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LM2675-5.0 Losing Regulation under Medium Load

Jeff Mullen
Prodigy 20 points

Hello there!

I am replacing a 7805 regulator on production device with an LM2675-5.0, to eliminate the heat sink parts required when the product is drawing 300-500mA in short bursts of  perhaps 1 minute, several times per day (typically it is approx 170mA).

My initial layout works, sort of...  Perfect voltage output up to about 220mA, using the recommended components from datasheet. Using slightly higher load, >250mA, causes loss of regulation and voltage drop, which recovers in stages as load is reduced, eventually back to +4.97v at around 80mA. Stable again as load increases until about 250ma.

Critical items used are:
47uH coil, Digikey # ASPI-0804T-470M-TCT-ND
Alum caps, Digikey # 493-3938-1-ND & PCE3806CT-ND
Schotk Diode, Digikey # SK32A-TPCT-ND
CB cap Digikey # 399-1167-1-ND
 
I found that placing a standard scope probe on either of the CB cap pins, that the regulation would work to about 320mA. Using a 100pf cap here did not change anything at all.

 
Is there a 'typical' error with such a circuit that causes this type of loss of regulation under medium (and higher) load? Perhaps a known/common layout error causes this - or a common mistake in component selection/type?? It is on a 1oz copper 2-sided PCB, so no internal ground plane, etc. I have ordered the EVAL module for further tests, but I really don't know what to look for next. Please assist.

Cheers!
  • 0
    TI__Genius 16025 points

    Hi Jeff,

    Very sorry for the extended delay. Let's get right to your circuit. You've done a good job with the schematic and providing the important details, there are a few things to check:

    a) Your CB cap is only rated to 16V, Please increase this to at least 35V preferably 50V.
    b) Please place a .1uF or 1uF ceramic capacitor between Input and GND as close to the IC as possible. This will serve as a high frequency bypass and clean up any noise into the regulator.
    c) Please place the diode close to the IC as well for the same noise benefits.
    d) Your electrolytics look fine.
    e) Make sure you're power supply isn't current limited.
    f) What is your load?

    g) Try increasing your inductor to 100uH if none of the above help.


    Finally, Can we see your layout?

    Let us know how we can help!

    Thanks,
    Anston

  • 0 in reply to Anston Lobo
    TI__Guru 73115 points

    Sounds like layout.  A good layout example can be found using the Webench tool.

  • 0 in reply to Anston Lobo
    Prodigy 20 points

    Thank you *very* much for the suggestions. It was indeed a poor layout. I purchased an LM2675EVAL module and made a layout as tight as the EVAL unit. Just got the respun PCBs back and tested, full +4.94V out of it under 800ma load, looks like we have it solved. Thank you again for reinforcing that the layout is (indeed!) critical. Cheers!