Linear LT3518EFE Full-featured led driver with 2.3a switch current Datasheet

LT3518
Full-Featured LED Driver
with 2.3A Switch Current
FEATURES
DESCRIPTION
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The LT®3518 is a current mode DC/DC converter with an
internal 2.3A, 45V switch specifically designed to drive
LEDs. The LT3518 operates as a LED driver in boost, buck
mode and buck-boost mode. It combines a traditional
voltage loop and a unique current loop to operate as a
constant-current source or constant-voltage source. Programmable switching frequency allows optimization of the
external components for efficiency or component size. The
switching frequency of the LT3518 can be synchronized
to an external clock signal. The LED current is externally
programmable with a 100mV sense resistor. The external
PWM input provides 3000:1 LED dimming. The CTRL pin
provides further 10:1 dimming ratio.
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3000:1 True Color PWM™ Dimming Ratio
2.3A, 45V Internal Switch
100mV High Side Current Sense
Open LED Protection
Adjustable Frequency: 250kHz to 2.5MHz
Wide Input Voltage Range:
Operation from 3V to 30V
Transient Protection to 40V
Operates in Boost, Buck Mode and Buck-Boost Mode
Gate Driver for PMOS LED Disconnect
Constant-Current and Constant-Voltage Regulation
CTRL Pin Provides 10:1 Analog Dimming
Low Shutdown Current: <1μA
Available in (4mm × 4mm) 16-Lead QFN and 16-Pin
TSSOP Packages
APPLICATIONS
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The LT3518 is available in the tiny footprint 16-lead QFN
(4mm × 4mm) and the 16-pin TSSOP package. The LT3518
provides a complete solution for both constant-voltage
and constant-current applications.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color
PWM is a trademark of Linear Technology Corporation. Patent Pending. All other trademarks
are the property of their respective owners. Protected by U.S. Patents, including 7199560,
7321203, 7746300.
Display Backlighting
Automotive and Avionic Lighting
Illumination
Scanners
TYPICAL APPLICATION
Efficiency
1.5A Buck Mode LED Driver
100
CTRL = VREF
10μF
M1
2.2μF
1.5A
15μH
VIN
3.3V
2.2μF
ISP
ISN
TG
SW
VIN
SHDN
LT3518
VREF
CTRL
FB
PWM
PWM
SS
80
70
60
50
SYNC
RT
TGEN VREF VC GND
0.1μF
90
EFFICIENCY (%)
PVIN
24V
68mΩ
40
0
16.9k
1MHz
0.1μF
20
40
60
80
PWM DUTY CYCLE (%)
100
3518 TA01b
3518 TA01a
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LT3518
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, SHDN, PWM, TGEN (Note 3)..............................40V
SW, ISP, ISN, TG ........................................................45V
TG Pin Below ISP Pin ................................................10V
FB, SYNC, SS, CTRL ...................................................6V
VC, RT, VREF ................................................................3V
Operating Junction Temperature Range (Notes 2, 4)
LT3518E ............................................. –40°C to 125°C
LT3518I .............................................. –40°C to 125°C
LT3518H ............................................ –40°C to 150°C
Storage Temperature Range
QFN.................................................... –65°C to 150°C
TSSOP ............................................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
TSSOP .............................................................. 300°C
PIN CONFIGURATION
TOP VIEW
16 SW
2
15 SW
12 FB
VREF
3
11 VC
RT
4
SYNC
5
SS
6
11 TGEN
PWM
7
10 FB
CTRL
8
9
ISN
1
SHDN
ISP
VIN
16 15 14 13
TG
TGEN
TOP VIEW
SW 1
SW 2
17
VIN 3
10 CTRL
SHDN 4
5
6
7
8
VREF
RT
SYNC
SS
9
PWM
UF PACKAGE
16-LEAD (4mm = 4mm) PLASTIC QFN
14 TG
17
GND
13 ISP
12 ISN
VC
FE PACKAGE
16-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 36°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
TJMAX = 150°C, θJA = 40°C/W, θJC(PAD) = 10°C/W
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3518EUF#PBF
LT3518EUF#TRPBF
3518
16-Lead (4mm × 4mm) Plastic QFN
–40°C to 125°C
LT3518IUF#PBF
LT3518IUF#TRPBF
3518
16-Lead (4mm × 4mm) Plastic QFN
–40°C to 125°C
LT3518HUF#PBF
LT3518HUF#TPBF
3518
16-Lead (4mm × 4mm) Plastic QFN
–40°C to 150°C
LT3518EFE#PBF
LT3518EFE#TRPBF
3518FE
16-Lead Plastic TSSOP
–40°C to 125°C
LT3518IFE#PBF
LT3518IFE#TRPBF
3518FE
16-Lead Plastic TSSOP
–40°C to 125°C
LT3518HFE#PBF
LT3518HFE#TRPBF
3518FE
16-Lead Plastic TSSOP
–40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
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/
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LT3518
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
Minimum VIN Operating Voltage
MAX
3
V
Maximum VIN Operating Voltage
Continuous Operation (Note 3)
Current Sense Voltage (VISP – VISN)
VCTRL = 2V, VISP = 24V, VC = 1V
VCTRL = 2V, VISP = 0V, VC = 1V
10% Scale Current Sense Voltage (VISP – VISN)
VCTRL = 100mV, VISP = 24V, VC = 1V
9
mV
Current Sense Voltage Line Regulation
2V < VISP < 45V
0.03
%/V
VIN Supply Current
PWM > 1.5V, VC = 0V
PWM = 0V
SHDN = 0V
6
4.5
0.1
10
1
mA
mA
μA
1.0
2.5
250
1.15
2.7
270
MHz
MHz
kHz
Switching Frequency
RT = 16.7k
RT = 4.03k
RT = 91.5k
30
UNITS
l
l
96
0.85
2.25
220
RT Voltage
V
100
100
1
Soft-Start Pin Current
SS = 0.5V, Out of Pin
SYNC Pull-Down Current (Into the Pin)
VSYNC = 2V
6
9
12
1.5
RT = 91.5k (250kHz)
SYNC = 300kHz Clock Signal, RT = 91.5k
RT = 16.7k (1MHz)
RT = 4.03k (2.5MHz)
l
Switch Current Limit
Switch VCESAT
ISW = 1.5A
Switch Leakage Current
VSW = 45V, PWM = 0V
CTRL Input Bias Current
Current Out of Pin, VCTRL = 0.1V
μA
μA
0.4
SYNC Input High
mV
mV
V
60
SYNC Input Low
Maximum Duty Cycle
103
V
V
95
94
85
97
96
90
74
2.3
2.8
%
%
%
%
3.5
400
20
A
mV
2
μA
100
nA
Error Amplifier Transconductance
550
μS
VC Output Impedance
1000
kΩ
VC Idle Input Bias Current
PWM = 0, VC = 1V
FB Pin Input Bias Current
Current Out of Pin, VFB = 0.5V
l
FB Pin Threshold
ISP , ISN Idle Input Bias Current
PWM = 0V
ISP , ISN Full-Scale Input Bias Current
ISP Tied to ISN, VISP = 24V, VCTRL = 2V
SHDN Voltage High
SHDN Voltage Low
–20
0.98
nA
20
100
nA
1.01
1.04
V
300
l
V
60
l
nA
μA
1.2
–40°C ≤ TJ ≤ 125°C
125°C < TJ ≤ 150°C
PWM Input High Voltage
PWM Pin Bias Current
20
20
SHDN Pin Bias Current
PWM Input Low Voltage
0
0.45
0.40
V
V
100
μA
1.2
V
–40°C ≤ TJ ≤ 125°C
125°C < TJ ≤ 150°C
60
0.45
0.40
V
V
120
μA
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LT3518
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TGEN Input High Voltage
TYP
MAX
1.5
UNITS
V
TGEN Input Low Voltage
0.4
V
100
200
μA
2
2.04
V
0.03
%/V
TGEN Pin Bias Current
TGEN = 5V
VREF Pin Voltage
IREF = –100μA
VREF Pin Voltage Line Regulation
3V < VIN < 40V
Gate Turn-On Delay
CLOAD = 1nF Between ISP and TG
200
ns
Gate Turn-Off Delay
CLOAD = 1nF Between ISP and TG
200
ns
Top Gate Drive VGS (VISP – VTG)
VISP = 24V, TGEN = 5V
PWM = 0V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT3518E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3518I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3518H is guaranteed over the full –40°C to
150°C operating junction temperature range. Operating lifetime is derated
at junction temperatures greater than 125°C.
l
1.96
7
0
0.3
V
V
Note 3: Absolute maximum voltage at VIN, SHDN, PWM and TGEN pins
is 40V for nonrepetitive 1 second transients and 30V for continuous
operation.
Note 4: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operation above
the specified maximum operating junction temperature may impair device
reliability.
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LT3518
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Current Limit
vs Duty Cycle
VISP – VISN Threshold vs VCTRL
80
60
40
2.0
1.0
0.5
0
0
0.4
0.2
0.6 0.8 1.0
VCTRL (V)
1.2
1.4
1000
1.5
20
TA = 25°C
0
1.6
20
100
40
60
DUTY CYCLE (%)
VISP – VISN Threshold
vs Temperature
1
100
2.5
VIN = 5V
CURRENT LIMIT (A)
2.8
99
100
Oscillator Frequency
vs Temperature
OSCILLATOR FREQUENCY (MHz)
3.0
VIN = 5V
103 VISP = 24V
VC = 1V
102 VCTRL = 2V
100
10
RT (kΩ)
3518 G03
Switch Current Limit
vs Temperature
104
VISP – VISN THRESHOLD (mV)
80
3518 G02
3518 G01
101
TA = 25°C
OSCILLATOR FREQUENCY (kHz)
2.5
CURRENT LIMIT (A)
VISP – VISN THRESHOLD (mV)
VIN = 5V
VISP = 24V
100 VC = 1V
TA = 25°C
0
Oscillator Frequency vs RT
10000
3.0
120
98
2.6
2.4
2.2
VIN = 5V
RT = 6.04k
2.3
2.1
1.9
1.7
97
96
–40 –20 0
2.0
–40 –15 –10 35 60 85 110 135 160
TEMPERATURE (°C)
20 40 60 80 100 120 140 160
TEMPERATURE (°C)
Quiescent Current vs VIN
8
TA = 25°C
7 VC = 0V
VIN = 5V
2.01
101
100
99
98
VIN CURRENT (mA)
102
VREF (V)
VISP – VISN THRESHOLD (mV)
2.02
VCTRL = 2V
VIN = 5V
TA = 25°C
VC = 1V
103
2.00
1.99
6
5
4
3
2
97
1
96
95
3518 G06
Reference Voltage
vs Temperature
VISP – VISN Threshold vs VISP
104
20 40 60 80 100 120 140 160
TEMPERATURE (°C)
3518 G05
3518 G04
105
1.5
–40 –20 0
0
10
30
20
VISP (V)
40
50
3518 G07
1.98
–40 –20 0
20 40 60 80 100 120 140 160
TEMPERATURE (°C)
3518 G08
0
0
10
20
30
40
VIN (V)
3518 G09
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LT3518
TYPICAL PERFORMANCE CHARACTERISTICS
FB Pin Threshold vs Temperature
1.04
PMOS Turn-Off
VIN = 5V
1.03
FB PIN THRESHOLD (V)
PMOS Turn-On
5V
5V
PWM
1.02
PWM
0V
0V
40V
40V
1.01
1.00
TG
0.99
0.98
–40 –20 0
TG
30V
20 40 60 80 100 120 140 160
TEMPERATURE (°C)
30V
VISP = 40V
200ns/DIV
3518 G11
VISP = 40V
200ns/DIV
3518 G12
3518 G10
PIN FUNCTIONS
SW: Switch Pin. Minimize trace at this pin to reduce EMI.
SHDN: Shutdown Pin. Tie to 1.5V or higher to enable
device or 0.4V or less to disable device.
CTRL: LED Current Adjustment Pin. Sets voltage across
sense resistor between ISP and ISN. Connect directly to
VREF for full-scale threshold of 100mV, or use signal values
between GND and 1V to modulate LED current. Tie the CTRL
pin to the VREF pin if not used.
VREF: Reference Output Pin. This pin can supply up to
100μA.
VC: gm Error Amplifier Output Pin. Stabilize the loop with
an RC network or compensating C.
RT: Switching Frequency Adjustment Pin. Set switching
frequency using a resistor to GND (see Typical Performance
Characteristics for values). For SYNC function, choose
the resistor to program a frequency 20% slower than the
SYNC pulse frequency. Do not leave this pin open.
FB: Voltage Loop Feedback Pin. Works as overvoltage
protection for LED drivers. If FB is higher than 1V, the
main switch is turned off.
VIN: Input Supply Pin. Must be locally bypassed.
SYNC: Frequency Synchronization Pin. Tie an external
clock signal here. RT resistor should be chosen to program a switching frequency 20% slower than SYNC pulse
frequency. Synchronization (power switch turn-on) occurs
a fixed delay after the rising edge of SYNC. Tie the SYNC
pin to ground if this feature is not used.
SS: Soft-Start Pin. Place a soft-start capacitor here. Leave
the pin open if not in use.
PWM: Pulse Width Modulated Input Pin. Signal low turns
off channel, disables the main switch and makes the TG
pin high. Tie the PWM pin to SHDN pin if not used. There
is an equivalent 50k resistor from PWM pin to ground
internally.
TGEN: Top Gate Enable Input Pin. Tie to 1.5V or higher
to enable the PMOS driver function. Tie the TGEN pin to
ground if TG function is not used. There is an equivalent
40k resistor from TGEN pin to ground internally.
ISN: Current Sense (–) Pin. The inverting input to the
current sense amplifier.
ISP: Current Sense (+) Pin. The noninverting input to the
current sense amplifier. Also serves as positive rail for
TG pin driver.
TG: Top Gate Driver Output. An inverted PWM signal drives series PMOS device between VISP and
(VISP – 7V). An internal 7V clamp protects the VISP PMOS
gate. Leave TG unconnected if not used.
Ground: Exposed Pad. Solder paddle directly to ground
plane.
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LT3518
BLOCK DIAGRAM
LED ARRAY
CIN
CFILT
RSENSE
PVIN
ISP
+
ISN
TG
–
TGEN
PWM
SW
SW
VISP
CURRENT
SENSE
AMPLIFIER
X10
VISP – 7V
SHDN
–
+
+
CTRL
1V
MOSFET DRIVER
A1
ERROR
AMPLIFIER
+
A3
+
+
1.01V
MAIN SWITCH
DRIVER
–
+
A4
R
S
Q1
MAIN
SWITCH
Q
A2
FB
PWM
COMPARATOR
–
+
VC
A8
SYNC
RAMP
GENERATOR
+
VIN
A5
1V
GND
–
SS
2.5MHz TO 250kHz
OSCILLATOR
–
100μA
VREF
VIN
10μA
SS
RT
1V
+
+
–
+
A6
A7
Q2
2V
FREQ
ADJUST
–
VIN
3518 F01
Figure 1. Buck Mode LED Driver
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LT3518
OPERATION
The LT3518 is a constant frequency, current mode regulator with an internal power switch. Operation can be best
understood by referring to the Block Diagram in Figure 1.
At the start of each oscillator cycle, the SR latch is set,
which turns on the Q1 power switch. 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, A4. When this voltage exceeds the level at the
negative input of A4, the SR latch is reset, turning off the
power switch. The level at the negative input of A4 is set
by the error amplifier A3. A3 has two inputs, one from the
voltage feedback loop and the other one from the current
loop. Whichever feedback input is lower takes precedence,
and forces the converter into either constant-current or
constant-voltage mode. The LT3518 is designed to transition cleanly between these two modes of operation. The
current sense amplifier senses the voltage across RSENSE
and provides a pre-gain to amplifier A1. The output of A1
is simply an amplified version of the difference between
the voltage across RSENSE and the lower of VCTRL/10
or 100mV. In this manner, the error amplifier sets the
correct peak switch current level to regulate the current
through RSENSE. If the error amplifier’s output increases,
more current is delivered to the output; if it decreases,
less current is delivered. The current regulated in RSENSE
can be adjusted by changing the input voltage VCTRL.
The current sense amplifier provides rail-to-rail current
sense operation. The FB voltage loop is implemented by
the amplifier A2. When the voltage loop dominates, the
error amplifier and the amplifier A2 regulate the FB pin to
1.01V (constant-voltage mode).
Dimming of the LED array is accomplished by pulsing the
LED current using the PWM pin. When the PWM pin is
low, switching is disabled and the error amplifier is turned
off so that it does not drive the VC pin. Also, all internal
loads on the VC pin are disabled so that the charge state
of the VC pin will be saved on the external compensation
capacitor. This feature reduces transient recovery time.
When the PWM input again transitions high, the demand
current for the switch returns to the value just before
PWM last transitioned low. To further reduce transient
recovery time, an external PMOS is used to disconnect
the LED array current loop when PWM is low, stopping
CFILT from discharging.
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LT3518
APPLICATIONS INFORMATION
Dimming Control
There are two methods to control the current source for
dimming using the LT3518. The first method uses the
PWM pin to modulate the current source between zero
and full current to achieve a precisely programmed average current. To make this method of current control more
accurate, the switch demand current is stored on the VC
node during the quiescent phase. This feature minimizes
recovery time when the PWM signal goes high. To further
improve the recovery time, a disconnect switch is used in
the LED current path to prevent the output capacitor from
discharging in the PWM signal low phase. The minimum
PWM on or off time will depend on the choice of operating
frequency through RT input pin or SYNC pin. When using the SYNC function, the SYNC and PWM signals must
have the aligned rising edges to achieve the optimized
high PWM dimming ratio. For best current accuracy, the
minimum PWM low or high time should be at least six
switching cycles (3μs for fSW = 2MHz). Maximum PWM
period is determined by the system and is unlikely to be
longer than 12ms. The maximum PWM dimming ratio
(PWMRATIO) can be calculated from the maximum PWM
period (tMAX) and the minimum PWM pulse width (tMIN)
as follows:
PWMRATIO =
tMAX
tMIN
(1)
Example:
PWMRATIO = 9ms/3μs = 3000:1
The second method of dimming control uses the CTRL
pin to linearly adjust the current sense threshold during
the PWM high state. When the CTRL pin voltage is less
than 1V, the LED current is:
VCTRL
10 • RSENSE
ILED =
100mV
RSENSE
(3)
The LED current programming feature possibly increases
total dimming range by a factor of ten.
2V
VREF
45.3k
49.9k
CTRL
5k
PTC
3518 F02
Figure 2
The CTRL pin should not be left open (tie to VREF if not
used). The CTRL pin can also be used in conjunction with
a PTC thermistor to provide overtemperature protection
for the LED load.
Setting Output Voltage
For a boost application, the output voltage can be set by
selecting the values of R1 and R2 (see Figure 3) according
to the following equation:
⎛ R1 ⎞
VOUT = ⎜ + 1⎟ • 1.01V
⎝ R2 ⎠
tMAX = 9ms, tMIN = 3μs (fSW = 2MHz)
ILED =
When VCTRL is higher than 1V, the LED current is clamped
to be:
(4)
VOUT
LT3518
R1
FB
R2
(2)
3518 F03
Figure 3
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LT3518
APPLICATIONS INFORMATION
For a buck or a buck-boost configuration, the output voltage
is typically level-shifted to a signal with respect to GND as
illustrated in the Figure 4. The output can be expressed as:
VOUT
R1
=
• 1.01V + VBE(Q1)
R2
(5)
Table 1 provides some recommended inductor vendors.
Table 1. Inductor Manufacturers
VENDOR
PHONE
WEB
Sumida
(408) 321-9660
www.sumida.com
Toko
(408) 432-8281
www.toko.com
Cooper
(561) 998-4100
www.cooperet.com
Vishay
(402) 563-6866
www.vishay.com
+
R1
RSENSE
VOUT
LT3518
–
Q1
Input Capacitor Selection
LED
ARRAY
For proper operation, it is necessary to place a bypass
capacitor to GND close to the VIN pin of the LT3518. A
1μF or greater capacitor with low ESR should be used. A
ceramic capacitor is usually the best choice.
FB
R2
3518 F04
Figure 4
Inductor Selection
The inductor used with the LT3518 should have a saturation current rating of 2A or greater. For buck mode LED
drivers, the inductor value should be chosen to give a
ripple current “ΔI” of ~30% to 40% of the LED current.
In the buck mode, the inductor value can be estimated
using the formula:
L (μH) =
DBUCK • tSW (μs) • ( VIN – VLED )
ΔI
DBUCK =
VLED
VIN
DBOOST • tSW (μs) • VIN
ΔI
DBOOST =
VLED – VIN
VLED
IIN(RMS) = ILED • (1– D) • D
(8)
where D is the switch duty cycle. A 2.2μF ceramic type
capacitor is usually sufficient.
Output Capacitor Selection
(6)
VLED is the voltage across the LED string, VIN is the input
voltage to the converter, and tSW is the switching period.
In the boost configuration, the inductor can be estimated
using the formula:
L (μH) =
In the buck mode configuration, the capacitor at the input
to the power converter has large pulsed currents due to
the current returned though the Schottky diode when the
switch is off. For best reliability, this capacitor should have
low ESR and ESL and have an adequate ripple current
rating. The RMS input current is:
(7)
The selection of output capacitor depends on the load
and converter configuration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output filter capacitor should be
sized to attenuate the current ripple.
To achieve the same LED ripple current, the required filter
capacitor value is larger in the boost and buck-boost mode
applications than that in the buck mode applications. For
LED buck mode applications, a 1μF ceramic capacitor
is usually sufficient. For the LED boost and buck-boost
mode applications, a 2.2μF ceramic capacitor is usually
sufficient. Very high performance PWM dimming applications may require a larger capacitor value to support
the LED voltage during PWM transitions.
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LT3518
APPLICATIONS INFORMATION
Use only ceramic capacitors with X7R, X5R or better dielectric as they are best for temperature and DC bias stability
of the capacitor value. All ceramic capacitors exhibit loss
of capacitance value with increasing DC voltage bias, so it
may be necessary to choose a higher value capacitor to get
the required capacitance at the operation voltage. Always
check that the voltage rating of the capacitor is sufficient.
Table 2 shows some recommended capacitor vendors.
Table 3. Schottky Diodes
Table 2. Ceramic Capacitor Manufacturers
International Rectifier
VENDOR
PHONE
WEB
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
AVX
(843) 448-9411
www.avxcorp.com
Murata
(770) 436-1300
www.murata.com
TDK
(847) 803-6100
www.tdk.com
Loop Compensation
The LT3518 uses an internal transconductance error amplifier whose VC output compensates the control loop. The
external inductor, output capacitor, and the compensation resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at VC are selected to optimize control loop
stability. For typical LED applications, a 10nF compensation
capacitor at VC is adequate, and a series resistor is not
required. A compensation resistor may be used to increase
the slew rate on the VC pin to maintain tighter regulation
of LED current during fast transients on VIN or CTRL.
Diode Selection
The Schottky diode conducts current during the interval
when the switch is turned off. Select a diode rated for
the maximum SW voltage. If using the PWM feature for
dimming, it is important to consider diode leakage, which
increases with the temperature, from the output during the
PWM low interval. Therefore, choose the Schottky diode
with sufficiently low leakage current. Table 3 has some
recommended component vendors.
PART NUMBER
VR (V)
IAVE (A)
60
2
40
1
40
2.2
60
1.5
On Semiconductor
MBRS260T3
Diodes Inc.
DFLS140L
Zetex
ZLLS2000TA
10MQ060N
Board Layout
The high speed operation of the LT3518 demands careful
attention to board layout and component placement. The
Exposed Pad of the package is the only GND terminal of
the IC and is also important for thermal management of
the IC. It is crucial to achieve a good electrical and thermal
contact between the Exposed Pad and the ground plane of
the board. To reduce electromagnetic interference (EMI),
it is important to minimize the area of the SW node. Use
a GND plane under SW and minimize the length of traces
in the high frequency switching path between SW and
GND through the diode and the capacitors. Since there is
a small DC input bias current to the ISN and ISP inputs,
resistance in series with these inputs should be minimized
and matched, otherwise there will be an offset. Finally, the
bypass capacitor on the VIN supply to the LT3518 should
be placed as close as possible to the VIN terminal of the
device.
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 0.1μF.
3518fe
11
LT3518
APPLICATIONS INFORMATION
Switching Frequency
There are two methods to set the switching frequency of
LT3518. Both methods require a resistor connected at RT
pin. Do not leave the RT pin open. Also, do not load this pin
with a capacitor. A resistor must always be connected for
proper operation. One way to set the frequency is simply
connecting an external resistor between the RT pin and
GND. See Table 4 below or see the Oscillator Frequency vs
RT graph in the Typical Performance Characteristics for
resistor values and corresponding switching frequencies.
Table 4. Switching Frequency vs RT
Switching Frequency (kHz)
RT ( kΩ )
250
90.9
500
39.2
1000
16.9
1500
9.53
2000
6.04
2500
4.02
The other way is to make the LT3518 synchronize with
an external clock via SYNC pin. For proper operation, a
resistor should be connected at the RT pin and be able
to generate a switching frequency 20% lower than the
external clock when external clock is absent.
In general, a lower switching frequency should be used
where either very high or very low switching duty cycle
operation is required, or high efficiency is desired. Selection
of a higher switching frequency will allow use of smaller
value external components and yield a smaller solution
size and profile.
Thermal Considerations
The LT3518 is rated to a maximum input voltage of 30V
for continuous operation, and 40V for nonrepetitive one
second transients. Careful attention must be paid to the
internal power dissipation of the LT3518 at higher input
voltages to ensure that the maximum junction temperature
is not exceeded. This junction limit is especially important
when operating at high ambient temperatures. The Exposed
Pad on the bottom of the package must be soldered to a
ground plane. This ground should then be connected to
an internal copper ground plane with thermal vias placed
directly under the package to spread out the heat dissipated
by the LT3518.
3518fe
12
LT3518
TYPICAL APPLICATIONS
Buck Mode 1.5A LED Driver
RSENSE
68mΩ
PVIN
24V
VIN
3.3V
C3
10μF
M1
C2
2.2μF
1.5A
ISP
1000:1 PWM Dimming at 120Hz
L1
15μH
PWM
5V/DIV
D1
ISN TG
SW
VIN
ILED
1A/DIV
C1
2.2μF
SHDN
VREF
CTRL
FB
PWM
PWM
SS
LT3518
IL
1A/DIV
SYNC
RT
TGEN VREF VC GND
RT
16.9k
1MHz
C4
0.1μF
PVIN = 24V
fOSC = 1MHz
ILED = 1.5A
C5
0.1μF
3518 TA02b
2μs/DIV
3518 TA02a
C1: KEMET C0805C225K4RAC
C2: MURATA GRM31MR71E225KA93
C3: MURATA GRM32DR71E106KA12B
C4, C5: MURATA GRM21BR71H104KA01B
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-150M
LEDS: LUXEON K2 (WHITE)
M1: ZETEX ZXMP6A13GTA
500mA, 5V to 12V Boost Converter with Accurate Input Current Limit
RSENSE
50m1
VIN
5V
L1
4.3μH
Efficiency
D1
VOUT
12V
500mA
C2
2.2μF
R1
549k
80
SW
FB
CTRL
PWM
SHDN
LT3518
C2
10μF
SYNC
TGEN
VREF
VC
R2
49.9k
RT
GND
R3
10k
C4
10nF
EFFICIENCY (%)
ISP TG ISN
VIN
SHDN
90
70
60
SS
C3
0.1μF
RT
6.04k
2MHz
50
3518 TA03a
0
100
300
200
ILOAD (mA)
400
500
3518 TA03b
C1: KEMET C0805C225K4RAC
C2: KEMET C1206C106K4RAC
C3: MURATA GRM21BR71H104KA01B
C4: MURATA GCM033R71A103KA03
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-4R3N
3518fe
13
LT3518
TYPICAL APPLICATIONS
Buck-Boost Mode LED Driver
L1
4.3μH
VIN
8V TO 16V
SHDN VIN
D1
C5
0.22μF
R1
3.92M
SW
FB
R2
124k
PWM
PWM
TGEN
LT3518
RSENSE
330mΩ
VREF
ISN
CTRL
C1
2.2μF
TG
C4
0.1μF
RT SS
C2
4.7μF
M1
SYNC
VC
300mA
ISP
GND
RT
6.04k
2MHz
C3
0.1μF
3518 TA04a
C1: KEMET C0806C225K4RAC
C2: KEMET C1206C475K3RAC
C3, C4: MURATA GRM21BR71H104KA01B
C5: MURATA GRM21BR71H224KA01B
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-4R3N
LEDS: LUXEON I (WHITE)
M1: ZETEX ZXMP6A13GTA
3000:1 PWM Dimming at 120Hz
Efficiency
90
PWM
5V/DIV
VIN = 10V
CTRL = VREF
80
ILED
200mA/DIV
EFFICIENCY (%)
70
IL1
1A/DIV
VIN = 10V
fOSC = 2MHz
ILED = 300mA
500ns/DIV
3518 TA04b
60
50
40
30
20
0
20
40
60
80
100
PWM DUTY CYCLE (%)
3518 TA04c
3518fe
14
LT3518
TYPICAL APPLICATIONS
Buck Mode 1A LED Driver with Open LED Protection and Sync Input
RSENSE
100mΩ
PVIN
32V
M1
C2
2.2μF
R1
49.9k
LED1
1A
FB
R2
2.00k
LED6
R3
5.62k
VIN
3.3V
ISP
ISN
C3
10μF
Q1
L1
10μH
TG
D1
SW
VIN
C1
2.2μF
SHDN
VREF
CTRL
FB
PWM
PWM
SS
SYNC
3.3V, 1.2MHz
SYNC
RT
TGEN VREF VC GND
LT3518
C4
0.1μF
FB
RT
16.9k
1MHz
C5
0.1μF
3518 TA05a
C1: KEMET C0806C225K4RAC
C2: MURATA GRM31MR71E225KA93
C3: MURATA GRM32DR71E106KA12B
C4, C5: MURATA GRM21BR71H104KA01B
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-100M
LEDS: LUXEON III (WHITE)
M1: ZETEX ZXMP6A13GTA
Q1: PHILIPS PMBT3906
1000:1 PWM Dimming at 120Hz
Efficiency
100
CTRL = VREF
PWM
5V/DIV
90
EFFICIENCY (%)
ILED
1A/DIV
IL1
1A/DIV
PVIN = 32V
fOSC = 1.2MHz
ILED = 1A
2μs/DIV
3518 TA05b
80
70
60
50
40
0
20
40
60
80
PWM DUTY CYCLE (%)
100
3518 TA05c
3518fe
15
LT3518
TYPICAL APPLICATIONS
Boost 300mA LED Driver with LED Open Protection
L1
8.2μH
VIN
8V TO 16V
SHDN VIN
D1
SW
R1
1M
FB
R2
30.1k
PWM
ISP
PWM
LT3518
RSENSE
330mΩ
TGEN
C1
2.2μF
VREF
ISN
CTRL
TG
M1
C2
6.8μF
SYNC
LED1
VC
RT SS
GND
C1: KEMET C1206C225K2RAC
C2: TDK C5750X7R1H685M
C3, C4: MURATA GRM21BR71H104KA01B
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-8R2N
LEDS: LUXEON I (WHITE)
M1: ZETEX ZXMP6A13GTA
LED2
RT
16.9k
1MHz
C4
0.1μF
300mA
C3
0.1μF
LED8
3518 TA06a
3000:1 PWM Dimming at 100Hz
Efficiency
100
PWM
5V/DIV
90
ILED
200mA/DIV
EFFICIENCY (%)
80
IL1
1A/DIV
VIN = 12V
fOSC = 1MHz
ILED = 300mA
1μs/DIV
70
60
50
3518 TA06b
40
30
20
VIN = 12V
CTRL = VREF
0
20
60
80
40
PWM DUTY CYCLE (%)
100
3518 TA06c
3518fe
16
LT3518
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UF Package
16-Lead Plastic QFN (4mm w 4mm)
(Reference LTC DWG # 05-08-1692 Rev Ø)
0.72 t0.05
4.35 t0.05
2.15 t0.05
2.90 t0.05 (4 SIDES)
PACKAGE OUTLINE
0.30 t0.05
0.65 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
4.00 t0.10
(4 SIDES)
R = 0.115
TYP
0.75 t0.05
15
PIN 1 NOTCH R = 0.20 TYP
OR 0.35 w 45s CHAMFER
16
0.55 t0.20
PIN 1
TOP MARK
(NOTE 6)
1
2.15 t0.10
(4-SIDES)
2
(UF16) QFN 10-04
0.200 REF
0.00 – 0.05
0.30 t0.05
0.65 BSC
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC)
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
3518fe
17
LT3518
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
FE Package
16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663 Rev I)
Exposed Pad Variation BA
4.90 – 5.10*
(.193 – .201)
2.74
(.108)
2.74
(.108)
16 1514 13 12 1110
6.60 t0.10
9
2.74
(.108)
4.50 t0.10
2.74 6.40
(.108) (.252)
BSC
SEE NOTE 4
0.45 t0.05
1.05 t0.10
0.65 BSC
1 2 3 4 5 6 7 8
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50*
(.169 – .177)
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
MILLIMETERS
2. DIMENSIONS ARE IN
(INCHES)
3. DRAWING NOT TO SCALE
0.25
REF
1.10
(.0433)
MAX
0s – 8s
0.65
(.0256)
BSC
0.195 – 0.30
(.0077 – .0118)
TYP
0.05 – 0.15
(.002 – .006)
FE16 (BA) TSSOP REV I 0211
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
3518fe
18
LT3518
REVISION HISTORY
(Revision history begins at Rev D)
REV
DATE
DESCRIPTION
D
01/11
Revised Electrical Characteristics
PAGE NUMBER
3
E
08/12
Clarified Abs Max Table, Pin Configuration, and Order Information
2
Clarified Electrical Specification Table
3
Clarified Pin Functions
6
Clarified Typical Application
16
3518fe
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.
19
LT3518
TYPICAL APPLICATION
Efficiency
5.5V SEPIC Converter with Short-Circuit Protection
C1
2.2μF
C2
10μF
L2
2.4μH
VIN
CTRL
PWM
SHDN
80
R1
221k
TG
ISN
GND
60
40
RT
VREF
70
50
ISP
LT3518
TGEN
VC
90
VOUT
5.5V
500mA
SW
FB
SYNC
SHDN
RSENSE
0.15Ω
D1
EFFICIENCY (%)
L1
2.4μH
VIN
3V
100
C5
10μF
R2
49.9k
SS
30
0
R3
10k
C4
10nF
C3
0.1μF
RT
6.04k
2MHz
100
200
300
400
500
ILOAD (mA)
3518 TA07b
3518 TA07a
C1: KEMET C0805C225K4RAC
C2, C5: KEMET C1206C106K4RAC
C3: MURATA GRM21BR71H104KA01B
C4: MURATA GCM033R71A103KA03
D1: ZETEX ZLLS2000TA
L1, L2: TOKO 962BS-2R4M
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1618
Constant Current, 1.4MHz, 1.5A Boost Converter
VIN: 5V to 18V, VOUT(MAX) = 36V, Dimming = Analog/PWM, ISD < 1μA,
MSOP10 Package
LT3003
3-Channel LED Ballaster with PWM Dimming
VIN: 3V to 48V, Dimming = 3000:1 True Color PWM, ISD < 5μA,
MSOP10 Package
LT3474
36V, 1A (ILED), 2MHz, Step-Down LED Driver
VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 400:1 True Color PWM,
ISD < 1μA, TSSOP16E Package
LT3475
Dual 1.5A (ILED), 36V 2MHz Step-Down LED Driver
VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 3000:1 True Color PWM,
ISD < 1μA, TSSOP20E Package
LT3476
Quad Output 1.5A, 36V, 2MHz High Current LED Driver
with 1,000:1 Dimming
VIN: 2.8V to 16V, VOUT(MAX) = 36V, Dimming = 1000:1 True Color PWM,
ISD < 10μA, 5mm × 7mm QFN Package
LT3477
3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver
VIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1μA,
QFN, TSSOP20E Packages
LT3478/LT3478-1
4.5A, 42V, 2.5MHz High Current LED Driver with 3,000:1
Dimming
VIN: 2.8V to 36V, VOUT(MAX) = 42V, Dimming = 3000:1 True Color PWM,
ISD < 3μA, TSSOP16E Packages
LT3479
3A, Full Featured DC/DC Converter with Soft-Start and
Inrush Current Protection
VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD < 1μA,
DFN and TSSOP Packages
LT3486
Dual 1.3A, 2MHz High Current LED Driver
VIN: 2.5V to 24V, VOUT(MAX) = 36V, Dimming = 1000:1 True Color PWM,
ISD < 1μA, 5mm × 3mm DFN, TSSOP16E
LT3496
Triple Output LED Driver
VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 3000:1 True Color PWM,
ISD < 10μA, 4mm × 5mm QFN Package
LT3517
Full-Featured LED Driver with 1.5A Switch Current
VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 5000:1 True Color PWM,
ISD < 1μA, 4mm × 4mm QFN and TSSOP Packages
LT3590
48V Buck Mode 50mA LED Driver
VIN: 4.5V to 55V, Drives Up to 10 LEDs, 200:1 Dimming,
ISO = 15mA, 2mm × 2mm DFN SC70
LT3595
16 Channel Buck LED Driver Mode
VIN: 4.5V to 45V, Drives Up to 160 LEDs, 5000:1 Dimming,
5mm × 9mm QFN
LTC®3783
High Current LED Controller
VIN: 3V to 36V, VOUT(MAX) = Ext FET, Dimming = 3000:1 True Color PWM,
ISD < 20μA, 5mm × 4mm QFN10, TSSOP16E Packages
3518fe
20 Linear Technology Corporation
LT 0812 REV E • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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