Bus Pirate v4 oscillates too. Tantalum cap to the rescue!

This week I finally received my Bus Pirate v4 from Seeed Studio.

The first thing I did after opening the box was to probe the 3.3V rail with the BP internal power supply enabled.

Here’s what I saw:

Bus Pirate v4 3.3V rail oscillation

That looks familiar!  Except this time it’s an oscillation at 5.9kHz and 400mVpp!  (almost 4 times what I saw on the SFE Bus Pirate).

I replaced C11 with a 4.7uF tantalum surface mount cap that I scavenged from another board, and the oscillation went away completely:

Bus Pirate v4 - C11 = 4.7uF tant


Ahh, that’s better!

Again, I strongly suspect the low ESR of the ceramic cap is to blame.  The 3.3V regulator really needs to see the higher ESR of a tantalum or electrolytic cap for stability.

My advice to all Bus Pirate owners is to check your 3.3V rail with a scope, if possible, and replace C11 with a tantalum or electrolytic cap.  Alternatively you may be able to add a small amount of resistance in series with the ceramic cap that comes with the Bus Pirate, but I haven’t verified this.


2 thoughts on “Bus Pirate v4 oscillates too. Tantalum cap to the rescue!”

  1. MIC5205 datasheet says:

    The output capacitor should have an ESR (effective
    series resistance) of about 5Ω or less and a resonant
    frequency above 1MHz. Ultra-low-ESR capacitors can
    cause a low amplitude oscillation on the output and/or
    underdamped transient response. Most tantalum or
    aluminum electrolytic capacitors are adequate; film types
    will work, but are more expensive. Since many aluminum
    electrolytics have electrolytes that freeze at about –
    30°C, solid tantalums are recommended for operation
    below –25°C.

  2. I’ve checked my Logic sniffer from DP – 3v3 rail is stable inspite of single ceramic cap at its output, but 2v5 rail is oscillating 200mV peak-to-peak and 1v2 rail at 30mV peak-to-peak.

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