VIPER11 datasheet(20/37 Pages) STMICROELECTRONICS

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General description

VIPer11

20/37

DS11873 Rev 5

5.7

Pulse-skipping

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



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

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

reached, FOSC_MIN (15 kHz).



Each time the DRAIN peak current does not exceed IDLIM within tON_MIN, one switching

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

reached, FOSC (30, 60 or 120 kHz).

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 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 a net

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 Figure 29 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 29. Pulse-skipping during startup

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