App note: Low drop out, phase margin, and stability

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Old app note from Fairchild Semiconductor about external component effects on LDOs. Link here (PDF!)

Low drop-out (LDO) regulators are one of the basic building blocks of most power supplies used in today’s electronics. Their ease of use, low cost, and small size makes them ideal in portable and handheld applications. They are available in wide variety of voltage & current levels with their primary function, to provide power to low-voltage digital circuits. The only other necessary components are external capacitances, which, in conjunction with the LDO, create a complete power solution. This application note reviews the effect of these components and their phase shifts on the LDO’s loop stability and show how to tailor the compensation to obtain a stable circuit.

 

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3 Comments

  1. This is valid for older LDO types, which use external compensation and whose datasheets either tells you to (a) use electrolytic or tantalum at the output or (b) keep the output ESR at 0.1-10ohm. Example: LP2951. No point for hobbyists to buy these any more.
    Most newer LDO parts have some form of internal compensation and can work with zero ESR, for use with SMD ceramic caps with very low ESR. Some don’t even need output capacitance.
    Rummaging through my pile of PDFs, here are some useful papers to find and read:
    (a) A User’s Guide to Compensating Low-Dropout Regulators
    (b) AN-1482 LDO Regulator Stability Using Ceramic Output Capacitors
    (c) SLVA079 Understanding LDO Regulators
    (d) SLVA115 ESR, Stability, and the LDO Regulator

  2. Another troublesome thing about LDOs are MCUs with integrated LDOs, many Microchip PIC parts (and others) do this, you will find a Vcap pin (or sometimes disguised as Vusb, huh).
    A lot of the older Microchip MCUs of this type _seems_ to be of the older LDO kind, since they tend to recommend tantalum caps, and while newer parts talk ceramics too, I would rather they just put it down crystal-clearly and give a stability chart. Drives me nuts trying to parse some of these datasheets…

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