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TPS7A7001: Feedback resistor R2 selection

Jasla p
Prodigy 10 points
Part Number: TPS7A7001

Tool/software:

Hi,

I am using TPSTA7001 LDO in my design and referring to the datasheet for selecting feedback resistors. The datasheet recommends choosing R2 between 27K and 33K when setting the output voltage.

I would like to understand:

1.What is the reason for this specific range?

2.What happens if I choose a value outside this range?

I have attached a snapshot from the datasheet for the reference.

I appreciate any insights on this. Thank You

  • +1
    TI__Expert 5241 points
    1. Reason for this specific range?

    Feedback current requirements: 

    • The LDO’s internal reference and error amplifier require a certain range of current through the feedback network to regulate accurately. Selecting R2 in the recommended range helps ensure that enough current flows through the feedback pin, keeping it within the ideal operating conditions.

    Stability and noise performance:

    •  LDOs rely on a feedback loop that has been characterized for a certain feedback impedance. If the resistors are too high, noise susceptibility can increase (because high-value resistors pick up more noise). If too low, the feedback loop can draw significant current and might not behave as intended.

    Accuracy and offset considerations:

    • The internal offset currents of the error amplifier can become more significant if R2 is too large, leading to inaccuracies in the output voltage. Conversely, very low resistor values increase quiescent current and power dissipation unnecessarily.

    2. What happens if you choose a value outside this range?

    • Too high (>33 kΩ):
      • Reduced feedback current may lead to regulation errors, since the amplifier input bias currents and offset voltages become more prominent.
      • Potentially higher noise pickup, as the feedback node becomes more sensitive to interference.
    • Too low (<27 kΩ):
      • Higher quiescent current consumption through the feedback network, which impacts power efficiency.
      • Possible shift in the loop dynamics (e.g., gain and phase margins) and, in rare cases, stability concerns.
      • The LDO will still regulate, but you could see unnecessary power loss and no real performance benefit.

    Hope this helps,

    Vahnroy Hosear