7

LT1170/LT1171/LT1172

1.24V

REF

1170/1/2 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

0.02

(0.04

(0.16

Ω

Ω LT1171)

Ω LT1172)

0.16

Ω

CURRENT

AMP

GAIN

6

≈

0.15V

ALWAYS CONNECT E1 TO THE GROUND PIN ON MINIDIP, 8- AND 16-PIN SURFACE MOUNT PACKAGES.

E1 AND E2 INTERNALLY TIED TO GROUND ON TO-3 AND TO-220 PACKAGES.

†

MODE

SELECT

E2

(LT1170 AND LT1171 ONLY)

LT1172

The LT1170/LT1171/LT1172 are current mode switchers.

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 midfrequencies 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 LT1170/LT1171/LT1172. This

low dropout design allows input voltage to vary from 3V to

60V 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 saturation in the power switch and adjusts driver

current instantaneously 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

OPERATIO

BLOCK DIAGRA