L6566B datasheet(34/51 Pages) STMICROELECTRONICS

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Application information

L6566B

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The soft-start pin is also invoked whenever the control voltage (COMP) saturates high,

which reveals an open-loop condition for the feedback system. This condition very often

occurs at start-up, but may be also caused by either a control loop failure or a converter

overload/short circuit. A control loop failure results in an output overvoltage that is handled

by the OVP function of the L6566B (see next section). In case of QR operation, a short

circuit causes the converter to run at a very low frequency, then with very low power

capability. This makes the self-supply system that powers the device unable to keep it

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 L6566B, 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 keeps on charging CSS. As the voltage reaches 5 V the device is disabled, if it

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

resulting behavior will be identical to that under short circuit illustrated in Figure 7 on

page 20

; 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 5 V the ISS2 generator will be

turned off and the voltage gradually brought back down to 2 V. Refer to the “Application

examples and Ideas” section (Table 7 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 L6566B 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

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