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L7987

Functional description

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4.4

Low VIN operation

In normal operation (i.e. VOUT programmed lower than input voltage) when the HS MOS is

In case the input voltage falls close or below the programmed output voltage (low dropout,

LDO) the L7987 control loop is able to increase the duty cycle up to 100%. However, in

order to keep the boot capacitor properly recharged, a maximum HS MOS on time is limited

between LX and GND is turned on until one of the following conditions is met:



A negative current limit (300 mA typ.) is reached



A timeout (1

s typ.) is reached.

So doing the L7987 device is able to work in low dropout operation, due to the advanced

boot capacitor management, and the effective maximum duty cycle is about

12 µs / 13 µs = 92%.

4.5

Overcurrent protection

The L7987 device implements an 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 an erroneous detection of a 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 inductor current constant and equal to the current limit, assuming only a

slight drift due to input and output voltage variation.

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 8).

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/short-circuit protection acts by switching 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.