Data Sheet

LT8641A

analog.com

Rev 0

17 of 35

BLOCK DIAGRAM

THEORY OF OPERATION

The LT8641A 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 0.81V

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 5.8A flowing through the bottom switch, the

next clock cycle is delayed until switch current returns to a safe level.

If the EN/UV pin is low, the LT8641A is shut down and draws 0.75μA from the input. When the EN/UV pin is above

1.01V, the switching regulator becomes active.

To optimize efficiency at light loads, the LT8641A 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 is consumed from the input supply when regulating with no load. The

SYNC/MODE pin is tied low to use Burst Mode operation and can be floated to use pulse-skipping mode. If a clock is

applied to the SYNC/MODE pin, the part synchronizes to an external clock frequency and operates 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 becomes

+

+

–

SLOPE COMP

INTERNAL 0.8V REF

OSCILLATOR

200kHz TO 3MHz

BURST

DETECT

3.4V

REG

M1

M2

60kΩ

600kΩ

CBST

COUT

VOUT

SW

L

BST

8, 9, 21, 22

SWITCH

LOGIC

AND

ANTI-

SHOOT

THROUGH

ERROR

AMP

SHDN

±8%

VC

INTVCC

SHDN

THERMAL SHDN

INTVCC UVLO

VIN UVLO

SHDN

THERMAL SHDN

VIN UVLO

EN/UV

1V

+

–

14

4

3

18

GND

INTVCC

2

BIAS

1

VIN2

13

GND1

6, 7

GND2

10, 11

PG

19

FB

R1

C1

R3

OPT

R4

OPT

R2

RT

CSS

OPT

VOUT

20

TR/SS

1.9µA

16

RT

15

SYNC/MODE

17

VIN1

VIN

CIN1

CIN3

CVCC

CIN2