LT1170 View Datasheet(PDF) - Linear Technology

Part Name

Description

View to exact match

LT1170

100kHz, 5A, 2.5A and 1.25A High Efficiency Switching Regulators

Linear Technology 

BLOCK DIAGRA

VIN

2.3V

REG

100kHz

OSC

LT1170/LT1171/LT1172

SWITCH

16V

OUT

LOGIC

FLYBACK

ERROR

AMP

DRIVER

5A, 75V

SWITCH

LT1172

MODE

SELECT

COMP

ANTI-

SAT

FB

–

ERROR

AMP

+

1.24V

REF

VC

SHUTDOWN

CIRCUIT

0.15V

+

CURRENT

AMP

–

GAIN ≈ 6

(LT1170 AND LT1171 ONLY)

0.02 Ω

(0.04Ω LT1171)

(0.16Ω LT1172)

0.16 Ω

E1†

† 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.

E2

1170/1/2 BD

U

OPERATIO

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

7

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