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TPS77601 Region of stability

Daniel Voggel
Prodigy 20 points

I´m using the TPS77601 to generate 5V from a 7V supply.

I want to use following parallel ceramic Output capacitors:

one 100nF Kemet C0402C104K4RAC; Minimum ESR=23milli-ohms @10GHz

and two 10uF Kemet C1206C106K4RAC; minimum ESR=4milli-ohm @1.3GHz

ESR values are from http://ksim.kemet.com

At first shot i simply added a series resistor with 100milli-ohms to be compliant to the required ESR-range (50milli-ohms to 1.5ohms). But this series resistor causes a voltage ripple because of my not constant load current...

Is there a better way to get a stable Output voltage?

Is the Minimum ESR-requirement even critical at all?

Best Regards

Daniel Voggel

  • 0
    TI__Guru 55940 points
    Hi Daniel,

    In order to help debug could you provide a scope shot with Vin, Vout, and Iload? Also could you provide a schematic (just around the LDO is fine)?

    This particular LDO was developed before the modern low ESR ceramic capacitors were common. The ESR requirements must be met for a stable application.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 20 points

    Hi Ryan,

    thanks for your fast Support. Here is my Setup:

    I have to use MLCCs.

    To meet the required ESR-Range of TPS77601 with my low ESR-MLCCs I inserted R534.

    Because my load current is not constant (mainly CAN-Transceivers are consumers) and the Output voltage is decoupled from the Regulation Feedback, my Output voltage has a ripple with the Magnitude of R534 * Iload.

    Because the ESR-Requirements have to be met, I´m looking for a better way to get a stable Output without a Series Element.

    (My laboratory prototypes are stable without R534, but for series production i have the meet the specifications from the datasheet)

    One possibility would be one series resistor for each capacitor, but this would increase the needed layout space because on my +5V-net every consumer has additional MLCC-Buffers...

    Is there another,better possibility?

    One Thing that came in my mind is to add a parallel capacitor to the Feedback resistor R85. This could slow down the high gain compensation of TPS77601 and could make the Regulation stable even with a low ESR MLCC at the Output...(the decreased transient Performance can be accepted because of my additional buffers/filters for each consumer)

    Is my idea correct/possible?

    Best Regards

    Daniel

  • 0 in reply to Daniel Voggel
    TI__Guru 55940 points
    Hi Daniel,

    My first thought would be to move R534 to the bottom of all three output capacitors. Essentially tie the bottom of the capacitors together and then go through the resistor before GND. This should not cause any DC error due to your load, while still allowing you to use only one resistor rather than three.

    If you choose to parallel a capacitor into the feedback network, you would want to place it parallel to R87. This additional capacitor would become a feedforward cap. While a feedforward cap would help stability, we would still need you to meet the ESR requirements. Here is a document that further discusses the use of Feedforward Capacitors with an LDO:

    www.ti.com/.../sbva042.pdf

    Very Respectfully,
    Ryan