VIPER37HE datasheet(24/35 Pages) STMICROELECTRONICS

24 / 35 page

Operation description

VIPER37

24/35

Doc ID 022218 Rev 1

8.8

Overvoltage protection (OVP)

The device can monitor the converter output voltage. This operation is done by the CONT

pin during Power MOSFET OFF-time, when the voltage generated by the auxiliary winding

tracks the converter's output voltage, through turn ratio

(see

Figure 28).

In order to perform the output voltage monitor, the CONT pin must be connected to the aux.

winding through a resistor divider made up of RLIM and ROVP (see Figure 21 or Figure 29). If

the voltage applied to the CONT pin exceeds the internal reference VOVP (see Table 8) for

four consecutive times, the controller recognizes an overvoltage condition. This special

feature uses an internal counter; that is to reduce sensitivity to noise and prevent the latch

from being erroneously activated (see

Figure 28). The counter is reset every time the OVP

signal is not triggered in one oscillator cycle.

Referring to

Figure 21, the resistors’ divider ratio kOVP is given by:

Equation 2

Equation 3

where:

●

VOVP is the OVP threshold (see Table 8)

●

VOUT OVP is the converter output voltage value to activate the OVP set by the user

●

NAUX is the auxiliary winding turns

●

NSEC is the secondary winding turns

●

VDSEC is the secondary diode forward voltage

●

VDAUX is the auxiliary diode forward voltage

●

ROVP together with RLIM make up the output voltage divider.

Then, once the RLIM value is fixed, according to the desired IDlim, the ROVP can be

calculated by:

Equation 4

AUX

SEC

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

OVP

AUX

SEC

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

OUTOVP

DSEC

() V

DAUX

–

⋅

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

OVP

LIM

OVP

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

OVP

LIM

OVP

–

OVP

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

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