diodessparkfunusb-c
I'm looking at the Sparkfun USB-C PD Board schematic and noticed this portion under ESD protection as seen below.
It looks like two Zener diodes facing each other thus prevent current flow in either direction. I looked at the properties and it said TVS diode. But with some lookup on the TVS diode, it didn't give me it's importance in the role here.
Can someone explain what this diode exactly is with this symbol and what its role is in the overall function.
It's just the basic operating principle of a diode. An ideal diode would allow current to flow in one direction but block current flow in the other direction. This is based on how it's made, with a p-type region, an n-type region, and a depletion zone in between. Like the bottom diode in this picture:
When you apply a some voltage, in your case 0.9V then the p-type holes and the n-type electrons move into the depletion region because they are repelled by their respective battery terminal. With enough voltage (0.9V in your case) the free electrons in the depletion region get moving and current begins to flow like this:
Now in the ideal case if you were to reverse that battery the opposite will happen and you'll get no current flowing:
In the real world though you can only apply so much reverse voltage or push before you hit the breakdown voltage and current begins to flow freely in the reverse direction. Zener diodes take advantage of this fact and are constructed to breakdown at lower voltages such as your 3.3V.
Sources:
You can read more about how zeners are made here
Or see the article I got all the pictures from here
The device is made to handle signals which normally operate between GND and V++. Any signal line transient that goes below GND is clamped by the lower diode to a voltage that is equal to the forward voltage drop of the lower diode. So you can see that clamping will be to less than about -1V in that case. (Clamping level really depends upon how much current surge there is caused by the negative voltage spike as the forward voltage varys depending on the diode forward current).
The device can be used with or without the pin 5 connected. If pin 5 is open any positive voltage spike on the signal pins is clamped to a level that is the sum of the forward voltage drop of the upper diode plus the reverse zener voltage of the TVS diode. Once again actual clamp level depends upon the amount of energy in the voltage spike and how much current it causes to flow through the clamp diode path.
If pin 5 is attached to a low impedance V++ voltage rail then the positive voltage spikes are clamped to a level about one forward voltage drop of the upper diode over the supply voltage level. In many cases though the leading edge of the positive voltage spike will have a very fast rise time and any series inductance in the wiring from pin 5 back to the V++ supply will prevent the supply from immediately being able to sop up the energy in the spike. This will cause the voltage on the pin 5 to rise up and then eventually be clamped by the TVS device.
On these devices the TVS device is not normally meant to be used for clamping an over voltage condition on the V++ supply rail. The device is just not rated for that behavior and the V++ supply output is rated for much higher energy output so as to be able to supply power to the rest of the circuit. If over voltage of the V++ rail is something that needs to be protected against then it is advisable to use a proper circuit for this purpose. Usually that would be called a supply crowbar and usually consists of a high power SCR and a trigger circuit. The SCR, once triggered is designed to short the supply output in either its current limited load or to overload the internal fusing circuit and cause that to blow out.
Best Answer
It’s a bidirectional TVS or transient voltage suppressor. Used for ESD (electro static discharge) and other types of surge protection.
Just regard it as the AC equivalent of a zener diode in that it will start to conduct if the magnitude of the applied voltage is above a certain level.