Power supply schematic help

Discussion in 'The Projects Forum' started by Mathews M John, Jul 14, 2015.

  1. Mathews M John

    Thread Starter New Member

    Jul 14, 2015
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    Hi,
    I am pretty new here but have used many of the resources from the website for a couple of my past projects. I am working on a handheld device and want to use +/-3.3V to power some opamps. I have been searching quite a lot to learn how to make a dual power supply. During my search, I came across this schematic. However, I don't understand how the inductors L1 and L2 helps in stepping down the voltage. Can someone help explain this circuit ?
    Converter.jpg
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    U4 is a 12V input, +/-5V output DC to Dc converter smps, which will have high frequency noise on its outputs. L2 and L3 acting with C17 and C18 form a low-pass filter to get rid of the noise.
     
    Last edited: Jul 14, 2015
  3. ScottWang

    Moderator

    Aug 23, 2012
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  4. Mathews M John

    Thread Starter New Member

    Jul 14, 2015
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    As can be seen here , U4 is a 5 to +/-12 V DC/DC converter. What I don't understand is how the inductors and resistors are used to get the voltage down from 12V to 5V
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    No, the input to the brick is 12Vdc. The output from the brick is +/-5Vdc, but noisy. The inductors are there to help filter the noisy 5V.
     
  6. Mathews M John

    Thread Starter New Member

    Jul 14, 2015
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    This is the rating from the data sheet. I know input isn't 12, because J2 draws power from a USB

    Screenshot_2015-07-14-08-28-22s.png
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Ok, I see Scott found the data sheet. The schematic is mislabeled. The output terminals should say +12V and -12V. The inductors are still part of the final filtering to get the smps switching noise off the outputs.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    They don't. The outputs on your schematic are labeled incorrectly, but the title is correct. The outputs are +/-12 V, and that's that. Each inductors is part of an output L-C filter to remove switching noise. The converter, not the filters, is doing the converting.

    ak
     
  9. Mathews M John

    Thread Starter New Member

    Jul 14, 2015
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    Thank you guys. I was trying to understand this circuit for ever. Are R15 and R16 also a part of the filter?
     
  10. crutschow

    Expert

    Mar 14, 2008
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    The spec sheet says the output ripple of that supply is 75mVpp with a fundamental switching frequency of 80kHz.
    The output filter will reduce that noise (likely needed if you are powering sensitive analog circuits).

    I think R15 and R16 are to add damping to the filter to minimize ringing at the LC resonant frequency.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    The picture posted (link) elsewhere doesn't look like a heatsink job, so unlikely a linear - my guess is those inductors are external buck inductors. It may require further filtering.

    Technically; the buck coil and output capacitor are a LPF in their own right - the question is; which are those shown in the application example?
     
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