LINER LTC3100 1.5mhz synchronous dual channel dc/dc converter and 100ma ldo Datasheet

LTC3100
1.5MHz Synchronous Dual
Channel DC/DC Converter
and 100mA LDO
DESCRIPTION
FEATURES
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Extremely Compact Triple-Rail Solution
Burst Mode® Operation, IQ = 15μA
1.5MHz Fixed Frequency Operation
Power Good Indicators
700mA Synchronous Step-Up DC/DC
0.65V to 5V VIN Range
1.5V to 5.25V VOUT Range
94% Peak Efficiency
VIN > VOUT Operation
Output Disconnect
250mA Synchronous Step-Down DC/DC
1.8V to 5.5V VIN Range
0.6V to 5.5V VOUT Range
LDO (VIN Internally Tied to VBST)
0.6V to 5.25V VOUT Range
200mV Dropout Voltage at 100mA
Available in a 16-Lead 3mm × 3mm QFN Package
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A switching frequency of 1.5MHz minimizes solution footprint by allowing the use of tiny, low profile inductors and
ceramic capacitors. The switching regulators use current
mode control and are internally compensated, reducing
external parts count. Each converter automatically transitions to Burst Mode operation to maintain high efficiency
over the full load range. Burst Mode operation can be
disabled for low noise applications. The integrated LDO
provides a third low noise, low dropout supply.
Anti-ringing circuitry reduces EMI by damping the boost
inductor in discontinuous mode. Additional features
include shutdown current of under 1μA and overtemperature shutdown. The LTC3100 is housed in a 16-lead
3mm × 3mm 0.75mm QFN package.
APPLICATIONS
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The LTC®3100 combines a high efficiency 700mA synchronous step-up converter, a 250mA synchronous
step-down converter and a 100mA LDO regulator. The
LTC3100 features a wide input voltage range of 0.65V to
5V. The step-down converter can be powered by the output
of the step-up converter or from a separate power source
between 1.8V and 5.5V. The LDO can also be used as a
sequencing switch on the output of the boost.
Bar Code Readers
Medical Instruments
Low Power Portable Electronic Devices
, 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
Efficiency and Power Loss
vs Load Current, VIN = 2.4V
Two-Cell, Triple Output Converter
3.3μH
SWBST VINBK
VINBST
1.87M
VBST
10μF
s2
80
BOOST_GOOD
3V AT 50mA
VLDO
PGBST
VLDO
102k
2.2μF
FBLDO
1.8V AT 200mA
VBUCK
4.7μH
BOOST
LDO
OFF ON
RUNBST
OFF ON
RUNLDO
OFF ON
RUNBK
SWBK
2M
FBBK
1M
BUCK
GND
10μF
10
50
40
1
BOOST
BUCK
PL, BOOST
PL, BUCK
20
10
0
0.01
1M
0.1
1
10
100
LOAD CURRENT (mA)
0.1
0.01
1000
3100 TA01b
PGBK
3100 TA01a
60
30
25.5k
MODE
100
70
POWER LOSS (mW)
1.07M
LTC3100
1000
90
FBBST
2.2μF
FF EN_BURST
100
1M
EFFICIENCY (%)
VBATT
1.6V TO 3.2V
3.3V AT 100mA
VBOOST
BUCK_GOOD
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LTC3100
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
MODE
RUNBST
VINBST
PGBST
TOP VIEW
VINBST and VINBK Voltage .............................. –0.3 to 6V
SWBST, SWBK DC Voltage ............................. –0.3 to 6V
SWBST, SWBK Pulsed (< 100ns) Voltage ...... –0.3 to 7V
FBBST, FBBK, FBLDO, PGBST, PGBK Voltage . –0.3 to 6V
MODE, RUNBST, RUNBK, RUNLDO Voltage ... –0.3 to 6V
VBST, VLDO ...................................................... –0.3 to 6V
Operating Temperature (Notes 2, 5) .........–40°C to 85°C
Storage Temperature Range...................–65°C to 125°C
16 15 14 13
SWBST 1
12 FBBST
VBST 2
11 FBLDO
17
VLDO 3
10 RUNLDO
SWBK 4
6
7
8
VINBK
PGBK
GND
RUNBK
9
5
FBBK
UD PACKAGE
16-LEAD (3mm s 3mm) PLASTIC QFN
TJMAX = 125°C, θJA = 68°C/W, 4-LAYER BOARD
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB (NOTE 6)
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3100EUD#PBF
LTC3100EUD#TRPBF
LDJR
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: STEP-UP CONVERTER
The l denotes the specifications
which apply over the full operating temperature range. Extended commercial grade: –40°C to 85°C, VINBST = 1.2V, VBST = 3.3V,
TA = 25°C, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum Start-Up Voltage
ILOAD = 1mA
l
0.65
0.90
V
Input Voltage Range
After Start-Up (Minimum Voltage Is Load Dependent)
l
0.5
5
V
Output Voltage Adjust Range
l
1.5
5.25
V
l
Feedback Voltage
1.182
1.200
1.218
V
1
50
nA
RUNBST = 0V, Not Including Switch Leakage, VBST = 0V, VINBK = 0V
0.01
1
μA
Quiescent Current: Active
Measured on VBST (Note 4), RUNBK = 0V, RUNLDO = 0V
300
500
μA
Quiescent Current: Burst Mode
Operation
Measured on VBST, FBBST > 1.25V
MODE = 1V, RUNLDO = 0V
MODE = 1V, RUNLDO = 1V
15
28
25
45
μA
μA
N-Channel MOSFET Switch Leakage
Current
SWBST = 5V, VBST= 5V
0.1
5
μA
P-Channel MOSFET Switch Leakage
Current
SWBST = 0V, VBST = 5V
0.1
10
μA
Feedback Input Current
FBBST = 1.2V
Quiescent Current (VIN): Shutdown
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LTC3100
ELECTRICAL CHARACTERISTICS: STEP-UP CONVERTER
The l denotes the specifications
which apply over the full operating temperature range. Extended commercial grade: –40°C to 85°C, VINBST = 1.2V, VBST = 3.3V,
TA = 25°C, unless otherwise noted.
PARAMETER
CONDITIONS
N-Channel MOSFET Switch-On
Resistance
VBST = 3.3V
0.3
Ω
P-Channel MOSFET Switch-On
Resistance
VBST = 3.3V
0.4
Ω
N-Channel MOSFET Current Limit
MIN
MAX
UNITS
l
700
850
mA
87
90
%
Maximum Duty Cycle
VFBBST = 1.15V
l
Minimum Duty Cycle
VFBBST = 1.3V
l
0
Switching Frequency
l
1.2
RUNBST Input High Voltage
l
0.9
RUNBST Input Low Voltage
l
RUNBST Input Current
TYP
RUNBST = 1.2V
1.5
MHz
V
0.8
Soft-Start Time
1.8
%
0.3
V
2
μA
0.8
ms
PGBST Threshold, Falling
Referenced to Feedback Voltage
–8
%
PGBST Hysteresis
Referenced to Feedback Voltage
3
%
PGBST Voltage Low
5mA Load
65
mV
PGBST Leakage Current
PGBST = 5.5V
0.01
10
μA
ELECTRICAL CHARACTERISTICS: STEP-DOWN CONVERTER
The l denotes the
specifications which apply over the full operating temperature range. Extended commercial grade: –40°C to 85°C, VINBK = 3.3V,
TA = 25°C, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Voltage Range
l
1.8
5.5
V
Output Voltage Adjust Range
l
0.61
5.5
V
Feedback Voltage
l
590
600
610
mV
Feedback Input Current
FBBK = 600mV
1
30
nA
Quiescent Current: Shutdown
Measured on VINBK, RUNBK = 0V, VINBST = 0V, VBST = 0V
Not Including Switch Leakage
0.01
1
μA
Quiescent Current: Active
Measured on VINBK (Note 4), RUNBST = 0V
240
350
μA
Quiescent Current: Burst Mode Operation
Measured on VINBK, FBBK = 620mV, MODE = OPEN,
RUNBST = 0V
16
30
μA
N-Channel MOSFET Switch Leakage Current
VINBK = SWBK = 5V
0.1
5
μA
P-Channel MOSFET Switch Leakage Current
SWBK = 0V, VINBK = 5V
0.1
5
μA
N-Channel MOSFET Switch-On Resistance
VINBK = 3.3V
0.45
Ω
P-Channel MOSFET Switch-On Resistance
VINBK = 3.3V
0.55
Ω
450
mA
P-Channel MOSFET Current Limit
l
340
Maximum Duty Cycle
FBBK < 590mV
l
100
Minimum Duty Cycle
FBBK > 610mV
l
Switching Frequency
l
%
0
1.2
1.5
1.8
%
MHz
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LTC3100
ELECTRICAL CHARACTERISTICS: STEP-DOWN CONVERTER
The l denotes the
specifications which apply over the full operating temperature range. Extended commercial grade: –40°C to 85°C, VINBK = 3.3V,
TA = 25°C, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
RUNBK Input High Voltage
l
RUNBK Input Low Voltage
l
RUNBK Input Current
TYP
MAX
0.9
V
0.8
RUNBK = 1.2V
Soft-Start Time
UNITS
0.3
V
2
μA
1.3
ms
PGBK Threshold, Falling
Referenced to Feedback Voltage
–8
%
PGBK Hysteresis
Referenced to Feedback Voltage
3
%
PGBK Voltage Low
5mA Load
65
mV
PGBK Leakage Current
PGBK = 5.5V
0.01
10
μA
ELECTRICAL CHARACTERISTICS: LDO REGULATOR
The l denotes the specifications which
apply over the full operating temperature range. Extended commercial grade: –40°C to 85°C, VBST = 3.3V, VLDO = 3V, TA = 25°C, unless
otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
1.8
5.25
V
0.618
5.25
V
618
mV
l
l
Feedback Voltage
l
582
600
Maximum Output Current
l
100
120
Input Voltage Range
Output Voltage Adjust Range
Feedback Input Current
(Note 3)
FBLDO = 600mV
1
mA
30
nA
Line Regulation
VIN = 3.3V to 5.25V
0.1
%/ V
Load Regulation
From 10mA to 100mA Load
0.1
%
Dropout Voltage
IOUT = 100mA
Ripple Rejection (PSRR)
Frequency = 1.5MHz at ILOAD = 50mA, COUT = 2.2μF (Note 3)
Short-Circuit Current Limit
FBLDO < 582mV
l
130
200
35
l
120
Soft-Start Time
dB
160
0.3
RUNLDO Input High Voltage
l
RUNLDO Input Low Voltage
l
mV
mA
ms
0.9
V
0.3
V
RUNLDO Input Current
RUNLDO = 1.2V
0.8
2
μA
Quiescent Current—Active
RUNLDO = 3.3V, Measured on VBST
RUNBST = RUNBK = 0V, VINBK = 0V
26
40
μA
ELECTRICAL CHARACTERISTICS: COMMON CIRCUITRY
The l denotes the specifications
which apply over the full operating temperature range. Extended commercial grade: –40°C to 85°C, VBST or VINBK = 3.3V, TA = 25°C,
unless otherwise noted.
PARAMETER
CONDITIONS
MIN
MODE Input High Voltage
l
MODE Input Low Voltage
l
MODE Input Current
MODE = 0V
MODE = 5V
TYP
MAX
0.9
UNITS
V
–3.3
1.7
0.3
V
–5
3
μA
μA
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LTC3100
ELECTRICAL CHARACTERISTICS
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 LTC3100E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over –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.
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.
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise specified.
Step-Up DC/DC Converter
Efficiency vs Load Current
and VIN for VO = 1.8V
Efficiency vs Load Current
and VIN for VO = 3.3V
100
90
90
80
10
50
40
1
30
VIN = 1.2V
VIN = 1.5V
PL, VIN = 1.2V
PL, VIN = 1.5V
20
10
0
0.01
0.1
1
10
100
LOAD CURRENT (mA)
EFFICIENCY (%)
60
70
10
60
50
1
40
VIN = 1.2V
VIN = 2.4V
VIN = 3V
PL, VIN = 1.2V
PL, VIN = 2.4V
PL, VIN = 3V
30
20
0.1
10
0
0.01
0.01
1000
0.1
1
10
100
LOAD CURRENT (mA)
Efficiency vs Load Current
and VIN for VO = 5V
0.01
1000
3.3V, 100mA Efficiency vs VIN
100
1000
100
100
80
90
60
10
50
40
1
VIN = 1.8V
VIN = 2.4V
VIN = 3.6V
PL, VIN = 1.8V
PL, VIN = 2.4V
PL, VIN = 3.6V
30
20
10
0.1
1
10
100
LOAD CURRENT (mA)
POWER LOSS (mW)
70
0.1
0.01
1000
3100 G03
EFFICIENCY (%)
90
80
EFFICIENCY (%)
0.1
3100 G02
3100 G01
0
0.01
POWER LOSS (mW)
70
100
80
100
POWER LOSS (mW)
EFFICIENCY (%)
1000
100
1000
70
60
50
40
30
20
10
VBST = 3.3V AT 100mA
0
1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2
VINBST (V)
3100 G04
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LTC3100
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise specified.
Step-Up DC/DC Converter
No-Load Input Current vs VIN,
Mode = Open, LDO and Buck Off
600
180
160
1000
VOUT = 3.3V
500
120
100
80
60
VOUT = 5V
40
VOUT = 1.8V
LOAD CURRENT (mA)
OUTPUT CURRENT (mA)
140
INPUT CURRENT (μA)
Maximum Load Current During
Start-Up vs VIN
Maximum Output Current vs VIN
400
300
VOUT = 5V
200
100
10
100
20
VOUT = 1.8V
VOUT = 3.3V
0
0
1.0
1.5
2.0
2.5 3.0
VINBST (V)
3.5
4.0
0
4.5
0.5
1
1.5
2
2.5 3
VINBST (V)
Burst Mode Threshold
Current vs VIN
1
4.5
0.7
0.8
0.9
1
1.1
VINBST (V)
1.2
1.3
3100 G07
VBST = 1.8V, RESISTIVE LOAD
VBST = 1.8V, CONSTANT-CURRENT LOAD
VBST = 3.3V, RESISTIVE LOAD
VBST = 3.3V, CONSTANT-CURRENT LOAD
VBST = 5V, RESISTIVE LOAD
VBST = 5V, CONSTANT-CURRENT LOAD
Start-Up Voltage vs Temperature
0.85
L = 3.3μH
0.80
40
INPUT VOLTAGE (V)
50
LOAD CURRENT (mA)
4
3100 G06
3100 G05
60
3.5
VOUT = 3.3V
VOUT = 1.8V
30
VOUT = 5V
20
10
0.75
0.70
0.65
0.60
0.55
0
1.0
1.5
2.0
2.5 3.0
VINBST (V)
3.5
4.0
4.5
0.50
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
3100 G09
3100 G08
Output Voltage Ripple in Fixed
Frequency and Burst Mode Operation
VOUT and IIN During Soft-Start
VBST COUT = 20μF
0.5μs/DIV
100mA LOAD
20mV/DIV
IIN
200mA/DIV
20μs/DIV
5mA LOAD
20mV/DIV
VBST
1V/DIV
500μs/DIV
3100 G10
3100 G11
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LTC3100
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise specified.
Step-Up DC/DC Converter
Load Step Response, 50mA-150mA
Fixed Frequency Mode
Load Step Response, 5mA-100mA
Burst Mode Operation Enabled
VBST COUT = 10μF
VBST COUT = 20μF
IOUT
100mA/DIV
IOUT
100mA/DIV
VBST
50mV/DIV
VBST
50mV/DIV
3100 G12
100μs/DIV
100μs/DIV
3100 G13
LDO Regulator
Dropout Voltage vs VOUT
and Temperature (IOUT = 100mA)
Ripple Rejection
VLDO = 1.5V
0.225
LDO COUT = 2.2μF
35
0.200
30
0.175
VLDO = 2.5V
0.150
0.125
RUNLDO
2V/DIV
25
20
15
VLDO
1V/DIV
VLDO = 5V
0.100
0.075
Soft-Start Time
40
PSRR (dB)
DROPOUT VOLTAGE (VBST_VLDO)
0.250
10
VLDO = 3.3V
0.050
–45 –30 –15
0
15
30
45
60
90
75
VOUT = 3V
5 I
OUT = 50mA
COUT = 2.2μF
0
0.1
1
TEMPERATURE (°C)
100μs/DIV
10
100
FREQUENCY (Hz)
1000
6105 G14
10000
3100 G15
Burst Mode Operation
Ripple Rejection
Load Step Response, 10mA-60mA
LDO COUT = 2.2μF
LDO COUT = 2.2μF
BOOST RIPPLE
20mV/DIV
50mA/DIV
LDO RIPPLE
20mV/DIV
VLDO
100mV/DIV
5μs/DIV
3100 G16
3100 G17
200μs/DIV
3100 G18
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LTC3100
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise specified.
Step-Down DC/DC Converter
Efficiency vs Load Current and VIN
for VO = 1.2V
100
Efficiency vs Load Current and VIN
for VO = 1.8V
90
90
80
50
40
1
VIN = 1.8V
VIN = 2.4V
VIN = 3.3V
PL, VIN = 1.8V
PL, VIN = 2.4V
PL, VIN = 3.3V
30
20
10
0
0.01
0.1
EFFICIENCY (%)
10
70
50
1
40
VIN = 2.4V
VIN = 3.3V
VIN = 5V
PL, VIN = 2.4V
PL, VIN = 3.3V
PL, VIN = 5V
30
20
0.1
10
0
0.01
0.01
1000
1
10
100
LOAD CURRENT (mA)
10
60
0.1
1
10
100
LOAD CURRENT (mA)
Burst Mode Operation Threshold
Current vs VIN
20
LOAD CURRENT (mA)
INPUT CURRENT (μA)
70
10
5
VOUT = 1.2V
INPUT
CURRENT
50mA/DIV
50
VOUT = 1.5V
30
VOUT
0.5V/DIV
20
VOUT
= 1.8V
10
0
1.5
2
2.5
3
3.5
4
4.5
STARTUP, 200mA LOAD
VIN = 2.4V
VOUT = 1.2V
COUT = 10μF
60
40
0.01
1000
VOUT and IIN During Soft-Start
80
15
0.1
3100 G20
3100 G19
No-Load Input Current
vs VINBK (Mode = Open)
POWER LOSS (mW)
60
100
80
100
70
POWER LOSS (mW)
EFFICIENCY (%)
1000
100
1000
VOUT = 2.5V
2ms/DIV
3100 G23
0
5
2
2.5
VINBK (V)
3
3.5
4
4.5
5
VINBK (V)
3100 G21
3100 G22
Load Step Response,
Fixed Frequency Mode
10mA to 100mA
Output Voltage Ripple in Fixed
Frequency and Burst Mode Operation
Load Step Response,
Burst Mode Operation Enabled
10mA to 100mA
COUT = 10μF
COUT = 10μF
COUT = 10μF
100mA/DIV
50mV/DIV
50mA/DIV
50mV/DIV
50mV/DIV
50mV/DIV
5μs/DIV
3100 G24
200μs/DIV
3100 G25
200μs/DIV
3100 G26
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LTC3100
TYPICAL PERFORMANCE CHARACTERISTICS
RUN Pin Threshold Voltage
TA = 25°C, unless otherwise specified.
Start-Up Delay Times vs VIN
450
0.625
400
0.575
FALLING
0.550
BUCK
350
RISING
DELAY TIME (μs)
THRESHOLD (V)
0.600
BOOST
300
250
200
LDO
150
100
0.525
50
0.500
1
1.5
2
2.5
3
3.5
4
4.5
5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
VIN (V)
VIN (V)
3100 G28
3100 G27
PIN FUNCTIONS
SWBST (Pin 1): Switch Pin for the Boost Converter.
Connect the boost inductor between SWBST and VINBST.
Keep PCB trace lengths as short and wide as possible to
reduce EMI. If the inductor current falls to zero, an internal
anti-ringing switch is connected from SWBST to VINBST
to minimize EMI.
VBST (Pin 2): Output Voltage for the Boost Converter
(which is the drain of the internal synchronous rectifier)
and Input Voltage for the LDO. PCB trace length from VBST
to the output filter capacitor (10μF minimum) should be
as short and wide as possible.
VLDO (Pin 3): Output Voltage of the LDO Regulator. Connect
a 1μF ceramic capacitor between VLDO and GND. Larger
values of capacitance may be used for higher PSRR or
improved transient response.
SWBK (Pin 4): Switch Pin for the Buck Converter. Connect
the buck inductor between SWBK and the buck output
filter capacitor. Keep PCB trace lengths as short and wide
as possible to reduce EMI.
VINBK (Pin 5): Input Voltage for the Buck Converter. Connect a minimum of 4.7μF ceramic decoupling capacitor
from this pin to ground.
PGBK (Pin 6): Open-Drain Output That Pulls Low When
FBBK Is More Than 8% Below Its Regulated Voltage. Con-
nect a pull-up resistor from this pin to a positive supply
less than 6V.
GND (Pin 7): Signal Ground. Provide a short, direct PCB
path between GND and the PC board ground plane connected to the Exposed Pad.
RUNBK (Pin 8): Logic-Controlled Shutdown Input for the
Buck Converter. There is an internal 4MΩ pull-down on
this pin.
RUNBK = High: Normal operation
RUNBK = Low: Shutdown
FBBK (Pin 9): Feedback Input to the gm Error Amplifier
for the Buck Converter. Connect the resistor divider tap
to this pin. The output voltage can be adjusted from 0.6V
to 5.5V by:
⎛ R6 ⎞
VOUT _ BUCK = 0 . 600 V • ⎜ 1 + ⎟
⎝ R5 ⎠
RUNLDO (Pin 10): Logic-Controlled Shutdown Input for
the LDO Regulator. There is an internal 4MΩ pull-down
on this pin.
RUNLDO = High: Normal operation
RUNLDO = Low: Shutdown
3100fa
9
LTC3100
PIN FUNCTIONS
FBLDO (Pin 11): Feedback Input to the gm Error Amplifier
for the LDO Regulator. Connect the resistor divider tap to
this pin. The output voltage can be adjusted from 0.6V
to 5.25V by:
⎛ R4 ⎞
VOUT _ LDO = 0 . 600 V • ⎜ 1 + ⎟
⎝ R3 ⎠
FBBST (Pin 12): Feedback Input to the gm Error Amplifier
for the Boost Converter. Connect the resistor divider tap
to this pin. The output voltage can be adjusted from 1.5V
to 5.25V by:
⎛ R2 ⎞
VOUT _ BOOST = 1 . 20 V • ⎜ 1 + ⎟
⎝
R1⎠
MODE (Pin 13): Logic-Controlled Mode Select Pin for
Both the Boost and Buck Converters. There is an internal
1MΩ pull-up on this pin to the higher of VINBST, VBST or
VINBK.
RUNBST (Pin 14): Logic-Controlled Shutdown Input for
the Boost Converter. There is an internal 4MΩ pull-down
on this pin.
RUNBST = High: Normal operation
RUNBST = Low: Shutdown
PGBST (Pin 15): Open-Drain Output That Pulls to Ground
When FBBST Is More Than 8% Below Its Regulated Voltage. Connect a pull-up resistor from this pin to a positive
supply less than 6V.
VINBST (Pin 16): Input Voltage for the Boost Converter.
Connect a minimum of 1μF ceramic decoupling capacitor
from this pin to ground.
Exposed Pad (Pin 17): The Exposed Pad must be soldered
to the PCB ground plane. It serves as the power ground
connection, and as a means of conducting heat away
from the die.
MODE = Float or High: Enables Burst Mode operation for
both the boost and the buck.
MODE = Low: Disables Burst Mode operation. Both converters will operate in fixed frequency mode regardless
of load current.
3100fa
10
LTC3100
BLOCK DIAGRAM
L1, 3.3μH
VBATT1, 0.65V TO 5V
VBOOST, 1.5V TO 5.25V
CIN
2.2μF
R2
16
1
VINBST
2
SWBST
VBST
12
VSEL
VINBK
R1
WELL
SWITCH
VBEST
PGBST
VB
VBEST
VBEST OF 3
15
GATE
DRIVERS
AND
ANTI-CROSS
CONDUCTION
1.1V
– +
VREF
VREF_GD
VREF
VREF_GD
3
+
–
IPK
START_OSC
IZERO
MODE
CONTROL
THERMAL
SHUTDOWN
WELL
SWITCH
TSD
BURST
R4
ERROR
AMPLIFIER
100mA
–
+
ILIM
–
+
VBST
1M
5
ILIM REF
ISENSE
FROM VBST, VBATT1
OR VBATT2
CIN
4.7μF
UVLO
LEVEL
SHIFT
SHUTDOWN
CLK
TSD
SHUTDOWN
LOGIC
4M
GND
VBUCK
0.6V TO 5V
R6
COUT
4.7μF
R5
3
+
–
PWM
0.55V
4
IZERO
COMPARATOR
SLOPE
COMPARATOR
PGBK
6
L1
3.3μH
SWBK
ISENSE
ERROR
AMPLIFIER
+
–
SHUTDOWN
LOGIC
GATE
DRIVERS
AND
ANTI-CROSS
CONDUCTION
+
–
RUNLDO
RUNBK
OFF ON
+
–
IPK
COMPARATOR
WAKE
8
OFF ON
4M
VINBK
4M
MODE
R3
10
SHUTDOWN
13
0.6V
GATE
CONTROL
TSD
ISET
CLAMP
SOFT-START
COUT
1μF
FBLDO
11
RUNLDO
1.2V
RUNBST
14
VLDO
0.6V TO 5V
VLDO
3
ERROR
AMPLIFIER/
SLEEP
COMPARATOR
FB
LOGIC
–
+
0.15Ω
+
–
CLK
1.5MHz
OSC
IZERO
COMPARATOR
SLOPE
COMPARATOR
IPK
COMPARATOR
START-UP
OFF ON
COUT
10μF
FBBST
VBST
FBBK
9
0.6V
PAD
7
PGND
3100 BD
3100fa
11
+
–
LTC3100
OPERATION
The LTC3100 includes an 700mA synchronous step-up
(boost) converter, a 250mA synchronous step-down
(buck) converter and a 100mA low dropout (LDO) linear
regulator housed in a 16-lead 3mm × 3mm QFN package.
Both converters utilize current mode PWM control for
exceptional line and load regulation and operate from the
same 1.5MHz oscillator. The current mode architecture
with adaptive slope compensation also provides excellent
transient load response, requiring minimal output filtering. Both converters have internal soft-start and internal
loop compensation, simplifying the design process and
minimizing the number of external components.
requirement for a large input capacitor. The limiting factor for the application becomes the ability of the power
source to supply sufficient energy to the output at low
input voltages, and maximum duty cycle of the converter,
which is clamped at 90% (typical). Note that at low input
voltages, even small input voltage drops due to series
resistance become critical, and greatly limit the power
delivery capability of the converter.
With its low RDS(ON) and low gate charge internal MOSFET
switches and synchronous rectifiers, the LTC3100 achieves
high efficiency over a wide range of load current. Burst
Mode operation maintains high efficiency at very light loads,
but can be disabled for noise-sensitive applications.
The internal soft-start circuitry ramps the peak boost
inductor current from zero to its peak value of 700mA in
approximately 800μs, allowing start-up into heavy loads.
The soft-start circuitry is reset in the event of a commanded
shutdown or an overtemperature shutdown.
With separate power inputs for the boost and buck converters, along with independent enable and power good
functions, the LTC3100 is very flexible. The two converters
can operate from the same input supply, or from two different sources, or can even be cascaded by powering the
buck converter from the output of the boost converter. By
using the LDO as well, three different output voltages can
be generated from a single alkaline/NiMH cell (or the LDO
can be used for power sequencing the boost output).
Operation can be best understood by referring to the
Block Diagram.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
Oscillator
An internal oscillator sets the switching frequency to
1.5MHz. The oscillator allows a maximum duty cycle of
90% (typical) for the boost converter.
Shutdown
The boost converter is shut down by pulling the RUNBST
pin below 0.3V, and activated by pulling the RUNBST pin
above 0.9V. Note that RUNBST can be driven above VIN
or VOUT, as long as it is limited to less than the absolute
maximum rating.
BOOST CONVERTER
Error Amplifier
Low Voltage Start-Up
The error amplifier is a transconductance type. The
non-inverting input is internally connected to the 1.20V
reference and the inverting input is connected to FBBST.
Clamps limit the minimum and maximum error amp output voltage for improved large signal transient response.
Power converter control loop compensation is provided
internally. A voltage divider from VBST to ground programs
the output voltage (via FBBST) from 1.5V to 5.25V, according to the formula:
⎛ R2 ⎞
VBST = 1 . 20 V • ⎜ 1 + ⎟
⎝
R1⎠
The LTC3100 boost converter includes an independent
start-up oscillator designed to start up at an input voltage
of 0.65V (typical). Soft-start and inrush current limiting are
provided during start-up, as well as in normal mode.
When either VINBST or VBST exceeds 1.4V (typical), the
IC enters normal operating mode. Once the output voltage exceeds the input by 0.24V, the IC powers itself from
VBST instead of VINBST. At this point, the internal circuitry
has no dependency on the input voltage, eliminating the
3100fa
12
LTC3100
OPERATION
Current Sensing
Lossless current sensing converts the peak current signal
of the N-channel MOSFET switch into a voltage which
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.
Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. Peak switch
current is no less than 700mA, independent of input or
output voltage, unless VOUT falls below 1V, in which case
the current limit is cut in half to minimize power dissipation into a short-circuit.
Slope Compensation
Current mode control requires the use of slope compensation to prevent subharmonic oscillations in the inductor
current waveform at high duty cycle operation. This is accomplished internally on the LTC3100 through the addition
of a compensating ramp to the current sense signal. The
LTC3100 performs current limiting prior to addition of the
slope compensation ramp and therefore achieves a peak
inductor current limit that is independent of duty cycle.
Zero Current Comparator
The zero current comparator monitors the boost inductor
current to the output and shuts off the synchronous rectifier
once this current reduces to approximately 30mA. This
prevents the inductor current from reversing in polarity,
improving efficiency at light loads.
Synchronous Rectifier
To control inrush current and to prevent the inductor
current from running away when VOUT is close to VIN ,
the P-channel MOSFET synchronous rectifier is only fully
enabled when VOUT > (VIN + 0.24V).
Anti-Ringing Control
The anti-ring circuitry connects a resistor across the
boost inductor to prevent high frequency ringing on the
SW pin during discontinuous current mode operation.
The ringing of the resonant circuit formed by L and CSW
(capacitance on SWBST pin) is low energy, but can cause
EMI radiation.
PGOOD Comparator
The PGBST pin is an open-drain output which indicates the
status of the boost converter output voltage. If the boost
output voltage falls 8% below the regulation voltage, the
PGBST open-drain output will pull low. The output voltage
must rise 3% above the falling threshold before the pulldown will turn off. In addition, there is a 60μs (typical)
deglitching delay in order to prevent false trips due to
voltage transients on load steps. The PGBST output will
also pull low if the boost converter is disabled. The typical
PGBST pull-down switch resistance is 13Ω when VBST or
VINBST equals 3.3V.
Output Disconnect
The LTC3100 boost converter is designed to allow true
output disconnect by eliminating body diode conduction
of the internal P-channel MOSFET rectifier. This allows for
VOUT to go to 0V during shutdown, drawing no current
from the input source. It also allows for inrush current
limiting at turn-on, minimizing surge currents seen by the
input supply. Note that to obtain the advantages of output
disconnect, there must not be an external Schottky diode
connected between SWBST and VBST. The output disconnect feature also allows VOUT to be pulled high without
any reverse current into the battery.
VIN > VOUT Operation
The LTC3100 boost converter will maintain voltage regulation even when the input voltage is above the desired
output voltage. Note that the output current capability is
slightly reduced in this mode of operation. Refer to the
Typical Performance Characteristics section.
Burst Mode Operation (for Boost and Buck Converters)
Burst Mode operation for both converters can be enabled
or disabled using the MODE pin. If MODE is grounded,
Burst Mode operation is disabled for both the boost and
3100fa
13
LTC3100
OPERATION
buck converters. In this case, both converters will remain in
fixed frequency operation, even at light load currents. If the
load is very light, they will exhibit pulse-skip operation.
If MODE is raised above 0.9V, or left open, Burst Mode
operation will be enabled for both converters. In this case,
either converter may enter Burst Mode operation at light
load, and return to fixed frequency operation when the
load current increases. Refer to the Typical Performance
Characteristics section to see the output load Burst Mode
threshold vs VIN and VOUT. The two converters can enter or
leave Burst Mode operation independent of each other.
In Burst Mode operation, each converter still switches at
a frequency of 1.5MHz, 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 regulation value, then the LTC3100 transitions to
sleep mode where the outputs are off and the LTC3100
consumes only 15μA of quiescent current from VBST. Once
the output voltage has drooped slightly, switching resumes
again. This maximizes efficiency at very light loads by
minimizing switching and quiescent losses. Burst Mode
operation output ripple is typically 1% peak-to-peak.
Burst Mode operation for the boost converter is inhibited
during start-up, and until soft-start is complete and VBST
is at least 0.24V greater than VINBST.
Short-Circuit Protection
The LTC3100 output disconnect feature allows output
short-circuit while maintaining a maximum internally set
current limit. To reduce power dissipation under shortcircuit conditions, the boost peak switch current limit is
reduced to 400mA (typical).
Schottky Diode
Although it is not required, adding a Schottky diode from
SWBST to VBST will improve efficiency by about 2%. Note
that this defeats the boost output disconnect and shortcircuit protection features.
BUCK CONVERTER OPERATION
The buck converter provides a high efficiency, lower voltage
output and supports 100% duty cycle operation to extend
battery life. The buck converter uses the same 1.5MHz
oscillator used by the boost converter.
PWM Mode Operation
When the MODE pin is held low, the LTC3100 buck converter
uses a constant-frequency, current mode control architecture. Both the main (P-channel MOSFET) and synchronous
rectifier (N-channel MOSFET) switches are internal. At
the start of each oscillator cycle, the P-channel switch
is turned on and remains on until the current waveform
with superimposed slope compensation ramp exceeds the
error amplifier output. At this point, the synchronous
rectifier is turned on and remains on until the inductor
current falls to zero or a new switching cycle is initiated.
As a result, the buck converter operates with discontinuous
inductor current at light loads which improves efficiency.
At extremely light loads, the minimum on-time of the main
switch will be reached and the buck converter will begin
turning off for multiple cycles (pulse-skipping) in order
to maintain regulation.
Burst Mode Operation
When the MODE pin is forced high, or left open, the buck
converter will automatically transition between Burst Mode
operation at sufficiently light loads (below approximately
10mA) and PWM mode at heavier loads. Burst Mode operation entry is determined by the peak inductor current and
therefore the load current at which Burst Mode operation
will be entered depends on the input voltage, the output
voltage and the inductor value. Typical curves for Burst
Mode operation entry threshold are provided in the Typical
Performance Characteristics section of this data sheet. The
quiescent current on VINBK in Burst Mode operation is only
15μA. If the boost converter is enabled and VINBST or VBST
are at a higher potential than VINBK, some of the quiescent
current will be supplied by the boost converter, reducing
the burst quiescent current on VINBK to just 9μA.
3100fa
14
LTC3100
OPERATION
Dropout Operation
As the input voltage decreases to a value approaching the
output regulation voltage, the duty cycle increases toward
the maximum on-time. Further reduction of the supply
voltage will force the main switch to remain on for more
than one cycle until 100% duty cycle operation is reached
where the main switch remains on continuously. In this
dropout state, the output voltage will be determined by
the input voltage less the resistive voltage drop across the
main switch and series resistance of the inductor.
Slope Compensation
Current mode control requires the use of slope compensation to prevent subharmonic oscillations in the inductor
current waveform at high duty cycle operation. This is accomplished internally on the LTC3100 through the addition
of a compensating ramp to the current sense signal. In
some current mode ICs, current limiting is performed by
clamping the error amplifier voltage to a fixed maximum.
This leads to a reduced output current capability at low
step-down ratios. In contrast, the LTC3100 performs current limiting prior to addition of the slope compensation
ramp and therefore achieves a peak inductor current limit
that is independent of duty cycle.
Short-Circuit Protection
When the buck output is shorted to ground, the error amplifier will saturate high and the P-channel MOSFET switch
will turn on at the start of each cycle and remain on until
the current limit trips. During this minimum on-time, the
inductor current will increase rapidly and will decrease very
slowly during the remainder of the period due to the very
small reverse voltage produced by a hard output short.
To eliminate the possibility of inductor current runaway
in this situation, the buck converter switching frequency
is reduced to approximately 375kHz when the voltage on
FBBK falls below 0.3V.
Soft-Start
respond to output load transients which occur during
this time. In addition, the output voltage rise time has
minimal dependency on the size of the output capacitor
or load current.
Error Amplifier and Compensation
The LTC3100 buck converter utilizes an internal transconductance error amplifier. Compensation of the feedback
loop is performed internally to reduce the size of the
application circuit and simplify the design process. The
compensation network has been designed to allow use of
a wide range of output capacitors while simultaneously
ensuring rapid response to load transients.
Undervoltage Lockout
If the VINBK supply voltage decreases below 1.6V (typical),
the buck converter will be disabled. The soft-start for the
buck converter will be reset during undervoltage lockout
to provide a smooth restart once the input voltage rises
above the undervoltage lockout threshold.
PGOOD Comparator
The PGBK pin is an open-drain output which indicates the
status of the buck converter output voltage. If the buck
output voltage falls 8% below the regulation voltage, the
PGBK open-drain output will pull low. The output voltage
must rise 3% above the falling threshold before the pulldown will turn off. In addition, there is a 60μs typical deglitching delay in order to prevent false trips due to voltage
transients on load steps. The PGBK output will also pull
low during overtemperature shutdown and undervoltage
lockout to indicate these fault conditions, or if the buck
converter is disabled. The typical PGBK pull-down switch
resistance is 13Ω when VINBK = 3.3V.
Schottky Diode
Although it is not required, adding a Schottky diode from
SWBK to the ground plane will improve efficiency by
about 2%.
The buck converter has an internal voltage mode soft-start
circuit with a nominal duration of 1.3ms. The converter
remains in regulation during soft-start and will therefore
3100fa
15
LTC3100
OPERATION
LDO REGULATOR OPERATION
COMMON FUNCTIONS
The LDO regulator utilizes an internal 1.3Ω (typical)
P-channel MOSFET pass device to supply up to 100mA
of load current with a typical dropout voltage of 130mV.
The input voltage to the LDO is internally connected to
the boost output (VBST pin), and can share the same filter
capacitor. The LDO can be operated independently of the
boost (or buck) converter, providing a sufficient voltage
is present on VBST.
Oscillator
Soft-Start and Current Limit
The MODE pin is used to force fixed frequency operation (MODE < 0.3V) or to enable Burst Mode operation
(MODE > 0.9V) for both the boost and buck converters.
With Burst Mode operation enabled, the two converters
will automatically enter or leave Burst Mode operation
independently, based on their respective load conditions.
There is an internal 1MΩ pull-up on MODE, in the event
that the pin is left open.
The LDO has an independent current limit circuit that limits
output current to 120mA (typical). To minimize loading on
the boost converter output when enabling the LDO, the LDO
current limit is soft-started over a 500μs period. Therefore
the rise time of the LDO output voltage will depend on the
amount of capacitance on the VLDO pin.
Reverse Current Blocking
The LDO is designed to prevent any reverse current from
VLDO back to the VBST pin, both in normal operation and
in shutdown. If VLDO is pulled above VBST and VBST is
above 1V, there will be a small (1μA typical) current from
VLDO to ground.
The 1.5MHz oscillator is shared by the boost and buck
converters. It will be oscillating if either converter is enabled. If both converters are enabled, the boost N-channel
MOSFET switch will be turned on coincident with the buck
P-channel MOSFET switch.
MODE Control
Note: Leaving the pin open, or connecting it to the highest of VINBK or VBST, will result in the lowest Burst Mode
quiescent current.
Overtemperature Shutdown
If the die temperature exceeds 150°C (typical) both converters and the LDO regulator will be disabled. All power
devices will be turned off and all switch nodes will be high
impedance. The soft-start circuits for both converters
and the LDO are reset during overtemperature shutdown
to provide a smooth recovery once the overtemperature
condition is eliminated. Both converters and the LDO will
restart (if enabled) when the die temperature drops to
approximately 130°C.
3100fa
16
LTC3100
APPLICATIONS INFORMATION
PC Board Layout Guidelines
should be placed as close to the IC as possible and
should have the shortest possible paths to ground.
The LTC3100 switches large currents at high frequencies. Special care should be given to the PC board layout
to ensure stable, noise-free operation. You will not get
advertised performance with a careless layout. Figure 1
depicts the recommended PC board layout. 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.
2. To prevent large circulating currents from disrupting
the output voltage sensing, the ground for each resistor divider should be returned directly to the ground
plane near the IC.
3. Use of vias in the die attach pad of the IC will enhance
the thermal environment of the converter, especially if
the vias extend to a ground plane region on the exposed
bottom surface of the PC board.
A few key guidelines follow:
16
SWBST 1
15
14
MODE
RUNBST
PGBST
4. Keep the connection from the resistor dividers to the
feedback pins as short as possible and away from the
switch pin connections.
VINBST
1. All circulating high current paths should be kept as
short as possible. Capacitor ground connections should
via down to the ground plane in the shortest route possible. The bypass capacitors on all VIN and VOUT pins
13
12 FBBST
LTC3100
VBST 2
11 FBLDO
VLDO 3
10
SWBK 4
RUNLDO
9 FBBK
8
RUNBK
7
GND
6
PGBK
VINBK
5
VBUCK
3100 F01
Figure 1. Recommended Component Placement for Two-Layer PC Board
3100fa
17
LTC3100
APPLICATIONS INFORMATION
COMPONENT SELECTION
Boost Output Voltage Programming
The boost output voltage is set by a resistive divider according to the following formula:
⎛ R2 ⎞
VOUT = 1 . 200 V • ⎜ 1 + ⎟
⎝
R1⎠
Boost Inductor Selection
The LTC3100 boost converter can utilize small surface
mount and chip inductors due to the fast 1.5MHz switching frequency. Inductor values between 2.2μH and 4.7μH
are suitable for most applications. Larger values of inductance will allow slightly 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.
The minimum boost inductance value is given by:
(
VIN(MIN) • VOUT(MAX ) − VIN(MIN)
Table 1. Recommended Boost Inductors
VENDOR
The external divider is connected to the output as shown
in the Block Diagram. A feedforward capacitor may be
placed in parallel with resistor R2 to improve the noise
immunity of the feedback node, improve transient response
and reduce output ripple in Burst Mode operation. A value
of 33pF will generally suffice.
L>
do not have enough core area to support the peak inductor currents of 800mA seen on the LTC3100. To minimize
radiated noise, use a shielded inductor. See Table 1 for
suggested components and suppliers.
)
1 . 5 • RIPPLE • VOUT(MAX )
Where:
RIPPLE = Allowable Inductor Current Ripple (Amps Peakto-Peak)
VIN(MIN) = Minimum Input Voltage
VOUT(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 DCR
(series resistance of the winding) to reduce the I2R power
losses, and must not saturate at peak inductor current
levels. Molded chokes and some chip inductors usually
Coilcraft
(847) 639-6400
www.coilcraft.com
PART/STYLE
LPS4012, LPS4018
MSS4020, MSS5131
Coiltronics
SD14, SD3814, SD3118
FDK
MIPSA2520
MIPW3226
Murata
www.murata.com
LQH43C
Sumida
(847) 956-0666
www.sumida.com
CDRH2D18, CDRH2D16
CDRH3D14, CDRH3D16
CDRH4D14, CDRH4D16
Taiyo-Yuden
www.t-yuden.com
NR3015
NP03SB
TDK
www.tdk.com
VLP
VLF, VLCF
Toko
(408) 432-8282
www.tokoam.com
D518LC
D52LC
DP418C
Würth
(201) 785-8800
www.we-online.com
WE-TPC Type S, M
Boost Input and Output Capacitor Selection
The internal loop compensation of the LTC3100 boost converter is designed to be stable with output capacitor values
of 4.7μF or greater. 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 fixed frequency applications. For applications
where Burst Mode operation is enabled, a minimum value
of 20μF is recommended. Larger values may be used to
obtain very low output ripple and to improve transient
response. X5R and X7R dielectric materials are preferred
for their ability to maintain capacitance over wide voltage
and temperature ranges. Y5V types should not be used.
Case sizes smaller than 0805 are not recommended due
to their increased DC bias effect.
3100fa
18
LTC3100
APPLICATIONS INFORMATION
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. A 2.2μF input capacitor on the VINBST
pin is sufficient for most applications. Larger values may
be used without limitations. For applications where the
power source is more than a few inches away, a larger
bulk decoupling capacitor is recommended on the input
to the boost converter.
Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers directly for detailed
information on their selection of capacitors.
Note that even X5R and X7R type ceramic capacitors have
a DC bias effect which reduces their capacitance with a DC
voltage applied. This effect is particularly bad for capacitors
in the smallest case sizes. Consult the manufacturer’s data
for the capacitor you select to be assured of having the
necessary capacitance in your application.
Table 2.Capacitor Vendor Information
SUPPLIER
PHONE
WEB SITE
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
Buck Inductor Selection
The choice of buck inductor value influences both the
efficiency and the magnitude of the output voltage ripple.
Larger inductance values will reduce inductor current ripple
and will therefore lead to lower output voltage ripple. For
a fixed DC resistance, a larger value inductor will yield
higher efficiency by lowering the peak current to be closer
to the average. However, a larger value inductor within the
same family will generally have a greater series resistance,
thereby offsetting this efficiency advantage. Given a desired
peak to peak current ripple, ΔIL , the required inductance
can be calculated via the following expression, where f
represents the switching frequency in MHz:
L=
1
fΔ IL
⎛
⎞
V
• ⎜ 1 − OUT ⎟ ( μ H)
VIN ⎠
⎝
A reasonable choice for ripple current is ΔIL = 100mA which
represents 40% of the maximum 250mA load current.
The DC current rating of the inductor should be at least
450mA to avoid saturation under overload or short-circuit
conditions. To optimize efficiency the inductor should have
a low series resistance. In particularly space restricted applications it may be advantageous to use a much smaller
value inductor at the expense of larger ripple current. In
such cases, the converter will operate in discontinuous
conduction for a wider range of output loads and efficiency
will be reduced.
In addition, there is a minimum inductor value required
to maintain stability of the current loop (given the fixed
internal slope compensation). Specifically, if the buck
converter is going to be utilized at duty cycles over 40%,
the inductance value must be at least LMIN as given by the
following equation:
LMIN = 2.5 • VOUT (μH)
Table 3 depicts the minimum required inductance for
several common output voltages.
Table 3.Buck Minimum Inductance
OUTPUT VOLTAGE
MINIMUM INDUCTANCE
0.6V
1.5μH
0.8V
2μH
1.2V
3μH
2V
5μH
2.7V
6.8μH
3.3V
8.3μH
Larger values of inductor will also provide slightly greater
output current capability before reaching current limit (by
reducing the peak-to-peak ripple current).
3100fa
19
LTC3100
APPLICATIONS INFORMATION
Table 4. Recommended Buck Inductors
VENDOR
Coilcraft
(847) 639-6400
www.coilcraft.com
PART/STYLE
LPS3008, LPS3010, LPS3015
Coiltronics
SD3114, SD3118, SD3112
FDK
MIPF2016
MIPF2520, MIPS2520
Murata
www.murata.com
LQH32C
LQM31P
Sumida
(847) 956-0666
www.sumida.com
CDRH2D11, CDRH2D09
CMD4D06-4R7MC
CMD4D06-3R3MC
Taiyo-Yuden
www.t-yuden.com
NR3010, NR3012
TDK
www.tdk.com
the value of the feedforward capacitor in parallel with the
upper resistor divider resistor.
Note that even X5R and X7R type ceramic capacitors have
a DC bias effect which reduces their capacitance with a DC
voltage applied. This effect is particularly bad for capacitors
in the smallest case sizes. Consult the manufacturer’s data
for the capacitor you select to be assured of having the
necessary capacitance in your application.
Table 5. Buck Output Capacitor Range
VOUT
CMIN
CMAX
0.6V
15μF
300μF
0.8V
15μF
230μF
VLF3010, VLF3012
LEMC3225, LBC2518
1.2V
10μF
150μF
1.8V
6.8μF
90μF
Toko
(408) 432-8282
www.tokoam.com
D3010
DB3015
D312, D301F
2.7V
6.8μF
70μF
3.3V
6.8μF
50μF
Würth
(201) 785-8800
www.we-online.com
WE-TPC Type XS, S
Buck Output Capacitor Selection
A low ESR output capacitor should be utilized at the buck
output in order to minimize voltage ripple. Multilayer ceramic capacitors are an excellent choice as they have low
ESR and are available in small footprints. In addition to
controlling the output ripple magnitude, the value of the
output capacitor also sets the loop crossover frequency and
therefore can impact loop stability. There is both a minimum
and maximum capacitance value required to ensure stability of the loop. If the output capacitance is too small, the
loop crossover frequency will increase to the point where
switching delay and the high frequency parasitic poles of
the error amplifier will degrade the phase margin. In addition, the wider bandwidth produced by a small output
capacitor will make the loop more susceptible to switching
noise. At the other extreme, if the output capacitor is too
large, the crossover frequency can decrease too far below
the compensation zero and also lead to degraded phase
margin. Table 5 provides a guideline for the range of allowable values of low ESR output capacitors. Larger value
output capacitors can be accommodated provided they
have sufficient ESR to stabilize the loop or by increasing
Buck Input Capacitor Selection
The VINBK pin provides current to the buck converter
power switch and is also the supply pin for the buck’s
internal control circuitry. It is recommended that a low
ESR ceramic capacitor with a value of at least 4.7μF be
used to bypass this pin. The capacitor should be placed
as close to the pin as possible and have a short return to
ground. For applications where the power source is more
than a few inches away, a larger bulk decoupling capacitor
is recommended.
Buck Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
⎛ R6 ⎞
VOUT = 0 . 600 V • ⎜ 1 + ⎟
⎝ R5 ⎠
The external divider is connected to the output as shown
in the Block Diagram. It is recommended that a feedforward capacitor be placed in parallel with resistor R6 to
improve the noise immunity of the feedback node and
reduce output ripple in Burst Mode operation. A value of
10pF will generally suffice.
3100fa
20
LTC3100
APPLICATIONS INFORMATION
LDO Output Capacitor Selection
LDO Output Voltage Programming
The LDO is designed to be stable with a minimum 1μF
output capacitor. No series resistor is required when using
low ESR capacitors. For most applications, a 2.2μF ceramic
capacitor is recommended. Larger values will improve
transient response, and raise the power supply rejection
ratio (PSRR) of the LDO. Refer to the Typical Performance
Characteristics for the allowable range of output capacitor
to ensure loop stability.
The output voltage is set by a resistive divider according
to the following formula:
⎛ R4 ⎞
VOUT = 0 . 600 V • ⎜ 1 + ⎟
⎝ R3 ⎠
The external divider is connected to the output as shown
in the Block Diagram. For improved transient response,
a feedforward capacitor may be placed in parallel with
resistor R4.
TYPICAL APPLICATIONS
Single-Cell Boost and Buck with Voltage Sequencing
L1
3.3μH
VBATT
0.9V TO 1.5V
+
16
3.5V
1
5
SWBST VINBK
2
VBST
FBBST
VINBST
CIN
2.2μF
12
13
14
10
8
SWBK
9
RUNLDO
RUNBK
GND
R4
115k
11
4
FBBK
15
PGBST
16
PGBK
VBST, 1V/DIV
VI/O, 1V/DIV
+3.3V AT 50mA
V_I/O
3
MODE
RUNBST
C1
10μF
s2
R1
523k
FBLDO
OFF ON
R2
1M
LTC3100
VLDO
FF EN_BURST
Output Voltages During Soft-Start
for Sequenced Converter
L2
3.3μH
R3
25.5k
C2
2.2μF
VCORE, 1V/DIV
120mA AT VBATT = 0.9V
220mA AT VBATT = 1.2V
V_CORE = 1.2V
R6
1M
R5
1M
1ms/DIV
3100 TA02b
C3
10μF
R7
1M
R8
1M
7
BOOST_GOOD
BUCK_GOOD
3100fa
21
LTC3100
TYPICAL APPLICATIONS
Li-Ion Input, Triple Output Converter
Efficiency vs Load Current
100
L1
3.3μH
16
R2
2M
12
FBBST
VINBST
CIN
4.7μF
2
VBST
R1
634k
R4
115k
11
FBLDO
BOOST
LDO
BUCK
OFF ON
OFF ON
OFF ON
14
10
8
MODE
9
FBBK
RUNLDO
RUNBK
4
SWBK
RUNBST
GND
C2
2.2μF
L2
4.7μH
CFF2
10pF
R6
976k
80
100
70
60
10
50
40
1
30
+1.8V AT
250mA
V_CORE
1.8V BUCK
0.1
5V BOOST
BUCK POWER LOSS
BOOST POWER LOSS
0.01
0.1
1
10
100
1000
LOAD CURRENT (mA)
10
0
0.01
C3
10μF
R7
100k
R5
487k
1000
VIN = 3.6V
20
R3
25.5k
15
PGBST
16
PGBK
7
+3.3V AT
50mA
V_I/O
3
VLDO
LTC3100
13
C1
10μF
s2
EFFICIENCY (%)
1
5
SWBST VINBK
90
3100 TA03a
POWER LOSS (mW)
VIN 2.5V TO 5V
Li-Ion
+5V AT 200mA
VBOOST
R8
100k
3100 TA03b
BOOST_GOOD
BUCK_GOOD
Single-Cell/Two-Cell or USB Input to 3.3V/1.8V Converter
MBR0520
VBATT
0.9V TO
3.3V
3.3V AT: 100mA FOR VBATT = 1.2V
300mA FOR VBATT = 2.4V
250mA FOR USB INPUT
L1
3.3μH
C1
4.7μF
16
C4
4.7μF
1
5
SWBST VINBK
VINBST
2
VBST
FBBST
R1
1.07M
12
LTC3100
FBLDO
11
R3
301k
R5
200k
1.8V AT 50mA
R6
100k
13
14
R7
64.9k
10
8
R4
20k
MODE
SWBK
RUNBST
FBBK
RUNLDO
PGBST
RUNBK
GND
PGBK
4
C4
2.2μF
L2
10μH
C3
10μF
9
VLDO
3.3V OUTPUT
90
VOUT
C2
10μF
R2
324k
VLDO 3
100
80 VIN = 2.4V
EFFICIENCY (%)
USB
INPUT
Efficiency vs Load Current
70
VIN = 1.2V
60
50
40
VIN = 5V USB
30
20
10
0
0.01
0.1
1
10
100
LOAD CURRENT (mA)
1000
3100 TA04b
15
16
7
3100 TA04a
3100fa
22
LTC3100
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
3100fa
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.
23
LTC3100
TYPICAL APPLICATION
Single-Cell to 1.2V/1.8V Converter
L1
3.3μH
+
16
CIN
2.2μF
1
5
SWBST VINBK
2
VBST
VINBST
FBBST
VLDO
R1
1M
12
FBLDO
OFF ON
14
10
8
MODE
SWBK
RUNBST
GND
7
R3
100k
R6
1M
9
RUNLDO
RUNBK
4
C2
2.2μF
R4
200k
L2
3.3μH
FBBK
15
PGBST
6
PGBK
R5
1M
80
1.8V AT
50mA
VLDO
3
11
90
C1
10μF
s2
R2
1M
LTC3100
13
100
2.4V
EFFICIENCY (%)
VBATT
0.9V TO 1.6V
Efficiency vs Load Current
(VBUCK)
70
60
50
40
30
20
1.2V AT: 120mA FOR VBATT = 0.9V
250mA FOR VBATT = 1.2V
VBUCK
C3
10μF
R7
100k
10
0
0.01
VIN = 0.9V
VIN = 1.2V
VIN = 1.5V
0.1
1
10
100
LOAD CURRENT ON VBUCK (mA)
1000
3100 TA05b
3100 TA05a
BUCK_GOOD
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC3442
1.2A (IOUT), 2MHz Synchronous Buck-Boost
DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(RANGE): 2.4V to 5.25V, IQ = 35μA, ISD < 1μA,
DFN Package
LTC3455
Dual DC/DC Converter with USB Power Manager and Li-Ion 96% Efficiency, Seamless Transition Between Inputs, IQ = 110μA,
Battery Charger
ISD < 2μA, QFN Package
LTC3456
2-Cell Multi-Output DC/DC Converter with USB Power
Manager
92% Efficiency, Seamless Transition Between Inputs, IQ = 180μA,
ISD < 1μA, QFN Package
LTC3520
Synchronous 1A Buck-Boost and 600mA Step-Down
DC/DC Converter
VIN: 2.2V to 5.5V, VOUT(MIN) = 0.6V, IQ = 55μA, ISD < 1μA,
4mm × 4mm QFN Package
LTC3522
Synchronous 400mA Buck-Boost and 200mA Step-Down
DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(MIN) = 0.6V, IQ = 25μA, ISD < 1μA,
3mm × 3mm QFN-16 Package
LTC3527/LTC3527-1
Dual (400mA/800mA) Synchronous Boost Converter
VIN: 0.5V to 5V, VOUT: 1.5V to 5.25V, IQ = 12μA, ISD < 2μA,
3mm × 3mm QFN Package
LTC3530
600mA (IOUT), 2MHz Synchronous Buck-Boost
DC/DC Converter
VIN: 1.8V to 5.5V, VOUT(RANGE): 1.8V to 5.5V, IQ = 40μA, ISD < 1μA,
DFN and MSOP Packages
LTC3532
500mA (IOUT), 2MHz Synchronous Buck-Boost
DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(RANGE): 2.4V to 5.25V, IQ = 35μA, ISD < 1μA,
DFN and MSOP Packages
LTC3537
600mA (ISW), 2.2MHz Synchronous Boost Converter with
100mA LDO
VIN: 0.68V to 5V, VOUT(MAX) = 5.5V, IQ = 30μA, ISD < 1μA,
3mm × 3mm QFN Package
LTC3538
600mA (IOUT), 2MHz Synchronous Buck-Boost
DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(RANGE): 1.5V to 5.5V, IQ = 35μA, ISD < 1μA,
DFN Package
LTC3544/LTC3544B
300mA, 200mA ×2, 100mA, 2.25MHz Quad Output
Synchronous Step-Down DC/DC Converter
VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 70μA, ISD < 1μA,
QFN Package
LTC3545
Triple Output, 3mA × 800mA, 2.25MHz Synchronous
Step-Down DC/DC Converter
VIN: 2.25V to 5.5V, VOUT(MIN) = 0.6V, IQ = 58μA, ISD < 1μA,
QFN Package
3100fa
24 Linear Technology Corporation
LT 1108 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008