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TPS734: Data sheet interpretation needed - Selecting R1 & R2 values

Part Number: TPS734

Hello,

The part I'm using is the TPS73401DDC.  Page 8 of the TPS734 datasheet in the Feedback Capacitor Requirements section (on the middle right hand side) says:
The feedback capacitor, CFB, shown in Figure 21 is
required for stability. For a parallel combination of R1
and R2 equal to 250kΩ, any value from 3pF to 1nF
can be used. Fixed voltage versions have an internal
30pF feedback capacitor that is quick-charged at
quick-charge circuit, so values below 5pF should be
used to ensure fast startup; values above 47pF can
be used to implement an output voltage soft-start.
Larger value capacitors also improve noise slightly.
The TPS73401 is stable in unity-gain configuration
(OUT tied to FB) without CFB.
How do I interpret the part in red?  Does this mean that R1 and R2 values should be selected so that the equivalent resistance of R1 and R2 in parallel should be as close to 250k Ohms as possible? Is there a range that is acceptable? What are the consequences of being above?  What are the consequences of being below?
Thanks for the clarification!
Cheers,
Mike
  • Hi Mike,

    I believe the wording is confusing, it looks like they mean the total value of the FB node should be targeted at 250k. The range can be modified, the concern is stability, by changing the resistance, you modify the RC network for ac stability performance. the CFB will need to be adjusted if significant deviations are made in the total resistance of the FB resistors.

    Consequences for your circuit, by increasing the FB network resistors larger, you cause the leakage current of IFB to cause a larger voltage deviation on VOUT. By being below you cause the current going through the FB node to increase decreasing the allowable current for IOUT and increasing the leakage current the devices circuit consumes.

    Additionally both will modify the necessary CFB necessary

    Regards,

    John