AN1882

Short-circuit protection

Doc ID 10209 Rev 4

13/30

Figure 10.

Equivalent circuit during the OFF time

Considering the figures above, in particular during the OFF time, in despite of the output

voltage is zero, the output current generates, on the parasitic resistances, the voltage drop

necessary to produce a negative slope. So, the higher will be the output current, the higher

will be the negative slope during the OFF time; in this way, the inductor current will find a

stable value. This value is given by:

Equation 6

conditions, the maximum current value depends both on the application conditions (like VIN

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 low

side MOSFET: this operates as a valley current limit, as shown in

Figure 11. The high side

MOSFET does not turn-on until the inductor current exceeds the valley current limit. This

implies that, depending on the over current conditions, the device skips some cycles, so

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

I

LIM

V

IN

T

MIN

F

SW

⋅

()

⋅

R

N

R

L

+

() 1T

MIN

F

SW

⋅

–

()

R

P

R

L

+

() T

MIN

F

SW

⋅

()

⋅

+

⋅

[]

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

=

I

LIM

I

VALLEY

V

L

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

+

=