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LM2587 adding noise to input supply

Other Parts Discussed in Thread: LM2587

In my LM2587 design, I'm boosting 5V to 20V.  The output is good, but as soon as I start drawing current noise appears on the 5V rail (see scope screen cap below; this is actually an output from my CPU, but the rail looks the same).  I've also included my schematic and layout.  Any tips on filtering this?  Thanks.

lm2587.pdf

  • This can be improved by better PCB layout. Layout plays an importance role in switching power supplies. In boost converter, the critical components for switching noise reduction is the power diode, Cout and the power FET inside the IC between SW to GND pins. The key is to minimize the loop area covered by Diode to Cout to GND pin to SW pin and back to diode. Please place the diode as close as possible to the IC. Add a 1uF ceramic capacitor from VOUT to GND to bypass high frequency noise. Place the ceramic capacitor close to the GND pin and the diode. The large Cout capacitor should be placed close to the IC as well.

    Also when the IC is switching, switching noise could couple in the scope ground loop as well when probing Vin. Please minimize the ground loop on your scope as well.

    Hope this helps.

    Yang

  • Hello Karl,

    In addition to what Yang mentioned, here are some additional notes on ripple measurement techniques. This is written for Buck converter, but the measurement concepts are the same. It is easy to pick up extra switching spikes with bad probe grounding. See how much spiking you really have and add high frequency caps to filter the spike/ring frequency.

    Cheers, 

    Denislav

  • Thanks Yang and Denislav. I'll try your suggestions.
  • Soldering a short ground wire to the board and then wrapping it around the bare scope probe reduced the measured noise to acceptable levels. I'll see what adding a 1 uF cap to the output does when I get my hands on one.