LT3477IFE#TRPBF [Linear]
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| 型号： | LT3477IFE#TRPBF |
| 厂家： | 
Linear |
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LT3477
3A, DC/DC Converter
with Dual Rail-to-Rail
Current Sense
U
FEATURES
DESCRIPTIO
The LT®3477 is a current mode, 3A DC/DC step-up con-
verter with dual rail-to-rail current sense amplifiers and an
internal 3A, 42V switch. It combines a traditional voltage
feedback loop and two unique current feedback loops to
operate as a constant-current, constant-voltage source.
■
Dual 100mV Rail-to-Rail Current Sense Amplifiers
■
Wide Input Voltage Range: 2.5V to 25V
■
3A, 42V Internal Switch
■
High Efficiency Power Conversion: Up to 93%
■
Drives LEDs in Boost, Buck-Boost or Buck Mode
■
Frequency Set by External Resistor: 200kHz to 3.5MHz Both current sense voltages are set at 100mV and can be
■
■
■
Programmable Soft-Start
adjusted independently using the IADJ1 and IADJ2 pins.
Efficiency of up to 91% can be achieved in typical applica-
tions.TheLT3477featuresaprogrammablesoft-startfunc-
tion to limit inductor current during start-up. Both inputs
oftheerroramplifierareavailableexternallyallowingposi-
tiveandnegativeoutputvoltages(Boost,Inverting,SEPIC,
Flyback). The switching frequency is programmable from
200kHz to 3.5MHz through an external resistor.
Low VCESAT Switch: 0.3V at 2.5A
Capable of Positive and Negative Output Voltages
(Boost, Inverting, SEPIC, Flyback)
Available in Thermally Enhanced 20-Lead
■
(4mm × 4mm) QUFN and 20-Lead TSSOP Packages
APPLICATIO S
■
Available in thermally enhanced 20-pin (4mm × 4mm)
QFN and 20-pin TSSOP packages, the LT3477 provides a
complete solution for both constant-voltage and con-
stant-current applications.
High Power LED Driver
■
DSL Modems
■
Distributed Power
■
Input/Output Current Limited Boost, SEPIC,
Inverting, Flyback Converters
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
■
Constant-Voltage, Constant-Current Source
U
TYPICAL APPLICATIO
330mA LED Driver with Open LED Protection
Efficiency
10µH
V
IN
90
85
80
75
70
65
60
55
5V
3.3µF
3.3µF
200k
10k
I
I
SW
SP1
SN1
V
FBN
IN
I
I
ADJ1
ADJ2
LT3477
SHDN
SHDN
I
SP2
0.3Ω
V
V
I
C
SN2
R
REF
T
FBP
GND
SS
1k
330mA
50
22k
0.2
(A)
0
0.1
0.3
0.4
33nF
4.7nF
I
OUT
3477 TA01b
3477 TA01a
3477fb
1
LT3477
W W U W
ABSOLUTE MAXIMUM RATINGS (Note 1)
SW Pin Voltage....................................................... 42V
VIN, SHDN Pin Voltage............................................ 25V
FBP, FBN Pin Voltage ................................................ 6V
Junction Temperature.......................................... 125°C
Operating Temperature Range (Note 2)
LT3477E ............................................. – 40°C to 85°C
LT3477I ............................................ – 40°C to 125°C
Storage Temperature Range ................. –65°C to 125°C
Lead Temperature (Soldering, 10 sec)
V
REF Pin Voltage ....................................................... 6V
RT, VC, SS Pin Voltage.............................................. 6V
IADJ1, IADJ2 Pin Voltage........................................... 25V
ISP1, ISP2, ISN1, ISN2 Pin Voltage .............................. 42V
TSSOP .............................................................. 300°C
U
W U
PACKAGE/ORDER INFORMATION
TOP VIEW
TOP VIEW
V
1
2
20
19
18
17
16
15
14
13
12
11
NC
IN
R
NC
20 19 18 17 16
T
SHDN
SS
3
NC
I
I
I
I
I
NC
NC
1
2
3
4
5
15
14
13
12
11
SP1
4
SW
SW
GND
SN2
SP2
V
C
5
21
V
21
8
IN
FBN
FBP
6
R
T
ADJ1
ADJ2
7
I
SN1
SHDN
V
8
I
REF
SP1
I
9
I
6
7
9 10
ADJ2
SN2
I
10
I
ADJ1
SP2
FE PACKAGE
20-LEAD PLASTIC TSSOP
UF PACKAGE
20-LEAD (4mm × 4mm) PLASTIC QFN
TJMAX = 125°C, θJA = 37°C/W
TJMAX = 150°C, θJA = 40°C/W
EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)
EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)
ORDER PART NUMBER
SW PART MARKING
ORDER PART NUMBER
UF PART MARKING
3477
LT3477EUF
LT3477IUF
LT3477EFE
LT3477IFE
3477
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
●
indicates specifications which apply over the full operating
= 2.5V.
temperature range, otherwise specifications are at T = 25°C. V = 2.5V, V
SHDN
A
IN
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum Input Voltage
Quiescent Current
●
2.3
2.5
V
V
V
= 0V
0.1
5.0
1.0
7.5
µA
mA
SHDN
SHDN
= 2.5V, V = 0.3V (Not Switching)
C
Reference Voltage
E Grade
I Grade
●
●
1.216
1.210
1.235
1.235
1.250
1.260
V
V
Reference Voltage Line Regulation
2.5V < V < 25V, V = 0.3V
0.01
0.03
100
%/V
IN
C
Maximum V Pin Current
Out of Pin
µA
REF
3477fb
2
LT3477
ELECTRICAL CHARACTERISTICS
The
●
indicates specifications which apply over the full operating
= 2.5V.
temperature range, otherwise specifications are at T = 25°C. V = 2.5V, V
A
IN
SHDN
PARAMETER
CONDITIONS
MIN
TYP
9
MAX
UNITS
µA
Soft-Start Pin Current
SS = 0.5V, Out of Pin
FBP Pin Bias Current
25
25
2
100
100
6
nA
FBN Pin Bias Current
nA
Feedback Amplifier Offset Voltage
Feedback Amplifier Voltage Gain
Voltage Feedback Amplifier Transconductance
Feedback Amplifier Sink Current
Feedback Amplifier Source Current
Current Sense Amplifier Sense Voltage
FBP – FBN, V = 1V
–2
mV
V/V
µS
C
500
500
10
10
V
V
= 1.25V, V
= 1.25V, V
= 1.5V, V = 1V
µA
FBP
FBN
FBN
C
= 1V, V = 0.5V
µA
FBP
C
Positive Rail, V = 25V, E Grade
●
●
97.5
97.5
88
100
100
100
102.5
103
112
mV
mV
mV
CM
Positive Rail, V = 25V, I Grade
CM
Ground
Switching Frequency
R = 17.2k
0.9
160
2.7
1
200
3.5
1.15
240
4.3
MHz
kHz
MHz
T
R = 107.4k
T
R = 2.44k
T
Maximum Switch Duty Cycle
Switch Current Limit
R = 17.2k
●
87
3
93
4
%
A
T
(Note 3)
5
200
5
Switch V
I
= 1A (Note 3)
SW
150
0.2
mV
µA
CESAT
Switch Leakage Current
SHDN Pin Current
SW = 40V
V
SHDN
V
SHDN
= 5V
= 0V
30
0.1
60
1
µA
µA
SHDN Pin Threshold
0.3
1.5
2
V
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.
junction temperature range are assured by design, characterization and
correlation with statistical process controls. The LT3477I is guaranteed
over the full –40°C to 125°C operating junction temperature range.
Note 3: Switch current limit and switch V
for UF package guaranteed
CESAT
Note 2: The LT3477E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
by design and/or correlation to static test.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Switch V
Switch Current Limit
V
REF
CE(SAT)
5
4
3
2
1
0
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
1.27
1.26
1.25
1.24
1.23
1.22
1.21
125°C
–50°C
25°C
V
V
= 25V
IN
= 2.5V
IN
50
75 100 125
75 100
–50 –25
0
25
–50 –25
0
25 50
125 150
0
0.5
1.5
2
2.5
3
1
TEMPERATURE (°C)
TEMPERATURE (°C)
SWITCH CURRENT (A)
3477 G03
3477 G01
3477 G02
3477fb
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LT3477
TYPICAL PERFOR A CE CHARACTERISTICS
U W
SHDN Pin Turn-On Threshold
SHDN Pin Current
Quiescent Current
50
40
30
20
10
0
1.6
6
5
4
3
V
C
= 0.3V
–50°C
1.4
25°C
1.2
1.0
125°C
2
0
5
10
V
15
(V)
20
25
–50 –25
0
25 50 75 100 125 150
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
TEMPERATURE (°C)
SHDN
3477 G05
3477 G06
3477 G04
Soft-Start Pin Current
Oscillator Frequency
Feedback Amplifier Offset Voltage
4
3
2.0
20
15
10
5
R
= 10kΩ
T
1.6
1.2
2
V
C
= 1V
C
1
R
R
= 15kΩ
= 20kΩ
T
T
0
V
= 0.5V
0.8
0.4
0
–1
–2
–3
–4
0
50 75
TEMPERATURE (°C)
–50 –25
0
25
100 125 150
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
3477 G09
3477 G07
3477 G08
FBP Pin Bias Current
FBN Pin Bias Current
50
50
“+” INDICATES THE CURRENT
FLOWS OUT OF PIN
40
“+” INDICATES THE CURRENT
FLOWS OUT OF PIN
40
30
20
30
20
10
0
10
0
–10
–10
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–50 –25
0
25
75
3477 G10
3477 G11
3477fb
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LT3477
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Current Sense Voltage
Current Sense Voltage
vs Temperature
vs I
ADJ
104
120
100
80
60
40
20
0
V
CM
= 10V
103
102
101
100
99
V
V
= 10V
= 42V
CM
CM
98
97
96
–25
0
50
75 100 125
–50
25
500 600
0
100 200 300 400
700 800
TEMPERATURE (°C)
I
ADJ
VOLTAGE (mV)
3477 G14
2477 G13
U
U
U
PI FU CTIO S (QFN/TSSOP)
NC(Pins 1, 2, 20/Pins 18, 19, 20): No Connect Pin. Okay
FBP (Pin 9/Pin 7): The Noninverting Input to the Error
Amplifier. Connect resistive divider tap here for negative
output voltage.
to connect to ground or VIN, or to float.
VIN (Pin 3/Pin 1): Input Supply. Must be locally bypassed.
Powers the internal control circuitry.
VREF (Pin 10/Pin 8): Bandgap Voltage Reference. Inter-
nally set to 1.235V. Connect this pin to FBP if generating
a positive output or to an external resistor divider if
generating a negative voltage. This pin can provide up to
100µAofcurrentandcanbelocallybypassedwitha100pF
capacitor.
RT (Pin 4/Pin 2): Timing Resistor Pin. Adjusts the switch-
ing frequency. Connect a 17.2k resistor between RT and
GNDfora1MHzswitchingfrequency. Donotleavethispin
open. See Table 4 for additional RT values and switching
frequencies.
I
ADJ2 (Pin 11/Pin 9): Second Current Sense Adjustment.
SHDN (Pin 5/Pin 3): Shutdown. Tie to 2V or greater to
Setting IADJ2 to be less than 625mV leads to adjustment of
the sensed voltage of the second current sense amplifier
linearly. If IADJ2 is tied to higher than 650mV, the default
current sense voltage is 100mV. If current sense ampli-
fier 2 is not used, always tie IADJ2 to higher than 650mV.
enable the device. Tie below 0.3V to turn off the device.
SS (Pin 6/Pin 4): Soft-Start. Place a soft-start capacitor
here. Leave floating if not in use.
VC (Pin 7/Pin 5): Compensation Pin for Error Amplifier.
Connect a series RC from this pin to GND. Typical values
are 1kΩ and 4.7nF.
IADJ1 (Pin 12/Pin 10): First Current Sense Adjustment.
Setting IADJ1 to be less than 625mV leads to adjustment of
the sensed voltage of the first current sense amplifier
linearly. If IADJ1 is tied to higher than 650mV, the default
current sense voltage is 100mV. If current sense ampli-
fier 1 is not used, always tie IADJ1 to higher than 650mV.
FBN (Pin 8/Pin 6): The Inverting Input to the Error Ampli-
fier. Connect resistive divider tap here for positive output
voltage.
3477fb
5
LT3477
U
U
U
PI FU CTIO S (QFN/TSSOP)
ISP2 (Pin 13/ Pin 11): Second Current Sense (+) Pin. The
noninverting input to the second current sense amplifier.
Connect to ISN2 if not used.
GND (Pins 17/Pin 15): Ground. Tie directly to local
ground plane.
SW (Pins 18, 19/Pins 16, 17): Switch Pins. Collector of
the internal NPN power switch. Connect the inductor and
diode here and minimize the metal trace area connected to
this pin to minimize electromagnetic interference.
ISN2 (Pin 14/ Pin 12): Second Current Sense (–) Pin. The
inverting input to the second current sense amplifier.
Connect to ISP2 if not used.
Exposed Pad (Pin 21/Pin 21): Power Ground. Must be
soldered to PCB ground for electrical contact and rated
thermal performance.
ISP1 (Pin 15/Pin 13): First Current Sense (+) Pin. The
noninverting input to the first current sense amplifier.
Connect to ISN1 if not used.
I
SN1 (Pin 16/Pin 14): First Current Sense (–) Pin. The in-
verting input to the first current sense amplifier. Connect
to ISP1 if not used.
W
BLOCK DIAGRA
SS
V
SW
C
I
I
SP1
+
–
IA1
–
+
+
SN1
V
V
A1
A2
ADJ
I
ADJ1
I
I
SP2
+
–
IA2
–
+
+
SN2
A3
ADJ
–
+
I
ADJ2
A4
R
Q
Q1
FBP
FBN
SLOPE
S
+
–
+
–
VA
Σ
V
REF
V
REF
1.25V
OSCILLATOR
V
SHDN
IN
R
T
3477 F01
Figure 1. LT3477 Block Diagram
3477fb
6
LT3477
U
OPERATIO
The LT3477 uses a fixed frequency, current mode control
scheme to provide excellent line and load regulation.
OperationcanbebestunderstoodbyreferringtotheBlock
Diagram in Figure 1. The start of each oscillator cycle sets
the SR latch and turns on power switch Q1. The signal at
thenoninvertinginputofthePWMcomparator(A4SLOPE)
is proportional to the sum of the switch current and
oscillator ramp. When SLOPE exceeds VC (the output of
the feedback amplifier), the PWM comparator resets the
latch and turns off the power switch. In this manner, the
feedback amplifier and PWM comparators set the correct
peak current level to keep the output in regulation. Ampli-
fier A3 drives A4 inverting input. A3 has three inputs, one
from the voltage feedback loop and the other two from the
currentfeedbackloop. Whicheverfeedbackinputishigher
takes precedence, forcing the converter into either a
constant-currentoraconstant-voltagemode.TheLT3477
isdesignedtotransitioncleanlybetweenthetwomodesof
operation. Current sense amplifier IA1 senses the voltage
between the ISP1 and ISN1 pins and provides a pre-gain to
amplifier A1. When the voltage between ISP1 and ISN1
reaches 100mV, the output of IA1 provides VADJ to the
inverting input of A1 and the converter is in constant-
currentmode.Ifthecurrentsensevoltageexceeds100mV,
the output of IA1 will increase causing the output of A3 to
decrease,thusreducingtheamountofcurrentdeliveredto
the output. In this manner the current sense voltage is
regulated to 100mV. The current sense level is also pin
adjustable by IADJ1. Forcing IADJ1 to less than 625mV will
overwrite VADJ voltage that’s set internally, thus providing
current level control. The second current sense amplifier,
IA2, works the same as the first current sense amplifier
IA1. Both current sense amplifiers provide rail-to-rail
current sense operation. Similarly, for positive output
voltage operation where FBP is tied to VREF, if the FBN pin
increases above VREF, the output of A3 will decrease to
reduce the peak current level and regulate the output
(constant-voltage mode). For negative output voltage
operation where FBN is tied to GND, if the FBP pin
decreases below GND level, the output of A3 will decrease
to reduce the peak current level and regulate the output
(constant-voltage mode).
The LT3477 also features a soft-start function. During
start-up, 9µA of current charges the external soft-start
capacitor. The SS pin directly limits the rate of voltage rise
on the VC pin, which in turn limits the peak switch current.
The switch current is constantly monitored and not al-
lowed to exceed the nominal value of 3A. If the switch
current reaches 3A, the SR latch is reset regardless of the
output of the PWM comparator. Current limit protects the
power switch and external components.
3477fb
7
LT3477
W U U
U
APPLICATIO S I FOR ATIO
Capacitor Selection
percentage of its nominal value—typically 65%. An
inductor can pass a current larger than its rated value
without damaging it. Aggressive designs where board
space is precious will exceed the maximum current rating
of the inductor to save board space. Consult each manu-
facturer to determine how the maximum inductor current
ismeasuredandhowmuchmorecurrenttheinductorcan
reliably conduct.
Low ESR (equivalent series resistance) ceramic capaci-
tors should be used at the output to minimize the output
ripple voltage. Use only X5R or X7R dielectrics, as these
materials retain their capacitance over wider voltage and
temperature ranges better than other dielectrics. A 4.7µF
to 10µF output capacitor is sufficient for most high output
current designs. Converters with lower output currents
may need only a 1µF or 2.2µF output capacitor.
Diode Selection
Table 1. Ceramic Capacitor Manufacturers
Schottky diodes, with their low forward voltage drop and
fast switching speed, are ideal for LT3477 applications.
Table 3 lists several Schottky diodes that work well with
the LT3477. The diode’s average current rating must ex-
ceed the average output current. The diode’s maximum
reverse voltage must exceed the output voltage. The diode
conducts current only when the power switch is turned off
(typically less than 50% duty cycle), so a 3A diode is suf-
ficient for most designs. The companies below also offer
Schottky diodes with higher voltage and current ratings.
MANUFACTURER
Taiyo Yuden
AVX
PHONE
WEB
(408) 573-4150
(803) 448-9411
(714) 852-2001
(847) 803-6100
www.t-yuden.com
www.avxcorp.com
www.murata.com
www.component.tdk.com
Murata
TDK
Inductor Selection
SeveralinductorsthatworkwellwiththeLT3477arelisted
in Table 2. However, there are many other manufacturers
and devices that can be used. Consult each manufacturer
for more detailed information and their entire range of
parts. Ferrite core inductors should be used to obtain the
best efficiency. Choose an inductor that can handle the
necessary peak current without saturating, and ensure
that the inductor has a low DCR (copper-wire resistance)
to minimize I2R power losses. A 4.7µH or 10µH inductor
will suffice for most LT3477 applications.
Table 3. Suggested Diodes
MANUFACTURER
PART NUMBER
MAX
MAX REVERSE
CURRENT (A) VOLTAGE (V) MANUFACTURER
UPS340
UPS315
3
3
40
15
Microsemi
www.microsemi.com
B220
B230
B240
B320
B330
B340
SBM340
2
2
2
3
3
3
3
20
30
40
20
30
40
40
Diodes, Inc
www.diodes.com
Inductor manufacturers specify the maximum current
rating as the current where the inductance falls to some
Table 2. Suggested Inductors
MANUFACTURER
PART NUMBER
I
INDUCTANCE
MAX DCR
(mΩ)
L × W × H
(mm)
DC
(A)
(µH)
MANUFACTURER
CDRH6D283R0
CDRH6D28100
CDRH4D284R7
3
1.7
1.32
3
10
4.7
24
65
72
6.7 × 6.7 × 3.0
6.7 × 6.7 × 3.0
5.0 × 5.0 × 3.0
Sumida
www.sumida.com
LM N 05D B4R7M
LM N 05D B100K
2.2
1.6
4.7
10
49
10
5.9 × 6.1 × 2.8
5.9 × 6.1 × 2.8
Taiyo Yuden
www.t-yuden.com
LQH55DN4R7M01L
LQH55DN100M01K
2.7
1.7
4.7
10
57
130
5.7 × 5.0 × 4.7
5.7 × 5.0 × 4.7
Murata
www.murata.com
FDV0630-4R7M
4.2
4.7
49
7.0 × 7.7 × 3.0
Toko
www.toko.com
3477fb
8
LT3477
W U U
APPLICATIO S I FOR ATIO
U
Setting Positive Output Voltages
For designs needing an adjustable current level, the IADJ1
and IADJ2 pins are provided for the first and the second
current sense amplifiers, respectively. With the IADJ1 and
IADJ2 pins tied higher than 650mV, the nominal current
sense voltage is 100mV (appearing between the ISP1 and
ISN2 or ISP2 and ISN2 pins). Applying a positive DC voltage
less than 600mV to the IADJ1 and IADJ2 pins will decrease
the current sense voltage according to the following
formula:
Tosetapositiveoutputvoltage,selectthevaluesofR1and
R2 (see Figure 2) according to the following equation:
R1
R2
⎛
⎝
⎞
⎟
⎠
VOUT = 1.235V 1+
⎜
FBP
LT3477
V
OUT
V
REF
100mV
RSENSE 618mV
V
IADJ
R1
ISENSE
=
•
FBN
R2
For example, if 309mV is applied to the IADJ1 pin and
RSENSE is 0.5Ω, the current sense will be reduced from
200mA to 100mA. The adjustability allows the regulated
current to be reduced without changing the current sense
resistor (e.g., to adjust brightness in an LED driver or to
reduce the charge current in a battery charger).
3477 F02
Figure 2. Positive Output Voltage Feedback Connections
Setting Negative Output Voltages
To set a negative output voltage, select the values of R3
and R4 (see Figure 3) according to the following equation:
Considerations When Sensing Input Current
R3
R4
⎛
⎜
⎝
⎞
⎟
⎠
In addition to regulating the DC output current for current-
source applications, the constant-current loop of the
LT3477 can also be used to provide an accurate input
current limit. Boost converters cannot provide output
short-circuit protection, but the surge turn-on current can
be drastically reduced using the LT3477 current sense at
the input. SEPICs, however, have an output that is DC-
isolated from the input, so an input current limit not only
helps soft-start the output but also provides excellent
short-circuit protection.
VOUT = –1.235V
–V
OUT
R3
FBP
R4
LT3477
V
REF
FBN
3477 F03
When sensing input current, the sense resistor should be
placed in front of the inductor (between the decoupling
capacitor and the inductor). This will regulate the average
inductor current and maintain a consistent inductor ripple
current, which will, in turn, maintain a well regulated input
current. Do not place the sense resistor between the input
source and the input decoupling capacitor, as this may
allow the inductor ripple current to vary widely (even
though the average input current and the average inductor
current will still be regulated). Since the inductor current
Figure 3. Negative Output Voltage Feedback Connections
Selecting RSENSE/Current Sense Adjustment
Using the following formula to choose the correct current
sense resistor value (for constant current or fail-safe
operation).
100mV
ISENSE
RSENSE
=
3477fb
9
LT3477
W U U
U
APPLICATIO S I FOR ATIO
is a triangular waveform (not a DC waveform like the
output current) some tweaking of the compensation val-
ues (RC and CC on the VC pin) may be required to ensure
a clean inductor ripple current while the constant-current
loop is in effect. For these applications, the constant-
current loop response can usually be improved by reduc-
ing the RC value or by adding a capacitor (with a value of
approximately CC/10) in parallel with the RC and CC com-
pensation network.
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and output
voltageovershoot. Atypicalvalueforthesoft-startcapaci-
tor is 10nF.
Switching Frequency
The switching frequency of the LT3477 is set by an
external resistor attached to the RT pin. Do not leave this
pin open. A resistor must always be connected for proper
operation. See Table 4 and Figure 4 for resistor values and
corresponding frequencies.
Frequency Compensation
TheLT3477hasanexternalcompensationpin(VC), which
allows the loop response to be optimized for each applica-
tion. An external resistor and capacitor (or sometimes just
a capacitor) are placed at the VC pin to provide a pole and
azero(orjustapole)toensureproperloopcompensation.
Several other poles and zeroes are present in the closed-
loop transfer function of a switching regulator, so the VC
pin pole and zero are positioned to provide the best loop
response. A thorough analysis of the switching regulator
control loop is not within the scope of this data sheet, and
will not be presented here, but values of 1k and 4.7nF will
be a good choice for many designs. For those wishing to
optimize the compensation, use the 1k and 4.7nF as a
starting point.
Increasing switching frequency reduces output voltage
ripple but also reduces efficiency. The user should set the
frequencyforthemaximumtolerableoutputvoltageripple.
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz)
R (kΩ)
T
3.5
3
2.43
3.65
4.87
6.81
10.2
17.4
43.2
107
2.5
2
1.5
1
0.5
0.2
Board Layout
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
Tomaximizeefficiency, switchriseandfalltimesaremade
as short as possible. To prevent radiation and high fre-
quency resonance problems, proper layout of the high
frequency switching path is essential. Minimize the length
and area of all traces connected to the SW pin and always
use a ground plane under the switching regulator to
minimize interplane coupling. The signal path including
3.5
3.0
2.5
2.0
1.5
1.0
0.5
the switch, output diode D1 and output capacitor COUT
,
contains nanosecond rise and fall times and should be
kept as short as possible.
0
0.1
100
10
R
(kΩ)
T
3477 F04
Figure 4. Switch Frequency
3477fb
10
LT3477
W U U
APPLICATIO S I FOR ATIO
U
PWM Dimming
through the LEDs. When the PWM input is taken low, the
LEDs are disconnected and turn off. This unique external
circuitry produces a fast rise time for the LED current,
resulting in a wide dimming range of 500:1 at a PWM
frequency of 100Hz.
For LED applications where a wide dimming range is
required, two competing methods are available: analog
dimming and PWM dimming. The easiest method is to
simply vary the DC current through the LED—analog
dimming—but changing LED current also changes its
chromaticity, undesirableinmanyapplications. Thebetter
method is PWM dimming, which switches the LED on and
off, using the duty cycle to control the average current.
PWM dimming offers several advantages over analog
dimming and is the method preferred by LED manufactur-
ers. By modulating the duty cycle of the PWM signal, the
average LED current changes proportionally as illustrated
in Figure 5. The chromaticity of the LEDs remains un-
changedinthisschemesincetheLEDcurrentiseitherzero
or at programmed current. Another advantage of PWM
dimming over analog dimming is that a wider dimming
range is possible.
The LED current can be controlled by feeding a PWM
signal with a broad range of frequencies. Dimming below
80Hz is possible, but not desirable, due to perceptible
flashing of LEDs at lower PWM frequencies. The LED
current can be controlled at higher frequencies, but the
dimmingrangedecreaseswithincreasingPWMfrequency,
as seen in Figure 6.
PWM dimming can be used in Boost (shown in Figure 7),
Buck mode (shown in Figure 8) and Buck-Boost mode
(shown in Figure 9). For the typical boost topology,
efficiency exceeds 80%. Buck mode can be used to in-
crease the power handling capability for higher current
LED applications. A Buck-Boost LED driver works best in
applicationswheretheinputvoltagefluctuatestohigheror
lower than the total LED voltage drop.
The LT3477 is a DC/DC converter that is ideally suited for
LED applications. For the LT3477, analog dimming offers
a dimming ratio of about 10:1; whereas, PWM dimming
with the addition of a few external components results in
a wider dimming range of 500:1. The technique requires a
PWM logic signal applied to the gate of both NMOS (refer
to Figure 7). When the PWM signal is taken high the part
runs in normal operation and ILED = 100mV/RSENSE runs
Inhightemperatureapplications,theleakageoftheSchot-
tky diode D1 increases, which in turn, discharges the
output capacitor during the PWM “off” time. This results
in a smaller effective LED dimming ratio. Consequently,
the dimming range decreases to about 200:1 at 85°C.
1000
100
R
T
= 6.81k
R
T
= 6.81k
10
1
100
10
1
0.1
V
= 5V
IN
BOOST
4 LEDs
PWM FREQUENCY = 100Hz
0.01
0.1
1
10
100
0.1
1
10
100
PWM DUTY CYCLE (%)
PWM FREQUENCY (kHz)
3477 F05
3477 F06
Figure 5. LED Current vs PWM Duty Cycle
Wide Dimming Range (500:1)
Figure 6. Dimming Range vs PWM Frequency
3477fb
11
LT3477
APPLICATIO S I FOR ATIO
W U U
U
L1
2.0µH
D1
OUT
V
IN
5V
C2
10µF
C1
3.3µF
1M
I
I
SW
SP1
SN1
V
FBN
FBP
IN
I
I
ADJ1
ADJ2
75k
SHDN
LT3477
V
REF
I
SP2
SS
R
SENSE
0.33Ω
C
33nF
SS
I
SN2
LED1
V
C
R
T
GND
LED2
LED3
LED4
6.81k
300mA
D2
PWM
5V
0
NMOS1
R
C
100Hz
NMOS2
2.4k
100k
C
C
10nF
3477 F07a
C1: TAIYO YUDEN EMK316BJ335ML
C2: TAIYO YUDEN UDK325BJ106MM
L1: TOKO D53LC (PN# A915AY-2ROM)
D1: ZETEX ZLLS1000
D2: DIODES INC 1N4148
NMOS1: ZETEX 2N7002
NMOS2: FAIRCHILD FDG327N
LED1 TO LED4: LUMILEDS LXHL-BW02
Figure 7a. 5V to 4 White LEDs: Boost with PWM Dimming
85
350
300
EFFICIENCY
80
PWM
5V/DIV
75
70
65
60
55
50
250
200
150
100
50
LED CURRENT
I
L
1A/DIV
I
LED
V
= 5V
IN
200mA/DIV
BOOST
4 LEDs, 300mA
PWM FREQUENCY = 100Hz
3477 F07b
0
100
10µs/DIV
= 5V PWM FREQ = 100Hz
4 LEDs
300mA
20
40
60
0
80
V
IN
PWM DUTY CYCLE (%)
BOOST
3477 F07c
Figure 7b. PWM Dimming Waveforms
Figure 7c. Efficiency and LED Current
vs PWM Duty Cycle
3477fb
12
LT3477
W U U
APPLICATIO S I FOR ATIO
U
PV
IN
32V
C1
2.2µF
R
SENSE
0.33Ω
LED1
300mA
•
•
•
C1: NIPPON NTS40X5R1H225M
LED6
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ335KL
L1: TOKO D53LC (PN# A915AY-100M)
D1: ZETEX ZLLS400
1k
1k
PWM
PMOS
NMOS2
D2: DIODES INC 1N4148
C2
1µF
NMOS1, NM0S2: ZETEX 2N7002
PMOS: SILICONIX Si2303BDS
LED1 TO LED6: LUMILEDS LXHL-BW02
L1
10µH
D1
280k
10k
I
I
SW
SP1
SN1
V
IN
V
FBN
FBP
IN
3.3V
C3
3.3µF
I
I
ADJ1
ADJ2
SHDN
LT3477
V
REF
I
SP2
SS
C
33nF
SS
I
SN2
V
C
R
T
GND
6.81k
3477 F08a
D2
PWM
5V
0
NMOS1
100Hz
C
C
0.1µF
100k
Figure 8a. 32V to 6 White LEDs: Buck Mode with PWM Dimming
PWM
5V/DIV
I
L
500mA/DIV
I
LED
500mA/DIV
3477 F08b
2ms/DIV
PV = 32V PWM FREQUENCY = 100Hz
6 LEDs
300mA
IN
BUCK MODE
Figure 8b. PWM Dimming Waveforms
3477fb
13
LT3477
W U U
U
APPLICATIO S I FOR ATIO
1k
1k
NMOS2
PWM
LED2
C1: TAIYO YUDEN LMK316BJ335ML
C2: TAIYO YUDEN UDK325BJ106MM
L1: TOKO D53LC (PN# A915AY-4R7M)
D1: ZETEX ZLLS1000
300mA
LED1
D2: DIODES INC 1N4148
NMOS1, NMOS2: ZETEX 2N7002
PMOS: SILICONIX Si2303BDS
LED1, LED2: LUMILEDS LXHL-BW02
PMOS
L1
4.7µH
R
SENSE
D1
0.33Ω
V
IN
10V
C1
3.3µF
1M
I
I
SW
SP1
SN1
V
FBN
FBP
IN
I
I
ADJ1
ADJ2
49.9k
SHDN
V
REF
LT3477
GND
I
SP2
SS
C
33nF
SS
I
SN2
V
R
T
C
C2
10µF
6.81k
3477 F09a
D2
PWM
5V
0
NMOS1
100Hz
R
1.5k
C
100k
C
C
10nF
Figure 9a. 10V to 2 White LEDs: Buck-Boost Mode with PWM Dimming
PWM
10V/DIV
I
L
1A/DIV
I
LED
500mA/DIV
3477 F09b
2ms/DIV
V
= 10V
PWM FREQUENCY = 100Hz
BUCK-BOOST MODE
IN
2 LEDs
300mA
Figure 9b. PWM Dimming Waveforms
3477fb
14
LT3477
U
TYPICAL APPLICATIO S
Efficiency
5.5V SEPIC Converter with Short-Circuit Protection
90
C2
10µF
L1
4.7µH
V
IN
= 3V
R1
0.04Ω
R3
0.15Ω
D1
85
80
V
5.5V
670mA
IN
3V TO
16V
C1
3.3µF
R4
34.8k
L2
4.7µH
I
I
SW
SP1
SN1
75
70
65
60
55
V
I
I
IN
ADJ1
ADJ2
FBN
LT3477
GND
SHDN
SHDN
I
SP2
V
V
I
C
SN2
50
R
REF
T
0.1
0.2
0.4 0.5 0.6 0.7
(A)
0
0.3
I
R
C
FBP
SS
1k
OUT
3477 TA02b
C4
33nF
C3
10µF
R5
10k
C
R2
18.2k
C
4.7nF
3477 TA02a
C1: TAIYO YUDEN LMK316BJ335ML
C2: TAIYO YUDEN LMK325BJ106MN
C3: TAIYO YUDEN LMK316BJ106ZL
D1: DIODES INC. DFLS130L
L1, L2: TOKO FDV0630-4R7M
800mA, 5V to 12V Boost Converter with Accurate Input Current Limit
Efficiency
R1
L1
90
85
80
75
70
65
60
55
50
D1
0.033Ω
4.7µH
V
12V
0.8A
IN
5V
C1
2.2µF
R3
I
I
SW
SP1
SN1
200k
C2
V
I
I
FBN
IN
ADJ1
ADJ2
10µF
R4
23.2k
I
SP2
LT3477
SHDN
SHDN
I
SN2
V
C
V
R
T
REF
R
C
FBP
GND
SS
1k
0.2
0
0.1
0.3 0.4 0.5 0.6 0.7 0.8
(A)
C3
10nF
R2
17.8k
C
C
4.7nF
I
OUT
3477 TA04a
3477 TA04b
C1: TAIYO YUDEN LMK316BJ225MD
C2: AVX 1206YD106MAT
D1: DIODES INC. B320A
L1: TOKO FDV0630-4R7M
3477fb
15
LT3477
U
TYPICAL APPLICATIO S
87% Efficient, 4W LED Driver
Efficiency
90
85
80
75
70
65
60
55
50
R4
L2
D1
0.05Ω
10µH
V
IN
5V
C2
C1
R2
3.3µF
3.3µF
I
I
SW
SP1
SN1
200k
V
I
I
FBN
IN
R1
10k
ADJ1
ADJ2
330mA
LT3477
GND
SHDN
SHDN
I
SP2
R6
0.3Ω
V
V
I
C
SN2
LED1
R
REF
T
R
C
FBP
SS
LED2
LED3
LED4
0.2
0
0.1
0.3
0.4
1k
I
(A)
OUT
R3
22k
C3
3477 TA01b
C
33nF
C
4.7nF
C1: TAIYO YUDEN LMK316BJ335ML
3477 TA03a
C2: TAIYO YUDEN TMK325BJ335MN
D1: DIODES INC. DFLS120L
L1: TOKO A915AY-100M
1A Buck Mode High Current LED Driver
PV
IN
32V
C1
2.2µF
R1
0.1Ω
LED1
LED4
Efficiency
•
•
•
LED
STRING
1A
C2
1µF
100
90
80
L1
33µH
D1
70
60
50
R3
I
I
SW
FBN
280k
SP1
SN1
V
IN
V
I
I
IN
ADJ1
ADJ2
3.3V
C3
40
30
20
R4
10k
3.3µF
LT3477
GND
SHDN
SHDN
I
SP2
0.2
0.4
0.6
0.8
0
1
V
V
I
LED CURRENT (A)
C
SN2
3477 TA05b
R
REF
T
R
C
FBP
SS
1k
C4
33nF
C
R2
22k
C
4.7nF
3477 TA05a
C1: NIPPON UNITED CHEMICON NTS40X5R1H225M
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ475
L1: TOKO A814AY-330M
D1: DIODES INC DFLS140
3477fb
16
LT3477
U
TYPICAL APPLICATIO S
Buck-Boost Mode LED Driver
LED2 LED1
L1
4.7µH
R1
0.1Ω
D1
V
IN
2.7V TO 10V
C1
R3
200k
3.3µF
I
I
SW1
FBN
SP1
SN1
V
I
IN
ADJ1
ADJ2
LED BRIGHTNESS
CONTROL
0mV TO 650mV
I
LT3477
GND
SHDN
SHDN
I
SP2
V
V
I
C
SN2
R
T
REF
FBP
SS
C3
33nF
C2
4.7µF
R4
10k
C
C
10nF
R2
18k
C1: TAIYO YUDEN LMK316BJ335ML
C2: MURATA GRM31CR71E475KA88L
D1: DIODES, INC. B320A
3477 TA06a
L1: TOKO FDV0630-4R7M
Efficiency
90
85
80
75
V
= 8V
IN
V
(V)
I
(A)
IN
2.7
OUT
0.57
3.6
4.2
5
0.74
0.83
0.93
1.0
V
= 4.2V
IN
70
65
8
60
55
50
0.2
0.4
0.8
0
1.0
0.6
(A)
I
OUT
3477 TA06b
3477fb
17
LT3477
U
PACKAGE DESCRIPTIO
UF Package
20-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1710)
0.70 ±0.05
4.50 ± 0.05
3.10 ± 0.05
2.45 ± 0.05
(4 SIDES)
PACKAGE
OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
R = 0.115
PIN 1 NOTCH
R = 0.30 TYP
0.75 ± 0.05
4.00 ± 0.10
(4 SIDES)
TYP
19 20
0.38 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
2.45 ± 0.10
(4-SIDES)
(UF20) QFN 10-04
0.200 REF
0.25 ± 0.05
0.50 BSC
0.00 – 0.05
NOTE:
1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220
VARIATION (WGGD-1)—TO BE APPROVED
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
3477fb
18
LT3477
U
PACKAGE DESCRIPTIO
FE Package
20-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation CB
6.40 – 6.60*
(.252 – .260)
3.86
(.152)
3.86
(.152)
20 1918 17 16 15 14 1312 11
6.60 ±0.10
2.74
(.108)
4.50 ±0.10
6.40
(.252)
BSC
2.74
(.108)
SEE NOTE 4
0.45 ±0.05
1.05 ±0.10
0.65 BSC
5
7
8
1
2
3
4
6
9 10
RECOMMENDED SOLDER PAD LAYOUT
1.20
(.047)
MAX
4.30 – 4.50*
(.169 – .177)
0.25
REF
0° – 8°
0.65
(.0256)
BSC
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
0.05 – 0.15
(.002 – .006)
FE20 (CB) TSSOP 0204
0.195 – 0.30
(.0077 – .0118)
TYP
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
MILLIMETERS
(INCHES)
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
3477fb
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
19
LT3477
U
TYPICAL APPLICATIO
Buck Mode High Current LED Driver
PV
IN
32V
C1
2.2µF
R1
0.1Ω
Efficiency
LED1
LED4
100
90
•
•
•
LED
STRING
1A
C2
1µF
80
70
L1
33µH
D1
60
50
R3
I
I
SW
FBN
280k
SP1
SN1
V
IN
3.3V
40
30
20
V
IN
C3
3.3µF
I
I
ADJ1
ADJ2
R4
10k
0.2
0.4
0.6
LED CURRENT (A)
0.8
0
1
LT3477
GND
SHDN
SHDN
I
SP2
3477 TA05b
V
V
I
C
SN2
R
REF
T
R
C
FBP
SS
1k
C4
33nF
C
R2
22k
C
4.7nF
3477 TA07
C1: NIPPON UNITED CHEMICON NTS40X5R1H225M
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ475
L1: TOKO A814AY-330M
D1: DIODES INC DFLS140
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
V : 1.6V to 18V, V
LT1618
Constant Current, Constant Voltage 1.4MHz,
High Efficiency Boost Regulator
= 5.5V, I = 2.5mA, I < 1µA, QFN16 Package
OUT(MAX) Q SD
IN
LT3436
LTC®3453
3A (I ), 800kHz, 34V Step-Up DC/DC Converter
V : 3V to 25V, V
= 34V, I = 0.9mA, I < 6µA, TSSOP16E Package
SW
IN
OUT(MAX) Q SD
Synchronous Buck-Boost High Power White
LED Driver
V : 2.7V to 5.5V, V
IN
= 5.5V, I = 2.5mA, I < 1µA, QFN16 Package
Q SD
OUT(MAX)
LT3466
Dual Constant Current, 2MHz, High Efficiency
White LED Boost Regulator with Integrated
Schottky Diode
V : 2.7V to 24V, V
= 40V, I = 5mA, I < 16µA, DFN Package
Q SD
IN
OUT(MAX)
LT3479
3A, 42V Full Featured Boost/Inverter Converter
with Soft-Start
V : 2.5V to 24V, V
= 40V, I = 5mA, I < 1µA, DFN/TSSOP Packages
OUT(MAX) Q SD
IN
LTC3490
Single Cell 350mA, 1.3MHz LED Driver
V : 1V to 3.2V, V
= 4.7V, I < 1µA, DFN/SO8 Packages
OUT(MAX) SD
IN
3477fb
LT 0207 REV B • PRINTED IN THE USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
20
●
●
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相关型号：
LT3477IUF#PBF
LT3477 - 3A, DC/DC Converter with Dual Rail-to-Rail Current Sense; Package: QFN; Pins: 20; Temperature Range: -40°C to 85°C
Linear
LT3477IUF#TR
LT3477 - 3A, DC/DC Converter with Dual Rail-to-Rail Current Sense; Package: QFN; Pins: 20; Temperature Range: -40°C to 85°C
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LT3478EFE#PBF
LT3478 - 4.5A Monolithic LED Drivers with True Color PWM Dimming; Package: TSSOP; Pins: 16; Temperature Range: -40°C to 85°C
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LT3478EFE-1#PBF
LT3478 - 4.5A Monolithic LED Drivers with True Color PWM Dimming; Package: TSSOP; Pins: 16; Temperature Range: -40°C to 85°C
Linear
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