Functional description

AST1S31HF

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DocID026968 Rev 2

6.4

Overcurrent protection

The AST1S31 device implements overcurrent protection sensing the current flowing through

the high-side current switch.

If the current exceeds the overcurrent threshold the high-side is turned off, implementing

cycle-by-cycle current limitation. Since the regulation loop is no more fixing the duty cycle,

the output voltage is unregulated and the FB pin falls accordingly to the new duty cycle.

The mechanism to adjust the switching under the current foldback condition exploits the

low-side current sense circuitry.

If FB is lower than 0.2 V, the high-side power MOSFET is turned off after the minimum

conduction time (approximately 100 nsec typ.), then, after a proper deadtime that avoids the

cross conduction, the low-side is turned on until the low-side current is lower than

a valley threshold (1.5 A typ.). Once the low-side is turned off the high-side is immediately

turned on.

In this way the frequency is adjusted to keep the inductor current ripple between peak

current value that could be evaluated with the following equation:

Equation 16

current threshold (1.5 A typ.), so properly limiting the output current in case of the

overcurrent or short-circuit .

The overcurrent protection is always effective when VFB < 0.2 V thanks to the natural

frequency reduction.

No frequency foldback is otherwise implemented when VFB > 0.2 V. In this case, when the

current ripple during the on phase is bigger than the one during the off phase, there will be

a peak current level higher than the current limit threshold.

The following equations show the inductor current ripple during the ON and OFF phases in

case of overcurrent condition:

On phase:

Equation 17

Where:

Equation 18

HS

+

 I

Valley



–

+

L

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





+

=

I

Ton

–

HS

+

 I



–

L

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





=

0.8

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



=