hello, I made a circuit (see pic) where the reference voltage comes from two anti-parallel diodes, but the stability is not very good (ca. 16mV per 1V Vcc). This is relatively independent of whether the series resistor is large or small, too.

Can anyone suggest a better way to make a Schmitt with a hysteresis that is independent of supply voltage? It doesn't have to be a certain voltage (0.7V, 1V, 1.4V or any number will do) and it doesn't have to be centered around Vcc/2 either.

As to why such admittedly strange requirement: no reason, this is a hobby project and the Schmitt is used by another part of the circuit and I found that the math becomes a bit simpler if the trigger has a fixed hysteresis.

 

Hi,

Do you mean you want it to be some absolute value like turn on at 2v and turn off at 1v, or do you want it to be ratiometric with the supply voltage?

Ratiometric would mean that with 10v if it turns on at 4v and off at 2v then at 5v it would turn on at 2v and off at 1v.

For absolute level, it would turn on at 4v and off at 2v at either 5v or 10v, or anything in between like 8v or even 15v supply voltage.

Or, do you mean that the DIFFERENCE has to be constant, such as on at 8v and off at 6v (difference of 2v) at either 5v, 10v, 15v or any other supply voltage?
This would mean on at 8v off at 6v, or on at 10v and off at 8v, or on at 20v and off at 18v, always a difference of 2v.

 

If you need absolute levels rather than ratiometric then you could perhaps use a voltage reference IC, tap off a couple of reference levels from that, and use a pair of comparators with feedback.

 

Below is the LTspice simulation of a LM339/393 comparator with stable hysteresis value derived from the 2.5V TL431 reference voltage.
The trigger point and hysteresis delta are determined by the relative values of R3, R4, R5, and the 2.5V reference.
The values shown give a simulated delta of 780mV.

If you want a higher or different output voltage or a signal inversion, you can use a second comparator triggered by the first comparator for that.