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Below is the circuit for backup power from BAT2 when BAT1 is missing. BAT1 in my case is the LD1117 regulator.

schematic

simulate this circuit – Schematic created using CircuitLab

Suppose only two situations are possible: (1) BAT1 and BAT2 both exists, (2) BAT1 missing. Can I exclude D2?

UPD. "BAT1 missing" mean LD1117 still exists in the circuit, but its input voltage missing.

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  • \$\begingroup\$ Will BAT1 actually be removed from the circuit, or just unpowered since its an LD1117? \$\endgroup\$ – Kris Bahnsen Aug 19 '13 at 18:25
  • \$\begingroup\$ @KrisBahnsen: I've updated my question. \$\endgroup\$ – user54579 Aug 19 '13 at 18:29
  • \$\begingroup\$ As well as what wouter said, if the the LM1117 has a voltage setting resistive divider this will draw current from BAT2. || Why do you wish to eliminate D2? Voltage drop or other reason?. If voltage drop is the issue then a MOSFET can be arranged to produce minimal drop when on but still block correctly. \$\endgroup\$ – Russell McMahon Aug 20 '13 at 16:55
  • \$\begingroup\$ @Russell: Yes, diode dropout is 0,6V -- too many for my voltages (3V). Can I use MOSFET with this small voltages? \$\endgroup\$ – user54579 Aug 21 '13 at 1:52
  • \$\begingroup\$ Yes - as long as you can trigger the MOSFET on when you want it to conduct and off when conduction is not wanted then you can use a MOSFET. This is not hard but may need a little thinking to ensure all possible conditions are met. A MOSFET drops Iload x Rdson. Rdson is FET on resistance and can be a few milliOhms if desired. Usually something around 100 milliOhms will be adequate and cheap. Note that the reverse body diode across FET Drain-Source in most MOSFETs means it will always conduct in the reverse direction. \$\endgroup\$ – Russell McMahon Aug 21 '13 at 22:06
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When you exclude the diode and you do not provide input power to the LM1117 it will (at least potentially) experience a reverse voltage (output higher than input). The 7805 regulator can be damaged by such a situation, but I have not heard of similar effects on an LM1117. But it is still outside its normal mode of operation, so I would advise against it.

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