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Analog Devices just brought out several LDO regulators claiming double the power supply rejection ratio of competing devices, and targeting these devices at battery powered devices.

However, I thought that batteries were quite noiseless, as opposed to powering up your device via a switchmode power supply where (i would have thought) a very good PSRR would be handy.

Why would PSRR be a crucial factor in battery powered systems?

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The reason why PSR is important even in battery supplied devices is that one can (and usually) have several different kinds of circuits supplied from the same battery in the same device. Due to internal resistance of the battery, changes in the current consumption of the one part of the device will be seen as voltage variation for all the circuitry supplied by thereof.

One dramatic example is GSM phone, where power amplifier for transmitter will consume significant amount of current (amps range) every time the frame is transmitted with almost no current consumption in between frames. Assuming* 0.5 Ohm internal resistance and 1A consumption that means 500mV voltage variation repeating several hundreds times a second.

*) those are example numbers, too lazy to check actual GSM numbers

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  • \$\begingroup\$ Good decoupling should help with this, but it never hurts if one device is pulling very high current compared to the others to avoid this. \$\endgroup\$ – Kortuk Feb 3 '11 at 23:32
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An LDO may not be powered directly from the battery. Often the battery goes to a switching regulator that's very efficient at getting the battery voltage down to the main power rail (say 3.3V), good enough for digital but a bit noisy for sensitive circuits, especially RF and probably audio. So an LDO would be added for each sensitive subsection to re-regulate the main rail to a slightly lower voltage that's much cleaner. The linear regulator doesn't have to throw away the whole voltage difference between the battery and the main rail as efficiency loss.

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