L6727 Datasheet, PDF (13/22 Pages) STMicroelectronics – Single phase PWM controller

L6727
7
Over current protection
Over current protection
The over current feature 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_dsON. This method reduces cost and enhances converter efficiency by avoiding
the use of expensive and space-consuming sense resistors.
The low side R_dsON current sense is implemented by comparing the voltage at the PHASE
node when LS MOSFET is turned on with the programmed OCP threshold voltage,
internally held. If the monitored voltage drop (GND to PHASE) exceeds this threshold, an
Over Current Event is detected. If two Over Current Events are detected in two consecutive
switching cycles, the protection will be triggered and the device will turn off both LS and HS
MOSFETs for 2048 clock cycles (plus internal SS remaining time, if triggered during a SS
phase); then it will begin a new Soft Start.
If the over current condition is not removed, the continuous fault will cause L6727 to go into
a hiccup mode with a typical period of 13.6ms (Figure 5), guaranteeing safe load protection
and very low power dissipation.
7.1
Over current threshold setting
When supplied with VCC = 5V, L6727 allows to easily program an Over Current Threshold
ranging from 50mV to 500mV, simply by adding a resistor (R_OCSET) between COMP and
VCC.
During a short period of time (5.5ms - 6.5ms) following the first enable (given VCC over
UVLO threshold), an internal 60µA current (I_OCSET) is sunk from COMP pin, determining a
voltage drop across R_OCSET. This voltage drop, differentially sensed between VCC and
COMP, divided by a factor 3, will be sampled and internally held by the device as Over
Current Threshold until next VCC cycling. Differential sensing versus VCC allows OCSET
procedure to be fully independent from VIN rail. The OC setting procedure overall time
length ranges from 5.5ms to 6.5ms, proportionally to the threshold being set.
Connecting an R_OCSET resistor between COMP and VCC, the programmed threshold will
be:
I_OCth
=
1_{-- â‹} ^I_--O_----C_----S_---E_---T_----^â‹_---^R_-----O_----C_---S_----E_---T_-
3
R_dsON
R_OCSET values range from 2.5kΩ to 25kΩ.
If the voltage drop across R_OCSET is too low, the system will be very sensitive to start-up
inrush current and noise. This can result in a continuous OCP triggering and hiccup mode.
In this case, consider to increase R_OCSET value.
In case R_OCSET is not connected (and VCC = 5V), the device will set the maximum
threshold.
If the device is supplied with a VCC higher than 7V, R_OCSET must be not connected. In this
case, as soon as VCC rises over V_{CC_OC} (8V typ.), L6727 switches OC threshold to 400mV
(internally fixed value).
See Figure 5 for OC threshold setting and soft start oscilloscope sample waveforms.
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