Comparator with Hysteresis_Design Calculations

[HT O.1]

I need to design a comparator with Vih=1.3V and Vil=1.2V. Therefore hysteresis is 100mV. The circuit will be working on a 3.3V supply. During high state, the o/p of the comparator will be 3.3V and at the low state, it will be 0V. I will be using a comparator with a push-pull output configuration. Please see my design calculations in the file attached. I would like to know two things

1)May I know my design calculation is correct or not.

2)For finding Vref where I need to plug the value of R1 and R2.In equation 1 or 2

Regards

HARI

[Peter BENSCH]

Please refer to the non-inverting Schmitt trigger, where you also find the calculations.

The wrong step in your calculation is equating (1) and (2), because we are talking about two different states.

But you are close: using R1=5k, R2=330k gives +-50mV hysteresis, i.e. in total 100mV. Vref just gives the trigger voltage and in your case should be set to 1.25V, so this Schmitt trigger turns on at Vref+50mV=1.3V and turns off at Vref-50mV=1.2V.

Good luck!

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

/Peter

[HT O.1]

Hi Peter,

Thank you very much. I will go through the link you provided.

Vref is constant. That is why I equated equations 1 & 2.

Regards

HARI

[Pierre SENNEQUIER · Accepted solution]

Hi Hari,

You are correct.

Back to your second question and using your way, you have to replace Vl by Vih in equation 2 as you are in low state untill the input voltage reaches Vih. (i.e. untill what you name Vl is equal to Vih)

So, it means : Vref=1.3*330/340=1.262V.

(You could do similarly by replacing Vh by Vil in equation 1 for similar reason).

Please refer to https://www.st.com/resource/en/application_note/dm00050759-introduction-to-comparators-their-parameters-and-basic-applications-stmicroelectronics.pdf (equation 5)

Also, this may be obvious, but be careful that it is valid for push-pull comparators only.

You can find what you need here in term of products : https://www.st.com/en/amplifiers-and-comparators/comparators.html?querycriteria=productId=SC60$$3847=Push%20Pull&aggOrder=0%7C1%7C2%7C3%7C4%7C5%7C6%7C7%7C8%7C9%7C10%7C11#products

Take care,

Pierre