6

LT1107

1107fa

I

DAGRA

BLOCK

S

OPERATIO

The LT1107 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 LT1107 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 63kHz 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 11

µs ON time and 5µs

OFF time in step-up mode, optimizing the device for

duty cycle and the current limit feature enables continuous

mode operation in many applications, increasing available

output power.

OSCILLATOR

1.25V

REFERENCE

GND

SENSE

COMPARATOR

A2

A1

DRIVER

SW1

SW2

AO

GAIN BLOCK/

ERROR AMP

SET

1107 BD02

R1

R2

220k

LT1107-5: R1 = 73.5k

LT1107-12: R1 = 25.5k

OSCILLATOR

1.25V

REFERENCE

GND

FB

COMPARATOR

A2

A1

DRIVER

SW1

SW2

AO

GAIN BLOCK/

ERROR AMP

SET

1107 BD01

LT1107

LT1107-5/LT1107-12

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 LT1107-5 and LT1107-12 are functionally identical to

the LT1107. 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.