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Low ESR caps for LDO regulator

Discussion in 'Electronic Design' started by Scott Miller, Jan 16, 2005.

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  1. Scott Miller

    Scott Miller Guest

    I'm looking at replacing a 78L05 regulator with a TI TL750L05 low-dropout
    regulator - the TI part has a lower dropout voltage, better transient and
    reverse bias protection, and can provide more current (though it'd probably
    toast my board and melt the enclosure in the process.) Of course, it needs
    a low ESR capacitor for stable operation. I've used LDO regs before with
    standard tantalums, but this data sheet has me scratching my head.

    The data sheet seems to indicate that the ESR needs to be below 0.4 ohms for
    best stability. When I start hunting for low ESR tantalums on Digi-Key, I
    find that at 10 uF, an ESR of 1.6 ohms or so is about as low as they go. Am
    I reading the regulator datasheet wrong, misinterpreting the ESR rating, or
    maybe just looking in the wrong place for my parts?

    Thanks,

    Scott
     

  2. Use a ceramic cap.

    Small sintered tantalums are well known for somewhat
    high ESR. If you want to spend more money, a foil
    tantalum would have lower ESR. But the ceramic
    will be cheaper and more stable in the long term, (at
    least if you choose the right dielectric).
     
  3. Andy

    Andy Guest

    Look for T494C226M016AS at Digi-Key. But why use TL750L05 and not
    something more standard, like LM1117?
     
  4. Do note that instability is possible for ESR too low as well as too
    high. ;-)


    Best regards,
    Spehro Pefhany
     
  5. Scott Miller

    Scott Miller Guest

    Look for T494C226M016AS at Digi-Key. But why use TL750L05 and not
    Sorry, I should have specified through-hole. 0.1" radial lead spacing.

    Aside from the TL75L05 being quite a bit cheaper, I need something in a
    TO-92 package - or at least, something that'll fit in the same footprint.
    This is for a kit that I've already produced boards for. I've substituted
    other, more expensive LDO regs before for special applications that needed
    very low dropout voltage (mostly due to very cold batteries), but the idea
    here is mostly to make it more resilient to stupid mistakes (reversed
    polarity, for example) and harsh conditions like automotive transients.

    I've also got another board revision in the works that gives me some more
    flexibility - I've added a transient suppressor, RFI protection, and might
    have enough room for a TO-220 regulator. This board will be larger and more
    expensive, though, and I'll probably keep both versions available.

    Scott
     
  6. Anders F

    Anders F Guest

    Yep - ceramics rule!
    Yep - and that s*cks big time. Seems I _need_ to put a big nasty
    electrolytic cap on my LM1117 in stead of the nice 22uF ceramics I thought I
    could use :-(

    Cheers,
    Anders
     
  7. Anders F

    Anders F Guest

    Because the LM1117 dislikes ceramic caps? ;-)

    /Anders
     
  8. If this is the data sheet you are looking at:
    http://rocky.digikey.com/WebLib/Texas Instruments/Web data/TL750L,TL751L Series.pdf

    I think that figure 4 tells you that the ESR of a 10 uf output cap
    should fall between .024 ohms and .4 ohms, only a 1 to 16 range.
    Satisfying this in a production situation may cost a lot more than the
    regulator does. They are not very helpful about how the regulator
    performs with other capacitances, either, so you will have to sweat
    what adding additional bypass caps to the rails will do to the
    stability. I would probably try a fairly good aluminum bypass cap a
    bit bigger than 10 uf, in parallel with a 1 uf stacked film Panasonic
    V series) that is in series with a .1 ohm resistor.

    Then I would connect the rest of my system and torment the upstream
    side with pulses to see how the output was behaving. That is a long
    way from guaranteeing stability with the next thousand regulators,
    though. The stability of low drop out regulators is not only
    dependent on the load current and capacitor characteristics, but on
    the upstream voltage, also. They give me heartburn.
     
  9. Terry Given

    Terry Given Guest

    Yep. Very frustrating, the tight ESR range spec. An Al electro aint much
    use if the temperature gets down to say -20C, as Al electrolytic ESR
    then skyrockets. Sanyo OSCON caps have very low, and (cf Al) very stable
    ESR. I normally use X7R caps & series R's, also ensuring the peak pulse
    power rating of the resistor can hack it (IOW not an 0402 :).

    Cheers
    Terry
     
  10. The current lasts a very short time and is linited by the regulator,
    so I doubt you will ever blow out even an 0402 on start up. But do
    you have any trouble finding ESR data for your ceramic caps? I would
    probably go with a 1210 size rated for a bit extra voltage (say, 10 or
    16 volts, rather than a 6.3 volt unit) to make it more likely that the
    ESR was low. And no Y5V or Z5U. X7R or X5R only.
     
  11. Terry Given

    Terry Given Guest

    Hi John,

    Very true wrt regulator. OTOH a load short will dump the cap into the
    series R.....hows that for paranoid :)

    I always measure the ESR. And some manufacturers provide good data (eg
    TDK, AVX). Good call wrt voltage rating, and especially the dielectric.
    Although by now there should be no excuse for SED patrons to use the
    evil Z5U/Y5V dielectrics (except as transducers :)

    Cheers
    Terry
     
  12. Greg Neff

    Greg Neff Guest

    One reason why Z5U/Y5V can be a problem is temperature range and
    tempco. In my (industrial) world I have to use X5R or better.

    In my experience using small resistors in series with ceramics works
    well. We are prohibited from using chip tantalums, and the big
    hermetic tantalums tend to have a fairly high ESR.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  13. Rich Grise

    Rich Grise Guest

    According to
    http://www.revistapcecia.com.br/downloads/datasheets/LM1117.pdf
    page 20, the TO-263 package has little leads on nominally .1 centers -
    problem is, they're ground, output, input, which is about as
    ass-inside-out from a 78L05 as you can get.

    Good Luck!
    Rich
     
  14. Why not put a very low value resistor in series with the ceramic?
     
  15. Tilmann Reh

    Tilmann Reh Guest

    --> LP 2950
     
  16. legg

    legg Guest

    Not so paranoid. Supply rail mechanical-intermittent short is the kind
    of fault that should cause no damage to a power supply. No small smd
    film resistors are really appropriate in series with larger caps - but
    below 220uF and at low voltage, is less than 100mJ per pop.

    RL
     
  17. Anders F

    Anders F Guest

    Whart size are you using for what cap sizes? Any tests done?
    I actually also considered using a 0805 ferrite. But I'm not quite sure it
    will better the EMC on the LDO input anyway...

    /Anders
     
  18. Greg Neff

    Greg Neff Guest

    We have thousands of boards in the field using this approach. Our
    boards are put through extensive testing because they are used in rail
    vehicle applications. What we usually do is use 1 ohm 1206 resistors
    in series with 2.2 to 4.7 uF ceramics (X5R or better). We use
    multiples of these RC networks to get the desired capacitance and ESR.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  19. Andy

    Andy Guest

    What about L4931CZ50?
     
  20. Andy

    Andy Guest

    The datasheet for REG1117 specifies only the upper boundary on the ESR
    and the lower boundary on the capacity. Does it mean that one can add
    the ceramic caps on the output rail without a stability problem, once
    the base 10uF ESR<0.5 Ohm tantalum capacitor is there?

    -- Andy
     
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