lm2917 low frequencies?

[kindred]

Hello everyone, first time poster

I was reviewing the lm2917 and i was looking for some information on how well it copes with low frequencies in terms of precision, I assume it should be no problem since its only calberation to an extent (rite?) but id like to find out for sure. The low frequencies Im refering to are 60hz and below.

Also, I seem to be having difficulty calculating my Vo for the lm2917, according to the data sheet i need to keep I3 below 240uA.

They gave me two formulas to help calculate this:\

VO =VCC xfIN xC1xR1xK ........(K typically 1.0)
I3 = Vo/R1

Now these are fairly simple calculations but I find that its impossible for me to reduce my current regardless of my resistance based on these formulas since v=ir, have I missed some logic here?

Thank you.

[pastera]

You need to choose a scale factor (V/Hz) and a maximum frequency (Fmax)

Lets assume the following:
Vcc: 12V
Scale factor: 10mV/Hz (60Hz = 0.6V)
Fmax = 600Hz (6V)

Fmax = I2/(C1 x Vcc)

C1 = I2/(Fmax x Vcc) = 140uA/(600Hz x 12v) = 19444pF = 0.02uF (closest standard)

Find R1:
Vo = Vcc x Fin x C1 x R1 x K

R1 = Vo/(Vcc x Fin x C1 x K) = 0.6v/(12v x 60Hz x 0.02uF x 1) = 41667 ohms = 41.2k

So with standard values:

R1 = 41.2k
C1 = 0.02uF

Vo = 12 x Fin x 20e-9 x 41.2e3 = 0.09888 Fin (~1.1% error)

So lets set ripple a 1% at 60Hz
Vr = Vcc/2 x C1/C2 x (1 - (VccxFinxc1)/I2)

C2 = Vcc/2 x C1/Vr x (1 - (VccxFinxc1)/I2)
= 12/2 x 0.02uF/(0.6x0.01) x (1-(12x60x0.02uF)/(0.6/41.2k))
= 19.8 uF (20uF)

Using a 20uF capacitor for C2 would result in a very slow settling time. If the output is going to be fed to a mechanical meter, the value could be reduced by a factor of ten (10% ripple)

Aaron