L6726ATR datasheet(13/24 Pages) STMICROELECTRONICS

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L6726A

Protections

13/24

Protections

7.1

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 in a latched condition.

To recover from Over Current Protection triggered, VCC power supply must be cycled.

7.1.1

Over current threshold setting

L6726A allows to easily program an Over Current Threshold ranging from 50mV to 550mV,

simply by adding a resistor (ROCSET) between LGATE and GND.

During a short period of time (5.5ms - 6.5ms) following VCC rising over UVLO threshold, an

internal 10µA 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 Over

Current Threshold. The OC setting procedure overall time length ranges from 5.5ms to

6.5ms, proportionally to the threshold being set.

Connecting a ROCSET resistor between LGATE and GND, the programmed threshold will be:

ROCSET values range from 5kΩ to 55kΩ.

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 undesired OCP triggering. In this case, consider

increasing ROCSET value.

In case ROCSET is not connected, the device switches the OCP threshold to a 375mV default

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

700mV (typ), enabling the 375mV default threshold and suddenly ending OC setting phase.

See Figure 6 for OC threshold setting procedure timings picture and oscilloscope sample

waveforms.

7.2

Feedback disconnection protection

In order to provide load protection even if FB pin is not connected, a 100nA bias current is

always sourced from this pin. If FB pin is not connected, bias current will permanently pull

up FB: this forces COMP pin low, avoiding output voltage rising to dangerous levels.

OCth

OCSET

⋅

dsON

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