RT9202CS datasheet(12/16 Pages) RICHTEK

12 / 16 page

RT9202

www.richtek.com

DS9202-02 August 2002

Input / Output Capacitor

High frequency/long life decoupling capacitors should

be placed as close to the power pins of the load as

physically possible. Be careful not to add inductance

the

PCB

trace,

could

eliminate

the

performance from utilizing these low inductance

components. Consult with the manufacturer of the

load on specific decoupling requirements.

The output capacitors are necessary for filtering

output and stabilizing the close loop (see the PWM

loop stability). For powering advanced, high-speed

processors,

required

meet

with

the

requirement of fast load transient, high frequency

capacitors

with

low

ESR/ESL

capacitors

are

recommended.

Another concern is high ESR induced ripple may

trigger UV or OV protections.

PWM Loop Stability

The RT9202 is a voltage mode buck controller

designed for 5V step-down applications. The gain of

error amplifier is fixed at 35dB for simplified design.

The output amplitude of ramp oscillator is 1.6V, the

loop gain and loop pole/zero are calculated as

follows:

DC loop gain GA = 35dB ×

LC filter pole PO =

× π ×

Error Amp pole PA = 300kHz

ESR zero ZO =

× π × ESR × C

The RT9202 Bode plot as shown Fig.6 is stable in

most of application conditions.

Reference Voltage

Because RT9202 use a low 35dB gain error amplifier,

shown in Fig. 7. The voltage regulation is dependent

on VIN & VOUT setting. The FB reference voltage of

0.8V were trimmed at VIN = 5V & VOUT = 2.5V

condition. In a fixed VIN = 5V application, the FB

reference voltage vs. VOUT voltage can be calculated

as Fig. 8.

Fig. 7

Fig. 8

Feedback Divider

The reference of RT9202 is 0.8V. The output voltage

can be set using a resistor based divider as shown in

Fig.9. Put the R1 and R2 as close as possible to FB

pin and R2 should less than 1 k

Ω to avoid noise

coupling. The C1 capacitor is a speed-up capacitor

for

reducing

output

ripple

meet

with

the

requirement of fast transient load. Typically a 1nF ~

0.1

µF is enough for C1.

Loop Gain

VOUT = 1.5V

VOUT = 2.5V

VOUT = 3.3V

ZO = 3.2kHz

PO = 2.9kHz

L=2

µH

COUT = 1500µF(33mΩ)

VOUT = 3.3V

100k

10k

100

Fig. 6

PWM

REP

0.8V

56K

RAMP

1.75V

VOUT (V)

VIN = 5V

3.5

1.5

2.5

0.82

0.78

0.80

0.81

0.79

0.5

4.5

VOUT

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