App note: How to successfully apply low dropout regulators

Posted on Sunday, August 30th, 2015 in app notes by DP


Application note regarding proper usage of LDO from Analog Devices. Link here (PDF)

A low dropout (LDO) regulator is capable of maintaining its specified output voltage over a wide range of load currents and input voltages, down to a very small difference between input and output voltages. This difference, known as the dropout voltage or headroom requirement, can be as low as 80 mV at  2 A.
Current portable devices often require up to 20 low dropout linear regulators. Many of the LDOs in today’s portable devices are integrated into multifunction power management ICs (PMICs), highly integrated systems with many power domains for audio, battery charging, housekeeping, lighting, communications, and other functions.
As portable systems rapidly evolve, however, the integrated PMIC cannot keep up with peripheral power requirements. Dedicated LDOs must be added in the later stages of system development to power such optional items as camera modules, Bluetooth®, WiFi®, and other bolt-on functions. LDOs have also been used to reduce noise, to solve voltage-regulation problems caused by electromagnetic interference (EMI) and printed circuit board (PCB) routing, and to improve system power efficiency by switching off unneeded functions.
This application note reviews the basic LDO topology, explains key specifications, and shows the application of low dropout voltage regulators in systems.

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One Response to “App note: How to successfully apply low dropout regulators”

  1. KH says:

    This is useful, but the operation of the LDO regulator is described in a hand-waving sort of way and the reader is only advised to follow manufacturers’ datasheets. “Trust us”, eh. Hmmm…
    Assuming they are talking about PFET (and less commonly, PNP) LDO regulators, and noting that they recommend X5R and X7R caps at the output, this means that they imply modern LDO regulators with internal compensation that does not oscillate when paired with low internal resistance ceramic caps (usually multilayer ceramic SMD types) at the output.
    DO NOT follow this guide and then buy cheap older generation PFET/PNP LDO regulators. Older LDO regulators need adequate resistance at the output cap, else there might be oscillations, and that usually means electrolytic or tantalum caps. Or you need to do external compensation. So better check the part docs and make sure they are ceramic cap capable or internally compensated, why worry about things like feedback loop stability…

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