RT9205GS datasheet(13/15 Pages) RICHTEK

13 / 15 page

RT9205/A

Preliminary

DS9205/A-08 March 2007

www.richtek.com

Reference Voltage

Because RT9205/A uses a low 35dB gain error amplifier,

as shown in Figure 10. The v oltage 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.

Feedback Divider

The reference of RT9205/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 k

Ω 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, value between 1nF and 0.1

µF is enough for C1.

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 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 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 RT9205/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.

Figure 11

Figure 10

PWM

I 3

56K

I 2

REP

0.8V

RAMP

1.75V

Duty (%)

0.82

0.78

0.80

0.81

0.79

VIN = 5V

Duty - 50

= 0.8 - (

)

6.25mV

100

Figure 12

RT9205/A

OUT

<1K

OUT

= V

(1 +)

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