AN4248 datasheet(7/46 Pages) STMICROELECTRONICS

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AN4248

AC_OK setting for brownout

This pin is used for brownout function when the mains voltage goes below minimum

operating level. This is not a latched shutdown. The purpose of this protection feature is to

prevent any overheating at the primary side due to brownout condition. A voltage below 0.45

V shuts down (not latched) the IC, lowers its consumption and clears the latch set by latched

protections. The IC's operation is re-enabled as the voltage exceeds 0.45 V. The protection

is done by using a simple resistor divider network RH and RL. By setting AC_OK pin voltage

at 0.45 V with proper selection of divider resistor at < 90 Vac, we can make the controller

shut down at desired mains undervoltage level.

The brownout comparator is provided with current hysteresis in addition to voltage

hysteresis: an internal 15

μA current sink is ON as long as the voltage applied on the

AC_OK pin is such that the AC_FAIL signal is high.

Figure 2. Brownout protection: internal block diagram

The following relationships can be established for the ON (VsenON) and OFF (VsenOFF)

thresholds of the sensed voltage:

Equation 1

From the above relations, we derive the upper resistor and lower resistor values of the

divider network as below:

Equation 2

After calculating, referring to the schematic we have following values :

R4 = R9 = R13 = 330 K, R21 = 5.1 K and R22 = 510E approximately.

AN4248 Datasheet(PDF) 7 Page - STMicroelectronics