Capacitor Discharge Time to Specific Voltage

[HardwareChap]

I would like to hold the logic level on pin 'D' of a D-type Latch high for 100 ns after the pulse driving it has gone to zero. The pulse will be from a NOT gate, 3 V and will be high for 0.5 ms (min). I was thinking of using a series R and C to ground to hold it above the logic threshold of 2 V for 100 ns. I know that \[\tau\] = RC. How do I calculate suitable values so that the voltage on the 'D' pin is > 2.1 V for at least 100 ns after the falling edge of the input pulse?

 

[BradtheRad]

It appears as though you want a pulse extender. It might be effective to include a series diode in addition to the RC network.

for at least 100 ns after the falling edge of the input pulse

The capacitor charges and discharges through different R values, creating different RC time constants. Small current is available, which suggests R values in the area of 1k-10k.

Take 100 ns as the RC time constant (just to make an estimate using some approximate value). Suppose your R is 1k. Calculate a C value which yields 100 nS time constant. See if the discharge curve creates the desired behavior. Adjust values until it works properly.

 

[d123]

Hi,

The below is a bit hit and miss, but that could be all sorts of reasons like breadboards, etc.

T = R*C
So presumably, C = T/R
or R = T/C

T = seconds, R = ohms, C = Farads.

It's been a while, so you'd need to check this, I think T means something like charged or discharged to respectively 66% and 33% of the supply voltage, basically similar to how the 555 works is one way of describing it, but don't take my word for that.

Alternatively, this pdf about capacitor charging and discharging is pretty confusing:

 

[KlausST]

Hi,

for higher precision you should consider to use an analog switchh instead of a logic IC output.
It creates reliable high-voltage and low-voltage.
(for TTL ICs a high level is 2.0V ... 5V, wich is not very precise)

Additionaly you should consider to use a comparator instead of an logic IC input. You can precisely setup the threshold voltage.
(for TTL ICs the threshold is somewhere inbetween 0.7V and 2.0V, wich is not precise also)

Klaus