Datasheet

L6591 Application information
Doc ID 14821 Rev 6 33/41
Figure 51. Soft-start pin operation under different operating conditions
While, in the 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:
the 2 V clamp on the SS pin is removed and a second internal current generator I
SS2
= I
SS1
/4 keeps on charging Css. If the voltage reaches 5 V, the IC is disabled, if it is allowed to
reach 2 V
BE
over 5 V, the IC is latched off. In the former case the resulting behavior is
identical to that under short-circuit illustrated in
Figure 46 ; in the latter case the result is
identical to that of
Figure 47 . The time delay before stopping switching upon overload is:
Equation 9
If the overload disappears before the SS voltage reaches 5 V, the I
SS2
generator is 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 user to select either an
auto-restart mode or a latch-mode behavior upon overload.
If latch-mode behavior is desired also for converter short-circuit, make sure that the supply
voltage of the IC never falls below the UVLO threshold before activating the latch.
Figure 20
shows soft-start pin behavior under different operating conditions.
Figure 52 shows a typical high-power adapter application that uses the L6591 in conjunction
with the L6563 PFC controller.
AM13267v1
1SS
SS
delay
I
C
12T =