L5988D datasheet(21/52 Pages) STMICROELECTRONICS

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L5988D

Functional description

5.5.2

Current limiting

The current limiting feature acts in different ways depending on the operative conditions.



In case an overcurrent detection happens after the soft-start phase, the internal logic

will trigger the “HICCUP” mode. Both switches are turned off and the soft-start

capacitor is discharged with a constant current of 22 A (typ.). When the SS/INH

voltage drops below the INH threshold a new SS cycle will start.



During the soft-start phase the overcurrent information is used to provide a constant

current protection. In this way additional current is available to charge the output

capacitor during power up.

The most common way is to sense the current flowing through the power MOSFETs.

However, due to the noise created by the switching activity of the power MOSFETs, the

current sense 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”. For this reason, the current cannot be sensed through the high-side

MOSFET in the case of extremely low duty cycles, nor through the low-side MOSFET in the

case of very high duty cycles.

The L5988D assures the effective protection sensing the current flowing in both embedded

switches. The protection achieved by sensing the current in the high-side MOSFET is called

“peak overcurrent protection”, while the protection achieved by sensing the current in the

low-side MOSFET is called “valley overcurrent protection”. When the current limit is reached

during normal operation, the so called HICCUP mode is triggered, and the soft-start cap is

discharged and recharged. However, during the start-up phase, additional current is

required to charge the output capacitor. This could continuously trigger the HICCUP

intervention preventing the system from reaching a steady working condition. For this

reason the HICCUP feature is disabled during the start-up phase and a constant current

mode is active to charge the output capacitor. In this case, when the peak current limit is

triggered after a conduction time equal to the “masking time”, the high-side MOSFET is

turned off and the low side MOSFET is kept on until the flowing current goes below the

“valley” current limit. If necessary, some switching pulses are skipped, as illustrated in

Figure 11. Thus, the combination of the “peak” and “valley” current limits assure the

effectiveness of the overcurrent protection even in extreme duty cycle conditions. The

current threshold of the low side is designed higher than the high side one to guarantee the

proper protection.

The constant current mode during the soft-start phase limits the maximum current up to:

Equation 7

The overcurrent limit protection is adjustable (higher or lower than the nominal value)

through an external resistor. To guarantee effective protection, both thresholds (valley and

peak) are in tracking.

IMAX

IVALLEY_TH

VIN VOUT

–

------------------------------ TMASK



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