ST AN832 Application note
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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 insul ation from the mai ns. Both sect ions use a P.W.M. technique eac h 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 s l ave (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)OSCfsw1 > fsw2
16 1
D95IN293
A
MASTER SLAVE
L4981AL4990
(fsw1)OSC (fsw2)OSCfsw1 > fsw2
1
16
D95IN294
A
Ref
47pFMASTER SLAVE
by C. Adragna
L4981A SYNCHRONIZATION
AN832 APPLICATION NO TE
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The L4981A needs a m inimum set-up time to recognize the sync. signal (see datasheet) and for this reason the
reported circuit is suggested. Using the UC3842 with the L4981A it is necessary to consider that the UC3842
does not have a dedicated sync. pin.
The suggested circuits are shown in fig. 3 and 4 for both master/slave configurations. For the above described
solut ions , it is n ece ss a ry to ta ke c are of pro pe rly s ett ing the osc illa ti on frequency of the two devices: in practice
the master oscillator frequency has to be higher than the slave one in any condition.
Finally, in fig. 5, it is mentioned an example in which an external frequency synchronizes both the PFC and the
PWM controller (L4990 or UC3842).
This can be a real situation in a TV or monitor application.
Figure 3. Sync. example between L4981A and a curren t mode con trolle r.
Figure 4.
Figure 5.
L4981A
UC3842
5
16
4
D95IN295A
1K
10nF
8
100 8.2K
47
MASTER
SLAVE
L4981AUC3842
616
D95IN296B
120Ω 1N4148
3.3K 6.2V470pF 2.2K
MASTER SLAVE
L4981A
1
D95IN297A
120Ω
3.3K 6.2V
470pF
1K
L4990
16
UC3842
5
4
10nF
8
1008.2K
47
VP ≈40V
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