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COnfused over MAX5035 math for Cin value

gruvin

May 25, 2022
4
Hi. Hoping someone can tell me where I'm going wrong here with this simple looking formula but which I cannot get any sense out of.

In the MAX5035 Datasheet (link) on Page 12 there is are formulas for calculating ESRIN and CIN.

Initially, I just smashed buttons on my calculator but came to stupid results. So, I made a spreadsheet to break things down. Same stupid results! Please see attached image.

Clearly, I'm waaaay off trying to figure this out. I have similar trouble trying to calculate inductance later on in the datasheet. What on Earth am I missing here?

Attachments

• ESR_C_Math.png
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gruvin

May 25, 2022
4
Oh and, I'm confident the mathematics itself is good ... just maybe my understanding of the inputs or what I'm actually calculating is an issue, I think? Like, perhaps the result is actually the complex output impedance of the circuit or something and not a formula to calculate a desirable value for CIN itself?? That would be weird though, wouldn't it?

On that note though, what is the use of calculating ESRIN anyway? I suppose we could ensure the selected capacitor has a resistive impedance of less than 80mΩ at 125KHz or something but it's not like there's much practical choice out there. Seems dodgy.

Clearly I'm a n0ob hre but to my credit, last week I couldn't even spell inginaire!

CircutScoper

Mar 29, 2022
300
Hi. Hoping someone can tell me where I'm going wrong here with this simple looking formula but which I cannot get any sense out of.

In the MAX5035 Datasheet (link) on Page 12 there is are formulas for calculating ESRIN and CIN.

Initially, I just smashed buttons on my calculator but came to stupid results. So, I made a spreadsheet to break things down. Same stupid results! Please see attached image.

Clearly, I'm waaaay off trying to figure this out. I have similar trouble trying to calculate inductance later on in the datasheet. What on Earth am I missing here?

Please note that D = Vout/Vin, but you instead calculated Vin/Vout. Try it again with D = 0.069 instead of 14.55

gruvin

May 25, 2022
4
Please note that D = Vout/Vin, but you instead calculated Vin/Vout. Try it again with D = 0.069 instead of 14.55

OMG. Thank you! So simple and right in front of me the whole time!

I bet my inductor calculations (which also use D=Vout/Vin) have the exact same problem. Good grief.

You made my day!

CircutScoper

Mar 29, 2022
300
OMG. Thank you! So simple and right in front of me the whole time!

I bet my inductor calculations (which also use D=Vout/Vin) have the exact same problem. Good grief.

You made my day!

Happy to help. Maybe one day you'll reciprocate -- again. Sorry -- a (very) little arithmetical humor there.

gruvin

May 25, 2022
4
You definitely got one over me there!

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