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RT9259

12V Synchronous Buck PWM DC-DC and Linear Power Controller

Richtek Technology 

RT9259

Application Information

Introduction

The RT9259 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 RT9259 with well-

decoupled 12V to VCC12 pin. VCC12 powers the RT9259

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 RT9259 integrates

MOSFET gate drives that are powered from the VCC12

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

that consist of RT9259 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 RT9259 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.

fOSC

≅

230kHz +

7700 (kHz)

RRT

When let open, the free running frequency is 230kHz

typically. Figure 1 shows the operation frequency vs. RRT

for quick reference.

1400

1200

1000

800

600

400

200

0

10

100

1000

RRT (koΩh)m)

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 RT9259 continuously generates a 10kHz pulse train

with 1μs 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 RT9259 initiates its soft

start cycle as described in next section.

VIN1 POR_H

PHASE_M

+

- 1.5V

PHASE

UGATE

1st 2nd 3rd 4th PHASE

waveform

Internal Counter will count (VPHASE > 1.5V)

four times (rising & falling) to recognize

VIN1 is ready.

Figure 2

www.richtek.com

8

DS9259-03T00 August 2007

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