VIPER012LSTR datasheet(20/36 Pages) STMICROELECTRONICS

20 / 36 page

General description

VIPer01

20/36

DocID028751 Rev 1

If the converter is operated at FOSC_MIN, the IC is turned off after the time tOVL_MAX (100 ms or

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

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

The protection is intended to avoid the so called "flux-runaway" condition often present at

converter startup and due to the fact that the primary MOSFET, which is turned on by the

internal oscillator, cannot be turned off before than the minimum on-time.

During the on-time, the inductor is charged by the input voltage and if it cannot be

discharged by the same amount during the off-time, in every switching cycle there is an

increase of the average inductor current, that can reach dangerously high values until the

output capacitor is not charged enough to ensure the inductor discharge rate needed for

the volt-second balance. This condition may happen at converter startup, because of the

low output voltage.

In the following Figure 28: "Pulse-skipping during startup" the effect of pulse-skipping

feature on the DRAIN peak current shape is shown (solid line), compared with the DRAIN

peak current shape when pulse-skipping feature is not implemented (dashed line).

Providing more time for cycle-by-cycle inductor discharge when needed, this feature is

effective by keeping low the maximum DRAIN peak current avoiding the flux-runaway

condition.

Figure 28: Pulse-skipping during startup

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