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LT1309

OPERATIO

The LT1309 is a current limited, Burst Mode (gated

oscillator) type switching regulator that produces a 12V

output from a 3.3V or 5V input. Operation can best be

understood by referring to the Block Diagram. When the

voltage at the Sense pin (Pin 7) is less than 12V, compara-

tor C1 enables the oscillator, turning switches Q1 and Q2

on. The switch turns off when current limit is reached or

when the oscillator maximum on-time is reached. When

Q2 turns off, current built up in the inductor flows into the

output capacitor and load. This action occurs until the

output voltage reaches 12V. During start-up, the Sense pin

voltage is either 3.3V or 5V, depending on the input. The

VPP VALID signal remains high until the output voltage

reaches 12V, signaling correct voltage level to program

flash memory. Connecting a 1

µF/0.1µF parallel RC to the

Soft Start pin limits inrush current during start-up.

BLOCK DIAGRA

8

ON/OFF

SENSE

7

4k

864k

200k

VPP VALID

OFF

Q3

Q2

× 1

Q1

× 50

100k

–

+

C2

R1

3

Ω

–

+

C3

36mV

PGND

1309 BD

SW

R2

700

Ω

5

3

4

GND

1

SOFT START

BIAS

+

–

500kHz

OSCILLATOR

C1

2

POWER ON

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