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Low Noise Switching Regulator

Discussion in 'Electronic Design' started by Kunal, Jan 6, 2005.

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  1. Kunal

    Kunal Guest

    I need +11V from a +15V supply using a switching regulator since I dont
    have space for heat sinks for linear ones. The main concern is that I
    am using it for a video board and low noise is very important.
    I have looked at the Linear's LT3439, but they require an external
    transformer which is 2cm on the side (too big) and lots of other
    external components.
    http://www.linear-tech.com/pc/productDetail.do?navId=H0,C1,C1003,C1042,C1034,P2228
    Any suggestions of low noise step down switching voltage regulators
    with low/small external parts?
     
  2. Rob Gaddi

    Rob Gaddi Guest

    You say you don't have room for heat sinks, implying that you're moving
    some hefty current. How much are we talking about?
     
  3. Any switcher that produces very little noise is going to include "lots
    of other
    external components" since these will be needed to filter out the
    noise that is inherent in the operation of a switcher. A simple buck
    regulator will probably need at least two inductors and several
    capacitors in addition to the switching components and control logic.

    What output current do you need?
    Is cost a serious limitation?
     
  4. Kunal

    Kunal Guest

    I should have put in the Current required(sorry). I need >1.5A.
    Cost is not a limitation. I dont mind spending more on the regulator
    now rather than have my video all messed up later on.

    The way I understand it is that the 'low noise' regulators are low
    noise because they smooth out the corners of the switching waveform and
    hence reduce harmonic energy and is not due to external component
    filtering. I have seen other low noise regulators on the linear site
    with low external component count.

    Thanks.
     
  5. Kunal

    Kunal Guest

    1.5A
     
  6. Rounding the corners of the switching waveform reduces the highest
    harmonics that tend to radiate from any antenna. But it also
    decreases the efficiency, making it more likely that you will need
    heat sinks. But no amount of rounding gets rid of the interrupted
    current going into the buck regulator which conducts noise back out
    into the 15 volt supply and ground busses. I prefer to contain the
    radiated noise of fast switching, high efficiency designs with good
    layout and grounding techniques and shielding. Then I address the
    conducted noise with low pass filtering components on both sides of
    the regulator.

    If you pick a candidate design, we can discuss how to apply those
    noise containment techniques on that example.
     
  7. Ken Smith

    Ken Smith Guest

    How about syncing the switcher to the pixel clock in the video board so
    that the noise aliases to DC?
     
  8. Mac

    Mac Guest

    Maybe you could use the switcher to get down to 12.5 Volts and use an LDO
    or an LDO + NPN power transitor for the last 1.5 V?

    Is there any airflow?

    Also, don't forget that switchers also put noise on the power input
    voltage rail. So you will need to filter it, too. This increases the
    board space of a switcher to the point where it may not save space over a
    linear regulator, even for your 6+ Watt drop. Especially if there is
    guaranteed airflow.

    --Mac
     
  9. Nico Coesel

    Nico Coesel Guest

    I would try to go for a linear regulator. A switcher at 80% efficiency
    will dissipate 3.3W. A linear regulator will dissipate 6W. If you use
    3 diodes to drop 1.8V (2.7W), you'll need to dissipate 3.3W in a
    linear regulator.
     
  10. I'm talking to a UK transformer company at the moment, for the LT3439
    since the coiltronics supplier in the UK have a MOQ of 300, and are
    not at all helpful.

    I've been trying to catch up on transformer design (groan)and have
    come across Planar E cores from Ferroxcube, who now own the Philips
    breed. These cores sit in a cut out in the PCB.

    www.ferroxcube.com
    and
    http://www.ferroxcube.com/prod/assets/planar_er.htm
    This may save you some height.


    martin

    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
     
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