AN2115

Operation description

Doc ID 11165 Rev 5

15/33

So the higher the output current, the higher the negative slope during OFF time. In this way

the inductor current will find a stable find value, which can be derived using the equation

below:

Equation 6

these extreme conditions the maximum current value depends on both the application

The maximum current value does not depend on the peak current limit at all. In order to limit

the output current to a safe value even in extreme short circuit conditions, a current limit has

also been introduced on the N-channel MOSFET, which operates as a valley current limit.

Figure 11 shows its operation. The P-channel MOSFET does not turn on until the inductor

current exceeds the valley current limit. This implies that the device skips some cycles

depending on the overcurrent conditions, reducing the equivalent switching frequency in

order to limit the output current. With this approach, the maximum peak current is definitively

limited to:

Equation 7

Figure 12.

Valley current limit intervention

I

LIM

V

in

T

MIN

F

SW

•

()

•

R

N

R

L

+

()

1T

MIN

F

SW

•

–

()

R

P

R

L

+

()

T

MIN

F

SW

•

()

•

+

•

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=

I

LIM

I

VALLEY

V

L

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

+

=