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TPS7A90: LDO Feedback loop resistor selection

Harisha BS
Prodigy 50 points
Part Number: TPS7A90

Hi,

I am using TPS7A9001 LDO in one of the design.

Input is 4.2V and output is designed for 3.4V

As per datasheet (Page 18) for 3V3 the recommended Feedback resistor values is  3.57K and 1.15K.

Since, it is connecting to internal error amplifier and error amplifier is voltage driven, i am planning to use 100K and 30K so that, the Feedback current will be further reduced.

Will this cause any issues for the performance of the IC for long term?

Please suggest, what is the recommended range of feedback pin current?

Can we use 100K and 30K in our design?

or do we need to follow as per datasheet recommendation values?

Regards,

Harish.

  • 0
    TI__Mastermind 44750 points

    Hi Harish,

    Using larger resistors will have a negative impact on accuracy, Noise,  and PSRR. But as long as you choose the divider that meets the criteria of equation 3, the device will work:

    So why the negative impact?

    1. Accuracy - the input leakage into the FB pin has more of an impact if there is less leakage through R1+R2.

    2. Noise - Thermal noise increases as a function of the square-root of resistance (this is minimal but real)

    3. PSRR - The increase resistance in R1+R2 slows down the loop response. This can be mitigated with a CFF capacitor in parallel with R1. 

    You can find more details on this in our LDO basics series. 

    Does this help to answer your question?

  • 0 in reply to JCHK
    Prodigy 50 points

    Hi,

    Thanks for the explanation.

    Since i am not using CFF capacitor due to space constrains, i will go with your recommendation.

    I would like to use 3.3K and 1K instead of 3.57K and 1.15K is it ok?

    Regards,

    Harish.

  • 0 in reply to Harisha BS
    TI__Mastermind 44750 points

    Hi Harish,

    Sure, that would yield a nominal 3.28V at the output. 

    Regards,