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I want to use this LDO: AP7346D-1828FS6-7.

This LDO has two outputs and it has "Enable" controls. I want to connect these outputs 1.8V and 2.8V for one input. I can control this with EN pins, but I didn't see any information about this usage in datasheet. Is it okay or not?

What I want is something like this:

enter image description here

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  • \$\begingroup\$ You want to connect both outputs, enabled at the same time or alternate between them? Are you okay with using a diode OR circuit? Your concerned about the backfeeding of the outputs. \$\endgroup\$
    – Passerby
    Jun 9, 2022 at 7:08
  • \$\begingroup\$ @Passerby I want to connect both outputs and alternate between them (not enabled same time). Yes i have some concerned about the backfeeding of the outputs. \$\endgroup\$
    – mtoprak
    Jun 9, 2022 at 7:12

2 Answers 2

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No, you cannot connect the outputs of the AP7346D directly even if you only enable one at a time.

They each have an internal discharge transistor that is enabled whenever the output is disabled, and that would heavily load the enabled regulator (about 50\$\Omega\$) .

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  • \$\begingroup\$ Thank you for the answer. Can you give me an advice about these usage? I have an Sim Card and i want to supply it 2.8v and 1.8v also. \$\endgroup\$
    – mtoprak
    Jun 9, 2022 at 7:17
  • \$\begingroup\$ Do you want to be able to select between 2.8V and 1.8V? \$\endgroup\$ Jun 9, 2022 at 7:22
  • \$\begingroup\$ Yes, it is what I want. \$\endgroup\$
    – mtoprak
    Jun 9, 2022 at 7:32
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    \$\begingroup\$ I suggest a single adjustable LDO with three resistors and a MOSFET to select voltage. \$\endgroup\$ Jun 9, 2022 at 7:59
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What you can do is to Power-OR them:

schematic

simulate this circuit – Schematic created using CircuitLab

When OUT1_en is high and OUT2_en is low the combined output will come from OUT1 only. Likewise, when OUT2_en is high and OUT1_en is low the combined output will come from OUT2 only.

If you have both enable inputs high then the combined output will come from the highest output voltage.

There are a few issues:

  • The load current will pass through the activated output's series diode and therefore the diodes will dissipate some power.
  • The output voltage will be one diode-drop less than desired. So a diode with very low Vf drop must be selected.
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  • \$\begingroup\$ If the voltages weren't slow I wonder if a mosfet body diode would work well here, solving both of the issues presented. \$\endgroup\$
    – Passerby
    Jun 9, 2022 at 7:58
  • \$\begingroup\$ Actually I tried this solution with two different LDOs and it didn't work well because of the mosfet body diode. \$\endgroup\$
    – mtoprak
    Jun 9, 2022 at 8:01

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