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LM 3478 Problem

Sotirios Gakopoulos
Prodigy 10 points

Other Parts Discussed in Thread: LM3478

Hello,

I need some help for the LM3478. I'm using this device to build a Step Up converter with the following specs: V input = 4,8-6 VDC and V out = 60VDC, 0.25A. The circuit according to the Webench is the following one

 

but I don't get 60V in the output, I get around 6.5V...

So, I decided to make some changes to the circuit according to the LM3478 data sheet. You can see them in the picture above. 

I used these schematics as a reference.

      

I calculated that the inductor can be from 1uH to 10uH depending on the switching frequency.

After these changes, my new output is 8.6V (still far much lower than the desirable 60V). What am I doing wrong? I would appreciate any help!

Best regards,

Sotirios Gakopoulos

  • 0
    TI__Expert 6365 points

    Hi Sotirios,

    Looking at your WEBENCH design, it was designed with highest efficiency which means it’s running at the lowest frequency. So the entire BOM is generated for 100kHz operating frequency.

    Looking your schematic, the inductor value is 1/10 of WEBENCH suggested and the rest of component values seem fine except for Cin (Need more bulk capacitance). Maybe just by increasing the inductor and Cin may work. Also, a good board layout is critical whether the circuit performances to expectation or not.  If the circuit is still not behaving, please post the layout and the waveforms of the switching node, comp and Vout? Our product application team will be gladly to help.