LM2901-Q1: The calculation of the structure of positive feedback to add hysteresis

Hello Pin,

Because they share common input and reference dividers, they will affect each other depending on the state of each output. This becomes quite complex.

First, I would recommend reading the following Appnotes to get acquainted with the hysteresis and window comparator fundamentals:

Inverting comparator with hysteresis circuit (Rev. A)

Comparator With Hysteresis Design Guide (Rev. A)

TIPD144 Comparator with Hysteresis Reference Design

Window Comparator Design Guide

TIPD178 Window Comparator Reference Design

Here is a nice, simple hysteresis calculator (external site) to verify your calculations:

 http://sim.okawa-denshi.jp/en/compkeisan.htm

The tough part comes while trying to determine the hysteresis feedback effects during each of the four possible output states, as each will have different reference and input values...

Hi Pin,

you could do it this way:

pin_lm2901.TSC

R8 is necessary because of the existence of C1. Having the +input of comparator directly hanging at C1 would be counterproductive. The positive feedback via R11 shall move the +input quickly out of the threshold region, but a cap there would slow down this process again. Because of this R8 is introduced. It isolates the +input of LM2901 from C1

Kai

Thanks Kai, that is a great point

Pin

Hopefully Paul and Kai have helped address your needs.

Chuck

Hi Chuck, 

The response above is really helpful, and I will need to go through these information clearly. 

I would follow up with team once I have further question.  

Thank you,

Pin

Thanks for following up Pin

Glad to hear and thanks for the feedback.

I will keep the post open so you have time to review their inputs.  Definitely a complex circuit with several variables to consider.

Chuck