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TPS777: Suitable Output Capacitor (ceramic, ESR)

Part Number: TPS777
Other Parts Discussed in Thread: TPS767,

From the TPS77725 datasheet:

"Like all low dropout regulators, the TPS777xx and TPS778xx require an output capacitor connected between OUT and GND to stabilize the internal control loop. The minimum recommended capacitance value is 10 μF and the ESR (equivalent series resistance) must be between 50 mΩ and 1.5 Ω. Capacitor values 10 μF or larger are acceptable, provided the ESR is less than 1.5 Ω. Solid tantalum electrolytic, aluminum electrolytic, and multilayer ceramic capacitors are all suitable, provided they meet the requirements described previously." 

We would like to use a ceramic capacitor for the output.  However, ESR data for ceramic parts is hard to come by.  Most datasheets do not include this parameter, and those that do offer a plot over frequency.  Finding a ceramic cap the fits the 0.050 to 1.50 Ohm range does not seem possible (the ESR value drops too low in the middle frequencies, on the handful we have been able to find ESR plots for).

Can you provide some guidance on how to select an appropriate ceramic component?  Would you have any capacitor part number examples you can share that would satisfy the TPS77725 output?

  • Hi Kevin,

    Ceramic capacitors may be difficult here by themselves. Note that you can get the ESR from the impedance over frequency plots, as shown below in a figure pulled from Digikey's forums. The ESR can be taken as the lowest point on the impedance plot over frequency, and the frequency at which this point occurs is the resonant frequency of the capacitor. It is recommended to choose a capacitor with a resonant frequency outside of the bandwidth of the internal error amplifier of the LDO, which typically extends up to 100kHz - 1MHz. 

    A (maybe not so ideal) solution to the low ESR of a ceramic cap is to use a series 50mΩ resistor to artificially add the required ESR. This solution will work just fine, but of course this increases the solution size and cost. 

    If this is not an option, I can recommend another part if you provide more details of your application requirements.



  • Hi Nick,

    Thanks for the info.  I think I'll change gears to an electrolytic cap.  My hope for a ceramic solution came from a similar datasheet for the TPS767.  The external capacitor section is nearly word-for-word the same as the TPS777, with the additional line:

    "Most of the commercially available 10 μF surface-mount ceramic capacitors, including devices from Sprague and Kemet, meet the ESR requirements stated above."

    There may be a contradiction in the TPS767 datasheet, however.  The text asks for a min 50 mOhms, but the detailed Region of Stability charts do not show a low end requirement on ESR.  If the charts are reality, then the quoted comment makes sense.