L5987A 데이터 시트보기 (PDF) - STMicroelectronics

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L5987A

3 A step-down switching regulator

STMicroelectronics 

L5987

5.4

Functional description

Overcurrent protection

The L5987 device implements the overcurrent 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 200 ns.

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

operating condition.

 Output voltage in regulation. When the overcurrent 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 overcurrent limit is reached, the Power MOSFET is turned off

implementing the pulse by pulse overcurrent protection. During the soft-start phase,

under overcurrent 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 overcurrent 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 overcurrent threshold, the number of

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

voltage is in regulation and if the overcurrent persists, the behavior explained above

takes place.(see Figure 8.b).

So the overcurrent protection can be summarized as a “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 overcurrent is triggered and the device

starts cycling with a period of 2048 clock cycles between the “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 33 on

page 357 for short-circuit behavior.

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