1/5

May 2004

AN555

APPLICATION NOTE

AUTOMOTIVE PROTECTION

WITH THE RBOxx SERIES

REV. 2A - 3585

INTRODUCTION

The harsh electrical environment in automobiles poses problems for the electronic modules present. Even

in normal operation, large positive and negative overvoltages due to switch bounce, ignition coil switching

and other phenomena are a potential cause of destruction. An additional hazard is the possibility of supply

reversal, perhaps caused by faulty wiring. Another danger is the "load dump" effect, caused by battery

disconnection while the engine is running. This causes the energy stored in the alternator coils to manifest

itself as an 80 V transient lasting around 300 ms : lethal to semiconductor circuits. See application note

4.1: "Protection standards in automotive applications" for more details.

Protection is therefore required, which can be centralized or distributed. Centralized protection attempts

to suppress disturbances at their source, for example crowbar devices at the alternator to counter the load

dump effect. Distributed protection aims to dissipate disturbances at their destination. Components per-

forming this function are present in the electronic modules themselves and are thus relatively numerous.

The RBOxx (Reversed Battery and Overvoltage) series of protection devices from STMicroelectronics has

been designed to reduce this distributed protection component count.

Figure 1. Classical protection circuit

CLASSICAL DISTRIBUTED PROTECTION

The circuit shown in Figure 1 protects the module against battery reverse as well as impulsive and con-

tinuous overvoltages, both positive and negative. In addition, overvoltages generated by the module are

prevented from reaching the car supply network. Rs, in cases where it can be used, limits the power dis-

sipated in the protection device P. Note that diode D is used in reverse-bias to block negative overvoltag-

es, so its peak reverse voltage limit (VRRM) must be taken into account.

Motor driver protection is complicated by the presence of transistor circuits which control the direction and

magnitude of current flow through the motor. A bidirectional clamping device needs to be added in order

to ensure protection of both halves of this circuit. (see Figure 2).

ELECTRONIC

MODULE

V

Bat

R

S

D

P