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# Feedforward Capacitor formula for LTC3129

Could you share me the formula to find feedforward capacitor, if the feedback resistors are used in the range of Kohm and Mohm.

The datasheet formula given the results interms of microfarad but the application circuits has CFF interms of pF.

Could you clarify me?

Datasheet suggested below formula, Let me given some example from application schematic,

if R1=2M,

Cff= 33uF

But 33pF used in the below application circuit (refer left side figure).

Similarly if R=3.01M, Cff would be 21.92uF (~22uF), by referring right side figure it was given as 22pF.  Please give me the solution for the above.

Regards,

Malathi T

• Hi,

The feedforward formula capacitor in datasheet has R1 in Mohms and resulted cap is in pF. So for R1=2M, Cff=66/2=33pF. Similarly for R1=3.01M,Cff=66/3.01=22pF.

Thanks

• Hi Anisha,

Could you please recheck whether will getting pF result or uF result by using R1 interms of Mohms.

1M=10^6. by using Mohm result will fall interms of uF not pico farad.

Regards,

Malathi T

• Hi,

If R1 is 2M, you have to put only value 2 in the formula and the result is 33 which will be in pF.So the cap will be 33pF. The formula has been simplified for calculation. I hope this clarifies your confusion.

Thanks

• Hi Anish,

Yes. Thank you

Regards,

Malathi T