AN832
APPLICATION NOTE
L4981A SYNCHRONIZATION
by C. Adragna
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.
L4981A
16
MASTER
1
L4990
SLAVE
(fsw1)OSC
fsw1 > fsw2
(fsw2)OSC
D95IN293A
Figure 2. Sync. example with L4981A used as slave
Ref
L4990
MASTER
1
16
47pF
L4981A
SLAVE
(fsw1)OSC
fsw1 > fsw2
(fsw2)OSC
D95IN294A
November 2003
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AN832 APPLICATION NOTE
The L4981A needs a minimum 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
solutions, it is necessary to take care of properly setting the oscillation 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 current mode controller.
L4981A
1K
16
MASTER
100
8.2K
4
10nF
8
UC3842
5
SLAVE
47
D95IN295A
Figure 4.
UC3842
6
MASTER
470pF
120Ω
3.3K
1N4148
6.2V
2.2K
L4981A
16
SLAVE
D95IN296B
Figure 5.
VP
≈40V
470pF
120Ω
3.3K
6.2V
1
L4990
8
8.2K
4
10nF
100
1K
L4981A
16
UC3842
5
47
D95IN297A
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AN832 APPLICATION NOTE
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics.
All other names are the property of their respective owners
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