AN832 Datasheet, PDF (2/3 Pages) STMicroelectronics – L4981A SYNCHRONIZATION

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.
Figure 4.
L4981A
16
MASTER
100 8.2K
1K
10nF
47
D95IN295A
8
4 _UC3842
5
SLAVE
Figure 5.
UC3842
6
MASTER
120Ω
470pF 3.3K
1N4148
L4981A
16
6.2V 2.2K
SLAVE
D95IN296B
VP ≈40V
470pF
120Ω
3.3K
6.2V
1
L4990
8
UC3842 ⁴
5
8.2K 100
1K
10nF
47
L4981A
16
D95IN297A
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