VLF10045 datasheet(3/15 Pages) RICHTEK

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RT2808A

DS2808A-00

November 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Operation

The RT2808A is a constant frequency, current mode

asynchronous step-down converter. In normal operation,

the high side N-MOSFET is turned on when the S-R latch

is set by the oscillator and is turned off when the current

comparator resets the S-R latch. While the N-MOSFET

is turned off, the inductor current conducts through the

external diode.

Error Amplifier

The error amplifier adjusts its output voltage by comparing

the feedback signal (VFB) with the internal 1.222V

reference. When the load current increases, it causes a

drop in the feedback voltage relative to the reference, the

error amplifier's output voltage then rises to allow higher

inductor current to match the load current.

Oscillator

The internal oscillator runs at fixed frequency 500kHz. In

short circuit condition, the frequency is reduced to 150kHz

for low power consumption.

Internal Regulator

The regulator provides low voltage power to supply the

internal control circuits and the bootstrap power for high

side gate driver.

Enable

The converter is turned on when the EN pin is higher than

1.4V and turned off when the EN pin is lower than 0.4V.

When the EN pin is open, it will be pulled up to logic-high

by 1

μA current internally.

Soft-Start (SS)

An internal current source charges an internal capacitor

to build a soft-start ramp voltage. The FB voltage will track

the internal ramp voltage during soft-start interval. The

typical soft-start time is 5ms.

Thermal Shutdown

The over temperature protection function will shut down

the switching operation when the junction temperature

exceeds 150

°C. Once the junction temperature cools

down by approximately 30

°C, the converter will

automatically resume switching.

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