SN65HVD72: Protection circuitry and bus performance

Hello Muhammad,

You are right in that the capacitance and the leakage current of the TVS diodes has an effect on the bus performance. The short answer is 9600 bps data rate is slow enough that these effects will not come into play and everything will work normally.

According to Bourns' CDS0T23-SM712 datasheet the typical capacitance that one TVS diode adds to a bus pin is 75pF and the max leakage current from the bus pin to ground (pin 1 - pin 3) is 1uA.

For the capacitance, having 128 of these TVS diodes in parallel on the bus adds 9.6nF worth of capacitance onto each bus pin. The SN65HVD72 has a driver that is capable of putting out a maximum of 160mA worth of pull up / pull down current (Ios specification). For the ease of calculation let's say that typically the output driver puts out something closer to 100mA of current. Using I = C (dV/dt), if we have 100mA trying to slew 9.6nF of capacitance by 5V, it would take a time of 480ns. So this additional slew rate caused by the increase of bus capacitance from having 128 TVS diodes on the bus would cause a lot of problems at data rates of 1Mbps or higher, but will have little to no impact at a data rate of 9600 bps.

As for the leakage current, even if all 128 TVS diodes were leaking 1uA on the bus pin, the driver of the SN65HVD72, which is capable of putting out over 100mA of current, can easily drive the extra 128uA of leakage current. Also the voltage drop across 120 Ohm cabling caused by 128uA of leakage current would be very small, in the 10s of mV range.

Best Regards,
Richard Broughton
Transceiver Interface - Applications Engineer