# PMOS as a switch - W and L selection

1. ## PMOS as a switch - W and L selection

1. How to determine the W/L ration for a PMOS used as a switch, can we simple use the normal method of using the drain currents and overdrive to determine it?

2. How to decide the W and L values for a PMOS used as a switch? Is there any thumb rule to decide the value of L as in Analog design ie 3 to 4 times the min lenght?

Thanks a lot!!

2. ## Re: PMOS as a switch - W and L selection

hey....
W/L should be decided according to current requirement...and as you said L as 3 to 4 times of the min length. One more equation you can add up for W/L determination that is Rds,on...It should be low as possible as..because pmos is a switch.

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3. ## Re: PMOS as a switch - W and L selection

Don't forget about the capacitance and charge injection you will have with a switch 3 to 4 times minimum. There isn't a rule for switches being 3 to 4.. its all in what you need! If you need to switch this PMOS fast you should have minimum length. Also just to note PMOS will only pass high voltages! Make sure you have the correct switch!
Jgk

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4. ## Re: PMOS as a switch - W and L selection

THank you for the suggestions.

The switch is to be used in a current scaling DAC. PMOS devices are being used as the current sources, based on the current and overdrive requirements i have chosen the length of these transistors to be 4um(min length allowed is .35um).

1. To maintain matching, if i use the same length(4um) for the switches then turning the swtiches on/off will take more time because of the relatively large capacitances.
2. To have a better switching speed, if i choose the length of the switches to be the min length ie. .35um the lengths of the currnet source transistors and the switches are not the same, non uniformity of lenght of devices, and this can lead to unexpected behaviour of the circuit. Is it not?

Thank you!!

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5. ## Re: PMOS as a switch - W and L selection

So i agree you need large area current sources and a large overdrive to make all of your sources match to a specific resolution. but the switches will not effect your matching of your bits in your DAC. They can have mismatch, you just need to make sure they switch fast enough so that your current sources do not enter triode and, thus effecting your resolution again! I would say you need your switches min length then just make sure they have a small enough resistance thus to not effect your DAC.
Jgk

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6. ## Re: PMOS as a switch - W and L selection

Switches should always be of minimum length.
A non-ideal switch will have finite on resistance and finite switching time, which are essentially trade-offs of each other.
How to size switches will depend on your requirements for these non-idealities.
For large switches, you may need to spend time to design a driver that can provide an acceptable switching time.

7. ## Re: PMOS as a switch - W and L selection

Can i simple use the minimum allowed values for length and width as this would reduce both the switching
time(small area, less capacitance) and less on time resistance( small lenght) OR is there
any other critera that needs to be considered?

Thank you!!

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8. ## Re: PMOS as a switch - W and L selection

A small switch implies high on resistance, as you can easily get by differentiating the MOSFET Sah model operating in the linear region.
In other words, switch sizing is a trade off between switching speed and on resistance.
How low a resistance you need depends on whether the switch is carrying current, and the source impedance of the signal that the switch is supposed to pass.

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9. ## Re: PMOS as a switch - W and L selection

Originally Posted by checkmate
How low a resistance you need depends on whether the switch is carrying current, and the source impedance of the signal that the switch is supposed to pass.
@checkmate:
1. can you please elaborate on " the source impedance of the signal that the switch is supposed to pass". I could not understand it properly.
2. I have not been provided with any specifications for the 'on time resistance' or 'switching speed'. Can you please let me know the generally acceptable values for these paramteres so tat i can base my design on them. The swithc does carry current of the order of 320uA.