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I am going to use ST DALC112S1 (http://www.st.com/web/en/resource/technical/document/datasheet/CD00001317.pdf) to protect the SIM card connection between uBlox Leon G100 and the SIM card holder (VSIM, VRST, SIM_IO, SIM_CLK lines) against ESD.

This DALC112S1 has REF1 connected to the anodes and REF2 to the cathodes and it is clear that REF1 shall be a GND in this case (I have one GND for the whole circuit) but I don’t know if I should connect the REF2 to VSIM or better to a circuit Vcc (VSIM < Vcc(3.8V)).

Connecting it to VSIM is hazardous to Leon as there are no capacitors to absorb the voltage peak if it occurs. The circuit Vcc has lots of them but on the other hand there is a small risk that the whole circuit gets damaged with the ESD on the SIM. So, what is the usual way to do it?

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Datasheet page 2: Typical application.:

Vref1 is connected to GND

Vref2 is connected to Vcc.

The question is what is the IO voltage? You should connect to a Vcc voltage that is a least higher than the max voltage into the IO you want to protect.

And it might be very good to add a decoupling cap between the Vref2 pin and the GND plane to provide a path of small impedance for high frequency signals (such as a spike...). This, with all the caps on the VCC plane, should protect your other parts connected to that Vcc.

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  • \$\begingroup\$ When I first started to consider this sort of protection I asked the local Guru and he said if you model this the amount of spike you see in Vcc is very small. The supply capacitance is so much larger than the capacitance of the ESD source. When you add the resistances of the connectors, solder (other components) and joints etc the real life measurements are even smaller. Since then I have never seen any design in 20 years that dumps ESD or fault currents to anywhere except the Vcc. Not even into the unregulated supply from the bridge rectifier. \$\endgroup\$
    – Spoon
    Commented Nov 19, 2013 at 13:48
  • \$\begingroup\$ @Spoon, that means that connecting the REF2 to global Vcc (that is of course no less than the protected IO voltages) is ok, because it is capable of absorbing the spikes that we are talking about when considering ESD protection, right? \$\endgroup\$
    – eltomek
    Commented Nov 19, 2013 at 15:08
  • \$\begingroup\$ Adding a resistor to the IO pin and your diodes before the resistor is very effective. Have a look at electronics.stackexchange.com/questions/78205/… or electronics.stackexchange.com/questions/28954/… or electronics.stackexchange.com/questions/77700/… for better explanation than I can give. \$\endgroup\$
    – Spoon
    Commented Nov 19, 2013 at 23:18

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