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 ther-
mally enhanced TSSOP package.
n
n
n
n
n
n
n
n
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
n
n
n
n
n
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
V
IN
PV
IN2
SW2
SW1A
SW1B
V
OUT1
FB2
LTC3521
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
OFF
BURST
PWM
FB1
SW3
SHDN1
FB3
SHDN2
SHDN3
PGOOD1
PWM
PGOOD2
PGND1A
PGOOD3
PGND1B GND PGND2
V
OUT3
1.2V
10µF 600mA
3521 TA01a
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
3521fb
For more information
www.linear.com/LTC3521
1
LTC3521
absoluTe MaxiMuM raTings
(Note 1)
PV
IN1
, PV
IN2
, V
IN
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
(Notes 2, 5) ............................................ –40°C to 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
PGND1A
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
PGND1A
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 PGND1A 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 PGND1A AND MUST BE SOLDERED TO PCB GROUND
orDer inForMaTion
LEAD FREE FINISH
LTC3521EFE#PBF
LTC3521IFE#PBF
LTC3521EUF#PBF
LTC3521IUF#PBF
TAPE AND REEL
LTC3521EFE#TRPBF
LTC3521IFE#TRPBF
LTC3521EUF#TRPBF
LTC3521IUF#TRPBF
PART MARKING*
LTC3521FE
LTC3521FE
3521
3521
PACKAGE DESCRIPTION
20-Lead Plastic TSSOP
20-Lead Plastic TSSOP
24-Lead (4mm × 4mm) Plastic QFN
24-Lead (4mm × 4mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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/
2
3521fb
For more information
www.linear.com/LTC3521
LTC3521
elecTrical characTerisTics
PARAMETER
Input Voltage
Quiescent Current—Shutdown
Burst Mode Quiescent Current
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
A
= 25°C (Note 2). V
IN
, PV
IN1
, PV
IN2
= 3.6V, V
OUT1
= 3.3V, unless
otherwise noted.
CONDITIONS
l
MIN
1.8
l
TYP
0.01
30
MAX
5.5
2
1.35
0.4
UNITS
V
µA
µA
MHz
V
V
V
µA
Ω
Ω
V
SHDN1
= V
SHDN2
= V
SHDN3
= 0V (Note 6)
V
FB1
= 0.66V, V
FB2
= 0.66V, V
FB3
= 0.66V, V
PWM
= 0V
l
l
l
0.85
1.4
1.1
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 tested under pulsed load conditions such that
T
J
≈
T
A
. The LTC3521E is guaranteed to meet performance specifica-
tions from 0°C to 85°C. Specifications over the –40°C to 125°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls. The LTC3521I is guaranteed
over the full –40°C to 125°C operating junction temperature range. The
maximum ambient temperature is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors.
3521fb
For more information
www.linear.com/LTC3521
3
LTC3521
elecTrical characTerisTics
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
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