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I am trying to build an undervoltage protector for a 6S LIPO battery. My idea was to use a comparator to switch off an NMOSFET when the battery fell below a preset voltage, 20V: enter image description here

R20, R21 and C20 create a crude voltage supply for the LTC1440, VCC varies between 6.6V and 8V in the simulation. R10 and R11 bring the 20V down to 5V. R1, R2, R3, R4 and R5 are calculated according to the datasheet.

When I run this through LTSpice, Vin drops linearly from 1.24V to 1.06V (thus crossing the VRef 1.182 at about 20V input): enter image description here but VOut falls slowly from about 6.5V to 0V after the 20V threshold is reached: enter image description here as a result, the load is switched off slowly: enter image description here

I expected it to switch off completely at exactly the threshold; why is this not happening?

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  • \$\begingroup\$ Why not use .TRAN for that, and instead of an input source, an input capacitor of large value with initial conditions for the voltage? Or even a PWL, or an EXP source. The dynamics are much better seen then. Otherwise, what Spehro said. \$\endgroup\$ – a concerned citizen Jun 8 '18 at 6:51
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You've only plotted four points on the curve (which are joined by straight lines). Try decreasing the increment in the DC sweep to 0.01 or something like that.

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    \$\begingroup\$ You're spot on, decreasing the step makes the switch-off arbitrarily steep - 0.001 works nicely. Thank you kindly, I learn a lesson. \$\endgroup\$ – smirkingman Jun 8 '18 at 15:22

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