L6566ATR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566ATR' PDF : 51 Pages View PDF
L6566A
Application information
5.12
Brownout protection
Brownout protection is basically a not-latched device shutdown function activated when a
condition of mains undervoltage is detected. There are several reasons why it may be
desirable to shut down a converter during a brownout condition, which occurs when the
mains voltage falls below the minimum specification of normal operation.
Firstly, a brownout condition may cause overheating of the PFC front-end due to an excess
of RMS current. Secondly, brownout can also cause the PFC pre-regulator to work open
loop. This could be dangerous to the PFC itself and the downstream converter, should the
input voltage return abruptly to its rated value, given the slow response of PFC to transient
events. Finally, spurious restarts may occur during converter power-down, therefore causing
the output voltage not to decay to zero monotonically.
The L6566A shutdown upon brownout is accomplished by means of an internal comparator,
as shown in the block diagram of Figure 25, which shows the basic circuit usage. The
inverting input of the comparator, available on the AC_OK pin (16), is supposed to sense a
voltage proportional to either the RMS or the peak mains voltage; the non-inverting input is
internally referenced to 0.485 V with 35 mV hysteresis. If the voltage applied on the AC_OK
pin before the device starts operating does not exceed 0.485 V or if it falls below 0.45 V
while the device is running, The AC_OK signal goes high, the Vcc_PFC pin is open and the
device shuts down, with the soft-start capacitor discharged and the gate-drive output low.
Additionally, in case the device has been latched off by some protection function (in which
case Vcc is oscillating between VccON and VccON - 0.5 V), the AC_OK voltage falling below
0.45 V clears the latch. This feature can be used to allow a quicker restart as the input
source is removed.
Figure 25. Brownout protection: internal block diagram and timing diagram
Sensed
voltage
Vcc
L6566A
5
RH
AC_OK
-
16
0.485V +
15 µA 0.45V
RL
AC_FAIL
Sensed voltage
VsenON
VsenOFF
VAC_OK
(pin 16) 0.485V
AC_FAIL
IHYS
15 µA
Vcc
(pin 5)
GD
(pin 4)
Vout
Vcc_PFC
(pin 6)
t
0.45V
t
t
t
t
t
t
t
Doc ID 13794 Rev 4
37/52
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