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# LT1963 / LT3015

Hello,

I would like to know if there is a maximum limit to the bypass capacitor (CFF).

In my simulator it also plays a role in the output impedance (low frequencies), I came up to 47µF !

The LT3015 Data sheet says : CFF ≥ 10 nF/100µA • IFB-DIVIDER

IFB-DIVIDER = VOUT/(R1+R2)

In my case IFB-DIVIDER = 14V5/(1.8k+19.6k) = 0,68 mA

CFF  ≥ 68 nF

Is 47µF far too much ?

Should I limit this capacitor to 100 nF ?

What about the LT1963a, can I do the same ?

Thank you,

Jean

## Top Replies

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• Hi Jean,

If you have computed Cff >=68nF, that would be your Minimum requirement Cff.
Using 47uF would be far too much from the minimum required. 100nf would be advisable.
*Note that the start-up time is affected by the use of a feedforward capacitor. Start-up time is directly proportional to the size of the feedforward capacitor and the output voltage, and is inversely proportional to the feedback resistor divider current

You can check the typical performance characteristics graphs for further details related to Cff and its trade offs depending on the value.

In regards with LT1963a, LDO's formula for Feedforward cap is different.
Please use the formula on page 7 (Equation 8) of this datasheet for computation of Cff.

Note that this is only applicable with an adjustable model and cannot be used with fixed output model LDO.

regards,
Ash

• Hi Jean,

If you have computed Cff >=68nF, that would be your Minimum requirement Cff.
Using 47uF would be far too much from the minimum required. 100nf would be advisable.
*Note that the start-up time is affected by the use of a feedforward capacitor. Start-up time is directly proportional to the size of the feedforward capacitor and the output voltage, and is inversely proportional to the feedback resistor divider current

You can check the typical performance characteristics graphs for further details related to Cff and its trade offs depending on the value.

In regards with LT1963a, LDO's formula for Feedforward cap is different.
Please use the formula on page 7 (Equation 8) of this datasheet for computation of Cff.