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Buck converter getting started ?

Discussion in 'General Electronics Discussion' started by phaseone, Nov 1, 2013.

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  1. phaseone


    Nov 1, 2013
    I need to incorporate an integrated 5V supply on a small run custom PCB project to drive an Arduino and associated peripheral circuit, and not sure which buck converter IC to select for this app. Here's my spec.
    1. Source is a 2s lipol batt - which means the converter should operate in the range of 8.4v down to around a battery discharge of 6.5v
    2. Total current load, including the Arduino, is estimated at 1.5a so anything in the 1.5a - 2a range (or better) will be fine
    3. Space on the PCB is finite so choice of IC packaging and support component space required is not unlimited
    4. ventilation is limited so any significant heat dissipation, if required, has to be factored in.
    5. The converter supply will not break if the peripheral load is accidentally shorted. soft failure.
    6. Through hole packaging OK.
    6. An IC that can be purchased in small quantity. 1-20 pcs.

    I've been looking at the LM2596 as a candidate but not sure if it will work all the way down to 6.5v - and which one of the many flavors offered (just in the 5v range) to pick - and if available in small quantity. If not, perhaps there is a better switching regulator IC for my app someone can recommend.

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I would look on Digikey - they have a very wide range in the section "Integrated Circuits (ICs) > PMIC - Voltage Regulators - DC DC Switching Regulators".

    Unfortunately they don't have a minimum dropout voltage figure in their selection guide so you will have to download the data sheets of anything you like the look of.

    Personally I recommend Linear Technology devices over National Semiconductor (TI) devices for ease of use, based on personal experience.

    You'll probably find the most appropriate devices for this application are the newest devices; these will be in the most modern packages, with the highest frequencies, etc.
  3. phaseone


    Nov 1, 2013
    Thx much. You're right. LT seems to have a second gen approach to their buck converter offerings. Unfortunately all but one, their 8-pin DIP LT1376, is in some form of surface mount packaging so that makes breadboarding these other offerings, some of which look like they might work well in my app, beyond my reach.

    That said, at this point I've narrowed my choices to the LM2596, MC340063, and now the LT1376 - unless someone has a better suggestion. The LT requires more peripheral components than the others but there is one thing in particular with the other designs that concerns me. The Mc34063 requires a very low resistor - point 22 ohms or thereabouts - to charge a cap that determines the converter's freq. Seems to me that any number of things could go wrong in the layout of the PCB to screw up implementation of that very low R value. I'm just surprised that the IC design would call for such a low value. I'm not used to seeing that in a circuit.

    The LT cost substantially more than the others - especially when adding the extra components- but it might be worth it in the long run so I plan to order a few samples for breadboard testing.

    Thanks again for the suggestion.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    The MC34063 is a very old device and uses a Darlington emitter follower which isn't rated for 1.5A. I doubt very much that you can get 5V out of it with only 6.5V input. You would have to use an external PNP or P-channel MOSFET. I've used them before and I don't like them much; there are much better alternatives.

    Yes, Linear Technology products are often quite expensive. I think that's because they put so much effort into the design and the documentation. The LM2596 is a nice chip and I've used them successfully before, but I still recommend Linear Technology if you want a trouble-free solution.

    Edit: Regarding prototyping the SMT packages. Linear Technology probably offer evaluation boards with the device already soldered in. You can remove the external components and replace them with the ones you intend to use.
    Last edited: Nov 3, 2013
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Another option (if you only want a couple) is to ask if anyone can solder them to a breakout board for you.

    For those of us with the right equipment, most SO outline stuff is simple.

    One caveat -- the breakout boards often cost more than the parts! (I'm talking up to a couple of dollars each)
  6. duke37


    Jan 9, 2011
    As I read the data, the 0.22 resistor is there to limit the current when the voltage drop is 300mV. I do not know whether this would affect the frequency or just add a delay.
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