L6566ATR datasheet(38/51 Pages) STMICROELECTRONICS

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Application information

L6566A

38/51

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, hence causing

the output voltage not to decay to zero monotonically.

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 pin Vcc_PFC 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 will clear 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

L6566A

AC_FAIL

AC_OK

Vcc

0.485V

0.45V

15 µA

Sensed

voltage

Vcc

(pin 5)

(pin 4)

Vout

VAC_OK

(pin 16)

0.45V

IHYS

15 µA

Vcc_PFC

(pin 6)

AC_FAIL

Sensed voltage

VsenON

VsenOFF

0.485V

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