[TriEmbed] Power Supply Testing - Am I over or under thinking this?

[Charles McClelland]

To all, 

In my never ending pursuit of longer battery life and / or less batteries, I have been exploring moving from a linear DC-DC converter to a switching power converter.

TI was gracious enough to send me some samples of their TPS63031 Buck-Boost Switching Power Supply which requires few external components and with their WEBench and Switcher Pro tools, Pete and I were able to create a simple board to test this chip.  Pete lent his expertise and lessons learned in working with these tiny QFN packages and we were able to build one of these boards over the weekend.  

Now I need to test it.  I wanted to share my approach with this group to make sure I am headed down the right track.  

Major components:
	- The TPS63031 which should maintain a constant 3.3V output as the batteries drain from 5.5V to 1.8V
	- A data logger 
	- Arduino (5V)
	- Battery packs of various types up to 5.5V
	- Some sort of load (like a 13.2 ohm 1W resistor)
	- A “high-side” current sensor like this one  from Adafruit which has an i2C output
	- A Real Time Clock like the DS1307

Here is the under / over thinking part.  In the end, I want to have a rough idea how efficient this device is and whether is can maintain a constant 3.3V voltage and 250mA current while the batteries run down.  

My proposed approach is:
	- Three power rails (Vin for the Power Supply, Vout for the Power Supply and 5V for the Arduino) and a common ground 
	- A “load” resistor connecting the Vout to ground
	- A current / voltage sensor to measure on Vin
	- I would then ask the Arduino to log the following every 30 seconds or so - Vin, Iin and Vout 
	- As long as the supply is delivering the 3.3V output, calculating Iout would be the voltage divided by the fixed load reisitance

After the batteries die (Vout drops below 3.3V), I could calculate the efficiency as the following:

	- Instantaneous Efficiency % = (Vout * Iout) / (Vin * Iin) *100
	- I think I could simply integrate the instantaneous efficiency over the duration of the test to get the overall efficiency.

Am I thinking of this correctly?

Thanks,  Chip

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