DN275 - Single Inductor, Tiny Buck-Boost Converter Provides 95% Efficiency in Lithium-Ion to 3.3V Applications

Single Inductor, Tiny Buck-Boost Converter Provides 95%
Efficiency in Lithium-Ion to 3.3V Applications – Design Note 275
Mark Jordan
Introduction
In portable applications powered by a single lithium-ion
cell, the input voltage can typically change from 4.2V
initially, down to 2.5V at end of life. It is a challenging
task to provide a regulated voltage within the range of
the battery. Until now, the most popular solution has
been the SEPIC converter, but its mediocre efficiency
and requirement of both a coupled inductor and a high
current flyback capacitor make it a less than optimal
solution. Another option is to cascade a Boost converter with either an LDO or a Buck converter, but the
additional area and cost of the extra components, as
well as low efficiency, are major drawbacks. Linear
Technology’s new LTC ®3440 Buck-Boost converter
provides the most compact solution with the highest
efficiency, thereby reducing cost, increasing battery
life and saving precious real estate.
light loads, the part offers user controlled Burst Mode ®
operation to maximize battery life, drawing only 25μA
of quiescent current. The operating frequency can be
programmed from 300KHz to 2MHz by changing the
value of the timing resistor on the RT pin. Users can
synchronize the operating frequency by connecting an
external clock to the MODE/SYNC pin. The part can also
be commanded to shut down by pulling the SHDN/SS
pin low. In shutdown, the part draws less than 1μA of
quiescent current and disconnects the output from
the input supply. During start-up, the ramp rate of the
output voltage is controlled by the external soft-start
components. This controlled ramp rate provides for
inrush current limiting. Housed in a thermally enhanced 10-lead MSOP package, the LTC3440 is ideal
for portable power applications requiring less than 2W
of output power.
The LTC3440 incorporates a patent-pending control
technique to efficiently regulate an output voltage
above, below or equal to the input source voltage with
a single inductor by properly phasing the four internal
switches. Efficiencies well over 90% are achieved for
the entire battery range without the use of Schottky
rectifier diodes. The low RDS(ON) (0.19Ω NMOS, 0.22Ω
PMOS), low gate charge synchronous switches, along
with minimal break-before-make times, provide high
frequency, low noise operation with high efficiency. For
All Ceramic Capacitor, Single Inductor,
2W Li-Ion to 3.3V Converter
An all-ceramic capacitor, lithium-ion to 3.3V application
at 600mA is shown in Figure 1. The operating frequency
is programmed to 1MHz and soft-start is incorporated
with R4 and C3. The efficiency curves versus load current
for the Li-Ion battery range are shown in Figure 2. With
Burst Mode operation enabled at light loads, efficiencies
L1
10μH
VIN = 2.5V TO 4.2V
R4 1M
Li-Ion
SW1
SW2
LTC3440
6
VIN
VOUT
8
9
2
+
C3
0.1μF
C1
10μF
SD
R5
1k
*
1
RT
60k
*1 = Burst Mode OPERATION
0 = FIXED FREQUENCY
4
7
SHDN/SS
FB
MODE/SYNC
RT
VC
GND
R1
340k
C2
22μF
C4 330pF
10
5
R3
100k
90
Burst Mode OPERATION
80
70
60
50
VIN = 4.2V
VIN = 2.5V
VIN = 3.3V
40
30
R2
200k
20
10
fOSC = 1MHz
C1: TAIYO YUDEN JMK212BJ106MG
C2: TAIYO YUDEN JMK325BJ226MM
L1: SUMIDA CDRH4D28-100
DN275 F01
Figure 1. Simple Lithium-Ion to 3.3V Converter at 600mA
01/02/275_conv
100
VOUT
3.3V
600mA
EFFICIENCY (%)
3
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
0
0.1
1.0
10
100
OUTPUT CURRENT (mA)
1000
DN275 F02
Figure 2. Li-Ion to 3.3V Efficiency
switch looses gate drive at low output voltage. Figure 4
demonstrates the efficiency of the converter versus input
voltage at various load currents. The transient response
of the power converter for a 1.5V output voltage change,
commanded by the DAC, is shown in Figure 5.
100
98
VOUT = 3.4V
96
EFFICIENCY (%)
of over 85% are achieved for more than three decades
of load current. At 200μA, the efficiency remains above
70%, primarily due to the low 25μA quiescent current
in Burst Mode operation. In many applications the
decreased load demand on the converter is known by
the application and the converter can be commanded
to enter power saving Burst Mode operation by driving
the MODE/SYNC pin high.
WCDMA Dynamically Controlled Power Amp
Power Supply
For the new 3rd generation (3G) cellular phones, the
high speed data transmission imposes a stringent power
demand on the battery. Maximum overall efficiency
and operation over the entire battery voltage range are
required to maximize run time. A 2W, dynamically
controlled power supply for a WCDMA cell phone
power amplifier (PA) is shown in Figure 3. By adjusting
the voltage across the PA, the overall efficiency to the
antenna is improved, and a linear PA can be utilized. At
peak power, the PA requires the highest programmed
voltage, typically 3.4V to 4V depending on the PA. At
the lowest power level, when only voice is transmitted
and the user is close to the basestation, the PA draws
less than 100mA and requires a lower voltage, typically
between 0.4V to 2V. Since the LTC3440 can regulate
an output voltage above, below or equal to the battery
voltage, the maximum transmit power can be maintained
over the entire voltage range. For applications requiring a program voltage below 2V, a Schottky diode is
required from the SW2 to VOUT pins to provide a low
impedance power path since the internal synchronous
IOUT = 100mA
94
92
IOUT = 250mA
90
88
IOUT = 600mA
86
84
82
80
2.5
3
4
4.5
3.5
INPUT VOLTAGE (V)
5
DN275 F04
VOUT 1V/DIV
Figure 4. Efficiency of the WCDMA
Power Amp Power Supply
4.5V
2.6V
0V
50μs/DIV
4.5V
COMMAND
DN275 F05
2.6V
COMMAND
Figure 5. Output Voltage Transient Response
of the WCDMA Power Supply
VOUT = 3.3V – 1.7V (VPROG – 1.22)
DAC
L1
3.3μH
D1
C3
33pF
4
SW1
SW2
LTC3440
6
7
VIN
VOUT
3
VIN = 2.5V TO 4.2V
8
Li-Ion
+
C1
*
10μF
2
1
RT
30k
SHDN/SS
FB
MODE/SYNC
VC
RT
GND
R1
340k
9
10
R3 15k
C4 150pF
5
C5 10pF
VOUT
0.4V TO 5V
600mA
R5
10k
R6
200k
R2
200k
C2
10μF
fOSC = 1MHz
*1 = Burst Mode OPERATION C1, C2: TAIYO YUDEN JMK212BJ106MM
0 = FIXED FREQUENCY
D1: ON SEMICONDUCTOR MBRM120T3
L1: SUMIDA CDRH4D28-3R3
DN275 F03
Figure 3. WCDMA Power Amp Power Supply with Dynamic Voltage Control
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn275f_conv LT/TP 0102 371.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
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