VIPER38HD datasheet(18/31 Pages) STMICROELECTRONICS

18 / 31 page

Operation

VIPER38

18/31

DocID025703 Rev 2

7.7

Current mode conversion with adjustable current limit

setpoint

The device is a current mode converter. The drain current is sensed and converted to

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

compared with the one on the feedback pin through a voltage divider on a cycle-by-cycle

basis.

The device has a default current limit value, IDlim, that can be adjusted according to the

electrical specification, by the RLIM resistor connected to the CONT pin.

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 set to the default value (see IDlim,

Table 8: Controller section).

7.8

Overvoltage protection (OVP)

The device has an integrated logic for the monitoring of the output voltage using as an 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 shown in Figure 22: CONT pin configuration. When, during the

OFF time, the voltage VCONT exceeds four consecutive times the reference voltage VOVP

(see Table 8: Controller section), the overvoltage protection will stop the power MOSFET

and the converter enters 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:

Controller section and the Figure 21: OVP timing diagram. The sampled signal, if higher

than VOVP, triggers 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 Figure 22: CONT pin configuration, the resistor divider ratio kOVP will be given

by:

Equation 2

Equation 3

OVP

AUX

SEC

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

OUTOVP

DSEC

() V

DAUX

–

⋅

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

OVP

LIM

OVP

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

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