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Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 898
Keywords: LDO, linear regulator, low dropout, regulators, PSRR, low noise, cellphone, cellular, handset,
RF, baseband, audio, GSM
APPLICATION NOTE 898
Selecting LDO Linear Regulators for Cellphone
Designs
Oct 01, 2002
Abstract: Low-dropout LDO linear regulators are used to power many sections of a typical cellular
handset. However, these baseband, RF, and audio sections have different requirements that influence
which LDO is most appropriate. After discussion of the specific requirements, different LDOs are
recommended. Also, some LDO design techniques are briefly discussed to demonstrate how an LDO
may be optimized for a specific level of performance.
Cellular phone designs require linear regulators with low-dropout, low-noise, high PSRR, low quiescent
current (Iq), and low-cost. They need to deliver a stable output and use small-value output capacitors.
Ideally, one device would have all these characteristics and one low-dropout linear regulator (LDO) could
be used anywhere in the phone without worry. But in practice, the various cell phone blocks are best
powered by LDOs with different performance characteristics. This application note provides guidance in
choosing the right LDO to power each cell phone block.
Baseband Chipset Power Supply
Most cellular phone baseband chipsets require power supplies for three circuit blocks: internal digital
circuitry, analog circuitry, and peripheral interface circuitry. BB internal digital circuits typically operate
from 1.8V to 2.6V. Since most phones shut off when the Li+ battery voltage falls to 3.2V to 3.3V, there is
usually at least 500mV to 600mV of headroom for the BB digital LDO, so dropout is not critical. Output
noise and the PSRR are not critical specs for the digital circuits. Instead, this supply requires low
quiescent current at light loads because this LDO stays on at all times. Figure 1 shows how the digital
supply current of a representative GSM chipset core (ADI AD20msp425) varies as a function of time. In
the standby mode, the microprocessor consumes only around 200µA. Since the phone stays in standby
for the longest percentage of time, using a 2µA quiescent current LDO, instead of 100µA LDO, saves
98µA and extends the standby time by 300µA /202µA, or 1.485 times.
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