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AN832

APPLICATION NOTE

November 2003

Devices Description

In most of SMPS using power factor correction, the convertion is accomplished with a PFC stage, in boost to-

pology, that delivers a preregulated DC voltage to a downstream section. This provides the regulated final DC

buses, ensuring also the galvanic insulation from the mains. Both sections use a P.W.M. technique each with

their own control.

This means that two different switch mode controllers work very close one to the other producing some potential

problem (eg. interferences, beating etc.).

The L4981A controller is provided with an input/output (I/O) synchronization pin (see datasheet pin16 descrip-

tion) which allows the device to be used both as a master or a slave in synchronised configuration.

Let us take into consideration how to interface the L4981A with other PWM controllers each requiring different

interconnections. The devices that have been considered in the following examples are the L4990 (PWM pri-

mary contoller provided with I/O sync.) and the UC3842 PWM controller without any dedicated pin for sync.).

When L4990 with L4981A are used, different situations can be considered :

The PFC controller L4981A is the master and the L4990 is the slave (see fig. 1). Since the L4990 is provided

with a positive edge input sync., the two sync. pins can be simply wired together.

The L4990 is the master and the L4981A is the slave (see fig. 2).

Figure 1. Sync. example with L4981A used as master.

Figure 2. Sync. example with L4981A used as slave

L4981A

L4990

(fsw1)OSC

(fsw2)OSC

fsw1 > fsw2

16

1

D95IN293A

MASTER

SLAVE

L4981A

L4990

(fsw1)OSC

(fsw2)OSC

fsw1 > fsw2

1

16

D95IN294A

Ref

47pF

MASTER

SLAVE

by C. Adragna

L4981A SYNCHRONIZATION