RT9174CS datasheet(6/8 Pages) RICHTEK

6 / 8 page

RT9174

Preliminary

www.richtek-ic.com.tw

DS9174-00 April 2001

Applications Information

Capacitors Selection

A 0.1

µF minimum capacitor is recommended

between VDD and ground to increase stability. The

linear controllers use dominant pole compensation

integrated into the error amplifier and are sensitive to

output capacitor selection. When driving external

pass transistor, normally a minimum 470

µF capacitor

must be placed at output for stability and good

transient response. When using internal pass

transistor, just need 10

µF capacitor for stable.

Pass Transistor Selection

The RT9174 are designed specifically to driver

external NPN or NMOSFET pass transistor for up to

5A output current. The driver could deliver more

100mA when driving NPN transistor and sinking for a

pull up resistor load when driving a NMOSFET.

Typically pass transistor with NPN hfe > 50 at Imax or

NMOSFET Rds-on 100m

Ω are suitable in most

application.

Output Voltage Setting

The RT9174 develop a 0.8V reference voltage,

especially suit for low voltage application. As shown

in Fig. 4, the output voltage could be easy set by R1

& R2 resistor. The divided resistor must keep below

Ω for quick response and stability.

Tri-state Operation

A special Tri-state design is empolyed in the RT9174

for AGP 2X/4X power supply. Typical application

shown the detail circuit. In AGP 2X SELECT = open,

FB1 will be pulled up over 1.3V through R8 to trip the

Tri-state threshold. R8 has to meet the following

relationship:

R8 < 0.75 (V33 - 1.3)

×R2,

1.5

0.8

When in AGP 4X SELECT = GND, VDDQ will work in

normal 1.5V setting.

Layout Considerations

There are two critical layout considerations. One is

the divider resistors should be located to RT9174 as

possible to avoid inducing any noise. The second is

COUT should be put at near the pass transistor to

avoid oscillation.

RT9174

µF

Fig 4

VDD

DRI1

DRI2

DRI3

FB1

FB2

FB3

VDD

VIN

COUT

VOUT

As close as to RT9174

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