VIPER06XS datasheet(16/28 Pages) STMICROELECTRONICS

16 / 28 page

Oscillator

VIPER06

16/28

Doc ID 022794 Rev 1

Oscillator

The switching frequency is internally fixed at 30 kHz or 60 kHz or 115 kHz (respectively part

numbers VIPER06Xx, VIPER06Lx and VIPER06Hx).

The switching frequency is modulated by approximately ±3 kHz (30 kHz version) or ±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.

Soft startup

During the converter’s startup phase, the soft-start function progressively increases the

cycle-by-cycle drain current limit, up to the default value IDlim. In 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

in Table 7 on page 8, and the function is activated for any attempt of converter startup and

after a fault event.

This function helps prevent saturation of the transformer during startup and short-circuit.

Adjustable current limit set point

The VIPER06 includes a current-mode PWM controller. The drain current is sensed cycle-

by-cycle through the integrated resistor RSENSE and the voltage is applied to the non-

inverting input of the PWM comparator, see Figure 2 on page 4. 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 4, checks

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

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

The level of the drain current limit IDlim can be reduced using a resistor RLIM connected

between the LIM and GND pins. Current is sunk from the LIM pin through the resistor RLIM

and the setup of IDlim depends on the level of this current. The relation between IDlim and

RLIM is shown in Figure 13 on page 11.

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

fixed to its default value, IDlim, as given in Table 7 on page 8.

VIPER06XS Datasheet(PDF) 16 Page - STMicroelectronics