UM1556 datasheet(9/87 Pages) STMICROELECTRONICS

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UM1556

General items

Doc ID 023520 Rev 2

9/87

1.2

Reverse battery protection

1.2.1

Reverse battery protection of monolithic HSDs

The reverse battery protection is inserted to the GND terminal of the driver. There are three

possible solutions: resistor only, resistor plus diode and MOSFET. There is a relatively low

current in the GND path and therefore no high power components are needed. However,

this protection circuit still must be able to handle the clamped ISO pulse current, as well as

the ISO pulse voltage. We also have to consider the fact that this simple "ground" circuitry

doesn't provide any protection to the connected load. If a reverse battery condition occurs,

the load is supplied in reverse polarity through internal body diode of the HSD and the

power dissipation on the HSD can become critical (depending on connected load and

thermal connection of the HSD). With a typical voltage drop on the internal body diode of

about 0.7 V, the resulting power dissipation = 0.7 ILOAD [W].

Reverse battery protection using resistor

Figure 4.

Voltage levels during reverse battery – resistor protection

A resistor Rgnd in the GND line prevents a short circuit through the internal substrate diode

of the HSD during a reverse battery condition. The minimum resistor value is limited by the

DC reverse ground pin current of the HSD. The maximum resistor value is limited by the

drop voltage, caused by the on-state supply current (IS) of the HSD. The voltage drop

across this resistor elevates the minimum input High threshold and normally should not

exceed 1 V (depending on microcontroller I/O levels).

Equation 1

Equation 2

GND

S(on)max

------------------------

≤

GND

BAT

GND(reverse)max

-------------------------------------------

≥

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