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RT9174CS

Triple Linear Regulator Controllers

Richtek Technology 

RT9174

Applications Information

Preliminary

Capacitors Selection

A 0.1µF 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 470µF capacitor

must be placed at output for stability and good

transient response. When using internal pass

transistor, just need 10µF 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.

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.

VDD

1µF

DR I1

VDD RT9174 DRI2

DR I3

FB1

FB2

FB3

R1

R2

As close as to RT9174

Fig 4

+ COUT

VIN

VOUT

Typically pass transistor with NPN hfe > 50 at Imax or

NMOSFET Rds-on 100mΩ 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

1KΩ 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,

if

R3 + R2

R3

× 0.8

= 1.5 .

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

normal 1.5V setting.

www.richtek-ic.com.tw

6

DS9174-00 April 2001

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