AN3286 datasheet(32/42 Pages) STMICROELECTRONICS

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Testing the board

AN3286

32/42

Doc ID 17998 Rev 1

Figure 41.

Brownout function block diagram

The current hysteresis provides an additional degree of freedom, allowing the designer to

set the ON and OFF thresholds of the input voltage separately from each other, by properly

choosing the values of the resistors of the voltage divider on the BR pin. The following

relationships can be established:

Equation 5

Equation 6

resulting in VIN_OFF = 83 V and VIN_ON = 114 V for the setting of the board. This can be

verified in the figures below, showing how brownout protection works in the VIPer25. For

a better understanding, DC input voltage has been applied.

At mains plug-in, the IBRhyst current generator is activated as soon as the drain voltage

exceeds VDRAIN_START, pulling down the BR pin voltage. If VIN < VIN_ON, VBR is lower than

0.45 V and the PWM is not activated, as shown in Figure 42. If VIN > VIN_ON, the BR pin

voltage is higher than 0.45 V, the PWM is enabled and the switching starts as the VDD

voltage reaches VDDon (Figure 43).

As the input voltage decreases, the IC continues switching until VIN > VIN_OFF, which means

VBR > 0.45 V (Figure 44). When VIN falls below VIN_OFF, the IBRhyst current generator is

turned on, therefore pulling the BR pin voltage below 0.45 V and shutting down the system.

The load is no longer supplied, and the output voltage decays monotonically to zero

(Figure 44).

IN OFF

BRth

---------------------

⎝⎠

⎛⎞

⋅

IN ON

BRth

BRhyst

()

---------------------

⎝⎠

⎛⎞ R

BR hyst

⋅

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