LM46002: Formula 22 for Cff may have an error

Hi Charles,

I believe the equation is correct for Cff. I calculated the application example in the datasheet and came up with the correct value. Make sure you are using the value for fx in kHz and calculating the parallel value for Rfbt//Rfbb correctly.

What values are you using for fx, Rfbt and Rfbb? 

Also, look at Table 2 of the datasheet for recommended values.

It looks like you tried posting an image in you post, but it didn't show up. Not sure what you were trying to post.

Regards, Jason

Hi Jason.

I am using fx as Hz. For all formulas, I am using ohms, farads, hertz, etc.

The data sheet shows fx=4.35/(Vout X Cout), what should the Cout units be?

Also, the parameter for voltage undershoot is not given in the design example, as far as I can see. What is the value for the example?

Hi Charles,

Use uF for Cout.

Regards, Jason

Hi Charles,

I can't find the Vout undershoot that was used in the example either.

For a 3.3V rail, a typical value might be 50mV to 100mV.

Regards, Jason

Hi Jason,

For the design example given with Vout =3.3V and 47uF X 3 capacitors, I get fx=0.009348 (using 141uF. If I use capacitance in Farads, it is 9348. Is that what you get?

Next, for Cff calculation I am not getting anything like 47pF. Would you be able to write out the detail of the calculations?

Hi Charles,

Yes, that's what I get for fx. Sorry, I said uF, but I meant F.

For Cff = 1/(2*pi*9348) x 1/√(1000000*303000) = 30.1pF

I know it's not 47pF, but it gives you a result in the tens of pF.

They probably used the value shown in table 2 for selecting typical components.

Thanks! My numbers are all good now!