Switching Regulator Step-up DC-DC Converters problem

Discussion in 'Embedded Systems and Microcontrollers' started by Matt Waldner, Sep 6, 2010.

  1. Matt Waldner

    Thread Starter New Member

    Sep 6, 2010
    Switching Regulator Step-up DC-DC Converters problem

    I have been working on a PIC18F microcontroller project for a long time now and I’m at the point where I have to get a working (efficient) power supply for it. This is a hand held testing device used for the industry I work in. I started off using a 9v battery with a L7805 voltage regulator. Real quick I learned how inefficient this combination is. The microcontroller never turns off just goes into sleep/doormat state when it’s not being used.
    I decided to go with a pair of AA batteries and a MAX756 for its vastly superior efficiency. I built the circuit using the Typical Operating Circuit from the data sheet. The fist time I used a solderless bread board just to see how it worked and it worked great for the most part. The efficiency was well within my designee envelope.
    The problem that I’m having now it with a .1v ripple in both the output from the battery and the supply to the system. This is causing erratic readings in my analog to digital converters in the IC.
    I then transitioned to a SMD board to try to solve the problem and no luck. I have tried larger and larger filtering capacitors to solve it and found no help until I got in the 2000uf range. This is a bit on the large side for the hand held project I’m working on. With a very light load the problem goes away.
    I’m sure that my lack of understanding switching power is biting me.
    Any help or suggestions would be greatly appreciated.

    Input is 2xAA batteries
    Output is 5v
    Max current draw is 120mA

    Parts for power supply
    Inductor 22μH (PO770.223T)
    Capacitors 2x 100μF (595D107x9016D2T)
    Diode Schottky (1N5817)
  2. Ghar

    Active Member

    Mar 8, 2010
    So, when you first built it did this problem not exist?
    Have you tried fresh batteries?
    Having ripple at the battery terminals would indicate the battery's impedance is fairly high.
  3. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    A switching supply has a minimum current output that it can support.

    You confirmed that this is the problem when you said that with even a light load the oscillations go away.

    Try putting a low current super mini LED with resistor to limit it to about 500μA current across the power supply output as a power on light.

    It will hurt your battery life, but AA batteries are cheap.
  4. eblc1388

    Senior Member

    Nov 28, 2008
    2xAA battery plus a 2,000uF capacitor plus a DC-DC converter.

    Can't you consider just using 3xAA to power the uC instead?

    Are there other limitations you haven't mentioned yet?
  5. Wendy


    Mar 24, 2008
    Also, don't just use capacitors. Look up a PI filter, which is a combination of coils and caps that is a bit more efficient on the filtering front.
  6. Matt Waldner

    Thread Starter New Member

    Sep 6, 2010
    Thanks everyone for your quick response.

    Ghar – Yes I had the same problem with the solder less breadboard as with the SMD Board. I have tried sourcing it with my bench top adjustable voltage power supply as well. Same problem. I tried new batteries a will back but I have made some changes since then. I will try it again but they appear to be good still.

    Potato Pudding – The problem goes away if there is no load or a very light load.

    eblc1388 – I guess I could try 3-AA batteries but I’m running out of space as it is. I would still need some sort of voltage regulator. That would be a big drain on the battery.

    Bill_Marsden – I will look into a PI filter. I think I read about that someplace. Thanks for the pointer.

    The 2000uF capacitors were more of a hell Marie long shot just to see if it worked. I didn’t really intend for that to stay in the design. I would like to find the cause of the problems not just patch the symptoms.
    According to the data sheet for the Max756 what I’m doing is well within the design specifications for the chip.
    The other big question is will this problem go away when I make the entire system into one board. Now only the power supply and some of the sub systems are on solder boards. A lot of it is still in a solder less breadboard. I don’t want to waist a bunch of boards trying different things to solve the problem if I don’t have to.
    Thanks again for your help. I really appreciate it.
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    Sorry for the misunderstanding about a light load making the ripple go away.

    A PI filter should work.

    Use a small capacitor on the output of the switcher something between 1μF to 3.3μF, follow that with a choke, and then put your large filter cap.

    Some oscillation can be normal. If you use a small capacitor after the switcher it exaggerates the oscillation and should correct better. With a large capacitor right behind the switcher the switcher will have less control. It takes longer for it to raise and lower the voltage.

    The final output capacitor should be larger as it is important to load regulation. Be careful of the output capacitors resonant balance with the choke as it is possible to have oscillations beyond the switchers notice caused by heavy pulsed cycle loads.
  8. Matt Waldner

    Thread Starter New Member

    Sep 6, 2010
    Sorry for taking so long to get back to this post.
    Thanks again for all the help. The PI filter worked wonderfully.
    I ended up having a hardware and software problem but after implementing the PI filter the software problems became obvious.
    Whenever I have a problem that just seems to be impossible to solve I generally find that I have more then one problem.