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RT9203

Dual Regulators- Synchronous Buck PWM DC-DC and Linear Controller

Richtek Technology 

RT9203/A

Preliminary

Reference Voltage

Because RT9203/A uses a low 35dB gain error amplifier,

as shown in Figure 10. The voltage regulation is

dependent on VIN and VOUT settings. The FB reference

voltage of 0.8V were trimmed at VIN = 5V and VOUT =

2.5V. In a fixed VIN = 5V application, the FB reference

voltage vs. VOUT voltage can be calculated as Figure 11.

FB

+

-

R1

1K

REP

0.8V

R1

56K

-

EA

+

RAMP

1.75V

Figure 10

+

PWM

-

0.82

0.81

0.80

0.79

0.78

10 20

30 40 50 60

Duty (%)

70 80 90

Figure 11

Feedback Divider

The reference of RT9203/A is 0.8V. The output voltage can

be set using a resistor-divider as shown in Figure 12. Put

the R1 and R2 as close as possible to FB pin. R2 value

should be less than 1 kΩ to avoid noise coupling issue.

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

VOUT

COUT

R1

C1

R1 R2

VOUT = VFB × (1+ ) <1K

R2

RT9203/A

FB

Figure 12

PWM Layout Considerations

MOSFETs switch very fast in efficiency. 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 over-voltage stress on devices. Careful the

layout for component placement 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

lower MOSFET or Schottky diode. Any inductance in the

switched current path generates a large voltage spike

during the switching interval. Care with component

selections, layout of the critical components, and use

shorter and wider PCB traces that help in minimizing the

magnitude of voltage spikes.

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

converter using the RT9203/A. 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.

The power components and the PWM controller should

be placed firstly. Place the input capacitors, especially the

high-frequency ceramic decoupling capacitors, close to

the power switches. Place the output inductor and output

capacitors between the MOSFETs and the load. Also

locate the PWM controller near by MOSFETs.

www.richtek.com

14

DS9203/A-10 March 2007

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