# hysterisi in undervoltage circuit

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#### poster_girl

##### Newbie level 3
I have this undervoltage protection circuit which was published in Linear Technology magazine. I have attached the design equations as well. I dont understand the hysterisis equation. Can somebody pls explain it? The equation mentions VB and VB'.
One must be the voltage at point B when output is high.
The other would be voltage at B when output is low.

The hysteresis is in volts. Must be the range of hysteresis action. The higher it is, the better the circuit resists spurious ups and downs.

However they could clarify things with another equation or two. Such as one that says at what declining supply V the designer set the output to change, and at what rising supply V the designer set the output to change back.

Because the hysteresis action is operating under two situations.

Ordinarily if the supply were constant, the hysteresis voltages might be expressed as percent values of supply V. Behavior could be explained with fewer equations, or a simpler equation. Example, schmitt trigger.

The R1 and R2 divider are typically used to give a stable reference.

However in this case there's more going on. The supply V rising and falling is what makes the circuit change state. It's an accomplishment that the designer could figure out a single mathematical equation for it. The equation might not be the most intuitive equation but math appeals to some electronics buffs more than to others.

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