UM1556 datasheet(14/87 Pages) STMICROELECTRONICS

14 / 87 page

General items

UM1556

14/87

Doc ID 023520 Rev 2

Reverse battery protection using MOSFET

Figure 9.

Voltage levels during reverse battery – MOSFET protection

The HSD is protected by a MOSFET which is switched-off during reverse battery conditions.

This MOSFET circuitry also provides full ISO pulse clamping at supply line and causes no

ground level shift. A capacitor between gate and source keeps the gate charged during

negative ISO pulses as well. The time constant given by RC values should be longer than

2 ms (duration of reverse battery protection (monolithic HSDs only) - comparison of negative

ISO7637 pulse 1).

Table 2.

Reverse battery protection (of monolithic HSDs only) – comparison

Protection type

(monolithic HSD)

Resistor

– Any type of load.

– Voltage drop fluctuation.

– Calculation of R value necessary.

– Positive and negative ISO-pulse

transfer to input and diagnostics pin

(serial protection resistors

necessary).

– Relatively high power dissipation on

the RGND (~1 W) during reverse

conditions

 higher price of the

resistor

– Device turns-off in case of positive

ISO pulses

Diode

– Fixed voltage drop.

– Positive ISO-pulse clamping

(> 50 V).

– Resistive load only.

– Negative ISO-pulse transfer to input

and diagnostics pin (serial

protection resistors necessary).

Rgnd

GND

HSD

GND

HSD

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