Motivation

The Arduino single-board microcontroller (https://www.arduino.cc)  and its spinoffs have opened up a world of opportunities for makers, educators, and researchers. Intended for classroom and lab prototypes, these boards are generally not designed to be used in a remote setting. So the powering options are limited to a USB cable or a 7-12V wall adapter.

The Adafruit Feather family (https://learn.adafruit.com/category/feather) partly remedies this by offering a JST power supply connector for a lithium ion polymer (LiPo) battery (https://www.adafruit.com/product/1570), and a built-in recharger . One downside, however, is the lack of support for standard disposable batteries such as AA, C, D, or 9V alkalines. There are several reasons one would want to use these batteries. LiPo batteries have known safety issues (https://en.wikipedia.org/wiki/Lithium-ion_battery#Safety) as well as limitations on their shipment and transport; in a field setting such as environmental monitoring, it can be more convenient to carry replacement batteries out to the installation than to recharge LiPo batteries; and disposable batteries have a lower initial cost than LiPo.

The present design is a board in the standard FeatherWing form factor (https://learn.adafruit.com/adafruit-feather/feather-specification) that is intended to fill this gap. This board has its own 2.1-mm barrel connector (compatible with many battery holders) that accepts a dc power input between 2V (3V to start up) and 15V. The buck-boost converter efficiently converts this input to a 3.3-V level that is output to the 3.3V pin of the Feather stack. As this is a switching converter, it will in most cases be more efficient than the standard LDO that is supplied with the Feather, resulting in longer battery life and more complete use of the battery capacity.

Design Details

Summary of features: 

• Texas Instruments TPS63070 buck-boost switching converter.
• Adafruit Feather board size (0.9 in x 2.0 in) and pinout.
• 2.1-mm barrel connector for supply input.
• Input voltage range: 2V – 15V (3V required to start).
• Output current capability: TBD but at least 500mA.
• Power-save mode for low-load efficiency (default).
• Jumper option to force PWM mode for high-load efficiency.
• Slide switch to select switcher or LDO operation; unused regulator disabled.
• Reset pin.
• Battery monitor pin (optional).  
• JST-PH connector as alternative

 The TPS63070 switching converter can start operation at any input voltage from 3V to 15V; once started, the TPS63070 will continue to run down to 2V if the supply is not interrupted. In this way, a battery can be almost completely drained before discarding. 

The TPS63070 is specified to output up to 2A; due to the current capability of the pins, however, no more than 500mA is recommended. The typical user will need much less than this.

This board shares the Feather pinout and, depending on the headers installed, can be stacked above or below the Feather board and any optional wings. It should not interfere with the function of any of the digital or analog pins.

 The 3.3-V output from the converter is fed to the 3.3V pin to supply the Feather and any Wings in the stack. Due to the design of the Feather, this pin is also powered by the LDO regulator on the main board; in order to avoid having competing current sources, a SPDT slide switch is provided, by which the user selects which source is to supply the 3.3V. The unused source is disabled at the same time.

Low Power Mode: the converter is designed to be highly efficient, but in its standard PWM mode, the efficiency is optimized for a high-current regime. However, there is a power-saving mode that in enabled with a pin; this is the default of this design, but it can be overridden (PWM mode selected) by soldering across jumper pads JP4.

A resistor divider is provided for monitoring the input voltage. This is not connected by default, but soldering across jumper pads JP1 connects the monitor to Pin A2. A resistor population option (see schematic) allows the monitor to be ranged for either the 1-V or 3.3-V ADC reference of the Feather’s SAMD21 (or similar) processor.  

A push-button momentary switch is provided as an auxilliary RESET button, in case this board is on top of the stack. 

A note on the development process: board was developed using Eagle for design capture and layout. Bare PCBs were fabricated by OSHPark. I assembled them myself  with solder paste and a toaster oven for reflow for the SMT parts;  TH parts (slide switch, barrel connector, and headers) I hand-soldered. 

Test Results

Coming soon. Initial tests show typical efficiency between 80-90% between 15 and 50mA load.  

Next Steps

This board is a proof of concept. I will be studying the options for the next revision, which will be the one I plan to manufacture. Likely changes I am considering include:

• Replace slide switch with logic to automatically select source of 3.3-V supply.
• Remove JST connector for cost and space reasons.
• The next version may be two separate designs, with either a buck or a boost converter. The desire to cover a wide range with one buck-boost converter entails compromises in efficiency and cost.
• I have had problems reliably reflowing the TPS63070 footprint. I would like to select a regulator IC in a package that is easier to mount.
• A regulator with a lower switching speed should provide improved efficiency in the low-current regime.
• Remove excess output filtering, which makes routing difficult and reduces efficiency. The design was originally developed for an analog application that requires very low noise.