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LP2992: Output capacitor choice

Part Number: LP2992

Hello,

The following is stated page 1 of the LP 2992 datasheet: "The LP2992 is stable with output capacitor equivalent series resistance (ESR) as low as 5 mΩ, which allows the use of ceramic capacitors on the output."

I've looked at several ceramic capacitor options and all of them have ESR < 5 mΩ at some frequency.

For example: AVX, PN: 12066D106KAT2A (10 µF, 6.3V, X5R, 1206) is just under 5 mΩ at 1 MHz. Other options are typically somewhere between 1 mΩ and 5 mΩ.

Is there an output capacitor series that is recommended for use with the LP2992?

On a separate note, based on other posts on E2E, it seems like TI is discouraging customers from using the LP2992. Is there reason to think this part will become obsolete in the near future?

Thank you in advance,

Leo

  • Hi Leo,

    it's very simple to solve this problem: Just put a 0603 or 0805 resistor of 20mR...100mR in series to the output cap. This will imitate the needed ESR of cap.

    Kai
  • Thanks, Kai, that's a good idea.

    I'm curious what the response from TI will be, since the LP2992 was designed to work with ceramic caps. (Did ceramic caps use to have higher ESR?)
  • Hi Leo,

    I agree with Kai. If you worry about your ESR to be lower than 5mOhms, you should consider adding a resistor in series with the output cap. The datasheet does have the chart for stability, please make sure your ESR is within the stable region. Please see here for the chart:

    Regards, 
    Jason Song

  • Thanks, Jason.

    I did see the stability chart. I was just surprised that I couldn't find any ceramic capacitor that satisfied both the minimum capacitance value requirement and the ESR requirement. Are you aware of any?

    Best Regards,
    Leo
  • Hi Leo,

    going back in time, when the LP2992 was released and the datasheet was written, ceramic caps had no such low ESR as today. Nevertheless you can still find suited ceramic caps if you take X5R in big 1206 package:

    LMK316BJ475KLHT_SS.pdf

    By the way, Taiyo Yuden has a nice search machine for their ceramic caps which provide spec sheets showing full data:

    Kai

  • Hi Kai,

    Thanks for your comment.

    Hi Leo,
    If you could not find caps within the stability chart, you will need to add resistor in series to the cap. It's a good practice to have a zero Ohm resistor as a placeholder on your PCB in series to the output cap if you have concern on the stability of the LDO.

    Regards,
    Jason Song
  • Hi Kai,

    Thanks very much! I think that you're right about ceramic caps having higher ESR at the time when the datasheet was written. The following statement from section 8.2.2.2, page 17 confirms it:

    "The LP2992 was designed to work with ceramic capacitors on the output to take advantage of the benefits they offer. For capacitance values in the 2.2-μF to 10-μF range, ceramics are the least expensive and also have the lowest ESR values (which makes them best for eliminating high-frequency noise). The ESR of a typical 4.7-μF ceramic capacitor is in the range of 5 mΩ to 10 mΩ, which easily meets the ESR limits required for stability by the LP2992."

    It seems to me that if Texas Instruments is planning to keep this part around, they should update the datasheet given that most ceramic capacitors available currently don't meet the required minimum ESR requirement, and there is no mention of an additional resistor in series with the output capacitor in the application notes or in the Basic Application Circuit (Figure 34).

    Best Regards,
    Leo
  • Hi Leo and Kai, 

    Thanks for your feedback. I will note the comments and we will update the contents in our next datasheet revision on this device to keep it up-to-date. 

    Regards, 
    Jason Song