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LP3985: No Output

Part Number: LP3985
Other Parts Discussed in Thread: ADS1262, LMP7721

I have 100 custom boards each with four LP3985IM5-2.5/NOPB voltage regulators.

So far I have found six LP3985 regulators that don't output +2.5V. I have checked the capacitors to ensure the right ones were put on the board.

The voltages on the pins are,

V(Pin1-GND) = 5.16V

V(Pin2-GND) = -0.273mV

V(Pin3-GND) = 5.164V

V(Pin4-GND) = -0.449V

V(Pin5-GND) = -0.609V

Replacing the LP3985 does not appear to fix the problem.

Sometimes the regulator will randomly start working.

Does anyone have any suggestions on what could be wrong?

Thanks,

Allan

  • Hi Allan,

    Is there a large load at the output when the device attempts to turn on? I wonder if a large load is preventing from turning off it the device is already near short-circuit current limit.

    If replacing the device does not appear to solve the issue, to me that is an indication that there is something else going on on the board. Can you share more of the schematic near the LDO and/or part of the layout where the LDO is mounted?

    Thanks,

    Nick

  • I am getting closer to figuring out this problem. The LP3985 is getting its power from a CUI PDS1-S24-S5-M DC/DC converter. The LP3985 does not output +2.5V when I power cycle the 24V into the board using the On/Off button on the Keysight E3613A power supply. The LP3985 does output +2.5V when I simply unplug the power cable from the board. The +5V going into the LP3985 is different,

    Below is the +5V going into the LP3985 when pressing the On/Off button on the power supply,

    The blue channel is the +5V going into the LP3985.

    Below is the +5V going into the LP3985 when plugging in the power cable.

    Does anyone know why the LP3985 is so picky about its input voltage?

    Thanks,

    Allan

  • Hi Nick,

    There isn't a large load connected to the voltage regulator. It is powering a ADS1262 ADC and a LMP7721 op amp. A complete schematic and board layout is attached.

    5008.Schematic.pdf

    8686.PCB.pdf

    Thanks,

    Allan

  • Hi Allan,

    I looked and I was not able to find any documentation showing a sensitivity to the VIN rise time for this device. With that said, I have seen devices have startup issues when the input voltage rises too slowly, mostly with older devices. Maybe that is what is happening here. 

    I don't see anything else in the schematic that looks problematic. I didn't look much at the layout because it's hard to find anything when I can't use a search function. What is the normal way this board will be powered? Of course it won't be a bench supply like E3613A for normal use, and I assume there is another way of powering the board besides hot-plugging it. If it is powered for normal use in a way in which the +5V rises quickly enough for it to always output 2.5V, then I don't see any issue. 

    Regards,

    Nick

  • Hi Nick,

    This will eventually be powered with a 24V industrial power supply. So this shouldn't be an issue, just a strange behavior for a voltage regulator.

    Thanks,

    Allan