AN2929 datasheet(26/32 Pages) STMICROELECTRONICS

26 / 32 page

Testing the board

AN2929

26/32

Doc ID 15333 Rev 2

2.8

Brownout protection

The brownout protection is basically an unlatched device shutdown functionality whose

typical use is to sense flyback converter input undervoltage. The VIPER27LN has a pin (BR,

pin 5) dedicated to this function that must be connected to the DC HV bus. If the protection

is not required, it can be disabled by connecting the pin to ground. In the present converter

the brownout protection is implemented but can be disabled by changing the setting of

jumper J7 (see schematic in Figure 2). The settings of jumper J7 are shown in Figure 27

and Figure 28. The shutdown of the converter is accomplished by means of an internal

comparator internally referenced to 450 mV (typ, VBRth) that disables the PWM if the voltage

applied to the BR pin is below the internal reference, as shown in Figure 29. The PWM

operation is re-enabled as the BR pin voltage is more than 450 mV plus 50 mV of voltage

hysteresis that ensures noise immunity. The brownout comparator is also provided with

current hysteresis. An internal current generator is ON as long as the voltage applied to the

brownout pin is below 450 mV and is OFF if the voltage exceeds 450 mv plus the voltage

hysteresis.

Figure 29.

Brownout protection

Figure 27.

Jumper J7 setting, brownout

disabled

Figure 28.

Jumper J7 setting, brownout

enabled

AM02176v1

100mV Typ.

IBRhyst

VBRth

VAC_OK

Disable

VBRhyst

AN2929 Datasheet(PDF) 26 Page - STMicroelectronics