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L6566BH

Application information

Doc ID 16610 Rev 2

33/51

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

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

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

critical to handle.

The L6566BH, regardless of the operating option selected, makes it easier to handle such

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

ISS2 = ISS1 /4 continues to charge CSS. As the voltage reaches 5 V the device is disabled, if

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

resulting behavior is identical to that of short-circuit illustrated in

Figure 7; in the latter case

the result is identical to that of

Figure 20. See Section 5.9: Latched disable function 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 5 V, the ISS2

generator is turned off and the voltage gradually brought back down to 2 V. Refer to

Section 6: Application examples and ideas (Table 7) for additional information.

If latch-mode behavior is desired also for converter 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 auto-restart).

Note:

Unlike other PWM controllers provided with a soft-start pin, in the L6566BH grounding the

SS pin does not guarantee that the gate driver is disabled.

5.11

OVP block

The OVP function of the L6566BH monitors the voltage on the ZCD pin (11) in MOSFET

OFF-time, during which the voltage generated by the auxiliary winding tracks the converter’s

output voltage. If the voltage on the pin exceeds an internal 5 V reference, a comparator is

triggered, an overvoltage condition is assumed and the device is shut down. An internal

current generator is activated that sources 1 mA out of the VFF pin (15). If the VFF voltage

Figure 21.

Soft-start pin operation under different operating conditions and settings

AM11497v1

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

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