I have this circuit (as part of a larger design):
The resistors are with a tolerance of 1%.
On the input, V1, the circuit gets a pulse whose initial voltage is 28V to a certain voltage (shown later), with the Ton of the pulse being at a certain length (shown later).
The reference voltages are created using a micropower voltage reference IC and op-amps.
I need to make sure that when inputting these pulses, some will assert out2 to '0' and some will make it remain at '1'. (Please note that out1 is not really important here, I just included it in the schematic so you will know it is there).
Here are the voltages and pulse lengths that I am inputting to V1:
Out2 Should Remain at '1' Out2 Should Assert to '0' 30V/80mSec 30V/100mSec 40V/10mSec 40V/20mSec 50V/5mSec 50V/10mSec
The capacitor C1 is what concerns me. I don't know how to determine it's value. It is a SMT ceramic capacitor.
From what I understand, the capacitor has two jobs:
- Filter spikes from the power supply (the P.S is a generator).
- Provide timing (together with the resistors), so that certain pulses with certain lengths will assert Out2 to '0'.
The thing is, I have tried so many options for the capacitor and none seem to work for all conditions. I have tried both in simulation and in real life, and of course it doesn't always seem to match...
If I use too small capacitor (for example 1uF), then Vc will reach 3.62V too quickly, and Out2 will be '0' when it's supposed to be at '1'. If I use too big capacitor (for example 4.7uF), then Vc won't reach 3.62V, even at 30V/100mSec, meaning Out2 will not be '0' as intended, and so will remain at '1'. (I am 99% sure that I used LTSpice to simulate this circuit with a 4.7uF capacitor and it actually asserted Out2 to '0', but it did not happen with the actual device when I soldered a 4.7uF capacitor)
So here are my questions:
Lets assume I can use a capacitor and be done with it. How can I determine it's value? I have tried calculations and simulations and experimented with lots of capacitor values and none seem to work for all conditions.
Is there a solution that is smarter than just using a capacitor? Do note that the PCB already exists so I can't change my design too much, so using ICs is somewhat of a problem. I would however love to hear about these solutions, if they exist, both because I would see if it can somehow be implemented and because I would like to learn.
Thank you very much!
\$V(t) = V(inf) - (V(inf)-V(0))e^(-t/Tau)\$
\$V(t) = 3.62V\$
\$V(inf) = 3.68V\$ (Because of voltage divider when input is at 30V)
\$V(0) = 3.43V\$ (Because of voltage divider when input is at 28V)
\$t = 92mSec\$ (fall time is 5mSec so I took a margin)
At the end of my calculations I got:
\$Tau = 64.46mSec\$
\$Tau = R1||R2*C = 11.14k*C = 64.46m\$
\$C = 5.786uF\$
But this result does not help me, as like I said, for 4.7uF, Everything passed except 30V/100mSec.