UM1693 datasheet(32/196 Pages) STMICROELECTRONICS

32 / 196 page

Reverse battery protection

UM1693

32/196

DocID025505 Rev 1

Figure 14. Reverse polarity protection – reverse FET protection

Note that a MOSFET has always an intrinsic anti parallel body diode. If the IN voltage

versus drain is negative the device is turned ON. The MOSFET is fully turned on when

applying the battery voltage and the IN pin goes negative versus drain. Due to the fact that

the Source is at high potential, the MOSFET is a high-side switch not referring to ground; a

charge pump circuit is needed to boost the gate voltage over the source voltage to turn the

MOSFET on.

During reverse polarity of the battery, no voltage will supply the gate of the MOSFET which

will automatically switch off. When IN is left open, device is in OFF state and behaves like a

power diode between source and drain pins. The power losses of an N-channel MOSFET

for a reverse battery protection are determined by the RDS(on) of the device and the load

current. The device is able to withstand an abrupt high load current value, typical of an

application where several HSDs are activated simultaneously, and loads like motors or

bulbs can have a transient current above the devices’ DC Current maximum rating. The

diagram reported in Figure 15 gives information about the safe operating area as well as the

maximum pulsed drain current the device is able to manage during normal operation.

The usage of VN5R003H-E for reverse polarity protection is not recommended, when the

system must be compliant to ISO 7637-2:2011(E) pulse 1 test level IV. In this case in fact it

is needed that the pulse does not discharge through the HSD and the conducting

VN5R003H-E device as this might be destructive for the HSD.

The VNR003H-E is robust against “ISO 7637-2 2004 rev E” pulses in the configuration with

IN pin grounded through a resistance R > 5

Ω.

UM1693 Datasheet(PDF) 32 Page - STMicroelectronics