6

LT1111

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

negative input of A2 is the 1.25V reference. A resistor

the SET pin provides the trip voltage in a low-battery

detector application. AO can sink 300

µA (use a 22k

resistor pull-up to 5V).

maximum switch current. When the switch current ex-

ceeds the set value, the switch cycle is prematurely

circuitry is approximately 1

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 LT1111-5 and LT1111-12 are functionally identical to

the LT1111. 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.

LT11

ATI

11 OPER

The LT1111 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 LT1111 block diagram.

Comparator A1 compares the feedback (FB) pin voltage

with the 1.25V reference signal. When FB drops below

1.25V, A1 switches on the 72kHz 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 300

µA.

The oscillator is set internally for 7

µs ONtime and 7µs OFF

differ by roughly a factor of 2. Examples include a 3V to 5V

step-up converter or a 9V to 5V step-down converter.

I

DAGRA

BLOCK

S

LT1111 • BD01

IN

V

GND

SET

A0

A2

1.25V

REFERENCE

A1

OSCILLATOR

DRIVER

SW1

SW2

LIM

I

GAIN BLOCK/

ERROR AMP

COMPARATOR

+

–

+

–

FB

LT1111-5/LT1111-12

LT1111

LT1111 • BD02

IN

V

GND

SET

A0

A2

1.25V

REFERENCE

A1

OSCILLATOR

DRIVER

R1

SW1

SW2

LIM

I

R2

220k

SENSE

LT1111-5:

LT1111-12:

R1 = 73.5k

R1 = 25.5k

GAIN BLOCK/

ERROR AMP

COMPARATOR

+

–

+

–