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Número de pieza

componentes Descripción

Fabricante

RT9209

Synchronous Buck PWM DC-DC with Enable & PGOOD

Richtek Technology 

RT9209/A

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 RT9209/A 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 :

5 0.8

DC loop gain GA = 35 dB × ×

1.6 VOUT

1

LC filter pole PO = × π × LC

2

Error Amp pole PA = 300kHz

1

ESR zero ZO = × π × ESR × C

2

The RT9209/A Bode plot as shown Figure 5 is stable in

most of application conditions.

VOUT = 3.3V

COUT = 1500uF(33mΩ)

L = 2uH

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

Figure 5

Reference Voltage

Because RT9209/A use a low 35dB gain error amplifier,

shown in Figure 6. 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 Figure 7.

R2

FB

+

-

R1

56K

-

1K

EA

+

+

PWM

-

REF

0.8V

RAMP

1.75V

Figure 6

0.82

0.81

0.80

0.79

VIN = 5V

0.78

0.5 1 1.5 2 2.5 3 3.5 4 4.5

VOUT (V)

Figure 7

Feedback Divider

The reference of RT9209/A is 0.8V. The output voltage can

be set using a resistor based divider as shown in Figure 8.

Put the R1 and R2 as close as possible to FB pin and R2

should less than 1 kΩ to avoid noise coupling.

VIN

L

A

VOUT

COUT

R1

R2

RT9209/A

FB

Figure 8

www.richtek.com

12

DS9209/A-08 March 2007

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