L9779WD datasheet(21/170 Pages) STMICROELECTRONICS

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L9779WD

Functional description

169

Functional description

6.1

Ignition switch, main relay, battery pin

The system has an ignition switch pin KEY_ON and a pin VB for battery behind the main

relay connected at pin MRD.

L9779WD can also support the configuration where it is permanently supplied by VB; in this

case the MRD output can be used to connect the loads to VB.

At pin KEY_ON there is an external diode for reverse battery protection. An internal Pull-

down resistor is provided on the KEY_ON pin. The external components to be connected to

KEY pin are shown in the below schematic.

Internal functions and regulators are supplied by VB; only some basic functions required for

startup are supplied from KEY_ON as described below. Reverse protection for pin VB is

done by the main relay. Transient negative voltage at VB may be limited by an external

diode if necessary. There is no integrated reverse protection at pin VB.

The pin connected to the battery line can bear the ISO 7637/1 noise pulses without any

damage. The VB voltage must be externally limited to +40 V and -0.3 V (with external

components as in Figure 4). It is suggested the use of a transil.

Figure 4. Configuration supplied by VB

1. The external components connected to KEY_ON pin are mandatory in order to protect the device from ISO

7637 pulses.

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