LTC3127 - 1A Buck-Boost DC/DC Converter with Programmable Input Current Limit

LTC3127
1A Buck-Boost DC/DC
Converter with Programmable
Input Current Limit
Description
Features
Programmable (0.2A to 1A) ±4% Accurate Average
Input Current Limit
n Regulated Output with Input Voltages Above,
Below or Equal to the Output
n 1.8V to 5.5V (Input) and 1.8V to 5.25V (Output)
Voltage Range
n 0.6A Continuous Output Current: V > 1.8V
IN
n 1A Continuous Output Current: V > 3V
IN
n Single Inductor
n Synchronous Rectification: Up to 96% Efficiency
n Burst Mode® Operation: I = 35μA (Pin Selectable)
Q
n Output Disconnect in Shutdown
n <1μA Shutdown Current
n Small, Thermally Enhanced 10-Lead (3mm × 3mm ×
0.75mm) DFN and 12-Lead MSOP Packages
n
Applications
n
n
n
n
n
The LTC®3127 is a wide VIN range, highly efficient, 1.35MHz
fixed frequency buck-boost DC/DC converter that operates
from input voltages above, below or equal to the output
voltage. The LTC3127 features programmable average
input current limit, making it ideal for power-limited input
sources. The input current limit is programmed with a
single resistor and is accurate from 0.2A to 1A of average
input current.
The topology incorporated provides a continuous
transfer function through all operating modes. Other
features include <1μA shutdown current, pin-selectable
Burst Mode operation and thermal overload protection.
The LTC3127 is housed in thermally enhanced 10-lead
(3mm × 3mm × 0.75mm) DFN packages and 12-lead
MSOP packages.
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
and PowerPath and ThinSOT are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
USB Powered GSM Modems
Supercap Charger
Handheld Test Instruments
PC Card Modems
Wireless Terminals
Typical Application
USB or Li-Ion (500mA Maximum Input Current) to 3.3V
Efficiency vs VIN
100
L1
4.7µH
90
SW2
VOUT
MODE
OFF ON
10µF
320k
SHDN
FB
VC
PROG
SGND PGND
32.4k
2.2mF
1A LOAD
80
70
182k
100pF
60
499k
3127 TA01
L1: COILCRAFT XPL4020-472ML
VOUT
3.3V
EFFICIENCY (%)
SW1
VIN
USB OR Li-Ion
2.9V to 5.5V
300mA LOAD
50
2.5
VOUT = 3.3V
L = 4.7µH
F = 1.35MHz
3
3.5
4
VIN(V)
4.5
5
5.5
3127 TA01a
3127f
LTC3127
Absolute Maximum Ratings
(Note 1)
VIN , VOUT Voltage .......................................... –0.3 to 6V
SW1, SW2 DC Voltage.................................... –0.3 to 6V
SW1, SW2 Pulsed (<100ns) Voltage............... –0.3 to 7V
MODE, FB, VC Voltage..................................... –0.3 to 6V
SHDN Voltage ................................................ –0.3 to 6V
PROG Voltage................................................. –0.3 to 6V
Operating Junction Temperature Range
(Note 2).....................................................–40°C to 85°C
Maximum Junction Temperature (Note 5)............ 125°C
Storage Temperature Range................... –65°C to 125°C
Pin Configuration
TOP VIEW
TOP VIEW
SW1
1
10 SW2
VIN
2
9 VOUT
SHDN
3
MODE
4
PROG
5
11
PGND
PGND
SW1
VIN
SHDN
MODE
PROG
8 VC
7 FB
6 SGND
1
2
3
4
5
6
13
PGND
12
11
10
9
8
7
PGND
SW2
VOUT
VC
FB
SGND
MSE PACKAGE
12-LEAD PLASTIC MSOP
DD PACKAGE
10-LEAD (3mm s 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 43°C/W (NOTE 6)
EXPOSED PAD (PIN 11) IS PGND, MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 40°C/W (NOTE 6)
EXPOSED PAD (PIN 11) IS PGND, MUST BE SOLDERED TO PCB
order information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3127EDD#PBF
LTC3127EDD#TRPBF
LDYD
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
LTC3127EMSE#PBF
LTC3127EMSE#TRPBF
3127
12-Lead Plastic MSOP
–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/
3127f
LTC3127
Electrical
Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TJ = 25°C. VIN = 3.6V, VOUT = 3.3V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Operating Range
l
1.8
5.5
V
Output Voltage Adjust
l
1.8
5.25
V
Feedback Voltage
l
1.165
1.195
1.225
V
1
50
nA
Feedback Input Current
VFB = 1.25V
Quiescent Current—Burst Mode Operation
VFB > 1.225, VMODE = VIN (Note 4)
35
Quiescent Current—Shutdown
VSHDN = 0V, Including SW Leakage
0.1
Quiescent Current—Active
VFB > 1.225V, VMODE = 0V (Note 4)
400
Input Current Limit
RPROG = 32.4k (Note 3)
µA
4
µA
µA
480
500
520
mA
0°C to 85°C (Note 3)
l
465
500
540
mA
–40°C to 85°C (Note 3)
l
430
500
540
mA
l
2
2.5
0.15
0.3
0.45
A
4
µA
Peak Current Limit
Reverse-Current Limit
A
P-Channel MOSFET Leakage
Switches A and D
0.1
N-Channel MOSFET On-Resistance
Switch B
Switch C
140
170
mΩ
mΩ
P-Channel MOSFET On-Resistance
Switch A
Switch D
160
190
mΩ
mΩ
Maximum Duty Cycle
Boost( % Switch C On)
Buck (% Switch A On)
90
%
%
l
l
80
100
0
Minimum Duty Cycle
l
Frequency Accuracy
l
1
SHDN Input High Voltage
l
1.2
SHDN Input Low Voltage
SHDN Input Current
0.01
l
MODE Input Low Voltage
l
VMODE = 5.5V
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 LTC3127 is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls. Note that the maximum
ambient temperature is determined by specific operating conditions in
conjunction with board layout, the rated package thermal resistance and
other environmental factors.
1.7
MHz
V
l
VSHDN = 5.5V
MODE Input High Voltage
MODE Input Current
1.35
%
0.3
V
1
µA
1.2
V
0.01
0.3
V
1
µA
Note 3: Specification is guaranteed when the inductor current is in
continuous conduction.
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
40°C/W.
3127f
LTC3127
Typical Performance Characteristics
PWM
90
80 BURST
80
70
70
60
50
40
30
20
VIN = 1.8V
VIN = 3.6V
VIN = 5V
10
0
0.1
1
10
100
LOAD CURRENT (mA)
50
PWM
40
–2
–3
–4
3.4 3.8 4.2 4.6
VIN (V)
VIN = 2.9V
VIN = 3.6V
VIN = 4.3V
1
10
100
LOAD CURRENT (mA)
5
5.4
VIN = 4.5V
VIN = 5V
VIN = 5.5V
10
0
0.1
1000
1
10
100
1000
LOAD CURRENT (mA)
3127 G02
10000
3127 G03
Quiescent Current vs VIN (Fixed
Frequency Mode–Not Switching)
430
VOUT = 3.3V
RPROG = 32.4k
410
0
–1
–2
–3
390
370
350
330
310
290
–5
–45 –30 –15
3127 G04
0 15 30 45 60
TEMPERATURE (°C)
75
90
270
1.8 2.2 2.6
3
3127 G05
3.4 3.8 4.2 4.6
VIN (V)
5
5.4
3127 G06
No Load Input Current vs VIN in
Burst Mode Operation
52.5
VOUT = 3.3V
52.0
INPUT CURRENT (µA)
37
INPUT CURRENT (µA)
PWM
40
30
Burst Mode Quiescent Current
vs VIN
36
35
34
33
32
1.8 2.2 2.6
50
20
–4
38
60
30
1
–1
3
70
20
2
0
–5
1.8 2.2 2.6
60
Average Input Current Limit
vs Temperature (Normalized)
VOUT = 3.3V
RPROG = 32.4k
VOUT = 5V
80 BURST
3127 G01
INPUT CURRENT LIMIT (%)
INPUT CURRENT LIMIT (%)
1
90
BURST
0
0.1
Average Input Current Limit
vs VIN (Normalized)
2
VOUT = 3.3V
10
1000
Efficiency vs Load Current
100
INPUT CURRENT (µA)
VOUT = 1.8V
EFFICIENCY (%)
EFFICIENCY (%)
90
Efficiency vs Load Current
100
EFFICIENCY (%)
Efficiency vs Load Current
100
(TJ = 25°C, unless otherwise noted )
51.5
51.0
50.5
50.0
49.5
49.0
3
3.4 3.8 4.2 4.6
VIN (V)
5
5.4
3127 G07
48.5
1.8 2.2 2.6
3
3.4 3.8 4.2 4.6
VIN (V)
5
5.4
3127 G08
3127f
LTC3127
Typical Performance Characteristics
Feedback Voltage vs Temperature
(Normalized)
0.40
L = 4.7µH
VOUT = 3.3V
VIN = 3.6V
NMOS RDS(ON) vs VIN
300
VOUT = 3.3V
0.30
0.20
–0.20
–0.40
–0.60
250
0.10
RDS(ON) (mΩ)
CHANGE (%)
0.00
VOUT Regulation vs Load Current
(Normalized)
VOUT REGULATION (%)
0.20
(TJ = 25°C, unless otherwise noted )
0
–0.10
–0.20
–0.30
200
SWC
150
–0.40
SWB
–0.50
–0.80
–50
–30
50
–10 10
30
TEMPERATURE (°C)
70
–0.60
90
3127 G09
2250
2000
1750
LOAD CURRENT (mA)
275
RDS(ON) (mΩ)
1000
600
800
400
LOAD CURRENT (mA)
225
SWD
175
2.4
3
1.8
1500
4.2
4.8
5.4
3
3.6
VIN (V)
RPROG = 90k
4.8
5.4
3127 G11
ILOAD
1A/DIV
VOUT = 2.4V
IIN
1A/DIV
VOUT
50mV/DIV
VOUT = 3.3V
1250
1000
VOUT = 5V
750
IL
1A/DIV
200µs/DIV
0
1.8 2.2 2.6
4.2
Load Transient Response in Fixed
Frequency Mode, No Load to 1A,
Not in Input Current Limit
250
3.6
VIN (V)
2.4
3127 G10
500
SWA
1.8
100
Maximum Load Current vs VIN
PMOS RDS(ON) vs VIN
325
125
200
0
3
3.4 3.8 4.2 4.6
VIN (V)
5
3127 G14
VIN = 3.6V
VOUT = 3.3V
RPROG = 90k COUT = 4.4mF
R3 = 499k
C1 = 100pF
5.4
3127 G13
3127 G12
Load Transient Response in Fixed
Frequency Mode, No Load to 1A,
in Input Current Limit
Burst Mode Operation
MODE = 0V
ILOAD
1A/DIV
IL
500mA/DIV
IIN
500mA/DIV
VOUT
100mV/DIV
VOUT
20mV/DIV
200µs/DIV
VOUT = 3.3V
VIN = 3.6V
RPROG = 32.4k COUT = 4.4mF
R3 = 499k
C1 = 100pF
3127 G15
5µs/DIV
3127 G16
VOUT = 3.3V
VIN = 3.6V
RPROG = 32.4k COUT = 4.4mF
R3 = 499k
C1 = 100pF
3127f
LTC3127
Typical Performance Characteristics
Load Transient Response in
Burst Mode Operation, No Load
to 1A, Not in Input Current Limit
ILOAD
1A/DIV
(TJ = 25°C, unless otherwise noted )
Transition from Burst Mode
Operation to Fixed Frequency Mode
IIN
200mA/DIV
IIN
1A/DIV
VOUT
50mV/DIV
VOUT
20mV/DIV
MODE
5V/DIV
200µs/DIV
VIN = 3.6V
RPROG = 90k
R3 = 499k
3127 G17
MODE
5V/DIV
100µs/DIV
VOUT = 3.3V
COUT = 4.4mF
C1 = 100pF
3127 G18
VOUT = 3.3V
VIN = 3.6V
RPROG = 32.4k COUT = 4.4mF
R3 = 499k
C1 = 100pF
Load Transient Response in
Burst Mode Operation, No Load
to 1A, in Input Current Limit
Start-Up Waveform
ILOAD
1A/DIV
IIN
500mA/DIV
VOUT
100mV/DIV
VOUT
1V/DIV
IIN
500mA/DIV
MODE
5V/DIV
SHDN
5V/DIV
200µs/DIV
VOUT = 3.3V
VIN = 3.6V
RPROG = 32.4k COUT = 4.4mF
R3 = 499k
C1 = 100pF
3127 G19
5ms/DIV
3127 G20
VOUT = 3.3V
VIN = 3.6V
RPROG = 32.4k COUT = 4.4mF
R3 = 499k
C1 = 100pF
3127f
LTC3127
Pin Functions
(DD Package)
SW1 (Pin 1): Switch Pin Where Internal Switches A and
B Are Connected. Connect inductor from SW1 to SW2.
Minimize trace length to reduce EMI.
SGND (Pin 6): Signal Ground for the IC. Terminate the
PROG resistor, compensation components and the output
voltage divider to SGND.
VIN (Pin 2): Input Supply Pin. Internal VCC for the IC. A
10μF or greater ceramic capacitor should be placed as
close to VIN and PGND as possible.
FB (Pin 7): Feedback Pin. Connect resistor divider tap here.
The output voltage can be adjusted from 1.8V to 5.25V.
The feedback reference voltage is 1.195V.
SHDN (Pin 3): Logic-Controlled Shutdown Input.
 R2 
VOUT = 1.195 •  1+ R1 V
SHDN = High: Normal Operation
SHDN = Low: Shutdown
MODE (Pin 4): Pulse Width Modulation/Burst Mode
Selection Input.
MODE = High: Burst Mode Operation
MODE = Low: PWM Operation Only. Forced continuous
conduction mode.
PROG (Pin 5): Sets the Average Input Current Limit
Threshold. Connect a resistor from PROG to ground. See
below for component value selection.
VC (Pin 8): Error Amplifier Output. Place compensation
components from this pin to SGND.
VOUT (Pin 9): Output of the Synchronous Rectifier. Connect
the output filter capacitor from this pin to GND. A minimum
value of 22µF is recommended. Output capacitors must
be low ESR.
SW2 (Pin 10): Switch Pin Where Internal Switches C and
D Are Connected. Minimize trace length to reduce EMI.
PGND (Exposed Pad Pin 11): Power Ground. The exposed
pad must be soldered to the PCB ground plane.
RPROG = 54.92 • ILIMIT (A) + 4.94 (kΩ)
3127f
LTC3127
Block Diagram
SW1
L
SW2
VIN
VOUT
CIN
–
+
VC
IZERO
AMP
IPEAK
AMP
PWM
COMPARATOR
AND LOGIC
SHDN
MODE
PROG
RPROG
SAMPLE/HOLD
AND RESET
R2
+
–
FB
+
1.195V
–
R3
COUT
C1
R1
–
VCLAMP
+
SGND
3127 BD
3127f
LTC3127
Operation
The LTC3127 is an average input current controlled buckboost DC/DC converter offered in both a thermally enhanced
3mm × 3mm DFN package and a thermally enhanced 12lead MSOP package. The buck-boost converter utilizes a
proprietary switching algorithm which allows its output
voltage to be regulated above, below or equal to the input
voltage. The low RDS(ON), low gate charge synchronous
switches efficiently provide high frequency PWM control.
High efficiency is achieved at light loads when Burst Mode
operation is commanded.
the AC switch pair remains on for longer durations and
the duration of the BD phase decreases proportionally. As
the input voltage drops below the output voltage, the AC
phase will eventually increase to the point that there is no
longer any BD switching. At this point, switch A remains
on continuously while switch pair CD is pulse width modulated to obtain the desired output voltage. At this point,
the converter is operating solely in boost mode.
This switching algorithm provides a seamless transition
between operating modes and eliminates discontinuities
in average inductor current, inductor current ripple, and
loop transfer function throughout all three operational
modes. These advantages result in increased efficiency
and stability in comparison to the traditional 4-switch
buck-boost converter. In forced PWM mode operation,
the inductor is forced to have continuous conduction.
This allows for a constant switching frequency and better
noise performance.
PWM Mode Operation
The LTC3127 uses fixed frequency, average input current PWM control. The MODE pin can be used to select
automatic Burst Mode operation (MODE connected to
VIN) or to disable Burst Mode operation and select forced
continuous conduction operation for low noise applications
(MODE grounded).
A proprietary switching algorithm allows the converter
to switch between buck, buck-boost and boost modes
without discontinuity in inductor current or loop characteristics. The switch topology for the buck-boost converter
is shown in Figure 1.
Error Amplifier and Compensation
The buck-boost converter utilizes two control loops. The
outer voltage loop determines the amount of current required to regulate the output voltage. The voltage loop is
externally compensated and can be configured with either
integral compensation or proportional control. The inner
current loop is internally compensated and forces the input
current to equal the commanded current.
When the input voltage is significantly greater than the
output voltage, the buck-boost converter operates in
buck mode. Switch D turns on continuously and switch C
remains off. Switches A and B are pulse width modulated
to produce the required duty cycle to support the output
regulation voltage. As the input voltage decreases, switch
A remains on for a larger portion of the switching cycle.
When the duty cycle reaches approximately 85%, the
switch pair AC begins turning on for a small fraction of the
switching period. As the input voltage decreases further,
When VC is compensated via proportional control, the
dominant pole of the output capacitor is used to ensure
stability with a minimum of 1000µF of capacitance on the
output when a 499k resistor is used. There is no maximum
capacitance limitation with proportional compensation.
L
VIN
A
SW1
B
LTC3127
PGND
SW2
D
VOUT
C
PGND
3127 F01
Figure 1. Buck-Boost Switch Topology
3127f
LTC3127
Operation
Integral compensation is required if an output capacitor
less than 1000µF but greater than 44µF is used, otherwise
using proportional compensation is recommended.
When compensating the converter with integral compensation it is important to consider that the total bandwidth
of the network must be below 15kHz. The inner current
loop of the LTC3127 eliminates one of the double poles
caused by the inductor. The output capacitor causes a
dominant pole and also a zero, and the resistor divider
sets the gain.
GDC = 1 +
fPOLE1 =
f ZERO1 =
R2
R1
1
2 • π • RLOAD • COUT
1
2 • π • RESR • COUT
Using the compensation network show in Figure 2, the
voltage loop compensation can be approximated with the
following transfer function:
gm • (C1 • R A • s + 1)
H COMP (s) =
s • (C1 • C2 • R A • s + C1 + C2)
where gm = 150 • 10–6
This causes poles and zeros to occur at the following
locations:
fPOLE2 @ DC
fPOLE3 =
1
2 • π • R A • C2
f ZERO2 =
1
2 • π • R A • C1
The poles and zeros of the compensation should be determined by looking at where fPOLE1 lands at the minimum
load where the LTC3127 will be continuously conducting,
which places the dominant pole at its lowest frequency.
After setting the poles and zeros for the compensation, the
phase margin of the system should be greater than 45°
and the gain margin should be greater than 3dB. Following
these two criteria will help to ensure stability.
Current Limit Operation
The buck-boost converter has two current limit circuits.
The primary current limit is an average input current
limit circuit that clamps the output of the outer voltage
loop. This limits the amount of input current that can be
commanded, and the inner current loop regulates to that
clamped value.
VOUT
VOUT
LTC3127
–
PWM
+
MEASURED
INPUT CURRENT
+
–
1.195V
R2
FB
COUT
VC
R1
RA
C2
SGND
C1
3127 F02
Figure 2. Buck-Boost External Compensation
3127f
10
LTC3127
Operation
The input current limit is set by the RPROG resistor placed
on the PROG pin to SGND. The resistor value can be
calculated using the following formula:
RPROG = 54.92 • ILIMIT (A) + 4.94 (kΩ)
Where ILIMIT is the average input current limit in amps.
A secondary 2.5A (typical) current limit forces switches
B and D on and A and C off if tripped. This current limit
is not affected by the value of RPROG.
Reverse Current Limit
The reverse current comparator on switch D monitors
the inductor current supplied from the output. When this
current exceeds 300mA (typical) switch D will be turned
off for the remainder of the switching cycle.
Burst Mode Operation
When the MODE pin is held high the LTC3127 will function in Burst Mode operation as long as the load current
is typically less than 35mA. In Burst Mode operation, the
LTC3127 still switches at a fixed frequency of 1.35MHz,
using the same error amplifiers and loop compensation for
average input 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 the output voltage reaches the nominal
regulation value. At this point, the LTC3127 transitions to
sleep mode where the output switches are shut off and the
LTC3127 consumes only 35μA of quiescent current from
VIN . When the output voltage droops slightly, switching
resumes. This maximizes efficiency at very light loads by
minimizing switching and quiescent losses.
Anti-Ringing Control
The anti-ringing control connects a resistor from SW1 and
SW2 to PGND to prevent high frequency ringing during
discontinuous current mode operation in Burst Mode.
Although the ringing of the resonant circuit formed by L
and CSW (capacitance on SW pin) is low energy, it can
cause EMI radiation.
Shutdown
Shutdown of the converter is accomplished by pulling
SHDN below 0.3V and enabled by pulling SHDN above
1.2V. Note that SHDN can be driven above VIN or VOUT,
as long as it is limited to less than the absolute maximum
rating.
Thermal Shutdown
If the die temperature exceeds 150°C (typical) the LTC3127
will be disabled. All power devices will be turned off and
both switch nodes will be high impedance. The LTC3127
will restart (if enabled) when the die temperature drops
to approximately 140°C.
Thermal Regulator
To help prevent the part from going into thermal shutdown
when charging very large capacitive loads, the LTC3127 is
equipped with a thermal regulator. If the die temperature
exceeds 130°C (typical) the average current limit is lowered
to help reduce the amount of power being dissipated in the
package. The current limit will be approximately 0A just before
thermal shutdown. The current limit will return to its full value
when the die temperature drops back below 130°C.
Zero Current Comparator
Undervoltage Lockout
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. This comparator is
only active in Burst Mode operation.
If the input supply voltage drops below 1.7V (typical),
the LTC3127 will be disabled and all power devices will
be turned off.
3127f
11
LTC3127
Applications Information
A typical LTC3127 application circuit is shown on the front
page of this data sheet. The external component selection
is determined by the desired output voltage, input current
and ripple voltage requirements for each particular application. However, basic guidelines and considerations for the
design process are provided in this section.
Buck-Boost Output Voltage Programming
The buck-boost output voltage is set by a resistive divider
according to the following formula:
 R2 
VOUT = 1.195V •  1 +  V
R1

The external divider is connected to the output as shown
in Figure 3. The buck-boost converter utilizes input current
mode control, and the output divider resistance does not
play a role in the stability.
1.8V b VOUT b 5.25V
R2
FB
LTC3127
R1
GND
3127 F03
Figure 3. Setting the Buck-Boost Output Voltage
The LTC3127 can utilize small surface mount inductors
due to its fast 1.35MHz 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 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 2.5A seen on
the LTC3127. To minimize radiated noise, use a shielded
inductor. See Table 1 and the reference schematics for
suggested components and suppliers.
Table 1. Recommended Inductors
VENDOR
PART/STYLE
Coilcraft
847-639-6400
www.coilcraft.com
LPO2506
LPS4012, LPS4018
MSS6122
MSS4020
MOS6020
DS1605, DO1608
XPL4020
Coiltronics
www.cooperet.com
SD52, SD53
SD3114, SD3118
Murata
714-852-2001
www.murata.com
LQH55D
Sumida
847-956-0666
www.sumida.com
CDH40D11
Taiyo-Yuden
www.t-yuden.com
NP04SB
NR3015
NR4018
TDK
847-803-6100
www.component.tdk.com
VLP, LTF
VLF, VLCF
Würth Elektronik
201-785-8800
www.we-online.com
WE-TPC Type S, M, MH
Buck-Boost Inductor Selection
To achieve high efficiency, a low ESR inductor should
be utilized for the buck-boost converter. The inductor
must have a saturation rating greater than the worst case
average inductor current plus half the ripple current.
The peak-to-peak inductor current ripple will be larger
in buck and boost mode than in the buck-boost region.
The peak-to-peak inductor current ripple for each mode
can be calculated from the following formulas, where L
is the inductance in μH:
V
( V − VOUT )
∆ I L,P −P,BUCK = OUT IN
( A)
VIN • L • (1.35MHz)
∆ I L,P −P,BOOST
VIN ( VOUT − VIN )
=
( A)
VOUT • L • (1.35MHz)
3127f
12
LTC3127
Applications Information
Output and Input Capacitor Selection
The total output voltage droop is given by:
When selecting output capacitors for large pulsed loads,
the magnitude and duration of the pulse current, together
with the droop voltage specification, determine the choice
of the output capacitor. Both the ESR of the capacitor and
the charge stored in the capacitor each cycle contribute
to the output voltage droop. The droop due to the charge
is approximately:
VDROOP = VDROOP_LOAD + VDROOP_ESR (V)
V DROOP _ LOAD =

 VIN • IIN(MAX ) • h

− ISTANDBY   • D • T
I PULSE − 
VOUT

 

( V)
COUT
where
IPULSE = pulsed load current
ISTANDBY = static load current in standby mode
IIN(MAX) = programmed input current limit in amps
T = period of the load pulse
High capacitance values and low ESR can lead to instability
in typical internally compensated buck-boost converters. Using proportional compensation, the LTC3127 is
stable with low ESR output capacitor values greater than
1000µF.
Multilayer ceramic capacitors are an excellent choice for
input decoupling of the step-up converter as they have
extremely low ESR and are available in small footprints.
Input capacitors should be located as close as possible to
the device. While a 10µF input capacitor is sufficient for
most applications, larger values may be used to improve
input decoupling without limitation. Consult the manufacturers directly for detailed information on their selection
of ceramic capacitors. Although ceramic capacitors are
recommended, low ESR tantalum capacitors may be used
as well.
When using a large capacitance to help with pulsed load
applications, the maximum load for a given duty cycle, and
the minimum capacitance can be calculated by:
D = load pulse’s duty cycle
VDROOP = amount the output falls out of regulation in volts
I LOAD(MAX ) =
h = the efficiency of the converter at the input current
limit point
C OUT(MIN) =
The preceding equation is a worst-case approximation
assuming all the pulsing energy comes from the output
capacitor.

 VIN • IIN(MAX ) • h

= I PULSE − 
− ISTANDBY   • ESR ( V)
VOUT

 

Low ESR and high capacitance are critical to maintaining low output droop. Table 2 and the Typical Applications schematics show a list of several reservoir capacitor
manufacturers.
D • VOUT
( A)


 VIN • IIN(MAX ) • h
− ISTANDBY  
IPULSE − 
VOUT

 

The droop due to the capacitor equivalent series resistance
(ESR) is:
V DROOP _ ESR
VIN • IIN(MAX ) • h
•
D•T
(F)
VDROOP
Table 2. Capacitor Vendor Information
SUPPLIER
PHONE
WEB SITE
Vishay
402-563-6866
www.vishay.com
AVX
803-448-9411
www.avxcorp.com
Cooper Bussmann
516-998-4100
www.cooperbussmann.com
CAP-XX
843-267-0720
www.cap-xx.com
Panasonic
800-394-2112
www.panasonic.com
3127f
13
LTC3127
Applications Information
Capacitor Selection Example
In this example, a pulsed load application requires that
VOUT droops less than 300mV. The application is a Li-Ion
battery input to a 3.6V output. The pulsed load is a no-load
to a 1.5A step with a frequency of 217Hz and a duty cycle
of 12.5%. The input current limit is set to 500mA. In order
to meet the 300mV droop requirement, the amount of
capacitance must be calculated at the highest VIN to VOUT
step-up ratio. All of the following calculations assume a
minimum VIN of 3V and an efficiency of 90%.
Given the application, the following is known:
VIN = 3V
Step 2: Calculate the minimum output capacitance required.

3V • 500mA • 0 . 9 
COUT(MIN) ≥  1 . 5A −

3 . 6V

•
0 . 125 • 4 . 6ms
= 2 . 15mF
300mV
Step 3: For this application a 2.2mF Vishay Tantamount
tantalum, low ESR capacitor is selected. This capacitor has
a maximum ESR of 0.04Ω. With the selected capacitor,
the amount of droop must be calculated:
VDROOP _ LOAD =
VOUT = 3.6V

 3V • 500mA • 0 . 9

− 0 A   • 0 . 125 • 4 . 6ms
1 . 5 A − 
3 . 6V



2 . 2mF
= 0 . 294V
IIN(MAX) = 500mA
IPULSE = 1.5A
ISTANDBY = 0A
h = 0.9
D = 0.125
VDROOP _ ESR =
T = 1/217Hz = 4.6ms

 3V • 500mA • 0 . 9

− 0 A   • 0 . 04Ω
1 . 5 A − 
3 . 6V



= 0 . 045V
VDROOP = 300mV
Step 1: Check to make sure the application can provide
enough current to recover from the pulsed load using the
ILOAD(MAX) equation:
ILOAD(MAX ) =
3V • 500mA • 0 . 9
= 3A
0 . 125 • 3 . 6 V
The maximum load that can be pulsed at this VIN to VOUT
combination is 3A.
VDROOP = VDROOP _ LOAD + VDROOP _ ESR = 0 . 339 V
Due to the ESR of the capacitor, the total droop is greater
than 300mV. In this case, if the higher droop cannot be
accepted, a larger valued, lower ESR capacitor can be
selected.
3127f
14
LTC3127
Applications Information
PCB Layout Considerations
The LTC3127 switches large currents at high frequencies.
Special care should be given to the PCB layout to ensure
stable, noise-free operation. Figure 4 depicts the recommended PCB layout to be utilized for the LTC3127. A few
key guidelines follow:
1. All circulating high current paths should be kept as short
as possible. This can be accomplished by keeping the
routes to all bold components in Figure 4 as short and
as wide as possible. Capacitor ground connections
should via down to the ground plane in the shortest
route possible. The bypass capacitor on VIN should be
placed as close to the IC as possible and should have
the shortest possible path to ground.
2. The small-signal ground pad (SGND) should have a
single point connection to the power ground. A con-
VIA TO
GROUND
SW1
venient way to achieve this is to short the pin directly
to the Exposed Pad as shown in Figure 4.
3. The components shown in bold and their connections
should all be placed over a complete ground plane.
4. To prevent large circulating currents from disrupting
the output voltage sensing, the ground for the resistor
divider and RPROG should be returned directly to the
small signal ground pin (SGND).
5. Use of vias in the die attach pad 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 PCB.
6. Keep the connections to the FB and PROG pins as
short as possible and away from the switch pin connections.
VIA TO
GROUND
SW2
1
10
VIN
2
9
SHDN
3
MODE
4
PROG
5
PGND
8
7
VOUT
VC
FB
6
SGND
3127 F04
Figure 4. Recommended PCB Layout
3127f
15
LTC3127
Typical Applications
USB (500mA Max), 3.8V GSM Pulsed Load
L1
4.7µH
SW1
VIN
VIN
USB
SW2
VOUT
VOUT
3.8V
MODE
PWM BURST
2.15M
LTC3127
OFF ON
FB
SHDN
10µF
C2
2.2mF
1M
VC
PROG
SGND
PGND
100pF
32.4k
C1, C2: VISHAY TANTAMOUNT
TANTALUM, LOW ESR CAPACITORS
L1: COILCRAFT XPL4020-472ML
C1
2.2mF
499k
3127 TA02
PCMCIA/Compact Flash (3.3V or 5V/500mA Max), 3.8V GPRS, Class 10 Pulsed Load
L1
4.7µH
SW1
VIN
VIN
3.3V OR
5V
SW2
VOUT
2.15M
MODE
PWM BURST
LTC3127
OFF ON
SHDN
10µF
PROG
SGND
C1, C2, C3: VISHAY TANTAMOUNT
TANTALUM, LOW ESR CAPACITORS
L1: COILCRAFT XPL4020-472ML
VOUT
3.8V
32.4k
FB
VC
PGND
100pF
C3
2.2mF
1M
499k
C2
2.2mF
C1
2.2mF
3127 TA03
3127f
16
LTC3127
Typical Applications
Stacked Supercapacitor Charger (1000mA Max Input Current)
L1
4.7µH
VIN
1.8V to 5.5V
SW1
VIN
PWM BURST
MODE
SW2
VOUT
3.16M
LTC3127
FB
SHDN
OFF ON
VC
PROG
10µF
VOUT
5V
SGND
1M
100k
C2
100F
PGND
100pF
60.4k
100k
C1
100F
499k
L1: COILCRAFT XPL4020-472ML
3127 TA04
General Purpose Forced Continuous Conduction Application with 500µs Start-Up
L1
4.7µH
SW1
VIN
VIN
3V TO 4.3V
SW2
VOUT
VOUT
3.3V
316k 33pF
MODE
PWM BURST
FB
SHDN
OFF ON
PROG
SGND
10µF
0.01µF
L1: COILCRAFT XPL4020-472ML
LTC3127
60.4k
PGND
VC
47k
22µF
s2
182k
3300pF
3127 TA05
3127f
17
LTC3127
Package Description
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev B)
0.70 p0.05
3.55 p0.05
1.65 p0.05
2.15 p0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 p 0.05
0.50
BSC
2.38 p0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 p0.10
(4 SIDES)
R = 0.125
TYP
6
0.40 p 0.10
10
1.65 p 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.75 p0.05
0.00 – 0.05
5
1
(DD) DFN REV B 0309
0.25 p 0.05
0.50 BSC
2.38 p0.10
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
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
3127f
18
LTC3127
Package Description
MSE Package
12-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1666 Rev B)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 p 0.102
(.112 p .004)
5.23
(.206)
MIN
2.845 p 0.102
(.112 p .004)
0.889 p 0.127
(.035 p .005)
6
1
1.651 p 0.102 3.20 – 3.45
(.065 p .004) (.126 – .136)
0.12 REF
12
0.65
0.42 p 0.038
(.0256)
(.0165 p .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
4.039 p 0.102
(.159 p .004)
(NOTE 3)
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
7
NO MEASUREMENT PURPOSE
0.406 p 0.076
(.016 p .003)
REF
12 11 10 9 8 7
DETAIL “A”
0o – 6o TYP
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
GAUGE PLANE
0.53 p 0.152
(.021 p .006)
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.22 – 0.38
(.009 – .015)
TYP
1 2 3 4 5 6
0.650
(.0256)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.86
(.034)
REF
0.1016 p 0.0508
(.004 p .002)
MSOP (MSE12) 0608 REV B
3127f
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
LTC3127
Typical Application
Single Supercapacitor Charger (1000mA Max Input Current)
L1
4.7µH
SW1
VIN
VIN
1.8V TO 5V
SW2
VOUT
VOUT
2.5V
1.05M
MODE
PWM BURST
LTC3127
SHDN
OFF ON
PROG
10µF
SGND
C1: COOPER BUSSMANN POWERSTOR
B-SERIES, B1860-2R5107-R
L1: COILCRAFT XPL4020-472ML
PGND
60.4k
FB
1M
VC
100pF
C1
100F
499k
3127 TA06
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LTC3101
Wide VIN, 1MHz Multioutput DC/DC Converter and
PowerPath™ Controller
95% Efficiency, VIN: 1.8V to 5.5V, VOUT(MAX): 1.8V to 5.25V, IQ = 35µA,
ISD < 1µA, 4mm × 4mm QFN-24 Package
LTC3125
1.2A IOUT, 1.6MHz, Synchronous Boost DC/DC
Converter With Adjustable Input Current Limit
94% Efficiency, VIN: 1.8V to 5.5V, VOUT(MAX) = 5.25V, IQ = 15µA, ISD < 1µA,
2mm × 3mm DFN-8 Package
LTC3606B
800mA IOUT, Synchronous Step-Down DC/DC
Converter with Average Input Current Limit
95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MAX) = 0.6V, IQ = 420µA, ISD < 1µA,
3mm × 3mm DFN-8 Package
LTC3440
600mA IOUT, 2MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT: 2.5V to 5.5V, IQ = 25µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10 Packages
LTC3441/LTC3441-2/
LTC3441-3
1.2A IOUT, 2MHz, Synchronous Buck-Boost DC/DC
Converter
95% Efficiency, VIN: 2.4V to 5.5V, VOUT: 2.4V to 5.25V, IQ = 50µA,
ISD < 1µA, 3mm × 4mm DFN-12 Package
LTC3520
1A 2MHz, Synchronous Buck-Boost and 600mA
Buck Converter
95% Efficiency, VIN: 2.2V to 5.5V, VOUT(MAX) = 5.25V, IQ = 55µA, ISD < 1µA,
4mm × 4mm QFN-24 Package
LTC3530
600mA IOUT, 2MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN: 1.8V to 5.5V, VOUT: 1.8V to 5.25V, IQ = 40µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10 Packages
LTC3532
500mA IOUT, 2MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN: 2.4V to 5.5V, VOUT: 2.4V to 5.25V, IQ = 35µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10 Packages
LTC3533
2A IOUT, 2MHz, Synchronous Buck-Boost DC/DC
Converter
95% Efficiency, VIN: 1.8V to 5.5V, VOUT: 1.8V to 5.25V, IQ = 40µA,
ISD < 1µA, 3mm × 4mm DFN-14 Package
LTC3538
800mA IOUT, 1MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN: 2.4V to 5.5V, VOUT: 1.8V to 5.25V, IQ = 35µA,
ISD < 1µA, 2mm × 3mm DFN-8 Package
LTC3534
500mA IOUT, 1MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN: 2.4V to 7V, VOUT: 1.8V to 2V, IQ = 25µA,
ISD < 1µA, 3mm × 3mm DFN-16 and SSOP-16 Packages
3127f
20 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
LT 0210 • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2010