Functional description

ST1S31

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DocID022998 Rev 4

Figure 8. Phase bode plot

4.4

Overcurrent protection

The ST1S31 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

a cycle-by-cycle current limitation. Since the regulation loop is no longer fixing the duty

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

cycle.

If the FB falls below 0.2 V, the peak current limit is reduced to around 2.3 A and the

switching frequency is reduced to assure that the inductor current is properly limited below

the above mentioned value and above 1.2 A. This strategy is called “current foldback”.

The mechanism to adjust the switching frequency during 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 when the current reaches the current foldback threshold (2.3 A), 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 (1.2 A). 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 the current foldback value (2.3 A) and valley threshold

(1.2 A), so properly limiting the output current in case of overcurrent or short-circuit.

It should be noted that in some cases, mainly with very low output voltages, the hard

overcurrent can cause the FB to find the new equilibrium just over the current foldback

threshold (0.2 V). In this case no frequency reduction is enabled, then the inductor current

may diverge. This means that the ripple current during the minimum ON-time is higher than

the ripple current during the OFF-time (the switching period minus the minimum ON-time),

so pulse-by-pulse, the average current is rising, exceeding the current limit.

In order to avoid too high current, a further protection is activated when the high-side current

exceeds a further current threshold (OCP2) slightly over the current limit (OCP1). If the

current triggers the second threshold, the converter stops switching, the reference of the

error amplifier is pulled down and then it restarts with a soft-start procedure. If the

overcurrent condition is still active, the current foldback with frequency reduction properly

limits the output current to 2.3 A.