LMR16020: Higher efficiency at lower switch frequency

Christian Juhl

Part Number: LMR16020

Hello

I have implemented this IC for PoE application, and after playing around with some RT/SYNC resister values, I noticed a significant difference in the efficiency with a lower switch frequency. How come?
(I do already have a PCB with everything setup)
VIN = 48V, Vout=12V

The schematic:

10 months ago

0 Christian Juhl 10 months ago

Prodigy 20 points

I did notice a slight overshoot at the switch pin on the ic:

0 Andrew Kutzler 10 months ago in reply to Christian Juhl

TI__Genius 15380 points

Hi Christian,

It's likely that the inductor may be saturating, Have you tried using a larger inductor at the lower switching frequencies?

Thanks,
Andrew

0 Christian Juhl 10 months ago in reply to Andrew Kutzler

Prodigy 20 points

Hey Andrew
No, I havent tried that yet, but I would really like to.
What sizes do you have in mind? 22uH and 33uH?

0 Andrew Kutzler 10 months ago in reply to Christian Juhl

TI__Genius 15380 points

Hi Juhl,

Yeah. Something like XAL5050-223MEa or XAL6060-333ME should be ok for your application.

Thanks,

Andrew

0 Christian Juhl 10 months ago in reply to Andrew Kutzler

Prodigy 20 points

Hi Andrew
Thanks, I've ordered theese.
In the meantime, would you suggest changing the switch frequency back to approx 1MHz? Or rather keep it the 400KHz region?

0 Andrew Kutzler 10 months ago in reply to Christian Juhl

TI__Genius 15380 points

Hi Christian,

Yeah you can increase the frequency for now. You should be able to any frequency with the inductors I selected though.

Thanks,

Andrew

0 Christian Juhl 10 months ago in reply to Andrew Kutzler

Prodigy 20 points

Hi Andrew
Ok, I'll test with different frequencies once i have the inductors.

0 Christian Juhl 10 months ago in reply to Andrew Kutzler

Prodigy 20 points

Ok so recieved new inductors, but this was not the root cause of the issue. I chose another diode with lower forward voltage drop, and this made efficiency higher, and also improved the overall current limit.

Changing from 15uH to 22uH or 33uH with similar sat. current specs, made a very little difference, I would say did not matter that much.

Now able to push 2A out, while maintaining 82% efficiency.

Thank you for help and reply

0 Andrew Kutzler 10 months ago in reply to Christian Juhl

TI__Genius 15380 points

Hi Christian,

Got it yeah. The EVM uses a diode with a voltage drop of 0.5V~0.7V. I'll keep that mind if I hear about this device again.

Glad you figured out the issue.

Thanks,
Andrew

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LMR16020: Higher efficiency at lower switch frequency