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Original question:

REF5025: REFERENCE OF ESR RESISTOR

REF5025: ESR of output capacitor

Hao Wang14
Expert 4415 points
Part Number: REF5025

Hi Team,

The datasheet says the ESR of CL must be less than or equal to 1.5Ω to ensure output stability. My questions are:

1. Why ESR with 1.5Ω or less is required to ensure output stability?

2. What if the ESR is higher than 1.5Ω？

3. At which frequency range should the ESR be 1.5Ω or less?

From the block diagram, the Vout is from an amplifier, usually the phase margin of open loop gain is big at low frequency. The ESR of capacitor at low frequency such as 2.8Ω @100Hz is also higher than 230mΩ@ 1KHz. Does it mean that the ESR should be less than 1.5Ω from 100Hz to 10GHz or just at 1kHz？

Thanks and Best Regards!

Hao

  • 0
    TI__Expert 3835 points
    Hi Hao,

    Low ESR capacitors is recommend in to ensure stability of the output voltage. Higher ESR capacitors may lead to large ripples and oscillations. It also influences the internal control loop that may lead instability. The 1.5 ohm value is just a recommendation that we know ensures output stability. This does not mean capacitors with ESR higher than 1.5 ohm will have unstable outputs.

    The graph you show is the capacitor impedance across frequency and should not be confused with ESR. ESR is usually lower than the impedance across frequency. The only time Z = ESR is at its self-resonant frequency which is at the lowest impedance point of the graph. Based on the capacitor that you showed, this is fine for our device application in terms of ESR.

    Thanks,
    Ethan
  • 0 in reply to Ethan T
    Prodigy 60 points

    Hi Ethan,

    I think it maybe misunderstood that the picture by hao wang, it can be seen in more detail from the attached datasheet,

    from the datasheet, this curve is show the ESR, and the ESR is greater than 1.5 ohm at low frequencies.

    Does this have any effect on this application?

    Or we only need to pay attention to the value of ESR when Z=ESR?

    Thanks,

    Jerry


    GRM188R60J106ME47_EC-data.pdf

  • 0 in reply to yunlin an
    TI__Expert 3835 points
    Hi Jerry,

    I understand, sorry for the confusion. Since this is not a switching regulator, we need to pay attention where the impedance is dominated by the ESR which is at the resonant frequency (Z = ESR). Since this is less than 1.5 ohm, I would be confident in using this capacitor. That being said, the circuit should always be validated and tested for the application.

    Thanks,
    Ethan
  • 0 in reply to Ethan T
    Prodigy 60 points

    Hi Ethan,

    Got it, thak you very much!

    Jerry

  • 0 in reply to Ethan T
    Prodigy 60 points

    Hi Ethan

    the datasheet says that the ESR must less or equal than 1.5 ohm, and the recommend value is 1-1.5 ohm to minimize the noise

    so the question is 

    Does it means the noise about ESR between 1-1.5 ohm is better than the ESR of 10 mohm ? and do you have any output noise figure of different ESR capacitor?

    Thanks

    jerry.an

  • 0 in reply to yunlin an
    TI__Expert 3835 points

    Hi Jerry,

    This a lower than recommended ESR value does not necessarily mean more noise. The device has shown optimized noise performance between the recommended ESR values which why it is stated in the datasheet.

    I have attached an excel sheet with some typical bench data between ESR values. It does not measure noise, but shows the range of stability between ESR values.

    Hope this helps,

    Ethan

    REF5025 Cout ESR Test.xlsx