LINER LTC3523 Synchronous 600ma step-up and 400ma step-down dc/dc converter Datasheet

LTC3523/LTC3523-2
Synchronous 600mA Step-Up
and 400mA Step-Down
DC/DC Converters
FEATURES
DESCRIPTION
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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.
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Dual High Efficiency DC/DC Converters:
Step-Up (VOUT = 1.8V to 5.25V, ISW = 600mA)
Step-Down (VOUT = 0.615V to 5.5V, IOUT = 400mA)
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
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
Both the step-up and step-down converters are current
mode controlled and utilize an internal synchronous rectifier 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 automatically 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.
APPLICATIONS
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Digital Cameras
Medical Instruments
Industrial Handhelds
GPS Navigators
The LTC3523/LTC3523-2 provide a sub-3μA shutdown
mode, overtemperature shutdown and current limit protection on both converters. The LTC3523/LTC3523-2 are
housed in a 16-lead 3mm × 3mm × 0.75mm QFN package.
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
2-CELL
ALKALINE
100
+
47μF
4.7μH
VIN1
VIN2
SW1
VBAT
4.7μH
SW2
10pF
VOUT
10μF
634k
FB1
PGOOD1
365k
SHDN1
LTC3523
511k
FB2
MODE
VIN
511k
PGOOD2
VOUT2
STEP-DOWN
OUTPUT
10μF 1.2V
200mA
80
60
P0WER LOSS
50
40
20
OFF ON
3523 TA01a
0
0.1
1
STEP-UP
STEP-DOWN
10
OFF ON
10
VIN = 2.4V
VOUT1 = 3.3V
VOUT2 = 1.2V
fOSC = 1.2MHz
30
SHDN2
GND1 GND2 GND3
100
EFFICIENCY
70
POWER LOSS (mW)
VOUT1
STEP-UP
OUTPUT
3.3V
200mA
1000
90
EFFICIENCY (%)
VIN
1.8V TO 3.2V
1
10
100
LOAD CURRENT (mA)
0.1
1000
3523 TA01b
3523fb
1
LTC3523/LTC3523-2
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN1, VIN2, VBAT, VOUT Voltages .................... –0.3V to 6V
SHDN1, PGOOD1, PGOOD2, FB1 Voltages .. –0.3V to 6V
SHDN2, FB2, MODE Voltages ...... –0.3V to (VIN2 + 0.3V)
SW1 Voltage
DC.............................................................. 0.3V to 6V
Pulse < 100ns .......................................... –0.3V to 7V
SW2 Voltage Pulse < 100ns ......... –0.3V to (VIN2 + 0.3V)
Operating Temperature Range
(Notes 2, 3) .............................................. –40°C to 85°C
Storage Temperature Range................... –65°C to 125°C
SHDN2
GND3
VBAT
SHDN1
TOP VIEW
16 15 14 13
FB1 1
12 FB2
VIN1 2
11 PGOOD2
17
PG00D1 3
10 MODE
VOUT 4
6
7
8
SW1
GND1
GND2
SW2
9
5
VIN2
UD PACKAGE
16-LEAD (3mm s 3mm) PLASTIC QFN
TJMAX = 125°C, θJA = 68°C/W
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
16-Lead (3mm × 3mm) Plastic DFN
16-Lead (3mm × 3mm) Plastic DFN
–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
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
l
Minimum Start-Up Voltage
Frequency
MIN
LTC3523
LTC3523-2
l
l
0.9
1.8
TYP
MAX
UNITS
1.6
1.8
V
1.2
2.4
1.5
2.65
MHz
MHz
3
Quiescent Current–Shutdown
VSHDN1 = VSHDN2 = 0V, VOUT = 0V, VIN1 = VIN2 = VBAT
0.5
Quiescent Current –Sleep
Measured from VSUPPLY, VIN1 = VIN2 = VBAT = 2.4V
45
μA
Quiescent Current VOUT – Sleep
Measured from VOUT = 3.3V (Note 4)
15
μA
SHDN1, SHDN2 Input High
1
V
SHDN1, SHDN2 Input Low
SHDN1, SHDN2 Input Current
VSHDN = 5.5V
PGOOD1, PGOOD2 Threshold
Referenced to the Feedback Voltage
PGOOD1, PGOOD2 Low Voltage
IPGOOD = 1mA
PGOOD1, PGOOD2 Leakage
VPGOOD = 5.25V
MODE Input High
MODE Input Low
–6
μA
0.35
V
1.4
2
μA
–9
–14
%
0.35
0.01
V
1
1.0
μA
V
0.35
V
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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
MODE Leakage Current
VMODE = 5.5V
MIN
Soft-Start Time
TYP
MAX
0.01
1
500
UNITS
μA
μs
Step-Up Converter
Input Voltage Range
Output Voltage Adjust Range
(Note 6)
l
1.8
5.25
V
l
1.8
5.25
V
l
1.16
1.20
1.23
V
Feedback Input Current FB1
VFB1 = 1.25V
0
50
nA
N-Channel Switch Leakage
VSW = 5.5V
0.20
2
μA
P-Channel Switch Leakage
VSW = 5.5V, VOUT = 0V
0.20
2
μA
N-Channel Switch On Resistance
VOUT = 3.3V
VOUT = 5V
0.36
0.22
Ω
Ω
P-Channel Switch On Resistance
VOUT = 3.3V, ISW = 100mA
VOUT = 5V, ISW = 100mA
0.33
0.31
Ω
Ω
1000
mA
40
ns
Feedback Voltage FB1
l
Peak Inductor Current
(Note 7)
Current Limit Delay to Output
(Note 6)
Maximum Duty Cycle
VFB = 1V
l
Minimum Duty Cycle
VFB = 1.5V
l
600
80
87
%
0
%
Step-Down Converter
Input Voltage Range
Output Voltage Range
(Note 6)
Feedback Voltage FB2
l
1.8
5.5
V
l
0.615
5.5
V
l
585
600
615
mV
0
50
Feedback Input Current FB2
VFB2 = 0.625V
Reference Voltage Line Regulation
IOUT = 100mA (Notes 5, 6)
0.04
%/V
Output Voltage Line Regulation
IOUT = 100mA, 1.6V < VIN < 5.5V (Note 6)
0.04
%/V
Output Voltage Load Regulation
IOUT = 0mA to 600mA (Note 6)
1.0
%
100
%
Maximum Duty Cycle
l
Peak Inductor Current
(Note 7)
650
mA
N-Channel Switch On Resistance
VIN2 = 2.4V
0.33
Ω
P-Channel Switch On Resistance
VIN2 = 2.4V
0.58
Ω
SW Leakage
VSHDN2 = 0V, VSW2 = 0V or 5V, VIN2 = 5.5V
0.20
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 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 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.
400
nA
2
μA
Note 4: Current is measured into the VOUT pin since the supply is
bootstrapped to the output for the step-up. The current will reflect to the
input supply by: (VOUT/VIN) • Efficiency. The outputs are not switching in
sleep.
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 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.
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LTC3523/LTC3523-2
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
Normalized FBx Reference vs
Temperature
Normalized Oscillator Frequency
vs Temperature
1.00125
NORMALIZED FBx VOLTAGE (V)
NORMALIZED FREQUENCY (Hz)
1.05
1.00000
0.99875
0.99750
0.99625
0.99500
–45
–25
15
35
55
–5
TEMPERATURE (°C)
75
VOUT_BST
2V/DIV
IL_BST
200mA/DIV
SHDN1
2V/DIV
1.00
VOUT = 3.3V
VIN = 2.4V
COUT = 10μF
L1 = 4.7μH
0.95
–45
15
35
55
–5
TEMPERATURE (°C)
–25
Inrush Current Control for the
Step-Down Converter
OUTPUT
RIPPLE
20mV/DIV
SHDN2
2V/DIV
VOUT = 1.2V
VIN = 2.4V
COUT = 10μF
L1 = 4.7μH
200μs/DIV
3523 G04
LOAD
CURRENT
20mA/DIV
3523 G05
VOUT = 3.3V
500μs/DIV
VIN = 2.4V
COUT = 10μF
L1 = 4.7μH
20mA TO 70mA STEP
1.2
RDS(ON) vs Output Voltage for the
Step-Up Converter
0.50
0.80
BOOST CURRENT
LIMIT
1.0
0.45
0.70
0.40
0.60
BUCK CURRENT
LIMIT
0.6
0.4
0.50
0.40
0.30
NMOS
55
35
–5
15
TEMPERATURE (°C)
75
3523 G07
0
NMOS
0.25
0.20
0.10
0.10
–25
0.30
0.15
0.20
0.2
PMOS
0.35
PMOS
RDS(0N) (Ω)
RDS(ON) (Ω)
0.8
3523 G06
VOUT = 1.2V
500μs/DIV
VIN = 2.4V
COUT = 47μF
L1 = 4.7μH
CF = 68pF
10mA TO 30mA STEP
RDS(ON) vs Input Voltage for the
Step-Down Converter
Current Limit vs Temperature
0
–45
Load Transient Response
Step-Down
OUTPUT
RIPPLE
20mV/DIV
LOAD
CURRENT
20mA/DIV
IL_BCK
200mA/DIV
3523 G03
75
Load Transient Response Step-Up
VOUT_BCK
1V/DIV
200μs/DIV
3523 G02
3523 G01
CURRENT LIMIT (A)
Inrush Current Control for the
Step-Up Converter
0.05
1
1.5
2
2.5 3 3.5 4
INPUT VOLTAGE (V)
4.5
5
3523 G08
0
1
1.5
2
2.5 3
3.5 4
OUTPUT VOLTAGE (V)
4.5
5
3532 G09
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LTC3523/LTC3523-2
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
Step-Up No-Load Input Current
vs VIN
1.3
500
1.2
450
1.0
PMOS
0.9
NMOS
0.8
0.7
Mode Transition Response
VOUT_BST
50mV/DIV
400
1.1
INPUT CURRENT (μA)
NORMALIZED RDS(ON) (Ω)
Normalized RDS(ON) vs
Temperature
VOUT = 5V
350
VOUT_BCK
20mV/DIV
300
MODE
2V/DIV
250
200
VOUT = 3.3V
150
VOUT1 = 3.3V
200μs/DIV
VOUT2 = 1.2V
VIN = 2.4V
IOUT1 = 20mA
IOUT2 = 25mA
COUT1 = COUT2 = 10μF
L1 = L2 = 4.7μH
100
0.6
0.5
–45
50
–25
55
35
15
TEMPERATURE (°C)
–5
VOUT = 2.8V
0
75
1.5
2
3.5
3
2.5
4
INPUT VOLTAGE (V)
3523 G10
3523 G11
Maximum IOUT vs VIN for the
Step-Up Converter
VOUT = 3.3V
Maximum IOUT vs VIN for the
Step-Down Converter
450
VOUT = 5V
MAXIMUM OUTPUT CURRENT (mA)
MAXIMUM OUTPUT CURRENT (mA)
500
450
VOUT = 2.5V
400
350
300
250
200
150
100
50
1
2
3
4
INPUT VOLTAGE (V)
5
3523 G13
VOUT = 1.8V
400
VOUT = 2.5V
350
VOUT = 1.2V
300
250
200
150
100
50
0
0
5
4.5
3523 G12
1
1.5
2
2.5 3 3.5 4
INPUT VOLTAGE (V)
4.5
5
3523 G14
PIN FUNCTIONS
FB1 (Pin 1): Step-Up Converter Feedback Input to the Error Amplifier. Connect resistor divider tap to this pin. The
output voltage can be adjusted from 1.8V to 5.25V by:
⎛ R1⎞
VOUT(STEP-UP) = 1.2V • ⎜1+ ⎟
⎝ R2⎠
See Block Diagram.
VIN1 (Pin 2): Step-Up Converter Power Voltage Input. This
pin can be connected to a different supply than VIN2. This
pin must be connected to a valid supply voltage.
PGOOD1 (Pin 3): Step-Up Converter Power Good Comparator Output. This open-drain output is pulled low when
VFB1 < –9% of its regulation voltage.
VOUT (Pin 4): Step-Up Converter Output Voltage Sense Input
and Drain of the Internal Synchronous Rectifier MOSFET.
Driver bias is derived from VOUT. PCB trace length from
VOUT to the output filter capacitor(s) should be as short
and wide as possible.
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LTC3523/LTC3523-2
PIN FUNCTIONS
SW1 (Pin 5): Step-Up Converter Switch Pin. Connect the
inductor between SW1 and VIN1. Keep these PCB trace
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
connected from SW1 to VIN1 to minimize EMI.
GND1 (Pin 6): Step-Up Converter Power Ground. Connect
this pin to the ground plane.
GND2 (Pin 7): Step-Down Converter Power Ground. Connect this pin to the ground plane.
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.
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.
SHDN2 (Pin 13): Step-Down Converter Logic Controlled
Shutdown Input. Do not leave this pin floating.
• SHDN2 = High: Normal free-running operation,
1.2MHz/2.4MHz typical operating frequency.
• SHDN2 = Low: Shutdown, quiescent current < 1μA.
This pin cannot exceed the voltage on VIN2.
GND3 (Pin 14): Analog Ground. The feedback voltage
dividers for each converter must be returned to GND3
for best performance.
VIN2 (Pin 9): Step-Down Converter Power Voltage Input.
This pin can be connected to a different supply than VIN1.
This pin must be connected to a valid supply voltage.
Note: When laying out your PCB provide a short direct
path between GND1 and the (–) side of the step-up output
capacitor(s) and GND2 and the step-down output capacitor.These pins are not connected together internally.
MODE (Pin 10): Step-Up and Step-Down Converter Mode
Selection Pin. Do not leave this pin floating.
VBAT (Pin 15): Analog Voltage Input. Connect this pin to
the higher of VIN1 or VIN2.
• MODE = Low: PWM mode
SHDN1 (Pin 16): Step-Up Converter Logic Controlled
Shutdown Input.
• MODE = High: Automatic Burst Mode operation
PGOOD2 (Pin 11): Step-Down Converter Power Good
Comparator Output. This open-drain output is pulled low
when VFB2 < –9% of its regulation voltage.
FB2 (Pin 12): Step-Down Converter Feedback Input to the
Error amplifier. Connect resistor divider tap to this pin. The
output voltage can be adjusted from 0.6V to 5.5V by:
⎛ R3⎞
VOUT(STEP-DOWN) = 0.6 V • ⎜1+ ⎟
⎝ R4⎠
• SHDN1 = High: Normal free-running operation,
1.2MHz/2.4MHz typical operating frequency.
• SHDN1 = Low: Shutdown, quiescent current < 1μA.
This pin cannot exceed the voltage on VIN1.
Exposed Pad (Pin 17): Die attach pad must be soldered
to PCB ground for electrical contact and optimum thermal
performance.
See Block Diagram.
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LTC3523/LTC3523-2
BLOCK DIAGRAM
L1
4.7μH
+
16
CIN
47μF
2
SHUTDOWN
AND
VBIAS
5
VIN1
SHDN1
SW1
BULK CONTROL
SIGNALS
VOUT
ANTI-RING
SHDN
OSC
+ –
CURRENT
SENSE
PWM/ILIM
COMP
MODE
+
ILIM
REF
CC1
+
CC2
RZ
FB1
R1
FB1
+
–
COUT
10μF
1
R2
1.2V
SLP
1.2V
–9%
STEP-UP
OSC
OSCILLATOR
15
MODE
START-UP
SOFT-START
AND
THERM REG
PGOOD1
–
+
IZERO
COMP
gm ERROR
AMPLIFIER
SLOPE COMPENSATION
3
VOUT
STEP-UP
1.8V TO 5.25V
4
PWM
LOGIC
AND
DRIVERS
MODE
–+–
VBAT
0.6V
1.2V
1V
REFERENCE
SLP
MODE 10
THERMAL SHDN
SHARED
9
STEP-DOWN
VIN2
+
SLOPE COMPENSATION
+
ZERO CURRENT
COMP
SLP
PWM/ILIM
COMP
ILIM
REF
11
13
+
–
–
PGOOD2
–
+
SHDN2
SHUTDOWN
AND
VBIAS
+
–
0A
PWM
LOGIC
AND
DRIVERS
MODE
OSC
SW2
L2
4.7μH
8
VOUT
STEP-DOWN
0.615V TO 5.5V
FB2
VOUT
LIMIT
COMP
0.6V
–9%
MODE
SHDN
RZ
0.66V
MODE
gm ERROR
AMPLIFIER
CC1
GND2
+
–
VIN
1.8V TO 5.5V
–
+
R3
FB2
COUT
10μF
12
R4
0.6V
START-UP
SOFT-START
AND
THERM REG
GND1
GND2
GND3
6
7
14
3523 BD
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LTC3523/LTC3523-2
OPERATION
The LTC3523 and LTC3523-2 are synchronous step-up and
step-down converters housed in a 16-pin QFN package.
Operating from inputs down to 1.8V, the devices feature
fixed frequency, current mode PWM control for exceptional
line and load regulation and transient response. With
low RDS(ON) and internal MOSFET switches, the devices
maintain high efficiency over a wide range of load current. Operation can be best understood by referring to
the Block Diagram.
PWM Comparators
Soft-Start
The current limit comparator shuts off the N-channel switch
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.
Both the step-up and step-down converters on the LTC3523
/LTC3523-2 provide soft-start. The soft-start time is typically 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 oscillator to a nominal 1.2MHz for the LTC3523 and nominal
2.4MHz for the LTC3523-2. The oscillator is shared by
both converters.
Shutdown
The step-up and the step-down converters have independent shutdown pins. To shut down a converter, pull
SHDNx below 0.35V. To enable a converter, pull SHDNx
above 1.0V.
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
and 0.6V for the step-down. The inverting input is connected
to the respective FBx for both converters. Internal clamps
limit the minimum and maximum error amp output voltage
for improved large signal transient response. A voltage
divider from VOUT to ground programs the output voltage
via the respective FBx pins from 1.8V to 5.25V for the stepup and 0.615V to 5.5V for the step-down. From the Block
Diagram the design equation for programming the output
voltages is VOUT = 1.2V • [1 + (R1/R2)] for the step-up and
VOUT = 0.6V • [1 + (R3/R4)] for the step-down.
The PWM comparators are used to compare the converters
external inductor current to the current commanded by
the error amplifiers. When the inductor current reaches
the current commanded by the error amplifier the inductor charging cycle is terminated and the rectification cycle
commences.
Current Limit
Zero Current Comparator
The zero current comparator monitors the inductor current 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.
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
(VOUT) has exceeded 91% of it final value. If the output
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
The step-down overvoltage comparator guards against
transient overshoots greater than 10% of the output voltage by turning the P-channel switch off until the transient
has subsided.
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LTC3523/LTC3523-2
OPERATION
Step-Up Anti-Ringing Control
The anti-ring circuitry connects a resistor across the inductor to prevent high frequency ringing on the SW1 pin
during discontinuous current mode operation. The ringing
of the resonant circuit formed by L and CSW (capacitance
on SW pin) is low energy, but can cause EMI radiation.
Step-Up Output Disconnect
The LTC3523/LTC3523-2 step-up is designed to provide
true output disconnect by eliminating body diode conduction of the internal P-channel MOSFET rectifier. This allows
for VOUT to go to zero volts during shutdown, drawing no
current from the input source. Controlling the P-channel
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 disconnect, an external Schottky diode cannot be connected
between SW1 and VOUT.
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.
APPLICATIONS INFORMATION
PCB LAYOUT GUIDELINES
COMPONENT 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 temperature. A multilayer board with a separate ground plane
is ideal, but not absolutely necessary.
Inductor Selection
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 (IP). High
Figure 1. Recommended Component Placement for Double Layer Board
3523fb
9
LTC3523/LTC3523-2
APPLICATIONS INFORMATION
frequency ferrite core inductor materials reduce frequency
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
I2R power losses, and must be able to handle the peak
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.
Step-Up: For the step-up converter a minimum inductance
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.
Step-Down: For most applications, the value of the inductor
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.
Note that 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. A 4.7μH inductor will work well for most Li-Ion
or 2-cell alkaline/NiMH cell applications
Output and Input Capacitor Selection
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
transient response. An additional phase lead capacitor connected between VOUT and FB1 may be required with output
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: 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
Table 1. Recommended Inductors
L (μH)
MAXIMUM CURRENT
(mA)
DCR (Ω)
DIMENSIONS (mm)
(L × W × H)
ME3220
4.7 to 15
1200 to 700
0.19 to 0.52
3.2 × 2.5 × 2.0
LPS3010
4.7 to 10
720 to 510
0.3 to 0.54
3.0 × 3.0 × 1.0
DO2010
4.7 to 15
800 to 510
0.8 to 1.84
2.0 × 2.0 × 1.0
SD3112
4.7 to 15
740 to 405
0.25 to 0.65
3.1 × 3.1 × 1.2
Cooper
www.cooperet.com
MIP3226D
4.7 to 10
600 to 200
0.1 to 0.16
3.2 × 2.6 × 1.0
FDK
www.fdk.com
LQH32CN
4.7 to 15
650 to 300
0.15 to 0.58
3.2 × 2.5 × 1.5
LQH2MC
4.7 to 15
300 to 200
0.8 to 1.6
2 × 1.6 × 0.9
Murata
www.murata.com
CDRH3D16
4.7 to 15
900 to 450
0.11 to 0.29
3.8 × 3.8 × 1.8
CDRH2D14
4.7 to 12
680 to 420
0.12 to 0.32
3.2 × 3.2 × 1.5
NR3010
4.7 to 15
750 to 400
0.19 to 0.74
3.0 × 3.0 × 1.0
NR3015
4.7 to 15
1000 to 560
0.12 to 0.36
3.0 × 3.0 × 1.5
PART
MANUFACTURER
Coil Craft
www.coilcraft.com
Sumida
www.sumida.com
Taiyo Yuden
www.t-yuden.com
3523fb
10
LTC3523/LTC3523-2
APPLICATIONS INFORMATION
will increase loop bandwidth resulting in a faster transient
response. Maximum capacitance values will produce lower
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.
Table 2. Step-Down Output Capacitor Range vs Programmed
Output Voltage
VOUT
MINIMUM CAPACITANCE (μF) MAXIMUM CAPACITANCE (μF)
0.8
8.4
33.6
1.2
5.6
22.4
1.5
4.5
17.9
1.8
3.7
14.9
2.5
2.7
10.7
5
1.3
5.4
Table 3. Capacitor Vendor Information
SUPPLIER
PHONE
WEBSITE
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
Taiyo-Yuden
(408) 573-4150
www.t-yuden.com
STEP-UP VIN > VOUT OPERATION
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,
the maximum output current is limited in order to maintain
an acceptable junction temperature and is given by:
IOUT(MAX ) =
250 – TA
T
136 • ⎡⎣(VIN + 1.5) – VOUT ⎤⎦
where TA = ambient temperature.
For example, at VIN = 4.5V, VOUT = 3.3V and TA = 85°C, the
maximum output current is limited to 449mA.
SHORT-CIRCUIT PROTECTION
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 VOUT less than 950mV,
the current limit folds back to 2/3 the normal current limit.
This 2/3 current limit remains in effect until VOUT exceeds
1V, at which time the normal internal set current limit is
restored.
When the LTC3523/LTC3523-2 step-down converters output is shorted to ground, the step-down uses a comparator
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.
THERMAL CONSIDERATIONS
To deliver the LTC3523/LTC3523-2’s full-rated power, it is
imperative that a good thermal path be provided to dissipate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal pad
on the underside of the LTC3523/LTC3523-2. It is recommended 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.
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
VIN
1.8V TO 5.25V
4.7μF
10μH
VIN1
VOUT1
STEP-UP
OUTPUT
5V
100mA
VBAT
VIN2
4.7μH
SW1
SW2
VOUT
FB2
10pF
10μF
768k
LTC3523
FB1
PGOOD1
10μF
182k
PGOOD2
SHDN2
SHDN1
243k
VIN
MODE
825k
VOUT2
STEP-DOWN
OUTPUT
3.3V
50mA
GND1 GND2 GND3
100k
VIN
3523 F02a
OFF ON
100
5V OUTPUT
90
EFFICIENCY (%)
80
70
3.3V OUTPUT
60
50
40
30
VIN = 2.4V
VOUT1 = 5V
VOUT2 = 3.3V
fOSC = 1.2MHz
BURST ENABLED
20
10
0
0.1
10
1
100
OUTPUT CURRENT (mA)
1000
3523 F02b
Figure 2. Dual Converter Efficiency (Load Applied
to Step-Down Output)
3523fb
12
LTC3523/LTC3523-2
TYPICAL APPLICATIONS
Power Sequence Operation
VIN
1.8V TO 3.2V
2-CELL
ALKALINE
+
4.7μF
4.7μH
VIN1
VOUT1
STEP-UP
OUTPUT
3.3V
200mA
VIN2
VBAT
4.7μH
SW1
SW2
VOUT
FB2
10pF
4.7μF
634k
FB1
PGOOD1
365k
LTC3523
MODE
511k
10μF
VOUT2
STEP-DOWN
OUTPUT
1.2V
200mA
511k
PGOOD2
SHDN1
SHDN2
GND1 GND2 GND3
100k
VIN
OFF ON
3523 TA02a
VOUT1
2V/DIV
PGOOD2
VOUT2
1V/DIV
SHDN2
500μs/DIV
3523 TA02b
3523fb
13
LTC3523/LTC3523-2
TYPICAL APPLICATIONS
Li-Ion to 5V/150mA, 2.5V/200mA
VIN
2.5V TO 4.2V
Li-Ion
+
4.7μF
10μH
VIN1
VOUT1
STEP-UP
OUTPUT
5V
150mA
VIN2
VBAT
4.7μH
SW1
SW2
VOUT
FB2
10pF
10μF
FB1
768k
PGOOD1
SHDN1
243k
LTC3523
MODE
VIN
768k
10μF
VOUT2
STEP-DOWN
OUTPUT
2.5V
200mA
243k
PGOOD2
SHDN2
GND1 GND2 GND3
OFF ON
OFF ON
3523 TA03
Efficiency and Power Loss
vs Load Current
1000
100
EFFICIENCY
90
100
70
60
P0WER
LOSS
50
40
10
VIN = 3.6V
VOUT1 = 5V
VOUT2 = 2.5V
fOSC = 1.2MHz
30
20
POWER LOSS (mW)
EFFICIENCY (%)
80
1
STEP-UP
STEP-DOWN
10
0
0
10
1
100
LOAD CURRENT (mA)
0
1000
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
1.45 p 0.05
2.10 p 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 p0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 p 0.10
(4 SIDES)
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
15
16
PIN 1
TOP MARK
(NOTE 6)
0.40 p 0.10
1
1.45 p 0.10
(4-SIDES)
2
(UD16) QFN 0904
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
0.25 p 0.05
0.50 BSC
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 representation 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
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600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converters
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IQ = 19μA/300μA, ISD < 1μA, ThinSOTTM Package
LTC3401
1A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter
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IQ = 38μA, ISD < 1μA, 10-Pin MS Package
LTC3402
2A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter
97% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5.5V,
IQ = 38μA, ISD < 1μA, 10-Pin MS Package
LTC3421
3A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter
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IQ = 12μA, ISD < 1μA, 24-Pin (4mm × 4mm) QFN Package
LTC3422
1.5A (ISW), 3MHz, Synchronous Step-Up DC/DC with Output
Disconnect Converter
94% Efficiency, VIN: 0.85V to 4.5V, VOUT(MAX) = 5.25V,
IQ = 25μA, ISD < 1μA, 10-Pin (3mm × 3mm) DFN Package
LTC3426
2A (ISW), 1.5MHz, Step-Up DC/DC Converter
92% Efficiency, VIN: 1.6V to 5.5V, VOUT(MAX) = 5V,
IQ = 600μA, ISD < 1μA, ThinSOT Package
LTC3427
500mA (ISW), 1.25MHz, Synchronous Step-Up DC/DC with Output
Disconnect Converter
94% Efficiency, VIN: 1.8V to 5V, VOUT(MAX) = 5.25V,
IQ = 350μA, ISD < 1μA, 6-Pin (2mm × 2mm) DFN Package
LTC3429/LTC3429B
600mA (ISW), 550kHz, Synchronous Step-Up DC/DC Converters
Soft-Start/Output Disconnect
96% Efficiency, VIN: 0.85V to 4.3V, VOUT(MAX) = 5V,
IQ = 20μA, ISD < 1μA, ThinSOT Package
LTC3459
80mA (ISW), Synchronous Step-Up DC/DC Converter
92% Efficiency, VIN: 1.5V to 5.5V, VOUT(MAX) = 10V,
IQ = 10μA, ISD < 1μA, ThinSOT Package
LTC3525-3
LTC3525-3.3
LTC3525-5
400mA (ISW), Synchronous Step-Up DC/DC Converters with Output 94% Efficiency, VIN: 0.85V to 4V, VOUT(MAX) = 5V,
Disconnect
IQ = 7μA, ISD < 1μA, SC-70 Package
LTC3526/LTC3526L
LTC3526B
500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converters with
Output Disconnect
94% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5.25V,
IQ = 9μA, ISD < 1μA, 6-Pin (2mm × 2mm) DFN Package
LTC3528/LTC3528B
1A (ISW), 1MHz Synchronous Step-Up DC/DC Converters with
Output Disconnect
94% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5.25V,
IQ = 10μA, ISD < 1μA, 8-Pin (2mm × 3mm) DFN Package
ThinSOT is a trademark of Linear Technology Corporation.
3523fb
16 Linear Technology Corporation
LT 1108 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
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