Overloading of ESD protection diode on CAN lines through current during Bulk current injection - test
NUP2105L specs.


  current injection (BCI). Adding 5.0pF RF ceramic capacitors in parallel with the TVS diodes, as shown in Figure 14, provides a simple solution to attenuate high frequency noise signals.
  Figure 13. The NUP2105L functions as a capacitor at frequencies below the resonant frequency (fR); however, the inductance term must be included at frequencies greater than fR=616MHz @0Vdc


Thus ultra low ESR (1 Ohm) low ESL, low capacitance diode has a  Q (~5) at resonance near 616MHz implying some voltage ringing at this frequency.  This Q means the diode conducts more reactive energy than resistive energy at this resonance. Yet at 0Vdc the diode should not be conducting much but due to this stored energy, and now high impedance parallel resonance it is damped by the CANBUS shunt R impedance.    So if we add a capacitor with about 50 Ohms impedance at 616MHz then we can expect a well damped response at some lower resonant frequency. 

By injecting a bulk cap across this low-capacitance TVS diode , we reduce the circuit resonant frequency and eliminate the current sharing of the diode  ESL when it is conducting by diverting the current at these frequencies. But one must ensure the layout has low ESL in the Bulk Cap’s path to not introduce another high Q resonance.

This 5 pF cap is about 50 Ohms at 616 MHz and higher impedance at its new lower fR.  CAN bus impedance is 60 Ohms. Matching these approximately, gives a well-damped low-ringing , low EMI result.  2-3pF is also suggested.

The EMI test results are improved in this region for Bulk Current Injection Test’s when this bypass bulk cap is applied correctly at the board interface with the TVS diode to achieve 30kV ESD protection.