Data Sheet

LT8638-2

analog.com

Rev.0

16 of 35

BLOCK DIAGRAM

Figure 43. Block Diagram

THEORY OF OPERATION

The LT8638-2 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 0.6V 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 in Burst Mode

operation, inductor current falls to zero. If overload conditions result in more than 15.5A 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 LT8638-2 is shut down and draws approximately 6µA from the input. When the EN/UV pin

is above 0.98V, the switching regulator becomes active.

+

+

–

SLOPE COMP

INTERNAL 0.6V REF

OSCILLATOR

200kHz TO 3MHz

BURST

DETECT

3.4V

REG

M1

M2

COUT

VOUT

SW

5 TO 8

L

BST

SWITCH

LOGIC

AND

ANTI-SHOOT

THROUGH

ERROR

AMP

SHDN

±7.75%

SHDN

THERMAL SHDN

INTVCC UVLO

VIN UVLO

SHDN

THERMAL SHDN

VIN UVLO

EN/UV

0.98V

+

–

21

4

INTVCC

3

BIAS

2

VIN

17 TO 18

GND

9 TO 14, 20,

29 TO 32

CLKOUT

PHMODE

PG

25

FB

R1

CPL

R3

OPT

R4

OPT

R2

RT

CSS

OPT

VOUT

28

SS

2µA

27

RT

22

SYNC/MODE

24

VIN

15 TO 16

VIN

CIN1

CIN3

23

CIN2

CVCC

CBST

INTVCC

60kΩ

600kΩ

INTVCC

INTVCC

60kΩ

600kΩ

26

CC

VC

RC

CF

1

PLL

0.1μF