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RT9232

Programmable Frequency Synchronous Buck PWM Controller

Richtek Technology 

Function Block Diagram

10uA

SS

0.6V

+

UV

-

VCC EN

Power-On

VIN

POR

+

Reset (POR)

-

POR

Soft Start

and

Fault Logic

INHIBIT

1.5V

FB

COMP

+

- EA

0.8V +

Reference

+

+

PWM

-

Oscillator

Driver Logic

RT9232

SENSE

BOOT

UGATE

PHASE

PVCC

LGATE

PGND

GND

RT

Operation

Startup

RT9232 initializes automatically after receiving both VCC and VIN power. Special power-on sequence is not necessary.

The Power-On Reset (POR) function continually monitors input supply voltages and enable voltage. POR function

monitors IC power via VCC pin and external MOSFET power via SENSE pin. Voltage on SENSE pin is a fixed voltage

drop less than VIN. When voltages on VCC, SENSE, and EN pins exceed their thresholds, POR function initializes soft-

start operation. POR inhibits driver operation while EN pin pulls low. Transitioning EN pin high after input supply voltages

ready initializes soft-start operation.

Soft-Start

After POR function releases soft-start operation, an internal 10uA current source charges an external capacitor on SS

pin (Css) to 5V. Soft-start function clamps both COMP & FB pins to SS pin voltage & a fixed voltage drop less than SS

pin voltage respectively. Thus upper MOSFET turns on at a limited duty and output current overshoot can be reduced.

This method provides a rapid and controlled output voltage rise.

Under Voltage Protection

The under voltage protection function protects the converter from an shorted output by detecting the voltage on FB pin

to monitor the output voltage. The UVP function cycles soft-start function in a hiccup mode. When output voltage lower

than 75% of designated voltage, UVP function initializes soft-start cycles. The soft-start function discharges Css with

10uA current sink and disable PWM operation. Then soft-start function recharges Css and PWM operation resumes.

The soft-start hiccup restarts after SS voltage fully charges to 4V if the output short event still remains. The converter is

shutdown permanently after 3 times hiccup and only restarting supply voltages can enable the converter.

DS9232-06 March 2007

www.richtek.com

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