L6728AH Datasheet, PDF (12/33 Pages) STMicroelectronics – High frequency single phase PWM controller with Power Good

Over-current protection
7
Over-current protection
L6728AH
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, R_DS(on). This method reduces cost and enhances converter efficiency by
avoiding the use of expensive and space-consuming sense resistors.
The low side R_DS(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:
● 1^st level threshold: it is the user externally set threshold. If the monitored voltage on
PHASE exceeds this threshold, a 1^st level over-current is detected. If four 1^st level OC
events are detected in four consecutive switching cycles, over-current protection will be
triggered.
● 2^nd level threshold: it is an internal threshold whose value is equal to 1^st 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, V_CC power
supply must be cycled.
7.1
Over-current threshold setting
L6728AH allows to easily program a 1^st level over-current threshold ranging from 50 mV to
550 mV, simply by adding a resistor (R_OCSET) between LGATE and GND. 2^nd level threshold
will be automatically set accordingly.
During a short period of time (about 5 ms) following V_CC rising over UVLO threshold, an
internal 10 µA current (I_OCSET) is sourced from LGATE pin, determining a voltage drop
across R_OCSET. This voltage drop will be sampled and internally held by the device as 1^st
level over-current threshold. The OC setting procedure overall time length is about 5 ms.
Connecting a R_OCSET resistor between LGATE and GND, the programmed 1^st level
threshold will be:
I_OCth1
=
I_{--O----C----S---E---T----}⋅_---R_{-----O----C---S----E---T-}
R_dsON
the programmed 2^nd level threshold will be:
I_OCth2
=
1.5
⋅
-^I-^O----^C----^S---^E---^T----^â‹---^R-----^O----^C---^S----^E---^T-
R_dsON
In case R_OCSET 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.
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Doc ID 15726 Rev 1