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LP2985: Reverse polarity protection

Part Number: LP2985

Is there a recommended solution for reverse polarity protection, i.e. if the power source (battery pack) is connected the wrong way to the Vin and GND pins of the LP2985 voltage regulator?

I am using the variant with Vout = 3.3V. The battery packs can be up to 9V, but they are connected to Vin and GND via wires. So it is easy to connect them the wrong way.

My goal is to:

  1. Protect the voltage regulator from damage.
  2. Protect the device connected to Vout from voltage > 3.3V.
  • Hi Kaloyan, 

    LP2985 does not have built-in reverse polarity protection. Technically, a Schottky barrier diode or rectifier diode connected in series with the power supply could help to block the reverse polarity condition, but the diode will be on at all time and it may create additional power loss. You may also consider adding a diode forward-biased from GND to IN, and it will only be turned on when reverse polarity happens; for this implementation, you may need a LED or something to alert the users for the wrong connections. 

    For the protection at the output side, again, you may add a diode in series but it will also create additional power loss. 

    We have a LDO that has built-in reverse voltage/polarity and reverse current protection, which is TPS7B7701/2, it is a little different than LP2985, but you may take a look to see if it meets your needs. 

    Jason Song

  • Hi Jason,

    Thank you for the suggestion. I looked at TPS7B7701/2. It requires significantly larger space on the PCB than the LP2985 and the its price is higher. We are adding the voltage regulator to an existing product. The space on the PCB is limited and we want to keep the manufacturing cost low. The LP2985 seems to be a good fit. We just need to clarify the protection.

    I tried adding a Shottky diode BAT48 in series between the positive terminal of the power supply and the Vin of LP2985. ON/OFF is connected to Vin. Bypass is floating. The voltage drop from the BAT48 diode is acceptable.

    The issue is when I reverse the polarity of the battery pack, i.e. when the positive terminal is connected to the GND of LP2985. In this case, Vout gets the entire voltage (e.g. 6V) applied to GND, which is catastrophic for the device connected to Vout.

    Am I missing anything?


  • Ah, actually, I just found that I connect the multimeter incorrectly when I reverse the battery polarity. I connected the COM probe of the multimeter to the wire that goes to the negative terminal of the battery. So when I changed the other end of the wires on the breadboard, the COM probe was actually connected to the positive terminal of the battery, hence the false measures.

    I fixed this and everything is as expected.

    Thanks for the help!

  • Thanks for the feedback. I am glad your issue has been resolved.