L6727 datasheet(14/34 Pages) STMICROELECTRONICS

14 / 34 page

Output voltage monitor and protections

L6727

14/34

Doc ID 12933 Rev 4

Output voltage monitor and protections

L6727 monitors the voltage at FB pin and compares it to internal reference voltage in order

to provide Under Voltage and Over Voltage protections.

8.1

Undervoltage protection

If the voltage at FB pin drops below UV threshold (0.6 V typ), the device turns off both HS

and LS MOSFETs, waits for 2048 clock cycles and then performs a new soft start. If under

voltage condition is not removed, the device enters a hiccup mode with a typical period of

13.6 ms.

UVP is active from the end of soft start.

8.2

Overvoltage protection

If the voltage at FB pin rises over OV threshold (1 V typ), over voltage protection turns off HS

MOSFET and turns on LS MOSFET overriding PWM logic as long as over voltage is

detected.

OVP is always active with top priority as soon as over current threshold setting phase has

been completed.

8.3

Feedback disconnection protection

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

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

FB over OVP threshold: thus LS will be latched on preventing output voltage from rising out

of control.

8.4

Undervoltage lock out

In order to avoid anomalous behaviors of the device when the supply voltage is too low to

support its internal rails, UVLO is provided: the device will start up when VCC reaches

UVLO upper threshold and will shutdown when VCC drops below UVLO lower threshold.

The 4.1 V maximum UVLO upper threshold allows L6727 to be supplied from 5 V and 12 V

busses in or-ing diode configuration.

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