Step-Up DC-DC Converter
Step-up converters are invaluable in comparison with the more common (at least in DIY projects) voltage regulators. after a few months of prototyping using the 5V 7805 and 78L05 regulators, it was time to step up to the step-ups.
Initially, I started by using the 7805 or 78L05 to a power a breadboard arduinos with an attached Maxstream XBee Module and i needed to do this for a handful of devices. the 7805 regulator demands a significant minimum input voltage of 7.2V to produce a constant 5V, anywhere below that and the output gets erratic, and useless in this case. So, using 9v batteries meant that I was going through them like a fat kid in a candy store as soon as I added some more power consumption to the devices. And, the strange behavior of a failing microcontrollers gets in the way when you try to figure what is wrong.
The next best solution became the step-up DC-DC converters. Instead of starting with a higher voltage and bringing it down by dissipating heat like the voltage regulators, step-up converters manage to convert a low voltage to a higher one.
I started by experimenting with the Maxim Max1676 Evaluation Kit using 2-AA NiMH rechargeable batteries. The evaluation kit works just like butter. Wire the power and ground of the battery holder to the +VBATT and GND terminal, and you get the voltage you specified with the jumpers out of +VOUT. The chip, Max1676, is built to output 3.3v or 5v, and is also adjustable using a voltage regulator. The evaluation kit comes with a jumper (JU1) so you can set it easily to 3.3v or 5v, or you can leave it open and use the R5 and R6 resistor to select a different voltage. The Max1676 also allows you to select the inductor peak current or 0.5A or 1.0A using the JU2 jumper. And the third jumper (JU3) let you shutdown the chip using an external signal. The chip also has a low-battery indicator which can be set to set off at a specific voltage using another voltage divider. And finally, if you want to use a single-cell battery, you must installed a Schottky diode (D1), which doesn’t come with the kit.
The Max1676 chip also has a brother and sister, the Max1674 and Max1675, which are similar but have a fix limit for the inductor peak current of 0.5v or 1.0v respectively.
Considering the high price of the Max1676 evaluation kit (60$), i started looking at design my own board using similar parts. The parts alone, from Digikey, added up to around 15$, not counting the PCB. I think it would be valuable to have this board designed and accessible at ACE for many reasons, and I intend to design/integrate it in the future, but for the moment a new option came up.
Possible parts to design the Max1676 board (Digikey)
- 47uF 16V Tant Capacitor 20% : 495-1544-1-ND : $0.76/10
- 100uF 10V Tant Capacitor 20% : 478-1779-1-ND : $2.04/10
- 1uF Y5V Ceramic Capacitor : 587-1308-1-ND : $0.088/10
- 0.1uF X7R Ceramic Capacitor : 587-1279-1-ND : $0.089/10
- Diode MBR0520 : MBR0520TPMSCT-ND : $0.175/10
- 22uH Power Inductor 1.1A : 308-1533-1-ND : $1.06/10
- Max167X Series : MAX1674EUA+-ND : MAX1675EUA+-ND : MAX1676EUB+-ND : $5.06/1
Bodhilabs have a series of VPack (10$) that produces a constant voltage similar to the Max167X series. They have the 3.3V, 5.0V, single/double AA and AAA available. It uses the LM2623 which can produce a wide range of voltage based on the voltage divider present on the board.
For example, to modify a 5v VPack to output 3.3V, you need to change the RF2 resistor to 182K.
The LM2623 is significantly cheaper than the Max167X series so it might be better to look into designing a board similar to the VPack instead.
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