schmitt trigger, min VDD calculation

[invent]

Mentioned in most text, the circuit shown below is not suitable for low-voltage applications.
If I am not mistaken, this mean it is not suitable if VDD is low. correct?

How do I calculate the minimum VDD required for this schmitt trigger?

 

[keith1200rs]

How low do you want to go?. I have used that circuit for 2.8V on 0.8um process and it still simulates fine down to 1.5Vand below.

Its low voltage operation is limited because it has 4 transistors in series. I guess there are other configurations that would go lower by not having so many series transistors.

Keith.

 

[invent]

I am using 0.35um process.

I have also considered many other Schmitt triggers, but each has its own strength and weakness.

I have been receiving different answers on how to calculate its minimum VDD.
Some suggest it is: 4VDS
While others say it is: 2Vth+1VDS

 

[keith1200rs]

Without doing a detailed analysis, I'm not sure of the correct answer. I also don't know what 4VDS means.

I think it will be at least 2Vth, but not much more. That is confirmed by my quick check on an old design of mine which I designed for 2.8V working on a 0.8um process with Vth around 0.8V/-0.84V. It was working at 1.5V but the threshold start to shift. As an example, I just range a few simulations to find out what happens (I am too lazy to calculate it. The threshold was designed for around 2/3Vd at a nominal 2.8V, I think. The following is what happens as you drop the supply voltage:

VDD High threshold
5V 64%
2.8V 68%
2V 70%
1.8V 72%
1.5V 80%

So, at VthP+VthN + 0.16V it is still quite close to the design value.

Keith.