LT3477
3A, DC/DC Converter
with Dual Rail-to-Rail
Current Sense
Description
Features
Dual 100mV Rail-to-Rail Current Sense Amplifiers
nn Wide Input Voltage Range: 2.5V to 25V
nn 3A, 42V Internal Switch
nn High Efficiency Power Conversion: Up to 93%
nn Drives LEDs in Boost, Buck-Boost or Buck Mode
nn Frequency Set by External Resistor: 200kHz to 3.5MHz
nn Programmable Soft-Start
nn Low V
CESAT Switch: 0.3V at 2.5A
nn Capable of Positive and Negative Output Voltages
(Boost, Inverting, SEPIC, Flyback)
nn Available in Thermally Enhanced 20-Lead
(4mm × 4mm) QFN and 20-Lead TSSOP Packages
nn
Applications
The LT®3477 is a current mode, 3A DC/DC step-up converter
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. Both current sense voltages are set at 100mV and can be adjusted
independently using the IADJ1 and IADJ2 pins. Efficiency
of up to 93% can be achieved in typical applications. The
LT3477 features a programmable soft-start function to
limit inductor current during start-up. Both inputs of the
error amplifier are available externally allowing positive
and negative output voltages (boost, inverting, SEPIC,
Flyback). The switching frequency is programmable from
200kHz to 3.5MHz through an external resistor.
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 constantcurrent applications.
High Power LED Driver
nn DSL Modems
nn Distributed Power
nn Input/Output Current Limited Boost, SEPIC,
Inverting, Flyback Converters
nn Constant-Voltage, Constant-Current Source
nn
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Typical Application
Efficiency
330mA LED Driver With Open LED Protection
10µH
3.3µF
ISP1
90
VIN
IADJ1
IADJ2
SHDN
SHDN
3.3µF
200k
SW
ISN1
80
FBN
10k
LT3477
ISP2
0.3Ω
VC
VREF
1k
4.7nF
FBP
GND
85
EFFICIENCY (%)
VIN
5V
75
70
65
ISN2
60
RT
55
SS
330mA
33nF
50
22k
0
0.1
0.2
0.3
0.4
IOUT (A)
3477 TA01b
3477 TA01a
3477fd
For more information www.linear.com/LT3477
1
LT3477
Absolute Maximum Ratings
(Note 1)
SW Pin Voltage......................................................... 42V
VIN, SHDN Pin Voltage.............................................. 25V
FBP, FBN Pin Voltage.................................................. 6V
VREF Pin Voltage......................................................... 6V
RT, VC , SS Pin Voltage................................................ 6V
IADJ1, IADJ2 Pin Voltage............................................. 25V
ISP1, ISP2, ISN1, ISN2 Pin Voltage................................42V
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)
TSSOP............................................................... 300°C
Pin Configuration
TOP VIEW
ISN1
GND
SW
1
20 NC
NC
SW
TOP VIEW
VIN
20 19 18 17 16
4
17 SW
VC
5
16 SW
FBN
6
FBP
7
14 ISN1
VREF
8
13 ISP1
21
9
12 ISN2
11 ISP2
13 ISP2
21
VIN 3
12 IADJ1
RT 4
11 IADJ2
SHDN 5
6
7
8
9 10
UF PACKAGE
20-LEAD (4mm × 4mm) PLASTIC QFN
FE PACKAGE
20-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 40°C/W
EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)
order information
14 ISN2
NC 2
15 GND
IADJ1 10
IADJ2
15 ISP1
NC 1
VREF
SS
FBP
18 NC
FBN
19 NC
3
VC
2
SS
RT
SHDN
TJMAX = 125°C, θJA = 37°C/W
EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)
http://www.linear.com/product/LT3477#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3477EFE#PBF
LT3477EFE#TRPBF
PART MARKING
20-Lead Plastic TSSOP
–40°C to 85°C
LT3477IFE#PBF
LT3477IFE#TRPBF
20-Lead Plastic TSSOP
–40°C to 125°C
LT3477EUF#PBF
LT3477EUF#TRPBF
3477
20-Lead (4mm × 4mm) Plastic QFN
–40°C to 85°C
LT3477IUF#PBF
LT3477IUF#TRPBF
3477
20-Lead (4mm × 4mm) Plastic QFN
–40°C to 125°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/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
2
3477fd
For more information www.linear.com/LT3477
LT3477
Electrical
Characteristics l indicates specifications which apply over the full operating
The
temperature range, otherwise specifications are at TA = 25°C. VIN = 2.5V, VSHDN = 2.5V.
PARAMETER
CONDITIONS
MIN
Minimum Input Voltage
l
Quiescent Current
VSHDN = 0V
VSHDN = 2.5V, VC = 0.3V (Not Switching)
Reference Voltage
E Grade
I Grade
Reference Voltage Line Regulation
2.5V < VIN < 25V, VC = 0.3V
Maximum VREF Pin Current
Out of Pin
Soft-Start Pin Current
SS = 0.5V, Out of Pin
l
l
1.216
1.210
TYP
MAX
2.3
2.5
V
0.1
5.0
1.0
7.5
µA
mA
1.235
1.235
1.250
1.260
V
V
0.01
0.03
%/V
100
µA
9
UNITS
µA
FBP Pin Bias Current
25
100
nA
FBN Pin Bias Current
25
100
nA
2
6
mV
Feedback Amplifier Offset Voltage
FBP – FBN, VC = 1V
–2
Feedback Amplifier Voltage Gain
500
V/V
Voltage Feedback Amplifier Transconductance
500
µS
Feedback Amplifier Sink Current
VFBP = 1.25V, VFBN = 1.5V, VC = 1V
10
µA
Feedback Amplifier Source Current
VFBP = 1.25V, VFBN = 1V, VC = 0.5V
10
µA
Current Sense Amplifier Sense Voltage
Positive Rail, VCM = 25V, E Grade
Positive Rail, VCM = 25V, I Grade
Ground
Switching Frequency
RT = 17.2k
RT = 107.4k
RT = 2.44k
Maximum Switch Duty Cycle
RT = 17.2k
Switch Current Limit
(Note 3)
l
l
l
97.5
97.5
88
100
100
100
102.5
103
112
mV
mV
mV
0.9
160
2.7
1
200
3.5
1.15
240
4.3
MHz
kHz
MHz
87
93
3
4
5
%
A
Switch VCESAT
ISW = 1A (Note 3)
150
200
mV
Switch Leakage Current
SW = 40V
0.2
5
µA
SHDN Pin Current
VSHDN = 5V
VSHDN = 0V
30
0.1
60
1
µA
µA
1.5
2
V
SHDN Pin Threshold
0.3
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 LT3477E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the – 40°C to 85°C operating
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 VCESAT for UF package guaranteed
by design and/or correlation to static test.
3477fd
For more information www.linear.com/LT3477
3
LT3477
Typical Performance Characteristics
Switch Current Limit
Switch VCE(SAT)
VREF
5
0.50
1.27
0.45
CURRENT (A)
125°C
0.30
25°C
0.25
0.20
–50°C
0.15
0.10
1.25
3
VREF (V)
0.35
VCE(SAT) (V)
1.26
4
0.40
2
1
0
0.5
1.5
2
1
SWITCH CURRENT (A)
0
–50
3
2.5
50
0
75
25
TEMPERATURE (°C)
–25
3477 G01
SHDN Pin Turn-On Threshold
VIN = 25V
1.23
VIN = 2.5V
1.22
0.05
0
1.24
1.21
–50 –25
125
100
SHDN Pin Current
25 50 75 100 125 150
TEMPERATURE (°C)
3477 G03
Quiescent Current
50
1.6
0
3477 G02
6
VC = 0.3V
1.4
1.2
50
25
75
0
TEMPERATURE (°C)
–25
100
30
25°C
20
125°C
10
0
125
0
5
15
10
VSHDN (V)
5
4
3
2
–50 –25
25
Soft-Start Pin Current
Feedback Amplifier Offset Voltage
2.0
4
FREQUENCY (MHz)
15
3
RT = 10kΩ
1.6
5
25 50 75 100 125 150
TEMPERATURE (°C)
3477 G06
Oscillator Frequency
20
10
0
3477 G05
3477 G04
ISS (µA)
20
1.2
OFFSET VOLTAGE (mV)
1.0
–50
40
QUIESCENT CURRENT (mA)
SHDN PIN CURRENT (µA)
SHDN THRESHOLD (V)
–50°C
RT = 15kΩ
RT = 20kΩ
0.8
0.4
2
VC = 1V
1
0
VC = 0.5V
–1
–2
–3
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3477 G07
4
0
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
3477 G08
–4
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3477 G09
3477fd
For more information www.linear.com/LT3477
LT3477
Typical Performance Characteristics
FBP Pin Bias Current
40
FBN Pin Bias Current
50
“+” INDICATES THE CURRENT
FLOWS OUT OF PIN
FBN PIN BIAS CURRENT (nA)
FBP PIN BIAS CURRENT (nA)
50
30
20
10
0
–10
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
“+” INDICATES THE CURRENT
FLOWS OUT OF PIN
40
30
20
10
0
–10
–50 –25
125
50
25
75
0
TEMPERATURE (°C)
100
3477 G11
3477 G10
Current Sense Voltage
vs Temperature
Current Sense Voltage vs IADJ
120
103
VCM = 10V
100
102
VOLTAGE SENSE (mV)
CURRENT SENSE VOLTAGE (mV)
104
101
VCM = 10V
100
99
VCM = 42V
98
80
60
40
20
97
96
–50 –25
125
75
50
25
TEMPERATURE (°C)
0
100
125
0
0
100 200 300 400 500 600 700 800
IADJ VOLTAGE (mV)
3477 G14
2477 G13
3477fd
For more information www.linear.com/LT3477
5
LT3477
Pin Functions
(QFN/TSSOP)
NC (Pins 1, 2, 20/Pins 18, 19, 20): No Connect Pin. Okay
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.
RT (Pin 4/Pin 2): Timing Resistor Pin. Adjusts the switching frequency. Connect a 17.2k resistor between RT and
GND for a 1MHz switching frequency. Do not leave this pin
open. See Table 4 for additional RT values and switching
frequencies.
SHDN (Pin 5/Pin 3): Shutdown. Tie to 2V or greater to
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.
FBN (Pin 8/Pin 6): The Inverting Input to the Error Amplifier. Connect resistive divider tap here for positive output
voltage.
FBP (Pin 9/Pin 7): The Noninverting Input to the Error
Amplifier. Connect resistive divider tap here for negative
output voltage.
VREF (Pin 10/Pin 8): Bandgap Voltage Reference. Internally
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 100µA of
current and can be locally bypassed with a 100pF capacitor.
IADJ2 (Pin 11/Pin 9): Second Current Sense Adjustment.
Setting IADJ2 to be less than 625mV leads to adjustment of
the sensed voltage of the second current sense amplifier
6
linearly. If IADJ2 is tied to higher than 650mV, the default
current sense voltage is 100mV. If current sense amplifier 2 is not used, always tie IADJ2 to higher than 650mV.
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 amplifier 1 is not used, always tie IADJ1 to higher than 650mV.
ISP2 (Pin 13/Pin 11): Second Current Sense (+) Pin. The
noninverting input to the second current sense amplifier.
Connect to ISN2 if not used.
ISN2 (Pin 14/Pin 12): Second Current Sense (–) Pin. The
inverting input to the second current sense amplifier. Connect to ISP2 if not used.
ISP1 (Pin 15/Pin 13): First Current Sense (+) Pin. The
noninverting input to the first current sense amplifier.
Connect to ISN1 if not used.
ISN1 (Pin 16/Pin 14): First Current Sense (–) Pin. The inverting input to the first current sense amplifier. Connect
to ISP1 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.
Exposed Pad (Pin 21/Pin 21): Power Ground. Must be
soldered to PCB ground for electrical contact and rated
thermal performance.
3477fd
For more information www.linear.com/LT3477
LT3477
BLOCK DIAGRAM
VC
SS
ISN1
VADJ
–
+
+
A1
VADJ
–
+
+
A2
IA1
–
IADJ1
ISP2
ISN2
+
IA2
–
IADJ2
FBP
+
FBN
–
A3
–
SLOPE
+
A4
R
S
Q
VA
Q1
∑
–
VREF
SW
+
+
ISP1
VREF
1.235V
OSCILLATOR
SHDN VIN
RT
3477 F01
Figure 1. LT3477 Block Diagram
3477fd
For more information www.linear.com/LT3477
7
LT3477
OPERATION
The LT3477 uses a fixed frequency, current mode control
scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block
Diagram in Figure 1. The start of each oscillator cycle sets
the SR latch and turns on power switch Q1. The signal at the
noninverting input of the PWM comparator (A4 SLOPE) 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. Amplifier A3 drives A4
inverting input. A3 has three inputs, one from the voltage
feedback loop and the other two from the current feedback
loop. Whichever feedback input is higher takes precedence,
forcing the converter into either a constant-current or a
constant-voltage mode. The LT3477 is designed to transition cleanly between the two modes of 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-current mode. If the
current sense voltage exceeds 100mV, the output of IA1
will increase causing the output of A3 to decrease, thus
reducing the amount of current delivered to the output.
8
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
allowed 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.
3477fd
For more information www.linear.com/LT3477
LT3477
APPLICATIONS INFORMATION
Capacitor Selection
Low ESR (equivalent series resistance) ceramic capacitors 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.
Table 1. Ceramic Capacitor Manufacturers
MANUFACTURER
PHONE
WEB
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
TDK
(847) 803-6100
www.component.tdk.com
Inductor Selection
Several inductors that work well with the LT3477 are listed
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.
Inductor manufacturers specify the maximum current
rating as the current where the inductance falls to some
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
manufacturer to determine how the maximum inductor
current is measured and how much more current the
inductor can reliably conduct.
Diode Selection
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 exceed
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 sufficient for
most designs. The companies below also offer Schottky
diodes with higher voltage and current ratings.
Table 3. Suggested Diodes
MANUFACTURER
MAX
MAX REVERSE
PART NUMBER
CURRENT (A) VOLTAGE (V) MANUFACTURER
UPS340
UPS315
nn
B220
B230
B240
B320
B330
B340
SBM340
nn
3
2
2
3
3
3
3
3
nn
2
nn
15
30
40
20
30
40
40
40
Microsemi
www.microsemi.com
20
Diodes, Inc
www.diodes.com
Table 2. Suggested Inductors
MANUFACTURER
PART NUMBER
IDC
(A)
INDUCTANCE
(µH)
MAX DCR
(mΩ)
L×W×H
(mm)
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
MANUFACTURER
3477fd
For more information www.linear.com/LT3477
9
LT3477
APPLICATIONS INFORMATION
Setting Positive Output Voltages
To set a positive output voltage, select the values of R1
and R2 (see Figure 2) according to the following equation:
⎛ R1 ⎞
VOUT = 1.235V ⎜1+ ⎟
⎝ R2 ⎠
FBP
LT3477
VOUT
VREF
R1
FBN
R2
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:
VOUT
⎛ R3 ⎞
= 1.235V ⎜1+ ⎟
⎝ R4 ⎠
–VOUT
R3
FBP
R4
LT3477
VREF
FBN
3477 F03
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).
RSENSE =
10
100mV
ISENSE
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:
ISENSE =
100mV VADJ
•
RSENSE 618mV
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).
Considerations When Sensing Input Current
In addition to regulating the DC output current for currentsource 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 DCisolated from the input, so an input current limit not only
helps soft-start the output but also provides excellent
short-circuit protection.
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 is a triangular
waveform (not a DC waveform like the output current)
some tweaking of the compensation values (RC and CC
3477fd
For more information www.linear.com/LT3477
LT3477
APPLICATIONS INFORMATION
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 reducing the RC value or by
adding a capacitor (with a value of approximately CC/10)
in parallel with the RC and CC compensation network.
Frequency Compensation
The LT3477 has an external compensation pin (VC), which
allows the loop response to be optimized for each application. An external resistor and capacitor (or sometimes just
a capacitor) are placed at the VC pin to provide a pole and
a zero (or just a pole) to ensure proper loop compensation.
Several other poles and zeroes are present in the closedloop 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.
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 voltage overshoot. A typical value for the soft-start
capacitor 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.
Increasing switching frequency reduces output voltage
ripple but also reduces efficiency. The user should set the
frequency for the maximum tolerable output voltage ripple.
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz)
nn
nn
Board Layout
nn
nn
nn
2.43
3
nn
3.65
2.5
nn
4.87
2
nn
6.81
1.5
nn
10.2
17.4
1
nn
nn
0.5
nn
43.2
nn
0.2
nn
107
nn
3.5
3.0
SWITCH FREQUENCY (MHz)
As with all switching regulators, careful attention must
be paid to the PCB board layout and component placement. To maximize efficiency, switch rise and fall times
are made as short as possible. To prevent radiation and
high frequency 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
the switch, output diode D1 and output capacitor COUT,
contains nanosecond rise and fall times and should be
kept as short as possible.
nn
RT (kΩ)
3.5
2.5
2.0
1.5
1.0
0.5
0
0.1
10
RT (kΩ)
100
3477 F04
Figure 4. Switch Frequency
3477fd
For more information www.linear.com/LT3477
11
LT3477
APPLICATIONS INFORMATION
PWM Dimming
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, undesirable in many applications. The better 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
manufacturers. 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 unchanged in this scheme since the LED current is
either zero or at programmed current. Another advantage
of PWM dimming over analog dimming is that a wider
dimming range is possible.
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
100
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.
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 dimming range
decreases with increasing PWM frequency, 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 increase
the power handling capability for higher current LED
applications. A buck-boost LED driver works best in applications where the input voltage fluctuates to higher or
lower than the total LED voltage drop.
In high temperature applications, the leakage of the
Schottky 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
RT = 6.81k
RT = 6.81k
DIMMING RANGE: 1
LED CURRENT (mA)
10
1
0.1
VIN = 5V
BOOST
4 LEDs
PWM FREQUENCY = 100Hz
0.01
0.1
1
10
PWM DUTY CYCLE (%)
100
100
10
1
0.1
12
10
100
PWM FREQUENCY (kHz)
3477 F05
Figure 5. LED Current vs PWM Duty Cycle
Wide Dimming Range (500:1)
1
3477 F06
Figure 6. Dimming Range vs PWM Frequency
3477fd
For more information www.linear.com/LT3477
LT3477
APPLICATIONS INFORMATION
L1
2.0µH
VIN
5V
C1
3.3µF
ISP1
D1
FBN
75k
FBP
LT3477
VREF
ISP2
SS
CSS
33nF
RSENSE
0.33Ω
ISN2
LED1
RT
VC
GND
0
LED2
6.81k
D2
300mA
LED3
PWM
5V
C2
10µF
1M
SW
ISN1
VIN
IADJ1
IADJ2
SHDN
OUT
NMOS1
LED4
RC
2.4k
100Hz
100k
NMOS2
CC
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
350
85
EFFICIENCY
EFFICIENCY (%)
PWM
5V/DIV
IL
1A/DIV
80
300
75
250
70
150
65
60
ILED
200mA/DIV
VIN = 5V
BOOST
4 LEDs, 300mA
PWM FREQUENCY = 100Hz
55
50
VIN = 5V
4 LEDs
300mA
10µs/DIV
PWM FREQ = 100Hz
BOOST
3477 F07b
200
LED CURRENT
20
0
40
60
80
100
50
0
100
PWM DUTY CYCLE (%)
3477 F07c
Figure 7b. PWM Dimming Waveforms
Figure 7c. Efficiency and LED Current
vs PWM Duty Cycle
3477fd
For more information www.linear.com/LT3477
13
LT3477
APPLICATIONS INFORMATION
PVIN
32V
C1
2.2µF
RSENSE
0.33Ω
300mA
C1: NIPPON NTS40X5R1H225M
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ335KL
L1: TOKO D53LC (PN# A915AY-100M)
D1: ZETEX ZLLS400
D2: DIODES INC 1N4148
NMOS1, NM0S2: ZETEX 2N7002
PMOS: SILICONIX Si2303BDS
LED1 TO LED6: LUMILEDS LXHL-BW02
LED1
•
•
•
LED6
1k
PMOS
NMOS2
ISP1
ISN1
D1
280k
SW
VIN
IADJ1
IADJ2
C3
3.3µF
SHDN
FBN
10k
FBP
LT3477
VREF
ISP2
SS
CSS
33nF
ISN2
RT
VC
GND
0
D2
PWM
5V
PWM
C2
1µF
L1
10µH
VIN
3.3V
1k
6.81k
3477 F08a
NMOS1
100Hz
CC
0.1µF
100k
Figure 8a. 32V to 6 White LEDs: Buck Mode With PWM Dimming
PWM
5V/DIV
IL
500mA/DIV
ILED
500mA/DIV
2ms/DIV
PVIN = 32V
6 LEDs
300mA
3477 F08b
PWM FREQUENCY = 100Hz
BUCK MODE
Figure 8b. PWM Dimming Waveforms
14
3477fd
For more information www.linear.com/LT3477
LT3477
APPLICATIONS INFORMATION
1k
C1: TAIYO YUDEN LMK316BJ335ML
C2: TAIYO YUDEN UDK325BJ106MM
L1: TOKO D53LC (PN# A915AY-4R7M)
D1: ZETEX ZLLS1000
D2: DIODES INC 1N4148
NMOS1, NMOS2: ZETEX 2N7002
PMOS: SILICONIX Si2303BDS
LED1, LED2: LUMILEDS LXHL-BW02
NMOS2
1k
PMOS
VIN
10V
C1
3.3µF
ISP1
ISN1
RSENSE
0.33Ω
1M
FBN
49.9k
FBP
LT3477
VREF
ISP2
SS
CSS
33nF
ISN2
RT
VC
GND
D2
PWM
300mA
LED1
D1
SW
SHDN
0
LED2
L1
4.7µH
VIN
IADJ1
IADJ2
5V
PWM
6.81k
3477 F09a
C2
10µF
NMOS1
100Hz
100k
RC
1.5k
CC
10nF
Figure 9a. 10V to 2 White LEDs: Buck-Boost Mode With PWM Dimming
PWM
10V/DIV
IL
1A/DIV
ILED
500mA/DIV
VIN = 10V
2 LEDs
300mA
2ms/DIV
PWM FREQUENCY = 100Hz
BUCK-BOOST MODE
3477 F09b
Figure 9b. PWM Dimming Waveforms
3477fd
For more information www.linear.com/LT3477
15
LT3477
TYPICAL APPLICATIONS
Efficiency
5.5V SEPIC Converter With Short-Circuit Protection
C1
3.3µF
ISP1
ISN1
SHDN
FBN
LT3477
SHDN
GND
70
65
55
50
RT
FBP
75
60
ISN2
VREF
RC
1k
80
ISP2
VC
SS
C4
33nF
CC
4.7nF
VIN = 3V
85
5.5V
670mA
R4
34.8k
L2
4.7µH
SW
VIN
IADJ1
IADJ2
R3
0.15Ω
D1
EFFICIENCY (%)
VIN
3V TO
16V
90
C2
10µF
L1
4.7µH
R1
0.04Ω
C3
10µF
R2
18.2k
0
0.1
0.2
ISP1
ISN1
SHDN
R3
200k
LT3477
C2
10µF
R4
23.2k
ISP2
ISN2
75
70
65
55
SS
CC
4.7nF
80
60
RT
GND
85
12V
0.8A
FBN
VREF
FBP
D1
SW
VC
RC
1k
90
L1
4.7µH
VIN
IADJ1
IADJ2
SHDN
0.7
Efficiency
EFFICIENCY (%)
C1
2.2µF
0.6
3477 TA02b
R5
10k
800mA, 5V to 12V Boost Converter With Accurate Input Current Limit
VIN
5V
0.5
3477 TA02a
C1: TAIYO YUDEN LMK316BJ335ML
C2: TAIYO YUDEN LMK325BJ106MN
C3: TAIYO YUDEN LMK316BJ106ZL
D1: DIODES INC. DFLS130L
L1, L2: TOKO FDV0630-4R7M
R1
0.033Ω
0.3 0.4
IOUT (A)
50
C3
10nF
R2
17.8k
3477 TA04a
0
0.1
0.2
0.3
0.4 0.5
IOUT (A)
0.6
0.7
0.8
3477 TA04b
C1: TAIYO YUDEN LMK316BJ225MD
C2: AVX 1206YD106MAT
D1: DIODES INC. B320A
L1: TOKO FDV0630-4R7M
16
3477fd
For more information www.linear.com/LT3477
LT3477
TYPICAL APPLICATIONS
87% Efficient, 4W LED Driver
R4
L2
0.05Ω 10µH
C1
3.3µF
ISP1
ISN1
SW
FBN
R1
10k
LT3477
SHDN
ISP2
VC
RC
1k
330mA
GND
SS
C3
33nF
65
50
0
0.1
0.2
0.4
0.3
IOUT (A)
LED3
R3
22k
C1: TAIYO YUDEN LMK316BJ335ML
C2: TAIYO YUDEN TMK325BJ335MN
D1: DIODES INC. DFLS120L
L1: TOKO A915AY-100M
70
55
LED2
CC
4.7nF
75
60
LED1
RT
FBP
80
R6
0.3Ω
ISN2
VREF
85
C2
3.3µF
R2
200k
VIN
IADJ1
IADJ2
SHDN
90
D1
EFFICIENCY (%)
VIN
5V
Efficiency
3477 TA01b
LED4
3477 TA03a
1A Buck Mode High Current LED Driver
PVIN
32V
C1
2.2µF
R1
0.1Ω
LED1
LED4
•
•
•
Efficiency
LED
STRING C2
1µF
100
90
80
L1
33µH D1
ISP1
VIN
3.3V
C3
3.3µF
SHDN
ISN1
VIN
IADJ1
IADJ2
SHDN
FBN
R4
10k
LT3477
ISP2
VC
FBP
CC
4.7nF
70
60
50
40
30
20
0
ISN2
VREF
RC
1k
R3
280k
SW
EFFICIENCY (%)
1A
0.4
0.6
LED CURRENT (A)
0.8
1
3477 TA05b
RT
GND
0.2
SS
C4
33nF
R2
22k
3477 TA05a
C1: NIPPON UNITED CHEMICON NTS40X5R1H225M
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ475
L1: TOKO A814AY-330M
D1: DIODES INC DFLS140
3477fd
For more information www.linear.com/LT3477
17
LT3477
TYPICAL APPLICATIONS
Buck-Boost Mode LED Driver
LED2 LED1
L1
4.7µH
VIN
2.7V TO 10V
C1
3.3µF
ISP1
ISN1
SHDN
SHDN
R3
200k
SW1
VIN
IADJ1
IADJ2
LED BRIGHTNESS
CONTROL
0mV TO 650mV
R1
0.1Ω
D1
FBN
LT3477
ISP2
VC
ISN2
VREF
FBP
RT
GND
SS
C3
33nF
CC
10nF
R2
18k
R4
10k
C1: TAIYO YUDEN LMK316BJ335ML
C2: MURATA GRM31CR71E475KA88L
D1: DIODES, INC. B320A
L1: TOKO FDV0630-4R7M
C2
4.7µF
3477 TA06a
Efficiency
90
85
VIN = 8V
EFFICIENCY (%)
80
75
VIN = 4.2V
70
65
VIN (V)
IOUT (A)
2.7
3.6
4.2
5
8
0.57
0.74
0.83
0.93
1.0
60
55
50
0
0.2
0.4
0.6
IOUT (A)
0.8
1.0
3477 TA06b
18
3477fd
For more information www.linear.com/LT3477
LT3477
Package Description
Please refer to http://www.linear.com/product/LT3477#packaging for the most recent package drawings.
UF Package
20-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1710 Rev A)
0.70 ±0.05
4.50 ±0.05
3.10 ±0.05
2.00 REF
2.45 ±0.05
2.45 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ±0.10
0.75 ±0.05
R = 0.05
TYP
R = 0.115
TYP
19 20
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
4.00 ±0.10
PIN 1 NOTCH
R = 0.20 TYP
OR 0.35 × 45°
CHAMFER
BOTTOM VIEW—EXPOSED PAD
1
2.00 REF
2.45 ±0.10
2
2.45 ±0.10
(UF20) QFN 01-07 REV A
0.200 REF
0.00 – 0.05
0.25 ±0.05
0.50 BSC
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
3477fd
For more information www.linear.com/LT3477
19
LT3477
Package Description
Please refer to http://www.linear.com/product/LT3477#packaging for the most recent package drawings.
FE Package
20-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663 Rev L)
Exposed Pad Variation CB
DETAIL A
6.40 – 6.60*
(.252 – .260)
3.86
(.152)
3.86
(.152)
0.60
(.024)
REF
0.28
(.011)
REF
20 1918 17 16 15 14 13 12 11
6.60 ±0.10
2.74
(.108)
4.50 ±0.10
DETAIL A
6.40
2.74 (.252)
(.108) BSC
SEE NOTE 4
0.45 ±0.05
DETAIL A IS THE PART OF
THE LEAD FRAME FEATURE
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
1.05 ±0.10
0.65 BSC
1 2 3 4 5 6 7 8 9 10
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50*
(.169 – .177)
0.09 – 0.20
(.0035 – .0079)
0.25
REF
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
MILLIMETERS
2. DIMENSIONS ARE IN
(INCHES)
3. DRAWING NOT TO SCALE
20
1.20
(.047)
MAX
0° – 8°
0.65
(.0256)
BSC
0.195 – 0.30
(.0077 – .0118)
TYP
0.05 – 0.15
(.002 – .006)
FE20 (CB) TSSOP REV L 0117
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
3477fd
For more information www.linear.com/LT3477
LT3477
Revision History
(Revision history begins at Rev D)
REV
DATE
DESCRIPTION
D
03/17
Clarified efficiency in Description
PAGE NUMBER
1
Clarified VREF in Block Diagram
6
3477fd
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LT3477
21
LT3477
Typical Application
Buck Mode High Current LED Driver
PVIN
32V
C1
2.2µF
R1
0.1Ω
Efficiency
LED1
1A
LED4
•
•
•
LED
STRING C2
1µF
100
90
80
ISP1
VIN
3.3V
ISN1
SHDN
SHDN
FBN
LT3477
70
60
50
40
R4
10k
30
20
ISP2
VC
0
ISN2
VREF
RC
1k
R3
280k
SW
VIN
IADJ1
IADJ2
C3
3.3µF
EFFICIENCY (%)
L1
33µH D1
FBP
0.2
0.4
0.6
LED CURRENT (A)
0.8
1
3477 TA05b
RT
GND
CC
4.7nF
SS
C4
33nF
R2
22k
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
LT1618
Constant Current, Constant Voltage 1.4MHz,
High Efficiency Boost Regulator
VIN: 1.6V to 18V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1µA, QFN16 Package
LT3436
3A (ISW), 800kHz, 34V Step-Up DC/DC Converter
VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6µA, TSSOP16E Package
LTC 3453
Synchronous Buck-Boost High Power White
LED Driver
VIN: 2.7V to 5.5V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1µA, QFN16 Package
LT3466
Dual Constant Current, 2MHz, High Efficiency
White LED Boost Regulator With Integrated
Schottky Diode
VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 16µA, DFN Package
LT3479
3A, 42V Full Featured Boost/Inverter Converter
With Soft-Start
VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 1µA, DFN/TSSOP Packages
LTC3490
Single Cell 350mA, 1.3MHz LED Driver
VIN: 1V to 3.2V, VOUT(MAX) = 4.7V, ISD < 1µA, DFN/SO8 Packages
®
22 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3477
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3477
3477fd
LT 0317 REV D • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2005
工商网监
湘ICP备2023018690号
