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We have a signal that is alternates between being high for 7s and low for 2s. We want to limit any single high pulse to 20s.
Initially I was going to use a SN74LVC1G123 monostable multivibrator, with Cext=10uF and Rext=2M, as the datasheet does not mention maximum values. Alas TI support do not recommend having Rext greater than about 660K or Cext above 10uF - limiting the period time to 6.6s.
I am looking for alternatives. I will try a TLC555-Q1 set up as a monostable, invert our trigger signal (using a SN74LVC1GU04DBVT) and connect it to both the 555's trigger input and the reset signals. Will this work? Is there a better more modern single IC solution?
The circuit is powered from 3.0V and only needs to drive the gate of a N type MOSFET. The trigger signal comes from a PIC I/O pin and needs to be backwards compatible with older software (hence the inverter for the 555).
This multivibrator (or any other model) does not, by itself, have a capacity limit; the problem is the leakage current. If you manage to make the leakage current lower than the charging current, it will work. (And if the leakage is not much lower than the charging current, you will not get much accuracy.)
You could try film capacitors (large and expensive). For really low leakage, try PTFE (Teflon) film capacitors (really expensive).
Nowadays, I'd consider using a microcontroller that counts from its internal oscillator.
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In reply to Clemens Ladisch:
Thank you for your reply, now I understand the concern.
Alas we cannot use the suggested capacitors because our product needs to be non-magnetic and cannot use capacitors containing nickel.
I prototyped the circuit using a non-magnetic Kemet T495A106K010ANA2K0 10uF capacitor and it worked giving a 20s period as it should. This capacitor is specified as having a max leakage of 1uA at 25'C (alas they don't specify for other temperatures). Ah I see now, we are only initially charging at 1.5uA with 2M at 3V, so out test capacitor had a much lower leakage current than it could have.
What is the charging current to maximum leakage current ratio that TI would recommend, please?
In reply to Ian Kelly2:
There is no TI recommendation. The leakage is supposed to be so small that you would not need to care.
You could arrange multiple multivibrators in series, e.g., 4× 5 s with Rext = 500 kΩ.
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