RT8101APSP datasheet(13/15 Pages) RICHTEK

13 / 15 page

RT8101/A

Preliminary

DS8101/A-01 March 2007

www.richtek.com

Figure 11. The connections of the critical components

in the converter

LOAD

VCC

GND

RT8101/A

LGATE

UGATE

IQ1

VOUT

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 11 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 RT8101/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 10. Derating Curves for RT8101/A Packages

0.2

0.4

0.6

0.8

1.2

1.4

100

120

140

Ambient Temperature (°C)

SOP-8

PSOP-8

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