L6986ITR datasheet(20/60 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 = 12 V, fSW = 2 MHz short-circuit condition).

In a worst case scenario of the overcurrent protection the switch current is limited to:

IMAX= IVALLEYTH+VIN‐VOUTL∙TMASKHS

(2)

where IVALLEY_THis 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

This figure 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 (picture 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)

L6986I

Overcurrent protection

DS13647 - Rev 1

page 20/60

L6986ITR Datasheet(PDF) 20 Page - STMicroelectronics