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 inductor current

constant and equal to the current limit, assuming only a slight drift due to input and output voltage variation.

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 (refer to Figure 8. OCP and frequency scaling).

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.

As a consequence, 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 close to the current limit.

Figure 8. OCP and frequency scaling

Tmask

Ton

I_LIM

I_IND

Up to seven clock pulses can be skipped

If the sensed output voltage, monitored through FB pin, falls below the VFOLD threshold (400 mV typ.) the peak

current limit threshold is reduced to 1/3 of the nominal value. This additional feature helps to reduce the IC stress

in case of output short-circuit.

As soon as the FB pin increases above the VFOLD threshold, the full peak current limit threshold is restored. This

fold-back protection is disabled during the soft-start.

This kind of overcurrent protection is effective if the inductor can be completely discharged during HS MOS turn-

off time, so the inductor current does not run away. In case of output short-circuit, the maximum switching

frequency can be computed by the following equation:

FSW,MAX≤ 8⋅ VF+RDCR⋅ILIM

VIN− RON+RDCR ⋅ILIM⋅ 1TON,MIN

(4)

Assuming VF=0.6 V the free-wheeling diode direct voltage, RDCR=30 mΩ the inductor parasitic resistance,

ILIM=IPK=1.3 A the peak current limit during fold-back protection, RON=0.24 Ω the HS MOS resistance and

TON,MIN=160 ns the minimum HS MOS on duration, the maximum FSW frequency which prevents the inductor

current from running away in case of output short-circuit and VIN=61 V is about 530 kHz.

If the programmed switching frequency is higher than the above computed limit, an estimation of the inductor

current in case of output short-circuit fault is provided by the following equation:

ILIM≤ FSW⋅TON⋅VIN−8⋅VF

8⋅RDCR+FSW⋅TON,MIN(RON+RDCR)

(5)

The peak current limit threshold (ILIM) can be programmed in the range of 0.85 A-4 A by selecting the proper

RILIM resistor, as suggested below:

RILIM=20kΩ⋅IPKILIM

(6)

A7987

Overcurrent protection

DS12928 - Rev 3

page 13/36