Hysteresis on OVLO with LM2903YPT

[SUDOsarvesh]

Hello TechGeeks,

Please welcome me to here as this is my first query on EDA Forum, and Kudos to you all for gathering impressive forum here.

Query goes like this: "Trying to develop a HW feature as Overvoltage protection and Undervoltage protection on 12V net with the help of LM2903YPT (Link Datasheet) comparator IC. I have tried to adapt limit comparator for this feature (see LimitComparator.jpg). Ahead of this circuit I have added positive feedback to these comparators but it seems like hysteresis is only possible on UVLO not OVLO (see Circuit.jpg). There is no affect of resistive +ve feeback on OVLO IC.
[Note:] In Circuit.jpg the top LM2903 is for UVLO and bottom LM2903 is for OVLO.
Observations: In simulation results, the red rising and falling signal is 12V which needs to be monitored via 5V reference voltage, the blue signal is output of collective LM2903, 5V on normal range of 12V net and Zero on Undervoltage and Overvoltage of 12V net. And there are one red cursor(Y1) for UVLO and green cursor(Y2) OVLO activation value. If you look on 12VRising_simulation.jpg and 12VFalling_simulation.jpg, you will find we have hysteresis of 800mV on UVLO but for OVLO there is no.
Question:
1. I am getting UVLO proper with hysteresis but OVLO is ramped on activation if you see simulation results, How is it possible to make it step activation?
2. How is it possible to provide hysteresis on OVLO side?"

Thank you!

 

[KlausST]

Hi,

Ahead of this circuit I have added positive feedback to these comparators but it seems like hysteresis is only possible on UVLO not OVLO

I see the positive feedback only onl the upper COMP.

Just do it in the same way with tho other COMP.
Mind: for hystereis there are always two resistors involved: The source resistance and the feedabck resistance.

***

The circuit works as expected. Your upper COMP has positive feedback, but the lower COMP has not.
Please update your schematic with your hysteresis resistors for the lower COMP.

Klaus

 

[SUDOsarvesh]

Hi,


I see the positive feedback only onl the upper COMP.

Just do it in the same way with tho other COMP.
Mind: for hystereis there are always two resistors involved: The source resistance and the feedabck resistance.

***

The circuit works as expected. Your upper COMP has positive feedback, but the lower COMP has not.
Please update your schematic with your hysteresis resistors for the lower COMP.

Klaus

Hi Klaus,
First of all thanks for quick response.

I have already mentioned above that "
Ahead of this circuit I have added positive feedback to these comparators but it seems like hysteresis is only possible on UVLO not OVLO (see Circuit.jpg). There is no affect of resistive +ve feeback on OVLO IC.
[Note:] In Circuit.jpg the top LM2903 is for UVLO and bottom LM2903 is for OVLO.
"
But let me confirm again with other Rh values

--- Updated Oct 29, 2022 ---

Hi Klaus,
The issue persists, either we can have hysteresis on UVLO or hysteresis on OVLO with using only one comparator so that other's output do not affect other's reference voltage because for OVLO reference voltage and hysteresis both are on same pin.

Here attaching the simulation files for your reference.

 

[KlausST]

Hi,

yes, I do understand.

but if you say "ahead of this circuit" ... and show a different circuit ... we will never know what you really tried.
So we don´t see the "error" situation.
It makes it hard to give good advice to avoid this (invisible) error. It leads to "guessing" and false recommendations ... wasting a lot of your time.

Klaus

 

[SUDOsarvesh]

Hi Klaus,

Thanks for the hint, now I understood the catch here. With positive feedback on OVLO pin somehow we need to isolate the input from output hindrance for OVLO threshold pin. I selected diode for it, BJT buffer can also work fine!

Closing the thread!

 

[KlausST]

Hi,

I don´t know why you used a diode, while I recommended to use two resistors.....

But as long as you are satisfied... it´s a valid solution.

Klaus