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LM3481: LM3481 SEPICE Eval board measurments

Part Number: LM3481

Hello,

I have the LM3481SEPICEVAL board and I was doing some measurements and have a few questions/concerns.

1) The datasheet for the eval board has a graph on page 5 for "Efficiency at Vout = 12V. With an input of 12V it says it has almost 90% efficiency.  When I powered it up, I have 12V in the input and it reads 12.11 volts on the output. I have an electronic load with a constant current set for 1Amp. But the input current from my power supply is 1.48Amps.

Vin = 12V

Iin = 1.48Amps

Vout = 12.11V

Iout = 1Amp

Power in = 17.76W

Power out = 12.11W

Efficiency = 12.11/17.76 = 0.68 or 68% efficiency.  I must be missing something. Could you help me out? 

2) Eval board datasheet also says on page 1 that the 12V output option can have a input voltage range of 4.5-18V. With an output current of 0-1.25 Amps

Even with no load, and 4.5V on the input, it does not output 12V. It is not able to do it. The only way I can get it to output 12V is to start with a higher input, wait for it to get to 12V then lower it to 4.5V.



3) With a 5V input on power up, it is able to see the full 12V output. But this won't work with any load. I put a 100mA load on there and the voltage drops down to almost 2V.

I need reliable operation at 5. I need to boost it to 12V with 600mA. Which will be an input current of about 2Amps. Also this is a battery application, so this low efficiency is no good.

Please let me know your thoughts on this.

  • Hi Matt,

    Thank you for your interests. I need to order an SEPIC EVM to have a check.

    Thanks,
    Youhao Xi
  • In reply to Youhao Xi:

    Just an update on the efficiency issue.

    I did the same test with a 12V LiPo battery is the input instead of the power supply. I was getting 89% efficiency.

    But I went back to my power supply and was still getting no more than 70%.

    I have a Keithley 2231A-30-3 bench top supply. Do you think this is an issue with my power supply. Why would I get 89% efficiency with a 12V battery input, but only 70% with a 12V power supply input?
  • In reply to Matt Lang:

    Hi Matt,

    Where did you measure your Vin and I(IN)? Not sure about your Keithley but if you measure the Vin and I(IN) at the EVM input port, it should perform the same unless the Keithley does not hold the voltage or the circuit oscillates.

    Thanks,
    Youhao
  • In reply to Youhao Xi:

    The Vin and Iin where measured by the digital display on the power supply. The positive and negative cables are about 1.5 Feet in length each. 

    I will put a meter on the input terminals and see if I get anything different.

    The issues with the 5V input are still there. I even tried a different power supply.

  • In reply to Matt Lang:

    The efficiency issue was indeed the cable from my power supply. I swapped them out for some short and thick alligator clip jumpers and the efficiency is about 89%

    Vin = 12.00 V

    Iin = 1.22 A

    Vout = 12.03 V

    Iout = 1 A

    89% efficiency 

    Turns out it solves the low voltage issue as well! 

    Vin = 4.98V

    Iin = 2.766 A

    Vout = 12.03 V

    Iout = 1 A

    87% efficiency

    It still can not start up at 4.5V with a 1.2A load. But that is perfectly fine in my application. 

    Thank you for your help. Without your question about my measurement, I would not have swapped my cables. They are going in the trash.

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