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Switched-capacitor Bandgap Reference-Switch Sizing (Read 2931 times)
prietess
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Switched-capacitor Bandgap Reference-Switch Sizing
Jul 05th, 2017, 12:49am
 
Hello everyone!

I am now designing a switched-capacitor (SC) bandgap reference, but I am having doubt on the switches size that I should use.

Let say I do not have any issue with settling time and my op-amp is good enough about slew rate and response time.
Regardless of how is the structure of my SC bandgap, I am just asking a general question.

What kind of size I should set for my switches? Is it okay to put the minimum size of W/L?
Let say my process is 0.18um, can I put the switches' size to be all 0.18um/0.18um?

Let say I use all switches to be NMOS.
The reason why I want minimum is to minimize charge injection, because my charge injection is still very bad affecting my node voltages when the switches are turned off. (even with that old-school book of using half-sized NMOS with S+D connected.)
I noticed that when I put that 0.18u/0.18u size to all my switches, my deltaV as impact of switching on/off at my nodes are reduced.

When I say reduced, it is still 10-20mV change (higher or lower) in node voltage when the switches turn off (even with minimum size). If i use higher W, that deltaV could be in hundreds of mV range. Even this 20mV is kind of unacceptable since I am designing bandgap reference.

Would any Master here give me advice please? Is it okay to use minimum size of NMOS to all your switches in your SC circuit design? Because my friend is not convinced that this kind of minimum size NMOS switches are used in the real practice by others.
Anyone know any reference that people design SC circuits with minimum size NMOS?

So far all the papers that I am reading, they don't talk about using minimum size of switches at the SC circuit design.
I would highly appreciate your kind help and answer!

Thank you!

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deba
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Re: Switched-capacitor Bandgap Reference-Switch Sizing
Reply #1 - Jul 10th, 2017, 6:35am
 
The design of the switch is highly dependent on the circuit which is under consideration. Depending on the voltage which you are switching to, you have a number of switches available to you nMOS only, pMOS only, CMOS and special switches like bootstrap switches which are very common is sampling circuits.

If your problem is with charge injection, there are techniques available to handle it. Look up bottom plate sampling for example.
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prietess
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Re: Switched-capacitor Bandgap Reference-Switch Sizing
Reply #2 - Jul 10th, 2017, 6:43am
 
deba wrote on Jul 10th, 2017, 6:35am:
The design of the switch is highly dependent on the circuit which is under consideration. Depending on the voltage which you are switching to, you have a number of switches available to you nMOS only, pMOS only, CMOS and special switches like bootstrap switches which are very common is sampling circuits.

If your problem is with charge injection, there are techniques available to handle it. Look up bottom plate sampling for example.


Hi deba!

Ya there are techniques, but I have tried some of them, to no avail.
With those techniques, There are worse charge injection if I increase the size of my switches (let's say to a few microns).

So, after many trials, i still get the best (least) charge injection when my switches is minimum size (let say 0.5um/0.5um).
So my question was simple, whether this size of switch is safe to be used for design.
Let say I want to tape-out my design, is it safe or will have problem (cannot turn on/etc) across some variations?
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DanielLam
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Re: Switched-capacitor Bandgap Reference-Switch Sizing
Reply #3 - Jul 10th, 2017, 9:50am
 
Hi,

First, I think you should include a schematic diagram so everyone can see which switches you are talking about. For example, I can't tell if this structure is differential or single-ended. If it is differential, then the charge injection is cancelled to a first order.

In general, the only time I've seen minimum sized devices is probably on digital logic trying to save power/area. Otherwise, I'd do at least 2-3x in case of process variation, and perhaps mismatch. You really need to show us a diagram for us to judge.
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prietess
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Re: Switched-capacitor Bandgap Reference-Switch Sizing
Reply #4 - Jul 10th, 2017, 9:55am
 
DanielLam wrote on Jul 10th, 2017, 9:50am:
Hi,

First, I think you should include a schematic diagram so everyone can see which switches you are talking about. For example, I can't tell if this structure is differential or single-ended. If it is differential, then the charge injection is cancelled to a first order.

In general, the only time I've seen minimum sized devices is probably on digital logic trying to save power/area. Otherwise, I'd do at least 2-3x in case of process variation, and perhaps mismatch. You really need to show us a diagram for us to judge.


Hello, I attached the schematic diagram in this reply.
I am referring to all the switches, they all introduce deltaVoltage to me at all the nodes when they turn on/off.




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« Last Edit: Jul 10th, 2017, 6:24pm by prietess »  

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