LTC1159CS datasheet(6/20 Pages) LINER

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LTC1159/LTC1159-3.3/LTC1159-5

When the voltage on CT has discharged past VTH1, com-

parator T trips, setting the flip-flop. This causes the N-gate

output to go low (turning off the N-channel MOSFET) and

the P-gate output to also go low (turning the P-channel

MOSFET back on). The cycle then repeats. As the load

current increases, the output voltage decreases slightly.

This causes the output of the gain stage to increase the

current comparator threshold, thus tracking the load

current.

The sequence of events for Burst Mode operation is very

similar to continuous operation with the cycle interrupted

by the voltage comparator. When the output voltage is at or

above the desired regulated value, the P-channel MOSFET

is held off by comparator V and the timing capacitor

continues to discharge below VTH1. When the timing

capacitor discharges past VTH2, voltage comparator S

trips, causing the internal SLEEP line to go low and the

N-channel MOSFET to turn off.

The circuit now enters sleep mode with both power

MOSFETs turned off. In sleep mode, much of the cir-

cuitry is turned off, dropping the supply current from

several milliamps (with the MOSFETs switching) to

300

µA. When the output capacitor has discharged by

the amount of hysteresis in comparator V, the P-channel

MOSFET is again turned on and this process repeats. To

avoid the operation of the current loop interfering with

Burst Mode operation, a built-in offset is incorporated in

the gain stage.

To prevent both the external MOSFETs from being turned

on at the same time, feedback is incorporated to sense the

state of the driver output pins. Before the N-gate output can

go high, the P-drive output must also be high. Likewise, the

P-drive output is prevented from going low when the N-

gate output is high.

OPERATIO

(Refer to Functional Diagram)

Internal divider broken at VFB for adjustable versions.

FU CTIO AL DIAGRA

–

N-GATE

P-DRIVE

VCC

550k

P-GATE

P-GND

550k

CAP

LOW DROP SWITCH

VCC

EXTVCC

SHDN2

VIN

SENSE+

VTH1

25mV TO 150mV

13k

REFERENCE

1.25V

S-GND

ITH

VOS

SENSE–

LTC1159 • FD

SHDN1

VTH2

SLEEP

VFB

OFF-TIME

CONTROL

SENSE–

LOW DROPOUT

4.5V REGULATOR

100k

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