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RT9204CS

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

Richtek Technology 

Preliminary

RT9204

Reference Voltage

Because RT9204 use a low 35dB gain error amplifier,

shown in Fig. 8. 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. 9.

I3

56K

FB

I2

1K

EA

PWM

REP

0.8V

RAMP

1.75V

Fig. 8

0.82

0.81

VIN = 5V

0.8

0.79

0.88

0.5

1 1.5

2

2.5

3

3.5

4 4.5

VOUT (V)

Fig. 9

Feedback Divider

The reference of RT9204 is 0.8V. Both the PWm and

LDO output voltages can be set using a resistor

based divider as shown in Fig.9. Put the R1&R2 and

R3&R4 as close as possible to FB pin and R2&R4

should be 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.

VIN

L

+

COUT

VOUT1 = 0.8V × (1+ RR21)

VOUT2

=

0.8V

× (1+

R3

R4

)

VOUT1

DRV

R1 C1 RT9204

R2

R2 < 1K

FB FBL

VOUT2

R3

+

R4

R4 < 1K

Fig. 10

PWM Layout Considerations

MOSFETs switch very fast and efficiently. The speed

with which the current transitions from one device to

another causes voltage spikes across the

interconnecting impedances and parasitic circuit

elements. The voltage spikes can degrade efficiency

and radiate noise, that results in ocer-voltage stress

on devices. Careful component placement layout and

printed circuit design can minimize the voltage spikes

induced in the converter. Consider, as an example,

the turn-off transition of the upper MOSFET prior to

turn-off, the upper MOSFET was carrying the full load

current. During turn-off, current stops flowing in the

upper MOSFET and is picked up by the low side

MOSFET or Schottky diode. Any inductance in the

switched current path generates a large voltage spike

during the switching interval. Careful component

selections, layout of the critical components, and use

shorter and wider PCB traces help in minimizing the

magnitude of voltage spikes.

There are two sets of critical components in a DC-DC

converter using the RT9204. The switching power

components are most critical because they switch

large amounts of energy, and as such, they tend to

generate equally large amounts of noise. The critical

small signal components are those connected to

sensitive nodes or those supplying critical bypass

current.

DS9204-00 February 2002

www.richtek-ic.com.tw

11

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