AN2689 datasheet(32/42 Pages) STMICROELECTRONICS

32 / 42 page

Supply rail protection

AN2689

32/42

Doc ID 14310 Rev 3

For +85 °C ambient temperature, the derating factor is 1. So the power dissipation induced

by the load dump surge is supported by the RBO40-40 device.

The RBO40-40 combines a transient suppressor diode and also a reverse battery

protection. This function is helpful in case of battery polarity inversion that can occur by

mistake. A direct diode is placed in series with the power supply so that all power supply

inversion is safe for the protection diode (see Figure 43).

The direct diode reverse battery protection can also be replaced with a simple fuse.

However, upon battery inversion this fuse will blow and the module will need to be replaced

or repaired.

Figure 44.

Use of serial protection devices for reverse battery prevention

Besides the ISO pulse 2a the Transil has to eliminate, and the reverse battery protection,

there is another condition to consider in the automotive environment. This condition is the

starting aid.

The starting aid covers the possibility of starting 12 V vehicles with a 24 V truck battery in

case of emergency. Then, the Transil nominal voltage will need to be higher than this

starting aid voltage. That is the reason the Transil nominal breakdown voltage is 27 V

minimum. In this case the Transil is not active during starting aid test. No dissipation occurs

in the device so the Transil is safe.

Module to protect

Transil

Reverse battery protection

fuse or diode

AN2689 데이터시트(PDF) 32 Page - STMicroelectronics