3/5

AN555

Diode D1 protects against a reversed battery, while the "Transil" clamping device T2 suppresses positive

surges. Negative surges are blocked up to the breakdown voltage of the bidirectional clamp T1. Larger

negative surges are suppressed by T1 in avalanche breakdown in series with T2 forward-biased. Figure

4 shows the basic application of the RBOxx.

Figure 4. RBO basic application

Note that as the voltage across D1 is clamped by T1, the VRRM of the diode is no longer a concern. The

clamping voltage is the same as that of T2, given by the yy digits in the part number of the form RBOxx-

yy. The xx digits indicate the average forward current between pins 1 and 3.

Two devices are available at the time of writing: the RBO08-40 and the RBO40-40. The latter device is

specifically designed to protect against "load dump" surges due to the greater power capability of T2, while

the RBO08-40 is adequate for suppressing the other overvoltages present.

The presence of T1 makes possible an elegant solution to the motor driver protection. The RBOxx replac-

es the three protection components shown in Figure 2 and protects the motor driver circuit in the same

way.

CONCLUSION

A car power supply network is often contaminated with voltage surges potentially damaging to the semi-

conductor circuits present. The STMicroelectronics family of RBOxx protection devices enables full pro-

tection from these surges with the minimum component count.

D1

T1

T2

CAR ELECTRONIC MODULE

PROTECTED

+12 V

+12 V

-