LTC3523_15 [Linear]
Synchronous 600mA Step-Up and 400mA Step-Down DC/DC Converters;
| 型号： | LTC3523_15 |
| 厂家： | 
Linear |
| 描述： | Synchronous 600mA Step-Up and 400mA Step-Down DC/DC Converters |
| 文件： | 总16页 (文件大小：263K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3523/LTC3523-2
Synchronous 600mA Step-Up
and 400mA Step-Down
DC/DC Converters
FEATURES
DESCRIPTION
The LTC®3523/LTC3523-2 combine a 600mA step-up
DC/DC converter with a 400mA synchronous step-down
DC/DC converter in a tiny 3mm × 3mm package. The
1.2MHz/2.4MHz switching frequencies minimize the
solution footprint while maintaining high efficiency. Both
converters feature soft-start and internal compensation,
simplifying the design.
n
Dual High Efficiency DC/DC Converters:
Step-Up (V
= 1.8V to 5.25V, I = 600mA)
OUT
SW
Step-Down (V
= 0.615V to 5.5V, I
= 400mA)
OUT
OUT
n
n
n
n
n
1.8V to 5.5V Input Voltage Range
Up to 94% Efficiency
Pin Selectable Burst Mode® Operation
45μA Quiescent Current in Burst Mode Operation
1.2MHz (LTC3523) or 2.4MHz (LTC3523-2)
Switching Frequency
Both the step-up and step-down converters are current
mode controlled and utilize an internal synchronous rec-
tifier for high efficiency. The step-up supports 0% duty
cycle operation and the step-down converter supports
100% duty cycle operation to extend battery run time.
If the MODE pin is held high, both converters automati-
cally transition between Burst Mode operation and PWM
operation improving light load efficiency. Fixed, low noise
1.2MHz/2.4MHz PWM operation is selected when MODE
is grounded.
n
n
n
n
n
Independent Power Good Indicator Outputs
Integrated Soft-Start
Thermal and Overcurrent Protection
<3μA Quiescent Current in Shutdown
Small 16-Lead 3mm × 3mm × 0.75mm QFN Package
APPLICATIONS
n
Digital Cameras
The LTC3523/LTC3523-2 provide a sub-3μA shutdown
mode, overtemperature shutdown and current limit pro-
tection on both converters. The LTC3523/LTC3523-2 are
housedina16-lead3mm×3mm×0.75mmQFNpackage.
n
Medical Instruments
n
Industrial Handhelds
GPS Navigators
n
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
LTC3523 Efficiency and Power
Loss vs Load Current
V
IN
1.8V TO 3.2V
100
90
80
70
60
50
40
30
20
10
0
1000
100
10
+
2-CELL
ALKALINE
47μF
4.7μH
V
OUT2
V
V
V
BAT
IN1
IN2
4.7μH
V
STEP-DOWN
OUTPUT
1.2V
10μF
EFFICIENCY
SW1
V
SW2
FB2
V
OUT1
10pF
511k
511k
STEP-UP
OUTPUT
3.3V
200mA
P0WER LOSS
OUT
LTC3523
FB1
MODE
634k
IN
10μF
200mA
V
V
V
= 2.4V
PGOOD1
SHDN1
PGOOD2
SHDN2
IN
= 3.3V
= 1.2V
OUT1
OUT2
365k
1
GND1 GND2 GND3
f
= 1.2MHz
OSC
STEP-UP
STEP-DOWN
100
OFF ON
OFF ON
3523 TA01a
0.1
0.1
1
10
1000
LOAD CURRENT (mA)
3523 TA01b
3523fb
1
LTC3523/LTC3523-2
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
V
, V , V , V
Voltages .................... –0.3V to 6V
TOP VIEW
IN1 IN2 BAT OUT
SHDN1, PGOOD1, PGOOD2, FB1 Voltages .. –0.3V to 6V
SHDN2, FB2, MODE Voltages ......–0.3V to (V + 0.3V)
SW1 Voltage
IN2
16 15 14 13
FB1
1
2
3
4
12 FB2
DC.............................................................. 0.3V to 6V
Pulse < 100ns.......................................... –0.3V to 7V
V
11 PGOOD2
IN1
17
PG00D1
MODE
10
9
V
V
SW2 Voltage Pulse < 100ns.........–0.3V to (V + 0.3V)
Operating Temperature Range
OUT
IN2
IN2
5
6
7
8
(Notes 2, 3).............................................. –40°C to 85°C
Storage Temperature Range................... –65°C to 125°C
UD PACKAGE
16-LEAD (3mm s 3mm) PLASTIC QFN
T
= 125°C, θ = 68°C/W
JA
JMAX
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3523EUD#PBF
LTC3523EUD-2#PBF
LTC3523EUD#TRPBF
LTC3523EUD-2#TRPBF
LCYC
LDDR
–40°C to 85°C
–40°C to 85°C
16-Lead (3mm × 3mm) Plastic DFN
16-Lead (3mm × 3mm) Plastic DFN
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 The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN1 = VIN2 = VBAT = 2.4V, VOUT = 3.3V, unless otherwise specified.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
Minimum Start-Up Voltage
Frequency
1.6
1.8
V
l
l
LTC3523
LTC3523-2
0.9
1.8
1.2
2.4
1.5
2.65
MHz
MHz
Quiescent Current–Shutdown
Quiescent Current –Sleep
V
= V
= 0V, V
= 0V, V = V = V
BAT
0.5
45
15
3
μA
μA
μA
V
SHDN1
SHDN2
OUT
IN1
IN2
Measured from V
Measured from V
, V = V = V = 2.4V
SUPPLY IN1 IN2 BAT
Quiescent Current V
– Sleep
= 3.3V (Note 4)
OUT
OUT
SHDN1, SHDN2 Input High
SHDN1, SHDN2 Input Low
SHDN1, SHDN2 Input Current
PGOOD1, PGOOD2 Threshold
PGOOD1, PGOOD2 Low Voltage
PGOOD1, PGOOD2 Leakage
MODE Input High
1
0.35
2
V
V
= 5.5V
1.4
–9
μA
%
V
SHDN
Referenced to the Feedback Voltage
–6
1.0
–14
I
= 1mA
0.35
0.01
PGOOD
V
= 5.25V
1
μA
V
PGOOD
MODE Input Low
0.35
V
3523fb
2
LTC3523/LTC3523-2
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN1 = VIN2 = VBAT = 2.4V, VOUT = 3.3V, unless otherwise specified.
PARAMETER
CONDITIONS
= 5.5V
MIN
TYP
0.01
500
MAX
UNITS
μA
MODE Leakage Current
Soft-Start Time
V
1
MODE
μs
Step-Up Converter
l
l
l
Input Voltage Range
1.8
1.8
5.25
5.25
1.23
50
V
V
Output Voltage Adjust Range
Feedback Voltage FB1
Feedback Input Current FB1
N-Channel Switch Leakage
P-Channel Switch Leakage
N-Channel Switch On Resistance
(Note 6)
1.16
1.20
0
V
V
FB1
V
SW
V
SW
= 1.25V
= 5.5V
nA
μA
μA
0.20
0.20
2
= 5.5V, V
= 0V
2
OUT
V
OUT
V
OUT
= 3.3V
= 5V
0.36
0.22
Ω
Ω
P-Channel Switch On Resistance
V
OUT
V
OUT
= 3.3V, I = 100mA
0.33
0.31
Ω
Ω
SW
= 5V, I = 100mA
SW
l
Peak Inductor Current
(Note 7)
(Note 6)
600
80
1000
40
mA
ns
%
Current Limit Delay to Output
Maximum Duty Cycle
l
l
V
FB
V
FB
= 1V
87
Minimum Duty Cycle
= 1.5V
0
%
Step-Down Converter
l
l
l
Input Voltage Range
1.8
0.615
585
5.5
5.5
615
50
V
V
Output Voltage Range
(Note 6)
Feedback Voltage FB2
600
0
mV
nA
%/V
%/V
%
Feedback Input Current FB2
Reference Voltage Line Regulation
Output Voltage Line Regulation
Output Voltage Load Regulation
Maximum Duty Cycle
V
= 0.625V
FB2
OUT
OUT
OUT
I
I
I
= 100mA (Notes 5, 6)
0.04
0.04
1.0
= 100mA, 1.6V < V < 5.5V (Note 6)
IN
= 0mA to 600mA (Note 6)
100
650
0.33
0.58
0.20
%
l
Peak Inductor Current
(Note 7)
400
mA
Ω
N-Channel Switch On Resistance
P-Channel Switch On Resistance
SW Leakage
V
V
V
= 2.4V
IN2
IN2
= 2.4V
= 0V, V
Ω
= 0V or 5V, V = 5.5V
2
μA
SHDN2
SW2
IN2
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 4: Current is measured into the V
bootstrapped to the output for the step-up. The current will reflect to the
pin since the supply is
OUT
input supply by: (V /V ) • Efficiency. The outputs are not switching in
OUT IN
sleep.
Note 2: The LTC3523/LTC3523-2 are guaranteed to meet performance
specifications from 0°C to 85°C. Specifications over the –40°C to 85°C
operating temperature range are assured by design, characterization and
correlation with statistical process control.
Note 5: The LTC3523/LTC3523-2 are tested in a propriety test mode that
connects FB2 to the output of the error amplifier.
Note 6: Specification is guaranteed by design and not 100% tested in
production.
Note 3: The LTC3523/LTC3523-2 include an overtemperature
shutdown that is intended to protect the device during momentary
overload conditions. Junction temperature will exceed 125°C when the
overtemperature shutdown is active. Continuous operation above the
specified maximum operating junction temperature may impair device
reliability.
Note 7: Current measurements are performed when the LTC3523/
LTC3523-2 are not switching. The current limit values in operation will be
somewhat higher due to the propagation delay of the comparator.
3523fb
3
LTC3523/LTC3523-2
(T = 25°C unless otherwise noted)
A
TYPICAL PERFORMANCE CHARACTERISTICS
Normalized FBx Reference vs
Temperature
Normalized Oscillator Frequency
vs Temperature
Inrush Current Control for the
Step-Up Converter
1.00125
1.00000
0.99875
0.99750
0.99625
0.99500
1.05
1.00
0.95
V
OUT_BST
2V/DIV
I
L_BST
200mA/DIV
SHDN1
2V/DIV
3523 G03
V
V
C
= 3.3V
= 10μF
200μs/DIV
OUT
IN
OUT
= 2.4V
L1 = 4.7μH
15
35
55
–45 –25
–5
75
15
35
55
–45 –25
–5
75
TEMPERATURE (°C)
TEMPERATURE (°C)
3523 G02
3523 G01
Load Transient Response
Step-Down
Inrush Current Control for the
Step-Down Converter
Load Transient Response Step-Up
OUTPUT
RIPPLE
20mV/DIV
OUTPUT
RIPPLE
20mV/DIV
V
OUT_BCK
1V/DIV
LOAD
CURRENT
20mA/DIV
I
L_BCK
200mA/DIV
LOAD
CURRENT
20mA/DIV
SHDN2
2V/DIV
3523 G04
3523 G06
3523 G05
V
V
C
= 1.2V
= 10μF
200μs/DIV
V
V
C
= 1.2V
= 47μF
500μs/DIV
V
V
C
= 3.3V
= 10μF
500μs/DIV
OUT
IN
OUT
L1 = 4.7μH
OUT
IN
OUT
L1 = 4.7μH
OUT
IN
OUT
L1 = 4.7μH
= 2.4V
= 2.4V
= 2.4V
C = 68pF
10mA TO 30mA STEP
20mA TO 70mA STEP
F
RDS(ON) vs Input Voltage for the
Step-Down Converter
RDS(ON) vs Output Voltage for the
Step-Up Converter
Current Limit vs Temperature
1.2
1.0
0.8
0.6
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0
BOOST CURRENT
LIMIT
PMOS
NMOS
PMOS
NMOS
BUCK CURRENT
LIMIT
0.4
0.2
0
55
–45 –25
–5
15
35
75
3
3.5
1
1.5
2
2.5
4
4.5
5
1
4.5
1.5
2
2.5
3
3.5
4
5
OUTPUT VOLTAGE (V)
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3523 G07
3523 G08
3532 G09
3523fb
4
LTC3523/LTC3523-2
(T = 25°C unless otherwise noted)
A
TYPICAL PERFORMANCE CHARACTERISTICS
Step-Up No-Load Input Current
vs VIN
Normalized RDS(ON) vs
Temperature
Mode Transition Response
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
500
450
400
350
300
250
200
150
100
50
V
OUT_BST
50mV/DIV
V
= 5V
OUT
V
OUT_BCK
20mV/DIV
PMOS
NMOS
MODE
2V/DIV
V
= 3.3V
3523 G12
OUT
V
V
V
= 3.3V
= 1.2V
200μs/DIV
OUT1
OUT2
IN
= 2.4V
I
I
= 20mA
= 25mA
= C
OUT1
OUT2
V
= 2.8V
2.5
OUT
2
C
= 10μF
OUT1
OUT2
0
L1 = L2 = 4.7μH
–25
–5
35
55
75
–45
15
1.5
3
3.5
4
4.5
5
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3523 G10
3523 G11
Maximum IOUT vs VIN for the
Step-Up Converter
Maximum IOUT vs VIN for the
Step-Down Converter
500
450
400
350
300
250
200
150
100
50
450
400
350
300
250
200
150
100
50
V
= 5V
V
= 1.8V
OUT
OUT
V
= 3.3V
OUT
V = 2.5V
OUT
V
= 2.5V
OUT
V
= 1.2V
OUT
0
0
3
3.5
1
1.5
2
2.5
4
4.5
5
1
2
3
INPUT VOLTAGE (V)
4
5
INPUT VOLTAGE (V)
3523 G14
3523 G13
PIN FUNCTIONS
FB1 (Pin 1): Step-Up Converter Feedback Input to the Er-
ror Amplifier. Connect resistor divider tap to this pin. The
output voltage can be adjusted from 1.8V to 5.25V by:
PGOOD1 (Pin 3): Step-Up Converter Power Good Com-
parator Output. This open-drain output is pulled low when
V
FB1
< –9% of its regulation voltage.
V
(Pin4):Step-UpConverterOutputVoltageSenseInput
R1
⎛
⎝
⎞
OUT
VOUT(STEP-UP) = 1.2V • 1+
⎜
⎟
and Drain of the Internal Synchronous Rectifier MOSFET.
⎠
R2
Driver bias is derived from V . PCB trace length from
OUT
V
to the output filter capacitor(s) should be as short
See Block Diagram.
OUT
and wide as possible.
V
(Pin2):Step-UpConverterPowerVoltageInput. This
IN1
pin can be connected to a different supply than V . This
IN2
pin must be connected to a valid supply voltage.
3523fb
5
LTC3523/LTC3523-2
PIN FUNCTIONS
SW1 (Pin 5): Step-Up Converter Switch Pin. Connect the
inductor between SW1 and V . Keep these PCB trace
If large feedback resistors, above 500k are used, then it
will be necessary to use a lead capacitor connected to the
output voltage and FB2.
IN1
lengths as short and wide as possible to reduce EMI and
voltage overshoot. If the inductor current falls to zero or
SHDN1 is low, an internal 150Ω anti-ringing resistor is
SHDN2 (Pin 13): Step-Down Converter Logic Controlled
Shutdown Input. Do not leave this pin floating.
connected from SW1 to V to minimize EMI.
IN1
• SHDN2 = High: Normal free-running operation,
GND1(Pin6):Step-UpConverterPowerGround. Connect
this pin to the ground plane.
1.2MHz/2.4MHz typical operating frequency.
• SHDN2 = Low: Shutdown, quiescent current < 1μA.
GND2 (Pin 7): Step-Down Converter Power Ground. Con-
nect this pin to the ground plane.
This pin cannot exceed the voltage on V
.
IN2
GND3 (Pin 14): Analog Ground. The feedback voltage
dividers for each converter must be returned to GND3
for best performance.
SW2 (Pin 8): Step-Down Converter Switch Pin. Connect
one end of the inductor to SW2. Keep these PCB trace
lengths as short and wide as possible to reduce EMI and
voltage overshoot.
Note: When laying out your PCB provide a short direct
pathbetweenGND1andthe(–)sideofthestep-upoutput
capacitor(s)andGND2andthestep-downoutputcapaci-
tor.These pins are not connected together internally.
V
IN2
(Pin 9): Step-Down Converter Power Voltage Input.
This pin can be connected to a different supply than V
.
IN1
This pin must be connected to a valid supply voltage.
V
(Pin 15): Analog Voltage Input. Connect this pin to
MODE (Pin 10): Step-Up and Step-Down Converter Mode
Selection Pin. Do not leave this pin floating.
BAT
the higher of V or V .
IN1
IN2
SHDN1 (Pin 16): Step-Up Converter Logic Controlled
• MODE = Low: PWM mode
Shutdown Input.
• MODE = High: Automatic Burst Mode operation
• SHDN1 = High: Normal free-running operation,
PGOOD2 (Pin 11): Step-Down Converter Power Good
1.2MHz/2.4MHz typical operating frequency.
Comparator Output. This open-drain output is pulled low
• SHDN1 = Low: Shutdown, quiescent current < 1μA.
when V < –9% of its regulation voltage.
FB2
This pin cannot exceed the voltage on V
.
FB2 (Pin 12): Step-Down Converter Feedback Input to the
Erroramplifier. Connectresistordividertaptothispin. The
output voltage can be adjusted from 0.6V to 5.5V by:
IN1
Exposed Pad (Pin 17): Die attach pad must be soldered
to PCB ground for electrical contact and optimum thermal
performance.
R3
R4
⎛
⎝
⎞
VOUT(STEP-DOWN) = 0.6V • 1+
⎜
⎟
⎠
See Block Diagram.
3523fb
6
LTC3523/LTC3523-2
BLOCK DIAGRAM
L1
4.7μH
V
IN
1.8V TO 5.5V
+
16
SHDN1
2
5
C
IN
47μF
V
SW1
BULK CONTROL
SIGNALS
IN1
V
V
OUT
OUT
ANTI-RING
SHUTDOWN
AND
STEP-UP
4
SHDN
1.8V TO 5.25V
V
BIAS
PWM
LOGIC
AND
MODE
OSC
DRIVERS
+
–
CURRENT
SENSE
I
ZERO
COMP
PWM/I
COMP
LIM
MODE
MODE
R1
R2
–
–
+
FB1
C
OUT
10μF
+
–
1
1.2V
g
ERROR
m
+
+
AMPLIFIER
C
C1
I
LIM
REF
R
C
C2
Z
START-UP
SLOPE COMPENSATION
SOFT-START
AND
PGOOD1
3
THERM REG
FB1
SLP
–
+
1.2V
–9%
STEP-UP
0.6V
1.2V
1V
OSC
SLP
OSCILLATOR
REFERENCE
V
V
BAT
MODE
10
15
9
THERMAL SHDN
SHARED
STEP-DOWN
IN2
+
+
SLOPE COMPENSATION
ZERO CURRENT
COMP
+
SLP
0A
–
PWM/I
LIM
COMP
I
LIM
REF
L2
+
PWM
LOGIC
AND
4.7μH
–
V
OUT
SW2
–
8
STEP-DOWN
MODE
0.615V TO 5.5V
PGOOD2
DRIVERS
OSC
11
13
FB2
–
+
V
OUT
GND2
0.6V
–9%
LIMIT
COMP
–
+
0.66V
SHUTDOWN
AND
MODE
MODE
SHDN2
R3
SHDN
FB2
C
OUT
10μF
12
–
+
V
BIAS
R4
0.6V
g
m
ERROR
AMPLIFIER
C
C1
R
Z
START-UP
SOFT-START
AND
THERM REG
GND1
GND2
GND3
14
6
7
3523 BD
3523fb
7
LTC3523/LTC3523-2
OPERATION
TheLTC3523andLTC3523-2aresynchronousstep-upand
step-down converters housed in a 16-pin QFN package.
Operating from inputs down to 1.8V, the devices feature
fixedfrequency,currentmodePWMcontrolforexceptional
line and load regulation and transient response. With
PWM Comparators
ThePWMcomparatorsareusedtocomparetheconverters
external inductor current to the current commanded by
the error amplifiers. When the inductor current reaches
the current commanded by the error amplifier the induc-
tor charging cycle is terminated and the rectification cycle
commences.
low R
and internal MOSFET switches, the devices
DS(ON)
maintain high efficiency over a wide range of load cur-
rent. Operation can be best understood by referring to
the Block Diagram.
Current Limit
ThecurrentlimitcomparatorshutsofftheN-channelswitch
for the step-up and P-channel switch for the step-down
once its current limit threshold is reached. The current
limit comparator delay to output is typically 40ns. Peak
switch current is limited to approximately 1000mA for
the step-up and 650mA for the step-down independent
of input or output voltage.
Soft-Start
Boththestep-upandstep-downconvertersontheLTC3523
/LTC3523-2 provide soft-start. The soft-start time is typi-
cally 500μs. The soft-start function resets in the event of
a commanded shutdown or thermal shutdown.
Oscillator
The frequency of operation is set by an internal oscilla-
tor to a nominal 1.2MHz for the LTC3523 and nominal
2.4MHz for the LTC3523-2. The oscillator is shared by
both converters.
Zero Current Comparator
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifier
once this current reduces to approximately 20mA. This
prevents the inductor current from reversing in polarity
improving efficiency at light loads.
Shutdown
The step-up and the step-down converters have inde-
pendent shutdown pins. To shut down a converter, pull
SHDNx below 0.35V. To enable a converter, pull SHDNx
above 1.0V.
Power Good Comparator
Both converters have independent open drain power good
comparators which monitor the output voltage via their
respective FBx pins. The comparator output will allow the
PGOODx to be pulled up high when the output voltage
Error Amplifiers
Power converter control loop compensation is provided
internally for each converter. The noninverting input is
internally connected to the 1.2V reference for the step-up
and0.6Vforthestep-down.Theinvertinginputisconnected
to the respective FBx for both converters. Internal clamps
limittheminimumandmaximumerrorampoutputvoltage
for improved large signal transient response. A voltage
(V ) has exceeded 91% of it final value. If the output
OUT
voltage decreases below 91%, the comparator will pull
the PGOODx pin to ground. The step-up comparator has
3.3% of hysteresis and the step-down has 6.6% relative
to FBx voltage for added noise immunity.
Step-Down Overvoltage Comparator
divider from V
to ground programs the output voltage
The step-down overvoltage comparator guards against
transient overshoots greater than 10% of the output volt-
age by turning the P-channel switch off until the transient
has subsided.
OUT
viatherespectiveFBxpinsfrom1.8Vto5.25Vforthestep-
up and 0.615V to 5.5V for the step-down. From the Block
Diagram the design equation for programming the output
voltages is V
OUT
= 1.2V • [1 + (R1/R2)] for the step-up and
OUT
V
= 0.6V • [1 + (R3/R4)] for the step-down.
3523fb
8
LTC3523/LTC3523-2
OPERATION
Step-Up Anti-Ringing Control
MOSFET body diode also enables inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output dis-
connect, an external Schottky diode cannot be connected
The anti-ring circuitry connects a resistor across the in-
ductor to prevent high frequency ringing on the SW1 pin
duringdiscontinuouscurrentmodeoperation.Theringing
of the resonant circuit formed by L and C (capacitance
on SW pin) is low energy, but can cause EMI radiation.
between SW1 and V
.
OUT
SW
Thermal Shutdown
If the die temperature reaches 160°C, the part will go into
thermal shutdown. All switches will be turned off and
the soft-start capacitor will be discharged. The device
will be enabled again when the die temperature drops by
approximately 15°C.
Step-Up Output Disconnect
The LTC3523/LTC3523-2 step-up is designed to provide
true output disconnect by eliminating body diode conduc-
tionoftheinternalP-channelMOSFETrectifier.Thisallows
for V
to go to zero volts during shutdown, drawing no
OUT
current from the input source. Controlling the P-channel
APPLICATIONS INFORMATION
PCB LAYOUT GUIDELINES
COMPONENT SELECTION
Inductor Selection
The high speed operation of the LTC3523/LTC3523-2
demands careful attention to board layout. You will not
get advertised performance with careless layout. Figure 1
shows the recommended component placement. A large
ground pin copper area will help to lower the chip tem-
perature. A multilayer board with a separate ground plane
is ideal, but not absolutely necessary.
The LTC3523/LTC3523-2 can utilize small surface mount
and chip inductors due to its fast 1.2MHz switching
frequency and for the 2.4MHz version, the values are
halved. The Inductor current ripple is typically set for
20% to 40% of the peak inductor current (I ). High
P
Figure 1. Recommended Component Placement for Double Layer Board
3523fb
9
LTC3523/LTC3523-2
APPLICATIONS INFORMATION
frequencyferritecoreinductormaterialsreducefrequency
dependent power losses compared to cheaper powdered
iron types, improving efficiency. The inductor should have
low ESR (series resistance of the windings) to reduce the
current.Increasingtheinductanceabove10μHwillincrease
size while providing little improvement in output current
capability. A 4.7μH inductor will work well for most Li-Ion
or 2-cell alkaline/NiMH cell applications
2
I R power losses, and must be able to handle the peak
Output and Input Capacitor Selection
inductor current without saturating. Molded chokes and
some chip inductors usually do not have enough core to
support the peak inductor currents of 1000mA seen on
the LTC3523/LTC3523-2. To minimize radiated noise, use
a toroid, pot core or shielded bobbin inductor. See Table
1 for suggested inductors and suppliers.
Low ESR (equivalent series resistance) capacitors should
be used to minimize the output voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
extremely low ESR and are available in small footprints.
Step-Up: A 2.2μF to 10μF output capacitor is sufficient for
most applications. Larger values up to 22μF may be used
to obtain extremely low output voltage ripple and improve
transientresponse.Anadditionalphaseleadcapacitorcon-
Step-Up:Forthestep-upconverteraminimuminductance
value of 3.3μH is recommended for 3.6V and lower output
voltage applications, and a 4.7μH for output voltages
greater than 3.6V. Larger values of inductance will allow
greater output current capability by reducing the inductor
ripple current. Increasing the inductance above 10μH will
increase size while providing little improvement in output
current capability.
nectedbetweenV
andFB1mayberequiredwithoutput
OUT
capacitors larger than 10μF to maintain acceptable phase
margin. X5R and X7R dielectric materials are preferred
for their ability to maintain capacitance over wide voltage
and temperature ranges.
Step-Down:Formostapplications,thevalueoftheinductor
will fall in the range of 3.3μH to 10μH, depending upon
the amount of current ripple desired. A reasonable point
to start is to set the current ripple at 30% of the output
current.
Step-Down: Low ESR input capacitors reduce input
switching noise and reduce the peak current drawn from
the battery. It follows that ceramic capacitors are also a
good choice for input decoupling and should be located
as close as possible to the device. Table 2 shows the
range of acceptable capacitors for a given programmed
output voltage. Minimum capacitance values in the table
Note that larger values of inductance will allow greater
output current capability by reducing the inductor ripple
Table 1. Recommended Inductors
MAXIMUM CURRENT
DIMENSIONS (mm)
PART
L (μH)
(mA)
DCR (Ω)
0.19 to 0.52
0.3 to 0.54
0.8 to 1.84
0.25 to 0.65
(L × W × H)
3.2 × 2.5 × 2.0
3.0 × 3.0 × 1.0
2.0 × 2.0 × 1.0
3.1 × 3.1 × 1.2
MANUFACTURER
ME3220
LPS3010
DO2010
SD3112
4.7 to 15
4.7 to 10
4.7 to 15
4.7 to 15
1200 to 700
720 to 510
800 to 510
740 to 405
Coil Craft
www.coilcraft.com
Cooper
www.cooperet.com
MIP3226D
4.7 to 10
600 to 200
0.1 to 0.16
FDK
www.fdk.com
3.2 × 2.6 × 1.0
LQH32CN
LQH2MC
CDRH3D16
CDRH2D14
NR3010
4.7 to 15
4.7 to 15
4.7 to 15
4.7 to 12
4.7 to 15
4.7 to 15
650 to 300
300 to 200
900 to 450
680 to 420
750 to 400
1000 to 560
0.15 to 0.58
0.8 to 1.6
Murata
3.2 × 2.5 × 1.5
2 × 1.6 × 0.9
www.murata.com
0.11 to 0.29
0.12 to 0.32
0.19 to 0.74
0.12 to 0.36
Sumida
www.sumida.com
3.8 × 3.8 × 1.8
3.2 × 3.2 × 1.5
3.0 × 3.0 × 1.0
3.0 × 3.0 × 1.5
Taiyo Yuden
www.t-yuden.com
NR3015
3523fb
10
LTC3523/LTC3523-2
APPLICATIONS INFORMATION
SHORT-CIRCUIT PROTECTION
will increase loop bandwidth resulting in a faster transient
response.Maximumcapacitancevalueswillproducelower
ripple. Table 3 shows a list of several ceramic capacitor
manufacturers. Consult the manufacturers directly for
detailed information on their entire selection of ceramic
parts.
The LTC3523/LTC3523-2’s step-up output disconnect
feature allows output short circuit while maintaining
a maximum internally set current limit. However, the
LTC3523/LTC3523-2 also incorporate internal features
such as current limit foldback and thermal shutdown for
protection from an excessive overload or short circuit.
During a prolonged short circuit of V
the current limit folds back to 2/3 the normal current limit.
This 2/3 current limit remains in effect until V exceeds
Table 2. Step-Down Output Capacitor Range vs Programmed
Output Voltage
less than 950mV,
OUT
V
MINIMUM CAPACITANCE (μF) MAXIMUM CAPACITANCE (μF)
OUT
0.8
1.2
1.5
1.8
2.5
5
8.4
5.6
4.5
3.7
2.7
1.3
33.6
22.4
17.9
14.9
10.7
5.4
OUT
1V, at which time the normal internal set current limit is
restored.
WhentheLTC3523/LTC3523-2step-downconvertersout-
putisshortedtoground,thestep-downusesacomparator
to limit the current through the synchronous rectifying
N-channel switch to 650mA. If this limit is exceeded, the
P-channel switch is inhibited from turning on until the
current through the synchronous rectifying N-channel
switch falls below 650mA.
Table 3. Capacitor Vendor Information
SUPPLIER
AVX
PHONE
WEBSITE
(803) 448-9411
(714) 852-2001
(408) 573-4150
www.avxcorp.com
www.murata.com
www.t-yuden.com
Murata
Taiyo-Yuden
THERMAL CONSIDERATIONS
To deliver the LTC3523/LTC3523-2’s full-rated power, it is
imperative that a good thermal path be provided to dis-
sipate the heat generated within the package. This can be
accomplishedbytakingadvantageofthelargethermalpad
on the underside of the LTC3523/LTC3523-2. It is recom-
mended that multiple vias in the printed circuit board be
used to conduct heat away from the LTC3523/LTC3523-2
and into the copper plane with as much area as possible.
In the event that the junction temperature gets too high,
the LTC3523/LTC3523-2 will go into thermal shutdown
and all switching will cease until the internal temperature
drops to a safe level at which point the soft-start cycle
will be initiated.
STEP-UP V > V
OPERATION
IN
OUT
The LTC3523/LTC3523-2 step-up converters will maintain
voltage regulation when the input voltage is above the
output voltage. Since this mode will dissipate more power,
themaximumoutputcurrentislimitedinordertomaintain
an acceptable junction temperature and is given by:
250 – TA
IOUT(MAX)
=
T
⎡
⎤
OUT
136 • V + 1.5 – V
(
)
IN
⎣
⎦
where T = ambient temperature.
A
For example, at V = 4.5V, V
= 3.3V and T = 85°C, the
A
IN
OUT
maximum output current is limited to 449mA.
3523fb
11
LTC3523/LTC3523-2
APPLICATIONS INFORMATION
DUAL BUCK-BOOST AND STEP-UP CONVERTER
OPERATION
into the step-down’s SHDN2 pin. Note that the overall
3.3V converter efficiency is the product of the individual
efficiencies.
The LTC3523/LTC3523-2 can be operated in a cascaded
configuration as shown in Figure 2, allowing buck-boost
and step-up converter operation. Supply rail sequencing
is achieved by feeding the step-up converter PGOOD1
V
IN
1.8V TO 5.25V
4.7μF
10μH
V
OUT2
V
V
V
BAT
4.7μH
IN1
IN2
STEP-DOWN
OUTPUT
3.3V
SW1
SW2
FB2
V
OUT1
825k
182k
10pF
10μF
STEP-UP
OUTPUT
5V
50mA
V
OUT
LTC3523
V
768k
FB1
MODE
10μF
IN
100mA
PGOOD1
PGOOD2
SHDN1
SHDN2
243k
GND1 GND2 GND3
100k
V
IN
3523 F02a
OFF ON
100
90
80
70
60
50
40
30
20
10
0
5V OUTPUT
3.3V OUTPUT
V
V
V
f
= 2.4V
IN
= 5V
OUT1
OUT2
= 3.3V
= 1.2MHz
OSC
BURST ENABLED
1000
100
OUTPUT CURRENT (mA)
0.1
1
10
3523 F02b
Figure 2. Dual Converter Efficiency (Load Applied
to Step-Down Output)
3523fb
12
LTC3523/LTC3523-2
TYPICAL APPLICATIONS
Power Sequence Operation
V
IN
1.8V TO 3.2V
+
2-CELL
4.7μF
ALKALINE
4.7μH
V
OUT2
V
V
IN2
V
BAT
IN1
4.7μH
STEP-DOWN
OUTPUT
1.2V
SW1
SW2
FB2
V
OUT1
10pF
511k
511k
10μF
STEP-UP
OUTPUT
3.3V
200mA
V
OUT
LTC3523
FB1
MODE
634k
4.7μF
200mA
PGOOD1
PGOOD2
SHDN1
SHDN2
365k
GND1 GND2 GND3
100k
V
IN
OFF ON
3523 TA02a
V
OUT1
2V/DIV
PGOOD2
V
OUT2
1V/DIV
SHDN2
3523 TA02b
500μs/DIV
3523fb
13
LTC3523/LTC3523-2
TYPICAL APPLICATIONS
Li-Ion to 5V/150mA, 2.5V/200mA
V
IN
2.5V TO 4.2V
+
4.7μF
Li-Ion
10μH
V
OUT2
V
V
IN2
V
BAT
IN1
4.7μH
STEP-DOWN
OUTPUT
2.5V
SW1
SW2
FB2
V
OUT1
10pF
768k
243k
10μF
STEP-UP
OUTPUT
5V
200mA
V
OUT
LTC3523
FB1
MODE
V
768k
10μF
IN
150mA
PGOOD1
PGOOD2
SHDN1
SHDN2
243k
GND1 GND2 GND3
OFF ON
OFF ON
3523 TA03
Efficiency and Power Loss
vs Load Current
100
1000
100
10
90
80
70
60
50
40
30
20
10
0
EFFICIENCY
P0WER
LOSS
V
V
V
f
= 3.6V
IN
= 5V
OUT1
OUT2
= 2.5V
1
= 1.2MHz
OSC
STEP-UP
STEP-DOWN
0
0
1
10
100
1000
LOAD CURRENT (mA)
3523 TA03b
3523fb
14
LTC3523/LTC3523-2
PACKAGE DESCRIPTION
UD Package
16-Lead Plastic QFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1691)
0.70 p0.05
3.50 p 0.05
2.10 p 0.05
1.45 p 0.05
(4 SIDES)
PACKAGE OUTLINE
0.25 p0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH R = 0.20 TYP
OR 0.25 s 45o CHAMFER
R = 0.115
TYP
0.75 p 0.05
3.00 p 0.10
(4 SIDES)
15 16
PIN 1
TOP MARK
(NOTE 6)
0.40 p 0.10
1
2
1.45 p 0.10
(4-SIDES)
(UD16) QFN 0904
0.25 p 0.05
0.50 BSC
0.200 REF
0.00 – 0.05
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3523fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3523/LTC3523-2
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
92% Efficiency, V : 0.85V to 5V, V
LTC3400/LTC3400B
600mA (I ), 1.2MHz, Synchronous Step-Up DC/DC Converters
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IN
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SD
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SD
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94% Efficiency, V : 0.85V to 4.5V, V
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Q SD
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2A (I ), 1.5MHz, Step-Up DC/DC Converter
92% Efficiency, V : 1.6V to 5.5V, V = 5V,
OUT(MAX)
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IN
I = 600μA, I < 1μA, ThinSOT Package
Q
SD
LTC3427
500mA (I ), 1.25MHz, Synchronous Step-Up DC/DC with Output 94% Efficiency, V : 1.8V to 5V, V = 5.25V,
OUT(MAX)
SW
IN
Disconnect Converter
I = 350μA, I < 1μA, 6-Pin (2mm × 2mm) DFN Package
Q SD
LTC3429/LTC3429B
LTC3459
600mA (I ), 550kHz, Synchronous Step-Up DC/DC Converters
96% Efficiency, V : 0.85V to 4.3V, V
Q SD
= 5V,
= 10V,
= 5V,
SW
IN
OUT(MAX)
Soft-Start/Output Disconnect
I = 20μA, I < 1μA, ThinSOT Package
80mA (I ), Synchronous Step-Up DC/DC Converter
92% Efficiency, V : 1.5V to 5.5V, V
IN OUT(MAX)
SW
I = 10μA, I < 1μA, ThinSOT Package
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LTC3525-3
LTC3525-3.3
LTC3525-5
400mA (I ), Synchronous Step-Up DC/DC Converters with Output 94% Efficiency, V : 0.85V to 4V, V
SW IN OUT(MAX)
Disconnect
I = 7μA, I < 1μA, SC-70 Package
Q SD
LTC3526/LTC3526L
LTC3526B
500mA (I ), 1MHz Synchronous Step-Up DC/DC Converters with 94% Efficiency, V : 0.85V to 5V, V = 5.25V,
OUT(MAX)
SW
IN
Output Disconnect
I = 9μA, I < 1μA, 6-Pin (2mm × 2mm) DFN Package
Q SD
LTC3528/LTC3528B
1A (I ), 1MHz Synchronous Step-Up DC/DC Converters with
94% Efficiency, V : 0.85V to 5V, V
= 5.25V,
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IN
OUT(MAX)
Output Disconnect
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Q SD
ThinSOT is a trademark of Linear Technology Corporation.
3523fb
LT 1108 REV B • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2008
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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