RT9205SLASHACS datasheet(13/16 Pages) RICHTEK

13 / 16 page

Preliminary

RT9205/A

DS9205/A-03 May 2003

www.richtek.com

Reference Voltage

Because RT9205/A uses a low 35dB gain error

amplifier, as shown in Fig.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 Fig.11.

Fig. 10

Fig. 11

Feedback Divider

The reference of RT9205/A is 0.8V. The output

voltage can be set using a resistor-divider as shown

in Fig.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.

Fig. 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 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.

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

PWM

REP

0.8V

56K

RAMP

1.75V

Duty (%)

VIN = 5V

0.82

0.78

0.80

0.81

0.79

VFB = 0.8V –













−

100

Duty

× 6.25mV

VOUT = VFB × (1+

)

VIN

COUT

VOUT

RT9205/A

< 1K

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