L6566B View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566B' PDF : 51 Pages View PDF
Application information
L6566B
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.
Figure 21. Soft-start pin operation under different operating conditions and settings
Vcc
(pin 5)
SS 5V+2Vbe
(pin 14) 5V
2V
COMP
(pin 9)
GD
(pin 4)
START-UP
NORMAL
OPERATION
TEMPORARY
OVERLOAD
NORMAL
OPERATION
Vcc falls below UVLO
before latching off
here the IC
shuts down
OVERLOAD
here the IC
latches off
UVLO
t
t
t
SHUTDOWN
LATCHED
AUTORESTART
t
RESTART
Note:
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).
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.
34/51
Share Link: 
