LT3471EDD#TRPBF [Linear]
LT3471 - Dual 1.3A, 1.2MHz Boost/Inverter in 3mm x 3mm DFN; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C;
| 型号： | LT3471EDD#TRPBF |
| 厂家： | 
Linear |
| 描述： | LT3471 - Dual 1.3A, 1.2MHz Boost/Inverter in 3mm x 3mm DFN; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C 开关 光电二极管 |
| 文件： | 总16页 (文件大小：294K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3471
Dual 1.3A, 1.2MHz
Boost/Inverter in
3mm × 3mm DFN
FEATURES
DESCRIPTION
n
1.2MHz Switching Frequency
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
n
Low V
Switches: 330mV at 1.3A
CESAT
n
n
n
n
n
n
n
n
n
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
V
CESAT
bipolar switches enable the device to deliver high
current outputs in a small footprint. The LT3471 switches
at1.2MHz,allowingtheuseoftiny,lowcostandlowprofile
inductors and capacitors. High inrush current at start-up
is eliminated using the programmable soft-start function,
whereanexternalRCsetsthecurrentramprate.Aconstant
frequency current mode PWM architecture results in low,
predictable output noise that is easy to filter.
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
n
Organic LED Power Supply
n
Digital Cameras
n
White LED Power Supply
Cellular Phones
Medical Diagnostic Equipment
Local 5V or 12V Supply
TFT-LCD Bias Supply
xDSL Power Supply
n
n
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.
n
n
n
TYPICAL APPLICATION
OLED Driver
2.2μH
OLED Driver Efficiency
V
OUT1
V
95
90
85
80
75
70
65
60
55
50
IN
7V
3.3V
350mA
90.9k
15k
4.7μF
V
OUT1
= 7V
CONTROL 1
SW1
4.7k
SHDN/SS1
FB1N
FB1P
0.33μF
V
OUT1
= –7V
V
REF
0.1μF
V
V
LT3471
IN
IN
10μF
4.7k
15k
FB2N
FB2P
CONTROL 2
SHDN/SS2
GND
SW2
0.33μF
75pF
3471 TA01
105k
1μF
10μH
15μH
V
–7V
250mA
OUT2
200
0
100
300
400
V
IN
I
(mA)
OUT
10μF
3471 TA01b
3471fb
1
LT3471
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V Voltage................................................................16V
IN
SW1, SW2 Voltage.....................................–0.4V to 42V
FB1N
FB1P
1
2
3
4
5
10 SW1
FB1N, FB1P, FB2N, FB2P Voltage......... 12V or V – 1.5V
9
8
7
6
SHDN/SS1
IN
V
REF
11
V
IN
SHDN/SS1, SHDN/SS2 Voltage ............................... 16V
FB2P
FB2N
SHDN/SS2
V
Voltage.............................................................1.5V
REF
SW2
Maximum Junction Temperature ........................ 125°C
Operating Temperature Range (Note 2) ...–40°C to 85°C
Storage Temperature Range...................–65°C to 125°C
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
T
= 125°C, θ = 43°C/ W, θ = 3°C/W
JMAX
JA JC
EXPOSED PAD (PIN 11) IS GND MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3471EDD#PBF
LEAD BASED FINISH
LT3471EDD
TAPE AND REEL
LT3471EDD#TRPBF
TAPE AND REEL
LT3471EDD#TR
PART MARKING
LBHM
PACKAGE DESCRIPTION
10-Lead (3mm × 3mm) Plastic DFN
TEMPERATURE RANGE
–40°C to 85°C
PART MARKING
LBHM
PACKAGE DESCRIPTION
TEMPERATURE RANGE
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/
The ● denotes specifications which apply over the full operating
ELECTRICAL CHARACTERISTICS
temperature range, otherwise specifications are TA = 25°C. VIN = VSHDN = 3V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum Operating Voltage
Reference Voltage
2.1
2.4
V
0.991
0.987
1.000
1.009
1.013
V
V
l
Reference Voltage Current Limit
Reference Voltage Load Regulation
Reference Voltage Line Regulation
Error Amplifier Offset
(Note 3)
1
1.4
0.1
0.03
2
mA
%/100μA
%/V
0mA ≤ I
≤ 100μA (Note 3)
0.2
0.08
3
REF
2.6V ≤ V ≤ 16V
IN
Transition from Not Switching to Switching, V
= V
= 1V
FBN
mV
FBP
l
FB Pin Bias Current
V
V
V
= 1V (Note 3)
60
100
4
nA
FB
Quiescent Current
= 1.8V, Not Switching
2.5
0.01
1.2
94
mA
SHDN
SHDN
Quiescent Current in Shutdown
Switching Frequency
= 0.3V, V = 3V
1
μA
IN
1
1.4
MHz
Maximum Duty Cycle
90
86
%
%
l
Minimum Duty Cycle
Switch Current Limit
15
%
At Minimum Duty Cycle
At Maximum Duty Cycle (Note 4)
1.5
0.9
2.05
1.45
2.6
2.0
A
A
Switch V
I
= 0.5A (Note 5)
= 5V
150
250
1
mV
μA
CESAT
SW
Switch Leakage Current
V
0.01
SW
SHDN/SS Input Voltage High
1.8
V
3471fb
2
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
MIN
TYP
MAX
UNITS
SHDN Input Voltage Low
SHDN Pin Bias Current
Quiescent Current ≤ 1μA
0.3
V
V
V
= 3V, V = 4V
22
0
36
0.1
μA
μA
SHDN
SHDN
IN
= 0V
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
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: V
is 100% tested at wafer level only.
CESAT
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
vs Temperature
VREF Voltage vs VREF Current
VREF Voltage vs Temperature
2.6
2.4
2.2
2.0
1.8
1.6
1.010
1.005
1.000
0.995
0.990
V
REF
VOLTAGE
100mV/DIV
V
CURRENT 200μA/DIV
REF
3471 G03
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3471 G01
3471 G02
SHDN/SS Current
vs SHDN/SS Voltage
Switch Saturation Voltage
vs Switch Current
Current Limit vs Duty Cycle
800
700
600
500
400
300
200
100
0
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
T
= 25°C
A
V
= 3.3V
IN
TYPICAL
90°C
SHDN/SS
CURRENT
20μV/DIV
GUARANTEED
25°C
V
> V
SHDN/SS
IN
SHDN/SS VOLTAGE 1V/DIV
3471 G04
0
20
40
60
80
100
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
DUTY CYCLE (%)
SW CURRENT (A)
3471 G05
3471 G06
3471fb
3
LT3471
TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency
vs Temperature
Peak Switch Current
Start-Up Waveform
(Figure 2 Circuit)
vs SHDN/SS Voltage
1.50
1.45
1.40
1.35
1.30
1.25
1.20
1.15
1.10
1.05
1.00
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
T
= 25°C
A
I
SUPPLY
1A/DIV
V
OUT1
2V/DIV
V
OUT2
5V/DIV
CONTROL 1
AND 2
5V/DIV
0.5ms/DIV
3471 G09
–50
0
25
50
75 100 125
–25
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8 2.0
TEMPERATURE (°C)
V
(V)
SHDN/SS
3471 G07
3471 G08
PIN FUNCTIONS
FB1N (Pin 1): Negative Feedback Pin for Switcher 1.
and minimize the metal trace area connected to this pin
to minimize EMI.
Connect resistive divider tap here. Minimize trace area at
FB1N. Set V
= V
(1 + R1/R2), or connect to ground
FB1P
OUT
SHDN/SS2 (Pin 7): Shutdown and Soft-Start Pin. Tie to
1.8V or more to enable device. Ground to shut down. Soft-
start function is provided when the voltage at this pin is
ramped slowly to 1.8V with an external RC circuit.
for inverting topologies.
FB1P(Pin2):PositiveFeedbackPinforSwitcher1.Connect
eithertoV oradivideddownversionofV ,orconnect
REF
REF
to a resistive divider tap for inverting topologies.
V (Pin 8): Input Supply. Must be locally bypassed.
IN
V
(Pin 3): 1.00V Reference Pin. Can supply up to
REF
SHDN/SS1(Pin9):SameasSHDN/SS2butforSwitcher 1.
Note: taking either SHDN/SS pin high will enable the part.
Each switcher is individually enabled with its respective
SHDN/SS pin.
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.
SW1 (Pin 10): Same as SW2 but for Switcher 1.
Exposed Pad (Pin 11): Ground. Connect directly to local
ground plane. This ground plane also serves as a heat
sink for optimal thermal performance.
FB2P (Pin 4): Same as FB1P but for Switcher 2.
FB2N (Pin 5): Same as FB1N but for Switcher 2.
SW2 (Pin 6): Switch Pin for Switcher 2 (Collector of in-
ternal NPN power switch). Connect inductor/diode here
3471fb
4
LT3471
BLOCK DIAGRAM
10 SW1
Q1
FB1P
2
+
–
+
DRIVER
A1
FB1N
R
1
A2
R
Q
C
–
S
C
C
+
–
V
V
REF
IN
1.00V
0.01Ω
8
9
3
∑
REFERENCE
RAMP
GENERATOR
SHDN/SS1
LEVEL
SHIFTER
GND
SW2
11
6
FB2P
FB2N
4
5
+
–
+
DRIVER
A3
R
Q2
A4
R
Q
C
–
S
C
C
+
–
SHDN/SS2
LEVEL
SHIFTER
7
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 regu-
lation. Refer to the Block Diagram. At the start of each
oscillator cycle, the SR latch is set, which turns on the
powerswitch,Q1(Q2).Avoltageproportionaltotheswitch
current is added to a stabilizing ramp and the resulting
sum is fed into the positive terminal of the PWM compara-
tor 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
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
3471fb
voltage, usually tied to the reference voltage V . In
REG
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
5
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 compara-
tors 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
For inverting topologies, V
is tied to ground and V
FBN
FBP
FBP
(see the Ap-
is connected between R1 and R2. R2 is between V
and V and R1 is between V and V
The typical maximum duty cycle of the LT3471 is 94%.
The duty cycle for a given application is given by:
REF
FBP
OUT
plications section for examples). In this case:
|VOUT |+|VD |–|V |
|VOUT |+|VD |–|VCESAT
IN
ꢀ
ꢃ
ꢅ
DC=
R1
R2
VOUT = VREF
|
ꢂ
ꢁ
ꢄ
Where V is the diode forward voltage drop and V
is in the worst case 330mV (at 1.3A)
D
CESAT
Select values of R1 and R2 according to the following
equation:
The LT3471 can be used at higher duty cycles, but it must
beoperatedinthediscontinuousconductionmodesothat
the actual duty cycle is reduced.
ꢀ
ꢂ
ꢁ
ꢃ
V
OUT ꢅ
R1=R2
V
ꢄ
REF
A good value for R2 is 15k, which sets the current in the
resistor divider chain to 1.00V/15k = 67μA.
Setting Output Voltage
Setting the output voltage depends on the topology used.
For normal noninverting boost regulator topologies:
Switching Frequency and Inductor Selection
ꢀ
ꢃ
R1
TheLT3471switchesat1.2MHz, allowingforsmallvalued
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
V
OUT = VFBP 1+
ꢂ
ꢅ
ꢁ
R2ꢄ
where V
is connected between R1 and R2 (see the
Typical Applications section for examples).
FBN
2
(copper-wire resistance) to minimize I R 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
ofthetotalswitchcurrent.Forbetterefficiency,usesimilar
valuedinductorswithalargervolume.Manydifferentsizes
and shapes are available from various manufacturers.
Choose a core material that has low losses at 1.2 MHz,
such as ferrite core.
Select values of R1 and R2 according to the following
equation:
ꢆ
ꢈ
ꢇ
ꢉ
ꢀ
ꢂ
ꢁ
ꢃ
ꢅ
ꢄ
VOUT
R1= R2
– 1
ꢋ
V
ꢊ
REF
A good value for R2 is 15k which sets the current in the
resistor divider chain to 1.00V/15k = 67μA.
V
is usually just tied to V = 1.00V, but V can also
REF FBP
FBP
Table 1. Inductor Manufacturers
be tied to a divided down version of V
or some other
REF
Sumida
TDK
(847) 956-0666
(847) 803-6100
(714) 852-2001
www.sumida.com
www.tdk.com
voltage as long as the absolute maximum ratings for the
feedback pins are not exceeded (see Absolute Maximum
Ratings).
Murata
www.murata.com
3471fb
6
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 differ-
ent 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
Low ESR (equivalent series resistance) capacitors should
beusedattheoutputtominimizetheoutputripplevoltage.
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.
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
AVX
(408) 573-4150
(803) 448-9411
(714) 852-2001
www.t-yuden.com
www.avxcorp.com
www.murata.com
on whether the SHDN/SS is above or below V . Normally
Murata
IN
SHDN/SS will not be driven above V , and thus the imped-
IN
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 ca-
pacitors, 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.
ance looks like 100kΩ in series with a diode. If the voltage
of the SHDN/SS pin is above V , the impedance looks
IN
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 V . (See V voltage reference
REF
REF
section of the Applications Information for details.)
Aphaseleadzerocanbeintentionallyintroducedbyplacing
The soft-start feature is of particular importance in ap-
plications where the switch will see voltage levels of 30V
orhigher.Intheseapplications,thesimultaneouspresence
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 rug-
gedness of the design.
a capacitor (C ) in parallel with the resistor (R3) between
PL
V
OUT
and V as shown in Figure 2. The frequency of the
zero is determined by the following equation.
FB
1
ƒZ =
2π •R3•CPL
3471fb
7
LT3471
APPLICATIONS INFORMATION
L1
2.2μH
D1
V
OUT1
V
9
IN
7V
10
SW1
R3
C
C3
R
PL
SS1
CONTROL 1
1.8V
0V
90.9k
33pF
4.7μF
4.7k
1
2
3
SHDN/SS1
FB1N
FB1P
R4
15k
C
SS1
0.33μF
V
REF
V
IN
8
7
C2
0.1μF
V
LT3471
2.6V TO 4.2V
Li-Ion
IN
10μF
5
4
R2
15k
R
FB2N
FB2P
SS2
CONTROL 2
1.8V
0V
4.7k
SHDN/SS2
GND
SW2
6
C
SS2
3471 F02
0.33μF
11
C5
1μF
C6
75pF
L3
15μH
R1
105k
L2
10μH
V
OUT2
V
IN
–7V
C4
10μF
D2
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 10V
C5: XR5 OR X7R 16V
D1, D2: ON SEMICONDUCTOR MBRM-120
L1: SUMIDA CR43-2R2
L2: SUMIDA CDRH4D18-100
L3: SUMIDA CDRH4D18-150
C
: OPTIONAL
PL
Supply Current of Figure 2 During
Start-Up without Soft-Start RC Network
Supply Current of Figure 2 During
Start-Up with Soft-Start RC Network
I
SUPPLY
0.5A/DIV
I
SUPPLY
0.5A/DIV
V
V
OUT1
OUT1
2V/DIV
2V/DIV
3471 F02b
3471 F02c
0.1ms/DIV
0.2ms/DIV
Figure 2. Li-Ion OLED Driver
3471fb
8
LT3471
APPLICATIONS INFORMATION
V
VOLTAGE REFERENCE
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
REG
Pin3oftheLT3471isabandgapvoltagereferencethathas
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
noiseonthispin.Thebufferhasabuilt-incurrentlimitofat
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
capacitor C . Although adequate for many applications,
PL
phase margin is not ideal as evidenced by 2-3 “bumps”
in both the output voltage and inductor current. A 33pF
capacitor for C results in ideal phase margin, which
PL
is revealed in Figure 4 as a more damped response and
less overshoot.
the finite current limit of the buffer for the V
take some time to charge up the bypass capacitor. During
this time, the voltage at the V pin will ramp up, and
pin, it will
REG
V
OUT
200mV/DIV
AC COUPLED
REG
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
I
L1
0.5A/DIV
AC/COUPLED
that would be provided from the V
pin is:
REF
LOAD CURRENT
100mA/DIV
AC/COUPLED
1μF •1.00V
=1.0ms
1.0mA
50μs/DIV
Choose the RC network such that the soft-start time is
longerthanthistime,orchooseasmallerbypasscapacitor
Figure 3. Transient Response of Figure 2’s Step-Up
Converter without Phase Lead Capacitor
for the V pin (but always larger than 0.01μF) so that the
REF
RCnetworkdominatesthesoft-startingoftheLT3471.The
voltage at the V pin can also be divided down and used
REF
V
OUT
for one of the feedback pins for the error amplifier. This
is especially useful in LED driver applications, where the
currentthroughtheLEDsissetusingthevoltagereference
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.
200mV/DIV
AC COUPLED
I
L1
0.5A/DIV
AC/COUPLED
LOAD CURRENT
100mA/DIV
AC/COUPLED
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
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.
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.
Wheretheswitchvoltageexceeds20V,usetheMBRM140,
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).
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure6showsthekeyequivalentelementsofaboostcon-
verter. Because of the fast current control loop, the power
stage of the IC, inductor and diode have been replaced by
LAYOUT HINTS
theequivalenttransconductanceamplifierg .g actsas
mp mp
The high speed operation of the LT3471 demands care-
ful attention to board layout. You will not get advertised
performance with careless layout. Figure 5 shows the
recommended component placement.
a current source where the output current is proportional
to the V voltage. Note that the maximum output current
C
of g is finite due to the current limit in the IC.
mp
CONTROL 1
CONTROL 2
–
C
C
SS2
SS1
R
SS1
R
SS2
g
mp
V
OUT
GND
GND
GND
+
C
R
ESR
R
L
PL
C4
C
OUT
C1
1.00V
REFERENCE
+
–
V
OUT2
V
C
g
ma
R1
R2
L1
L2
L3
R
R
O
C
V
CC
•
V
OUT1
D1
C5
•
C
C
SW1
10
SW2
6
3471 F06
9
8
7
C : COMPENSATION CAPACITOR
C
D2
GND
C3
GND
C
C
: OUTPUT CAPACITOR
SHDN/SS1
SHDN/SS2
OUT
: PHASE LEAD CAPACITOR
PL
ma
mp
C
L
g
g
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R : COMPENSATION RESISTOR
LT3471
R : OUTPUT RESISTANCE DEFINED AS V
DIVIDED BY I
LOAD(MAX)
OUT
R : OUTPUT RESISTANCE OF g
PIN 11 GND
O
ma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
R
: OUTPUT CAPACITOR ESR
ESR
V
FB1N FB1P
FB2P FB2N
REF
Figure 6. Boost Converter Equivalent Model
1
2
3
4
5
R4
R2
R3
R1
V
OUT1
V
OUT2
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:
Using the circuit of Figure 2 as an example, Table 3 shows
the parameters used to generate the Bode plot shown in
Figure 7.
2
Output Pole: P1=
Table 3. Bode Plot Parameters
2• π •RL •COUT
Parameter
Value
20
Units
Ω
Comment
1
R
Application Specific
Application Specific
Application Specific
Not Adjustable
Not Adjustable
Adjustable
L
Error Amp Pole: P2=
2• π •RO •CC
C
4.7
10
μF
OUT
R
mΩ
MΩ
pF
ESR
1
Error Amp Zero: Z1=
R
0.9
90
O
C
2• π •RC •CC
C
VREF
VOUT
1
2
C
33
pF
PL
DC GAIN: A=
•gma •RO •gmp •RL •
R
55
kΩ
kΩ
kΩ
V
Not Adjustable
Adjustable
C
R1
R2
90.9
15
1
ESR Zero: Z2=
Adjustable
2• π •RESR •COUT
V
7
Application Specific
Application Specific
Not Adjustable
Not Adjustable
Application Specific
Not Adjustable
OUT
V
2 •RL
IN
V
3.3
50
V
IN
RHP Zero: Z3=
2• π • VOUT2 •L
g
g
μmho
mho
μH
ma
9.3
2.2
1.2
mp
fS
3
High Frequency Pole: P3>
L
f
MHz
S
1
Phase Lead Zero: Z4=
2• π •R1•CPL
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.
1
Phase Lead Pole: P4=
R1•R2
R1+R2
2• π •CPL •
70
60
0
–50
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.
50
–100
–150
–200
–250
–300
–350
–400
40
30
20
10
0
–10
–20
–30
GAIN
PHASE
100
1k
10k
100k
1M
FREQUENCY (Hz)
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
V
OUT1
V
9
IN
7V
10
SW1
R3
C6
C3
4.7μF
R
500mA WHEN V = 4.2V
SS1
IN
IN
CONTROL 1
1.8V
0V
90.9k
33pF
4.7k
350mA WHEN V = 3.3V
1
SHDN/SS1
FB1N
250mA WHEN V = 2.6V
IN
2
R4
15k
C
FB1P
SS1
3
0.33μF
V
REF
V
V
IN
8
7
CONTROL
0V TO 1V
C2
0.1μF
V
LT3471
2.6V TO 4.2V
Li-Ion
IN
C1
10μF
R5
20k
5
4
R2
15k
FB2N
FB2P
CONTROL 2
1.8V
0V
R
4.7k
SS2
SHDN/SS2
R6
10k
GND
SW2
6
C
SS2
0.33μF
3471 TA02
11
C5
1μF
C6
75pF
L3
15μH
R1
105k
L2
15μH
V
OUT2
V
IN
–7V TO –4V
–7V WHEN V
–4V WHEN V
C4
10μF
= 0V
= 1
D2
CONTROL
CONTROL
–7V, 300mA WHEN V = 4.2V
IN
–7V, 250mA WHEN V = 3.3V
IN
–7V, 200mA WHEN V = 2.6V
IN
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 10V
C5: XR5 OR X7R 16V
C6: OPTIONAL
D1, D2: ON SEMICONDUCTOR MBRM-120
L1: SUMIDA CR43-2R2
L2: SUMIDA CDRH4D18-100
L3: SUMIDA CDRH4D18-150
Li-Ion OLED Driver Efficiency
95
90
85
80
75
70
65
60
55
V
= 7V
OUT
V
= 4.2V
IN
V
IN
= 3.3V
V
= 2.6V
IN
V
= 4.2V
IN
= 3.3V
V
IN
V
= 2.6V
IN
V
OUT
= –7V
100
50
0
400
500
200
300
(mA)
I
OUT
3471 TA02b
3471fb
12
LT3471
TYPICAL APPLICATIONS
Single Li-Ion Cell to 5V, 12V Boost Converter
L1
3.3μH
V
OUT1
D1
5V
V
900mA IF V = 4.2V
IN
IN
10
630mA IF V = 3.3V
IN
R1
C5
100pF
C3
10μF
R
SS1
CONTROL 1
1.8V
OV
425mA IF V = 2.6V
IN
20k
4.7k
SW1
9
1
2
3
SHDN/SS1
FB1N
FB1P
R2
4.99k
C
SS1
0.33μF
V
REF
8
7
V
C2
0.1μF
IN
V
LT3471
IN
2.6V TO 4.2V
C1
4
5
4.7μF
R
FB2P
FB2N
SS2
CONTROL 2
1.8V
0V
4.7k
SHDN/SS2
GND
SW2
6
C
SS2
0.33μF
3471 TA03
11
L2
6.8μH
V
OUT2
D2
12V
300mA IF V = 4.2V
V
IN
IN
C4
10μF
C6
220pF
210mA IF V = 3.3V
IN
R3
54.9k
145mA IF V = 2.6V
IN
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
V
9
IN
C3
I
OUT1
10
SW1
0.22μF
R
20mA
SS1
CONTROL 1
4.7k
1
2
3
1.8V
OV
SHDN/SS1
FB1N
FB1P
C
SS1
0.33μF
V
REF
R1
90.9k
8
7
V
C2
0.1μF
IN
V
IN
LT3471
10 WHITE LEDs
2.6V TO 4.2V
C1
4
5
4.7μF
R2
10k
R
4.7k
FB2P
FB2N
SS2
CONTROL 2
1.8V
OV
SHDN/SS2
GND
SW2
6
C
SS2
0.33μF
3471 TA04
11
4.99Ω
L2
2.2μH
D2
V
IN
C4
I
OUT2
0.22μF
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
V
OUT1
D1
3.3V
640mA AT V = 6.5V
V
IN
IN
550mA AT V = 5V
C4
15μF
IN
L2
470mA AT V = 4V
IN
C7
56pF
10μH
410mA AT V = 3.3V
IN
10
SW1
R1
R
340mA AT V = 2.6V
IN
SS1
CONTROL 1
34.8k
4.7k
9
1
2
3
1.8V
SHDN/SS1
FB1N
FB1P
OV
R2
15k
C
SS1
0.33μF
V
REF
8
7
C2
0.1μF
V
IN
V
LT3471
IN
2.6V TO 6.5V
C1
4
5
4.7μF
R
FB2P
FB2N
SS2
CONTROL 2
1.8V
OV
4.7k
SHDN/SS2
GND
SW2
6
C
SS2
0.33μF
3471 TA05
11
C5
10μF
L3
10μH
V
OUT2
D2
5V
500mA AT V = 6.5V
V
IN
IN
C6
15μF
420mA AT V = 5V
IN
C8
R3
L4
10μH
360mA AT V = 4V
56pF 60.4k
C1, C3, C5: X5R OR X7R 10V
C4, C6: X5R OR X7R 6.3V
D1, D2: ON SEMICONDUCTOR MBRM-120
L1-L4: MURATA LQH43CN100K032
IN
300mA AT V = 3.3V
IN
250mA AT V = 2.6V
IN
R4
15k
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
2.15 0.05 (2 SIDES)
1.65 0.05
3.00 0.10
(4 SIDES)
1.65 0.10
(2 SIDES)
PIN 1
PACKAGE
OUTLINE
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
1
0.25 0.05
0.50 BSC
0.75 0.05
0.200 REF
0.25 0.05
0.50
BSC
2.38 0.10
(2 SIDES)
2.38 0.05
(2 SIDES)
0.00 – 0.05
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,noresponsibilityisassumedforitsuse.LinearTechnologyCorporationmakesnorepresenta-
t ion t h a t t he in ter c onne c t ion o f i t s cir cui t s a s de s cr ib e d her ein w ill no t in fr inge on ex is t ing p a ten t r igh t s.
15
LT3471
TYPICAL APPLICATIONS
5V to 12V Dual Supply Boost/Inverting Converter
L1
D1
10μH
V
OUT1
12V
V
IN
320mA
10
SW1
R1
C6
C3
4.7μF
CONTROL 1
54.9k
56pF
4.7k
1.8V
9
1
2
3
SHDN/SS1
FB1N
FB1P
OV
R2
4.99k
0.33μF
V
REF
R3
15k
8
7
V
IN
5V
C2
V
LT3471
IN
0.1μF
C1
4.7μF
4
5
FB2P
FB2N
CONTROL 2
4.7k
1.8V
SHDN/SS2
OV
C7
GND
SW2
6
56pF
0.33μF
3471 TA06
11
R4
182k
V
•
•
OUT2
–12V
V
IN
L2
10μH
L3
10μH
200mA
C4
4.7μF
D2
C5
1μF
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 16V
C5: X5R OR X7R 25V
L1: SUMIDA CR43-10
L2, L3: SUMIDA CLS63-10
D1, D2: ON SEMICONDUCTOR MBRM-120
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1611
550mA (I ), 1.4MHz, High Efficiency Micropower Inverting
V : 1.1V to 10V, V
= –34V, I = 3mA, I < 1μA,
Q SD
SW
IN
OUT(MAX)
= 34V, I = 3mA, I < 1μA,
OUT(MAX) Q SD
DC/DC Converter
ThinSOT Package
LT1613
LT1614
550mA (I ), 1.4MHz, High Efficiency Step-Up
V : 0.9V to 10V, V
SW
IN
DC/DC Converter
ThinSOT Package
750mA (I ), 600kHz, High Efficiency Micropower Inverting
V : 1V to 12V, V
= –24V, I = 1mA, I < 10μA,
OUT(MAX) Q SD
SW
IN
DC/DC Converter
MS8, S8 Packages
LT1615/LT1615-1 300mA/80mA (I ), High Efficiency Step-Up
V
= 1V to 15V, V
= 34V, I = 20μA, I < 1μA,
OUT(MAX) Q SD
SW
IN
DC/DC Converters
ThinSOT Package
LT1617/LT1617-1 350mA/100mA (I ), High Efficiency Micropower Inverting
V
= 1.2V to 15V, V
= –34V, I = 20μA, I < 1μA,
OUT(MAX) Q SD
SW
IN
DC/DC Converters
ThinSOT Package
LT1930/LT1930A 1A (I ), 1.2MHz/2.2MHz, High Efficiency Step-Up
V : 2.6V to 16V, V
= 34V, I = 4.2mA/5.5mA,
Q
SW
IN
OUT(MAX)
DC/DC Converters
I
< 1μA, ThinSOT Package
SD
LT1931/LT1931A 1A (I ), 1.2MHz/2.2MHz High Efficiency Micropower Inverting
V
= 2.6V to 16V, V
= –34V, I = 5.8mA, I < 1μA,
Q SD
SW
IN
OUT(MAX)
OUT(MAX)
OUT(MAX)
DC/DC Converters
ThinSOT Package
LT1943 (Quad)
LT1945 (Dual)
Quad Boost, 2.6A Buck, 2.6A Boost, 0.3A Boost, 0.4A Inverter
1.2MHz TFT DC/DC Converter
V
= 4.5V to 22V, V
= 40V, I = 10μA, I < 35μA,
Q SD
IN
TSSOP28E Package
Dual Output, Boost/Inverter, 350mA (I ), Constant Off-Time,
V
= 1.2V to 15V, V
= 34V, I = 40μA, I < 1μA,
Q SD
SW
IN
High Efficiency Step-Up DC/DC Converter
10-Lead MS Package
LT1946/LT1946A 1.5A (I ), 1.2MHz/2.7MHz, High Efficiency Step-Up
V : 2.45V to 16V, V
= 34V, I = 3.2mA, I < 1μA,
OUT(MAX) Q SD
SW
IN
DC/DC Converters
MS8 Package
LT3436
3A (I ), 1MHz, 34V Step-Up DC/DC Converter
V
: 3V to 25V, V
= 34V, I = 0.9mA, I < 6μA,
OUT(MAX) Q SD
SW
IN
TSSOP16E Package
LT3462/LT3462A 300mA (I ), 1.2MHz/2.7MHz, High Efficiency Inverting
V
= 2.5V to 16V, V
= –38V, I = 2.9mA, I < 1μA,
Q SD
SW
IN
OUT(MAX)
OUT(MAX)
OUT(MAX)
DC/DC Converters with Integrated Schottkys
ThinSOT Package
LT3463/LT3463A Dual Output, Boost/Inverter, 250mA (I ), Constant Off-Time,
V
= 2.3V to 15V, V
= 40V, I = 40μA, I < 1μA,
Q SD
SW
IN
High Efficiency Step-Up DC/DC Converters with Integrated Schottkys DFN Package
LT3464
85mA (I ), High Efficiency Step-Up DC/DC Converter with
V
= 2.3V to 10V, V
= 34V, I = 25μA, I < 1μA,
Q SD
SW
IN
Integrated Schottky and PNP Disconnect
ThinSOT Package
3471fb
LT 1008 REV B • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2004
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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