UM1557 datasheet(13/111 Pages) STMICROELECTRONICS

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UM1557

General items

Doc ID 023521 Rev 2

13/111

Reverse battery protection using diode plus resistor

Figure 6.

Voltage levels during reverse battery using diode-resistor protection

A diode at the GND terminal prevents a short circuit through the internal substrate diode of

the HSD during a reverse battery condition.

A resistor (max. 4.7 k

Ω) connected in parallel to the diode is recommended (not mandatory)

in case the device drives a high inductance load with a demagnetization time longer than

tstby. The purpose of this resistor is to suppress a negative voltage on the GND pin during

standby mode if the demagnetization phase is still ongoing. Without this resistor, the low

supply current in standby mode (2 µA typically) allows the GND pin to be pulled negative by

the demagnetization voltage on the output (~ -40 V) via an internal pull-down resistor

(~120 k

Ω, not specified) on the output (see Figure 9, Figure 10). If the negative ground shift

exceeds the input high level threshold (VIH = 2,1 V min.), the device leaves standby mode

and tends to turn on. The GND pin is immediately pulled high (~600 mV) by the increased

supply current so that standby mode is activated again after tstby delay. As a result, we may

see short negative peaks on the GND pin with a period of tstby during the whole

demagnetization phase. These peaks are not long enough to activate the HSD output,

which means the device works safely even without the GND resistor. However, this resistor

is still recommended in order to suppress the described parasitic oscillations (if TDEMAG >

tstby).

This ground network can be safely shared amongst several different HSDs. The presence of

the ground network produces a shift (~ 600 mV) in the input threshold. This shift does not

vary if more than one HSD share the same diode/resistor. A diode at the GND terminal

allows the High-Side Driver to clamp positive ISO pulses above 64 V (the typical clamping

voltage of the HSD). Negative ISO pulses still pass GND and logic terminals. The diode

should withstand clamped ISO currents in case of positive ISO pulses and reverse voltages

in case of negative ISO pulses.

4.7k

GND

56k

Vdd

GND

out

Microcontroller

GND

-27V

-26.3V

-19.3V

-0.7V

Logic

OUT

FR_Stby

GND

Vcc

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