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VIPer26K

General description

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4.5

Oscillator

The switching frequency is internally fixed at 60 kHz.

The switching frequency is modulated by approximately ±4 kHz 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.

4.6

Soft-start

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. 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, and the function is activated for

any attempt of converter startup and after a fault event.

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

4.7

Current limit set point

The VIPer26K 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. 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 checks the level of the drain

current and switches OFF the power MOSFET in case the current is higher than the

threshold IDlim.

The IC is available with two different drain current limitations: the VIPer267K has a 700 mA

(typical value), whereas the VIPer265K is available with 500 mA current limitation.

Both values are ensured with tolerance reported in Table 3.

4.8

FB pin and COMP pin

The device can be used both in non-isolated and in isolated topology. In the case of non-

isolated topology, the feedback signal from the output voltage is applied directly to the FB

pin as inverting input of the internal error amplifier having the reference voltage, VREF_FB.

The output of the error amplifier sources and sinks the current, ICOMP, respectively to and

from the compensation network connected on the COMP pin. This signal is then compared,

in the PWM comparator, with the signal coming from the sense-FET; the power MOSFET is

switched off when the two values are the same on a cycle-by-cycle basis.

When the power supply output voltage is equal to the error amplifier reference voltage,

VREF_FB, a single resistor has to be connected from the output to the FB pin. For higher

output voltages the external resistor divider is needed. If the voltage on the FB pin is

accidentally left floating, an internal pull-up protects the controller.

The output of the error amplifier is externally accessible through the COMP pin and it's used

for the loop compensation: usually an RC network.