AN1424: Extending Cell Phone Battery Life with the ISL9109

Extending Cell Phone Battery Life
with the ISL9109
®
Application Note
In this day and age practically everyone is armed with a cell
phone. Some of these smart phones not only work as a cell
phone but have enough processing power that they are
potentially compete with a notebook computer for surfing the
web; with a digital camera for taking pictures; with an MP3
player for listening to music; with hardcopy books by
allowing downloaded electronic version; with cash and credit
cards for paying bills – the possibilities are endless.
All these applications for the cell phone have put extreme
efficiency requirements from a systems point of view. Since
the cell phone is a portable device powered by a single Li-ion
cell, every aspect of the cell phone has to be optimized from
a power point of view. This application note will focus on
optimizing power to the power amplifier which is the heart of
a cell phone. In the past most of the power amplifier were
powered by a low dropout regulator. But now to keep up with
the current applications, the power amplifiers have become
real power hungry to maintain a certain signal-to-noise ratio.
In some GSM (global system for mobile system) handsets,
the power amplifier can draw up to approximately 40% of the
stored battery energy.
GSM900 is widely used in most parts of the world and uses
890MHz to 915MHz to send information to the base station
(uplink) and 935MHz to 960MHz to receive information from
the base station (downlink). This allows 124 RF-channels
spaced at 200kHz. In a typical GSM network, the handset
does not communicate with the base station at full RF power
level all the time as seen from the below GSM900 probability
density function which shows probability of use vs
transmitted RF power.
FIGURE 1. CLASS 4 GSM PROBABILITY DENSITY
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POUT in Figure 1, is the RF power generated by the power
amplifier (PA) for data communications and is usually directly
proportional to the PA input voltage. This very requirement
forms the basis for this application note for using the
ISL9109 DC/DC buck regulator as a power supply for the
power amplifier. Due to cost reason, traditionally PA
manufacturers have used a low dropout linear regulator to
power the PA, but now are migrating towards using a buck
regulator to optimize battery life. The efficiency of an LDO is
VO/VBAT which for a 1.8VOUT from a fully charged 4.2V
Lithium Ion battery is approximately 43%. On the other hand,
the ISL9109 buck regulator under the same test conditions is
approximately 89% efficient.
The schematic in Figure 2 shows a complete PA buck
regulator power supply. The input voltage on the ISL9109
can be from 2.7V to 5.5V, but in this case the input is limited
by the voltage range of a single Li-ion cell which is typically
3V to 4.2V. The part has internal synchronous MOSFETS
which can support 1.5A continuous load current and
switches at 1.6MHz and still achieves 95% efficiency. The
higher switching frequency saves board space by requiring
smaller inductor and capacitor on the output. The part also
has an EN pin which allows a nanoamp shutdown drawing
only 50nA. It also has a mode pin which allows PFM mode to
achieve higher efficiency at light loads (<50mA) by tying this
pin to the input supply; or PWM mode by connecting this pin
to ground. The constant PWM mode results in constant
switching frequency of 1.6MHz and is independent of load
variation. The part also has a STBY pin which allows the
output voltage to reach steady state in less than 60µs. The
part enters this mode by asserting STBY pin approximately
1ms before the part is Enabled. This mode can easily be
programmed with a simple RC network or a pre-transmit
signal from the baseband chip to pull the Stby pin high
approximately 1ms before enabling the part. This feature
allows keeping the ISL9109 in nanoamp shutdown mode
until the PA is ready to transmit data, hence conserving
battery life. Since the PA transmits at different output
voltages, this can easily be achieved by connecting a
resistor R3 from FB to a DAC output of 0V to 1V from the
baseband chip. The relationship between PA output and
DAC voltage is given in Equation 1 and plotted in Figure 3. It
can be seen that VPA is 3.4V when VDAC = 0V and 1.5V
when VDAC is 1V. Equation 1 can easily be used to program
different output voltages from a different DAC voltage by
using different values for R1, R2, and R3. Figure 4 shows the
efficiency for 1.8VPA from VBAT = 2.7V and 3.3V; Figure 5
shows the start-up time for VPA = 1.5V to be around 50µs;
Figure 6 shows the transient response for a 0A to 1A load
step on the PA when VBAT = 3.6V.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2008. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
Application Note 1424
2µH
VBAT
SW
VIN
L1
STBY
10µF
C3
10µF
C1
ISL9109
10kΩ
R4
VPA
190kΩ
R1
NC
100pF
C2
FB
EN
140.8kΩ
R2
MODE
0.1µF
C4
GND
100kΩ
VDAC
R3
FIGURE 2. ISL9109 PROGRAMMABLE PA POWER SUPPLY SCHEMATIC
R 1 R 1⎞
⎛
⎛ R 1⎞
V PA = V FB∗ ⎜ 1 + ------- + -------⎟ – V DAC∗ ⎜ -------⎟
R 2 R 3⎠
⎝
⎝ R 3⎠
(EQ. 1)
100
DAC CONTROL OF PA POWER
3.50
VBAT = 2.7V
90
EFFICIENCY (%)
VPA
3.00
2.50
2.00
80
VBAT = 3.3V
70
60
1.50
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
VDAC
FIGURE 3. VPA vs VDAC
2
0.8
0.9
1.0
50
0
250
500
750
1000
LOAD CURRENT (mA)
1250
1500
FIGURE 4. ISL9109 TYPICAL APPLICATION SCHEMATIC
EFFICIENCY
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Application Note 1424
FIGURE 5. ISL9109 VPA = 1.5V START-UP WAVEFORM
VSW
2V/DIV
VOUT (AC-COUPLED
100mV/DIV
IL
1A/DIV
100µs/DIV
FIGURE 6. VPA TRANSIENT RESPONSE AT 1.5V FOR A 0A TO 1A LOAD STEP
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Application Note 1424
ISL9109 Powers Anadigics AWE6174
Power Amplifier*
Figure 7 shows the ISL9109 providing power to Anadigics
AWE6174 Quad-Band GSM/GPRS/Polar Edge Power
Amplifier. Some other suppliers have a buck regulator and
an integrated bypass switch which they turn-on when the
baseband chip decides to transmit at full power. The
purpose for having this bypass switch is to allow the input
battery voltage to drop further down and still be able to
provide 3.4V on the output which is the PA voltage for
transmitting at 33dbm. The rDS(ON) of the bypass switch is
usually 75mΩ which is much less than the rDS(ON) of the
high side buck regulator and the DC resistance of the buck
inductor. For the ISL9109 the rDS(ON) is 150mΩ and the
inductor resistance is 100mΩ. The AWE6174 power
amplifier does not benefit from having an internal bypass
switch since it has a low dropout LDO in parallel which the
baseband chip enables when it wants to transmit at full
power. To empirically see the advantage of using a buck
regulator over the internal LDO, efficiency data was taken on
the AWE6174 PA operating at certain dBm in GSM 900
mode powered with the ISL9109 vs with its internal low
dropout regulator as shown in Figure 8. It can be seen from
Figure 9 that at 29dBm and 27dBm where the handsets
spend most of its time (~40%), the average current
consumption weighted by PDF is reduced by 40% at the
former and by 23% at the latter power level. This average
input current reduction results in about 20% gain in talk time
from 5 hours to 6 hours for 850mA-Hr battery in a typical
phone application.
*Data provided by Anadigics
ISL9109
FIGURE 7. ISL9109 POWERS ANADIGICS AWE6174 PA
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Application Note 1424
GSM900 EFFICIENCY OF AWE6174+ISL9109
60
55
50
45
35
Ch37_w DC/DC
30
25
EFFICIENCY (%)
40
20
15
Ch37_w/o DC/DC
10
5
6.5
8.5
10.5
12.5
14.5
16.5
18.5
20.5
22.5
24.5
26.5
28.5
30.5
32.5
34.5
0
POWER [dBm]
FIGURE 8. OVERALL SYSTEM EFFICIENCY FOR ISL9109 vs LDO
AVERAGE CURRENT CONSUMPTION WEIGHTED BY PDF AWE6174+ISL9109
30
W/O DC/DC
WITH DC/DC
AVERAGE CURRENT * PDF(mA)
25
20
15
10
5
0
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
POWER (dBm)
FIGURE 9.
AVERAGE INPUT CURRENT REDUCTION BY USING ISL9109 INCREASES TALK TIME BY 20%
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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