VIPER0PHDTR datasheet(19/36 Pages) STMICROELECTRONICS

19 / 36 page

VIPer0P

General description

DocID028423 Rev 2

19/36

Figure 27: Soft startup

4.6

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 with respect to the one that should be needed in absence

of jittering feature. Two options with different switching frequencies, FOSC, are available: 60

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

4.7

Pulse skipping

The IC embeds a pulse skip circuit that operates in the following way:



each time the DRAIN peak current exceeds IDLIM level within tON_MIN, the switching

cycle is skipped. The cycles can be skipped until the minimum switching frequency is

reached, FOSC_MIN (15 kHz, typ.).



each time the DRAIN peak current does not exceed IDLIM within tON_MIN, a switching

cycle is restored. The cycles can be restored until the nominal switching frequency is

reached, FOSC (60 or 120 kHz, typ.)

If the converter is indefinitely operated at FOSC_MIN, the IC is turned off after the time tOVL_MAX

(200 ms or 400 ms typ., depending on FOSC) and then automatically restarted with soft start

phase, after the time tRESTART (1 sec, typ.).

The protection is intended in order to avoid the so called "flux runaway" condition often

present at converter startup or in case of a dead-short at converter output and due to the

fact that the primary MOSFET, which is turned on by the internal oscillator, cannot be

turned off before the minimum on-time.

VIPER0PHDTR 数据表(PDF) 19 Page - STMicroelectronics