RT9259CGS datasheet(8/15 Pages) RICHTEK

8 / 15 page

RT9259C

DS9259C-03

August 2007

www.richtek.com

Figure 2

Application Information

Introduction

The RT9259C is a dual-channel DC/DC controller

specifically designed to deliver high quality power where

12V power source is available. This part consists of a

synchronous buck controller and an LDO controller. The

synchronous buck controller integrates internal MOSFET

drivers that support 12V+12V bootstrapped voltage for high

efficiency power conversion. The bootstrap diode is built-

in to simplify the circuit design and minimize external part

count. The LDO controller drives an external N-MOSFET

for lower power requirement.

Internal 5VDD Regulator

It is highly recommended to power the RT9259C with well-

decoupled 12V to VCC12 pin. VCC12 powers the RT9259C

control circuit, low side gate driver and bootstrap circuit for

high side gate driver. A bootstrap diode is embedded to

facilitates PCB design and reduce the total BOM cost. No

external Schottky diode is required. The RT9259C

integrates MOSFET gate drives that are powered from the

VCC12 pin and support 12V + 12V driving capability.

Converters that consist of RT9259C feature high efficiency

without special consideration on the selection of

MOSFETs.

An internal linear regulator regulates VCC12 input to a

5VDD voltage for internal control logic circuit. No external

bypass capacitor is required for filtering the 5VDD voltage.

This further facilitates PCB design and reduces the total

BOM cost.

Power On Reset

The RT9259C automatically initializes upon applying input

power (at the VCC12) pin. The power on reset function

(POR) continually monitors the input bias supply voltage

at the VCC12 pin. The VCC12V POR level is typically

9.6V at VCC12V rising.

Frequency Setting and Shut Down

Connecting a resistor RRT from the RT_DIS pin to GND

sets the operation frequency. The relation can be roughly

expressed in the equation.

When let open, the free running frequency is 230kHz

typically. Figure 1 shows the operation frequency vs. RRT

for quick reference.

Figure 1. RT vs. fsw at Low Frequency

Shorting the RT_DIS pin to GND with an external signal-

level MOSFET shuts down the device. This allows flexible

power sequence control for specified application. The

RT_DIS pin threshold voltage is 0.4V typically.

VIN1 Detection

The RT9259C continuously generates a 10kHz pulse train

with 1

μs pulse width to turn on the upper MOSFET for

detecting the existence of VIN1 after VCC12V POR and

RT_DIS enabled as shown in Figure 2. PHASE pin voltage

is monitored during the detection duration.

If the PHASE voltage crosses 1.5V four times, VIN1

existence is recognized and the RT9259C initiates its soft

start cycle as described in next section.

(kHz)

7700

kHz

OSC

≅

200

400

600

800

1000

1200

1400

100

1000

RRT (kohm)

Ω)

IN1 POR_H

PHASE

UGATE

PHASE_M

1.5V

Internal Counter will count (V

PHASE > 1.5V)

four times (rising & falling) to recognize

IN1 is ready.

1st 2nd 3rd 4th PHASE

waveform

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