L7987L datasheet(18/41 Pages) STMICROELECTRONICS

18 / 41 page

Functional description

L7987L

18/41

DocID026362 Rev 3

Figure 8. OCP and frequency scaling

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 foldback 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, in order to avoid the inductor current to run away. In case of

output short-circuit the maximum switching frequency can be computed by Equation 4.

Equation 4

Assuming VF = 0.6 V the freewheeling diode direct voltage, RDCR = 70 m the inductor

parasitic resistance, ILIM = IPK = 0.9 A the peak current limit during foldback protection,

RON = 0.30  the HS MOS resistance and TON,MIN = 120 ns the minimum HS MOS on

duration, the maximum FSW frequency which avoids the inductor current run away in case

of output short-circuit and VIN = 61 V is 728 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

Equation 5:

Equation 5

FSW MAX



 V

RDCR I



LIM



VIN RON RDCR



–

 I

LIM

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

TON MIN



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





ILIM

FSW TON VIN 8VF



–



8RDCR FSW TON MIN



 R

RDCR





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