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RT9202CS

Single Synchronous Buck PWM DC-DC Controller

Richtek Technology 

RT9202

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

to the PCB trace, as it 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, it is required to 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

=

1

2

×

π

×

5×

1.6

LC

0.8

VOUT

Error Amp pole PA = 300kHz

ESR zero ZO =

1 × π × ESR × C

2

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

most of application conditions.

VOUT = 3.3V

COUT = 1500µF(33mΩ)

L=2µH

40 VOUT = 1.5V

VOUT = 2.5V

30 VOUT = 3.3V

PO = 2.9kHz

ZO = 3.2kHz

20

Loop Gain

10

100

1k

10k

100k

1M

Fig. 6

www.richtek.com

12

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.

I3

56K

FB

I2

1K

EA

PWM

REP

0.8V

RAMP

1.75V

Fig. 7

0.82

0.81

VIN = 5V

0.80

0.79

0.78

0.5

1 1.5

2

2.5 3

3.5 4 4.5

VOUT (V)

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 kΩ to avoid noise

coupling. The C1 capacitor is a speed-up capacitor

for reducing output ripple to meet with the

requirement of fast transient load. Typically a 1nF ~

0.1µF is enough for C1.

DS9202-02 August 2002

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