In the following circuit, my understanding is that when there is a spike or rise in the data line higher than Vcc, the top diode conducts and the voltage flows into the path to protect the downstream IC/circuitry.
But how does that voltage flow into the top Vcc, which is a probably a power supply? Or am I missing something?
Best Answer
There are "four quadrant" power supplies which can do what you're thinking they won't, but even if the supply is not such, the energy will go to charging the capacitance between Vcc and GND (both the power supply's output capacitor and the board's decoupling).
This is better than not having the protection because the energy of an ESD event is spread out so the voltage is much smaller at any point.
Vcc will be somewhat higher for a moment but this has little effect.