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# Very tricky analog design

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#### tia_design

Give you only three voltage A, B, and C, no other voltage available. That means you don't know which one can be used as supply voltage for decision circuit. The other difficult part is that at certain time A could be the smallest voltage, while at other time B or C could be smallest voltage. Design a circuit to find the smallest voltage among these three and output it using a CMOS tri-well process. Be careful of parasitic diode or SCR when you design this circuit. How to do that?

I would diode OR the three voltages to get the comparator B+. Then divide each voltage down by half and have three comparators compare them in the three possible pairs. Then use logic on the three comparator outputs to select the lowest.

flatulent said:
I would diode OR the three voltages to get the comparator B+. Then divide each voltage down by half and have three comparators compare them in the three possible pairs. Then use logic on the three comparator outputs to select the lowest.

Thanks for reply. Then how you power those comparators? As I said, you don't know which of A, B or C can be used as Vdd and Vss.

highest voltage supply will revers bias the other two supply via diode.

hock

u can perform two cascated full wave rectification with proper loading R for example to get the lowest potential node if diod drop could be omitted. Diod for rectifier you can use tripple well D should be ok.

is it 3 phase line rectification?

i wonder that why not to select the lowest voltage as the VDD and the "0" as the GND. i mean that a single voltage power supply used as the PS.

I ever saw to use a bridge circuit(very simple) you can distinguish two unknown signal, that is , you in put a and b, but do not know which one is high and which one is low, then the output of the bridge circuit you got the certain high and certain low signals.

Then to three signals? maybe need more bridges?

Here is the rectification circuit you need. You can further regulate the output and use it to power the comparators!
Now we are half way there ...

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