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Good Switcher

Discussion in 'Electronic Basics' started by Richard, Oct 5, 2005.

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  1. Richard

    Richard Guest

    Any recommendations on a good, small switcher to go from 27volts to 5 volts
    @ 100ma.
     
  2. PeteS

    PeteS Guest

    Are you looking for a point of load converter board or a switcher
    device such that you can design your own switcher?

    If you want a POL stick, search google for Point of load converters.

    If designing your own switcher will the input voltage vary
    significantly and will the output current vary significantly?
    (Significant is a ratio of at 2:1 or more).

    If the input voltage varies but the output current does not, choose a
    current mode controller. If the output current varies but the input
    voltage does not, choose a voltage mode converter. If both vary, try a
    voltage mode converter that has feedforward compensation to make the
    device insensitive to input voltage variations (the LTC3703-5 is one,
    although it's rated for higher currents).

    The general rule for the loop compensation is:
    Voltage mode converters are sensitive to Vi/Vo variations (and for a
    fixed output that means just Vin) and insensitive to Io variations.
    Current mode converters are sensitive to effective Ro variations
    (Vo/Io, so for a fixed voltage output current variations) and
    insensitive to input voltage variations.
    Obviously, both are sensitive to Vo/Vi as it sets the main switch duty
    cycle but at your range that's not an issue. (Lower than 10% and higher
    than 90% can limit your choices)

    Cheers

    PeteS
     
  3. Richard

    Richard Guest

    Looking for a way to power a very small pic processor and a couple of optos
    from 18-30 vdc.
    Was looking at a LM2936MP-5.0 but 50 ma may be cutting it close.

    Richard
     
  4. PeteS

    PeteS Guest

    I would suggest one of the wide range of Linear Tech current mode
    controllers. Get one rated at least 36V (a little headroom helps).

    Go to http://www.linear.com/
    select Power Management -> Switching regulators -> Stepdown regulators

    At the next page select (from the drop down box)'High Input Voltage' ,
    then 'View category'

    At the next page, select 'View table'

    You'll see a whole list of devices that would suit your needs for Vin
    and Iout. Monolithic means the switch is internal (so all you need to
    provide is the external schottkys, caps and a small inductor) [I'm not
    trying to teach you basics, just trying to be complete]

    There are a number of devices there that should meet your needs, *and*
    happen to be easily available from your favourite mail order parts
    company.

    Cheers

    PeteS
     
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