LT8620 is available in small 16-Lead
MSOP and 3mm × 5mm QFN packages with exposed pads
nn Automotive and Industrial Supplies for low thermal resistance.
nn General Purpose Step-Down L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
nn GSM Power Supplies of Linear Technology Corporation. All other trademarks are the property of their respective
owners.

TYPICAL APPLICATION Efficiency at 5VOUT

5V 2A Step-Down Converter 100

VIN 95
VIN BST
5.5V TO 65V 0.1µF
4.7µF 90
EN/UV 4.7µH VOUT 85
PG SW 5V
EFFICIENCY (%)

LT8620 47µF 2A 80
SYNC BIAS
10nF 75
1M
TR/SS FB 70
1µF 10pF 65
INTVCC
60 fSW = 700kHz
RT GND
VIN = 12V
55
VIN = 24V
60.4k 243k
50
0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0
fSW = 700kHz 8620 TA01a LOAD CURRENT (A)
8620 TA01b

8620fa

www.datasheetall.com 1
LT8620
ABSOLUTE MAXIMUM RATINGS (Note 1)

VIN, EN/UV.................................................................65V SYNC Voltage ..............................................................6V

PG..............................................................................42V Operating Junction Temperature Range (Note 2)
BIAS...........................................................................25V LT8620E............................................. –40°C to 125°C
BST Pin Above SW Pin................................................4V LT8620I.............................................. –40°C to 125°C
FB, TR/SS, RT, INTVCC ................................................4V LT8620H............................................. –40°C to 150°C
LT8620MP.......................................... –55°C to 150°C
Storage Temperature Range................... –65°C to 150°C

PIN CONFIGURATION
TOP VIEW

NC
NC
NC
NC
24 23 22 21
SYNC 1 20 FB
TOP VIEW
TR/SS 2 19 PG
SYNC 1 16 FB
TR/SS 2 15 PG RT 3 18 BIAS
RT 3 14 BIAS
EN/UV 4 25 17 INTVCC
EN/UV 4 17 13 INTVCC
VIN 5 GND 12 BST VIN 5 GND 16 BST
VIN 6 11 SW
NC 7 10 SW VIN 6 15 SW
GND 8 9 SW NC 7 14 SW
MSE PACKAGE GND 8 13 SW
16-LEAD PLASTIC MSOP
9 10 11 12
θJA = 40°C/W, θJC(PAD) = 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
GND
NC
NC
NC
UDD PACKAGE
24-LEAD (3mm × 5mm) PLASTIC QFN
θJA = 46°C/W, θJC(PAD) = 5°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB

ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT8620EMSE#PBF LT8620EMSE#TRPBF 8620 16-Lead Plastic MSOP –40°C to 125°C
LT8620IMSE#PBF LT8620IMSE#TRPBF 8620 16-Lead Plastic MSOP –40°C to 125°C
LT8620HMSE#PBF LT8620HMSE#TRPBF 8620 16-Lead Plastic MSOP –40°C to 150°C
LT8620MPMSE#PBF LT8620MPMSE#TRPBF 8620 16-Lead Plastic MSOP –55°C to 150°C
LT8620EUDD#PBF LT8620EUDD#TRPBF LGGV 24-Lead (3mm × 5mm) Plastic QFN –40°C to 125°C
LT8620IUDD#PBF LT8620IUDD#TRPBF LGGV 24-Lead (3mm × 5mm) Plastic QFN –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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/

8620fa

2 www.datasheetall.com
 LT8620
ELECTRICAL CHARACTERISTICS 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
Minimum Input Voltage l 2.9 3.4 V
VIN Quiescent Current VEN/UV = 0V, VSYNC = 0V 1.0 3 µA
l 1.0 8 µA
VEN/UV = 2V, Not Switching, VSYNC = 0V 1.7 4 µA
l 1.7 10 µA
VEN/UV = 2V, Not Switching, VSYNC = 2V 0.28 0.5 mA
VIN Current in Regulation VOUT = 0.97V, VIN = 6V, Output Load = 100µA l 20 50 µA
VOUT = 0.97V, VIN = 6V, Output Load = 1mA l 200 350 µA
Feedback Reference Voltage VIN = 6V, ILOAD = 0.5A 0.964 0.970 0.976 V
VIN = 6V, ILOAD = 0.5A l 0.958 0.970 0.982 V
Feedback Voltage Line Regulation VIN = 4.0V to 42V, ILOAD = 0.5A l 0.004 0.02 %/V
Feedback Pin Input Current VFB = 1V –20 20 nA
INTVCC Voltage ILOAD = 0mA, VBIAS = 0V 3.23 3.4 3.57 V
ILOAD = 0mA, VBIAS = 3.3V 3.25 3.29 3.35 V
INTVCC Undervoltage Lockout 2.5 2.6 2.7 V
BIAS Pin Current Consumption VBIAS = 3.3V, ILOAD = 1A, 2MHz 7.2 mA
Minimum On-Time ILOAD = 1A, SYNC = 0V l 30 45 ns
ILOAD = 1A, SYNC = 3.3V l 30 45 ns
Minimum Off-Time 90 130 ns
Oscillator Frequency RT = 221k, ILOAD = 1A l 180 210 240 kHz
RT = 60.4k, ILOAD = 1A l 665 700 735 kHz
RT = 18.2k, ILOAD = 1A l 1.85 2.00 2.15 MHz
Top Power NMOS On-Resistance ISW = 1A 175 mΩ
Top Power NMOS Current Limit l 2.8 4.1 4.9 A
Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A 85 mΩ
Bottom Power NMOS Current Limit VINTVCC = 3.4V 2.9 3.9 4.7 A
SW Leakage Current VIN = 42V, VSW = 0V, 42V –1.5 1.5 µA
EN/UV Pin Threshold EN/UV Rising l 0.94 1.0 1.06 V
EN/UV Pin Hysteresis 40 mV
EN/UV Pin Current VEN/UV = 2V –20 20 nA
PG Upper Threshold Offset from VFB VFB Falling l 6 9.0 12 %
PG Lower Threshold Offset from VFB VFB Rising l –6 –9.0 –12 %
PG Hysteresis 1.3 %
PG Leakage VPG = 3.3V –40 40 nA
PG Pull-Down Resistance VPG = 0.1V l 680 2000 Ω
SYNC Threshold SYNC Falling 0.8 1.0 1.2 V
SYNC Rising 1.1 1.3 1.5 V
SYNC Pin Current VSYNC = 6V –100 100 nA
TR/SS Source Current l 1.2 2 2.7 µA
TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V 220 Ω
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may 150°C operating junction temperature range. The LT8620MP is 100%
cause permanent damage to the device. Exposure to any Absolute Maximum tested and guaranteed over the full −55°C to 150°C operating junction
Rating condition for extended periods may affect device reliability and lifetime. temperature range. High junction temperatures degrade operating lifetimes.
Note 2: The LT8620E is guaranteed to meet performance specifications Operating lifetime is derated at junction temperatures greater than 125°C.
from 0°C to 125°C junction temperature. Specifications over the –40°C Note 3: This IC includes overtemperature protection that is intended to protect
to 125°C operating junction temperature range are assured by design, the device during overload conditions. Junction temperature will exceed 150°C
characterization, and correlation with statistical process controls. The when overtemperature protection is active. Continuous operation above the
LT8620I is guaranteed over the full –40°C to 125°C operating junction specified maximum operating junction temperature will reduce lifetime.
temperature range. The LT8620H is guaranteed over the full −40°C to 8620fa

www.datasheetall.com 3
LT8620
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.

Efficiency at 5VOUT Efficiency at 3.3VOUT Efficiency at 5VOUT

100 100 100

95 95 90

90 90 80

85 85 70

EFFICIENCY (%)
EFFICIENCY (%)

EFFICIENCY (%)
80 80 60

75 75 50

70 fSW = 700kHz 70 fSW = 700kHz 40 fSW = 700kHz

L = IHLP2525CZ-01, 4.7µH L = IHLP2525CZ-01, 4.7µH 30 L = IHLP2525CZ-01, 4.7µH
65 65
VIN = 12V VIN = 12V VIN = 12V
60 60 20 VIN = 24V
VIN = 24V VIN = 24V
55 VIN = 36V 55 VIN = 36V 10 VIN = 36V
VIN = 48V VIN = 48V VIN = 48V
50 50 0
0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 0.01 0.1 1.0 10 100 1000
LOAD CURRENT (A) LOAD CURRENT (A) LOAD CURRENT (mA)
8620 G03
8620 G01 8620 G02

Efficiency at 3.3VOUT Efficiency vs Frequency at 1A Reference Voltage

100 94 0.979
VOUT = 3.3V
90 L = IHLP2525CZ-01, 4.7µH 0.977
92
80 0.975

REFERENCE VOLTAGE (V)

70 90
0.973
EFFICIENCY (%)
EFFICIENCY (%)

60
88 0.971
50
86 0.969
40 fSW = 700kHz
L = IHLP2525CZ-01, 4.7µH 0.967
30 84
VIN = 12V 0.965
20
VIN = 24V
82 VIN = 12V
10 VIN = 36V 0.963
VIN = 48V VIN = 24V
0 80 0.961
0.01 0.1 1.0 10 100 1000 0.25 0.75 1.25 1.75 2.25 –50 –25 0 25 50 75 100 125 150
LOAD CURRENT (mA) SWITCHING FREQUENCY (MHz) TEMPERATURE (°C)
8620 G04 8620 G05 8620 G06

EN Pin Thresholds Load Regulation Line Regulation

1.03 0.15 0.12
VOUT = 5V VOUT = 5V
1.02 VIN = 12V 0.10 ILOAD = 1A
0.1
0.08
1.01 EN RISING
0.06
CHANGE IN VOUT (%)
CHANGE IN VOUT (%)
EN THRESHOLD (V)

0.05
1.00 0.04
0.99 0 0.02

0.98 0.00
–0.05
–0.02
0.97 EN FALLING
–0.04
–0.1
0.96 –0.06
0.95 –0.15 –0.08
–50 –25 0 25 50 75 100 125 150 0 0.5 1 1.5 2 5 15 25 35 45 55 65
TEMPERATURE (°C) LOAD CURRENT (A) INPUT VOLTAGE (V)
8620 G07 8620 G08 8620 G09

8620fa

4 www.datasheetall.com
 LT8620
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.

No Load Supply Current Top FET Current Limit vs Duty Cycle Top FET Current Limit
5.0 4.0 5.0
VOUT = 3.3V
4.5 IN REGULATION
4.0
3.5 4.5
INPUT CURRENT (µA)

3.5

CURRENT LIMIT (A)

CURRENT LIMIT (A)
3.0
2.5 3.0 4.0
DUTY CYCLE = 5%
2.0
1.5
2.5 3.5
1.0
0.5
0 2.0 3.0
0 10 20 30 40 50 60 0 0.2 0.4 0.6 0.8 1.0 –55 –25 5 35 65 95 125 155
INPUT VOLTAGE (V) DUTY CYCLE TEMPERATURE (°C)
8620 G12
8620 G10 8620 G11

Bottom FET Current Limit Switch Drop Switch Drop

5.0 350 450
SWITCH CURRENT = 1A
400
300
350
4.5
250
SWITCH DROP (mV)

SWITCH DROP (mV)

CURRENT LIMIT (A)

300
TOP SWITCH
200 250
4.0 TOP SWITCH
150 200

150
100
3.5 100
BOTTOM SWITCH
50 BOTTOM SWITCH
50

3.0 0 0
–55 –25 5 35 65 95 125 155 –50 –25 0 25 50 75 100 125 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
TEMPERATURE (°C) TEMPERATURE (°C) SWITCH CURRENT (A)
8620 G13
8620 G14 8620 G15

Minimum On-Time Dropout Voltage Switching Frequency

34 600 740
ILOAD = 1A, VSYNC = 0V RT = 60.4k
ILOAD = 1A, VSYNC = 3V 730
32 ILOAD = 2A, VSYNC = 0V 500
SWITCHING FREQUENCY (kHz)

ILOAD = 2A, VSYNC = 3V 720

DROPOUT VOLTAGE (mV)
MINIMUM ON-TIME (ns)

30
400
710
28
300 700
26
690
200
24
680

22 100
670

20 0 660
–50 –25 0 25 50 75 100 125 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 –50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C) LOAD CURRENT (A) TEMPERATURE (°C)
8620 G16 8620 G17 8620 G18

8620fa

www.datasheetall.com 5
LT8620
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.

Minimum Load to Full Frequency

Burst Frequency (SYNC DC High) Frequency Foldback
800 100 800
VIN = 12V VOUT = 5V VOUT = 3.3V
700 VOUT = 5V fSW = 700kHz 700 VIN = 12V
VSYNC = 0V
SWITCHING FREQUENCY (kHz)

80

SWITCHING FREQUENCY (kHz)

600 600 RT = 60.4k

LOAD CURRENT (mA)

500 60 500

400 400

300 40 300

200 200
20
100 100

0 0 0
0 50 100 150 200 5 15 25 35 45 55 65 0 0.2 0.4 0.6 0.8 1
LOAD CURRENT (mA) INPUT VOLTAGE (V) FB VOLTAGE (V)
8620 G19 8620 G20 8620 G21

Soft-Start Tracking Soft-Start Current PG High Thresholds

1.2 2.2 12.0
VSS = 0.5V

PG THRESHOLD OFFSET FROM VREF (%)

11.5
1.0
11.0
2.1
SS PIN CURRENT (µA)

10.5
0.8
FB VOLTAGE (V)

FB RISING
10.0
0.6 2.0 9.5
9.0 FB FALLING
0.4
8.5
1.9 8.0
0.2
7.5
0 7.0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 –50 –25 0 25 50 75 100 125 150
TR/SS VOLTAGE (V) –55 –25 5 35 65 95 125 155 TEMPERATURE (°C)
TEMPERATURE (°C)
8620 G22 8620 G24
8620 G23

RT Programmed Switching
PG Low Thresholds Frequency VIN UVLO
–7.0 250 3.6
225
PG THRESHOLD OFFSET FROM VREF (%)

–7.5 3.4
–8.0 200
3.2
RT PIN RESISTOR (kΩ)

–8.5 175
INPUT VOLTAGE (V)

FB RISING 150 3.0

–9.0
–9.5 125 2.8
–10.0 100 2.6
FB FALLING
–10.5 75
2.4
–11.0 50
2.2
–11.5 25
–12.0 0 2.0
–50 –25 0 25 50 75 100 125 150 0.2 0.6 1 1.4 1.8 2.2 –55 –25 5 35 65 95 125 155
TEMPERATURE (°C) SWITCHING FREQUENCY (MHz) TEMPERATURE (°C)
8620 G25 8620 G26 8620 G27

8620fa

6 www.datasheetall.com
 LT8620
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.

Switching Waveforms, Full

Bias Pin Current Bias Pin Current Frequency Continuous Operation
4.5 10
VBIAS = 5V VBIAS = 5V
VOUT = 5V VOUT = 5V IL
ILOAD = 1A 8 VIN = 12V 1A/DIV
4.0 fSW = 700kHz ILOAD = 1A
BIAS PIN CURRENT (mA)

BIAS PIN CURRENT (mA)

6 VSW
5V/DIV
3.5
4
500ns/DIV 8620 G30

3.0 12VIN TO 5VOUT AT 1A

2

2.5 0
5 15 25 35 45 55 65 0 0.5 1 1.5 2 2.5
INPUT VOLTAGE (V) SWITCHING FREQUENCY (MHz)
8620 G28 8620 G29

Switching Waveforms, Transient Response; Load Current

Burst Mode Operation Switching Waveforms Stepped from 1A to 2A

IL
IL 1A/DIV ILOAD
200mA/DIV 1A/DIV

VSW VOUT
VSW 20V/DIV 100mV/DIV
5V/DIV

2µs/DIV 8620 G31

500ns/DIV 8620 G32
50µs/DIV 8620 G33

12VIN TO 5VOUT AT 10mA 48VIN TO 5VOUT AT 1A 1A TO 2A TRANSIENT

VSYNC = 0V 12VIN, 5VOUT
COUT = 47µF

Transient Response; Load Current

Stepped from 50mA
(Burst Mode Operation) to 1A Start-Up Dropout Performance Start-Up Dropout Performance

ILOAD
1A/DIV VIN VIN
VIN VIN
VOUT 2V/DIV 2V/DIV
VOUT VOUT
200mV/DIV VOUT VOUT
2V/DIV 2V/DIV

50µs/DIV 8620 G34

100ms/DIV 8620 G35
100ms/DIV 8620 G36

50mA (Burst Mode Operation) TO 1A TRANSIENT 2.5Ω LOAD 20Ω LOAD

12VIN, 5VOUT (2A IN REGULATION) (250mA IN REGULATION)
COUT = 47µF

8620fa

www.datasheetall.com 7
LT8620
PIN FUNCTIONS
SYNC: External Clock Synchronization Input. Ground this SW: The SW pins are the outputs of the internal power
pin for low ripple Burst Mode operation at low output loads. switches. Tie these pins together and connect them to the
Tie to a clock source for synchronization to an external inductor and boost capacitor. This node should be kept
frequency. Apply a DC voltage of 3V or higher or tie to small on the PCB for good performance.
INTVCC for pulse-skipping mode. When in pulse-skipping BST: This pin is used to provide a drive voltage, higher
mode, the IQ will increase to several hundred µA. Do not
than the input voltage, to the topside power switch. Place
float this pin. a 0.1µF boost capacitor as close as possible to the IC.
TR/SS: Output Tracking and Soft-Start Pin. This pin allows
INTVCC: Internal 3.4V Regulator Bypass Pin. The in-
user control of output voltage ramp rate during start-up. A ternal power drivers and control circuits are powered
TR/SS voltage below 0.97V forces the LT8620 to regulate from this voltage. INTVCC maximum output current is
the FB pin to equal the TR/SS pin voltage. When TR/SS 20mA. Do not load the INTVCC pin with external cir-
is above 0.97V, the tracking function is disabled and the cuitry. INTVCC current will be supplied from BIAS if
internal reference resumes control of the error amplifier. VBIAS > 3.1V, otherwise current will be drawn from VIN.
An internal 2μA pull-up current from INTVCC on this pin Voltage on INTVCC will vary between 2.8V and 3.4V when
allows a capacitor to program output voltage slew rate. VBIAS is between 3.0V and 3.6V. Decouple this pin to power
This pin is pulled to ground with an internal 220Ω MOS- ground with at least a 1μF low ESR ceramic capacitor
FET during shutdown and fault conditions; use a series placed close to the IC.
resistor if driving from a low impedance output. This pin
may be left floating if the tracking function is not needed. BIAS: The internal regulator will draw current from BIAS
instead of VIN when BIAS is tied to a voltage higher than
RT: A resistor is tied between RT and ground to set the 3.1V. For output voltages of 3.3V to 25V, this pin should
switching frequency. be tied to VOUT. If this pin is tied to a supply other than
EN/UV: The LT8620 is shut down when this pin is low and VOUT use a 1µF local bypass capacitor on this pin. If no
active when this pin is high. The hysteretic threshold volt- supply is available, tie to ground.
age is 1.00V going up and 0.96V going down. Tie to VIN PG: The PG pin is the open-drain output of an internal
if the shutdown feature is not used. An external resistor
comparator. PG remains low until the FB pin is within
divider from VIN can be used to program a VIN threshold
±9% of the final regulation voltage, and there are no fault
below which the LT8620 will shut down. conditions. PG is valid when VIN is above 3.4V, regardless
VIN: The VIN pins supply current to the LT8620 internal of EN/UV pin state.
circuitry and to the internal topside power switch. These
FB: The LT8620 regulates the FB pin to 0.970V.
pins must be tied together and be locally bypassed. Be
Connect the feedback resistor divider tap to this pin. Also,
sure to place the positive terminal of the input capacitor
connect a phase lead capacitor between FB and VOUT.
as close as possible to the VIN pins, and the negative Typically, this capacitor is 4.7pF to 10pF.
capacitor terminal as close as possible to the PGND pins.
GND: Ground. The exposed pad must be connected to the
NC: No Connect. This pin is not connected to internal negative terminal of the input capacitor and soldered to
circuitry.
the PCB in order to lower the thermal resistance.

8620fa

8 www.datasheetall.com
 LT8620
BLOCK DIAGRAM

VIN
VIN
CIN –
INTERNAL 0.97V REF
+ 3.4V BIAS
R3 1V + REG
OPT EN/UV
– SHDN
SLOPE COMP
R4 INTVCC
OPT
CVCC
OSCILLATOR
ERROR BST
PG 200kHz TO 2.2MHz
±9% AMP

+ VC
BURST SWITCH
CBST
+ DETECT LOGIC L
VOUT – AND
SW
VOUT
ANTI-
SHDN SHOOT COUT
C1 R1 THROUGH
THERMAL SHDN
INTVCC UVLO
R2 FB VIN UVLO

CSS SHDN
2µA THERMAL SHDN
OPT VIN UVLO
TR/SS

RT RT

SYNC

GND

8620 BD

8620fa

www.datasheetall.com 9
LT8620
OPERATION
The LT8620 is a monolithic, constant frequency, current from the input supply when regulating with no load. The
mode step-down DC/DC converter. An oscillator, with SYNC pin is tied low to use Burst Mode operation and can
frequency set using a resistor on the RT pin, turns on be tied to a logic high to use pulse-skipping mode. If a
the internal top power switch at the beginning of each clock is applied to the SYNC pin the part will synchronize to
clock cycle. Current in the inductor then increases until an external clock frequency and operate in pulse-skipping
the top switch current comparator trips and turns off the mode. While in pulse-skipping mode the oscillator operates
top power switch. The peak inductor current at which continuously and positive SW transitions are aligned to
the top switch turns off is controlled by the voltage on the clock. During light loads, switch pulses are skipped
the internal VC node. The error amplifier servos the VC to regulate the output and the quiescent current will be
node by comparing the voltage on the VFB pin with an several hundred µA.
internal 0.97V reference. When the load current increases
To improve efficiency across all loads, supply current to
it causes a reduction in the feedback voltage relative to
internal circuitry can be sourced from the BIAS pin when
the reference leading the error amplifier to raise the VC
biased at 3.3V or above. Else, the internal circuitry will draw
voltage until the average inductor current matches the new
current from VIN. The BIAS pin should be connected to
load current. When the top power switch turns off, the VOUT if the LT8620 output is programmed at 3.3V or above.
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload Comparators monitoring the FB pin voltage will pull the
conditions result in more than 3.9A flowing through the PG pin low if the output voltage varies more than ±9%
bottom switch, the next clock cycle will be delayed until (typical) from the set point, or if a fault condition is present.
switch current returns to a safe level. The oscillator reduces the LT8620’s operating frequency
If the EN/UV pin is low, the LT8620 is shut down and when the voltage at the FB pin is low. This frequency
draws 1µA from the input. When the EN/UV pin is above foldback helps to control the inductor current when the
1V, the switching regulator will become active. output voltage is lower than the programmed value which
occurs during start-up or overcurrent conditions. When
To optimize efficiency at light loads, the LT8620 operates
a clock is applied to the SYNC pin or the SYNC pin is
in Burst Mode operation in light load situations. Between
held DC high, the frequency foldback is disabled and the
bursts, all circuitry associated with controlling the output
switching frequency will slow down only during overcur-
switch is shut down, reducing the input supply current to
rent conditions.
1.7μA. In a typical application, 2.5μA will be consumed

8620fa

10 www.datasheetall.com
 LT8620
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current In order to achieve higher light load efficiency, more energy
To enhance efficiency at light loads, the LT8620 operates must be delivered to the output during the single small
in low ripple Burst Mode operation, which keeps the out- pulses in Burst Mode operation such that the LT8620 can
stay in sleep mode longer between each pulse. This can be
put capacitor charged to the desired output voltage while
achieved by using a larger value inductor (i.e. 4.7µH), and
minimizing the input quiescent current and minimizing
should be considered independent of switching frequency
output voltage ripple. In Burst Mode operation the LT8620
when choosing an inductor. For example, while a lower
delivers single small pulses of current to the output capaci-
inductor value would typically be used for a high switch-
tor followed by sleep periods where the output power is
supplied by the output capacitor. While in sleep mode the ing frequency application, if high light load efficiency is
LT8620 consumes 1.7μA. desired, a higher inductor value should be chosen.
While in Burst Mode operation the current limit of the top
As the output load decreases, the frequency of single cur-
switch is approximately 400mA resulting in output voltage
rent pulses decreases (see Figure 1a) and the percentage
ripple shown in Figure 2. Increasing the output capacitance
of time the LT8620 is in sleep mode increases, resulting in
will decrease the output ripple proportionally. As load ramps
much higher light load efficiency than for typical convert-
upward from zero the switching frequency will increase
ers. By maximizing the time between pulses, the converter
but only up to the switching frequency programmed by
quiescent current approaches 2.5µA for a typical application
when there is no output load. Therefore, to optimize the the resistor at the RT pin as shown in Figure 1a. The out-
put load at which the LT8620 reaches the programmed
quiescent current performance at light loads, the current
frequency varies based on input voltage, output voltage,
in the feedback resistor divider must be minimized as it
and inductor choice.
appears to the output as load current.

Burst Frequency Minimum Load to Full Frequency (SYNC DC High)

800 100
VIN = 12V VOUT = 5V
700 VOUT = 5V fSW = 700kHz
SWITCHING FREQUENCY (kHz)

80
600
LOAD CURRENT (mA)

500 60

400

300 40

200
20
100

0 0
0 50 100 150 200 5 15 25 35 45 55 65
LOAD CURRENT (mA) INPUT VOLTAGE (V)
8620 F01a 8620 F01b

(1a) (1b)

Figure 1. SW Frequency vs Load Information in Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)

8620fa

www.datasheetall.com 11
LT8620
APPLICATIONS INFORMATION
For some applications it is desirable for the LT8620 to where 1.7µA is the quiescent current of the LT8620 and
operate in pulse-skipping mode, offering two major differ- the second term is the current in the feedback divider
ences from Burst Mode operation. First is the clock stays reflected to the input of the buck operating at its light
awake at all times and all switching cycles are aligned to load efficiency n. For a 3.3V application with R1 = 1M and
the clock. In this mode much of the internal circuitry is R2 = 412k, the feedback divider draws 2.3µA. With VIN =
awake at all times, increasing quiescent current to several 12V and n = 80%, this adds 0.8µA to the 1.7µA quiescent
hundred µA. Second is that full switching frequency is current resulting in 2.5µA no-load current from the 12V
reached at lower output load than in Burst Mode operation supply. Note that this equation implies that the no-load
(see Figure 1b). To enable pulse-skipping mode, the SYNC current is a function of VIN; this is plotted in the Typical
pin is tied high either to a logic output or to the INTVCC Performance Characteristics section.
pin. When a clock is applied to the SYNC pin the LT8620
When using large FB resistors, a 4.7pF to 10pF phase-lead
will also operate in pulse-skipping mode.
capacitor should be connected from VOUT to FB.

Setting the Switching Frequency

VSW
5V/DIV The LT8620 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.2MHz
IL by using a resistor tied from the RT pin to ground. A table
500mA/DIV
VOUT
showing the necessary RT value for a desired switching
10mV/DIV frequency is in Table 1.
2µs/DIV 8620 F02
The RT resistor required for a desired switching frequency
12VIN TO 5VOUT AT 10mA
VSYNC = 0V can be calculated using:
Figure 2. Burst Mode Operation 46.5
RT = – 5.2 (3)
fSW
FB Resistor Network
where RT is in kΩ and fSW is the desired switching fre-
The output voltage is programmed with a resistor divider
quency in MHz.
between the output and the FB pin. Choose the resistor
Table 1. SW Frequency vs RT Value
values according to:
fSW (MHz) RT (kΩ)
 V  (1)
R1= R2  OUT – 1 0.2 232
 0.970V  0.3 150
0.4 110
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage 0.5 88.7

accuracy. 0.6 71.5

0.7 60.4
If low input quiescent current and good light-load efficiency 0.8 52.3
are desired, use large resistor values for the FB resistor 1.0 41.2
divider. The current flowing in the divider acts as a load
1.2 33.2
current, and will increase the no-load input current to the
14 28.0
converter, which is approximately:
1.6 23.7
 V  V  1  1.8 20.5
IQ = 1.7µA +  OUT  OUT   (2)
 R1+R2  VIN  n  2.0 18.2
2.2 15.8
8620fa

12 www.datasheetall.com
 LT8620
APPLICATIONS INFORMATION
where VIN(MIN) is the minimum input voltage without
Operating Frequency Selection and Trade-Offs skipped cycles, VOUT is the output voltage, VSW(TOP) and
Selection of the operating frequency is a trade-off between VSW(BOT) are the internal switch drops (~0.3V, ~0.15V,
efficiency, component size, and input voltage range. The respectively at maximum load), fSW is the switching fre-
advantage of high frequency operation is that smaller induc- quency (set by RT), and tOFF(MIN) is the minimum switch
tor and capacitor values may be used. The disadvantages off-time. Note that higher switching frequency will increase
are lower efficiency and a smaller input voltage range. the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
The highest switching frequency (fSW(MAX)) for a given
application can be calculated as follows: Inductor Selection and Maximum Output Current
VOUT + VSW(BOT) The LT8620 is designed to minimize solution size by
fSW(MAX) = (4)
tON(MIN) ( VIN – VSW(TOP) + VSW(BOT) ) allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
where VIN is the typical input voltage, VOUT is the output short-circuit conditions the LT8620 safely tolerates opera-
voltage, VSW(TOP) and VSW(BOT) are the internal switch tion with a saturated inductor through the use of a high
drops (~0.3V, ~0.15V, respectively at maximum load) speed peak-current mode architecture.
and tON(MIN) is the minimum top switch on-time (see the
A good first choice for the inductor value is:
Electrical Characteristics). This equation shows that a
slower switching frequency is necessary to accommodate VOUT + VSW(BOT)
a high VIN/VOUT ratio. L= (6)
fSW
For transient operation, VIN may go as high as the abso- where fSW is the switching frequency in MHz, VOUT is
lute maximum rating of 65V regardless of the RT value, the output voltage, VSW(BOT) is the bottom switch drop
however the LT8620 will reduce switching frequency as (~0.15V) and L is the inductor value in μH.
necessary to maintain control of inductor current to as-
sure safe operation. To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
The LT8620 is capable of a maximum duty cycle of ap- the maximum expected output load of the application. In
proximately 99%, and the VIN-to-VOUT dropout is limited addition, the saturation current (typically labeled ISAT)
by the RDS(ON) of the top switch. In this mode the LT8620 rating of the inductor must be higher than the load current
skips switch cycles, resulting in a lower switching frequency plus 1/2 of in inductor ripple current:
than programmed by RT.
1
For applications that cannot allow deviation from the pro- IL(PEAK) = ILOAD(MAX) + ∆IL (7)
grammed switching frequency at low VIN/VOUT ratios use 2
the following formula to set switching frequency: where ∆IL is the inductor ripple current as calculated in
VOUT + VSW(BOT) Equation 9 and ILOAD(MAX) is the maximum output load
VIN(MIN) = – VSW(BOT) + VSW(TOP) (5) for a given application.
1– fSW • tOFF(MIN)

8620fa

www.datasheetall.com 13
LT8620
APPLICATIONS INFORMATION
As a quick example, an application requiring 1A output The optimum inductor for a given application may differ
should use an inductor with an RMS rating of greater than from the one indicated by this design guide. A larger value
1A and an ISAT of greater than 1.3A. During long duration inductor provides a higher maximum load current and
overload or short-circuit conditions, the inductor RMS reduces the output voltage ripple. For applications requir-
routing requirement is greater to avoid overheating of the ing smaller load currents, the value of the inductor may
inductor. To keep the efficiency high, the series resistance be lower and the LT8620 may operate with higher ripple
(DCR) should be less than 0.04Ω, and the core material current. This allows use of a physically smaller inductor,
should be intended for high frequency applications. or one with a lower DCR resulting in higher efficiency. Be
aware that low inductance may result in discontinuous
The LT8620 limits the peak switch current in order to
mode operation, which further reduces maximum load
protect the switches and the system from overload faults.
current.
The top switch current limit (ILIM) is at least 3.8A at low
duty cycles and decreases linearly to 2.8A at DC = 0.8. The For more information about maximum output current
inductor value must then be sufficient to supply the desired and discontinuous operation, see Linear Technology’s
maximum output current (IOUT(MAX)), which is a function Application Note 44.
of the switch current limit (ILIM) and the ripple current. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
∆IL a minimum inductance is required to avoid sub-harmonic
IOUT(MAX) = ILIM – (8)
2 oscillation. See Application Note 19.

The peak-to-peak ripple current in the inductor can be Input Capacitor

calculated as follows:
Bypass the input of the LT8620 circuit with a ceramic ca-
VOUT  V  pacitor of X7R or X5R type placed as close as possible to
∆IL = • 1– OUT  (9) the VIN and PGND pins. Y5V types have poor performance
L • fSW  VIN(MAX)  over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
where fSW is the switching frequency of the LT8620, and
bypass the LT8620 and will easily handle the ripple current.
L is the value of the inductor. Therefore, the maximum
Note that larger input capacitance is required when a lower
output current that the LT8620 will deliver depends on
switching frequency is used. If the input power source has
the switch current limit, the inductor value, and the input
high impedance, or there is significant inductance due to
and output voltages. The inductor value may have to be
long wires or cables, additional bulk capacitance may be
increased if the inductor ripple current does not allow
necessary. This can be provided with a low performance
sufficient maximum output current (IOUT(MAX)) given the
electrolytic capacitor.
switching frequency, and maximum input voltage used in
the desired application. Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
In order to achieve higher light load efficiency, more energy
capacitor is required to reduce the resulting voltage
must be delivered to the output during the single small
ripple at the LT8620 and to force this very high frequency
pulses in Burst Mode operation such that the LT8620 can
switching current into a tight local loop, minimizing EMI.
stay in sleep mode longer between each pulse. This can be
A 4.7μF capacitor is capable of this task, but only if it is
achieved by using a larger value inductor (i.e. 4.7µH), and
placed close to the LT8620 (see the PCB Layout section).
should be considered independent of switching frequency
A second precaution regarding the ceramic input capaci-
when choosing an inductor. For example, while a lower
tor concerns the maximum input voltage rating of the
inductor value would typically be used for a high switch-
LT8620. A ceramic input capacitor combined with trace
ing frequency application, if high light load efficiency is
or cable inductance forms a high quality (under damped)
desired, a higher inductor value should be chosen.
8620fa

14 www.datasheetall.com
 LT8620
APPLICATIONS INFORMATION
tank circuit. If the LT8620 circuit is plugged into a live Burst Mode operation, the noise is typically very quiet to a
supply, the input voltage can ring to twice its nominal casual ear. If this is unacceptable, use a high performance
value, possibly exceeding the LT8620’s voltage rating. tantalum or electrolytic capacitor at the output. Low noise
This situation is easily avoided (see Linear Technology ceramic capacitors are also available.
Application Note 88). A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8620. As
Output Capacitor and Output Ripple
previously mentioned, a ceramic input capacitor combined
The output capacitor has two essential functions. Along with trace or cable inductance forms a high quality (un-
with the inductor, it filters the square wave generated derdamped) tank circuit. If the LT8620 circuit is plugged
by the LT8620 to produce the DC output. In this role it into a live supply, the input voltage can ring to twice its
determines the output ripple, thus low impedance at the nominal value, possibly exceeding the LT8620’s rating.
switching frequency is important. The second function This situation is easily avoided (see Linear Technology
is to store energy in order to satisfy transient loads and Application Note 88).
stabilize the LT8620’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and Enable Pin
provide the best ripple performance. For good starting
The LT8620 is in shutdown when the EN pin is low and
values, see the Typical Applications section.
active when the pin is high. The rising threshold of the EN
Use X5R or X7R types. This choice will provide low output comparator is 1.0V, with 40mV of hysteresis. The EN pin
ripple and good transient response. Transient performance can be tied to VIN if the shutdown feature is not used, or
can be improved with a higher value output capacitor and tied to a logic level if shutdown control is required.
the addition of a feedforward capacitor placed between Adding a resistor divider from VIN to EN programs the
VOUT and FB. Increasing the output capacitance will also LT8620 to regulate the output only when VIN is above a
decrease the output voltage ripple. A lower value of output desired voltage (see the Block Diagram). Typically, this
capacitor can be used to save space and cost but transient threshold, VIN(EN), is used in situations where the input
performance will suffer and may cause loop instability. See supply is current limited, or has a relatively high source
the Typical Applications in this data sheet for suggested resistance. A switching regulator draws constant power
capacitor values. from the source, so source current increases as source
When choosing a capacitor, special attention should be voltage drops. This looks like a negative resistance load
given to the data sheet to calculate the effective capacitance to the source and can cause the source to current limit or
under the relevant operating conditions of voltage bias and latch low under low source voltage conditions. The VIN(EN)
temperature. A physically larger capacitor or one with a threshold prevents the regulator from operating at source
higher voltage rating may be required. voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
Ceramic Capacitors they satisfy the following equation:
Ceramic capacitors are small, robust and have very low  R3 
ESR. However, ceramic capacitors can cause problems VIN(EN) =  + 1 • 1.0V (10)
 R4 
when used with the LT8620 due to their piezoelectric
nature. When in Burst Mode operation, the LT8620’s where the LT8620 will remain off until VIN is above VIN(EN).
switching frequency depends on the load current, and Due to the comparator’s hysteresis, switching will not stop
at very light loads the LT8620 can excite the ceramic until the input falls slightly below VIN(EN).
capacitor at audio frequencies, generating audible noise.
Since the LT8620 operates at a lower current limit during

8620fa

www.datasheetall.com 15
LT8620
APPLICATIONS INFORMATION
When operating in Burst Mode operation for light load An active pull-down circuit is connected to the TR/SS pin
currents, the current through the VIN(EN) resistor network which will discharge the external soft-start capacitor in
can easily be greater than the supply current consumed the case of fault conditions and restart the ramp when the
by the LT8620. Therefore, the VIN(EN) resistors should be faults are cleared. Fault conditions that clear the soft-start
large to minimize their effect on efficiency at low loads. capacitor are the EN/UV pin transitioning low, VIN voltage
falling too low, or thermal shutdown.
INTVCC Regulator
Output Power Good
An internal low dropout (LDO) regulator produces the 3.4V
supply from VIN that powers the drivers and the internal When the LT8620’s output voltage is within the ±9%
bias circuitry. The INTVCC can supply enough current for window of the regulation point, which is a VFB voltage in
the LT8620’s circuitry and must be bypassed to ground the range of 0.883V to 1.057V (typical), the output voltage
with a minimum of 1μF ceramic capacitor. Good bypassing is considered good and the open-drain PG pin goes high
is necessary to supply the high transient currents required impedance and is typically pulled high with an external
by the power MOSFET gate drivers. To improve efficiency resistor. Otherwise, the internal pull-down device will pull
the internal LDO can also draw current from the BIAS the PG pin low. To prevent glitching both the upper and
pin when the BIAS pin is at 3.1V or higher. Typically the lower thresholds include 1.3% of hysteresis.
BIAS pin can be tied to the output of the LT8620, or can The PG pin is also actively pulled low during several fault
be tied to an external supply of 3.3V or above. If BIAS is conditions: EN/UV pin is below 1V, INTVCC has fallen too
connected to a supply other than VOUT, be sure to bypass low, VIN is too low, or thermal shutdown.
with a local ceramic capacitor. If the BIAS pin is below
3.0V, the internal LDO will consume current from VIN. Synchronization
Applications with high input voltage and high switching
frequency where the internal LDO pulls current from VIN To select low ripple Burst Mode operation, tie the SYNC pin
will increase die temperature because of the higher power below 0.4V (this can be ground or a logic low output). To
dissipation across the LDO. Do not connect an external synchronize the LT8620 oscillator to an external frequency
load to the INTVCC pin. connect a square wave (with 20% to 80% duty cycle) to
the SYNC pin. The square wave amplitude should have val-
Output Voltage Tracking and Soft-Start leys that are below 0.4V and peaks above 1.5V (up to 6V).
The LT8620 allows the user to program its output voltage The LT8620 will not enter Burst Mode operation at low
ramp rate by means of the TR/SS pin. An internal 2μA output loads while synchronized to an external clock, but
pulls up the TR/SS pin to INTVCC. Putting an external instead will pulse skip to maintain regulation. The LT8620
capacitor on TR/SS enables soft starting the output to pre- may be synchronized over a 200kHz to 2.2MHz range. The
vent current surge on the input supply. During the soft-start RT resistor should be chosen to set the LT8620 switching
ramp the output voltage will proportionally track the TR/SS frequency equal to or below the lowest synchronization
pin voltage. For output tracking applications, TR/SS can input. For example, if the synchronization signal will be
be externally driven by another voltage source. From 0V to 500kHz and higher, the RT should be selected for 500kHz.
0.97V, the TR/SS voltage will override the internal 0.97V The slope compensation is set by the RT value, while the
reference input to the error amplifier, thus regulating the minimum slope compensation required to avoid subhar-
FB pin voltage to that of TR/SS pin. When TR/SS is above monic oscillations is established by the inductor size,
0.97V, tracking is disabled and the feedback voltage will input voltage, and output voltage. Since the synchroniza-
regulate to the internal reference voltage. The TR/SS pin tion frequency will not change the slopes of the inductor
may be left floating if the function is not needed. current waveform, if the inductor is large enough to avoid

8620fa

16 www.datasheetall.com
 LT8620
APPLICATIONS INFORMATION
subharmonic oscillations at the frequency set by RT, then (either by a logic signal or because it is tied to VIN), then
the slope compensation will be sufficient for all synchro- the LT8620’s internal circuitry will pull its quiescent current
nization frequencies. through its SW pin. This is acceptable if the system can
tolerate several μA in this state. If the EN pin is grounded
For some applications it is desirable for the LT8620 to
the SW pin current will drop to near 1µA. However, if the
operate in pulse-skipping mode, offering two major differ-
VIN pin is grounded while the output is held high, regard-
ences from Burst Mode operation. First is the clock stays
less of EN, parasitic body diodes inside the LT8620 can
awake at all times and all switching cycles are aligned to
pull current from the output through the SW pin and
the clock. Second is that full switching frequency is reached
at lower output load than in Burst Mode operation. These the VIN pin. Figure 3 shows a connection of the VIN and
EN/UV pins that will allow the LT8620 to run only when
two differences come at the expense of increased quiescent
the input voltage is present and that protects against a
current. To enable pulse-skipping mode, the SYNC pin is
shorted or reversed input.
tied high either to a logic output or to the INTVCC pin.
D1
The LT8620 does not operate in forced continuous mode
VIN VIN
regardless of SYNC signal. Never leave the SYNC pin LT8620
floating. EN/UV
GND
Shorted and Reversed Input Protection 8620 F03

The LT8620 will tolerate a shorted output. Several features

are used for protection during output short-circuit and Figure 3. Reverse VIN Protection
brownout conditions. The first is the switching frequency
will be folded back while the output is lower than the set PCB Layout
point to maintain inductor current control. Second, the
bottom switch current is monitored such that if inductor For proper operation and minimum EMI, care must be taken
current is beyond safe levels switching of the top switch during printed circuit board layout. Figure 4 shows the
will be delayed until such time as the inductor current recommended component placement with trace, ground
falls to safe levels. plane and via locations. Note that large, switched currents
flow in the LT8620’s VIN pins, GND pins, and the input
Frequency foldback behavior depends on the state of the capacitor. The loop formed by the input capacitor should
SYNC pin: If the SYNC pin is low the switching frequency be as small as possible by placing the capacitor adjacent
will slow while the output voltage is lower than the pro- to the VIN and GND pins. When using a physically large
grammed level. If the SYNC pin is connected to a clock input capacitor the resulting loop may become too large
source or tied high, the LT8620 will stay at the programmed in which case using a small case/value capacitor placed
frequency without foldback and only slow switching if the close to the VIN and GND pins plus a larger capacitor
inductor current exceeds safe levels. further away is preferred. These components, along with
There is another situation to consider in systems where the inductor and output capacitor, should be placed on the
the output will be held high when the input to the LT8620 same side of the circuit board, and their connections should
is absent. This may occur in battery charging applications be made on that layer. Place a local, unbroken ground
or in battery-backup systems where a battery or some plane under the application circuit on the layer closest to
other supply is diode ORed with the LT8620’s output. If the surface layer. The SW and BOOST nodes should be
the VIN pin is allowed to float and the EN pin is held high as small as possible. Finally, keep the FB and RT nodes

8620fa

www.datasheetall.com 17
LT8620
APPLICATIONS INFORMATION
small so that the ground traces will shield them from the must be soldered to a ground plane. This ground should
SW and BOOST nodes. The exposed pad on the bottom of be tied to large copper layers below with thermal vias;
the package must be soldered to ground so that the pad these layers will spread heat dissipated by the LT8620.
is connected to ground electrically and also acts as a heat Placing additional vias can reduce thermal resistance
sink thermally. To keep thermal resistance low, extend the further. The maximum load current should be derated
ground plane as much as possible, and add thermal vias as the ambient temperature approaches the maximum
under and near the LT8620 to additional ground planes junction rating. Power dissipation within the LT8620 can
within the circuit board and on the bottom side. be estimated by calculating the total power loss from an
efficiency measurement and subtracting the inductor loss.
High Temperature Considerations The die temperature is calculated by multiplying the LT8620
For higher ambient temperatures, care should be taken in power dissipation by the thermal resistance from junction
the layout of the PCB to ensure good heat sinking of the to ambient. The LT8620 will stop switching and indicate
LT8620. The exposed pad on the bottom of the package a fault condition if safe junction temperature is exceeded.

GND GND
NC NC NC NC
24 23 22 21

VOUT VOUT
SYNC 1 16 FB SYNC 1 20 FB

TR/SS 2 15 PG TR/SS 2 19 PG

RT 3 14 BIAS RT 3 18 BIAS

EN/UV 4 13 INTVCC EN/UV 4 17 INTVCC

5 12 BST 5 16 BST
VIN VIN
6 11 6 15

7 10 7 14

8 9 8 13
SW SW

GND 9 10 11 12
GND GND NC NC NC

VOUT
VOUT

8620 F04
VOUT LINE TO BIAS VIAS TO GROUND PLANE OUTLINE OF LOCAL
8620 F04
VOUT LINE TO BIAS VIAS TO GROUND PLANE OUTLINE OF LOCAL
GROUND PLANE
GROUND PLANE

Figure 4a. Recommended PCB Layout for the Figure 4b. Recommended PCB Layout for the
LT8620 MSOP Package LT8620 QFN Package

8620fa

18 www.datasheetall.com
 LT8620
TYPICAL APPLICATIONS
5V 2MHz Step-Down Converter
VIN
VIN BST
5.5V TO 65V 0.1µF
4.7µF EN/UV 2.2µH VOUT
LT8620 SW 5V
SYNC BIAS 47µF 2A
10nF 100k
TR/SS PG POWER GOOD
1µF 1M
FB
INTVCC 10pF
RT GND

18.2k 243k

fSW = 2MHz
8620 TA02
L: XFL4020

5V Step-Down Converter
VIN
VIN BST
5.5V TO 65V 0.1µF
4.7µF EN/UV 4.7µH VOUT
LT8620 SW 5V
SYNC BIAS 47µF 2A
10nF 100k
TR/SS PG POWER GOOD
1µF 1M
FB
INTVCC 10pF
RT GND

60.4k 243k

fSW = 700kHz
8620 TA03
L: IHLP2020CZ-01

8620fa

www.datasheetall.com 19
LT8620
TYPICAL APPLICATIONS
3.3V 2MHz Step-Down Converter
VIN
3.8V TO 65V VIN BST
4.7µF 0.1µF
EN/UV 1.8µH VOUT
PG LT8620 SW 3.3V
SYNC BIAS 47µF 2A
10nF
TR/SS
1µF 1M
FB
INTVCC 10pF
RT GND

18.2k 412k

fSW = 2MHz
8620 TA04
L: XFL4020

3.3V Step-Down Converter

VIN
VIN BST
3.8V TO 65V 0.1µF
4.7µF EN/UV 4.7µH VOUT
PG LT8620 SW 3.3V
SYNC BIAS 47µF 2A
10nF
TR/SS
1µF 1M
FB
INTVCC 10pF
RT GND

60.4k 412k

fSW = 700kHz
8620 TA05
L: IHLP2020CZ-01

8620fa

20 www.datasheetall.com
 LT8620
TYPICAL APPLICATIONS
12V Step-Down Converter
VIN
VIN BST
12.5V TO 65V 0.1µF
4.7µF EN/UV 10µH VOUT
LT8620 SW 12V
SYNC BIAS 47µF 2A
10nF 100k
TR/SS PG POWER GOOD
1µF 1M
FB
INTVCC 10pF
RT GND

41.2k 88.7k

fSW = 1MHz
8620 TA09
L: IHLP2525CZ-01

1.8V 2MHz Step-Down Converter

VIN
3.4V TO 22V VIN BST
(65V TRANSIENT) 4.7µF 0.1µF
EN/UV 1µH VOUT
PG LT8620 SW 1.8V
EXTERNAL SOURCE 2A
SYNC BIAS >3.1V OR GND 100µF
10nF 1µF
TR/SS
1µF 866k
FB
INTVCC 10pF
RT GND

18.2k 1M

fSW = 2MHz
8620 TA06
L: XFL4020

8620fa

www.datasheetall.com 21
LT8620
TYPICAL APPLICATIONS
1.8V Step-Down Converter

VIN
VIN BST
3.4V TO 65V 0.1µF
4.7µF EN/UV 4.7µH VOUT
PG LT8620 SW 1.8V
SYNC BIAS
EXTERNAL SOURCE 120µF 2A
>3.1V OR GND
10nF 1µF
TR/SS
1µF 866k
FB
INTVCC 10pF
RT PGND GND

110k 1M

fSW = 400kHz
8620 TA07
L: IHLP2020CZ-01

Ultralow EMI 5V 2A Step-Down Converter

FB1
BEAD 4.7µH
VIN
VIN BST
5.5V TO 65V 0.1µF
4.7µF 4.7µF 4.7µF EN/UV 2.2µH VOUT
PG SW 5V
LT8620 2A
SYNC BIAS
10nF 47µF
1M
TR/SS FB
1µF 10pF
INTVCC
RT GND

18.2k 243k
FB1: TDK MPZ2012S221A
L: XFL4020 fSW = 2MHz 8620 TA11

8620fa

22 www.datasheetall.com
 LT8620
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)

BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102 2.845 ±0.102
(.112 ±.004) 0.889 ±0.127 (.112 ±.004)
(.035 ±.005)
1 8 0.35
REF

5.10 1.651 ±0.102

1.651 ±0.102 3.20 – 3.45
(.201) 0.12 REF
(.065 ±.004) (.126 – .136) (.065 ±.004)
MIN
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
16 9 FOR REFERENCE ONLY
0.305 ±0.038 0.50 NO MEASUREMENT PURPOSE
(.0120 ±.0015) (.0197) 4.039 ±0.102
TYP BSC (.159 ±.004)
(NOTE 3) 0.280 ±0.076
RECOMMENDED SOLDER PAD LAYOUT
16151413121110 9 (.011 ±.003)
REF
DETAIL “A”
0.254
(.010) 3.00 ±0.102
0° – 6° TYP 4.90 ±0.152
(.118 ±.004)
(.193 ±.006)
GAUGE PLANE (NOTE 4)

0.53 ±0.152
(.021 ±.006)
1234567 8
DETAIL “A” 1.10 0.86
0.18 (.043) (.034)
(.007) MAX REF

SEATING
PLANE 0.17 – 0.27 0.1016 ±0.0508
(.007 – .011) (.004 ±.002)
TYP 0.50
NOTE: (.0197)
MSOP (MSE16) 0213 REV F

1. DIMENSIONS IN MILLIMETER/(INCH) BSC

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.

8620fa

www.datasheetall.com 23
LT8620
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

UDD Package
24-Lead Plastic QFN (3mm × 5mm)
(Reference LTC DWG # 05-08-1833 Rev Ø)

0.70 ±0.05

3.50 ±0.05
2.10 ±0.05 3.65 ±0.05
1.50 REF
1.65 ±0.05

PACKAGE OUTLINE

0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ±0.05
5.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
PIN 1 NOTCH
R = 0.20 OR 0.25
0.75 ±0.05
1.50 REF × 45° CHAMFER
3.00 ±0.10 R = 0.05 TYP
23 24
0.40 ±0.10

PIN 1 1
TOP MARK
2
(NOTE 6)

3.65 ±0.10
5.00 ±0.10 3.50 REF
1.65 ±0.10

(UDD24) QFN 0808 REV Ø

0.200 REF 0.25 ±0.05

R = 0.115
0.00 – 0.05 TYP 0.50 BSC
BOTTOM VIEW—EXPOSED PAD

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 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE

8620fa

24 www.datasheetall.com
 LT8620
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 05/15 Added H- and MP-Grade Versions ABS Max Table, Order Information 2
Clarified Specifications to 150°C and Note 2 3
Clarified Current Limit Graphs 5
Clarified RT Programmed Switching Frequency, Soft-Start Current 6
Clarified TR/SS and BIAS Pin Function Description 8
Clarified Overload Conditions from 3.8A to 3.9A 10

8620fa

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-
For more
tion that the interconnection information
of its circuits www.linear.com/LT8620
as described herein will not infringe on existing patent rights. 25
LT8620
TYPICAL APPLICATION
Ultralow IQ 2.5V, 3.3V Step-Down with LDO

VIN
VIN BST
3.8V TO 65V 0.1µF
EN/UV 1.8µH
4.7µF VOUT1
PG LT8620 SW 3.3V
2A
SYNC BIAS
10nF 47µF
TR/SS
1µF 1M
FB
INTVCC 10pF
VOUT2
RT GND
IN OUT 2.5V
20mA
18.2k 412k LT3008-2.5
SHDN SENSE 2.2µF
fSW = 2MHz
L: XFL4020 8620 TA10

RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
DC/DC Converter with IQ = 2.5µA ISD < 1µA, MSOP-16E Package
LT8610A/ 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
LT8610AB DC/DC Converter with IQ = 2.5µA ISD < 1µA, MSOP-16E Package
LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
DC/DC Converter with IQ = 2.5µA and Input/Output Current Limit/Monitor ISD < 1µA, 3mm × 5mm QFN-24 Package
LT8612 42V, 6A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 3µA,
DC/DC Converter with IQ = 2.5µA ISD < 1µA, 3mm × 6mm QFN-28 Package
LT8614 42V, 4A, 96% Efficiency, 2.2MHz Silent Switcher Synchronous VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
Micropower Step-Down DC/DC Converter with IQ = 2.5µA ISD < 1µA, 3mm × 4mm QFN-20 Package
LT3690 36V with 60V Transient Protection, 4A, 92% Efficiency, 1.5MHz VIN: 3.9V to 36V, VOUT(MIN) = 0.985V, IQ = 70µA,
Synchronous Micropower Step-Down DC/DC Converter with IQ = 70µA ISD < 1µA, 4mm × 6mm QFN-26 Package
LT3991 55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC VIN: 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.8µA,
Converter with IQ = 2.8µA ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3990 62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC VIN: 4.2V to 65V, VOUT(MIN) = 1.21V, IQ = 2.5µA,
Converter with IQ = 2.5µA ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-6E Packages
LT3980 58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High Efficiency VIN: 3.6V to 58V, Transient to 80V, VOUT(MIN) = 0.78V,
Step-Down DC/DC Converter with Burst Mode Operation IQ = 85µA, ISD < 1µA, 3mm × 4mm DFN-16 and
MSOP-16E Packages

8620fa

26 Linear Technology Corporation

LT 0515 REV A • PRINTED IN USA

1630 McCarthy Blvd., Milpitas, CA 95035-7417

For more information www.linear.com/LT8620
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT8620  LINEAR TECHNOLOGY CORPORATION 2014