VIPER28

Operation descriptions

19/29

7.7

Current mode conversion with adjustable current limit set

point

The device is a current mode converter: the drain current is sensed and converted in voltage

that is applied to the non inverting pin of the PWM comparator. This voltage is compared

with the one on the feed-back pin through a voltage divider on cycle by cycle basis.

page 12.

Table 8 on page 8).

7.8

Over-voltage protection (OVP)

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

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

tracks converter's output voltage, through turn ratio

See Figure 22.

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

applied to the CONT pin exceeds the internal 3 V reference for four consecutive times the

controller recognizes an over-voltage 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 22 on page 20. The counter is reset every time the OVP signal is not

triggered in one oscillator cycle.

Equation 2

Equation 3

N

AUX

N

SEC

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

k

OVP

V

OVP

N

AUX

N

SEC

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

V

OUTOVP

V

DSEC

+

() V

DAUX

–

⋅

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

=

k

OVP

R

LIM

R

LIM

R

OVP

+

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

=