XR2206 timing current formula

[marekpasek]

Hello, I have a problem deriving the formula which would descripe the relationship between Vin and output frequency on xr2206.
The datasheet says that f =1/RC, where C mean the timing capacity and R means the resistance between pins 3 and 12. Therefore I derived that f = dVin/3RC, where dVin means the voltage difference across resistor with given resistance R and C mean timing capacity.
Nevertheless the datasheet also says that f = 320000I/C.
Certainly both formulae cannot be correct therefore... where is the mistake
Thanks

[ledtester]

Well, dVin/R = I, right?

So the only difference is in the constant - you have 1/3 and the datasheet has 3200000.

How are you deriving your 1/3? Could it just be you are using different units? Note that 1/3 = .333333... which is close to 320000 if you multiply by 10^6.

[marekpasek]

Correct. The voltage on pin 7 (sorry, I was misled and I thought it was pin 3) with reference to pin 12 equals 3v and is constant. Therefore 1/V equals 0.33333. There is a difference only in that. Nevertheless it makes a serious variance in the output frequency. I want to construct a device on whose precision I could rely, so I need to be capable of predicting the output frequency accurately.

[ledtester]

Two things...

1. If you want to have an RC timing circuit produce a specific fixed frequency you need to make one of the components adjustable (i.e. either a variable resistor or variable capacitor) and use a frequency counter to tune that component to the desired frequency.

2. Below is the formula I found in the XR2206 datasheet. Note that "I measured in mA" is I*1000 and "C measured in microfarads" is C*1000000, so the frequency formula is:

Code: [Select]

      f = 320 * (I*1000) / (C * 1000000) = 0.320 * I / C

XR2206 datasheet: https://www.sparkfun.com/datasheets/Kits/XR2206_104_020808.pdf

[schmitt trigger]

If you are building your own XR2206 based generator, I would suggest to watch this very informative video: