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Question about using diode-connected transistor to limit suppoly voltage

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rockycheng

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Dear all,

I'm trying to use two diode-connected MOS transistors to limit the supply voltage (may reach 5.5V) for the 3.3V transistors in 0.13um process.

At normal operation, there is a static current through the diode-connected transistors, making enough voltage drop to keep the circuit safe. The problems is, if the current goes to zero at standby state, the diode-connected transistors go into the cut-off region, so the voltage drop gets much lower. The simulated supply voltage at this situation is over 4.8V.

Now here is my quesion. Will this 4.8V supply make any damage? Can a pure high voltage make damage, or it has to be with high current? In this case, if current large enough is need to break down the circuit, it has to go through the diode-connected transistors which will make the voltage drop on them increase immediatedly and thus lower the current in turn. Can anybody help me? Thanks a lot!!
 

fcfusion

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It would be easier if you had added an schematic to show the problem.

Anyway, high voltages cause damage because the PN junctions with reverse polarization to disrupt, and allow a high current to pass through even though the junction should be in cut-off. Therefore, if you exceed the maximum allowed voltage chances are that the junctions will disrupt, allowing a very high current to pass through, and you will damage your circuit. This current will be limited by the diode connected transistors, and the amount of current through them will be defined by their equivalent ON resistance.
 
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