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LM27762: Output noise voltage vs. Output voltage ripple

Gman
Prodigy 40 points
Part Number: LM27762
Other Parts Discussed in Thread: OPA2209, , LM7705

I need to generate a very low noise negative voltage supply (below ground) for an opamp's V- power supply. The opamp (OPA2209/2210/2211) does not have rail-to-rail input and my input signal is expected to include (or come close to) ground, which is why I need the negative power supply. The supply must be low noise because the application is an analogue front end requiring ultralow noise performance.

I am considering the LM27762 charge pump which is advertised as low noise, but I am confused by the specifications in the datasheet, specifically "Output noise voltage" which is given as 22 µVRMS (IL = 80 mA, 10 Hz to 100 kHz), while the graphs for output voltage ripple have the ripple as anywhere between 0.75 and 3 mV (IOUT/VIN dependent). What are the differences between these numbers, which are orders of magnitude different?

Is there a better (lower noise, similar price and complexity) solution which I should be considering?

  • 0
    TI__Genius 14856 points
    Hello Gareth,

    LM27762 is a switching regulator. The system will then charge and discharge, as part of its regulation. This will define the ripple at a switching frequency of 2MHz typical.

    Other constitutive signals of the output signal will be considered as noise.
    LM27762 features a LDO at the output which allow the device to provide a very low noise output and low ripple.
    Does it make more sense to you?

    This device is a good fit to your application.
    I could also recommend the following part LM7705, if you want to provide a fixed, just below zero, output voltage.

    Thank you very much!

    Best Regards,
    Dorian
  • 0 in reply to Dorian Brillet de Cande
    Prodigy 40 points
    Thank you Dorian,

    This answers my question.
    Ripple = primary/fundamental switching component of regulator
    Noise = everything else (10 Hz - 10 kHz)