Buck regulator PCB layout help

Discussion in 'General Electronics Chat' started by tom66, Oct 21, 2010.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I've been working on a 3.3V buck regulator for my Super OSD project. It needs to convert an input from 4.8V to 20V to 3.3V at up to 400mA continuous. As size is an issue I settled on the LM2734Z in a TSOT-23-6 package. It operates at 3 MHz meaning a smaller inductor (only 3.3µH) can be used. However, operating at 3 MHz creates many more problems for the layout - the traces to the inductor and catch diode must be very short and the small SOT-23-6 package means thermal dissipation can be an issue.

    So I gave it my best, and with some help from other sites and application notes, I have come out with this:

    [​IMG]

    There are vias under the LM2734Z to help cool it (these go to the ground plane and are also interconnected to the ground of the chip and the grounds of the cap and diode), along with the thick traces coming out of it, to draw heat out. It's a tiny package and the chip could overheat without this.

    • CONN1 is a power connector: pin #1 is 3.3V output, pin #2 is not connected, pin #3 is input, pin #4 is ground.
    • D1 and D2 are RB160M Schottky diodes rated at 1A. D1 is the input protection diode (protecting against a reverse battery) and D2 is the catch diode.
    • D3 is a small signal dual diode - BAV99W - in a SOT-23 package (one diode unused.)
    • C1 is a 10µF 25V ceramic capacitor - the input cap.
    • C2 is a 10nF ceramic capacitor - the boost cap.
    • C3 is a 22µF 6.3V ceramic capacitor - the output cap.
    • R1 is 10k, R2 is 31.6k.
    • R3, LED1 form the power indicator.
    • L1 is 3.3µH.
    • Z1 is ESD protection (a TVS.)
    • PTC1 is a resettable fuse protecting the output. The idea is there can be an external short on the output and the 3.3V supply remains for the rest of the board; it also complements the over-current limit of the LM2734.
    Many thanks!
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Do you have a schematic to go with that?
     
  3. tom66

    Thread Starter Senior Member

    May 9, 2009
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    See attached screenshot.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,764
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    A common trick for ECL (which run very hot indeed) is to glue a heat sink on top of the chip. They used to be pretty common. If I can find some references I'll post them on this post.

    http://www.aavidthermalloy.com/products/heatsinks.shtml

    http://www.google.com/search?q=dip+...resnum=11&ved=0CGQQrQQwCg&fp=9a951961ff9ba31d

    http://www.allelectronics.com/make-a-store/item/HS-332/HEATSINK_FOR_DIP_DEVICES/-/1.html

    The allelectronics is closest to what I was thinking of, they also have styles where the fins run laterally down the chip.
     
  5. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    I don't know how you are soldering it - if you are using the oven method ignore this.
    Most of the components have to be very close, but if you are hand soldering it makes thing a lot easier if you can get a bit more space in between the non critical ones. You could also put some on the reverse side.
     
  6. tom66

    Thread Starter Senior Member

    May 9, 2009
    2,613
    213
    I'm going to be doing reflow soldering, but some of the bigger components (e.g. inductor, 1206 input cap, power header) I may add myself... it's still undecided.
     
    Last edited: Oct 22, 2010
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