Hi team,
My customer use CD4047B in their project, and below is the question, could you please help give some comments?
Thank you.
According to the datasheet: tM(10,11)=2.48RC
The following application scenario output theoretical frequency: f=1/ tM=1/(2.48*47kΩ*22pF)=195kHZ. Is the calculation formula correct?
The following application scenario actual test frequency: f=170kHz. Why is the difference from the theoretical value?
Regards,
Ivy
Hi Ivy,
It looks like they are running the device in the "Free Running" mode rather than the "Monostable Multivibrator" mode, so the equation for operating frequency will be:
F = 1/(4.4 * R * C)
Since they are using a very small capacitance value, system parasitics will become much more of a concern than if using a larger capacitor.
I expect that their actual total capacitance as seen by the oscillator is approximately 28.4 pF, which will result in a 170 kHz output:
F = 1/(4.4 * 47k * 28.4p) = 170.3 kHz
If, for example, they were using a 1nF capacitor, the error from an additional 7pF of capacitance would be negligible.
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Looking at a similar device, CD4046A, we can see that the log-linear relationship for the output frequency to the timing capacitor value is only shown for values greater than 80pF:
I can't say for sure since I wasn't there when this part was defined, but I would guess that this was done because the values were no longer linear below that point.
