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MC34063 gives me ADC noise on AVR! :(

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shadewind:
Okay, here we go again with my MC34063 problems. Now I think I've managed to calculate everything correctly but when using this circuit to power an AVR circuit with an ATMega8 controlling some LEDs using the ADC inputs, I get lots of ADC noise which makes the LEDs flicker. Everything worked perfectly when I was using a linear regulator.

http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF

The one I have used is the one on page 7, bottom right (which isn't complete, I know but I've completed it with the one above).

The values and components are as follows:

Rsc = 0.22 ?
Input capacitor = 100 µF
Feedback resistors = 30 k? and 10 k?
Diode = SR304
External switching transistor = STX826
Timing cap = 680 pF
Inductor = 22 µH
Output cap = 100 µF

I'm feeding it 12V through a wall wart but I've calculated it to work with voltages down to 7 volts as well.

I've tried adding an extra LC-filter (another 22 µH and another 100 µF) but that didn't help. I've decoupled VCC and AVCC, each to ground with a 0.1 µF X7R cap. AREF is also decoupled in the same way. I've tried filtering AVCC through a series inductor as well but that does help. I've tried enabling the brown-out-detector to see if it resets because of crappy power but it runs fine. The only thing I have yet to try is look at the power through an oscilloscope since I don't have one :(

Is it my power circuit which is miscalculated or otherwise crappy? Is this a common problem? It all worked fine when using a linear regulator but I need more power with less heat.

tecman:
The issue of noise can be many faceted.  There is noise straight through the supply, common mode noise, and magnetically coupled noise.  You stated that you added an L-C filter with no effect.  So you now should look at common mode noise.  This can be coupled through the ground leg to the uProcessor.  Make sure that the leads are short and heavy enough for the peak currents.  Remember that the mail output cap on the 34063 sees large current spikes.  The wiring from the diode to the cap is also critical.  These runs should only contain power suppl currents as much as possible.  The output from the power supply should be pulled directly from the output cap, in terms of wiring.  The last item is the inductor.  What type of inductor is it ?  Is it too close to other sensitive wiring ?  Toroids are best for minimal leakage inductance.  Little straight wound chokes (look like resistors with windings on the outside) are the worst. 

One solution is to derive your AD ref from a small linear reg and use the 34063 for everything else.

I have a number of products using 34063s and see AD input noise under 10 mV.  Layout and design is critical.

paul

jahonen:
It shouldn't be a surprise to anyone if 34063 produces noisy output voltage. 34063 is used because it is cheap, absolutely not not state-of-the art switch-mode regulator in noise sense. Biggest problem is subharmonic switching noise (which is for example in audio band) due to its naturally erratic (some call it hysteretic) operating principle.

You can see my measurements of the output (in step-down configuration) in here.

Regards,
Janne

shadewind:
tecman: Well, I'm running everything as short as I can on the breadboard which is not easy. The inductor is a ferrite core radial one.

jahonen: Are saying this is to be expected when using the 34063? Because if I'll have to work really hard to make this work, it's probably better just getting another switcher. It's not like I'll have a fancy PCB with ground planes and stuff, it'll be a quick and dirty protoboard so if it doesn't work on a breadboard, it probably won't work there either. Any recommendations for nice but relatively cheap regulators?

As you've said, I could simply use a 7805 to power the MCU and analog stuff and then use the 34063 to power my LEDs (which is what'll require all ze power).

jahonen:
Good that you mentioned the Breadboard, as it probably is the major contributors of noise in this case. Breadboard is a difficult environment for a switch-mode regulator, since loops and parasitic capacitances tend to be large, whatever the placement. Also, radial inductors tend to have relatively high stray field. I'd change that to something more shielded, but that is again difficult on a breadboard. However, I have many times soldered wires to a SMD-inductor and used it on a non-SMD-way in some occasions :)

What is your regulator configuration, step-down or step-up? What is the input voltage/output voltage and current? For step-downs, my usual choice for casual use has been LM2675, it is very simple to use and it usually works satisfactorily even with non-perfect layouts. For step-ups, LM2588 (5 A switch current) or LM2585 (3 A switch current) might do the job.

If the regulator fails to solve the problem, you might build the regulator on a separate copper clad board (using Jim Williams high-speed style :)), using it as a ground plane, and just connect this "sub-board" to breadboard using wires. This will eliminate ground-induced noise.

Regards,
Janne

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