What's causing this high frequency ripple on the output of my LM317?

Question

I'm using an LM317T to regulate 5 V to 3.38 V for a microcontroller. R1 is 330 ohms and R2 is 560 ohms. There isn't any input capacitor and the output capacitor is 470 µF 25V, because I had that value in a pack of 50. The microcontroller is drawing at least 50 mA, so I'm fairly sure I'm meeting the minimum load specifications. The regulator is being powered from a computer SMPS, but I had a very similar configuration set up last time running off an SMPS without any problems.

Here's a scope trace of an I/O pin to show you what I mean:

The ripple is about 60 mV_p-p, but as you can see it also happens when the pin is low. I'm worried it might affect the microcontroller itself (for example, make it unstable.)

What I'm more worried about are these large bursts of noise which occur seemingly randomly about 100 times per second:

It turns out this noise is coming from my power line network adapters; it's leaking through from the power line into the computer SMPS and to the 5 V output.

Answer 1

Make sure that there is a small ceramic decoupling capacitor at the supply pin(s) of the microcontroller. Something 100nF - 1uF should be OK. (The 470uF cannot deal very well with HF noise).

How is the grounding on the microcontroller side? Is there a large ground plane?

Is the circuit return earth-referenced? Is the scope connected to the same earth reference?

If you don't have a spring-clip for the ground, and don't mind abusing a probe, you can solder short pieces of solid wire from the supply rail to the probe tip and ground ring (under the probe tip) which will cut down on CM noise pickup dramatically.

In general, most ripple measurements in the switching power world are done with short probes (or direct coax) with 100nF and 10uF ceramic / tantalum caps shunting the probe and the scope set @ 20MHz bandwidth limit. (CM noise <> PARD)

Answer 2

The ripple could come from a number of sources. It's a little hard to tell from the photo, but it looks like the ripple is at 100 kHz. Is there anything in you circuit that operates at that frequency? If anything is getting switched at that frequency it is most likely the source and you will have to find some way to isolate it from the power supply.

But first, check that the oscilloscope ground isn't picking up the noise. It could be picking up the noise it two ways. Make sure it is connected to a sensible reference point so that there is no common impedance that could be coupling the noise. Secondly, remove the ground wire and use a spring ground clip to ensure that no mutual inductance it coupling into the probe.

If there is no 100kHz frequency in your circuit and you have a bench supply, try running the circuit off that to remove any influence of the switch mode supply. You could also double check the bypassing on your LM317. These will often require input decoupling. (The data sheet suggests input decoupling if it is more that 6 inches from the power source)

Answer 3

The 317 has a rather big voltage drop (Vin-Vout). At room temperature, it is at least 1.5 V at light loads (20 mA), increasing to as much as 2.5 V for larger loads (roughly two diode drops). What may happen is that the uC draws a larger current every once in a while, the regulator can't maintain 3.3 V with as little as 5 V at its input and goes out of regulation, causing what looks like noise.

Try using a low dropout-regulator (Vin-Vout < 0.7 V) instead of the 317, or try increasing the input voltage.

Answer 4

I just looked at your oscillograms again, and there is one detail that may hint towards your problem: The ripple seems to be just as bad when the uC's output is low. If it was the 317, I would expect the problem to be worse at a high output voltage, and maybe even almost invisible with an output at the low state. Try tracing back the problem from there, by considering both the VCC and the GND rail.

Answer 5

Noise when the output is low...to me, that implies ground bounce. But nothing in your statement implies anything that could cause that kind of ground bounce.

Are you drawing enough load from the ATX supply? They have minimum load requirements.

Answer 6

The LM1117 instead of the LM317, with some protection diode and improved ripple rejection should do this job. Oh, and don't use an SMPS direcly with the LM317. You can use the SMPS itself to give many more voltages than it has.

Examples:

• +5 v to GND = 5 V
• +12 V to 5 V = 7 V

AND most "GOOD" computer PSUs have a "GOOD, stable" 3.3 V supply for the processor. Afraid of its current? Limit it with a 33 ohm resistor (100 mA maximum) and be happy.