LT3471 - Dual 1.3A, 1.2MHz Boost/Inverter in 3mm × 3mm DFN

LT3471
Dual 1.3A, 1.2MHz
Boost/Inverter in
3mm × 3mm DFN
DESCRIPTION
FEATURES
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The LT®3471 dual switching regulator combines two 42V,
1.3A switches with error amplifiers that can sense to
ground providing boost and inverting capability. The low
VCESAT bipolar switches enable the device to deliver high
current outputs in a small footprint. The LT3471 switches
at 1.2MHz, allowing the use of tiny, low cost and low profile
inductors and capacitors. High inrush current at start-up
is eliminated using the programmable soft-start function,
where an external RC sets the current ramp rate. A constant
frequency current mode PWM architecture results in low,
predictable output noise that is easy to filter.
1.2MHz Switching Frequency
Low VCESAT Switches: 330mV at 1.3A
High Output Voltage: Up to 40V
Wide Input Range: 2.4V to 16V
Inverting Capability
5V at 630mA from 3.3V Input
12V at 320mA from 5V Input
–12V at 200mA from 5V Input
Uses Tiny Surface Mount Components
Low Shutdown Current: < 1μA
Low Profile (0.75mm) 10-Lead 3mm × 3mm
DFN Package
The LT3471 switches are rated at 42V, making the device
ideal for boost converters up to ±40V as well as SEPIC
and flyback designs. Each channel can generate 5V at
up to 630mA from a 3.3V supply, or 5V at 510mA from
four alkaline cells in a SEPIC design. The device can be
configured as two boosts, a boost and inverter or two
inverters.
APPLICATIONS
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Organic LED Power Supply
Digital Cameras
White LED Power Supply
Cellular Phones
Medical Diagnostic Equipment
Local ±5V or ±12V Supply
TFT-LCD Bias Supply
xDSL Power Supply
The LT3471 is available in a low profile (0.75mm) 10-lead
3mm × 3mm DFN package.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
OLED Driver
2.2μH
VIN
3.3V
90.9k
4.7k
SHDN/SS1
4.7μF
SW1
15k
VREF
0.1μF
LT3471
10μF
CONTROL 2
15k
FB2N
4.7k
0.33μF
SHDN/SS2
GND
VOUT1 = –7V
75
70
65
55
75pF
105k
3471 TA01
10μH
80
60
FB2P
SW2
1μF
VOUT1 = 7V
85
FB1P
VIN
90
FB1N
0.33μF
VIN
95
EFFICIENCY (%)
CONTROL 1
OLED Driver Efficiency
VOUT1
7V
350mA
15μH
VIN
10μF
VOUT2
–7V
250mA
50
0
100
200
300
400
IOUT (mA)
3471 TA01b
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LT3471
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN Voltage ................................................................16V
SW1, SW2 Voltage ..................................... –0.4V to 42V
FB1N, FB1P, FB2N, FB2P Voltage......... 12V or VIN – 1.5V
SHDN/SS1, SHDN/SS2 Voltage ............................... 16V
VREF Voltage.............................................................1.5V
Maximum Junction Temperature ........................ 125°C
Operating Temperature Range (Note 2) ...– 40°C to 85°C
Storage Temperature Range...................– 65°C to 125°C
TOP VIEW
10 SW1
FB1N
1
FB1P
2
VREF
3
FB2P
4
7 SHDN/SS2
FB2N
5
6 SW2
9 SHDN/SS1
11
8 VIN
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 43°C/ W, θJC = 3°C/W
EXPOSED PAD (PIN 11) IS GND MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3471EDD#PBF
LT3471EDD#TRPBF
LBHM
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3471EDD
LT3471EDD#TR
LBHM
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
This product is only offered in trays. For more information go to: http://www.linear.com/packaging/
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = VSHDN = 3V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
2.1
2.4
V
0.991
0.987
1.000
1.009
1.013
V
V
1
1.4
Minimum Operating Voltage
Reference Voltage
l
UNITS
Reference Voltage Current Limit
(Note 3)
Reference Voltage Load Regulation
0mA ≤ IREF ≤ 100μA (Note 3)
0.1
0.2
%/100μA
Reference Voltage Line Regulation
2.6V ≤ VIN ≤ 16V
0.03
0.08
%/V
Error Amplifier Offset
Transition from Not Switching to Switching, VFBP = VFBN = 1V
±2
±3
mV
l
mA
FB Pin Bias Current
V FB = 1V (Note 3)
60
100
nA
Quiescent Current
VSHDN = 1.8V, Not Switching
2.5
4
mA
Quiescent Current in Shutdown
VSHDN = 0.3V, VIN = 3V
0.01
1
μA
1
1.2
1.4
90
86
94
%
%
15
%
Switching Frequency
Maximum Duty Cycle
l
Minimum Duty Cycle
Switch Current Limit
At Minimum Duty Cycle
At Maximum Duty Cycle (Note 4)
Switch VCESAT
ISW = 0.5A (Note 5)
Switch Leakage Current
VSW = 5V
SHDN/SS Input Voltage High
1.5
0.9
1.8
MHz
2.05
1.45
2.6
2.0
A
A
150
250
mV
0.01
1
μA
V
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LT3471
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = VSHDN = 3V unless otherwise noted.
PARAMETER
CONDITIONS
SHDN Input Voltage Low
Quiescent Current ≤ 1μA
MIN
SHDN Pin Bias Current
VSHDN = 3V, VIN = 4V
VSHDN = 0V
TYP
22
0
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 LT3471E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
MAX
UNITS
0.3
V
36
0.1
μA
μA
temperature range are assured by design, characterization and
correlation with statistical process controls.
Note 3: Current flows out of the pin.
Note 4: See Typical Performance Characteristics for guaranteed current
limit vs duty cycle.
Note 5: VCESAT is 100% tested at wafer level only.
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
vs Temperature
1.010
2.4
1.005
2.2
VREF
VOLTAGE
100mV/DIV
VREF (V)
1.000
2.0
0.995
1.8
1.6
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
VREF CURRENT 200μA/DIV
0.990
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
3471 G01
3471 G02
SHDN/SS Current
vs SHDN/SS Voltage
Switch Saturation Voltage
vs Switch Current
Current Limit vs Duty Cycle
2.2
SHDN/SS VOLTAGE 1V/DIV
3471 G04
CURRENT LIMIT (A)
700
TYPICAL
1.8
VIN > VSHDN/SS
800
TA = 25°C
2.0
VIN = 3.3V
SHDN/SS
CURRENT
20μV/DIV
3471 G03
125
600
1.6
90°C
GUARANTEED
1.4
VCESAT (mV)
QUIESCENT CURRENT (mA)
VREF Voltage vs VREF Current
VREF Voltage vs Temperature
2.6
1.2
1.0
0.8
0.6
25°C
400
300
200
0.4
100
0.2
0
500
0
0
20
60
40
DUTY CYCLE (%)
80
100
3471 G05
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SW CURRENT (A)
3471 G06
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LT3471
TYPICAL PERFORMANCE CHARACTERISTICS
Peak Switch Current
vs SHDN/SS Voltage
1.50
2.0
1.45
1.8
1.40
1.6
SWITCH CURRENT (A)
FREQUENCY (MHz)
Oscillator Frequency
vs Temperature
1.35
1.30
1.25
1.20
1.15
125
VOUT2
5V/DIV
CONTROL 1
AND 2
5V/DIV
0.8
0.6
0.2
100
VOUT1
2V/DIV
1.0
1.05
50
25
0
75
TEMPERATURE (°C)
ISUPPLY
1A/DIV
1.2
0.4
–25
TA = 25°C
1.4
1.10
1.00
–50
Start-Up Waveform
(Figure 2 Circuit)
0
0.5ms/DIV
0
3471 G09
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.0
VSHDN/SS (V)
3471 G07
3471 G08
PIN FUNCTIONS
FB1N (Pin 1): Negative Feedback Pin for Switcher 1.
Connect resistive divider tap here. Minimize trace area at
FB1N. Set VOUT = VFB1P(1 + R1/R2), or connect to ground
for inverting topologies.
FB1P (Pin 2): Positive Feedback Pin for Switcher 1. Connect
either to VREF or a divided down version of VREF, or connect
to a resistive divider tap for inverting topologies.
and minimize the metal trace area connected to this pin
to minimize EMI.
SHDN/SS2 (Pin 7): Shutdown and Soft-Start Pin. Tie to
1.8V or more to enable device. Ground to shut down. Softstart function is provided when the voltage at this pin is
ramped slowly to 1.8V with an external RC circuit.
VIN (Pin 8): Input Supply. Must be locally bypassed.
VREF (Pin 3): 1.00V Reference Pin. Can supply up to
1mA of current. Do not pull this pin high. Must be locally
bypassed with no less than 0.01μF and no more than 1μF.
A 0.1μF ceramic capacitor is recommended. Use this pin
as the positive feedback reference or connect a resistor
divider here for a smaller reference voltage.
SHDN/SS1 (Pin 9): Same as SHDN/SS2 but for Switcher 1.
Note: taking either SHDN/SS pin high will enable the part.
Each switcher is individually enabled with its respective
SHDN/SS pin.
FB2P (Pin 4): Same as FB1P but for Switcher 2.
Exposed Pad (Pin 11): Ground. Connect directly to local
ground plane. This ground plane also serves as a heat
sink for optimal thermal performance.
FB2N (Pin 5): Same as FB1N but for Switcher 2.
SW1 (Pin 10): Same as SW2 but for Switcher 1.
SW2 (Pin 6): Switch Pin for Switcher 2 (Collector of internal NPN power switch). Connect inductor/diode here
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LT3471
BLOCK DIAGRAM
2
FB1P
10 SW1
+
–
A1
1
FB1N
–
RC
+
CC
8
VIN
1.00V
REFERENCE
VREF
DRIVER
A2
R
S
Q1
Q
+
3
∑
0.01Ω
–
9
4
SHDN/SS1
FB2P
RAMP
GENERATOR
LEVEL
SHIFTER
6 SW2
+
–
A3
5
FB2N
–
RC
+
CC
7
SHDN/SS2
11 GND
LEVEL
SHIFTER
DRIVER
A4
R
S
Q2
Q
+
∑
0.01Ω
–
RAMP
GENERATOR
GND
1.2MHz
OSCILLATOR
3471 F01
Figure 1. Block Diagram
OPERATION
The LT3471 uses a constant frequency, current mode
control scheme to provide excellent line and load regulation. Refer to the Block Diagram. At the start of each
oscillator cycle, the SR latch is set, which turns on the
power switch, Q1 (Q2). A voltage proportional to the switch
current is added to a stabilizing ramp and the resulting
sum is fed into the positive terminal of the PWM comparator A2 (A4). When this voltage exceeds the level at the
negative input of A2 (A4), the SR latch is reset, turning
off the power switch Q1 (Q2). The level at the negative
input of A2 (A4) is set by the error amplifier A1 (A3) and
is simply an amplified version of the difference between
the negative feedback voltage and the positive feedback
voltage, usually tied to the reference voltage VREG. In
this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the error
amplifier’s output increases, more current is delivered to
the output. Similarly, if the error decreases, less current
is delivered. Each switcher functions independently but
they share the same oscillator and thus the switchers are
always in phase. Enabling the part is done by taking either
SHDN/SS pin above 1.8V. Disabling the part is done by
grounding both SHDN/SS pins. The soft-start feature of
the LT3471 allows for clean start-up conditions by limiting
the amount of voltage rise at the output of comparator A1
and A2, which in turn limits the peak switching current.
The soft-start feature for each switcher is enabled by
slowly ramping that switcher’s SHDN/SS pin, using an
RC network, for example. Typical resistor and capacitor
values are 0.33μF and 4.7k, allowing for a start-up time
on the order of milliseconds. The LT3471 has a current
limit circuit not shown in the Block Diagram. The switch
current is constantly monitored and not allowed to exceed
the maximum switch current (typically 1.6A). If the switch
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LT3471
OPERATION
current reaches this value, the SR latch is reset regardless
of the state of the comparator A2 (A4). Also not shown
in the Block Diagram is the thermal shutdown circuit. If
the temperature of the part exceeds approximately 160°C,
both latches are reset regardless of the state of comparators A2 and A4. The current limit and thermal shutdown
circuits protect the power switch as well as the external
components connected to the LT3471.
APPLICATIONS INFORMATION
Duty Cycle
The typical maximum duty cycle of the LT3471 is 94%.
The duty cycle for a given application is given by:
DC =
| VOUT |+| VD | – | VIN |
| VOUT |+| VD | – | VCESAT |
Where VD is the diode forward voltage drop and VCESAT
is in the worst case 330mV (at 1.3A)
The LT3471 can be used at higher duty cycles, but it must
be operated in the discontinuous conduction mode so that
the actual duty cycle is reduced.
Setting Output Voltage
Setting the output voltage depends on the topology used.
For normal noninverting boost regulator topologies:
R1 VOUT = VFBP 1+ R2 where VFBN is connected between R1 and R2 (see the
Typical Applications section for examples).
Select values of R1 and R2 according to the following
equation:
V
R1= R2
OUT – 1
VREF A good value for R2 is 15k which sets the current in the
resistor divider chain to 1.00V/15k = 67μA.
VFBP is usually just tied to VREF = 1.00V, but VFBP can also
be tied to a divided down version of VREF or some other
voltage as long as the absolute maximum ratings for the
feedback pins are not exceeded (see Absolute Maximum
Ratings).
For inverting topologies, VFBN is tied to ground and VFBP
is connected between R1 and R2. R2 is between VFBP
and VREF and R1 is between VFBP and VOUT (see the Applications section for examples). In this case:
R1 VOUT = VREF R2 Select values of R1 and R2 according to the following
equation:
V R1=R2 OUT VREF A good value for R2 is 15k, which sets the current in the
resistor divider chain to 1.00V/15k = 67μA.
Switching Frequency and Inductor Selection
The LT3471 switches at 1.2 MHz, allowing for small valued
inductors to be used. 4.7μH or 10μH will usually suffice.
Choose an inductor that can handle at least 1.4A without
saturating, and ensure that the inductor has a low DCR
(copper-wire resistance) to minimize I2R power losses.
Note that in some applications, the current handling
requirements of the inductor can be lower, such as in the
SEPIC topology where each inductor only carries one half
of the total switch current. For better efficiency, use similar
valued inductors with a larger volume. Many different sizes
and shapes are available from various manufacturers.
Choose a core material that has low losses at 1.2 MHz,
such as ferrite core.
Table 1. Inductor Manufacturers
Sumida
(847) 956-0666
www.sumida.com
TDK
(847) 803-6100
www.tdk.com
Murata
(714) 852-2001
www.murata.com
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LT3471
APPLICATIONS INFORMATION
Soft-Start and Shutdown Features
CAPACITOR SELECTION
To shut down the part, ground both SHDN/SS pins. To
shut down one switcher but not the other one, ground that
switcher’s SHDN/SS pin. The soft-start feature provides a
way to limit the inrush current drawn from the supply upon
start-up. To use the soft-start feature for either switcher,
slowly ramp up that switcher’s SHDN/SS pin. The rate of
voltage rise at the output of the switcher’s comparator (A1
or A3 for switcher 1 or switcher 2 respectively) tracks the
rate of voltage rise at the SHDN/SS pin once the SHDN/SS
pin has reached about 1.1V. The soft-start function will
go away once the voltage at the SHDN/SS pin exceeds
1.8V. See the Peak Switch Current vs SHDN/SS Voltage
graph in the Typical Performance Characteristics section.
The rate of voltage rise at the SHDN/SS pin can easily be
controlled with a simple RC network connected between
the control signal and the SHDN/SS pin. Typical values
for the RC network are 4.7kΩ and 0.33μF, giving start-up
times on the order of milliseconds. This RC time constant
can be adjusted to give different start-up times. If different values of resistance are to be used, keep in mind the
SHDN/SS Current vs SHDN/SS voltage graph along with
the Peak Switch Current vs SHDN/SS Voltage graph, both
found in the Typical Performance Characteristics section.
The impedance looking into the SHDN/SS pin depends
on whether the SHDN/SS is above or below VIN. Normally
SHDN/SS will not be driven above VIN, and thus the impedance looks like 100kΩ in series with a diode. If the voltage
of the SHDN/SS pin is above VIN, the impedance looks
more like 50kΩ in series with a diode. This 100kΩ or 50kΩ
impedance can have a slight effect on the start-up time if
you choose the R in the RC soft-start network too large.
Another consideration is selecting the soft-start time so
that the soft-start feature is dominated by the RC network
and not the capacitor on VREF. (See VREF voltage reference
section of the Applications Information for details.)
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multi-layer ceramic capacitors are an excellent choice,
as they have extremely low ESR and are available in very
small packages. X5R dielectrics are preferred, followed
by X7R, as these materials retain the capacitance over
wide voltage and temperature ranges. A 4.7μF to 15μF
output capacitor is sufficient for most applications, but
systems with very low output currents may need only a
1μF or 2.2μF output capacitor. Solid tantalum or OS-CON
capacitors can be used, but they will occupy more board
area than a ceramic and will have a higher ESR. Always
use a capacitor with a sufficient voltage rating.
The soft-start feature is of particular importance in applications where the switch will see voltage levels of 30V
or higher. In these applications, the simultaneous presence
of high current and voltage during startup may cause an
overstress condition to the switch. Therefore, depending
on input and output voltage conditions, higher RC time
constant values may be necessary to improve the ruggedness of the design.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT3471. A 4.7μF to 10μF input capacitor
is sufficient for most applications. Table 2 shows a list
of several ceramic capacitor manufacturers. Consult the
manufacturers for detailed information on their entire
selection of ceramic parts.
Table 2. Ceramic Capacitor Manufacturers
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
The decision to use either low ESR (ceramic) capacitors
or the higher ESR (tantalum or OS-CON) capacitors can
affect the stability of the overall system. The ESR of any
capacitor, along with the capacitance itself, contributes
a zero to the system. For the tantalum and OS-CON capacitors, this zero is located at a lower frequency due to
the higher value of the ESR, while the zero of a ceramic
capacitor is at a much higher frequency and can generally
be ignored.
A phase lead zero can be intentionally introduced by placing
a capacitor (CPL) in parallel with the resistor (R3) between
VOUT and VFB as shown in Figure 2. The frequency of the
zero is determined by the following equation.
ƒZ =
1
2π • R3 • CPL
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LT3471
APPLICATIONS INFORMATION
L1
2.2μH
D1
VIN
RSS1
4.7k
CONTROL 1
1.8V
0V
10
9
SHDN/SS1
SW1
FB1N
CSS1
0.33μF
VIN
2.6V TO 4.2V
Li-Ion
FB1P
8
10μF
RSS2
4.7k
CONTROL 2
1.8V
0V
CSS2
0.33μF
VREF
VIN
1
2
VOUT1
7V
C3
4.7μF
C2
0.1μF
SHDN/SS2
GND
SW2
11
6
FB2P
L2
10μH
R3
90.9k
R4
15k
3
LT3471
FB2N
7
CPL
33pF
5
R2
15k
4
3471 F02
C5
1μF
L3
15μH
R1
105k
C6
75pF
VOUT2
–7V
VIN
C4
10μF
D2
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 10V
C5: XR5 OR X7R 16V
CPL: OPTIONAL
D1, D2: ON SEMICONDUCTOR MBRM-120
L1: SUMIDA CR43-2R2
L2: SUMIDA CDRH4D18-100
L3: SUMIDA CDRH4D18-150
Supply Current of Figure 2 During
Start-Up without Soft-Start RC Network
ISUPPLY
0.5A/DIV
VIN = 3.3V
ISUPPLY
0.5A/DIV
VOUT1
2V/DIV
VIN > VSHDN/SS
0.1ms/DIV
Supply Current of Figure 2 During
Start-Up with Soft-Start RC Network
VOUT1
2V/DIV
VIN = 3.3V
VIN > VSHDN/SS
3471 F02b
0.2ms/DIV
3471 F02c
Figure 2. Li-Ion OLED Driver
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LT3471
APPLICATIONS INFORMATION
By choosing the appropriate values for the resistor and
capacitor, the zero frequency can be designed to improve
the phase margin of the overall converter. The typical
target value for the zero frequency is between 35kHz
to 55kHz. Figure 3 shows the transient response of the
step-up converter from Figure 2 without the phase lead
capacitor CPL. Although adequate for many applications,
phase margin is not ideal as evidenced by 2-3 “bumps”
in both the output voltage and inductor current. A 33pF
capacitor for CPL results in ideal phase margin, which
is revealed in Figure 4 as a more damped response and
less overshoot.
VOUT
200mV/DIV
AC COUPLED
VIN = 3.3V
IL1
0.5A/DIV
AC/COUPLED
LOAD CURRENT
100mA/DIV
AC/COUPLED
VIN > VSHDN/SS
50μs/DIV
Figure 3. Transient Response of Figure 2’s Step-Up
Converter without Phase Lead Capacitor
VOUT
200mV/DIV
AC COUPLED
VIN = 3.3V
IL1
0.5A/DIV
AC/COUPLED
LOAD CURRENT
100mA/DIV
AC/COUPLED
VIN > VSHDN/SS
VREG VOLTAGE REFERENCE
Pin 3 of the LT3471 is a bandgap voltage reference that has
been divided down to 1.00V and buffered for external use.
This pin must be bypassed with at least 0.01μF and no more
than 1μF. This will ensure stability as well as reduce the
noise on this pin. The buffer has a built-in current limit of at
least 1mA (typically 1.4mA). This not only means that you
can use this pin as an external reference for supplemental
circuitry, but it also means that it is possible to provide a
soft-start feature if this pin is used as one of the feedback
pins for the error amplifier. Normally the soft-start time
will be dominated by the RC time constant discussed in
the soft-start and shutdown section. However, because of
the finite current limit of the buffer for the VREG pin, it will
take some time to charge up the bypass capacitor. During
this time, the voltage at the VREG pin will ramp up, and
this action provides an alternate means for soft-starting
the circuit. If the largest recommended bypass capacitor
is used, 1μF, the worst-case (longest) soft-start function
that would be provided from the VREF pin is:
1μF • 1.00V
=1.0ms
1.0mA
Choose the RC network such that the soft-start time is
longer than this time, or choose a smaller bypass capacitor
for the VREF pin (but always larger than 0.01μF) so that the
RC network dominates the soft-starting of the LT3471. The
voltage at the VREF pin can also be divided down and used
for one of the feedback pins for the error amplifier. This
is especially useful in LED driver applications, where the
current through the LEDs is set using the voltage reference
across a sense resistor in the LED chain. Using a smaller
or divided down reference leads to less wasted power in
the sense resistor. See the Typical Applications section
for an example of LED driving applications.
50μs/DIV
Figure 4. Transient Response of Figure 2’s Step-Up
Converter with 33pF Phase Lead Capacitor
3471fb
9
LT3471
APPLICATIONS INFORMATION
DIODE SELECTION
Compensation—Theory
A Schottky diode is recommended for use with the
LT3471. For high efficiency, a diode with good thermal
characteristics at high currents should be used such as
the On Semiconductor MBRM120. This is a 20V diode.
Where the switch voltage exceeds 20V, use the MBRM140,
a 40V diode. These diodes are rated to handle an average
forward current of 1.0A. In applications where the average
forward current of the diode is less than 0.5A, use the
Philips PMEG 2005, 3005, or 4005 (a 20V, 30V or 40V
diode, respectively).
Like all other current mode switching regulators, the
LT3471 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT3471: a
fast current loop which does not require compensation,
and a slower voltage loop which does. Standard Bode
plot analysis can be used to understand and adjust the
voltage feedback loop.
LAYOUT HINTS
The high speed operation of the LT3471 demands careful attention to board layout. You will not get advertised
performance with careless layout. Figure 5 shows the
recommended component placement.
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure 6 shows the key equivalent elements of a boost converter. Because of the fast current control loop, the power
stage of the IC, inductor and diode have been replaced by
the equivalent transconductance amplifier gmp. gmp acts as
a current source where the output current is proportional
to the VC voltage. Note that the maximum output current
of gmp is finite due to the current limit in the IC.
CONTROL 2
CONTROL 1
CSS1
RSS1
GND
–
CSS2
RSS2
GND
gmp
GND
VOUT
+
CPL
RESR
RL
C4
C1
VOUT2
VC
gma
L2
L1
VOUT1 D1
SW1
10
9
L3
VCC
•
8
SW2
6
SHDN/SS1
C3
7
SHDN/SS2
C5
RC
•
FB1P
VREF
FB2P
FB2N
2
3
4
5
R3
R2
R1
–
R2
CC: COMPENSATION CAPACITOR
COUT: OUTPUT CAPACITOR
CPL: PHASE LEAD CAPACITOR
gma: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
gmp: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
RC: COMPENSATION RESISTOR
RL: OUTPUT RESISTANCE DEFINED AS VOUT DIVIDED BY ILOAD(MAX)
RO: OUTPUT RESISTANCE OF gma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
RESR: OUTPUT CAPACITOR ESR
D2
LT3471
1
COUT
1.00V
REFERENCE
3471 F06
PIN 11 GND
FB1N
RO
CC
GND
GND
R4
+
Figure 6. Boost Converter Equivalent Model
R1
VOUT2
VOUT1
C2
3471 F05
Figure 5. Suggested Layout Showing a Boost on SW1 and
an Inverter on SW2. Note the Separate Ground Returns for
All High Current Paths (Using a Multilayer Board)
3471fb
10
LT3471
APPLICATIONS INFORMATION
From Figure 6, the DC gain, poles and zeroes can be
calculated as follows:
Output Pole: P1=
2
2 • π • RL • COUT
Value
Units
RL
20
Ω
Application Specific
COUT
4.7
μF
Application Specific
RESR
10
mΩ
Application Specific
RO
0.9
MΩ
Not Adjustable
CC
90
pF
Not Adjustable
CPL
33
pF
Adjustable
RC
55
kΩ
Not Adjustable
1
R1
90.9
kΩ
Adjustable
2 • π • RESR • COUT
R2
15
kΩ
Adjustable
VOUT
7
V
VIN
3.3
V
gma
50
μmho
Not Adjustable
gmp
9.3
mho
Not Adjustable
L
2.2
μH
fS
1.2
MHz
Error Amp Zero: Z1=
1
2 • π • RC • CC
V
1
DC GAIN: A= REF • gma • RO • gmp • RL •
VOUT
2
RHP Zero: Z3=
Table 3. Bode Plot Parameters
Parameter
1
Error Amp Pole: P2=
2 • π • RO • CC
ESR Zero: Z2 =
Using the circuit of Figure 2 as an example, Table 3 shows
the parameters used to generate the Bode plot shown in
Figure 7.
VIN 2 • RL
2 • π • VOUT 2 • L
High Frequency Pole: P3>
fS
3
1
2 • π • R1• CPL
1
Phase Lead Pole: P4 =
R1• R2
2 • π • CPL •
R1+R2
Comment
Application Specific
Application Specific
Application Specific
Not Adjustable
Phase Lead Zero: Z4 =
70
0
60
–50
50
–100
GAIN (dB)
40
30
–150
20
–200
10
–250
0
–300
–10
–20
–30
100
PHASE (DEG)
The Current Mode zero is a right half plane zero which can
be an issue in feedback control design, but is manageable
with proper external component selection.
From Figure 7, the phase is –115° when the gain reaches
0dB giving a phase margin of 65°. This is more than
adequate. The crossover frequency is 50kHz.
–350
GAIN
PHASE
1k
–400
10k
100k
FREQUENCY (Hz)
1M
3471 F07
Figure 7. Bode Plot of 3.3V to 7V Application
3471fb
11
LT3471
TYPICAL APPLICATIONS
Li-Ion OLED Driver
L1
2.2μH
D1
VIN
CONTROL 1
1.8V
0V
VIN
2.6V TO 4.2V
Li-Ion
CONTROL 2
1.8V
0V
RSS1
4.7k
10
9
SHDN/SS1
SW1
FB1N
CSS1
0.33μF
FB1P
8
C1
10μF
RSS2 4.7k
CSS2
0.33μF
VREF
VIN
7
1
2
SHDN/SS2
GND
FB2P
VCONTROL
C2
0.1μF 0V TO 1V
R5
20k
5
4
6
L2
15μH
VOUT1
7V
500mA WHEN VIN = 4.2V
350mA WHEN VIN = 3.3V
250mA WHEN VIN = 2.6V
R2
15k
R6
10k
SW2
11
C3
4.7μF
R4
15k
3
LT3471
FB2N
R3
90.9k
C6
33pF
3471 TA02
C5
1μF
L3
15μH
R1
105k
VIN
C4
10μF
D2
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 10V
C5: XR5 OR X7R 16V
C6: OPTIONAL
C6
75pF
D1, D2: ON SEMICONDUCTOR MBRM-120
L1: SUMIDA CR43-2R2
L2: SUMIDA CDRH4D18-100
L3: SUMIDA CDRH4D18-150
VOUT2
–7V TO –4V
–7V WHEN VCONTROL = 0V
–4V WHEN VCONTROL = 1
–7V, 300mA WHEN VIN = 4.2V
–7V, 250mA WHEN VIN = 3.3V
–7V, 200mA WHEN VIN = 2.6V
Li-Ion OLED Driver Efficiency
95
VOUT = 7V
90
VIN = 4.2V
EFFICIENCY (%)
85
VIN = 3.3V
VIN = 2.6V
80
VIN = 4.2V
VIN = 3.3V
75
70
VIN = 2.6V
65
60
VOUT = –7V
55
50
0
100
300
200
IOUT (mA)
400
500
3471 TA02b
3471fb
12
LT3471
TYPICAL APPLICATIONS
Single Li-Ion Cell to 5V, 12V Boost Converter
L1
3.3μH
C3
10μF
VOUT1
5V
900mA IF VIN = 4.2V
630mA IF VIN = 3.3V
425mA IF VIN = 2.6V
C4
10μF
VOUT2
12V
300mA IF VIN = 4.2V
210mA IF VIN = 3.3V
145mA IF VIN = 2.6V
D1
VIN
CONTROL 1
1.8V
OV
9
SHDN/SS1
SW1
FB1N
CSS1
0.33μF
FB1P
8
VIN
2.6V TO 4.2V
CONTROL 2
1.8V
0V
10
RSS1
4.7k
RSS2
4.7k
VREF
VIN
CSS2
0.33μF
2
C2
0.1μF
FB2P
SHDN/SS2
GND
11
R1
20k
R2
4.99k
3
LT3471
C1
4.7μF
7
C5
100pF
1
FB2N
4
5
SW2
L2
6.8μH
6
3471 TA03
VIN
D2
C6
220pF
R3
54.9k
R4
4.99k
C1-C3: X5R OR X7R 6.3V
C4: X5R OR X7R 16V
D1, D2: ON SEMICONDUCTOR MBRM-120
L1: SUMIDA CR43-3R3
L2: SUMIDA CR43-6R8
3471fb
13
LT3471
TYPICAL APPLICATIONS
Li-Ion 20 White LED Driver
L1
2.2μH
D1
VIN
CONTROL 1
1.8V
OV
9
SHDN/SS1
SW1
FB1N
CSS1
0.33μF
FB1P
8
VIN
2.6V TO 4.2V
CONTROL 2
1.8V
OV
C3
0.22μF
10
RSS1
4.7k
RSS2
4.7k
VREF
VIN
CSS2
0.33μF
2
3
C2
0.1μF
FB2P
7
1
LT3471
C1
4.7μF
SHDN/SS2
GND
FB2N
IOUT1
20mA
4
R1
90.9k
10 WHITE LEDs
R2
10k
5
SW2
11
6
3471 TA04
4.99Ω
L2
2.2μH
VIN
D2
C4
0.22μF
IOUT2
20mA
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 50V
D1, D2: ON SEMICONDUCTOR MBRM-140
L1, L2: SUMIDA CDRH2D-2R2
10 WHITE LEDs
4.99Ω
3471fb
14
LT3471
TYPICAL APPLICATIONS
Li-Ion or 4-Cell Alkaline to 3.3V and 5V SEPIC
C3
4.7μF
L1
10μH
D1
VIN
CONTROL 1
1.8V
OV
10
RSS1
4.7k
9
SW1
SHDN/SS1
RSS2
4.7k
SHDN/SS2
GND
R1
34.8k
C2
0.1μF
LT3471
4
FB2P
7
C4
15μF
R2
15k
3
VREF
VIN
C1
4.7μF
CSS2
0.33μF
2
FB1P
8
C7
56pF
1
FB1N
CSS1
0.33μF
VIN
2.6V TO 6.5V
CONTROL 2
1.8V
OV
L2
10μH
VOUT1
3.3V
640mA AT VIN = 6.5V
550mA AT VIN = 5V
470mA AT VIN = 4V
410mA AT VIN = 3.3V
340mA AT VIN = 2.6V
5
FB2N
SW2
11
6
3471 TA05
C5
10μF
L3
10μH
D2
VIN
C1, C3, C5: X5R OR X7R 10V
C4, C6: X5R OR X7R 6.3V
D1, D2: ON SEMICONDUCTOR MBRM-120
L1-L4: MURATA LQH43CN100K032
L4
10μH
C6
15μF
C8
R3
56pF 60.4k
R4
15k
VOUT2
5V
500mA AT VIN = 6.5V
420mA AT VIN = 5V
360mA AT VIN = 4V
300mA AT VIN = 3.3V
250mA AT VIN = 2.6V
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698)
R = 0.115
TYP
6
0.38 ± 0.10
10
0.675 ±0.05
3.50 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
3.00 ±0.10
(4 SIDES)
PACKAGE
OUTLINE
1.65 ± 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
0.200 REF
0.25 ± 0.05
0.50
BSC
2.38 ±0.05
(2 SIDES)
1
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
0.00 – 0.05
2.38 ±0.10
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
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
3471fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LT3471
TYPICAL APPLICATIONS
5V to ±12V Dual Supply Boost/Inverting Converter
L1
10μH
D1
VIN
10
CONTROL 1
1.8V
OV
4.7k
9
SHDN/SS1
SW1
FB1N
FB1P
0.33μF
8
VIN
5V
CONTROL 2
1.8V
OV
VREF
VIN
0.33μF
SHDN/SS2
GND
FB2N
4
5
C7
56pF
6
3471 TA06
•
VIN
R3
15k
SW2
11
L2
10μH
C3
4.7μF
R2
4.99k
3
C2
0.1μF
FB2P
7
2
LT3471
C1
4.7μF
4.7k
R1
C6
56pF 54.9k
1
VOUT1
12V
320mA
R4
182k
•
C5
1μF
D2
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 16V
C5: X5R OR X7R 25V
D1, D2: ON SEMICONDUCTOR MBRM-120
L3
10μH
C4
4.7μF
VOUT2
–12V
200mA
L1: SUMIDA CR43-10
L2, L3: SUMIDA CLS63-10
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1611
550mA (ISW ), 1.4MHz, High Efficiency Micropower Inverting
DC/DC Converter
VIN: 1.1V to 10V, VOUT(MAX) = –34V, IQ = 3mA, ISD < 1μA,
ThinSOT Package
LT1613
550mA (ISW ), 1.4MHz, High Efficiency Step-Up
DC/DC Converter
VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA, ISD < 1μA,
ThinSOT Package
LT1614
750mA (ISW ), 600kHz, High Efficiency Micropower Inverting
DC/DC Converter
VIN: 1V to 12V, VOUT(MAX) = –24V, IQ = 1mA, ISD < 10μA,
MS8, S8 Packages
LT1615/LT1615-1
300mA/80mA (ISW ), High Efficiency Step-Up
DC/DC Converters
VIN = 1V to 15V, VOUT(MAX) = 34V, IQ = 20μA, ISD < 1μA,
ThinSOT Package
LT1617/LT1617-1
350mA/100mA (ISW ), High Efficiency Micropower Inverting
DC/DC Converters
VIN = 1.2V to 15V, VOUT(MAX) = –34V, IQ = 20μA, ISD < 1μA,
ThinSOT Package
LT1930/LT1930A
1A (ISW ), 1.2MHz/2.2MHz, High Efficiency Step-Up
DC/DC Converters
VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA,
ISD < 1μA, ThinSOT Package
LT1931/LT1931A
1A (ISW ), 1.2MHz/2.2MHz High Efficiency Micropower Inverting
DC/DC Converters
VIN = 2.6V to 16V, VOUT(MAX) = –34V, IQ = 5.8mA, ISD < 1μA,
ThinSOT Package
LT1943 (Quad)
Quad Boost, 2.6A Buck, 2.6A Boost, 0.3A Boost, 0.4A Inverter
1.2MHz TFT DC/DC Converter
VIN = 4.5V to 22V, VOUT(MAX) = 40V, IQ = 10μA, ISD < 35μA,
TSSOP28E Package
LT1945 (Dual)
Dual Output, Boost/Inverter, 350mA (ISW ), Constant Off-Time,
High Efficiency Step-Up DC/DC Converter
VIN = 1.2V to 15V, VOUT(MAX) = ±34V, IQ = 40μA, ISD < 1μA,
10-Lead MS Package
LT1946/LT1946A
1.5A (ISW ), 1.2MHz/2.7MHz, High Efficiency Step-Up
DC/DC Converters
VIN: 2.45V to 16V, VOUT(MAX) = 34V, IQ = 3.2mA, ISD < 1μA,
MS8 Package
LT3436
3A (ISW ), 1MHz, 34V Step-Up DC/DC Converter
VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6μA,
TSSOP16E Package
LT3462/LT3462A
300mA (ISW ), 1.2MHz/2.7MHz, High Efficiency Inverting
DC/DC Converters with Integrated Schottkys
VIN = 2.5V to 16V, VOUT(MAX) = –38V, IQ = 2.9mA, ISD < 1μA,
ThinSOT Package
LT3463/LT3463A
VIN = 2.3V to 15V, VOUT(MAX) = ±40V, IQ = 40μA, ISD < 1μA,
Dual Output, Boost/Inverter, 250mA (ISW ), Constant Off-Time,
High Efficiency Step-Up DC/DC Converters with Integrated Schottkys DFN Package
LT3464
85mA (ISW ), High Efficiency Step-Up DC/DC Converter with
Integrated Schottky and PNP Disconnect
VIN = 2.3V to 10V, VOUT(MAX) = 34V, IQ = 25μA, ISD < 1μA,
ThinSOT Package
3471fb
16 Linear Technology Corporation
LT 1008 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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© LINEAR TECHNOLOGY CORPORATION 2004