Oscillator

VIPER06

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Doc ID 022794 Rev 1

9

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.

10

Soft startup

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

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

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.

11

Adjustable current limit set point

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

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