L5983 datasheet(13/37 Pages) STMICROELECTRONICS

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Functional description

L5983

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4.4

Over-current protection

The L5983 implements the over current protection sensing current flowing through the

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

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

erroneous detection of a fault condition. This interval is generally known as “masking time”

or “blanking time”. The masking time is about 200ns.

When the over current is detected, two different behaviors are possible depending on the

operation condition.

Output voltage in regulation. When the over current is sensed, the power MOSFET is

switched off and the internal reference (VREF), that biases the non-inverting input of the

error amplifier, is set to zero and kept in this condition for a soft start time (TSS, 2048

clock cycles). After this time, a new soft start phase takes place and the internal

reference begins ramping (see Figure 8.a).

Soft start phase. If the over current limit is reached the power MOSFET is turned off

implementing the pulse by pulse over current protection. During the soft start phase,

under over current condition, the device can skip 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 over current threshold, the power MOSFET is turned off and it

will skip one pulse. If, at the next switching on at the end of the "masking time" the

current is still higher than the threshold, the device will skip two pulses. This

mechanism is repeated and the device can skip up to seven pulses. While, if at the end

of the "masking time" the current is lower than the over current threshold, the number of

skipped cycles is decreased of one unit. At the end of soft start phase the output

voltage is in regulation and if the over current persists the behavior explained above

take place. (see Figure 8.b)

So the over current protection can be summarized as an “hiccup” intervention when the

output is in regulation and a constant current during the soft start phase.

If the output is shorted to ground when the output voltage is on regulation, the over current is

triggered and the device starts cycling with a period of 2048 clock cycles between “hiccup”

(power MOSFET off and no current to the load) and “constant current” with very short on-

time and with reduced switching frequency (up to one eighth of normal switching frequency).

See figure 32 for short circuit behavior.

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