L6727 datasheet(13/22 Pages) STMICROELECTRONICS

13 / 22 page

L6727

Over current protection

13/22

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, RdsON. This method reduces cost and enhances converter efficiency by avoiding

the use of expensive and space-consuming sense resistors.

The low side RdsON 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 (ROCSET) 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 (IOCSET) is sunk from COMP pin, determining a

voltage drop across ROCSET. 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 ROCSET resistor between COMP and VCC, the programmed threshold will

be:

ROCSET values range from 2.5kΩ to 25kΩ.

If the voltage drop across ROCSET 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 ROCSET value.

In case ROCSET is not connected (and VCC = 5V), the device will set the maximum

threshold.

If the device is supplied with a VCC higher than 7V, ROCSET must be not connected. In this

case, as soon as VCC rises over VCC_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.

OCth

---

OCSET

⋅

dsON

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

⋅

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