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RT9204CS

Dual Regulator - Standard Buck PWM DC-DC and Linear Power Controller

Richtek Technology 

RT9204

Preliminary

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.

Linear Regulator Driver

The linear regulator of RT9204 was designed to drive

bipolar NPN or MOSFET pass transistor. For

MOSFET pass transistor, normally DRV need to

provide minimum VOUT2+VT+gate-drive voltage to

keep VOUT2 as setting voltage. When driving

MOSFET operating at 5V power supply system, the

gate-drive will be limited at 5V. In this situation shown

in Fig. 5, low VT threshold MOSFET (VT = 1V) and

Vout2 setting below 2.5V were suggested. In VBOOT

= 12V operation condition as Fig. 6, VCC is regulated

as higher to 6V providing more gate-drive for pass

MOSFET transistor, VOUT2 can be set as ≤ 3.3V.

VBOOT = 12V

DRV

BOOT

FBL

6V VCC

RT9204

VOUT2 ≤ 3.3V

R3

+

R4

R4 < 1K

Fig. 6

PWM Loop Stability

The RT9204 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

2

×

π

×

ESR

×

C

The RT9204 Bode plot as shown Fig.7 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

VCC = 5V

DRV

BOOT

FBL

VCC

RT9204

VOUT2 ≤ 2.5V

R3

+

R4

R4 < 1K

20

10

100

Loop Gain

1k

10k

100k

1M

Fig. 7

Fig. 5

www.richtek-ic.com.tw

10

DS9204-00 February 2002

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