DS11873 Rev 5

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VIPer11

General description

37

5.5

Soft-start

The internal soft-start function of the device progressively increases the cycle-by-cycle

current limitation set point from zero up to IDLIM in 8 steps. The soft-start time, tSS, is

internally set at 8 ms. This function is activated at any attempt of converter startup and at

any restart after a fault event. The feature protects the system at startup, when the converter

would run at its maximum drain current limitation because the output capacitor is fully

discharged and behaves like a short-circuit.

Figure 28. Soft startup

5.6

Oscillator

The IC embeds a fixed frequency oscillator with jittering feature. The switching frequency is

modulated by approximately ± 7% kHz FOSC at 260 Hz rate. The purpose of the jittering is to

get a spread-spectrum action that distributes the energy of each harmonic of the switching

frequency over a number of frequency bands, having the same energy on the whole but

smaller amplitudes. This helps to reduce the conducted emissions, especially when

measured with the average detection method or, which is the same, to pass the EMI tests

with an input filter of smaller size than that needed in absence of jittering feature.

Three options with different switching frequencies, FOSC, are available: 30 (X type),

60 kHz (L type) and 120 kHz (H type).