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 protection acts by turning 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 13. OCP and frequency scaling

Tmask

Ton

I_LIM

I_IND

Up to seven clock pulses can be skipped

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 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 = 4 A the peak current limit, RON = 0.25 Ω 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 801 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 equation below:

ILIM= FSW⋅TON⋅VIN−8⋅VF

8⋅RDCR+FSW⋅TON,MINRON+RDCR

(5)

The peak current limit threshold (ILIM) can be programmed in the range 0.85 A-4.0 A by selecting the proper RILIM

resistor, as suggested below:

RILIM=20kΩ⋅IPKILIM

(6)

IPK is the default ALED6000 current limit in case of RILIM not mounted, as shown in Table 5. Electrical

characteristics .

ALED6000

Overcurrent protection

DS13018 - Rev 4

page 17/45