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Electronics => Beginners => Topic started by: prosper on October 18, 2022, 03:54:39 am

Title: Help with my undervoltage shutdown circuit
Post by: prosper on October 18, 2022, 03:54:39 am

I've designed an undervoltage shutdown circuit. I'd like some tips on:

• decreasing current consumption, and
• (ideally) a way to eliminate the startup switch - (or, at least, to start the circuit in 'on' state when the battery is connected.)
• anything else I've overlooked, or should consider?

My design goals:

• Use jellybean parts that I have on hand
• Minimize quiescent current
• Zero (or, <10's of uA) level of current consumption when in shutdown
• Shutdown is triggered when Vbatt is < ~2.8V (anything from 2.75 - 3.1V is acceptable)
• Don't care whether it's high or low-side
• Tolerates up to 5.5V input (realistically, 4.4V is fine)
• My voltage reference requires 100uA
• Expected Iout is <250mA

(https://www.eevblog.com/forum/beginners/help-with-my-undervoltage-shutdown-circuit/?action=dlattach;attach=1617262;image)

I haven't mathed out the values of R4/R6/Rhyst yet. I believe they are in the correct order of magnitude, but I need to dial in the exact values, bearing the current requirement for U3 in mind.

A major current consumer is the reference, which needs 100uA (max). Perhaps I could use a zener or couple si- diodes in its place at a smaller current, although I believe that would be really temperature sensitive. Are there any other options or configurations (short of a specialty part) that I could use as a reference? I don't need a great precision.

R6/C1 is intended to ensure the op amp inputs are always less than it's supply, and to give it enough time to fully turn Q2 off when the battery voltage drops below threshold.

Title: Re: Help with my undervoltage shutdown circuit
Post by: ledtester on October 18, 2022, 04:42:48 am

Quote

R6/C1 is intended to ensure the op amp inputs are always less than it's supply ...

It seems that you would want to have the LM358's V+ and C1 to tap into the positive supply to the right of R6. Is that right?

Title: Re: Help with my undervoltage shutdown circuit
Post by: prosper on October 18, 2022, 05:01:55 am

Sure, but then the capacitor would just discharge through whatever load is connected, which might not give the ic enough time to shut the fet off hard, before all power is losg