FEATURES
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LT3471
Dual 1.3A, 1.2MHz
Boost/Inverter in
3mm × 3mm DFN
DESCRIPTION
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
V
CESAT
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.
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.
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.
1.2MHz Switching Frequency
Low V
CESAT
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
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
TYPICAL APPLICATION
OLED Driver
V
IN
3.3V
CONTROL 1
4.7k
0.33μF
2.2μH
90.9k
SHDN/SS1
SW1
FB1N
FB1P
V
REF
V
IN
10μF
CONTROL 2
FB2N
4.7k
0.33μF
SHDN/SS2
GND
10μH
V
IN
10μF
FB2P
SW2
3471 TA01
4.7μF
V
OUT1
7V
350mA
OLED Driver Efficiency
95
90
85
EFFICIENCY (%)
V
OUT1
= 7V
15k
0.1μF
15k
80
75
70
65
60
V
OUT1
= –7V
V
IN
LT3471
105k
15μH
75pF
V
OUT2
–7V
250mA
55
50
0
100
200
I
OUT
(mA)
3471 TA01b
1μF
300
400
3471fb
1
LT3471
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
FB1N
FB1P
V
REF
FB2P
FB2N
1
2
3
4
5
11
10 SW1
9
SHDN/SS1
8 V
IN
7
SHDN/SS2
6 SW2
V
IN
Voltage ................................................................16V
SW1, SW2 Voltage ..................................... –0.4V to 42V
FB1N, FB1P FB2N, FB2P Voltage......... 12V or V
IN
– 1.5V
,
SHDN/SS1, SHDN/SS2
Voltage ............................... 16V
V
REF
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
DD PACKAGE
10-LEAD (3mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/ W,
θ
JC
= 3°C/W
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
PART MARKING
LBHM
PACKAGE DESCRIPTION
10-Lead (3mm
×
3mm) Plastic DFN
PACKAGE DESCRIPTION
10-Lead (3mm
×
3mm) Plastic DFN
TEMPERATURE RANGE
–40°C to 85°C
TEMPERATURE RANGE
–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
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= V
SHDN
= 3V unless otherwise noted.
PARAMETER
Minimum Operating Voltage
Reference Voltage
Reference Voltage Current Limit
Reference Voltage Load Regulation
Reference Voltage Line Regulation
Error Amplifier Offset
FB Pin Bias Current
Quiescent Current
Quiescent Current in Shutdown
Switching Frequency
Maximum Duty Cycle
Minimum Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
SHDN/SS
Input Voltage High
At Minimum Duty Cycle
At Maximum Duty Cycle (Note 4)
I
SW
= 0.5A (Note 5)
V
SW
= 5V
1.8
1.5
0.9
l
l
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
0.991
0.987
1
TYP
2.1
1.000
1.4
0.1
0.03
±2
MAX
2.4
1.009
1.013
0.2
0.08
±3
100
4
1
1.4
UNITS
V
V
V
mA
%/100μA
%/V
mV
nA
mA
μA
MHz
%
%
%
(Note 3)
0mA ≤ I
REF
≤ 100μA (Note 3)
2.6V ≤ V
IN
≤ 16V
Transition from Not Switching to Switching, V
FBP
= V
FBN
= 1V
V
FB
= 1V (Note 3)
V
SHDN
= 1.8V, Not Switching
V
SHDN
= 0.3V, V
IN
= 3V
l
60
2.5
0.01
1
90
86
1.2
94
15
2.05
1.45
150
0.01
2.6
2.0
250
1
A
A
mV
μA
V
3471fb
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LT3471
ELECTRICAL CHARACTERISTICS
PARAMETER
SHDN
Input Voltage Low
SHDN
Pin Bias Current
CONDITIONS
Quiescent Current ≤ 1μA
V
SHDN
= 3V, V
IN
= 4V
V
SHDN
= 0V
22
0
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= V
SHDN
= 3V unless otherwise noted.
MIN
TYP
MAX
0.3
36
0.1
UNITS
V
μA
μA
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
CESAT
is 100% tested at wafer level only.
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
vs Temperature
2.6
1.010
V
REF
Voltage vs Temperature
V
REF
Voltage vs V
REF
Current
QUIESCENT CURRENT (mA)
2.4
1.005
V
REF
(V)
V
REF
VOLTAGE
100mV/DIV
2.2
1.000
2.0
1.8
0.995
V
REF
CURRENT 200μA/DIV
3471 G03
1.6
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
0.990
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3471 G01
3471 G02
SHDN/SS
Current
vs
SHDN/SS
Voltage
2.2
V
IN
= 3.3V
2.0
1.8
CURRENT LIMIT (A)
SHDN/SS
CURRENT
20μV/DIV
V
IN
> V
SHDN/SS
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Current Limit vs Duty Cycle
T
A
= 25°C
TYPICAL
800
700
600
V
CESAT
(mV)
GUARANTEED
Switch Saturation Voltage
vs Switch Current
90°C
500
25°C
400
300
200
100
0
SHDN/SS
VOLTAGE 1V/DIV
3471 G04
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
3471fb
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LT3471
TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency
vs Temperature
1.50
1.45
1.40
FREQUENCY (MHz)
1.35
1.30
1.25
1.20
1.15
1.10
1.05
1.00
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
SWITCH CURRENT (A)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.0
V
SHDN/SS
(V)
3471 G08
Peak Switch Current
vs
SHDN/SS
Voltage
T
A
= 25°C
I
SUPPLY
1A/DIV
V
OUT1
2V/DIV
V
OUT2
5V/DIV
CONTROL 1
AND 2
5V/DIV
Start-Up Waveform
(Figure 2 Circuit)
0.5ms/DIV
3471 G09
3471 G07
PIN FUNCTIONS
FB1N (Pin 1):
Negative Feedback Pin for Switcher 1.
Connect resistive divider tap here. Minimize trace area at
FB1N. Set V
OUT
= V
FB1P
(1 + R1/R2), or connect to ground
for inverting topologies.
FB1P (Pin 2):
Positive Feedback Pin for Switcher 1. Connect
either to V
REF
or a divided down version of V
REF
, or connect
to a resistive divider tap for inverting topologies.
V
REF
(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.
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
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. Soft-
start function is provided when the voltage at this pin is
ramped slowly to 1.8V with an external RC circuit.
V
IN
(Pin 8):
Input Supply. Must be locally bypassed.
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.
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.
3471fb
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LT3471
BLOCK DIAGRAM
2
FB1P
+
A1
DRIVER
A2
R
S
Q
10 SW1
1
FB1N
–
R
C
C
C
8
V
IN
1.00V
REFERENCE
V
REF
3
∑
–
RAMP
GENERATOR
9
SHDN/SS1
LEVEL
SHIFTER
4
FB2P
+
A3
DRIVER
A4
R
S
Q
5
FB2N
–
LEVEL
SHIFTER
R
C
C
C
7
SHDN/SS2
∑
–
RAMP
GENERATOR
1.2MHz
OSCILLATOR
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
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 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
voltage, usually tied to the reference voltage V
REG
. 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
3471fb
+
+
–
–
3471 F01
Q1
+
0.01Ω
11 GND
6 SW2
Q2
+
0.01Ω
GND
5
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