MAX868 issue

Discussion in 'Homework Help' started by atlas513, Nov 12, 2012.

  1. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    Hello everyone,

    I have an issue with the MAX868 regulating charge pump. I am currently using a 5V power supply and wish to convert that to a -10V power supply. I have attached the following schematic that shows how I am currently connecting it. Pin2 is just the output of the chip that should be at -10V but when I connect it together I get around -5V at most instead (using a voltmeter). My question is what am I doing wrong in which I am getting a lower value as an output.

    Let me know if you need more information.

    Thanks.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,993
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    I don't see any obvious problem with the circuit. Did you double-check all connections with an ohmmeter?
     
  3. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    Thank you for the reply.

    I have checked each portion with an ohmmeter and there didnt seemed to be a short anywhere. I kept getting around 2kOhms which seems quite normal for this application. I do have some capacitors (1uF) on my output connected to ground to regulate the voltage but I dont think that would be the issue.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet shows that the output resistor value must be double the input resistor value to double the output voltage.
    Your resistor values are the same so +5V in creates -5V out.

    Every circuit in the datasheet has a 1uF (ceramic) input capacitor to ground. Your circuit is missing this important capacitor.
     
  5. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    Thank you for your reply.

    I have changed the values of R16 to 100k and R17 to 200k (191k) and I have seen no change to the output values. As of your comment on the 1uF I thought I could change that by placing a 100pF on R17 to help balance that but will go ahead and try placing that 1uF cap to ground and see what happens.

    I know I forgot to mention this but everytime I connect this circuit the chip smokes after a couple of minutes.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The chip might smoke when the important input capacitor is missing.
    Add the input capacitor and replace the chip.

    What is its load?
     
  7. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    Currently waiting for replacement chips to come in. Will let you know what I get.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That's an important detail.....:D

    In the IC world it usually happens if the power and ground connections are reversed or some kind of "reverse voltage" is applied to two pins which turns on the substrate diode.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    +1

    It can't work without it because it supplies the current to switch and do the charge pump function.
     
  10. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    Sorry for the late reply but I finally got it working yesterday after a long wait for my parts to arrive. Thanks for your suggestion. I still have some slight concerns though.

    I have been using the surface mount package with an adapter to make it through hole to place it on a breadboard. The issue is that the chip still smoked but only for a short period of time but right after that it stopped and gave me around -10V (expected result). I am thinking it is a connection issue which I am still looking at. The other is that the output voltage gives me 0.038A. Is this too high or too low for this chip. I believe that it is reasonable then again I would like another opinion.

    Let me know if I need to clearify anything.
     
    Last edited: Nov 20, 2012
  11. tshuck

    Well-Known Member

    Oct 18, 2012
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    A smoking chip means your device is not working properly...
     
  12. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    True I will give you that but in this case it is still giving me the correct output. I understand I need to modify it so that it doesnt do so.
     
  13. tshuck

    Well-Known Member

    Oct 18, 2012
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    I once had a SMPS that I didn't spec the inductor properly and it smoked, but still held the desired voltage. When I applied a load, the voltage shot to nothing. It worked... as long as I didn't try to use it :)

    Was it the chip that was smoking? Could it have been something else nearby? Have you tried applying different loads to your current device?
     
  14. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    I do not know if it was the chip persay but it did seem that way when I was running it.

    I did place a resistor in the end to see if the desired output stays the same if that is what you meant. The results were that the current was reduced from 0.038A to 0.0015A with the same voltage.
     
  15. tshuck

    Well-Known Member

    Oct 18, 2012
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    It would seem that there is something wrong with your chip. It has an absolute maximum output current of 35mA, and you are seeing 38mA. (datasheet pg. 2)

    Also, your circuit shouldn't draw less current when given a higher load....
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    I think the iC was zapped the moment you tried it without the important input capacitor.
    Then you used an "adapter" instead of using the recommended pcb design that uses a ground plane for cooling. Yours had nothing for cooling.
     
  17. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    @tshuck
    I could try another chip and see if it gives me a different value then.

    @Audioguru
    I did use the input capacitor (1uF) for this new design though but I needed to add a cap (4.7uF) inbetween R2 and pin 1 (GND) to regulate the output.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    I don't know where you added this capacitor. At the output to ground?

    Please post an accurate schematic of what you have.
     
  19. atlas513

    Thread Starter New Member

    Nov 12, 2012
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    Good point. Here is my schematic so far. I am thinking that the 4.7uF is not necessary but we shall keep it there for now.

    Forgot to change two resisters in the schematic:
    R16 = 200k
    R17 = 100k
     
  20. tshuck

    Well-Known Member

    Oct 18, 2012
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    I'm curious as to why C9 is there.... the charge pump design on pg. 8 shows how your circuit should look. Also, the app notes are quoted as saying the device only needs 4 capacitors, two flying, two suppression. You should not have a capacitor in the feedback path like this....

    The 4.7uF cap should not only be there, you should increase it to 10uF...again, pg. 8
    This capacitor is the output capacitor.. betwen OUT and GND... check your polarity-this could lead to a smoking capacitor easy...

    Basically, you need to build the circuit I keep referencing on page 8, with your FB voltage divider different...

    Edit: I should note that I'm referring to the diagram on the left of the page...
     
    Last edited: Nov 21, 2012
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