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LP2985 unstable operation

Lewis Conrad
Prodigy 170 points
Other Parts Discussed in Thread: SM72485, SM72238, LP2985

Dear Sirs,

I am using the LP2985-3.3V and I'm having an output voltage stability problem.

The circuit powers from a 24V battery, then the SM72485 supplies 10VDC.

From this 10VDC, there is a SM72238 giving a perfect 5VDC output and there is the LP2985-3.3V.

When I connect the battery, the 10V and 5V work ok, but the LP2985 output goes up to 4V and periodically has a glitch where the voltage goes down to 3.3V and then up to 4V again, where it remains for most of the time.

But if I quickly disconnect and connect again the battery, it starts working properly at 3.3V.

LP2985-3.3V has a ceramic (MLCC, X5R) 1uF input capacitor and a ceramic (MLCC, X7R) 2.2uF output capacitor. It also has a 10nF, 25V, ceramic (MLCC, X7R) capacitor on the bypass pin.

How can I make it work correctly?

  • 0
    TI__Intellectual 1190 points

    Hello Lewis,

    Do you have the enable (on/off) pin of LP2985 tied to the input voltage? You say that it periodically glitches. What is that time period?

    Also, during the connection, what are the loading conditions (Iout)?

    Thanks,

    Andrew

  • 0
    TI__Intellectual 1190 points

    Hello Lewis,

    Do you have the enable (on/off) pin of LP2985 tied to the input voltage? You say that it periodically glitches. What is that time period?

    Also, during the connection, what are the loading conditions (Iout)?

    Thanks,

    Andrew

  • 0 in reply to Andrew Leverette
    Prodigy 170 points
    Hi,

    Yes, enable (on/off) pin is tied to input voltage (IN) pin directly.

    The voltage waveform on Output pin is:
    a) 150ms at 4.0V, 75ms at 3.75V and 25ms at 3.5V. Then goes up to 4.0V again and the cycle repeats itself indefinitely. (T=250ms)
    or b) 225ms at 4.2V and 25ms at 4.0V. Then goes up to 4.2V again and the cycle repeats itself indefinitely. (T=250ms)
    Input voltage is pretty steady at exactly 10.0V.

    The Iout is approximately 30mA.

    Thank you.
  • 0 in reply to Lewis Conrad
    TI__Guru*** 140126 points
    Lewis,

    Would a higher load result in a stable 3.3V output?
    Does the load change at a 4Hz rate?
    Of course the load current must always flow out of the LP2985. The LP2985 won't sink current to maintain regulation.
  • 0 in reply to Ron Michallick
    Prodigy 170 points

    Hi,

    After some experiences, I've noticed that there was something strange on the load: MCU not starting properly when the problem on the 3.3V occurred.

    I've always thought it was due to the power supply and not the other way around.

    It turns out that there was a floating pin that should be tied to ground. It sets the boot mode (Flash, system memory or RAM).

    After making that change, the problem on the LP2985 never happened again.

    I'm still curious to know what the problem was.

    Could the MCU be consuming too much power instantly at the startup?

    What could cause such behavior on the LP2985?

    Thanks.

  • 0 in reply to Lewis Conrad
    TI__Guru*** 140126 points
    Lewis,

    Over current should be output too low. Output too high seems to be reverse current.
    Sorry I can't be more helpful.