DocID022801 Rev 5

13/42

A7986A

Functional description

This kind of overcurrent protection is effective if the output current is limited. To prevent the

current from diverging, the current ripple in the inductor during the on-time must not be

higher than the current ripple during the off-time. That is:

Equation 3

So, from the above equation, the maximum switching frequency that guarantees to limit the

current results:

Equation 4

the maximum frequency to keep the output current limited during the short-circuit results

88 kHz.

Based on the pulse-by-pulse mechanism, that reduces the switching frequency down to one

output current limitation is 88 kHz*8 = 706 kHz.

condition the system finds a different equilibrium with higher current. For example, with

Equation 5

V

IN

V

–

OUT

R

DSON

I

OUT

⋅

DCR I

OUT

⋅

–

–

LF

SW

⋅

------------------------------------------------------------------------------------------------------------ D

⋅

V

OUT

V

F

R

DSON

I

OUT

⋅

DCR I

OUT

⋅

++

+

LF

SW

⋅

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

1D

–

()

⋅

=

F

SW

*

V

F

DCR I

⋅

+

LIM

()

V

IN

R

DSON

DCR

+

() I

LIM

⋅

–

()

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

1

T

ON_MIN

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

⋅

=

I

OUT

V

IN

F

SW

*

⋅

V

⁄

–

DRC T

ON_MIN

⁄

()

R

DSON

DCR

+

() F

SW

*

⋅

+

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

4.2A

==