• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 62 subscribers
  • Views 277 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.

TPS7A85: Noise calculation using a preregulator

Laszlo Garab
Intellectual 965 points
Part Number: TPS7A85
Other Parts Discussed in Thread: LM317, , UCC283

Hi,

I would like to ask some help from an experienced designer regarding nosie effect of preregulator on a total system.

Unfortunatelly, I have no instrument and I have little experience regarding noise effects. I am thinking on a low noise power supply based on TPS7A85. I would like to use LM317 as a preregulator which has a higher (40uV) noise. Could someone help me how this effects of the total output noise of the system?

Thank you in advance. 

Regards,

Laszlo 

  • 0
    TI__Mastermind 23361 points

    Laszlo,

    Please allow some time to investigate this topic and I will get back to you by Wednesday.

    Regards,

    Jason Song

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points

    Hi Laszlo, 

    To better assist you with the correct type of contents, will you share a little more on your application and what kind of application are you trying to use the LDOs? Do you have any noise requirements?

    One real-life example I can give regarding to different behaviors due to different levels of noise is that for a CMOS image sensor, the input stage has some ripple and the LDO used at that time does not have high PSRR at 100KHz, so the filtering for the ripple is not enough. When I put the sensor on with all black, I can see some ripples from the video through USB. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Intellectual 965 points

    Hi Jason,

    I am thinking on low noise power supply. It is nothing special, but use of the low noise behavior of the TPS7A85. The problem is that this IC has very low power dissipation properties, so it needs some preregulation to limit drop about 0.5V on the device.

    I have power source  with 5.7-6.6V basicly with zero ripple, but it is out of TPS7A85 limit, so I guessed to preregulate it to 5.5V. Probably, UCC283 preregulator is better, because it has lower drop, but I could not find info about it's noise, Before I start designing it is a predesign study whether it makes sense to build it with preregulated system in case the preregulator has higher noise behavior.

    Thank you for spending time on my idea!

    Regards,

    Laszlo

  • 0 in reply to Laszlo Garab
    Intellectual 965 points

    Hi Jason,

    I was surfing on the UCC283 product page for other reasons and by chance I have found SLAA412A document. This explanes many aspects of the noise issues, so I basicly get an answere to my question. Maybe not figures as would help more but design requironments surely.

    Thank you for your time!

    Regards,

    Laszlo