RT9277APF datasheet(3/12 Pages) RICHTEK

3 / 12 page

RT9277A/B

Preliminary

DS9277A/B-02 March 2007

www.richtek.com

Operation

The RT9277A/B is a high efficiency step-up Boost converter

with a fixed-frequency, current-mode PWM architecture.

It performs fast transient response and low noise operation

with appropriate component selection. The output voltage

is regulated through a feedback control consisting of an

error amplifier, a summing comparator, and several control

signal generators (as shown in function block diagram).

The feedback reference voltage is 1.24V. The error amplifier

varies the COMP voltage by sensing the FB pin. The slope

compensation signal summed with the current -sense

signal will be compared with the COMP voltage through

the summing comparator to determine the current trip point

and duty cycle. When driving light loads, the RT9277A/B

will perform the pulse-skipping mode to prevent

overcharging the output voltage. In this mode, the switching

frequency will be reduced to perform a higher efficiency.

Soft-Start

The RT9277B provides programmable soft-start function.

When the EN pin is connected to high, a 4

μA constant

current is sourced to charge an external capacitor. The

voltage rate of rise on the COMP pin is limited during the

charging period, and so is the peak inductor current.

When the EN pin is connected to GND, the external

capacitor will be discharged to ground for the next time

soft-start.

Current Limitation

The switch current is monitored to limit the value not to

exceed 1.6A typically. When the switch current reaches

1.6A, the output voltage will be pulled down to limit the

total output power to protect the power switch and external

components.

Shutdown

Connect the EN to GND to turn the RT9277A/B off and

reduce the supply current to 0.1

μA. In this operation, the

output voltage is the value of VIN to subtract the forward

voltage of catch diode.

Frequency Selection

The switching frequency of RT9277A/B can be selected to

operate at either 640kHz or 1.2MHz. When the FREQ pin

is connected to GND for 640kHz operation, and connected

to VIN for 1.2MHz operation. FREQ is preset to 640kHz

operation for allowing the FREQ pin unconnected.

RT9277B

Control

and

Driver

Logic

Current

Sense

Slope

Compen-

sation

Oscillator

Summing

Comparator

Clock

Error

Amplifier

1.24V

5uA

4uA

VIN

GND

COMP

FREQ

Protection

Soft-

Start

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