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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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Figure 1. ADI AD20msp425 GSM chipset core power consumption profile.
The supply voltage of the BB internal analog circuits is typically 2.4V to 3.0V, and it requires 200mV to
600mV dropout. Also, the BB analog LDO needs to be optimized for low-frequency (217Hz for GSM
phones) ripple rejection in order to reject the battery voltage ripple caused by the RF power amplifier.
This LDO is on all the time, so it requires low quiescent current as well.
RF Power Supply
The RF circuit is divided into receive and transmit sections. These circuits typically require 2.6V to 3.0V
supply voltage. The RF circuits such as LNA (low-noise amplifier), up/downconverter, mixer, PLL, VCO,
and IF stage require low-noise and high PSRR LDOs. In particular, the VCO and PLL blocks' overall
performance affects the radio's critical specifications, such as spectral purity of the transmitter, selectivity
of the receiver, noise and hum in analog transceivers, and phase errors in digital circuits. Noise can alter
the oscillator's phase and amplitude characteristic and the oscillator's loop circuitry amplifies the noise.
Consequently, it may modulate the carrier. The LDO output noise can be reduced by effective LDO
internal design and external bypass and compensation. Figure 2 shows a simplified block diagram of the
PMOS linear regulator.
Figure 2. A simplified PMOS linear regulator block diagram.
The major contributor to LDO output noise is the reference. Figure 3 shows a typical bandgap reference
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noise spectrum.
Figure 3. Typical noise spectrum of a bandgap reference.
In order to reduce the reference noise, a low pass filter needs to be added on the reference output. This
can be done internally or externally, but the internal bypass may take a large die size. LDOs designed
for low-noise will often provide a pin for a reference bypass capacitor. Increasing the bypass capacitor
value will reduce the output noise up to a point where the reference noise becomes a minor noise
contributor in overall LDO circuits. Output noise vs. bypass capacitance characteristics in the LDO data
sheet will provide information for an optimum bypass capacitor value. Figure 4 shows output noise vs.
BP capacitance of the MAX8867.
Figure 4. Output noise vs. BP capacitance of the MAX8867.
Other factors affecting the LDO output noise are LDO internal poles, zeros, and the output dominant
pole. Figure 5 shows the typical noise spectrum of the PMOS linear regulator except the bandgap
reference. By increasing the output capacitor's value or reducing output load, the high-frequency output
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noise will be reduced.When choosing an LDO for an RF circuit, carefully compare the noise
specifications to make sure that the bypass capacitance, output capacitance, and load conditions are the
same.
Figure 5. Typical noise spectrum of PMOS LDO except the bandgap reference.
TCXO Power Supply
The temperature-compensated crystal oscillator generates a reference frequency which is used for IF
sections. It requires an ultra-low noise LDO with an on/off control pin. Even though the TCXO circuit
does not require more than 5mA, it uses a dedicated LDO and isolates the LDO's output from other
noise sources. It also requires very high PSRR at RF power amplifier burst frequency. For example,
minimum 65dB PSRR at 217Hz is recommended for the GSM handsets.
RTC Power Supply
The RTC LDO charges a capacitor-type backup coin cell or a rechargeable coin cell and supplies current
for a real-time clock (See Figure 6). 3V or 2.5V cells are the most common for cellular phones. The
RTC LDO needs a very low quiescent current, since this LDO is on all the time, even though the
handset is powered off. It also requires a reverse current protection with a low leakage current.
Figure 6. RTC and rechargeable coin cell.
Audio Power Supply
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The 300mA to 500mA high current LDO may be required for recent audio demands such as hands-free,
game, MP3, and multimedia applications in cellular phones. Also, this power supply requires low-noise
and high PSRR at audio frequency range (20Hz to 20kHz) in order to provide good audio quality. CDMA
RF transmit current is around 600mA peak, but GSM transmit burst current is 1.7A peak. Figure 7
shows how the GSM transmit burst current creates noise on the battery when the RF power amplifier
runs directly from the battery. Since all LDOs in the phones use the battery voltage for their input power
source, this ripple influences output quality of the regulators. For example, if the LDO's PSRR at 217Hz
is 40dB, we will have 4.35mV peak-to-peak, 217Hz noise on the LDO output based on Figure 7. The
microphone's AC audio level is around 10mV to 20mV, so this noise may cause a serious clicking noise.
Figure 7. GSM transmit burst waveform.
Maxim LDOs for Cellular Phone Applications
The following table summarizes the Maxim LDO product line suitable for the wireless phone applications.
Part numbers Package
MAX8863
MAX8864
MAX8867
MAX8868
Dropout
SOT23-5 55mV at 50mA
SOT23-5 55mV at 50mA
Ground
current
Output
noise
PSRR
68µA
350µVRMS
at 10Hz–
1MHz,
C OUT =
1µF
62dB at
300Hz
85µA
30µVRMS
at 10Hz–
Audio
100kHz, > 60dB at
BB Analog
C OUT =
10Hz–
BB Digital
100kHz
10µF,
RF Circuit
C BYP =
0.01µF
Application
area
Remark
BB Digital 350µVRMS
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MAX8873
MAX8874
(Auto
Discharge)
MAX8877
MAX8878
(Auto
Discharge)
MAX8875
MAX8885
(Auto
Discharge)
MAX8887
MAX8882
MAX1725
MAX1726
MAX8880
MAX8881
SOT23-5 55mV at 50mA
SOT23-5 55mV at 50mA
SOT23-5 55mV at 50mA
Thin
SOT23-5
SOT23-6
SOT23-5
SOT23-5
100mV at
200mA
72mV at 80mA
300mV at
20mA
100mV at
50mA
at 10Hz–
1MHz,
C OUT =
1µF
60dB at
300Hz
BB Digital 85µA at
no load
30µVRMS
at 10Hz–
100kHz,
C OUT =
10µF,
C BYP =
0.01µF
60dB at
300Hz
Audio
BB Analog
BB Digital
RF Circuit
85µA at
no load
30µVRMS
at 10Hz–
100kHz,
C OUT =
10µF,
C BYP =
0.01µF
60dB at
300Hz
Audio
BB Analog
BB Digital
RF Circuit
55µA at
no load
60dB
42µVRMS
below
at 10Hz–
Audio
100kHz, 1kHz with
BB Analog
C OUT =
C OUT =
BB Digital
2.2µF,
2.2µF,
RF Circuit
C BYP =
C BYP =
0.01µF
0.01µF
165µA at
no load
40µV RMS
at 10Hz–
100kHz,
COUT =
4.7µF,
CBYP =
0.01µF,
I OUT =
1mA
2µA at
no load
350µVRMS
at 10Hz–
100kHz,
C OUT =
4.7µF,
I OUT =
1µA
45dB at
300Hz
RTC Circuit 3.5µA at
no load
300µVRMS
at 10Hz–
100kHz,
C OUT =
4.7µF,
I OUT =
1µA
45dB at
300Hz
BB Digital
RTC circuit
73µA at
no load
62dB at
BB Analog
300Hz
BB Digital
56dB up to
RF Circuit
100kHz
300mA
Dual LDO
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Related Parts
MAX1725
12V, Ultra-Low-IQ , Low-Dropout Linear Regulators
MAX1726
12V, Ultra-Low-IQ , Low-Dropout Linear Regulators
MAX8873
Low-Dropout 120mA Linear Regulators
Free Samples MAX8874
Low-Dropout 120mA Linear Regulators
Free Samples MAX8875
150mA, Low-Dropout Linear Regulator with Power OK
Output
Free Samples MAX8877
Low-Noise, Low-Dropout, 150mA Linear Regulators with
'2982 Pinout
Free Samples MAX8878
Low-Noise, Low-Dropout, 150mA Linear Regulators with
'2982 Pinout
Free Samples MAX8880
12V, Ultra-Low-IQ , Low-Dropout Linear Regulators with
POK
Free Samples MAX8881
12V, Ultra-Low-IQ , Low-Dropout Linear Regulators with
POK
Free Samples MAX8882
Dual, Low-Noise, Low-Dropout, 160mA Linear Regulators
in SOT23
Free Samples MAX8885
150mA, Low-Dropout Linear Regulator with Power OK
Output
Free Samples MAX8887
Low-Dropout, 300mA Linear Regulators in SOT23
Free Samples More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 898: http://www.maximintegrated.com/an898
APPLICATION NOTE 898, AN898, AN 898, APP898, Appnote898, Appnote 898
Copyright © by Maxim Integrated Products
Additional Legal Notices: http://www.maximintegrated.com/legal
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