L6928D datasheet(9/16 Pages) STMICROELECTRONICS

9 / 16 page

L6928

Operation description

9/16

4.2

Short circuit protection

During the device operation, the inductor current increases during the high side turn ON

phase and decrease during the high side turn off phase based on the following equations:

Equation 1

Equation 2

In strong overcurrent or shortcircuit conditions the VOUT can be very close to zero. In this

case

∆ION increases and ∆IOFF decreases. When the inductor peak current reaches the

current limit, the high side mosfet turns off and so the TON is reduced down to the minimum

value (250ns typ.) in order to reduce as much as possible

∆ION.

Anyway, if VOUT is low enough it can be that the inductor peak current further increases

because during the TOFF the current decays very slowly.

Due to this reason a second protection that fixes the maximum inductor valley current has

been introduced. This protection doesn't allow the high side MOSFET to turn on if the

current flowing through the inductor is higher that a specified threshold (valley current limit).

Basically the TOFF is increased as much as required to bring the inductor current down to

this threshold.

So, the maximum peak current in worst case conditions will be:

Equation 3

Where IPEAK is the valley current limit (1.4A typ.) and TON_MIN is the minimum TON of the

high side MOSFET.

∆I

OUT

–

()

----------------------------------

⋅

∆I

OFF

OUT

()

-------------------

OFF

⋅

PEAK

VALLEY

---------

ON_MIN

⋅

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