L7986TA

Functional description

Doc ID 022098 Rev 3

13/43

5.4

Overcurrent protection

The L7986TA implements overcurrent protection by sensing current flowing through the

power MOSFET. Due to the noise created by the switching activity of the power MOSFET,

the current sensing is disabled during the initial phase of the conduction time. This avoids an

erroneous detection of a fault condition. This interval is generally known as “masking time”

or “blanking time”. The masking time is about 200 ns.

If the overcurrent limit is reached, the power MOSFET is turned off implementing pulse-by-

pulse overcurrent protection. In the overcurrent condition, the device can skip turn-on pulses

in order to keep the output current constant and equal to the current limit. If, at the end of the

“masking time”, the current is higher than the overcurrent threshold, the power MOSFET is

turned off and one pulse is skipped. If, at the following switching on, when the “masking

time” ends, the current is still higher than the overcurrent threshold, the device skips two

pulses. This mechanism is repeated and the device can skip up to seven pulses. While, if at

the end of the “masking time”, the current is lower than the overcurrent threshold, the

number of skipped cycles is decreased by one unit (see Figure 7).

So, the overcurrent/short-circuit protection acts by switching off the power MOSFET and

reducing the switching frequency down to one eighth of the default switching frequency, in

order to keep constant the output current around the current limit.

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

from Equation 3, the maximum switching frequency that guarantees to limit the current

results:

Equation 4

maximum frequency to keep the output current limited during the short-circuit results 88

kHz.

The pulse-by-pulse mechanism, which reduces the switching frequency down to one eighth

limitation, is 88 kHz*8 = 706 kHz.

V

IN

V

–

OUT

R

DSON

I

OUT

⋅

DCR

I

OUT

⋅

–

–

LF

SW

⋅

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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

DC R

+

() I

LIM

⋅

–

()

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

1

T

ON_MIN

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

⋅

=