• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 64 subscribers
  • Views 1053 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.

TPS73501 minimum value of feedback resistors

prachi panse1
Intellectual 980 points
Other Parts Discussed in Thread: TPS735

Hi,

The TPS73501 datasheet specifies that for a parallel combination of R1 and R2 equal to 250 kΩ, a feed forward capacitor of value between 3 pF to 1 nF is required. However, the recommended or the minimum value of the resistors R1 and R2 is not specified. Can you please tell me the minimum required value?

Regards,

Prachi

  • 0
    TI__Expert 8905 points

    Hi Prachi,

    The recommended minimum value will depend on the desired output voltage - the parallel combination of R1 and R2 should = ~250 kOhm.

    Some examples, rounded to the nearest 1% standard value:

    Vout = ~5.0V, R1= 1M, R2 = 316k

    Vout = ~3.3V, R1 = 681k, R2 = 392k

    Vout = ~2.85V, R1 = 562k, R2 = 412k

    Vout = ~2.5V, R1 = 511k, R2 = 487k

    Vout = ~1.8V, R1 = 357k, R2 = 732k

    Vout = ~1.5V, R1 = 301k, R2 = 1.24M


    Regards,

    David

  • 0 in reply to David Hopkins
    Intellectual 980 points

    Dear David,

    Thank you for your reply. I have another question to follow.
    As per the TPS73501 datasheet, smaller value of resistors in the output resistor divider reduce noise. Since, as the parallel combination of R1 and R2 should be close to 250Kohm, the values of the resistors used is large. In this case, there will be larger noise in the output.
    Can you please tell me why will the noise increase with the resistors' value and also suggest how this can be taken care of?

    Regards,
    Prachi

  • 0 in reply to prachi panse1
    TI__Expert 8905 points
    Hi Prachi,

    The noise due to the resistors values is actually because of the thermal noise floor of the resistors. It is known as Johnson-Nyquist noise. The noise from the resistors doesn't matter as long as it's below the noise floor of the device (after the noise is gained up by the device), which for the TPS735 is optimized when R1||R2 = 250kOhm.

    This app note explains the impact of resistor noise on op-amps, and it's applicable to LDOs as they operate as a non-inverting op-amp.
    www.ti.com/.../slva043b.pdf

    Regards,
    David
  • 0 in reply to David Hopkins
    Intellectual 980 points
    Dear David,

    I understand that the thermal noise due to resistors increases as the resistance value increases. This is independent of the LDO device.
    Please confirm if my understanding is correct. In that case, will the noise in the TPS735 output be optimized for R1||R2=250Kohm, or it will be better for lower value of resistors?

    Regards,
    Prachi
  • 0 in reply to prachi panse1
    TI__Expert 8905 points
    The noise should be optimized at R1||R2 = 250 kOhm.

    Regards,
    David
  • 0 in reply to David Hopkins
    Prodigy 50 points
    Hi David,

    We usually choose the feedback resistor R2 = 10K as the starting point and calculate R1 based on the desired output voltage. In the current designs, our customer chose these resistor values:
    Vout = 3.3V, R1 = 17.4K, R2 = 10K
    Vout = 2.5V, R1 = 10.7K, R2 = 10K
    Both cases use the same feed-forward cap Cff = 47pF, Vin = 5V. Should we change these resistors as your recommendation to reduce noise and avoid potential issue in the next version?

    I find in thread e2e.ti.com/.../84316 , F_pole = 1/(2 x pi x (R1||R2) x Cfb) => Cfb = 1/(2 x pi x (R1||R2) x 175k). How to choose the value of zero, pole for feedback loop to calculate Cff. Is it the same for all value of R1||R2.

    Thanks & Regards,
    Tien Vo