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LP3990 Output overshoot during turn off

Other Parts Discussed in Thread: LP3990

Hi ,

We have a 1.2V generating circuit from 3.3V input using LP3990 LDO.

Input is having 1uF ceramic capacitor output is having a 1uF ceramic,a 10uF tantalum and 4x 0.1uF ceramic capacitors.

The rail is powering VDD core to a microcontroller.

During turn OFF the LDO output generates an over shoot which goes to around 1.5V as shown below.

Any suggestions why its happening will be greatly helpful

  • This is not a known issue with LP3990 if the 'turn OFF' is via the Enable pin.

    If the 'turn OFF' is from a falling Vin, when Vin falls below the Minimum Operating Vin (2V) the LP3990 loses the ability to regulate and the output voltage may rise to near Vin. For the LP3990 versions where the pre-set output voltage is above 2.0V, this is not a problem. For LP3990 versions where the preset output voltage is less than 2V it can be a problem, especially if Vin is falling slowly.

    This same issue exists for a rising Vin as well.

    In some cases increasing Cout will reduce, perhaps eliminate, the overvoltage spike

    Best fix is to use the EN pin to turn-off the LP3990 when Vin is below the 2V Minimum Operating voltage.