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LP2992 Output capacitor ESR

Other Parts Discussed in Thread: LP2992

Hey,

I'm in the design stage of a power supply for a little µC board. Parameters:

LP2992-5.0V

Vin = 7.2 V

Vout = 5 V

Imax = 100 mA

Question is: In the datasheet the ESR for the output capacitor for stable operation is limited from 5 mΩ to about 900 mΩ and it is mentioned, that virtually all ceramic capacitors are in this range. I wanted to use MC0805B475K100CT from Multicomp (4.7 uF) with a D.F. of 10 % @ 1 kHz. When I calculate the ESR from this value with:

ESR = DF * Xc = DF / (2 * Pi * f * C)

I get:

ESR = 0.1 / (2 * Pi * 1 kHz * 4.7 uF) = 3.4 Ω

which is much higher than anticipated. Did I make a gross error or is the capacitor plainly not the right one for this application? (In this case, can you suggest a working alternative. Most capacitors I found have a DF in the region of 2-10 %)

Cheers,

Thor

  • Hi Thor,

    That is a really good question; the DF given in the MULTICOMP datasheet is a maximum value, to obtain the exact value of that particular cap you will need to contact them directly.

    Other manufacturers spec a lower dissipation factor as shown in page 4 of this document http://www.avx.com/docs/techinfo/mlc-tant.pdf . This document also shows that DF is lower at higher rated voltage. Look for capacitors with higher rated voltage like 16V and 25V they usually have lower DS.

    Regards,  

    AC

  • Hi Antony,

    most of the ceramic capacitors I found had a DF of around 10% @ 1kHz and for reasons of reliability, I won't use Tantalum capacitors. My guess is, that in the relevant frequency range (about 500 kHz for the LP2992) the ESR will be low enough. Thanks for the paper you gave, it cleared some things up. Here in fact, the 1206 ceramic capacitor with 10V, 22 µF and Y5R has a too LOW ESR of only 3-4 mΩ, which would make the LP2992 unstable. I will try the 4,7 µF capacitors I mentioned above.

    Regards,

    Thor


    edit:

    PS: It's funny, that today I received a newsletter from TI regarding "Start your power design with one click" and redirecting to the Power Webench (http://webench.ti.com). There you find the relevant ESR values of the most commonly used ceramic caps. The one I wanted to use (GRM21BR61E475MA12L) has a too low ESR of only 4 mΩ. The best solution for me in this case would be the addition of a PCB "shunt" style resistor. Knowing that 35 µm copper PCB has 10 mΩ/cm resistance for a 0,5 mm wide trace, even a mm of this trace delivers the missing 1 mΩ for stable operation.