LM3481 Output

Anson Nunley

Other Parts Discussed in Thread: LM3481

Hello All,

I am working on a project where I will have the following requirements:

Input: 0-36V
Output: 3.8V
Support up to 2A for GSM transmission

I used the TI Webench Power Designer to help me meet these requirements. It gave me a design using the LM3481 device. I have Implemented the design on my board but the Output is only 0.6V. I have triple checked all my connections to make sure there were no shorts or unconnected pins but I didn't find anything wrong.

Can someone help point me in the right direction? I have attached the schematic and layout of my design. I would appreciate any help that can be offered!

Thank you,

Anson N.

1325.5050.Questions for TI.zip

over 9 years ago

0 Tommy Jewell - WEBENCH Design Center over 9 years ago

TI__Intellectual 2930 points

Hi Anson,

What is the Input voltage and Load current you are testing at?

Do you have the ability to take any scope shots or probe some points and describe what you see?

I'd like to see what is happening at the Drain of the MOSFET.

Best regards,
Tommy

0 Anson Nunley over 9 years ago in reply to Tommy Jewell - WEBENCH Design Center

Prodigy 210 points

Hello Tommy,

Thank you for such a quick response! For testing the board, I set the input voltage to 9V and for the Load current I'm not sure. I was just probing the output of the LM3481 circuit before I connected it to the rest of the board. I'm still populating the board, but I'm testing the power first so I don't damage any components.

I took a few screen captures. The image title is the area I was probing.

Thank you,

Anson

4111.Screen Shots.zip

0 Tommy Jewell - WEBENCH Design Center over 9 years ago in reply to Anson Nunley

TI__Intellectual 2930 points

Hi Anson,

It looks like the Drain.jpg is AC coupled. Whats the DC average? or just take a DC coupled pic? Maybe also Pics of the FET Gate and the ISEN Pin and COMP Pin?

Looks like your FET isn't turning on. Want to see the Gate pin to see if the IC is sending it a signal. ISEN to make sure its not current limiting and COMP pin just to make sure its working.

Best regards,
Tommy

0 Tommy Jewell - WEBENCH Design Center over 9 years ago in reply to Anson Nunley

TI__Intellectual 2930 points

Also whats the partnumber on L2?

0 Anson Nunley over 9 years ago in reply to Tommy Jewell - WEBENCH Design Center

Prodigy 210 points

Hello Tommy,

Thank you again for your quick responses. I have taken some more screen captures for you. The signals in the images are DC coupled. I have also included a BoM of the parts with the designators associated with each component.

From what I saw about the switching frequency, it is switching at about 272kHz which is what I originally calculated. Is there anything else you would like to know?

Best regards,

Anson N.

7802.More Screenshots.zip

0 Tommy Jewell - WEBENCH Design Center over 9 years ago in reply to Anson Nunley

TI__Intellectual 2930 points

Hi Anson,

I may be reading the datasheet wrong, but it looks like your L2 inductor is not connected. The datasheet says the primary inductance goes from Pin 1 to Pin 3, and the secondary from Pin2 to Pin4, but according to your layout you've only connected Pin 1 and Pin2 and left the other pins floating. Which would mean you just have 2 open ended inductors.

Take a look and let me know what you think.

Quick question, did WEBENCH recommend L2?

Best regards,

Tommy

0 Anson Nunley over 9 years ago in reply to Tommy Jewell - WEBENCH Design Center

Prodigy 210 points

Hello Tommy,

You seem to be correct about the pinout for the L2 inductor. I must've overlooked it when I was putting everything together. I'm going to try and fix it so that it's pinned out correctly, then I will let you know if that fixes the problem. Thank you for catching my mistake!

As for the recommended L2 from WEBENCH, it suggested using the XAL5050-153MEB from Coilcraft. At the time I was designing the circuit, it had a long lead-time so that's why I went with another inductor. Now that it's available, I will look at replacing my current inductor once I can get this problem fixed.

Thanks again for your help,

Anson N.

0 Anson Nunley over 9 years ago in reply to Tommy Jewell - WEBENCH Design Center

Prodigy 210 points

Hello Tommy,

I corrected the pins for the L2 inductor and the power supply is working properly now. Thank you so much for your help! You saved me a lot of time of stressing and debugging.

Best regards,

Anson N.

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LM3481 Output