VIPER26 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

VIPER26

Fixed frequency VIPer™ plus family

STMicroelectronics 

Oscillator

9

Oscillator

VIPER26

The switching frequency is internally fixed at 60 kHz (VIPER26LN or LD) or 115 kHz

(VIPER26HN or HD).

In both cases the switching frequency is modulated by approximately ±4 kHz (60 kHz

version) or ±8 kHz (115 kHz version) at 230 Hz (typical) rate, so that the resulting spread-

spectrum action distributes the energy of each harmonic of the switching frequency over a

number of sideband harmonics having the same energy on the whole but smaller

amplitudes.

10 Soft start-up

During the converters' start-up phase, the soft-start function progressively increases the

cycle-by-cycle drain current limit, up to the default value IDlim. By this way the drain current is

further limited and the output voltage is progressively increased reducing the stress on the

secondary diode. The soft-start time is internally fixed to tSS, see typical value on Table 8 on

page 7, and the function is activated for any attempt of converter start-up and after a fault

event.

This function helps prevent transformers' saturation during start-up and short-circuit.

11 Adjustable current limit set point

The VIPer26 includes a current mode PWM controller: cycle by cycle the drain current is

sensed through the integrated resistor RSENSE and the voltage is applied to the non

inverting input of the PWM comparator, see Figure 2 on page 3. As soon as the sensed

voltage is equal to the voltage derived from the COMP pin, the power MOSFET is switched

OFF.

In parallel with the PWM operations, the comparator OCP, see Figure 2 on page 3, checks

the level of the drain current and switch OFF the power MOSFET in case the current is

higher than the threshold IDlim, see Table 8 on page 7.

The level of the drain current limit, IDlim, can be reduced depending from the sunk current

from the pin LIM. The resistor RLIM, between LIM and GND pins, fixes the current sunk and

than the level of the current limit, IDlim, see Figure 13 on page 9.

When the LIM pin is left open or if the RLIM has an high value (i.e. > 80 kΩ) the current limit

is fixed to its default value, IDlim, as reported on Table 8 on page 7.

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Doc ID 17736 Rev 2

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