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LP2985A: Calculation of Cout ESR

Yama
Prodigy 40 points
Part Number: LP2985A
Other Parts Discussed in Thread: LP2985

Hi, could you let me know about LP2985AIM5-3.6/NOPB's Cout ESR?

I know LP2985's cout ESR should be greater than 5mohm.

Please refer to an image file that I uploaded. There are 3 patterns (A)~(C).

(A) When Cout ESR vaule is 8mohm, I think it isOK.

(B) When LP2985's output is connected by other power IC's VIN (TPS0518), which are LP2985's Cout?

Only C1? or C1 and C2?  (C2 is Cin for TPS0518.)

And what is the vaule of LP2985? 8mohm ? or 5.71ohm?

(C)When LP2985's output is connected by other two power IC's VIN(TPS0518 x2), which are LP2985's Cout?

Only C1? or C1, C2 and C3?(C2 and C3 are Cin for TPS0518.)

And what is the value of LP2985? 8mohm ? or 4.44ohm?

If 4.44 ohm, it is not meet the LP2985's spec 5mohm. Is it correct?

Thanks.

  • 0
    TI__Genius 12511 points

    Hi Yama,

    The Cout value along with the esr is computed using the total Cout and esr along the output rail.

    A) with 8mohm you are in the region of stability so it should land in the region of stability

    B) It will be both C1 and C2, and should still be in the region of stability

    C) It will be C1, C2, and C3, this will be outside the region of stability, meaning the output will be unstable

    Because the region of stability requires a 5mohm esr, configuration C is not within the region of stability, so resistance must be added in order for the part to be stable.

    Regards,

    John

  • 0 in reply to John Schneider
    Prodigy 40 points

    Thank you very much for your support.

    Please refer to an Image file (D) that I uploaded.

    Your advice is "resistance must be added" when pattern (C).

    So, I added 100mohm resistor (R1) between 3.6V and C1.

    Is it correct ? I think your advice is meaning this Image file.

    I don't know such an example, putting on a milli ohm order resistor between Vout and Cout.

    Is this circuit commonly used ?

    If this Image file is not correct, could you let me know reference circuit?

    Best Regards.

  • +1 in reply to Yama
    TI__Genius 12511 points

    Hi Yama,

    Yes, that is increasing the total rail resistance for the zero in the feedback loop.

    A majority of designs will intentionally select higher esr capacitors for these older parts, but when capacitor selection is fixed, yes it is necessary to implement a series resistance.

    In this datasheet for the LP2951, in the second paragraph on the 8.2.2.2 section in capacitor types, it explicitly talks about requiring a series resistance on an output rail that does not meet minimum esr resistance, so it is common enough for it to be brought up on a datasheet, we don't have reference designs or app notes on this practice, but it is brought up on some of these older parts datasheets.

    Regards,

    John

  • 0 in reply to John Schneider
    Prodigy 40 points

    Thank you very much for your support.
    I understood.
    My question is resolved.

    Best Regards.