i know the traditional wiring of two power sources with two diodes. But can it also work only with one?

Please take a look at the attached schematic:

The TPS706 pmos LDOs have build in reverse current protection so no current can flow from output to input. In my system i have three power sources:

1) Battery 3.6V

2) LiPo 3.7V (which can also be charged by the sources under 3)

3) 4.5V-6V external Supply (e.g. USB or solar panel for charging the LiPo)

If the power-good signal (#PG, meaning external supply > 4.5V) is asserted i want to disconnect the battery (1) and power the system (VCC) via the external supply while (eventually) charging the LiPo. The same becomes true even if PG is not asserted but i instruct my lipo charger (MCP73871, CE pin high) to connect the LiPO to Vout.

My question:

Is this schematic smart if one of the two jumpers in the upper right corner is closed. I want to implement this way some sort of power supply priority. But this approch leads to have two LDOs connected with only one of it isolated by a diode...Iam not quite sure if this is too clever ;-) Or is it possible to connect both LDO directly together? Have some doubts about the control loops inside...

Thank you for any comments!

enter image description here

  • \$\begingroup\$ With two diodes it might work. With one diode it is unpredictuable in my opinion. It might oscillate in a certain rare situation and the current might flow in reverse direction through a LDO, even if that LDO is not enabled. Without diodes the chance increases that it might start to oscillate. It is not exact science, design your circuit to prevent problems. \$\endgroup\$
    – Jot
    Aug 2 '17 at 19:05
  • \$\begingroup\$ This Ldo has a reverse current protection switching inside... \$\endgroup\$
    – wildcard
    Aug 2 '17 at 19:59
  • \$\begingroup\$ Thanks for correcting me. Indeed they do. I read the datasheet of the TPS706, and they have a reverse current protection and don't have a reverse current protection diode. \$\endgroup\$
    – Jot
    Aug 2 '17 at 21:35

Electronic industry has a solution for this case. It is solved by a special class of devices called "power distribution switch", within PMIC group of IC. Search Digi-key for this type, "PMIC - Power Distribution Switches, Load Drivers", then select, say, "2:1" in the column "Ratio - Input:Output".

Essentially these are switches based on FETs, as "ideal diodes". Here is an example, FPF1320UCX.

  • \$\begingroup\$ Hey thank you, i known the fpf1320/21 devices. But these are hard to pròduce since they are so tiny. Plus it is not easy to setup it in a similar way without Brown out while switching over... \$\endgroup\$
    – wildcard
    Aug 2 '17 at 19:56
  • \$\begingroup\$ @wildcard, I am sure there are similar devices in other, more manageable packages. The part also comes with a normal 1.27 mm pitch SO-8 package. \$\endgroup\$ Aug 2 '17 at 19:58
  • 1
    \$\begingroup\$ Would be nice, datasheet says it does not - only some 6 pin bga style \$\endgroup\$
    – wildcard
    Aug 2 '17 at 20:21
  • \$\begingroup\$ @Ale..chenski I couldn't find the FPF1320 in an SO-8 \$\endgroup\$
    – Alex
    Dec 23 '19 at 1:56

One diode won't work if you have two sources on because the voltage drop from the diode will cause one the current to be sourced more from one regulator than the other.

Another way that this could be done is a detection scheme with a microcontroller (if its on and has power all the time) or comparators that select specific voltage regulators that have a power enable pin. The power enable pins allows the regulator to be switched on. To prevent current leaking in from other regulators they would also need to support reverse current protection.

  • \$\begingroup\$ but this is the idea of the circuit...one LDO having the load but if it fails the other comes in. The IC10 LDO is unstable Vin e.g. from a solar panel. \$\endgroup\$
    – wildcard
    Aug 3 '17 at 8:29

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