AST1S31HF Datasheet, PDF (16/33 Pages) STMicroelectronics – Internal soft-start and enable

Functional description
AST1S31HF
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
I_Peak = I_Valley + -^V----^I-ⁿ----+-----^V-----^O----^u---^t--–-----^--^D-----^C-----^R----^L----+-_L----^R-----^D----^S---^--^o---ⁿ---^--^H---^S----^---^-----^I--^V----^a---^l-^l-^e---^y-  T_Onmin
where DCR_L is the series resistance of the inductor and the measured value of the valley
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
I_Ton = -^V----^I-ⁿ-----–----^V----^O----^u---^t---–----^---^D-----^C----^R--_L---^L----+-----^R----^D----^S----^-^o---ⁿ---^--^H----^S----^---^-----^I  T_Onmin
Where:
Equation 18
V_Out = V_FB  ^V-----^O---₀-^U--_.-^T-₈---^S---^e---^t
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