AN832 APPLICATION NOTE

2/3

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.

Figure 5.

L4981A

UC3842

5

16

4

D95IN295A

1K

10nF

8

100

8.2K

47

MASTER

SLAVE

L4981A

UC3842

616

D95IN296B

120

Ω

1N4148

3.3K

6.2V

470pF

2.2K

MASTER

SLAVE

L4981A

1

D95IN297A

120

Ω

3.3K

6.2V

470pF

1K

L4990

16

UC3842

5

4

10nF

8

100

8.2K

47

VP

≈40V