LTC3521
Wide V
IN
, 1A Buck-Boost
DC/DC and Dual 600mA
Buck DC/DC Converters
FeaTures
n
DescripTion
The LTC
®
3521 combines a 1A buck-boost DC/DC converter
and dual 600mA synchronous buck DC/DC converters. The
1.1MHz switching frequency minimizes the solution foot-
print while maintaining high efficiency. All three converters
feature soft-start and internal compensation to minimize
the solution footprint and simplify the design process.
The buck converters are current mode controlled and
utilize an internal synchronous rectifier to improve ef-
ficiency. The buck converters support 100% duty cycle
operation to extend battery life. If the PWM pin is held
low, the buck converters automatically transition from
Burst Mode operation to PWM mode at high loads. With
the PWM pin held high, the buck converters remain in low
noise, 1.1MHz PWM mode.
The buck-boost converter features continuous conduction
operation to maximize efficiency and minimize noise. At
light loads, the buck-boost converter can be operated in
Burst Mode operation to improve efficiency and reduce
no-load standby current.
The LTC3521 provides a < 2μA shutdown mode, over-
temperature shutdown and current limit protection
on all converters. The LTC3521 is available in a 24-pin
0.75mm
×
4mm
×
4mm QFN package, and a 20-pin
thermally enhanced TSSOP package.
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Three High Efficiency DC/DC Converters:
Buck-Boost (V
OUT
: 1.8V to 5.25V, I
OUT
: 1A)
Dual Buck (V
OUT
: 0.6V to V
IN
, I
OUT
: 600mA)
1.8V to 5.5V Input Voltage Range
Pin-Selectable Burst Mode
®
Operation
30µA Total Quiescent Current in Burst Mode
Operation
Independent Power Good Indicator Outputs
Integrated Soft-Start
Thermal and Overcurrent Protection
<2µA Current in Shutdown
Small 4mm
×
4mm QFN and Thermally Enhanced
TSSOP Packages
applicaTions
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Bar Code Readers
Medical Instruments
Handy Terminals
PDAs, Handheld PCs
GPS Receivers
L,
LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
and PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U. S. Patents, including 6404251, 6166527.
Typical applicaTion
V
IN
2.4V TO 4.2V
+
Li-Ion
4.7µF
4.7µH
PV
IN1
SW1A
SW1B
V
OUT1
PV
IN2
SW2
FB2
4.7µH
137k
68.1k
4.7µH
100k
100k
EFFICIENCY (%)
V
OUT1
3.3V
800mA
(1A, V
IN
> 3.0V)
V
OUT2
1.8V
10µF 600mA
22µF
1.0M
221k
ON
LTC3521
FB1
SHDN1
SHDN2
SHDN3
PWM
SW3
FB3
PGOOD1
PGOOD2
PGOOD3
PGND1 GND PGND2
3521 TA01a
OFF
V
OUT3
1.2V
10µF 600mA
BURST
PWM
100
98
96
94
92
90
88
86
84
82
80
78
76
74
72
70
2.4
Efficiency vs V
IN
V
OUT1
= 3.3V
I
OUT
= 500mA
V
OUT2
= 1.8V
I
OUT
= 200mA
V
OUT3
= 1.2V
I
OUT
= 200mA
3.4
V
IN
(V)
4.4
5.4
3521 TA01b
3521f
LTC3521
absoluTe MaxiMuM raTinGs
(Note 1)
PV
IN1
, PV
IN2
Voltage .................................... –0.3V to 6V
SW1A, SW1B, SW2, SW3 Voltage
DC............................................................ –0.3V to 6V
Pulsed < 100ns ........................................... –1V to 7V
Voltage, All Other Pins ................................. –0.3V to 6V
Operating Junction Temperature Range
(Note 2).................................................... –40°C to 85°C
Maximum Junction Temperature (Note 5)............. 125°C
Storage Temperature Range................... –65°C to 150°C
pin conFiGuraTion
PGND1A
TOP VIEW
FB2
FB3
FB2
SHDN2
PGOOD3
PGOOD2
PGOOD1
V
IN
GND
PWM
1
2
3
4
5
6
7
8
9
21
GND
20 PV
IN2
19 SW2
18 PGND2
17 SW3
16 V
OUT1
15 SW1A
14 SW1B
13 PV
IN1
12
SHDN1
11
SHDN3
SHDN2
1
PGOOD3 2
PGOOD2 3
PGOOD1 4
V
IN
5
GND 6
7
PWM
8
FB1
9 10 11 12
PGND1B
SHDN3
SHDN1
PV
IN1
25
GND
TOP VIEW
PV
IN2
SW2
FB3
NC
18 PGND2
17 SW3
16 V
OUT1
15 SW1A
14 SW1B
13 NC
24 23 22 21 20 19
FB1 10
FE PACKAGE
20-LEAD PLASTIC TSSOP
T
JMAX
= 150°C,
θ
JA
= 40°C/W (NOTE 4)
UNDERSIDE METAL INTERNALLY CONNECTED TO V
–
(PCB CONNECTION OPTIONAL)
EXPOSED PAD (PIN 21) IS GND AND MUST BE SOLDERED TO PCB GROUND
UF PACKAGE
24-LEAD (4mm 4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 37°C/W
EXPOSED PAD (PIN 25) IS GND AND MUST BE SOLDERED TO PCB GROUND
orDer inForMaTion
LEAD FREE FINISH
LTC3521EUF#PBF
LTC3521EFE#PBF
TAPE AND REEL
LTC3521EUF#TRPBF
LTC3521EFE#TRPBF
PART MARKING
3521
LTC3521FE
PACKAGE DESCRIPTION
24-Lead (4mm
×
4mm) Plastic QFN
20-Lead Plastic TSSOP
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
elecTrical characTerisTics
PARAMETER
Input Voltage
Quiescent Current—Shutdown
Burst Mode Quiescent Current
The
l
denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at T
J
= 25°C. V
IN
= 3.6V, V
OUT1
= 3.3V, unless otherwise noted.
CONDITIONS
l
MIN
1.8
l
TYP
0.01
30
MAX
5.5
2
UNITS
V
µA
µA
3521f
V
SHDN1
= V
SHDN2
= V
SHDN3
= 0V
V
FB1
= 0.66V, V
FB2
= 0.66V, V
FB3
= 0.66V, V
PWM
= 0V
LTC3521
elecTrical characTerisTics
PARAMETER
Oscillator Frequency
SHDN1, SHDN2, SHDN3,
PWM Input High Voltage
SHDN1, SHDN2, SHDN3,
PWM Input Low Voltage
Power Good Outputs Low Voltage
Power Good Outputs Leakage Current
Buck Converters
PMOS Switch Resistance
NMOS Switch Resistance
NMOS Switch Leakage Current
PMOS Switch Leakage Current
Feedback Voltage
Feedback Input Current
PMOS Current Limit
Maximum Duty Cycle
Minimum Duty Cycle
PGOOD Threshold
Power Good Hysteresis
Buck-Boost Converter
Output Voltage
PMOS Switch Resistance
NMOS Switch Resistance
NMOS Switch Leakage Current
PMOS Switch Leakage Current
Feedback Voltage
Feedback Input Current
Average Current Limit
Reverse Current Limit
Maximum Duty Cycle
Minimum Duty Cycle
PGOOD Threshold
Power Good Hysteresis
V
SW1A
= V
SW1B
= 5.5V, V
IN
= 5.5V
V
SW1A
= V
SW1B
= 0V, V
IN
= 5.5V
(Note 4)
V
FB1
= 0.6V
(Note 3)
(Note 3)
V
FB1
= 0.55V
V
FB1
= 0.66V
V
FB1
Falling
V
FB1
Returning Good
l
l
l
l
l
The
l
denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at T
J
= 25°C. V
IN
= 3.6V, V
OUT1
= 3.3V, unless otherwise noted.
CONDITIONS
l
l
l
MIN
0.85
1.4
TYP
1.1
MAX
1.35
0.4
UNITS
MHz
V
V
V
µA
Ω
Ω
I
PGOOD1
= I
PGOOD2
= I
PGOOD3
= 1mA
V
PGOOD1
= V
PGOOD2
= V
PGOOD3
= 5.5V
0.1
0.1
0.205
0.170
0.2
10
V
SW2
= V
SW3
= 5.5V, V
IN
= 5.5V
V
SW2
= V
SW3
= 0V, V
IN
= 5.5V
(Note 4)
V
FB2
= V
FB3
= 0.6V
(Note 3)
V
FB2
= V
FB3
= 0.55V
V
FB2
= V
FB3
= 0.66V
V
FB2,3
Falling
V
FB2,3
Returning Good
1.8
l
l
l
l
0.1
0.1
0.585
750
100
0.6
1
1050
5
10
0.612
50
µA
µA
V
nA
mA
%
0
–12
–9
2
5.25
0.110
0.085
0.1
0.1
0.585
1.65
85
–12
0.6
1
2.1
375
94
0
–9
3
–6
5
10
0.612
50
–6
%
%
%
V
Ω
Ω
µA
µA
V
nA
A
mA
%
%
%
%
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LTC3521 is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls.
Note 3:
Current measurements are performed when the LTC3521 is not
switching. The current limit values in operation will be somewhat higher
due to the propagation delay of the comparators.
Note 4:
The LTC3521 is tested in a proprietary test mode that connects
each FB pin to the output of the respective error amplifier.
Note 5:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
3521f
LTC3521
Typical perForMance characTerisTics
Buck-Boost Efficiency,
Li-Ion to 3.3V
100
90
80
EFFICIENCY (%)
60
Burst Mode
40 OPERATION
30
20
10
0
0.1
1
Burst Mode
POWER LOSS
100
10
LOAD CURRENT (mA)
50
EFFICIENCY (%)
70
100
80
60
40
20
0
1000
3521 G01
T
A
= 25°C, unless otherwise noted.
Buck Efficiency, Li-Ion to 2.5V
140
120
POWER LOSS (mW)
100
90
80
70
60
50
40
30
20
10
0
0.1
1
100
10
LOAD CURRENT (mA)
Burst Mode
POWER LOSS
V
IN
= 3.6V
V
IN
= 4.2V
Burst Mode
OPERATION
140
PWM MODE
120
POWER LOSS (mW)
100
80
60
40
20
0
1000
3521 G02
V
IN
= 2.7V
V
IN
= 4.2V
PWM MODE
Buck Efficiency, Li-Ion to 1.8V
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.1
1
100
10
LOAD CURRENT (mA)
Burst Mode
POWER LOSS
V
IN
= 2.7V
V
IN
= 4.2V
Burst Mode
OPERATION
PWM MODE
140
120
100
80
60
40
20
0
1000
3521 G03
Buck Burst Mode Current Threshold
60
50
LOAD CURRENT (mA)
40
30
20
10
0
1.5
V
OUT
= 1.2V
POWER LOSS (mW)
V
OUT
= 1.8V
V
OUT
= 2.5V
2
2.5
3
3.5
V
IN
(V)
4
4.5
5
5.5
3521 G04
160
Buck-Boost Switches R
DS(ON)
350
300
250
R
DS(ON)
(m )
200
150
100
50
0
20
40
60
80
100 120
3521 G05
Buck Switches R
DS(ON)
V
IN
= 3.6V
PMOS
NMOS
Switching Frequency
vs Temperature
1.0
0.8
0.6
CHANGE FROM 25°C (%)
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
V
IN
= 3.6V
140 V
OUT1
= 3.3V
PMOS
(SWITCHES A AND D)
NMOS
(SWITCHES B AND C)
120
R
DS(ON)
(m )
100
80
60
40
20
0
–40 –20
0
–40 –20
0
20
40
60
80
100 120
3521 G06
TEMPERATURE (°C)
TEMPERATURE (°C)
–1.0
–50 –30 –10 10 30 50 70
TEMPERATURE (°C)
90 110
3521 G07
3521f
LTC3521
Typical perForMance characTerisTics
CHANGE IN FEEDBACK VOLTAGE FROM 25°C (%)
2.0
1.5
CHANGE FROM V
IN
= 3.6V (%)
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
1.8
2.3
2.8
3.3
3.8
4.3
4.8
5.3
3521 G08
T
A
= 25°C, unless otherwise noted.
Buck Feedback Voltage
vs Temperature
CHANGE IN FEEDBACK VOLTAGE FROM 25°C (%)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–50
–25
0
25
50
75
100
125
3521 G10
Switching Frequency vs V
IN
Buck-Boost Feedback Voltage
vs Temperature
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–40 –20
0
20
40
60
80
100 120
3521 G09
V
IN
(V)
TEMPERATURE (°C)
TEMPERATURE (°C)
33
Burst Mode Quiescent Current
vs V
IN
ALL THREE CONVERTERS ENABLED
MAXIMUM LOAD CURRENT (mA)
Buck-Boost Maximum Load
Current, Burst Mode Operation
90
80
70
LOAD CURRENT (mA)
60
50
40
30
20
10
V
OUT
= 5V
V
OUT
= 3V
1500
1300
1100
900
700
500
300
2.3
2.8
3.3
3.8
4.3
4.8
5.3
3521 G12
Buck-Boost Maximum Load
Current, PWM Mode
L = 4.7µH
QUIESCENT CURRENT (µA)
31
V
OUT
= 3.3V
29
V
OUT
= 5V
27
25
1.8
2.3
2.8
3.3
3.8
4.3
4.8
5.3
3521 G11
0
1.8
100
1.8
2.3
2.8
V
IN
(V)
V
IN
(V)
3.3 3.8
V
IN
(V)
4.3
4.8
5.3
3521 G13
60
No Load Quiescent Current
vs V
IN
Buck-Boost Load Step,
0mA to 750mA
V
IN
= 3.6V, V
OUT
= 3.3V
L = 4.7µH
C
OUT
= 22µF
QUIESCENT CURRENT (µA)
55
V
OUT
100mV/DIV
50
INDUCTOR
CURRENT
500mA/DIV
100µs/DIV
40
1.8
2.3
2.8
3.3
3.8
4.3
4.8
5.3
3521 G14
3521 G15
45
V
IN
(V)
3521f
嵌入式系统
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