L6566ATR datasheet(34/51 Pages) STMICROELECTRONICS

34 / 51 page

Application information

L6566A

34/51

operating, so that the converter will work intermittently, which is very safe. In case of

overload the system has a power capability lower than that at nominal load but the output

current may be quite high and overstress the output rectifier. In case of FF operation the

capability is almost unchanged and both short circuit and overload conditions are more

critical to handle.

The L6566A, regardless of the operating option selected, makes it easier to handle such

conditions: the 2V clamp on the SS pin is removed and a second internal current generator

ISS2= ISS1 /4 keeps on charging Css. As the voltage reaches 5V the device is disabled, if it is

allowed to reach 2 VBE over 5V, the device will be latched off. In the former case the

resulting behavior will be identical to that under short circuit illustrated in Figure 6 on

page 19

; in the latter case the result will be identical to that of Figure 20 on page 33.

See Section 5.9: Latched disable function on page 32 for additional details.

A diode, with the anode to the SS pin and the cathode connected to the VREF pin (10) is the

simplest way to select either auto-restart mode or latch-mode behavior upon overcurrent. If

the overload disappears before the Css voltage reaches 5V the ISS2 generator will be turned

off and the voltage gradually brought back down to 2V. Refer to the Section 6: Application

examples and ideas on page 44

section (Figure 8 on page 45) for additional hints.

If latch-mode behavior is desired also for converter’s short circuit, make sure that the supply

voltage of the device does not fall below the UVLO threshold before activating the latch.

Figure 21

shows soft-start pin behavior under different operating conditions and with

different settings (latch-mode or autorestart).

Note:

Unlike other PWM controllers provided with a soft-start pin, in the L6566A grounding the SS

pin does not guarantee that the gate driver is disabled.

Figure 21. Soft-start pin operation under different operating conditions and settings

Vcc

(pin 5)

(pin 14)

COMP

(pin 9)

START-UP

TEMPORARY

OVERLOAD

(pin 4)

NORMAL

OPERATION

NORMAL

OPERATION

RESTART

here the IC

shuts down

here the IC

latches off

Vcc falls below UVLO

before latching off

SHUTDOWN

LATCHED

AUTORESTART

UVLO

5V+2Vbe

Vcc_PFC

(pin 6)

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