• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 62 subscribers
  • Views 214 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

TPS7A4701-EP: TPS7A4701

Pavel Margulis
Prodigy 70 points
Part Number: TPS7A4701-EP
Other Parts Discussed in Thread: TPS7A33, TPS7A94

Hello,

My question is about positive regulated ultra-low noise LDO TPS7A4701 and negative regulated ultra-low noise LDO TPS7A33.

For TPS7A4701's output current of 85uA, and for TPS7A33's output current of 25uA, (no load current condition, only a feedback circuit current) do their output noise become different (larger/smaller) than a nominal value for the same Vout?

In other words, is the absolute minimum Iout for these LDO-s a bad habbit and I should put an additional resistor at the output in order to create some larger Iout? If yes, how much of minimal load current should I assure for really low noise at the output?

Thank you

Pavel Margulis

  • 0
    TI__Mastermind 43585 points

    Hi Pavel,

    Can you tell me where you are seeing the 85uA and 25uA specification? 

    This question does not have a trivial answer. Noise is generally not directly effected by load current, but the poles and zeroes of the feedback loop do move around when the load current changes, so it is possible that the noise (in particular high frequency noise) will change. PSRR, however, is affected by load current - higher load current lowers PSRR - and so noise from the input is attenuated less and the total integrated noise can increase with increasing load current. 

    What are the operating requirements for the positive LDO? Our new TPS7A94 (now in pre-production and prototypes available) is the lowest-noise LDO on the market, but it is a lower VIN part than TPS7A4701.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 70 points

    Hello Nick,

     Thank you for your quick reply.

     There are no such specs for Iout, those are just values of Iout through the feedback circuit only, when there are no load current at all, and these are values in my circuit for TPS7A4701 and TPS7A33 correspondently. I use them in order to make a bias voltage for PIN photodiodes, so when there is no light, there is no current. I am wondering whether I have to arrange artificially a minimal load current in order to keep the noise of Vout be low.

     My requirements are as following: Vin is +-12V+-10%, Vout is programmable +-(2.5 to 7.5) V. I use a voltage output DAC in order to control each Vout. Iout is 0 to 5 mA. Minimal noise of Vout up to 100 MHz is absolutely required.

     Thank you

     Pavel Margulis

  • 0 in reply to Pavel Margulis
    TI__Mastermind 43585 points

    Hi Pavel,

    I don't know what the optimal minimum load current for low-noise performance for these devices without having some data to look at, but the difference in noise performance will likely be small anyways by using a minimum load current. However, there are a number of things you can do to improve noise performance, and fortunately you have selected 2 LDOs that have NR/SS and FB pins, which you can use to your advantage to get the noise lower. Check out this PPT that discusses methods of reducing LDO noise:

    1212.Measuring LDO Noise.pptx

    Are you controlling the output of the LDO with a DAC used like the following?

    Regards,

    Nick