A good power supply project



  • I set the objectives for a versatile 3.3V_output power supply for Omega 2+

    • low cost
    • can supply 3.3v (under heavy load: lowest 3.2v) output voltage at max 550mA (250 mA for the Omage 2+ itself and 300 mA extra for other peripheral circuits in the future)
    • input power supply can go as high as 5.5v and as low as 3.20v (when Omega 2+ is the only load) or 3.55v (when additional 300 mA load is added in)

    Observations:

    • by using a TD6817 based low-drop out switching IC, under 600 mA total load, the drop-out is less than 300mV and under 250 mA load (it is Omega 2+ by itself) the drop-out voltage is less than 70mV
    • naturally the "original power feed" into this voltage regulator must have low enough internal impedance and be able to handle the transient load demand (e.g., a 5V 2A power supply with large enough output filter capacitor, say 1000 uFD)
    • due to a low-cost CD54 4.7uH (1.2A current handling) inductor (and probably not designed for 1.5MHz operation) is used, the swtiching power efficiency is somewhere in between 70% to 90% (input voltage and output load dependent)
    • not applicable for Omega 2 use case (won't work reliably when its V_in is lower than 3.2v) but worthwhile to note that I picked TD6817 is it can still function when its voltage_in voltage is less than the preset voltage-out voltage. (I.e., v_out will not be cut off, just some drop-out voltage)
    • with the wide input range of such design, a Li-Ion cell can also be used to supply this unit (although I highly recommend a proper charger with protection circuit to be used, however it is out of scope for this project)

    Abbreviated design description:

    • I used the standard design in page 3 of TD6817 datasheet) <-- TD6817 100 piece price is less than $15, shipped (it is a SOT23-5 package)
    • 4.7uH inductor (CD54 size) <-- 50 piece price is less than $3, shipped
    • I bought SOT23-10 (I only use 5 pins out of the 2-by-5 layout) to 0.1" pitch blank adapter PC board as the base for this power supply <-- 100 piece price is less than $4, shipped
    • from my spare parts cabinet, I picked 1x low-ESR 47uF 16v cap for input, 1x low-ESR 47uF 16v for output capacitor, 1x 22pF cap, 150K 1/8W for R2 and 680K 1/8W for R1.

    ===

    • on the Omage 2+ side, I soldered a 330uF 10v low-ESR cap to its power supply pins
    • I used 20 AWG power cable (a pair of black-red wires) no more than 10cm long to connect the Omega 2+ with the TD6817 voltage regulator board

    Hope this info is useful for some design ideas.

    ccs_hello



  • Assembled pictures:
    PCB size: 12mm x 10mm

    0_1490226527347_IMG_0992.JPG 0_1490226539234_IMG_0994.JPG



  • Omega2+ side:

    Gnd and 3v3 pins are connected thru wire-wrap IC socket pins.
    330uFD 10V capacitor is used.
    0_1490227122963_IMG_0995.JPG
    0_1490227149122_IMG_0996.JPG



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