Re: CD4060 oscillator, max resistor value

On Fri, 03 Aug 2007 13:32:15 -0700, the renowned Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Hello Folks,

Got a design here that contains a few 4060 long timers with RC
oscillators. Battery operated so consumption counts. Some datasheets
state that the maximum value for any resistor in that area is 1M. Since
Rs (feedback resistor) must be at least twice Rt (timing resistor) this
leads to rather lowish timing resistor values. Since this is a logic
gate oscillator that requirement causes quite some current at voltages
around 10V.

Anyhow, the TI data*** does not seem to state that maximum:
http://focus.ti.com/lit/ds/symlink/cd4060b.pdf

... while ON-Semi does state a 1M maximum:
http://www.onsemi.com/pub/Collateral/MC14060-D.PDF

What gives? Why do some specify that max value as low as 1M for really
low leakage CMOS logic? A remnant from the olden days when this stuff
was leaky at times?

TI gives the maximum leakage as +/-1uA at 85°C. If we assume a
threshold of 0.5*Vdd, the unit would stop working entirely with a
2.5M resistor and 5V, so the 1M is more-or-less within reason assuming
you could allow something like 30% change in timing and given that the
threshold can be different from 50%, and not necessarily in the
direction that's favorable.

I usually use my own oscillators around CD40106 but this time those are
all used up and there ain't no space no more :-(

Using >1M on regular CMOS is living in unspecified land, but you may
well be able to get away with it for a non-critical application. Note
that there's a 4 order of magnitude difference between typical and
maximum at 25°C, which should track over temperature.


Best regards,
Spehro Pefhany
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