AN2689 datasheet(11/42 Pages) STMICROELECTRONICS

11 / 42 page

AN2689

Data line protection

Doc ID 14310 Rev 3

11/42

For a positive surge, shown in Figure 11, as the over voltage reaches the power supply

voltage Vcc plus the forward voltage drop of the upper diode, the surge current is diverted

into the power supply line. To prevent this power supply line oscillating or being raised too

much, a capacitor (47 nF suggested) is needed close to the rail-to-rail protection device.

The remaining voltage V at the module data line is limited to VCC plus the forward voltage

drop Vf of the upper diode.

Figure 11.

Positive surge suppression

For the negative surge case (Figure 12), the surge suppression is the same as described in

Figure 8.

Figure 12.

Negative surge suppression

In the same approach as for the previous topology, there is a possibility to manage this rail-

to-rail protection topology using as many single devices as there are data lines to protect or

one diode array device that fulfills all line protection needs as shown in Figure 13.

Figure 13.

Rail-to-rail diode array

Module to protect

Vnom

Vmax

Vcc

Isurge

Module to protect

Vnom

Vmax

Vcc

Isurge

Module to protect

Vnom

Vmax

Vcc

DiodeArray

AN2689 Datasheet(PDF) 11 Page - STMicroelectronics