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Having More capacitance in Output of LDO

Raju E
Prodigy 80 points
Other Parts Discussed in Thread: LMS8117A

Hello,

I am Using the LDO  for 5V to 3.3V(LMS8117A) Conversion to provide input to micro Controller.In the datasheet the recommended output capacitance is 10uf tantalum.

But in my application i wanted to go with higher capacitance to 470uf. is there any problem with output stability and inrush current.

In my application Micro controller needs power for few milli seconds  for writing the data to flash from RAM after the power failure. that is the reason i am going with higher capacitor in Output of LDO

Please suggest.

  • 0
    TI__Mastermind 22610 points
    The 10uF is the recommended minimum Cout value for LMS8117. Using 470uF for Cout presents no problem for stability.
    No problem for LMS8117 for in-rush current with larger Cout value, however I suggest that you check your 5V input to confirm that its behavior is acceptable during power-up.
  • 0
    TI__Intellectual 2010 points

    Hi Raju,

    The minimum output capacitance required by the LMS8117A is 10µF, if a tantalum capacitor is used because of low internal ESR. Any increase of the output capacitance will merely improve the loop stability and load and line transient response. The ESR of the output capacitor should be greater than 0.5Ω and less than 5Ω. So as long as the ESR is within the prescribed limit, I see no problem in using a larger capacitor. 

    You should consider using a discharge diode in the case of voltage dip or line transient. If you are using a large output cap, and Vin turns OFF, the cap will hold a charge for a long time because uC doesn't require a lot of current. In this situation, the device LMS8117 will see a negative voltage across it which can damage the device. This is true for all linear regulators and LDOs. Therefore you would require a safe discharge path for the charge being held by 470uF cap as shown below:

    Hope this helps. 

  • 0 in reply to Donald Jones
    Prodigy 80 points

    Hello Donald,

    Thanks for your response,

    I have one concern regarding inrush current

    With 470uf the capacitor is drawing 3A for 500us with is flowing through device ,but the device is rated for 1.5A Max will it create problem to device?

    I have attached a waveform for your reference

  • 0 in reply to Raju E
    Prodigy 80 points

    Hello

    Waiting for response

  • 0 in reply to Raju E
    TI__Mastermind 22610 points

    I agree that datasheet indicates that the short circuit current should be less than ~1.6A. Usually this parameter is measured after the device is powered-up.

    If this ramp starts right when Vin is first applied this might be due to the time needed for the internal control circuit for current limit to get biased-up and operating.

    It's not ideal, but I doubt that the device will be damaged.