ST1S32

Functional description

Doc ID 023246 Rev 1

13/29

Figure 7.

Phase Bode plot

4.3

Overcurrent protection

The ST1S32 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 pin 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 undercurrent 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 dead

time 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), therefore

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 make the FB 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. That is, 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.

AM11421v1

0.1

1

10

100

1 .10

3

1 .10

4

1 .10

5

1 .10

6

1 .10

7

210

182.5

155

127.5

100

72.5

45

17.5

10

Frequency [Hz]