78xx Buck Converter Project

Discussion in 'General Electronics Chat' started by tom66, Aug 18, 2011.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    The 78xx is a nice voltage regulator. It's simple, it's easy to implement, and it's small.

    HOWEVER, it's a linear regulator. A linear regulator reduces a voltage by dissipating it as heat. Power dissipated ≈ (Vin - Vout) × Iload. This means for high voltage inputs, and low output voltages, a heatsink is almost always necessary for any useful current.

    The board I have designed is exactly the same size as a 78xx in TO220, but does not require a heatsink. It is a switch mode buck converter. It's 0.415" x 0.615", excluding the pin connectors.

    It's based around the MCP16301 buck converter, and accepts from 4.5V - 30V or 6V - 30V and gives 3.3V or 5V out at 600mA (less than the 78xx but still enough for most applications), depending on the resistor configuration. The maximum configurable output voltage is about 5.5V and the minimum is 3V, due to how the integrated boost circuit works, but a few design modifications could be used for up to 15V outputs or down to 2V outputs. It's got better drop-out performance than a 78xx, requiring usually less than 1V to remain in regulation at full load. It is reverse polarity protected and can survive a load dump of up to 40V for a few hundred milliseconds, and is suitable for hot-plugged applications. It has integrated ceramic capacitors: 10u on the input, and 44u (22u x 2) on the output, requiring no external capacitance. Output ripple is about 50mVp-p under full load; depends on input voltage (higher voltage = more ripple) and output current (higher current = more ripple.) It even has a power LED (optional) which illuminates when the output is working. Typical efficiency is 85%+, up to 95% with low input-output differences and high output current.

    Quiescent current is about 2µA with no load. Operating frequency is 500kHz. Standard regulator features: short circuit / over current protection, under voltage lock out, and over temperature protection are all built into the tiny SOT26 chip. The chip may reach around 70°C at 25°C ambient under full load, and the catch diode may also get hot, but that is not a major issue. Convection could be used to cool them down, and they could be clamped onto a heatsink, but for almost all applications (except high temperature ones) a heatsink is unnecessary.

    It's still a work in progress; I am still waiting for my MCP16301 samples (they are backordered.)
     
  2. nickelflipper

    Active Member

    Jun 2, 2010
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    Great idea! This has to be a new chip?, as it seemed anything associated with MCHP was low voltage battery stuff, with really low Vin max (6-7V??). Are you using a ground flood on the back of the pcb for possible heat sink?
     
  3. nickelflipper

    Active Member

    Jun 2, 2010
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    Nice double post, hadn't done that in a while.

    P.S. Oh, I see the via's and some missing components from the top view, so there must be resistors/cap/diode on back.
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Nice work, Tom!
     
  5. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Yeah, Microchip's CMOS is usually low voltage, but this chip is rated to 30V with 40V transients. LT1933 is an equivalent part for 4x the price. Be careful of applying 40V for more than 10 seconds as I burnt up a demo board doing just this, the 40V is a transient rating only.

    Bottom of board attached, there are lots more components on the bottom.

    Heatsinking the IC isn't really crucial though, as even with no heatsink it will only hit 70°C, and it's rated for a junction temperature of 125°C.

    The PCB is only 2-layer which presents some difficulties in the layout but it worked out okay.
     
  6. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Thanks! :)
     
  7. nickelflipper

    Active Member

    Jun 2, 2010
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    Thanks for the peek, keep up the good work. Wow, are some of those 0402 pads?
     
  8. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Smallest is 0603, for the resistors.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Why does it need so many components?
     
  10. Barnaby Walters

    Member

    Mar 2, 2011
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    This is a wonderful idea — how much do you think these would be to manufacture?

    Thanks,
    Barnaby
     
  11. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Only half the components need to be placed for a functional one without bells and whistles like hot plug protection, reverse polarity protection, power LED or lower output ripple. However, if there's space, why not use it?

    Less than $5 each. The chip is less than $1 and the inductor is half that; those are the most expensive components.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thanks for the clarification. The reason I asked is that there are many SMPS buck ICs that just need the very minimum of parts like an inductor and a couple of caps, maybe a fet and diode too depending on which IC.
     
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