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TPS79901-EP: What's the suggestion CFB for 1.6V output ?

Shaquille Chen
Intellectual 1770 points
Part Number: TPS79901-EP

Dear Team,

My customer are working on design of TPS79901.

However, in datasheet 8.2.2.6 shown that the parallel combination of R1 and R2 equal to 250kohm, any value from 3pF to 1nF CFB can be use.

Customer design output voltage to 1.6V. They select R1 by 150k and R2 by 422k. What's the suggestion CFB range? Or just keep it NC?

Thanks

Shaq

  • 0
    TI__Mastermind 20045 points
    Hello Shaquille,

    I would recommend adding the Cfb. While we don't have an exact range computed in the datasheet for values without equal feedback resistors, I would imagine the range will be similar. It will also depend on what the customer wants to optimize. More information on calculating the capacitor value can be found in our application note on how to minimize noise in LDOs: www.ti.com/.../slyt489.pdf

    Thanks,
    Kyle
  • 0 in reply to Kyle Rakos
    TI__Intellectual 1770 points
    Hi Rakos,

    Thanks for your prompt feedback. The equal feedback resistor mean R1+R2 ? Or R1// R2?

    Thanks,
    Shaq.
  • 0 in reply to Shaquille Chen
    TI__Mastermind 20045 points
    Hi Shaq,

    Sorry, I misspoke. The suggested values hold true for R1 || R2 = 250k. R1 and R2 do not need to be equal. As per the datasheet:

    The feedback capacitor, CFB, shown in Figure 28 is required for stability. For a parallel combination of R1 and R2 equal to 250 kΩ, any value from 3 pF to 1 nF can be used. Values below 5 pF should be used to ensure fast startup; values above 47 pF can be used to implement an output voltage soft-start. Larger value capacitors also improve noise slightly. The TPS79901 is stable in unity-gain configuration (OUT tied to FB) without CFB

    Thanks,
    Kyle