A6986I datasheet(21/62 Pages) STMICROELECTRONICS

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Figure 22. Valley current sense operation in short-circuit

Figure 22 shows the switching frequency reduction during the valley current sense operation in case of extremely

low duty cycle (VIN = 24 V, fSW = 500 kHz short-circuit condition).

In a worst case scenario (like Figure 22) of the overcurrent protection the switch current is limited to:

IMAX= IVALLEYTH+VIN‐VOUTL∙TMASKHS

(2)

where IVALLEY_TH is the current threshold of the valley sensing circuitry (see Table 1) and TMASK_HS is the

masking time of the high-side switch (330 ns typ.).

In most of the overcurrent conditions the conduction time of the high-side switch is higher than the masking time

and so the peak current protection limits the switch current.

IMAX =IPEAK_TH

(3)

Figure 23. Peak current sense operation in overcurrent condition

Figure 23 shows the effect of the peak current protection. On the left the current (blue waveform) during the

on-time reaches the peak current protection threshold. From this point on, any increase of the output current

causes the on-time to be reduced (image on the right) so that the current provided to the output is limited. As a

consequence, the output voltage drops, as the purple waveform depicts.

The DC current flowing in the load in overcurrent condition is:

IDCOC (VOUT) = IMAX‐V IN‐VOUT2L∙TON

(4)

A6986I

Overcurrent protection

DS13481 - Rev 2

page 21/62

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