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TPS7A47: Low frequency noise

Part Number: TPS7A47
Other Parts Discussed in Thread: TL1963A, TPS7A45

Hello team,

Some TI's LDO have NR pin to achieve low noise performance such as TPS7A47. However its noise reduction at low frequency area is a little weak. For example, LT1963A has flat noise spectral density with level less than 0.1uV/rtHz at low frequency area. If customer is interested in noise at low frequencies, do we have better way to achieve low noise at also low frequency with our LDO which has NR pin?

Or is it better to introduce TPS7A45 or TL1963A in that case?

Best regards,

  • HI Sato-san,

    The LT1963A is showing a scope shot not a noise plot:

    If you are to believe this is the noise it shows `200uVpp. It does not show noise as a function of frequency, nor how changing the output voltage affects the noise. The TPS7A47's noise plot does:

    The TPS7A47 has ~ 1/10th the noise of the LT1963A.

    I hope this answers your question.

  • Hello John-san,

    Thank you for the comment.

    Is there any way to improve noise at low frequency area with TPS7A47?

    Noise at high frequency area may be improved by devising circuitry such as feedforward capacitor. This will work for LT1963A also. Then smaller noise density at low frequency area will contribute to total noise even though it might be still a little.

    Best regards,

  • HI Sato-san,

    Using a feedforward capacitor is a great way to help further reduce noise. This is discussed in our LDO Basics series

    Also increasing the CNR capacitor can help but this increases the soft-start time. One advantage of the TPS7A47 is that it has the Any-Out feature. This means you can set VOUT without feedback resistors which can also add some thermal noise to the system. 

    I hope this helps.