• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 965 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS734: some regulators spec noise as proportional vout, others as absolute value

Jeff Smith1
Prodigy 180 points
Part Number: TPS734
Other Parts Discussed in Thread: LP5900,

Some regulators spec output noise as a proportion of Vout, such as noise (uVrms) = 11 x Vout.  Other datasheets specify noise as an absolute number such as noise = 11 uVrms.  Are the differences in specifying noise correct?

If the differences are correct, the TPS734 for example has 11 times greater output noise than the LP5900.  An 11 times difference seems off for two regulators both spec'ed for ultra low noise. 

TPS734

LP5900 -

  • 0
    TI__Genius 14245 points
    Hi Jeff,

    Specifying noise can be done either way. If the noise scales with output voltage then it makes sense to have a line that covers all output of the devices output voltages and have a noise spec which is proportional to the output voltage. Sometimes the noise does not scale with the output voltage and a singe absolute number is valid for the output voltage specified.

    Unfortunately there are some challenges when using qualitative terms such as ultra-low or ultra-small. The first being that there is no recognized standard for these terms and therefore what is considered low for one application may be considered high when addressing a different application. Additionally, what was cutting edge in an earlier era becomes commonplace technological progression. Therefore, it is important to take qualitative terms with a grain of salt and look at the quantitative specs and compare accordingly.

    -Kyle
  • 0 in reply to Kyle Van Renterghem
    Prodigy 180 points

    Is a scaling output noise vs. an absolute value due to different regulator designs internally?  Why do some designs scale while others have an absolute value?

  • 0 in reply to Jeff Smith1
    TI__Genius 14245 points
    Hi Jeff,

    For applications or devices where you are not using a noise reduction cap (Cnr) or a feedforward cap (Cff), whether or not the output noise of the LDO is proportional to the output voltage is due to the design topology. If the output voltage is set by changing the gain configuration of the error amp (which is in a non-inverting configuration), then the output noise will scale proportionally to the output voltage divided by the reference/feedback voltage. If the output voltage is set by changing the value of the reference then the output noise will remain relatively constant across output voltages since the gain of the error amp isn't changing.

    Now, if a Cnr and/or Cff is used then the output noise will not scale directly with the output voltage due to the filtering effect of Cnr and the reduction in the AC gain by Cff.

    -Kyle