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)
- 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.
PCB size: 12mm x 10mm
Gnd and 3v3 pins are connected thru wire-wrap IC socket pins.
330uFD 10V capacitor is used.
What is the SMD marking code of the TD6817 chip please?
And of the TD6811 chip (the PSU of the Expansion Dock)?
Thank you in advance for your answer,
I would like to power the omega2 directly without a board. I understand it needs 3.3v input, of which there are no standard ones to purchase.
Would I just be able to use a standard usb cable (5V) w/ this 3.3v downstep from Banggood (https://goo.gl/zeb2eJ) to directly power the omega2?
Never mind. Answered my own question: https://docs.onion.io/omega2-docs/hardware-prep-no-dock.html
the TD6817 datasheet lists output @ 1.5v or 1.8v so i'm a bit lost in understanding how it can power the Omega2+.
Output Voltage Programming
In the adjustable version, the output voltage is set by a resistive divider according to the following formula:
Vout= 0.6 * ( 1 + R2 / R1 ) = 0.6 * (1 + 680k / 150k) = 3.32V nom
Make sure getting the TD6817 (with the blank suffix)
not part such as TD6817-1.8 (which is a fixed voltage part.)
TD6817 (without suffix) is an adjustable voltage buck regulator which is what we need.
yes, i know how to search. the problem is in the usa there are not any hits that offer this td6817 as you describe. it is 100% full moon tonight so i'm gonna wait a few days and try it again. after 22 years exposure to seach on internet i know when to fold and walk away. it is an interesting build, though. thank you for that!
Try Aliexpress :)
lm3671mf-3.3 (or LM3671 module) is easier to find. Circuit is simpler but it costs more.