WEBENCH® Tools/LM2735YMFEVAL: LM2735Y Custom Capatir

Ron Kolarovic

Tool/software: WEBENCH® Design Tools

I am adding 3 x 10uF, 4mohm per cap output capacitors that are a custom cap that will see 10V of DC bias. I have the manufacturers datasheet and I need to derate them for DC bias by -55%. In the Webench Designer if I setup a custom part what do I set the "Derated Cap Per Cap" to? If I set it to 10 and the ESR to 4mohms, the schematic shows 10uF 4mohms qty 3. When I click on the schematic symbol it shows the cap as 10uf and the "Total Derated Cap" as 30uf. Is Webench doing the linear approximation internally for calculations? The same question holds true for the input capacitors as well, are they internally derated?

over 7 years ago

0 Gerold Dhanabalan - High Performance Radar over 7 years ago

TI__Genius 14455 points

Hi Ron,

The custom inputs webform will ask for "derated cap per cap" and the "ESR per cap" and a quantity at the bottom.

The total capacitance = Derated capacitance per cap * Qty.

The total ESR = ESR per cap / Qty

Regards

Gerold

0 Amod Vaze over 7 years ago

TI__Mastermind 29010 points

Ron,

For a custom capacitor, we take input on derated cap value per cap and multiply it with quantity to get the "Total Derated Cap" value. The Total derated cap value is used internally for calculations as is.

Here is something you can do.

1. You can create a custom Cout by clicking on the Cout in schematic and clicking "Choose Alternate" option.

2. You can then specify 4.5uFin the "Derated Cap per Cap" field (-55% derated value of 10uF) and quantity of 3.

Once done, the Cout updates to use 4.5uFx3 = 14.5uF Total Derated Cap for internal calculations. Hope this helps.

Regards,

Amod

0 Ron Kolarovic over 7 years ago in reply to Gerold Dhanabalan - High Performance Radar

Prodigy 70 points

Thanks for the response. I get the total C/R but it is more of a question as to whether I input the derated value, in my example it would be 4.5uF per cap (for 55% derating). If that is the case I expect the internal calculation/simulation should not apply a derating. Or do I leave it at 10uF if the internal engine applies a linear derating?

0 Ron Kolarovic over 7 years ago in reply to Amod Vaze

Prodigy 70 points

Thanks, that's what I needed.

0 Gerold Dhanabalan - High Performance Radar over 7 years ago in reply to Ron Kolarovic

TI__Genius 14455 points

Hi Ron
For custom components, the value that you enter is the derated value. No internal derating is applied on top of that.

For components recommended by WEBENCH, the tool will apply appropriate derating based on the datasheet recommendations for that component. The derated value for each of these components can be viewed as a column in the Select Alternate Parts window in the tool.

Regards,
Gerold

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