7

LT1572

I

DAGRA

BLOCK

1.24V

REF

1572 BD

ERROR

AMP

100kHz

OSC

2.3V

REG

FB

+

–

+

–

SHUTDOWN

CIRCUIT

COMP

LOGIC

DRIVER

ANTI-

SAT

FLYBACK

ERROR

AMP

16V

SWITCH

OUT

5A, 75V

SWITCH

CURRENT

AMP

GAIN

6

≈

0.15V

0.16

Ω

*ALWAYS CONNECT E1 TO GROUND

MODE

SELECT

E1*

E2

LT1172

ANODE

CATHODE

OPERATIO

The LT1572 is a current mode switcher. This means that

switch duty cycle is directly controlled by switch current

rather than by output voltage. Referring to the block

diagram, the switch is turned “on” at the start of each

oscillator cycle. It is turned “off” when switch current

reaches a predetermined level. Control of output voltage is

obtained by using the output of a voltage sensing error

amplifier to set current trip level. This technique has

several advantages. First, it has immediate response to

input voltage variations, unlike ordinary switchers which

have notoriously poor line transient response. Second, it

reduces the 90

° phase shift at mid-frequencies in the

energy storage inductor. This greatly simplifies closed-

loop frequency compensation under widely varying input

voltage or output load conditions. Finally, it allows simple

pulse-by-pulse current limiting to provide maximum switch

protection under output overload or short conditions.

A low dropout internal regulator provides a 2.3V supply for

all internal circuitry on the LT1572. This low dropout

design allows input voltage to vary from 3V to 40V with

virtually no change in device performance. A 100kHz

oscillator is the basic clock for all internal timing. It turns

“on” the output switch via the logic and driver circuitry.

Special adaptive anti-sat circuitry detects onset of satura-

tion in the power switch and adjusts driver current instan-

taneously to limit switch saturation. This minimizes driver

dissipation and provides very rapid turn-off of the switch.

A 1.2V bandgap reference biases the positive input of the

error amplifier. The negative input is brought out for

output voltage sensing. This feedback pin has a second

function; when pulled low with an external resistor, it

programs the LT1572 to disconnect the main error ampli-

fier output and connects the output of the flyback amplifier