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

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 highcurrent outputs in a small footprint. The LT3471 switches
at 1.2MHz, allowing the use of tiny, low cost and low proﬁleinductors 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 constantfrequency current mode PWM architecture results in low,
predictable output noise that is easy to ﬁlter.

1.2MHz Switching Frequency
n Low VCESAT Switches: 330mV at 1.3A
n High Output Voltage: Up to 40V
n Wide Input Range: 2.4V to 16V
n Inverting Capability
n 5V at 630mA from 3.3V Input
n 12V at 320mA from 5V Inputn –12V at 200mA from 5V Input
n Uses Tiny Surface Mount Components
n Low Shutdown Current: < 1μA
n 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 fromfour alkaline cells in a SEPIC design. The device can be
conﬁgured as two boosts, a boost and inverter or two
inverters.

APPLICATIONSn

Organic LED Power Supply
Digital Camerasn White LED Power Supply
n Cellular Phonesn Medical Diagnostic Equipmentn Local ±5V or ±12V Supply
n TFT-LCD Bias Supply
n xDSL Power Supplyn

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

LT347110μF
CONTROL 2

15k

FB2N
4.7k
0.33μF

SHDN/SS2GND

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

3471fb1

LT3471
ABSOLUTE MAXIMUM RATINGSPIN 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 DFNTJMAX = 125°C, θJA = 43°C/ W, θJC = 3°C/W
EXPOSED PAD (PIN 11) IS GND MUST BE SOLDERED TO PCBORDER INFORMATIONLEAD FREE FINISH

TAPE AND REELPART MARKINGPACKAGE DESCRIPTIONTEMPERATURE RANGE

LT3471EDD#PBF

LT3471EDD#TRPBF

LBHM

10-Lead (3mm × 3mm) Plastic DFN–40°C to 85°C

LEAD BASED FINISH

TAPE AND REELPART MARKINGPACKAGE DESCRIPTIONTEMPERATURE 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.
PARAMETERCONDITIONS

MIN

TYP

MAX

UNITS

2.1

2.4

V

0.991
0.987

1.000

1.009
1.013

V
V

1

1.4

Minimum Operating Voltage
Reference VoltagelReference 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

MHz

90
86

94

%
%

15

%

Switching Frequency
Maximum Duty Cyclel

Minimum Duty Cycle
Switch Current Limit

At Minimum Duty Cycle
At Maximum Duty Cycle (Note 4)

Switch VCESATISW = 0.5A (Note 5)

Switch Leakage Current

VSW = 5V

SHDN/SS Input Voltage High

1.5
0.9

1.8

2.05
1.45

2.6
2.0

A
A

150

250

mV

0.01

1

μA
V
3471fb2

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.
PARAMETERCONDITIONSSHDN 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 andcorrelation 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/DIVVREF (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)

1003471 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/SSCURRENT
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 (%)

801003471 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

3471fb3

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/DIVCONTROL 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)

00.5ms/DIV

03471 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 FUNCTIONSFB1N (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 groundfor 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 isramped 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 locallybypassed 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 heatsink 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

3471fb4

LT3471
BLOCK DIAGRAM
2FB1P

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
GENERATORLEVELSHIFTER

6 SW2

+

–

A3
5

FB2N

–

RC

+

CC
7

SHDN/SS2

11 GNDLEVELSHIFTERDRIVER
A4

R

S

Q2

Q

+
∑

0.01Ω

–
RAMP
GENERATOR

GND

1.2MHzOSCILLATOR

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 thenegative 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) andis 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 peakcurrent level to keep the output in regulation. If the error
ampliﬁer’s output increases, more current is delivered tothe output. Similarly, if the error decreases, less current
is delivered. Each switcher functions independently but
they share the same oscillator and thus the switchers arealways in phase. Enabling the part is done by taking either
SHDN/SS pin above 1.8V. Disabling the part is done bygrounding 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 byslowly 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 timeon 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
3471fb5

LT3471
OPERATION
current reaches this value, the SR latch is reset regardlessof the state of the comparator A2 (A4). Also not shownin 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 thatthe 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 theresistor 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 othervoltage 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 theresistor 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 handlingrequirements of the inductor can be lower, such as in the
SEPIC topology where each inductor only carries one halfof the total switch current. For better efﬁciency, use similarvalued 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

3471fb6

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 thatswitcher’s SHDN/SS pin. The soft-start feature provides away 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 ofvoltage rise at the output of the switcher’s comparator (A1or A3 for switcher 1 or switcher 2 respectively) tracks therate 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 becontrolled 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 constantcan 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, bothfound 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 voltageof 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 sothat the soft-start feature is dominated by the RC networkand 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 verysmall packages. X5R dielectrics are preferred, followedby X7R, as these materials retain the capacitance overwide voltage and temperature ranges. A 4.7μF to 15μF
output capacitor is sufﬁcient for most applications, butsystems 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 30Vor higher. In these applications, the simultaneous presenceof 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 listof 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 canaffect 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 tothe 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 thezero is determined by the following equation.
ƒZ =1
2π • R3 • CPL
3471fb7

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 DuringStart-Up without Soft-Start RC Network

ISUPPLY
0.5A/DIV

VIN = 3.3V
ISUPPLY
0.5A/DIVVOUT1
2V/DIV

VIN > VSHDN/SS

0.1ms/DIV

Supply Current of Figure 2 DuringStart-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 Driver3471fb8

LT3471
APPLICATIONS INFORMATION
By choosing the appropriate values for the resistor andcapacitor, the zero frequency can be designed to improve
the phase margin of the overall converter. The typicaltarget value for the zero frequency is between 35kHz
to 55kHz. Figure 3 shows the transient response of thestep-up converter from Figure 2 without the phase leadcapacitor 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, whichis revealed in Figure 4 as a more damped response andless overshoot.

VOUT
200mV/DIVAC 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/DIVAC 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 morethan 1μF. This will ensure stability as well as reduce thenoise on this pin. The buffer has a built-in current limit of atleast 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 willtake some time to charge up the bypass capacitor. During
this time, the voltage at the VREG pin will ramp up, andthis action provides an alternate means for soft-startingthe circuit. If the largest recommended bypass capacitor
is used, 1μF, the worst-case (longest) soft-start functionthat 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 islonger 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. Thisis especially useful in LED driver applications, where thecurrent 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 inthe 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

3471fb9

LT3471
APPLICATIONS INFORMATIONDIODE SELECTION

Compensation—Theory

A Schottky diode is recommended for use with theLT3471. 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 thePhilips PMEG 2005, 3005, or 4005 (a 20V, 30V or 40V
diode, respectively).

Like all other current mode switching regulators, theLT3471 needs to be compensated for stable and efﬁcient
operation. Two feedback loops are used in the LT3471: afast current loop which does not require compensation,
and a slower voltage loop which does. Standard Bodeplot analysis can be used to understand and adjust thevoltage 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 therecommended 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 powerstage of the IC, inductor and diode have been replaced by
the equivalent transconductance ampliﬁer gmp. gmp acts asa current source where the output current is proportional
to the VC voltage. Note that the maximum output currentof 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/SS1C3

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 RESISTORRL: OUTPUT RESISTANCE DEFINED AS VOUT DIVIDED BY ILOAD(MAX)
RO: OUTPUT RESISTANCE OF gma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORKRESR: 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 andan Inverter on SW2. Note the Separate Ground Returns for
All High Current Paths (Using a Multilayer Board)
3471fb10

LT3471
APPLICATIONS INFORMATION
From Figure 6, the DC gain, poles and zeroes can becalculated as follows:
Output Pole: P1=

2
2 • π • RL • COUT

Value

Units

Comment

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

Adjustable2 • π • RESR • COUT

R2

15

kΩ

Adjustable

VOUT

7

V

Application Speciﬁc

VIN

3.3

V

gma

50

μmho

Not Adjustable

gmp

9.3

mho

Not Adjustable

L

2.2

μH

Application Speciﬁc

fS

1.2

MHz

Not Adjustable

Error Amp Zero: Z1=

1
2 • π • RC • CC

V
1
DC GAIN: A= REF • gma • RO • gmp • RL •VOUT2

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

fHigh Frequency Pole: P3> S
31
2 • π • R1• CPL
1Phase Lead Pole: P4 =R1• R22 • π • CPL •R1+R2

Application Speciﬁc

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 canbe an issue in feedback control design, but is manageablewith 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 thanadequate. 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

3471fb11

LT3471
TYPICAL APPLICATIONS
Li-Ion OLED Driver
L1
2.2μH

D1

VIN
CONTROL 1
1.8V
0V

VIN
2.6V TO 4.2V
Li-IonCONTROL 2
1.8V
0V

RSS1
4.7k

10
9

SHDN/SS1

SW1
FB1N

CSS1
0.33μF

FB1P
8

C1
10μF
RSS2 4.7k
CSS20.33μF

VREF
VIN

7

1
2

SHDN/SS2
GND

SW2

11

6

FB2P

L2
15μH

C3
4.7μF

VOUT1
7V
500mA WHEN VIN = 4.2V
350mA WHEN VIN = 3.3V
250mA WHEN VIN = 2.6V

R4
15k

3
VCONTROL
C2
0.1μF 0V TO 1V
R5
20k

LT3471
FB2N

R3
90.9k

C6
33pF

5
4

R2
15k

R6
10k

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

3471fb12

LT3471
TYPICAL APPLICATIONSSingle 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
8VIN
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

3471fb13

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
8VIN
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

LT3471C1
4.7μF
SHDN/SS2
GND

SW2

11

6

FB2N

IOUT1
20mA

4

R1
90.9k

10 WHITE LEDs

R2
10k

5

3471 TA04

4.99Ω
L2
2.2μH
VIN

D2
C4
0.22μF

IOUT220mA

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

3471fb14

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

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

SHDN/SS2
GND

SW2

11

6

5

FB2N
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 DESCRIPTIONDD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698)
R = 0.115
TYP
6

10

5

1

0.38 ± 0.10

0.675 ±0.05

3.50 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)

3.00 ±0.10(4 SIDES)
PACKAGEOUTLINE

PIN 1
TOP MARK(SEE NOTE 6)
0.200 REF

0.25 ± 0.05

1.65 ± 0.10
(2 SIDES)

0.50
BSC
2.38 ±0.05(2 SIDES)(DD) DFN 1103

0.25 ± 0.05
0.50 BSC

0.75 ±0.05

0.00 – 0.05

2.38 ±0.10(2 SIDES)
BOTTOM VIEW—EXPOSED PADRECOMMENDED 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
3471fbInformation 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

SW2

11

6

FB2N

R3
15k

4
5
C7
56pF

3471 TA06

•
VIN

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

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PART NUMBER

DESCRIPTIONCOMMENTS

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VIN: 1.1V to 10V, VOUT(MAX) = –34V, IQ = 3mA, ISD < 1μA,
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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
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Quad Boost, 2.6A Buck, 2.6A Boost, 0.3A Boost, 0.4A Inverter
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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

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VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6μA,
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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

LT346485mA (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,
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3471fb16 Linear Technology Corporation

LT 1008 REV B • PRINTED IN USA

1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com© LINEAR TECHNOLOGY CORPORATION 2004