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LTC4412 Voltage Drop after diode


i'm building a UPS around LTC4412 to switch between wall adapter and battery. I have the following problem:

When i connect a load, almost the whole voltage drops across the diode, so the load never turns on.

Did i connect mosfet wrong?

Below are my schematic and PCB layout.

Thank you for your help.

Regards Jan

  • Hi Jan,
    Can you provide a larger picture of the schematic? The picture is too small.

    What is the nominal input voltage of the “18650” supply?
    What kind of load are you connecting when you report that the load doesn’t turn on?

    It is suspicious how you see the entire supply voltage drop across the diode. If the “19V” supply is present, D3 is forward biased, so you should get ~18.7V on the output. Maybe D3 was installed backwards. Let’s check this:

    Power off the circuit. Using a multimeter in diode mode, place the positive lead on the “19V” supply, and the negative lead on the output. You should see 0.3V to 0.7V. If the meter says "OL", reverse the leads. If you see 0.3V to 0.7V now, D3 is installed backwards.

    Now place the positive lead on the “18650” supply, and the negative lead on the output. You should see 0.3V to 0.7V (from the FETs body diode).



  • Hi Aaron,

    the nominal voltage of the 18650 input is about 5 volts. And "19V" is 12 volts. I used these voltages for testing.

    The load that i connected is a DC motor and the LTC3780 board.

    As mentioned when there is no load connected i see the same voltage before and after the diode. As soon as i connect a load almost the whole voltage drops at the diode.

    In the meantime i tried the LTC4412 with two Mosfets as it's suggested in the datasheet. That seems to be working fine.

    Unfortunately my Multimeter doesn't support diode testing, so i attach a picture of the mounted PCB.

    Here is a larger picture of the schematic:

    Thank you for your help.

    Regards Jan

  • Little Update: When i connect the diode the other way around, the load actually turns on. But my batteries get really hot during that process, so im not sure if it's right. Here is the full schematic:

  • 12V wall and 5V battery are present, 12V should be delivered to load.
    If the 5V battery is getting hot, I suspect 12V wall supply is charging 5V battery.
    D1 should block the 12V wall from charging the battery, but doesn't seem to be.

    Your symptoms may be due to a reversed diode footprint.
    Let's look at your layout below:

    We know that when you reversed D3, power is delivered to the load.
    From the schematic, the cathode of D3 should connect to the source of Q2.
    The stripe on the silkscreen should be the cathode, but instead it is the anode.

    Now, looking at D1 and D2.
    From the schematic, the cathodes (striped sides) should be connected, but they aren't.
    Once again, the anode is incorrectly marked as the striped side.

    Reverse D1 and D2 and try again.


  • Sir, thank you so much. I reversed all diodes as they should be and that seems to solve the problem. I am still confused why my PCB program arranged the diodes the wrong way. I definitely learned something from this problem.

    Thank you 

  • In your layout program, you have to look at how the schematic symbol is connected to the PCB landing pattern.
    Glad the fix worked.


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