VIPER28LDTR datasheet(18/32 Pages) STMICROELECTRONICS

18 / 32 page

Operation descriptions

VIPER28

18/32

Doc ID 15028 Rev 5

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.

The device has a default current limit value, IDlim, that the designer can adjust according the

electrical specification, by the RLIM resistor connected to the CONT, see Figure 12 on

page 11.

The CONT pin has a minimum current sunk needed to activate the IDlim adjustment: without

RLIM or with high RLIM (i.e. 100 kΩ) the current limit is fixed to the default value (see IDlim,

Table 8 on page 7).

7.8

Overvoltage protection (OVP)

The device has integrated the logic for the monitor of the output voltage using as input signal

the voltage VCONT during the OFF time of the power MOSFET. This is the time when the

voltage from the auxiliary winding tracks the output voltage, through the turn ratio

The CONT pin has to be connected to the auxiliary winding through the diode DOVP and the

resistors ROVP and RLIM as shows the Figure 23 on page 20 When, during the OFF time,

the voltage VCONT exceeds, four consecutive times, the reference voltage VOVP (see Table 8

on page 7) the overvoltage protection will stop the power MOSFET and the converter enters

the auto-restart mode.

In order to bypass the noise immediately after the turn off of the power MOSFET, the voltage

VCONT is sampled inside a short window after the time TSTROBE, see Table 8 on page 7 and

the

Figure 22 on page 19. The sampled signal, if higher than VOVP, trigger the internal OVP

digital signal and increments the internal counter. The same counter is reset every time the

signal OVP is not triggered in one oscillator cycle.

Referring to the

Figure 23, the resistors divider ratio kOVP will be given by:

Equation 2

Equation 3

AUX

SEC

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

OVP

AUX

SEC

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

OUTOVP

DSEC

() V

DAUX

–

⋅

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

OVP

LIM

OVP

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

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