Frequently I run into situations where I need to power a 3.3V circuits from a single cell or from a pair of rechargeables that might put out only 2V. This requires a boost circuit to deliver a constant 3.3 volts to the PIC system and peripherals. I searched for circuits that would have a low parts count, low cost, small real-estate, be efficient and would still operate at less than 1 volt so I could run from one cell if I wanted to. My power budget is usually less than 100 milliamps but, depending on the project, I might need as much as 200 milliamps (mA).
I’ve updated the prices and the NCP parts are now available in single quantities from Mouser Electronics and Digikey.
One of the reasons for this exploration is that sometimes I only have enough room in my case for a single AA or AAA cell. Another reason is that I like to use rechargeable batteries such as the Sanyo Eneloop and Rayovac Hybrid, which are low self discharge (LSD) NiMH batteries. I tested them in a TV remote control and they retained a charge for over two years. The downside to these cells is the voltage is only 1.35V per cell when freshly charged and the capacity is roughly 2 Ahrs per cell.
There are many chips available for this task. I’m sure I didn’t find all of them and the others I did find I dismissed as either too expensive, too complicated to use, or both. Some were only available in a format I couldn’t use. Here are the candidates for my projects:
ON Semiconductor NCP1400 and NCP1402
My initial search for a boost converter yielded a breakout board from Sparkfun. This includes .1″ headers which made it easy for breadboarding. This board consisted of the NCP1400 (100mA version) chip, two capacitors, a Schottkey diode and an inductor. ON Semiconductor also makes a 200mA version, the NCP1402. I did some testing and the breakout board works well. The price from Sparkfun is $5.95. I was all set to design with this and discovered that I could only get sample quantities directly from ON Semiconductor. Mouser and Digikey do not stock this part. I did order 5 samples. They were free but the shipping was $8.00. At the time of this writing (June 2010) there is a 37 week lead time on the 3000 quantity reels!
Texas Instruments TPS61097
Having found the NCP1400 component, I now had sufficient descriptive information to search for other parts on the Mouser and Digikey websites. Mouser is my preferred supplier because ground shipping is only one day away, which is great for prototyping. My search yielded the Texas Instruments TPS61097. This handles 100mA and requires only two capacitors and one inductor. The downside is that the part costs $2.25 in single quantities and $1.80 each for 25.
Further searching found the Semtech SC120 component. This supports 200mA and is available from Digikey at $.70 for single quantities and $.47 each for 25. This also requires two capacitors and one inductor.
I followed the specifications in the data sheets for all three components and acquired the recommended capacitors, inductors and diodes. I then assembled my own booster circuits so I could test them. The pictures below show the four circuits I tested:
From left to right is the Sparkfun NCP1400 breakout, the TPS61097, the NCP1402 and the SC120. The largest package for these is SOT23-5 and SOT23-6 so it’s a lot of fun assembling these. The picture at the right shows one of these powering a prototype of an environmental logging application.
The following table shows the supporting components and costs. All part numbers are for Mouser Electronics, except for the SC120 which comes from Digikey. The prices show the single quantity price followed by a quantity price each per minimum quantity. For the capacitors I selected the cheapest of the type (tantlum or ceramic) in the required size. Please refer to the data sheets in the reference section below for connection diagrams and specifications.
|TPS61097||10uf Tantalum||10uf Tantalum||10uH Inductor|
|2.59 2.02/25||.25||.25||.45 .35/50|
|NCP1402||Schottkey Diode||10uf Tantalum||68uf Tantalum||47uH Inductor|
|NCP1402||MBR0540 (on hand)||generic||generic||652-SDR0604-470KL|
|.80 .62/10||.36 .10/100||.25||.65 .26/100||.43 .32/10|
|SC120||22uf Ceramic||22uf Ceramic||4.7uH Inductor|
|1.20 .74/100||.25 .22/100||.25 .22/100||.50 .35/10|
The table below compares the one-off cost of each, as well as some other factors. I did some simple efficiency testing using two Eneloop cells and some white LEDs. Comparing watts in and watts out, efficiency was 84%, 76% and 80% respectively. The Sparkfun breakout also had an efficiency of 76%. All versions were able to supply at least 50mA from a single cell. In summary we have:
Ultimately, I’ll be using the TPS61097. While it is more expensive by itself, I order parts from Mouser electronics regularly so it gets (practically) free shipping. I rarely order from Digikey so I have to add the price of shipping when I purchase the SC120 from Digikey. Naturally, I’ll have to revisit my choice if I need more than 100mA capacity.