<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div>To all, </div><div><br></div><div>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.</div><div><br></div><div>TI was gracious enough to send me some samples of their <a href="http://www.ti.com/product/tps63031">TPS63031 Buck-Boost Switching Power Supply</a> which requires few external components and with their WEBench and Switcher Pro tools, Pete and I were able to create a<a href="https://oshpark.com/shared_projects/WRmW4AQU"> simple board </a>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. </div><div><br></div><div>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. </div><div><br></div><div>Major components:</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- The TPS63031 which should maintain a constant 3.3V output as the batteries drain from 5.5V to 1.8V</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- A data logger </div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Arduino (5V)</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Battery packs of various types up to 5.5V</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Some sort of load (like a 13.2 ohm 1W resistor)</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- A “high-side” current sensor <a href="https://www.adafruit.com/products/904">like this one </a> from Adafruit which has an i2C output</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- A Real Time Clock like the DS1307</div><div><br></div><div>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. </div><div><br></div><div>My proposed approach is:</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Three power rails (Vin for the Power Supply, Vout for the Power Supply and 5V for the Arduino) and a common ground </div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- A “load” resistor connecting the Vout to ground</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- A current / voltage sensor to measure on Vin</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- I would then ask the Arduino to log the following every 30 seconds or so - Vin, Iin and Vout </div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- As long as the supply is delivering the 3.3V output, calculating Iout would be the voltage divided by the fixed load reisitance</div><div><br></div><div>After the batteries die (Vout drops below 3.3V), I could calculate the efficiency as the following:</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Instantaneous Efficiency % = (Vout * Iout) / (Vin * Iin) *100</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- I think I could simply integrate the instantaneous efficiency over the duration of the test to get the overall efficiency.</div><div><br></div><div>Am I thinking of this correctly?</div><div><br></div><div>Thanks, Chip</div><div><br></div></body></html>