UM1693 datasheet(29/196 Pages) STMICROELECTRONICS

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UM1693

Reverse battery protection

195

Figure 11. Generic schematic and test setup with P-channel MOSFET in VCC line

As soon as the battery voltage is applied and for the first start up, the body diode of the

MOSFET will conduct, until the channel is switched on in parallel. The Zener diode will

clamp the Gate of the MOSFET to its Zener voltage in case of over voltage on the battery

track. In normal operation only the leakage current of ZD1 Zener Diode is flowing through

R2 to GND. In order to minimize this current even at higher supply voltages, a diode with

higher Zener voltage (i.e. 18 V) might be chosen, however it shall be dimensioned to ensure

the Zener voltage is always safely below the maximum rated gate source voltage VGS of the

P-channel MOSFET.

The resistor R2 limits the current through the Zener diode at supply voltages higher than the

Zener Voltage and limits the charging/discharging current of the gate. In addition the resistor

R2 together with the gate capacitance of the P-channel MOSFET determines the turn-off

time when exposed to fast negative transients or abrupt reverse polarity according to

LV 124: 2009-10 standard. 1 k

Ω as demonstrated by the experiment reported below

appears to be a good compromise between minimizing the charging/discharging current

and ensuring a fast turn-off time. Due to the fact, that the P-channel MOSFET will carry also

the load current, it needs to be a lower ohmic component compared to an N-channel

reverse polarity protection MOSFET in the GND line. In consequence it will have a higher

gate capacitance, hence longer turn-off times for identical gate resistance R2.

UM1693 Datasheet(PDF) 29 Page - STMicroelectronics