LINER LTC3537EUD-PBF

LTC3537
2.2 MHz, 600mA
Synchronous Step-Up DC/DC
Converter and 100mA LDO
FEATURES
DESCRIPTION
n
The LTC®3537 combines a high efficiency, 2.2MHz step-up
DC/DC converter with an idependent 100mA low dropout
regulator (LDO). The step-up converter starts from an input
voltage as low as 0.68V and contains an internal 0.4Ω
switch and a 0.6Ω synchronous rectifier that disconnects
from the output when disabled in shutdown.
High Efficiency Step-Up DC/DC Converter and LDO
Step-Up
n V : 0.68V to 5V, V
IN
OUT: 1.5V to 5.25V
IOUT: 100mA at 3.3V, VIN >0.8V
n 2.2MHz Fixed Frequency Operation
n Synchronous Rectifier with Output Disconnect
n Burst Mode Operation (Pin Selectable)
Linear LDO Regulator
n V : 1.8V to 5.5V, V
IN
OUT: 0.6V to 5V
IOUT: 100mA
n 100mV Dropout Voltage at 50mA
n 24dB Ripple Rejection at f
SW
Combined
n Power Good Indicators
n Low-Battery Comparator
n 30μA I
Q
n Low Profile 3mm × 3mm × 0.75mm Package
A switching frequency of 2.2MHz minimizes solution
footprint by allowing the use of tiny, low profile inductors
and ceramic capacitors. The current mode PWM design
is internally compensated, reducing external parts count.
Fixed frequency switching is maintained until a light load
current is sensed, at which point Burst Mode® operation is
engaged to maximize efficiency. For low noise operation,
Burst Mode Operation can be disabled. Anti-ring circuitry
reduces EMI by damping the inductor in discontinuous
mode. Additional features include a low shutdown current
of under 1μA and thermal overload protection.
APPLICATIONS
The integrated LDO regulator provides a very low noise,
programmable low dropout supply.
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.
n
n
n
Wireless Microphones
Portable Medical instruments
Noise Cancelling/Portable Headsets
RF and Audio Power
TYPICAL APPLICATION
Efficiency and Power Loss
vs Load Current
100
2.2μH
1000
90
+
R5
1.0M
1μF
OFF ON
PWM BURST
SW
VOUTB
3.3V
VOUTB
LBI LTC3537 VINL
LBO
FBB
PGDB
PGDL
VOLDO
ENLDO
ENBST
MODE
FBL
SGND PGND
R2
1.74M
4.7μF
R4
2.05M
VOLDO
3V
R3
511k
3537 TA01a
100
EFFICIENCY
70
60
10
50
40
1
POWER LOSS
30
1μF
R1
1M
80
20
0.1
10
0
0.01
POWER LOSS (mW)
ALKALINE
0.8V
TO
1.6V
VINB
EFFICIENCY (%)
R6
665k
VIN, MODE = 1.8V
0.1
1
100
10
LOAD CURRENT (mA)
0.01
1000
3537 TA01b
3537fa
1
LTC3537
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VOUTB
SW
PGND
LBO
TOP VIEW
16 15 14 13
MODE 1
12 VINL
LBI 2
11 VOLDO
17
SGND 3
10 FBL
9 FBB
5
6
7
8
PGDB
PGDL
ENLDO
VINB 4
ENBST
VINB and VINL Voltage................................... –0.3V to 6V
SW DC Voltage............................................. –0.3V to 6V
SW Pulsed (<100ns) Voltage ....................... –0.3V to 7V
FBB, FBL, PGDB, PGDL Voltage ................... –0.3V to 6V
MODE, ENBST, ENLDO Voltage ................... –0.3V to 6V
LBI and LBO Voltage .................................... –0.3V to 6V
VOUTB, VOLDO ............................................... –0.3V to 6V
Operating Temperature (Notes 2, 5) ......... –40°C to 85°C
Junction Temperature ........................................... 125°C
Storage Temperature Range................... –65°C to 125°C
UD PACKAGE
16-LEAD (3mm s 3mm) PLASTIC QFN
TJMAX = 125°C, θJA = 68°C/W (Note 6)
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
LTC3537EUD#PBF
LTC3537EUD#TRPBF
LDBD
16-Lead (3mm × 3mm) Plastic QFN
–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. VINB = 1.2V, VOUTB = 3.3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.68
0.8
V
5.25
V
Boost Converter
VINMIN
Minimum Start-Up Voltage
ILOAD = 1mA
VOUTB
Output Voltage Range
l
1.5
VFBB
Feedback Voltage
l
1.179
IFBB
Feedback Input Current
IQSHDN
Quiescent Current - Shutdown
IQACTIVE
1.21
1.240
V
1
50
nA
VENBST = VENLDO = 0V, Not Including SW Leakage,
VOUTB = 0V
0.02
1
μA
Quiescent Current - Active
Measured on VOUTB, Nonswitching, MODE = 1.2V,
VENLDO = 0V
300
500
μA
IQBURST
Quiescent Current - Burst
Measured on VOUTB, FBB >1.24V, MODE = 1.2V,
VENLDO = 0V
15
INLEAK
NMOS Switch Leakage Current VSW = 5V
0.1
5
μA
IPLEAK
PMOS Switch Leakage Current VSW = 5V, VOUTB = 0V
0.1
10
μA
μA
3537fa
2
LTC3537
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VINB = 1.2V, VOUTB = 3.3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
RNMOS
NMOS Switch On Resistance
VOUTB = 1.8V
VOUTB = 3.3V
VOUTB = 5V
0.8
0.4
0.3
Ω
Ω
Ω
RPMOS
PMOS Switch On Resistance
VOUTB = 1.8V
VOUTB = 3.3V
VOUTB = 5V
1
0.6
0.4
Ω
Ω
Ω
ILIM
NMOS Current Limit
(Note 4)
750
mA
tLIMDELAY
Current Limit Delay Time to
Output
(Note 3)
40
ns
Max Duty Cycle
VFBB = 1.15V
l
Min Duty Cycle
VFBB = 1.3V
l
fSW
Switching Frequency
VENBSTH
ENBST Input High Voltage
VENBSTL
ENBST Input Low Voltage
IENBSTIN
ENBST Input Current
VMODEH
MODE Input High Voltage
VMODEL
MODE Input Low Voltage
IMODEIN
MODE Input Current
tSS
Soft-Start Time
VFBLBI
LBI Feedback Voltage
MIN
l
l
600
87
TYP
UNITS
92
%
0
2
2.2
%
2.4
MHz
0.8
V
0.3
VENBST = 5.5V
V
1.5
μA
0.8
V
0.3
VMODE = 5.5V
Falling Threshold
530
LBI Hysteresis Voltage
V
1.5
μA
0.5
ms
553
575
mV
35
ILBIIN
LBI Input Current
VLBI = 1V
10
VLBOLOW
LBO Voltage Low
ILBO = 5mA
200
ILBOLEAK
LBO Leakage Current
VLBO = 5.5V
0.01
VPGDBLOW
PGDB Voltage Low
IPGDB = 5mA
200
PGDB Leakage Current
VPGDB = 5.5V
0.01
PGDB Trip Point Voltage
VFBB Rising
IPGDBLEAK
MAX
PGDB Hysteresis
mV
50
nA
mV
1
μA
mV
1
μA
94
% VOUTB
6
%
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
VINL = 3.3V, VOLDO = 3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
1.8
5.5
V
ILOAD = 100mA
VFBL
5
V
LDO Regulator
VINL
Input Voltage Range
VOLDO
Output Voltage Range
IOUTMAX
Max Output Current
l
100
VFBL
Feedback Voltage
l
590
VDROPOUT
mA
600
610
mV
Line Regulation
VINL = 1.8V to 5.5V
0.1
%
Load Regulation
ILOAD = 10mA to 90mA
0.4
%
Dropout Voltage
IO = 50mA
100
mV
3537fa
3
LTC3537
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VINL = 3.3V, VOLDO = 3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
PSRR
Ripple Rejection
f = 2.2MHz at ILOAD = 100mA (Note 3)
ISHORT
Short Circuit Current Limit
VOLDO = 0V
l
110
TYP
MAX
UNITS
24
dB
150
mA
VENLDOH
ENLDO Input High Voltage
VENLDOL
ENLDO Input Low Voltage
IENLDO
ENLDO Input Current
VENLDO = 5.5V
1.5
μA
VPGDLLOW
PGDL Voltage Low
IPGDL = 5mA
200
mV
PGDL Leakage Current
VPGDL = 5.5V
0.01
PGDL Trip Point
VFBL Rising
IPGDLLEAK
0.8
V
0.3
PGDL Hysteresis
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 LTC3537 is 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 controls.
Note 3: Specification is guaranteed by design and not 100% tested in
production.
1
V
μA
96
% VOLDO
3
%
Note 4: Current measurements are made when the output is not switching.
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 result in device degradation or failure.
Note 6: Failure to solder the exposed backside of the package to the PC
board ground plane will result in a thermal resistance much higher than
68°C/W.
3537fa
4
LTC3537
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current and
VINB for VOUTB = 1.8V
Efficiency vs Load Current and
VINB for VOUTB = 3.3V
100
1
VINB = 1V
VINB = 1.2V
VINB = 1.5V
0.1
PLOSS AT VINB = 1V
PLOSS AT VINB = 1.2V
PLOSS AT VINB = 1.5V
0.01
1
100
1000
0.1
10
LOAD CURRENT (mA)
40
20
10
0
0.01
60
10
50
VINB = 1.2V
VINB = 1.8V 1
VINB = 2.4V
VINB = 2.8V
PLOSS AT VINB = 1.2V 0.1
PLOSS AT VINB = 1.8V
PLOSS AT VINB = 2.4V
PLOSS AT VINB = 2.8V
0.01
0.1
10
1
100
1000
LOAD CURRENT (mA)
40
30
20
10
0
0.01
Efficiency vs Load Current and
VINB for VOUTB = 5V
1000
10
0.5
700
500
400
3.5
3537 G03
100
VOUTB = 2.5V
300
VOUTB = 1.8V
100
0
0.5
1
1.5
2
2.5 3
VINB (V)
3.5
4
10
0.8 0.9
4.5
Burst Mode Threshold Current
vs VINB
45
VOUTB = 1.8V
= 4.7μF
C
30 OUT
L = 2.2μH
25
VOUTB = 2.5V
40 COUT = 4.7μF
L = 2.2μH
35
LEAVE BURST
LOAD CURRENT (mA)
LOAD CURRENT (mA)
50
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
VINB (V)
3537 G06
35
60
1
3537 G05
Burst Mode Threshold Current
vs VINB
20
3
VOUTB = 3.3V
600
200
Start-Up Delay Time vs VINB
30
2
2.5
VINB (V)
1.5
1000
3537 G04
40
1
Minimum Load Resistance During
Start-Up vs VINB
LOAD (Ω)
VINB = 1.2V
VINB = 2.4V 1
VINB = 3.6V
VINB = 4.2V
PLOSS AT VINB = 1.2V 0.1
PLOSS AT VINB = 2.4V
PLOSS AT VINB = 3.6V
PLOSS AT VINB = 4.2V
0.01
0.1
10
1
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
10
60
0
0.01
20
800
POWER LOSS (mW)
70
10
30
VOUTB = 5V
900
100
80
20
50
40
1000
90
30
60
Maximum Output Current vs VINB
100
40
70
3537 G02
3537 G01
50
80
IIN (μA)
EFFICIENCY (%)
50
100
70
POWER LOSS (mW)
10
POWER LOSS (mW)
60
VOUTB = 3.3V
90
80
100
70
30
EFFICIENCY (%)
100
1000
90
80
DELAY (μs)
No-Load Input Current vs VINB
100
1000
90
EFFICIENCY (%)
TA = 25°C unless otherwise noted.
20
15
10
LEAVE BURST
30
25
20
15
10
10
0
ENTER BURST
5
1
1.5
2
2.5
3
VINB (V)
3.5
4
4.5
5
3537 G07
0
0.8
5
ENTER BURST
0.9
1
1.1 1.2
VINB (V)
1.3
1.4
1.5
3537 G08
0
0.8
1
1.2
1.4
1.6
VINB (V)
1.8
2
3537 G09
3537fa
5
LTC3537
TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Threshold Current
vs VINB
Burst Mode Threshold Current
vs VINB
Oscillator Frequency Change
vs VOUTB
120
VOUTB = 3.3V
COUT = 4.7μF
50 L = 2.2μH
LOAD CURRENT (mA)
100
40
LEAVE BURST
30
20
10
60
LEAVE BURST
40
20
0
0.8
2.3
VINB (V)
1.3
1.8
2.3 2.8
VINB (V)
3.3
3.8
–4
–6
1.5
4.3
30
NORMALIZED TO 25°C
0.7
PMOS
0.6
0.5
NMOS
0.4
–1
–2
–3
2
2.5
3
3.5
VOUTB (V)
4
–4
–40
5
4.5
3
3.5
VOUTB (V)
4
–20
0
20
40
TEMPERATURE (°C)
60
3537 G12
10
0
PMOS
–10
80
NMOS
–30
–40
–20
0
20
40
TEMPERATURE (°C)
60
3537 G14
3537 G13
Voltage Feedback Change
vs Temperature
Burst Mode Quiescent Current
vs VOUTB
0.80
0.00
80
3537 G15
Start-Up Voltage vs Temperature
NORMALIZED TO 20°C
5
4.5
NORMALIZED TO 25°C
–20
0.3
0.2
1.5
2.5
20
0
RDS(ON) CHANGE (%)
FREQUENCY CHANGE (%)
0.8
2
RDS(ON) Change vs Temperature
1
0.9
RDS(ON) (Ω)
–3
Oscillator Frequency Change vs
Temperature
1.0
60
0.75
50
VFBB AND VFBL
0.70
–0.15
0.65
0.60
–0.25
0.55
–20
0
20
40
TEMPERATURE (°C)
60
80
3537 G16
40
30
–0.20
–0.30
–40
IQ (μA)
–0.10
VINB (V)
VOLTAGE CHANGE (%)
–2
3537 G11
3537 G10
RDS(ON) vs VOUTB
0.05
–1
–5
ENTER BURST
1.8
1.3
NORMALIZED TO 3.3V
0
80
ENTER BURST
0
0.8
1
VOUTB = 5V
COUT = 4.7μF
L = 2.2μH
FREQUENCY CHANGE (%)
60
LOAD CURRENT (mA)
TA = 25°C unless otherwise noted.
0.50
–40
20
–20
0
20
40
TEMPERATURE (°C)
60
80
3537 G17
10
1.8
ENLDO = HIGH
2.3
2.8
3.3
3.8
VOUTB (V)
4.3
4.8
3537 G18
3537fa
6
LTC3537
TYPICAL PERFORMANCE CHARACTERISTICS
Fixed Frequency Switching
Waveform and VOUTB Ripple
TA = 25°C unless otherwise noted.
VOUTB and IINB During Soft-Start
Burst Mode Waveforms
ENBST
VOUTB
20mV/DIV
SW
2V/DIV
VOUTB
2V/DIV
VOUTB
20mV/
DIV
IL
10mA/DIV
VINB = 2.4V
VOUTB = 3.3V
COUTB = 4.7μF
200ns/DIV
IVINB
200mA/
DIV
VINB = 2.4V
VOUTB = 3.3V
COUTB = 4.7μF
3537 G19
Load Current Step Response
(from Burst Mode Operation)
10μs/DIV
VINB = 1.2V
VOUTB = 3.3V
COUTB = 4.7μF
ILOAD = 10mA
3537 G20
Load Current Step Response
(Fixed Frequency)
Load Current Step Response
(Fixed Frequency)
VOUTB
100mV/
DIV
VOUTB
100mV/
DIV
VOUTB
100mV/
DIV
ILOAD
100mA/
DIV
ILOAD
100mA/
DIV
ILOAD
100mA/
DIV
VINB = 2.4V
VOUTB = 3.3V
COUT = 4.7μF
100μs/DIV
VINB = 2.4V
VOUTB = 3.3V
COUT = 4.7μF
3537 G22
100μs/DIV
VINB = 3.6V
VOUTB = 5V
COUTB = 4.7μF
3537 G23
LDO Dropout Voltage vs
Load Current
Load Current Step Response
(from Burst Mode Operation)
60
DROPOUT VOLTAGE (mV)
120
ATTENUATIOIN (dB)
80
60
40
100μs/DIV
3537 G25
0
40
30
20
10
20
VINB = 3.6V
VOUTB = 5V
COUTB = 4.7μF
VINL = 3.3V
VOLDO = 3V
CLOAD = 4.7μF
ILOAD = 50mA
50
100
ILOAD
100mA/
DIV
3537 G24
100μs/DIV
LDO Input Ripple Rejection vs
Frequency
140
VOUTB
100mV/
DIV
3537 G21
100μs/DIV
0
10 20 30 40 50 60 70 80 90 100
LOAD CURRENT (mA)
3537 G26
0
0.01
0.1
1
10
FREQUENCY (kHz)
100
3537 G29
3537fa
7
LTC3537
TYPICAL PERFORMANCE CHARACTERISTICS
LDO Current Limit vs Temperature
TA = 25°C unless otherwise noted.
LDO Load Current Step Response
7
NORMALIZED TO 25°C
6
LOAD CURRENT (%)
5
VOLDO
100mV/
DIV
4
3
2
1
ILOAD
100mA/
DIV
0
–1
–2
–40
–20
0
20
40
TEMPERATURE (°C)
80
60
VINL = 3.3V
VOLDO = 3V
COUT = 1μF
3537 G30
VOLDO
100mV/
DIV
VOLDO
100mV/
DIV
ILOAD
100mA/
DIV
ILOAD
100mA/
DIV
100μs/DIV
3537 G32
3537 G31
LDO Load Current Step Response
LDO Load Current Step Response
VINL = 5V
VOLDO = 3V
COUT = 1μF
100μs/DIV
VINL = 5V
VOLDO = 1.8V
COUT = 1μF
100μs/DIV
3537 G33
3537fa
8
LTC3537
PIN FUNCTIONS
MODE (Pin 1): Logic Controlled Input for the Auto-Burst
Mode Feature.
MODE = High: PWM operation with Burst Mode
Operation
MODE = Low: PWM operation only
LBI (Pin 2): Low-Battery Comparator Non-Inverting Input.
(Comparator enabled with ENBST or ENLDO)
SGND (Pin 3): Signal Ground. Provide a short direct PCB
path between GND and the (–) side of the input and output
capacitors.
VINB (Pin 4): Input Supply for the Step-Up Converter.
Connect a minimum of 1μF ceramic decoupling capacitor
from this pin to ground.
PGDB (Pin 5): Power Good Indicator for the Boost Converter. This is an open-drain output that sinks current when
VOUTB is greater than 94% of the programmed voltage.
ENBST (Pin 6): Logic controlled shutdown input for the
boost converter.
ENBST = High: Normal operation
ENBST = Low: Shutdown
PGDL (Pin 7): Power Good Indicator for the LDO Regulator.
This is an open-drain output that sinks current when VOLDO
is greater than 96% of the programmed voltage.
ENLDO (Pin 8): Logic Controlled Shutdown Input for the
LDO Regulator.
ENLDO = High: Normal operation
ENLDO = Low: Shutdown
FBB (Pin 9): Feedback Input to the gm Error Amplifier
of the Boost Converter. Connect resistor divider tap to
this pin. The output voltage can be adjusted from 1.5V
to 5.25V by:
VOUTB = 1.2V • [1 + (R2/R1)]
FBL (Pin 10): Feedback Input to the gm Error Amplifier of
the LDO. Connect resistor divider tap to this pin. The output
voltage can be adjusted from 0.6V (typical) to 5V by:
VOLDO = 0.6V • [1 + (R4/R3)]
VOLDO (Pin 11): LDO Regulator Output. PCB trace from
VOLDO to the output filter capacitor (1μF minimum) should
be as short and as wide as possible.
VINL (Pin 12): Input Supply for the LDO Regulator.
VOUTB (Pin 13): Output Voltage Sense Input and Drain
of the Internal Synchronous Rectifier. PCB trace length
from VOUTB to the output filter capacitor (4.7μF minimum)
should be as short and wide as possible.
SW (Pin 14): Switch Pin. Connect the inductor between
SW and VINB. Keep these PCB trace lengths as short and
wide as possible to reduce EMI. If the inductor current falls
to zero or ENBST is low, an internal anti-ringing switch is
connected from SW to VINB to minimize EMI.
PGND (Pin 15): Power Ground. Provide a short direct
PCB path between GND and the (–) side of the input and
output capacitors.
LBO (Pin 16): Low-Battery Comparator Output. (OpenDrain)
Exposed Pad (Pin 17): Power Ground. The Exposed Pad
must be soldered to the PCB.
3537fa
9
LTC3537
BLOCK DIAGRAM
SW
VOUT
VBEST
VINB
WELL
SWITCH
VBEST
VOUTB
R2
+
–
GATE DRIVERS
AND
ANTI-CROSS
CONDUCTION
FBB
R1
SHUTDOWN
SHUTDOWN
ENBST
3
–
VREF
+
ENLDO
SLOPE
COMPENSATION
1.2V
VREF
VREF
+
STARTUP
LOGIC
–
UVLO
MODE
CONTROL
2.2MHz
OSC
WELL
SWITCH
CLAMP
THERMAL
SHUTDOWN
+
1.13V
–
FBB
+
0.55V
–
FBL
VINL
VOLDO
PGDB
R4
GATE
DRIVER
–
R3
0.6V
+
PGDL
+
LBI
–
0.55V
LBO
MODE
SGND
PGND
FBL
3537 BD
VIN
R6
R5
3537fa
10
LTC3537
OPERATION
The LTC3537 is a 2.2MHz synchronous step-up (boost)
converter and LDO regulator housed in a 16-lead 3mm
× 3mm QFN package. Included with the ability to start
up and operate from inputs less than 0.7V, the LTC3537
features fixed frequency, current mode PWM control for
exceptional line and load regulation.
The current mode architecture with adaptive slope compensation provides excellent transient load response,
requiring minimal output filtering. Internal soft-start and
loop compensation simplifies the design process while
minimizing the number of external components. With its low
RDS(ON) and low gate charge internal N-channel MOSFET
switch and P-channel MOSFET synchronous rectifier, the
LTC3537 achieves high efficiency over a wide range of
load currents. Automatic Burst Mode operation maintains
high efficiency at very light loads, reducing the quiescent
current to just 30μA. Operation can be best understood
by referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3537 step-up converter includes an independent
start-up oscillator designed to operate at an input voltage
of 0.68V (typical). Soft-start and inrush current limiting
are provided during start-up, as well as normal mode.
When either VINB or VOUTB exceeds 1.4V typical, the IC
enters normal operating mode. When the output voltage
exceeds the input by 0.24V, the IC powers itself from VOUTB
instead of VINB. At this point the internal circuitry has no
dependency on the VINB input voltage, eliminating the
requirement for a large input capacitor. The input voltage
can drop as low as 0.5V after start-up is achieved. The
limiting factor for the application becomes the availability
of the power source to supply sufficient energy to the
output at low voltages, and maximum duty cycle, which
is clamped at 92% typical. Note that at low input voltages,
small voltage drops due to series resistance become
critical, and greatly limit the power delivery capability of
the converter.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
The LTC3537 contains internal circuitry to provide softstart operation. The soft-start circuitry slowly ramps the
peak inductor current from zero to its peak value of 750mA
(typical) in approximately 0.5ms, allowing start-up into
heavy loads. The soft-start circuitry is reset in the event
of a shutdown command or a thermal shutdown.
Oscillator
An internal oscillator sets the switching frequency to
2.2MHz.
Shutdown
Shutdown of the boost converter is accomplished by
pulling ENBST below 0.3V and enabled by pulling ENBST
above 0.8V. Note that ENBST can be driven above VINB or
VOUTB, as long as it is limited to less than the absolute
maximum rating.
Boost Error Amplifier
The non-inverting input of the transconductance error
amplifier is internally connected to the 1.2V reference
and the inverting input is connected to FBB. Clamps limit
the minimum and maximum error amp output voltage for
improved large-signal transient response. Power converter
control loop compensation is provided internally. An external resistive voltage divider from VOUTB to ground programs
the output voltage via FBB from 1.5V to 5.25V.
⎛ R2 ⎞
VOUTB = 1.2V ⎜ 1+ ⎟
⎝ R1⎠
Boost Current Sensing
Lossless current sensing converts the peak current signal of
the N-channel MOSFET switch into a voltage that is summed
with the internal slope compensation. The summed signal
is compared to the error amplifier output to provide a peak
current control command for the PWM.
3537fa
11
LTC3537
OPERATION
Boost Current Limit
Thermal Overload Protection
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. The current limit comparator delay to output is typically 40ns.
Peak switch current is limited to approximately 750mA,
independent of input or output voltage, unless VOUTB falls
below 0.8V, in which case the current limit is cut in half.
If the die temperature exceeds 160°C typical, the LTC3537
boost converter will shut down. All switches will be off
and the soft-start capacitor will be discharged. The boost
converter will be enabled when the die temperature drops
by approximately 15°C.
Boost Zero Current Comparator
The zero current comparator monitors the inductor current to the output and shuts off the synchronous rectifier
when this current reduces to approximately 30mA. This
prevents the inductor current from reversing in polarity,
improving efficiency at light loads.
Boost Synchronous Rectifier
To control inrush current and to prevent the inductor current from running away when VOUTB is close to VINB, the
P-channel MOSFET synchronous rectifier is only enabled
when VOUTB > (VINB + 0.24V).
Boost Anti-Ringing Control
The anti-ringing control connects a resistor across the
inductor to prevent high frequency ringing on the SW pin
during discontinuous current mode operation. Although
the ringing of the resonant circuit formed by L and CSW
(capacitance on SW pin) is low energy, it can cause EMI
radiation.
Boost Output Disconnect
The LTC3537 is designed to allow true output disconnect
by eliminating body diode conduction of the internal Pchannel MOSFET synchronous rectifier. This allows VOUTB
to go to zero volts during shutdown, drawing no current
from the input source. It also allows inrush current limiting at turn-on, minimizing surge currents seen by the
input supply. Note that to obtain the advantages of output
disconnect, there cannot be an external Schottky diode
connected between the SW pin and VOUTB. The output
disconnect feature also allows VOUTB to be pulled high,
above the nominal regulation voltage, without any reverse
current into the power source connected to VINB.
BOOST BURST MODE OPERATION
When enabled (MODE pin high), the LTC3537 will automatically enter Burst Mode operation at light load current
and return to fixed frequency PWM mode when the load
increases. Refer to the Typical Performance Characteristics
to see the Burst Mode Threshold Current vs VINB. The
load current at which Burst Mode operation is entered can
be changed by adjusting the inductor value. Raising the
inductor value will lower the load current at which Burst
Mode operation is entered.
In Burst Mode operation, the LTC3537 still switches at a
fixed frequency of 2.2MHz, using the same error amplifier
and loop compensation for peak current mode control.
This control method eliminates any output transient when
switching between modes. In Burst Mode operation, energy is delivered to the output until it reaches the nominal
voltage regulation value, then the LTC3537 transitions to
sleep mode where the outputs are off and the LTC3537
consumes only 30μA of quiescent current from VOUTB
including the current required to keep the LDO enabled.
When the output voltage droops slightly, switching resumes. This maximizes efficiency at very light loads by
minimizing switching and quiescent losses. Burst Mode
output voltage ripple, which is typically 1% peak-to-peak,
can be reduced by using more output capacitance (10μF
or greater), or with a small capacitor (10pF to 50pF) connected between VOUTB and FBB.
As the load current increases, the LTC3537 will automatically leave Burst Mode operation. Note that larger output
capacitor values may cause this transition to occur at lighter
loads. Once the LTC3537 has left Burst Mode operation and
returned to normal operation, it will remain there until the
output load is reduced below the burst threshold.
3537fa
12
LTC3537
OPERATION
Burst Mode operation is inhibited during start-up and softstart and until VOUTB is at least 0.24V greater than VINB.
The LTC3537 will operate at a continuous PWM frequency
of 2.2MHz by connecting MODE to GND. At very light loads,
the LTC3537 will exhibit pulse-skip operation.
LDO REGULATOR OPERATION
The LTC3537 includes an independent 100mA low dropout
linear regulator (LDO). The VINL pin can be connected to
an independent source or connected to the output of the
boost regulator. An input capacitor on VINL is optional, but
it will help to improve transient responses. The LDO will
operate with a VINL down to 1.5V, but specifications are
guaranteed with VINL from 1.8V to 5.5V.
Shutdown
Shutdown of the LDO is accomplished by pulling ENLDO
below 0.3V and enabled by pulling ENLDO above 0.8V. Note
that ENLDO can be driven above VINL or VOLDO, as long
as it is limited to less than the absolute maximum rating.
In the event that the LDO output voltage is held above the
input voltage, the LDO goes in to shutdown until the output
drops back below the input voltage. In shutdown the LDO
will block reverse current from VOLDO to VINL.
LDO Error Amplifier
The non-inverting input of the transconductance error
amplifier is internally connected to a 0.6V reference and
the inverting input is connected to FBL. The control loop
compensation is provided internally. An external resistive
voltage divider from VOLDO to ground programs the output
voltage via FBL from 0.6V to 5V.
⎛ R4 ⎞
VOLDO = 0.6V ⎜ 1+ ⎟
⎝ R3 ⎠
LDO Current Sensing and Limiting
Current is sensed across an internal resistor. The guaranteed minimum output current is 100mA.
LOW-BATTERY INDICATOR
The LTC3537 includes a low-battery comparator. The noninverting input of the comparator is internally connected
to a 0.6V reference and the inverting input is connected
to LBI. An external resistive voltage divider from VINL to
ground programs the threshold voltage. When the voltage at LBI drops below 0.6V, the open-drain N-channel
MOSFET will turn on. The N-channel MOSFET device is
forced off when both the step-up converter and LDO are
in shutdown.
⎛ R6 ⎞
VLBI = 0.6V ⎜ 1+ ⎟
⎝ R5 ⎠
BOOST POWER-GOOD INDICATOR
The LTC3537 includes a power-good comparator for the
step-up converter. The non-inverting input of the comparator is internally connected to a 1.08V reference and the
inverting input is connected to the FBB pin. The open-drain
MOSFET on PGDB will turn on when the output voltage is
typically within 6% of the programmed output voltage.
Output sequencing can be achieved by connecting PGDB to
the LDO enable pin (ENLDO). This would allow the user to
keep the LDO off until the step-up converter is regulating.
The N-channel MOSFET is forced on in shutdown.
LDO POWER-GOOD INDICATOR
The LTC3537 includes a power-good comparator for the
LDO. The non-inverting input of the comparator is internally
connected to a 540mV reference and the inverting input is
connected to the FBL pin. The open-drain MOSFET on the
PGDL pin will turn on when the output voltage is typically
within 4% of the programmed output voltage.
Output sequencing can be achieved by connecting PGDL to
the boost enable pin (ENBST). This would allow the user to
keep the step-up converter off until the LDO is regulating.
The N-channel MOSFET is forced on in shutdown.
3537fa
13
LTC3537
APPLICATIONS INFORMATION
VINB > VOUTB OPERATION
The LTC3537 step-up converter will maintain voltage regulation even when the input voltage is above the desired
output voltage. Note that the efficiency is much lower in this
mode, and the maximum output current capability will be
less. Refer to the Typical Performance Characteristics.
STEP-UP SHORT-CIRCUIT PROTECTION
The LTC3537 output disconnect feature provides output
short circuit protection. To reduce power dissipation under
short-circuit conditions, the peak switch current limit is
reduced to 400mA (typical).
SCHOTTKY DIODE
Although it is not required, adding a Schottky diode from
SW to VOUTB will improve efficiency by about 4%. Note
that this defeats the output disconnect and short-circuit
protection features.
The high speed operation of the LTC3537 demands careful
attention to board layout. A careless layout will result in
reduced performance. Figure 1 shows the recommended
component placement. A large ground pin copper area
16
15
SW
14
VOUTB
12 VINL
11 VOLDO
LBI 2
SGND 3
10 FBL
4
9 FBB
+
VINB
6
Inductor Selection
The LTC3537 can utilize small surface mount chip inductors due to its fast 2.2MHz switching frequency. Inductor
values between 1μH and 4.7μH are suitable for most applications. Larger values of inductance will allow slightly
greater output current capability (and lower the Burst
Mode threshold) by reducing the inductor ripple current.
Increasing the inductance above 10μH will increase size
while providing little improvement in output current capability. The minimum inductance value is given by:
L>
(
VINB(MIN) • VOUTB(MAX) − VINB(MIN)
)
Ripple• VOUTB(MAX)
7
Ripple = Allowable inductor current ripple (amps
peak-peak)
VINB(MIN) = Minimum converter input voltage
VOUTB(MAX) = Maximum output voltage
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High 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 support the peak inductor
current without saturating. Molded chokes and some chip
inductors usually do not have enough core area to support
the peak inductor currents of 750mA seen on the LTC3537.
To minimize radiated noise, use a shielded inductor. See
Table 1 for suggested components and suppliers.
13
MODE 1
5
COMPONENT SELECTION
where:
PCB LAYOUT GUIDELINES
LBO
will help to lower the die temperature. A multilayer board
with a separate ground plane is ideal, but not absolutely
necessary.
8
3537 F01
PGDB ENBST PGDL ENLDO
MULTIPLE VIAS
TO INNER GROUND LAYERS
Figure 1
3537fa
14
LTC3537
APPLICATIONS INFORMATION
VENDOR
PART/STYLE
Coilcraft
(847) 639-6400
www.coilcraft.com
LPO4815
LPS4012, LPS4018
MSS5131
MSS4020
MOS6020
ME3220
DS1605, DO1608
Coiltronics
www.cooperet.com
SD10, SD12, SD14, SD18,
SD20,
SD52, SD3114, SD3118
FDK
(408) 432-8331
www.fdk.com
MIP3226D4R7M,
MIP3226D3R3M
MIPF2520D4R7
MIPWT3226D3R0
Murata
(714) 852-2001
www.murata.com
LQH43C
LQH32C (-53 series)
301015
Sumida
(847) 956-0666
www.sumida.com
CDRH5D18
CDRH2D14
CDRH3D16
CDRH3D11
CR43
CMD4D06-4R7MC
CMD4D06-3R3MC
1.6
1.4
1.2
1.0
VLP
VLF, VLCF
Toko
(408) 432-8282
www.tokoam.com
D412C
D518LC
D52LC
D62LCB
WE-TPC Type S, M
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.
A 4.7μF to 10μF output capacitor is sufficient for most
boost applications. Larger values up to 22μF may be used
to obtain extremely low output voltage ripple and improve
transient response. X5R and X7R dielectric materials are
preferred for their ability to maintain capacitance over
0.8
0.6
REGION OF
OPERATION
0.4
0.2
0.0
NP03SB
NR3015T
NR3012T
TDK
(847) 803-6100
www.component.tdk.com
Wurth
(201) 785-8800
www.we-online.com
The internal loop compensation of the LTC3537 is designed
to be stable with output capacitor values of 4.7μF or
greater on the boost regulator and 1μF or greater on the
LDO regulator (without the need for any external series
resistor). Although ceramic capacitors are recommended,
low ESR tantalum capacitors may be used as well. For the
LDO, see Figures 2 and 3 for output capacitor value and
ESR requirements.
1
10
CAPACITANCE (μF)
100
3537 F02
Figure 2. LDO Regulator Output Capacitance vs ESR
5.0
MINIMUM OUTPUT CAPACITANCE (μF)
Taiyo-Yuden
www.t-yuden.com
wide voltage and temperature ranges. Y5V types should
not be used.
ESR (Ω)
Table 1: Recommended Inductors
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1
2
3
4
5
VINL/VOLDO
6
7
3537 F03
Figure 3. LDO Regulator Minimum Output Capacitance
vs VINL/VOLDO
3537fa
15
LTC3537
APPLICATIONS INFORMATION
For the step-up converter, a tantalum capacitor may be used
in demanding applications that have large load transients.
Another method of improving the transient response is to
add a small feedforward capacitor across the top resistor
of the feedback divider (from VOUTB to FBB). A typical value
of 22pF will generally suffice.
Ceramic capacitors are also a good choice for input decoupling of the step-up converter and should be located
as close as possible to the device. A 2.2μF input capacitor
is sufficient for most applications, although larger values
may be used without limitations. The LDO regulator will
have improved performance with an input capacitor, but
it is not required. Table 2 shows a list of several ceramic
capacitor manufacturers. Consult the manufacturers directly for detailed information on their selection of ceramic
capacitors.
Table 2. 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
TDK
(847) 803-6100
www.component.tdk.com
Samsung
(408) 544-5200
www.sem.samsung.com
TYPICAL APPLICATIONS
1-Cell to 1.8V, 1.5V
2.2μH
R6
665k
ALKALINE
0.8V
TO
1.6V
+
R5
1M
1μF
OFF ON
PVM BURST
VINB
SW
VOUTB
1.8V
VOUTB
LBI LTC3537 V
INL
LBO
FBB
PGDB
PGDL
VOLDO
ENDLO
ENBST
MODE
FBL
SGND PGND
R2
499k
4.7μF
R4
1.5M
VOLDO
1.5V
1μF
R1
1M
R3
1M
3537 TA02
3537fa
16
LTC3537
TYPICAL APPLICATIONS
1-Cell to 3.3V, 2.8V
2.2μH
R6
665k
ALKALINE
0.8V
TO
1.6V
VINB
+
R5
1M
1μF
OFF ON
PVM BURST
SW
VOUTB
3.3V
VOUTB
LBI LTC3537 V
INL
LBO
FBB
PGDB
PGDL
VOLDO
ENDLO
ENBST
MODE
FBL
SGND PGND
R2
1.74M
4.7μF
VOLDO
2.8V
R4
1.1M
1μF
R1
1M
R3
301k
3537 TA03
2-Cell to Low Noise 3.3V
2.2μH
2-CELL
ALKALINE
1.6V TO 3.2V
R6
2M
+
VINB
R5
1M
1μF
OFF ON
PVM BURST
SW
VOUTB
LBI LTC3537 VINL
LBO
FBB
PGDB
PGDL
VOLDO
ENDLO
ENBST
MODE
FBL
SGND PGND
R2
2M
4.7μF
VOLDO
3.3V
R4
2.37M
1μF
R1
1M
R3
523k
3537 TA04
2-Cell to 5V, 1.8V
2.2μH
2-CELL
ALKALINE
1.6V TO 3.2V
R6
2M
+
VINB
R5
1M
1μF
OFF ON
PVM BURST
SW
VOUTB
5V
VOUTB
LBI LTC3537 V
INL
LBO
FBB
PGDB
PGDL
VOLDO
ENDLO
ENBST
MODE
FBL
SGND PGND
R2
1.91M
4.7μF
VOLDO
1.8V
R4
2M
1μF
R1
604k
R3
1M
3537 TA05
3537fa
17
LTC3537
TYPICAL APPLICATIONS
Li-Ion to 5V, 3.3V
2.2μH
R6
2M
VINB
R5
499k
1μF
OFF ON
PVM BURST
VOUTB
5V
VOUTB
LBI LTC3537 V
INL
LBO
FBB
PGDB
PGDL
VOLDO
ENDLO
ENBST
MODE
FBL
SGND PGND
+
Li-Ion
2.8V
TO
4.2V
SW
R2
1.91M
4.7μF
R4
2.37M
VOLDO
3.3V
1μF
R1
604k
R3
523k
3537 TA06
Single Cell or 5V Input to 3.3V
USB
OR
0.8V TO 1.6V
5V ADAPTER
ALKALINE
2.2μH
+
+
R6
510k
1μF
R5
1.02M
OFF ON
PWM BURST
VINB
10μF
SW
VOUTB
LBI LTC3537
LBO
FBB
PGDB
V
INL
PGDL
ENLDO
VOLDO
ENBST
MODE
FBL
SGND PGND
3.3V/100mA
R3
1.74M
10μF
R2
511k
R1
487k
3537 TA07
3537fa
18
LTC3537
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
3537fa
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.
19
LTC3537
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PART NUMBER
DESCRIPTION
COMMENTS
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LTC3522
400mA Synchronous Buck-Boost and 200mA
Synchronous Buck Converter
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ISD < 1μA, 3mm × 3mm DFN Package
LTC3525-3/
LTC3525-3.3/
LTC3525-5
400mA Micropower Synchronous Step-Up DC/DC
Converter with Output Disconnect
95% Efficiency, VIN: 1V to 4.5V, VOUT(MAX) = 3.3V or 5V, IQ = 7μA,
ISD <1μA, SC-70 Package
LTC3525L-3
400mA Micropower Synchronous Step-Up DC/DC
Converter with Output Disconnect
90% Efficiency, VIN: 0.7V to 4.5V, VOUT = 3V, IQ = 7μA, ISD < 1μA,
SC70 Package
LTC3526/
LTC3526L
600mA Micropower Synchronous Step-Up DC/DC
Converter with Output Disconnect
95% Efficiency, VIN: 0.75V to 5V, VOUT(MAX): 1.5V to 5.25V, IQ = 12μA,
ISD <1μA, DFN6 Package
LTC3528/
LTC3528B
1A, 1MHz, Synchronous Step-Up DC/DC Converters
94% Efficiency, VIN: 0.7V to 5V, VOUT: 1.6V to 5.25V, IQ = 12μA, ISD < 1μA,
2mm × 3mm DFN Package, LTC3528B (PWM Mode Only)
ThinSOT is a trademark of Linear Technology Corporation.
3537fa
20 Linear Technology Corporation
LT 0608 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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