The normal 555 timer has Control Voltage pin that is intended to be decoupled to ground. For voltage controlled PWM circuits this pin is driven but must obviously be from a low impedance source.
Most datasheets don't even qualify this input resistance and was wondering if anyone knew of an IC with the same features as a 555 but where the Control Voltage input pin is high impedance?
Ya mean like a CMOS 555? How about a CMOS 555.
NT
The CMOS CV input impedance is around 100k, higher than a bipolar 555 but not as high as CMOS logic. The output impedance of whatever driving it will probably have to be significantly less than that as the CMOS input still feeds the midpoint of a voltage divider across the rails
It's easy to see what the input impedance of the control voltage input is from the equivalent schematic:
The CV input feeds some high-impedance comparator inputs and the upper voltage-divider point of a string of resistors, in the BJT version they're about 5k and in the CMOS version they're about 200k.
The comparator inputs draws negligible current by comparison when operating normally so the resistor string will dominate, so the small signal AC input impedance will be around 5k || 10k in the BJT version and 200k || 400k in the CMOS
Plus some capacitance maybe 20-50 pf?
I thought the purpose of the CV input was to provide a point to add a capacitor to remove sensitivity to power supply noise. I suppose if you are driving the pin with a signal this doesn't matter.
Rick C.
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The purpose of the Control Voltage pin is probably to add a "control voltage" dat's why they call it "Control Voltage"! Heh! IC designers are funny guys
By definition no, because the control pin is a resistor divider, period.
The highest I know of is this at ~6M (total),
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:
That would be a rather disingenious* use of the pin, but yes, there's definitely reason for its being there!
*Meaning the opposite of "ingenious", not "disingenuous". It feels kind of unfortunate that this isn't actually a word, or the application of it at least.Example:
Tim
Vectrex power supply board:
a 555 ran the CRT voltages, it looks like line regulation was accomplished via running the 555 off an unregulated input rail and connecting the CV pin to regulated 5 volts, and load regulation indirectly via the -30/-9 rail
Tim Williams wrote
Oh well, I had [1] a HeNe laser supply
No longer have a HeNe laser..... :-)
The old school 555 has 3 5K resistors in series** so the impedance is 3.3K +/- a lot.
The LMC555 has 3 100K resistors so 66.7K nominal impedance.
If you want higher, just add a unity gain buffer op-amp, like MCP6001 for
5V operation. 10^13 ohms typical input resistance, which should be adequate.--Spehro Pefhany
** apparently not the origin of the part numberHave something to add? Share your thoughts — no account required.
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