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A friend and I are working on building a smartwatch on a budget, so it’s a (very) low power project, but we are currently having a problem with the power supplying. We already chose this board, because of its size and the BLE chip: RFduino BLE SMT For now on, we are planning to use this battery 500 mAh LiPo battery (approx. 4V at full charge) with this voltage regulator Micrel MIC5209-3.6YS (the board supports 3.6V max.).

But the problem is that the Voltage Regulator will apparently drain by itself too much energy from the battery (by heating), reducing drastically the autonomy.

What (type of) regulator would be the best or do we need to find another battery?

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The battery you have chosen is a typical Lithium Poly with a maximum voltage of 4.2v, dropping to 3.7v according to the description linked to in the OP; however the datasheet says it can be discharged to 2.75v, which is more typical of these batteries..

The RFduino can operate with voltages from 1.9v to 3.6v, with a typical voltage of 3.0v. (It's too bad it can't operate up to 4.2v like a lot of cell modems do, then it could be connected directly to the battery.)

However if you put a silicon diode with a typical forward voltage drop (Vf) of around 0.7v in series with the battery, this will drop the battery voltage range to 3.5v down to 2.0v, which is just right for the RFduino. And diodes are much much cheaper than regulators.

The venerable 1N4148 has a forward drop Vf of 0.6v at 1 ma, and 0.8v at 20 ma.

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  • \$\begingroup\$ While being a little "hacky" in my opinion, this approach drastically reduces part count as well as price and I think the power loss over the diode is acceptable. \$\endgroup\$ – arne Apr 25 '14 at 9:09
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Your supply voltage and operating voltage are so close to each other (within 10%) that it isn't massively worth switching (no pun intended) to a switching regulator, especially not with such a small load. Use a low-power LDO linear regulator such as the LP2985-3.6. Whack it into your circuit, add a few caps, and you're done.

OTOH, if you can get away with a much lower operating voltage then it may be worth investigating switching regulators. But only you can make that call.

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  • \$\begingroup\$ Hi, Thank you for your fast answer. Indeed the voltages are really close. But I just thought : we can't use a linear regulator because it won't let the current pass through below 3.6V, right ? So yeah, maybe a switching regulator would be better. Perhaps you know a reference of one that would fit our needs ? \$\endgroup\$ – Black3v3r Apr 24 '14 at 17:38
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    \$\begingroup\$ A linear regulator's output will decrease gradually if the supply voltage drops below the minimum input for regulation rather than just cut out; you should be able to ride the LP2985 all the way down to the recharge point of the battery. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 24 '14 at 17:54

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