AN2929 datasheet(27/32 Pages) STMICROELECTRONICS

27 / 32 page

AN2929

Testing the board

Doc ID 15333 Rev 2

27/32

The current hysteresis provides an additional degree of freedom. It is possible to set the ON

threshold and the OFF threshold for the flyback input voltage separately by properly

choosing the resistors of the external divider. The following relationships can be established

for the ON (VIN_ON) and OFF (VIN_OFF) thresholds of the input voltage:

Equation 4

Equation 5

where IBRhyst = 8.5 µA (typ.) is the current hysteresis, VBRhyst = 50 mV (typ.) is the voltage

hysteresis and VBRth = 450 mV (typ.) is the brownout comparator internal reference.

One purpose of this protection is to stop operation of the converter when the line voltage is

too low, avoiding an excessive root mean square current value flowing inside the main

switch and consequently its overheating. Another purpose is to avoid a false restart of the

converter and then having a monotonic decay to zero of the output voltage when the

converter itself is unplugged from the mains. A typical situation, in most cases for converters

designed for the European range (230 VAC), could be a converter that, when unplugged,

shuts down due to the overload protection (due to the low input voltage the converter is not

able to supply the full power), but the voltage on the bulk capacitor is higher than VDRAIN

RESTART so the device restarts and the output voltage rises again. This situation could be

dangerous for some loads, and in many applications it is better to avoid it.

The following figures show how the brownout protection works in the VIPER27LN board

when used. Figure 30 shows the behavior of the board when the input voltage is changed

from 90 VAC to 75 VAC with a full load applied. The system stops switching and the output

load, no longer supplied, decays monotonically to zero.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

BRth

OFF

()

BRhyst

BRth

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

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