L6566B datasheet(37/51 Pages) STMICROELECTRONICS

37 / 51 page

L6566B

Application information

37/51

Equation 15

where TBLANK2 = 2 µs; this is also illustrated in the diagram of Figure 24.

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 primary power section due to an

excess of RMS current. Secondly, spurious restarts may occur during converter power

down, hence causing the output voltage not to decay to zero monotonically.

L6566B shutdown upon brownout is accomplished by means of an internal comparator, as

shown in the block diagram of Figure 25 on page 38, which shows the basic usage. The

inverting input of the comparator, available on the AC_OK pin (16), is supposed to sense a

voltage proportional to the RMS (peak) mains voltage; the non-inverting input is internally

referenced to 0.485V with 35 mV hysteresis. If the voltage applied on the AC_OK pin before

the device starts operating does not exceed 0.485V or if it falls below 0.45 V while the

device is running, the AC_FAIL signal goes high and the device shuts down, with the soft-

start capacitor discharged and the gate-drive output low. Additionally, if the device has been

latched off by some protection function (testified by Vcc oscillating between VccON and

VccON - 0.5V) the AC_OK voltage falling below 0.45 V clears the latch. This may allow a

quicker restart as the input source is removed.

While the brownout protection is active the start-up generator keeps on working but, being

there no PWM activity, the Vcc voltage continuously oscillates between the start-up and the

HV generator restart thresholds, as shown in the timing diagram of Figure 25.

Figure 24. Maximum allowed duty cycle vs. switching frequency for correct OVP

detection

BLANK

≤

5 .10

1 .10

1.5 .10

2 .10

2.5.10

3 .10

3.5.10

4 .10

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.725

fsw [Hz]

Dmax

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