Application information

L6591

24/32

Figure 16.

Soft-start pin operation under different operating conditions

While in case of feedback loop failure the system must be stopped quickly to prevent the

output voltage from reaching too high values, an overload or a short circuit does not

generally need such fast intervention. The L6591 makes it easier to handle such conditions:

/4 keeps on charging Css. If the voltage reaches 5 V the IC will be disabled, if it is allowed to

be identical to that under short circuit illustrated in Figure 11; in the latter case the result will

be identical to that of Figure 12. The time delay before stopping switching upon overload is:

Equation 9

turned off and the voltage gradually brought back down to 2 V. A diode, with the anode to the

SS pin and the cathode connected to the VREF pin (#6) allows the designer to select either

an auto-restart mode or a latch-mode behavior upon overload.

If latch-mode behavior is desired also for converter

’s short circuit, make sure that the supply

voltage of the IC never falls below the UVLO threshold before activating the latch. Figure 18

shows soft-start pin behavior under different operating conditions.

Figure 17

shows a typical high-power adapter application that uses the L6591 in conjunction

with the L6563 PFC controller.

1

SS

SS

delay

I

C

12

T

=