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Level Shifters Design

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diarmuid

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Hello,

Can anyone please show me where I can get infomation on how to build a
CMOS level shifter. I see various schematics have been uploaded but nothing
which clearly shows the operating principles and construction of such a circuit.

Any pointers to books/papers/online tutorials/etc would be most appreciated.

Thanks in advance,

Diarmuid
 

Hi,
the following paper discusses different structures of level shifter. (contention mitigated level shifter proposed).
"Level shifter Design for Low power applications" by Manuj Kumar https://airccse.org/journal/jcsit/1010ijcsit09.pdf
the sizing depends on your input and output voltage. you should also take care of short circuit power specially for larger voltage difference.
 

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Thats a very helpful paper. Thanks.

Seems the cross coupled structure is the way to go.

Do you have any idea why cascading two inverters would not work? i.e. apply a 3V3 volt input signal to the inverters (whose Vdd is 5V). Would this not also provide adequate shifting from 3V3 to 5V?

Thanks in advance,

Diarmuid
 

3V3 to 5V shifting in fact doesn't require a full featured level shifter. CMOS gates with suitable threshold voltages can work as well. You can refer to 74HCT series, that is perfect for this purpose.
 
Diiarmuid,
it won't work, because when you apply 3V to the input of inverter with supply of 5V, both nmos and pmos are on and therefore the output of the inverter does not go down all the way to zero, it will be some intermediate voltage and it will cause the same problem in next gates. cross coupled is the best way.
 
Very good explanation Muhammadasour. Its clear now why I cant cascade inverters to realise such a level shifter.

@ FvM - I didnt think of this. Will definitely look into it.

Given that both of you clearly know your level shifters one further question:

I also see buffers placed at the output of the level shifter. Are these always necessary or only necessary when the shifters must operate at high speed i.e. when the drive capability of the level shifter is not enough to charge up the proceeding gates in a specified time?

Thanks,

Diarmuid
 

Hi,
If the load of your level shifter is very small(less than Fan out of 4 of inverter) you can remove it, but notice that the more load you have at this node, speed and power will be affected. be careful of level shifter sizing and power if you wanna remove it.
I suggest not to remove these output inverter because they decouple the level shifter with output load so the speed and power are optimum in presence of output inverters unless your level shifter load is about 1or2X an inverter input cap.
 
Ok thats good advice. I'll leave them in.

Thanks very much for the helpful comments.

Much appreciated!
 

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