AN2783 datasheet(27/47 Pages) STMICROELECTRONICS

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AN2783

Layout guidelines

27/47

Layout guidelines

We suggest the following guidelines for the layout of the PM8800:

●

Place the component group including input ceramic capacitors, input side of

transformer, power MOSFET and sense resistors close to each other in order to keep

the interconnections as short as possible.

●

Place the component group including secondary rectifier diode, output side of

transformer, output ceramic capacitors close to each other in order to keep the

interconnections as short as possible.

●

Place the PM8800 in such a way as to have a short path to the gate of the power

MOSFET. Use a 20-30 mils wide path for this signal.

●

Ground: there are basically 4 different grounds on the board (VSS, GND, RTN and

chassis ground).

–

The exposed pad of PM8800 must be connected to VSS. Design a fill area with at

least 6 vias to the VSS plane. Try where possible to increase the number of VSS

power planes connected, at least below the PM8800 position, to improve the heat

dissipation of PM8800.

–

GND must be divided into power gnd (to connect input caps, Rsense, PM8800 pin

9, AUXII circuitry, isolation cap) and signal gnd (to connect the other components

around the PM8800, the circuitry powered by VCC voltage, and the IC pin 16). The

signal gnd must be connected to power gnd in one point only, close to the PM8800

pin 9. Keep the power path on RTN (output side of transformer, secondary diode,

output connector) separated from the feedback network gnd, which is connected

only at the connector side.

●

Design the power MOSFET area with at least 9-12 vias of connection to the internal

copper area. Try where possible to increase the number of power planes connected, at

least below the MOSFET position, to improve the heat dissipation.

●

Design the secondary rectifier diode with at least 9-12 vias of connection to the internal

copper area.

●

Try where possible to increase the number of power planes connected, at least below

the diode position, to improve the heat dissipation.

●

Chassis: design copper areas on both side of the PCB. Do not place other grounds or

signals under the RJ45 and the data transformer area. Place any termination network

on the bottom side.

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