LM3478 output noise with light load

Bruno Andrade

Intellectual 530 points

Other Parts Discussed in Thread: LM3478

Hi,

I have a SEPIC design using LM3478.

Input will vary from 5 to 30v and the output must be 18v. Maximum load will be 500mA.

I've used WebBench from TI.

The circuit:

The output has 18v.

But at low loads, for example 4mA, the output is terribly noisy: CA coupled images:

20mA:

I could not find where is the problem.

Can anyone give me some help?

Thanks!

over 11 years ago

0 Bruno Andrade over 11 years ago

Intellectual 530 points

There's one more thing:

That noise occurs just when the input voltage is beyond ~15v, when it drops below it, the noise almost disappear, around ~10-15 mV

0 Tommy Jewell - WEBENCH Design Center over 11 years ago in reply to Bruno Andrade

TI__Intellectual 2930 points

Hi Bruno,

It sounds like you are hitting the minimum on-time of the part and going into Over Voltage protection (Where the devices keeps switching till FB goes over spec and then turns off till FB goes under spec).

Check out this thread for a little more detail and then let me know if you have more questions:

http://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/p/333595/1165229.aspx#1165229

Best regards,

Tommy

0 Bruno Andrade over 11 years ago in reply to Tommy Jewell - WEBENCH Design Center

Intellectual 530 points

Hi Tommy,

Thanks, it clarified me about the situation.

Just another question, is there a quick calculation I can do to determine the minimum load using 3478 and 3481?

Thanks!

0 Anston Lobo over 11 years ago in reply to Bruno Andrade

TI__Genius 16025 points

Hi Bruno,

Typically anywhere between 10mA to 25mA should suffice and can be easily implemented with a resistor on the output or feedback resistors one decade lower.

Thanks,

Anston

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LM3478 output noise with light load