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799 Filtering

I’m using the 799 boost regulator to step up ~3.8V to 24V @ 0 - 100mA, and I am noticing a fair bit of noise in the output. I’m wondering if there is a recommended filter. I’m experiencing essentially a triangle wave at ~ 393.5 Hz with P-P amplitude of 456mV. For some regulators, more capacitance on the output is helpful, and for some it causes more problems. I can (and will) experiment, but I was hoping for a little guidance.


What exactly is the regulator driving?

What happens if you add a 470 uF electolytic cap from the output to GND?

I’m really just running a couple 4-20 sensors or accelerometers. Last night I was running with just a 4mA current source attached. I had a couple 33uF ceramic caps directly on the output and a couple more 10 uF sprinkled about. Removing the 22’s did not affect the amplitude, but the frequency shifted up to about 600 Hz. I’m starting to think less C is better, and higher frequency noise is better for me. I can try a 470, but that is not going to be a practical solution. I have also ordered some linear regulators that I could put on the output if I have to.

I’m really just running a couple 4-20 sensors or accelerometers.

Why 24V for sensors? It doesn’t make sense to have a separate regulator power a couple of sensors, especially a noisy switching regulator. Instead, power the sensors from the output of the regulator on the MCU board controlling them.

With a light load (a few mA) an RC low pass decoupling filter will remove as much ripple as you like. Typical values depend on the ripple and load, but might be 47 Ohms followed by 100-220 uF.

In any case, 456 mV ripple is quite excessive. If you actually want to solve this problem, post a schematic diagram of the entire setup.

I think the answer is the 799 doesn’t like much extra capacitance on its output. Small R and C is fine, but too much C seems to make it unstable.