LM27403: Calculation Confusion and disconnects, Webench, Design Spreadsheet, Datasheet, Reference Design, Evaluation Board

For OC calculation, just refer to Equation 8 in datasheet, and Which EVM do you have? Can you take a photo?

Yuchang

Here is a photo.  The board I have is the LM27403EVM-POL600.  Since this is a mature device, why does TI provide tools that don't work?  And, why doesn't TI clean up conflicting design information? 

Thanks,

Hi Gerold,

I generated a design using Webench and the SS.  After adjusting parameters so the two were close as far as Ldcr and current limit, noticed differences with Cc1, Rc1, Cc2, Cc3, Rc2, Rfbt & Rfbb.  So I'll go through the formulas provided in the datasheet.  The LM27403 PWM Regulator Design Tool.xlsm does require more active participation by the circuit designer, like calculating Desired Crossover Frequency and Pole & Zero placement.

Thanks,

-j

One other thing I found, when using the Rcs and Riset values calculated by Webench, 2.67k, I hit a current limit much lower than the required 35A.  When I change them to 4.99k as used in the 0.9V, 30A reference design, I was able to get 35A Iout.

Thanks,

-j

Hi John,

    What was the value of current limit that you got with the WEBENCH recommended resistor? What is the inductor DCR for your application ? What is the inductor part number - I like to verify if there is a lot of variation in the DCR that is causing this?

Can you please share the WEBENCH design ? You can post the WEBENCH shared design link here.

Regards,

Gerold

Hi Gerold,

I'm using a Coilcraft SLR1070-121KE with DCR of 0.29mOhms.

12V Vin, 0.9Vout, 35A

I used the paperclip to insert a pdf file, but it looks like it tried to embed it instead of attaching it.  

Here is what it looks like on my end.

LM27403 Design.pdf

Now that I sent it, the file looks to be correctly attached.

Hi John

  The 2.67k looks high for the 0.29mohm DCR. I have asked the WEBENCH team to look into this.

Will get back to you shortly.

Regards,

Gerold

Hi John,

      You can create a shared link for your design. If you go to tool, you will find three dots on the top right, you will find "Share this design". Then select a publick link tab. Then Get Link" and copy and paste the link here. The reason is because when I create the design, I am getting a custom inductor with 0.571mohm and the 2.67k Riset corresponds to the custom inductor. When you change the inductor, the Riset is not getting updated automatically - I will ask the team to check if this can be supported.

Here is a link to the design that I created.

webench.ti.com/.../SDP.cgi

Regards,

Gerold

Here it is.

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=5FD8B342B2CCD6F3

Thanks,

-j

Thanks John. The WEBENCH team is looking into it. To get you going, please use the value given in the excel tool for the corresponding OCP and DCR values entered.

Regards,

Gerold

Hey Gerold,

Sorry for the delay, I was diverted for a bit.  In the SS, I set the Required Current Limit Setpoint to 40A.  I'll configure my eval board with these values and give you an update.

I used Rcs & Riset = 1.33k and Rs = 2.05k.  I am able to get 0.9V at 35A out.

Does this look correct?

Thanks,

-j

Using and rearranging Rs*Cs = L/Rdcr from the datasheet, RS = L/(Cs*Rdcr), RS = .12u/(0.22u*0.29m) = 1.88k.  The spreadsheet gives me Rs = 2.05k.

Looking at the Rs Cell note, it says the RsCs time constant is set to 1.2 * L/Rdcr.  Rs = 1.2*(.12u/0.29m)/.22u giving me an Rs = 2.26k with Cs = 0.22uF.

A couple inconsistencies.

Hi Gerold,

The supply is working well stepping the load between 10A and 35A with a capacitive load of 1,507uF at room temp.

Next is selecting the correct Rc1, Cc1, Rc2, Rfb, Rc2 and Cc3.  Should I just stick with the datasheet formulas, or is the Webench tool adequate?

Thanks,

John