LED6000PHTR datasheet(21/49 Pages) STMICROELECTRONICS

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LED6000

Functional description

4.6

Overcurrent protection

The LED6000 implements overcurrent protection by sensing the current flowing through the

power MOSFET. Due to the noise created by the switching activity of the power MOSFET,

the current sensing circuitry is disabled during the initial phase of the conduction time. This

avoids erroneous detection of fault condition. This interval is generally known as “masking

time” or “blanking time”. The masking time is about 120 ns.

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 output current constant and equal to the current limit. 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 13).

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

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

the output short-circuit the maximum switching frequency can be computed by the following

equation:

Equation 4



MIN

LIM

DCR

LIM

DCR

MAX















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