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Two-stage open-loop comparator draw large instantaneous current

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bit_an

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Two stage open loop comparator draw large instantaneous current

Hi,

I have a basic two stage open loop comparator circuit. While checking the dynamic power I found that in the comparator second stage, the source transistor (PMOS), is drawing large instantaneous current (glitch). See the attached figure. Is there any way to avoid it without any major change in the circuit? Please give your feedback.

Thanks and Regards, PU_leakage_current.JPG
 

Re: Two stage open loop comparator draw large instantaneous current

Although readers might guess what the circuit is, it would be nice to show it for clarity.

Which current do we see, output transistor current (Id, Is ?) or total supply current?
Did you check where the current flows, load capacitance if any, or low-side current source?
Does the spike charge correspond to the respective capacitances and voltage swing?

In other words, use the information available in simulation to find out what exactly happens.
 

Re: Two stage open loop comparator draw large instantaneous current

We see the output M6 transistor source current (Is) in the figure. The circuit has capacitive load. Current flows through the load. The result is from transient simulation. You could check the circuit schematic in this link: https://pallen.ece.gatech.edu/Academic/ECE_6412/Spring_2003/L370-2StageOLCompI-2UP.pdf. The problem is no matter how I can not reject the spike. The moment output skews, it gives this large spike. Any idea?!
 

Re: Two stage open loop comparator draw large instantaneous current

When the Mosfet tries to charge the output capacitor quickly then it uses a high current to do it. Remove the output capacitor.
 

Re: Two stage open loop comparator draw large instantaneous current

Even if I remove the Load Cap, but this circuit will drive another MOS Cap in the following stage. In my understanding, still this large spike current will be present, which could surge the overall network. Is there any more reliable alternative?
 

Re: Two stage open loop comparator draw large instantaneous current

Your 'scope shows a current surge of only 300uA which is almost nothing. You should worry if it is 300mA.
 

Re: Two stage open loop comparator draw large instantaneous current

You didn't answer the additional questions, e.g. where does the current flow. If it's into the load capacitance, you should compare with expectable load current peak according to output dV/dt. If the current peak goes into the current source, it would be interesting if it's mainly caused by Cds or also Cgd and miller effect.

Presuming the current peak is caused by output dV/dt and you worry more about absolute current maximum than energy loss, you may want to slow down the output dV/dt, if the specification allows.
 

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