7

LT1082

OPERATIO

–

+

2.3V

REG

FLYBACK

ERROR

AMP

COMP

ERROR

AMP

SHUTDOWN

CIRCUIT

CURRENT

AMP

OSC

60kHz

14kHz

MODE SELECT

1.24V

REF

ANTI-SAT

DRIVER

LOGIC

0.15V

GND

* ALWAYS CONNECT E1 TO GROUND PIN ON MiniDIP PACKAGE.

EMITTERS TIED TO GROUND ON TO-220 PACKAGE.

GAIN

≈ 5

0.2

Ω

0.2

Ω

16.2V

SWITCH OUT

FB

E1* E2

1082 BD

I

DAGRA

BLOCK

low dropout internal regulator provides a 2.3V supply for

all internal circuitry on the LT1082. This low dropout

design allows input voltage to vary from 3V to 75V with

virtually no change in device performance. A 60kHz

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

function: when pulled low with an external resistor and

The LT1082 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