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I have a small board with ATtiny85 and it has a button cell battery socket on it. The battery that I'm using is CR2032 rated 3V. At the same time, while programming it there will be second power source from the USB programmer - that is usually 5V or 3.3V depending on the programmer.

In all cases the buttons cell battery will be connected in parallel with another power source of a higher voltage and that could cause problems.

I'm trying to avoid putting additional switch/jumper to have battery on/off while programming the micro-controller.

Q1: Is the assumption, especially in that particular case, that connecting battery and higher voltage in parallel could case problems correct?

One way to solve that issues is probably to put a diode in series to the battery - schottky will be better, but still there will be some drop on the diode and that could be critical for the ATtiny.

Q2: Are there any, other than using schottky diode, solutions for that issues that do not require too many additional components?

The power consumption of the board is about 15-20 mA.

Please note that the board is very inexpensive and the additional protection circuit that I'm looking for should be with inexpensive components as well.

Thanks in advance.

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A germanium diode will only produce a 0.2V forward voltage drop compared to a silicon diode with a 0.6~0.7V forward voltage drop. This can be a simple single part solution with a minimal change in operating voltage.

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  • \$\begingroup\$ Are there any advantages of using germanium diode over schottky? \$\endgroup\$ – Neven Boyanov Jun 2 '14 at 11:56
  • \$\begingroup\$ @neven I'm not too familiar with how a schottky works to say. The advantage of a germanium is mostly the lower forward voltage drop compared to a silicon diode. Both germanium and silicon work well to block reverse voltage, and when a higher voltage is on the line, the diode won't forward bias. \$\endgroup\$ – Passerby Jun 2 '14 at 16:06
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Yes, there are ICs designed specifically for this purpose, search for "ideal diode". They generally contain voltage-comparator logic that drives MOSFETs that do the power switching with low overhead.

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