L6728AH datasheet(12/33 Pages) STMICROELECTRONICS

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Over-current protection

L6728AH

12/33

Doc ID 15726 Rev 1

Over-current protection

The over-current function protects the converter from a shorted output or overload, by

sensing the output current information across the low side MOSFET drain-source on-

resistance, RDS(on). This method reduces cost and enhances converter efficiency by

avoiding the use of expensive and space-consuming sense resistors.

The low side RDS(on) current sense is implemented by comparing the voltage at the PHASE

node when LS MOSFET is turned on with the programmed OCP thresholds voltages,

internally held. If the monitored voltage is bigger than these thresholds, an over-current

event is detected.

For maximum safety and load protection, L6728AH implements a dual level over-current

protection system:

●

1st level threshold: it is the user externally set threshold. If the monitored voltage on

PHASE exceeds this threshold, a 1st level over-current is detected. If four 1st level OC

events are detected in four consecutive switching cycles, over-current protection will be

triggered.

●

2nd level threshold: it is an internal threshold whose value is equal to 1st level

threshold multiplied by a factor 1.5. If the monitored voltage on PHASE exceeds this

threshold, over-current protection will be triggered immediately.

When over-current protection is triggered, the device turns off both LS and HS MOSFETs in

a latched condition. To recover from over-current protection triggered condition, VCC power

supply must be cycled.

7.1

Over-current threshold setting

L6728AH allows to easily program a 1st level over-current threshold ranging from 50 mV to

550 mV, simply by adding a resistor (ROCSET) between LGATE and GND. 2

nd level threshold

will be automatically set accordingly.

During a short period of time (about 5 ms) following VCC rising over UVLO threshold, an

internal 10 µA current (IOCSET) is sourced from LGATE pin, determining a voltage drop

across ROCSET. This voltage drop will be sampled and internally held by the device as 1

level over-current threshold. The OC setting procedure overall time length is about 5 ms.

Connecting a ROCSET resistor between LGATE and GND, the programmed 1

st level

threshold will be:

the programmed 2nd level threshold will be:

In case ROCSET is not connected, the device sets the OCP thresholds to the maximum

values: an internal safety clamp on LGATE is triggered as soon as LGATE voltage reaches

600 mV, setting the maximum threshold and suddenly ending OC setting phase.

OCth1

OCSET

⋅

dsON

--------------------------------------------

OCth2

1.5

OCSET

⋅

dsON

--------------------------------------------

⋅

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