RT9259GS datasheet(12/15 Pages) RICHTEK

12 / 15 page

RT9259

DS9259-03T00

August 2007

www.richtek.com

Figure 12. The connections of the critical components in

the converter

LOAD

VCC

GND

RT9259

LGATE

UGATE

IQ1

OUT

IQ2

5V/12V

GND

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. A multi-layer printed

circuit board is recommended. Figure 12 shows the

connections of the critical components in the converter.

Note that the capacitors CIN and COUT each of them

represents numerous physical capacitors.

Use a dedicated grounding plane and use vias to ground

all critical components to this layer. Apply another solid

layer as a power plane and cut this plane into smaller islands

of common voltage levels. The power plane should support

the input power and output power nodes. Use copper filled

polygons on the top and bottom circuit layers for the PHASE

node, but it is not necessary to oversize this particular

island. Since the PHASE node is subjected to very high

dV/dt voltages, the stray capacitance formed between

these islands and the surrounding circuitry will tend to couple

switching noise. Use the remaining printed circuit layers

for small signal routing. The PCB traces between the PWM

controller and the gate of MOSFET and also the traces

connecting source of MOSFETs should be sized to carry

2A peak currents.

PCB 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 over-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 RT9259. 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. Derating Curves for RT9259 Packages

0.25

0.5

0.75

1.25

1.5

1.75

100

125

Ambient Temperature

QFN-16L 4x4

(

°C)

SOP-14

SSOP-16

4-Layers PCB

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