5

LT1108

ATI

1 OPER

negative input of A2 is the 1.245V reference. A resistor

the SET pin provides the trip voltage in a low battery

detector application. A0 can sink 100

µA (use a 47k resis-

tor pull-up to 5V).

maximum switch current. When the switch current ex-

ceeds the set value, the switch cycle is prematurely

limit circuitry is approximately 2

µs.

In step-up mode the switch emitter (SW2) is connected to

ground and the switch collector (SW1) drives the induc-

and the emitter drives the inductor.

The LT1108-5 and LT1108-12 are functionally identical to

the LT1108. The -5 and -12 versions have on-chip voltage

setting resistors for fixed 5V or 12V outputs. Pin 8 on the

fixed versions should be connected to the output. No

external resistors are needed.

The LT1108 is a gated oscillator switcher. This type

architecture has very low supply current because the

switch is cycled when the feedback pin voltage drops

below the reference voltage. Circuit operation can best be

understood by referring to the LT1108 block diagram.

Comparator A1 compares the feedback (FB) pin voltage

with the 1.245V reference signal. When FB drops below

1.245V, A1 switches on the 19kHz oscillator. The driver

amplifier boosts the signal level to drive the output NPN

power switch. The switch cycling action raises the output

voltage and FB pin voltage. When the FB voltage is suffi-

cient to trip A1, the oscillator is gated off. A small amount

of hysteresis built into A1 ensures loop stability without

external frequency compensation. When the comparator

output is low, the oscillator and all high current circuitry

is turned off, lowering device quiescent current to just

110

µA.

The oscillator is set internally for 36

µs ON-time and 17µs

OFF-time, allowing continuous mode operation in many

cases such as 2V to 5V converters. Continuous mode

greatly increases available output power.

Gain block A2 can serve as a low battery detector. The

I

DAGRA

BLOCK

S

LT1108

LT1108-5/LT1108-12

A2

A1

GND

FB

SET

A0

GAIN BLOCK/

ERROR AMP

COMPARATOR

DRIVER

SW1

SW2

1.245V

REFERENCE

OSCILLATOR

LT1108 • BD

A2

A1

GND

SET

A0

GAIN BLOCK/

ERROR AMP

COMPARATOR

DRIVER

SW1

SW2

1.245V

REFERENCE

OSCILLATOR

LT1108-5 • BD

SENSE

R1

R2

753k

LT1108-5: R1 = 250k

LT1108-12: R1 = 87.4k