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SN6501 has ringing on 5V output

Ross Johnson
Prodigy 230 points
Other Parts Discussed in Thread: SN6501, ALLIGATOR

Hello,


I am testing the SN6501 using a SN6501EVM-MULTI board. When I try any configuration with an output of 3V3, everything works as expected.

However, all configurations which output 5V, result in a very high level of ringing/ripple superimposed on the output voltage (whch is otherwise correct). The ringing is in some configurations as high as 2Vpp! The frequency of the 'ripple' varies but is usually around a few MHz. Looking at the board with a near field probe on a spectrum analyzer shows strong frequency spikes at the switching frequency (e.g. 360kHz for 3V3 in) and integer multiples of it all the way up to over 50MHz.

I've tried a variety of modifications such as output capacitors before and/or after the LDO, resistive loads, different power sources (incl. straight batteries). These things have a slight impact on the output, but are not significant with regards to the problem at hand.

Is this to be expected? Is there something wrong with my EVM? Or am I doing something wrong?

/Ross

  • Genius 3295 points
    Without actually seeing any waveforms it's hard to tell what the problem is. I can only make a few suggestions. Look at the noise and determine whether it is ringing or ripple noise. That's important because ripple noise problems and solutions are not the same as for voltage ringing.

    Check your voltage probe. That's probably ok since you are measuring the 3.3V configuration the same way with no problems. Don't use the probe alligator clip when you connect to ground. Use a tip ground or a piece of solder wick wrapped around the ground near the tip and find a ground close to the measurement point. You may just be reading pickup noise. Make sure you have a solid ground too.

    The ripple noise waveform tracks the switching frequency. You say the frequency of the noise is in the MHz, so it's probably voltage ringing. Ringing is usually caused by board or transformer parasitics -like stray capacitance and stray inductance. The MOSFET output capacitance and diode reverse recovery current also contributes to ringing noise.

    Once you are sure the noise measurements are real and not pickup, the check the grounds. Then I would swap out some parts with the good boards. Check and see if you can put the SN6501 on the quiet 3.3 V board into 5 V mode. If you can and the output is clean, you may have a leaky diode, or a high output capacitance MOSFET. Maybe one of the diodes has a higher than average reverse recovery current.

    These are only guesses since I don't know how you are measuring things or what the waveforms look like. Good luck.