LT1109

4

I

DAGRA

BLOCK

S

LT1109-5, -12 Z Package

The LT1109Z-5 and LT1109Z-12 are fixed output voltage

step-up DC/DC converters in a 3-pin TO-92 package.

Power for internal regulator circuitry is taken from the

operation can be best understood by referring to the block

negative input of comparator A1. When this voltage falls

below the 1.25V reference voltage, the oscillator is turned

on and the power switch Q1 cycles at the oscillator

frequency of 120kHz. Switch cycling alternately builds

current in the inductor, then dumps it into the output

capacitor, increasing the output voltage. When A1’s nega-

tive input rises above 1.25V, it turns off the oscillator. A

small amount of hysteresis in A1 obviates the need for

frequency compensation circuitry. When Q1 is off, current

from the battery will be higher because the device oper-

ates off the

stepped-up voltage.

LT1109-5, -12 N8, S8 Package

CC

HARA TERISTICS

A

TYPICALPERFOR

CE

LT1109Z

ATIO

OPER

Minimum/Maximum

Quiescent Current

Quiescent Current

INPUT VOLTAGE (V)

0

200

300

450

20

LT1109 • TPC09

210

400

18

250

4

8

12 14

350

616

TEMPERATURE (°C)

–50

200

280

400

100

LT1109 • TPC10

25

360

75

220

–25

0

50

320

240

260

300

340

380

IN

V

LT1109 • TA04

GND

1.25V

REFERENCE

A1

DRIVER

SW

120kHz

OSCILLATOR

COMPARATOR

R2

250k

R1

+

–

SENSE

SHUTDOWN

Q1

FB

ON FIXED VERSION PIN 8 IS SENSE

ON ADJUSTABLE VERSION PIN 8 IS FB AND R1 AND R2 ARE DISCONNECTED

OUT

V

LT1109 • TA03

GND

1.25V

REFERENCE

A1

DRIVER

SW

120kHz

OSCILLATOR

COMPARATOR

R2

250k

R1

+

–

Q1

LT1109-5: R1 = 83k

LT1109-12: R1 = 29k

2.0

80

110

160

6.0

LT1109 • TPC08

2.5

4.0

140

120

5.5

150

90

3.0

3.5

4.5

5.0

130

100

0

°C ≤ T