LT3518
Full-Featured LED Driver
with 2.3A Switch Current
FEATURES
n
3000:1 True Color PWM™ Dimming Ratio
n
2.3A, 45V Internal Switch
n
100mV High Side Current Sense
n
Open LED Protection
n
Adjustable Frequency: 250kHz to 2.5MHz
n
Wide Input Voltage Range:
Operation from 3V to 30V
Transient Protection to 40V
n
Operates in Boost, Buck Mode and Buck-Boost Mode
n
Gate Driver for PMOS LED Disconnect*
n
Constant-Current and Constant-Voltage Regulation
n
CTRL Pin Provides 10:1 Analog Dimming
n
Low Shutdown Current: <1μA
n
Available in (4mm × 4mm) 16-Lead QFN and 16-Pin
TSSOP Packages
APPLICATIONS
n
Display Backlighting
n
Automotive and Avionic Lighting
n
Illumination
n
Scanners
DESCRIPTION
The LT®3518 is a current mode DC/DC converter with an
internal 2.3A, 45V switch specifi cally 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 effi ciency 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.
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. *Patents Pending.
TYPICAL APPLICATION
1.5A Buck Mode LED Driver
ISP
V
IN
SHDN
CTRL
PWM
SYNC
TGEN GNDV
68mΩ M1
ISN
TG
LT3518
REFVC
0.1μF
SW
FB
SS
R
T
PV
24V
V
3.3V
IN
IN
2.2μF
V
REF
PWM
2.2μF
1.5A
16.9k
1MHz
15μH
0.1μF
10μF
3518 TA01a
100
CTRL = V
90
80
70
EFFICIENCY (%)
60
50
40
0
Effi ciency
REF
20 40 60 80
PWM DUTY CYCLE (%)
100
3518 TA01b
3518fb
1
LT3518
ABSOLUTE MAXIMUM RATINGS
VIN, SHDN, PWM, TGEN (Note 3)..............................40V
SW, ISP, ISN, TG ........................................................45V
TG Pin Below ISP Pin ................................................10V
FB, SYNC, SS, CTRL ...................................................6V
V
C
, RT, V
.................................................................3V
REF
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
PIN CONFIGURATION
TOP VIEW
TG
ISP
ISN
TGEN
16 15 14 13
FB
1SW
SW
2
V
IN
SHDN
16-LEAD (4mm × 4mm) PLASTIC QFN
T
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
JMAX
17
3
4
5 6 7 8
R
REF
V
UF PACKAGE
= 125°C, θJA = 36°C/W
12
V
11
C
CTRL
10
PWM
9
T
SS
SYNC
(Note 1)
Storage Temperature Range
QFN .................................................... –65°C to 125°C
TSSOP ............................................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
TSSOP .............................................................. 300°C
TOP VIEW
1
V
IN
2
SHDN
3
V
REF
4
R
T
5
SYNC
6
SS
7
PWM
8
CTRL
16-LEAD PLASTIC TSSOP
T
= 150°C, θJA = 40°C/W, θ
JMAX
17
GND
FE PACKAGE
16
15
14
13
12
11
10
9
JC(PAD)
SW
SW
TG
ISP
ISN
TGEN
FB
V
C
= 10°C/W
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3518EUF#PBF LT3518EUF#TRPBF 3518
LT3518IUF#PBF LT3518IUF#TRPBF 3518
16-Lead (4mm × 4mm) Plastic QFN
16-Lead (4mm × 4mm) Plastic QFN
–40°C to 125°C
–40°C to 125°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 3518HFE 16-Lead Plastic TSSOP –40°C to 150°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based fi nish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/
3518fb
2
LT3518
ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum VIN Operating Voltage 3V
Maximum V
Current Sense Voltage (V
10% Scale Current Sense Voltage (V
Current Sense Voltage Line Regulation 2V < V
Supply Current PWM > 1.5V, VC = 0V
V
IN
Switching Frequency R
R
Voltage 1V
T
Soft-Start Pin Current SS = 0.5V, Out of Pin 6 9 12 μA
SYNC Pull-Down Current (Into the Pin) V
SYNC Input Low 0.4 V
SYNC Input High 1.5 V
Maximum Duty Cycle R
Switch Current Limit 2.3 2.8 3.5 A
Switch V
Switch Leakage Current V
CTRL Input Bias Current Current Out of Pin, V
Error Amplifi er Transconductance 550 μS
Output Impedance 1000 kΩ
V
C
Idle Input Bias Current PWM = 0, VC = 1V –20 0 20 nA
V
C
FB Pin Input Bias Current Current Out of Pin, V
FB Pin Threshold
, ISN Idle Input Bias Current PWM = 0V 300 nA
ISP
ISP , ISN Full-Scale Input Bias Current ISP Tied to ISN, V
SHDN Voltage High 1.5 V
SHDN Voltage Low 0.4 V
SHDN Pin Bias Current 60 100 μA
PWM Input High Voltage 1.5 V
PWM Input Low Voltage 0.4 V
PWM Pin Bias Current 60 120 μA
TGEN Input High Voltage 1.5 V
TGEN Input Low Voltage 0.4 V
TGEN Pin Bias Current TGEN = 5V 100 200 μA
Pin Voltage I
V
REF
Operating Voltage Continuous Operation (Note 3) 30 V
IN
– V
ISP
CESAT
)V
ISN
– V
ISP
)V
ISN
= 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.
A
l
l
l
96 100
0.85
2.25
220
95
94
85
100
6
4.5
0.1
1.0
2.5
250
97
96
90
74
103 mV
mV
10
mA
mA
1
1.15
2.7
270
MHz
MHz
kHz
μA
V
CTRL
CTRL
CTRL
= 2V, V
= 2V, V
= 100mV, V
ISP
= 24V, VC = 1V
ISP
= 0V, VC = 1V
ISP
= 24V, VC = 1V 9 mV
ISP
< 45V 0.03 %/V
PWM = 0V
SHDN = 0V
= 16.7k
T
R
= 4.03k
T
R
= 91.5k
T
= 2V 60 μA
SYNC
= 91.5k (250kHz)
T
SYNC = 300kHz Clock Signal, R
R
= 16.7k (1MHz)
T
R
= 4.03k (2.5MHz)
T
= 91.5k
T
ISW = 1.5A 400 mV
= 45V, PWM = 0V 2 μA
SW
= 0.1V 20 100 nA
CTRL
= 0.5V 20 100 nA
FB
l
0.98 1.01 1.04 V
= –100μA
REF
= 24V, V
ISP
= 2V 20 μA
CTRL
l
1.96 2 2.04 V
%
%
%
%
3518fb
3
LT3518
ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T
PARAMETER CONDITIONS MIN TYP MAX UNITS
V
Pin Voltage Line Regulation 3V < VIN < 40V 0.03 %/V
REF
Gate Turn-On Delay C
Gate Turn-Off Delay C
(V
Top Gate Drive V
– VTG)V
GS
ISP
= 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.
A
= 1nF Between ISP and TG 200 ns
LOAD
= 1nF Between ISP and TG 200 ns
LOAD
= 24V, TGEN = 5V
ISP
PWM = 0V
7
0 0.3
V
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.
Note 2: The LT3518E is guaranteed to meet performance specifi cations
from 0°C to 125°C junction temperature. Specifi cations 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.
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 specifi ed maximum operating junction temperature may impair device
reliability.
4
3518fb
TYPICAL PERFORMANCE CHARACTERISTICS
V
– V
Threshold vs V
ISN
= 24V
CTRL
3.0
2.5
2.0
120
100
80
ISP
VIN = 5V
V
ISP
= 1V
V
C
= 25°C
T
A
LT3518
Switch Current Limit
vs Duty Cycle
60
THRESHOLD (mV)
ISN
40
– V
ISP
V
20
0
0
0.4 0.8
0.2 0.6
V
CTRL
Oscillator Frequency vs R
10000
TA = 25°C
1000
OSCILLATOR FREQUENCY (kHz)
100
1
RT (kΩ)
1.2
1.0
(V)
1.4
3518 G01
T
10 100
3518 G03
1.6
1.5
1.0
CURRENT LIMIT (A)
0.5
TA = 25°C
0
0
20 40 60 80
V
– V
ISP
Temperature
105
V
= 2V
CTRL
104
= 5V
V
IN
= 25°C
T
A
103
= 1V
V
C
102
101
100
THRESHOLD (mV)
99
ISN
98
– V
ISP
V
97
96
95
10
0
DUTY CYCLE (%)
Threshold vs
ISN
20
V
(V)
ISP
100
3518 G02
30
40
50
3518 G04
Switch Current Limit vs
Temperature
3.0
VIN = 5V
2.8
2.6
2.4
CURRENT LIMIT (A)
2.2
2.0
–40 –15
35
–10
TEMPERATURE (°C)
Oscillator Frequency vs
Temperature
2.5
VIN = 5V
= 6.04k
R
T
2.3
2.1
1.9
1.7
OSCILLATOR FREQUENCY (MHz)
60
85
110 135 160
3518 G05
1.5
–40 –20
40
20
0
TEMPERATURE (°C)
60
80
160100 120 140
3518 G06
3518fb
5
LT3518
TYPICAL PERFORMANCE CHARACTERISTICS
V
– V
ISN
= 2V
= 5V
10
Threshold vs V
30
20
V
(V)
ISP
ISP
40
3518 G07
2.02
2.01
(V)
2.00
REF
V
1.99
50
1.98
ISP
105
V
CTRL
104
V
IN
= 25°C
T
A
103
= 1V
V
C
102
101
100
THRESHOLD (mV)
99
ISN
98
– V
ISP
V
97
96
95
0
Reference Voltage vs
Temperature
VIN = 5V
–40
–20 0 20 40
60 80 100 120 140 160
TEMPERATURE (°C)
3518 G08
Quiescent Current vs V
8
TA = 25°C
= 0V
V
C
7
6
5
4
CURRENT (mA)
3
IN
V
2
1
0
0
10
VIN (V)
IN
1.04
1.03
1.02
1.01
1.00
FB PIN THRESHOLD (V)
0.99
0.98
20
30
40
3518 G09
–40
PMOS Turn-On PMOS Turn-Off
5V
PWM
0V
5V
PWM
0V
FB Pin Threshold vs Temperature
VIN = 5V
–20 0 20 40
60 80 140 160100 120
TEMPERATURE (°C)
3518 G10
6
40V
TG
30V
ISP
= 40V
200ns/DIVV
3518 G11
40V
TG
30V
ISP
= 40V
200ns/DIVV
3518 G12
3518fb
PIN FUNCTIONS
LT3518
SW: Switch Pin. Minimize trace at this pin to reduce
EMI.
: Input Supply Pin. Must be locally bypassed.
V
IN
SHDN: Shutdown Pin. Tie to 1.5V or higher to enable
device or 0.4V or less to disable device.
: Reference Output Pin. This pin can supply up to
V
REF
100μA.
: Switching Frequency Adjustment Pin. Set switching
R
T
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.
SYNC: Frequency Synchronization Pin. Tie an external
clock signal here. R
gram a switching frequency 20% slower than SYNC pulse
frequency. Synchronization (power switch turn-on) occurs
a fi xed 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.
resistor should be chosen to pro-
T
CTRL: LED Current Adjustment Pin. Sets voltage across
sense resistor between ISP and ISN. Connect directly to
for full-scale threshold of 100mV, or use signal values
V
REF
between GND and 1V to modulate LED current. Tie the CTRL
pin to the V
: gm Error Amplifi er Output Pin. Stabilize the loop with
V
C
an RC network or compensating C.
FB: Voltage Loop Feedback Pin. Works as overvoltage
protection for LED drivers. If FB is higher than 1V, the
main switch is turned off.
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 amplifi er.
ISP: Current Sense (+) Pin. The noninverting input to the
current sense amplifi er. Also serves as positive rail for
TG pin driver.
TG: Top Gate Driver Output. An inverted PWM signal drives series PMOS device between V
– 7V). An internal 7V clamp protects the V
(V
ISP
gate. Leave TG unconnected if not used.
Ground: Exposed Pad. Solder paddle directly to ground
plane.
pin if not used.
REF
ISP
PMOS
ISP
and
3518fb
7
LT3518
BLOCK DIAGRAM
C
R
SHDN
SENSE
ISP ISN
+
–
X10
CURRENT
SENSE
AMPLIFIER
PV
IN
–
CTRL
FB
V
C
SYNC
1.01V
+
A1
+
1V
+
A2
–
IN
TG
V
MOSFET DRIVER
ERROR
AMPLIFIER
+
A3
+
SS
+
A5
1V
–
V
IN
1V
SS
R
T
10μA
+
A6
+
–
FREQ
ADJUST
2.5MHz TO 250kHz
Q2
LED ARRAY
V
ISP
– 7V
ISP
RAMP
GENERATOR
OSCILLATOR
+
A4
–
PWM
COMPARATOR
V
IN
TGEN
C
FILT
PWM
MAIN SWITCH
DRIVER
R
Q
S
SW SW
Q1
MAIN
SWITCH
+
A8
–
100μA
GND
V
IN
V
REF
+
A7
2V
–
3518 F01
8
Figure 1. Buck Mode LED Driver
3518fb
OPERATION
LT3518
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 amplifi er 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 amplifi er senses the voltage across R
and provides a pre-gain to amplifi er A1. The output of A1
is simply an amplifi ed version of the difference between
the voltage across R
or 100mV. In this manner, the error amplifi er sets the
correct peak switch current level to regulate the current
and the lower of V
SENSE
CTRL
SENSE
/10
through R
more current is delivered to the output; if it decreases,
less current is delivered. The current regulated in R
can be adjusted by changing the input voltage V
The current sense amplifi er provides rail-to-rail current
sense operation. The FB voltage loop is implemented by
the amplifi er A2. When the voltage loop dominates, the
error amplifi er and the amplifi er 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 amplifi er is turned
off so that it does not drive the V
loads on the V
of the V
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
C
FILT
pin will be saved on the external compensation
C
from discharging.
. If the error amplifi er’s output increases,
SENSE
pin. Also, all internal
C
pin are disabled so that the charge state
C
SENSE
CTRL
.
3518fb
9
LT3518
APPLICATIONS INFORMATION
Dimming Control
There are two methods to control the current source for
dimming using the LT3518. The fi rst 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 V
C
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 R
input pin or SYNC pin. When using
T
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 f
= 2MHz). Maximum PWM period is
SW
determined by the system and is unlikely to be longer than
12ms. The maximum PWM dimming ratio (PWM
be calculated from the maximum PWM period (t
the minimum PWM pulse width (t
t
MAX
=
t
MIN
PWM
RATIO
) as follows:
MIN
RATIO
MAX
) can
) and
(1)
Example:
t
MAX
PWM
= 9ms, t
RATIO
= 3μs (fSW = 2MHz)
MIN
= 9ms/3μs = 3000:1
When V
is higher than 1V, the LED current is clamped
CTRL
to be:
LED
100mV
=
R
SENSE
(3)
I
The LED current programming feature possibly increases
V
REF
2V
45.3k
5k
PTC
Figure 2
49.9k
CTRL
3518 F02
total dimming range by a factor of ten.
The CTRL pin should not be left open (tie to V
REF
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
OUT
=
V
R2
+ 1
• 1.01V
(4)
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:
V
=
10 •R
CTRL
SENSE
(2)
I
LED
10
LT3518
Figure 3
V
OUT
R1
FB
R2
3518 F03
3518fb
APPLICATIONS INFORMATION
LT3518
For a buck or a buck-boost confi guration, 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:
OUT
R1
=
R2
• 1.01V + V
LT3518
FB
BE(Q1)
+
R1
V
–
R2
3518 F04
Figure 4
OUT
(5)
R
SENSE
LED
ARRAY
V
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:
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
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
pin of the LT3518. A
IN
1μF or greater capacitor with low ESR should be used. A
ceramic capacitor is usually the best choice.
In the buck mode confi guration, 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:
I
IN(RMS)
= I
•(1–D)•D
LED
(8)
where D is the switch duty cycle. A 2.2μF ceramic type
capacitor is usually suffi cient.
D
LµH
()
D
BUCK
BUCK•tSW
=
V
LED
=
V
IN
is the voltage across the LED string, VIN is the input
V
LED
voltage to the converter, and t
(µs) • VIN–V
()
LED
I
is the switching period.
SW
(6)
In the boost confi guration, the inductor can be estimated
using the formula:
LµH
()
D
BOOST
D
BOOST•tSW
=
V
LED–VIN
=
V
LED
I
(µs) • V
IN
(7)
Output Capacitor Selection
The selection of output capacitor depends on the load
and converter confi guration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output fi lter capacitor should be
sized to attenuate the current ripple.
To achieve the same LED ripple current, the required fi lter
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 suffi cient. For the LED boost and buck-boost
mode applications, a 2.2μF ceramic capacitor is usually
suffi cient. Very high performance PWM dimming applications may require a larger capacitor value to support
the LED voltage during PWM transitions.
3518fb
11
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
suffi cient. Table 2 shows some recommended capacitor
vendors.
Table 2. Ceramic Capacitor Manufacturers
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
amplifi er whose V
output compensates the control loop.
C
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 V
are selected to optimize control loop
C
stability. For typical LED applications, a 10nF compensation
capacitor at V
is adequate, and a series resistor is not
C
required. A compensation resistor may be used to increase
the slew rate on the V
of LED current during fast transients on V
pin to maintain tighter regulation
C
or CTRL.
IN
Table 3. Schottky Diodes
PART NUMBER VR (V) I
On Semiconductor
MBRS260T3 60 2
Diodes Inc.
DFLS140L 40 1
Zetex
ZLLS2000TA 40 2.2
International Rectifi er
10MQ060N 60 1.5
AVE
(A)
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 V
be placed as close as possible to the V
supply to the LT3518 should
IN
terminal of the
IN
device.
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 suffi ciently low leakage current. Table 3 has some
recommended component vendors.
12
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
signifi cantly reduces the start-up current spike and
output voltage overshoot. A typical value for the soft-start
capacitor is 0.1μF.
3518fb
APPLICATIONS INFORMATION
LT3518
Switching Frequency
There are two methods to set the switching frequency of
LT3518. Both methods require a resistor connected at R
pin. Do not leave the R
pin open. Also, do not load this pin
T
T
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 R
See Table 4 below or see the Oscillator Frequency vs R
pin and GND.
T
T
graph in the Typical Performance Characteristics for resistor values and corresponding switching frequencies.
Table 4. Switching Frequency vs R
Switching Frequency (kHz) RT ( kΩ )
250 90.9
500 39.2
1000 16.9
1500 9.53
2000 6.04
2500 4.02
T
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 R
pin and be able
T
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 effi ciency is desired. Selection
of a higher switching frequency will allow use of smaller
value external components and yield a smaller solution
size and profi le.
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.
3518fb
13
LT3518
TYPICAL APPLICATIONS
Buck Mode 1.5A LED Driver
R
SENSE
ISP
V
IN
SHDN
CTRL
PWM
SYNC
TGEN GNDV
68mΩ
LT3518
REFVC
C4
0.1μF
PV
IN
24V
V
IN
3.3V
2.2μF
V
PWM
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
C2
2.2μF
C1
REF
ISN
M1
TG
SW
SS
R
1.5A
FB
T
R
T
16.9k
1MHz
C3
10μF
L1
15μH
D1
C5
0.1μF
3518 TA02a
500mA, 5V to 12V Boost Converter with Accurate Input Current Limit
PWM
5V/DIV
I
LED
1A/DIV
1A/DIV
1000:1 PWM Dimming at 120Hz
I
L
f
I
OSC
LED
= 24V
IN
= 1MHz
= 1.5A
2μs/DIVPV
3518 TA02b
V
IN
5V
C2
2.2MF
V
CTRL
PWM
SHDN
C1: KEMET C0805C225K4RAC
C2: KEMET C1206C106K4RAC
C3: MURATA GRM21BR71H104KA01B
C4: MURATA GCM033R71A103KA03
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-4R3N
SHDN
TGEN
V
10k
C4
10nF
SENSE
TGISP
LT3518
GND SS
ISN
L1
4.3MH
SW
SYNC
FB
R
T
C3
0.1MF
100
Effi ciency
300
200
I
(mA)
LOAD
400
500
3518 TA03b
D1
R1
549k
R2
49.9k
R
T
6.04k
2MHz
3518 TA03a
C2
10MF
V
OUT
12V
500mA
90
80
70
EFFICIENCY (%)
60
50
0
R
50m7
IN
REF
V
C
R3
14
3518fb
TYPICAL APPLICATIONS
LT3518
Buck-Boost Mode LED Driver
V
8V TO 16V
IN
SHDN
PWM
C1
2.2μF
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
0.1μF
PWM
TGEN
V
REF
CTRL
SYNC
V
C4
C
6.04k
2MHz
R
V
T
IN
LT3518
L1
4.3μH
C3
0.1μF
SW
D1
C2
4.7μF
C5
0.22μF
3518 TA04a
R1
FB
ISP
ISN
TG
GNDRTSS
3.92M
R2
124k
300mA
R
SENSE
330mΩ
M1
PWM
5V/DIV
I
LED
200mA/DIV
1A/DIV
3000:1 PWM Dimming at 120Hz Effi ciency
90
VIN = 10V
CTRL = V
80
70
I
L1
IN
f
OSC
I
LED
= 10V
= 2MHz
= 300mA
500ns/DIVV
3518 TA04b
60
50
EFFICIENCY (%)
40
30
20
0
20 40 60 100
REF
80
PWM DUTY CYCLE (%)
3518 TA04c
3518fb
15
LT3518
TYPICAL APPLICATIONS
Buck Mode 1A LED Driver with Open LED Protection and Sync Input
PV
IN
32V
V
3.3V
IN
2.2μF
V
PWM
SYNC
C1
REF
3.3V, 1.2MHz
C2
2.2μF
V
SHDN
CTRL
PWM
SYNC
R
100mΩ
ISP
IN
TGEN GNDV
SENSE
LT3518
REFVC
0.1μF
M1
D1
C5
0.1μF
3518 TA05a
R1
49.9k
Q1
R2
2.00k
C3
10μF
FB
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
M2: PHILIPS PMBT3906
LED1
1A
LED6
R3
5.62k
L1
ISN
TG
SW
SS
C4
10μH
FB
FB
R
T
R
T
16.7k
1MHz
PWM
5V/DIV
I
LED
1A/DIV
I
1A/DIV
1000:1 PWM Dimming at 120Hz Effi ciency
100
CTRL = V
90
80
L1
70
EFFICIENCY (%)
IN
f
OSC
I
LED
= 32V
= 1.2MHz
= 1A
2μs/DIVPV
3518 TA05b
60
50
40
0
20 40 60 80
REF
100
PWM DUTY CYCLE (%)
3518 TA05c
16
3518fb
TYPICAL APPLICATIONS
LT3518
Boost 300mA LED Driver with LED Open Protection
PWM
5V/DIV
I
LED
200mA/DIV
1A/DIV
V
8V TO 16V
L1
R
V
T
8.2μH
IN
LT3518
SW
ISP
ISN
TG
FB
GNDRTSS
C3
0.1μF
IN
C4
0.1μF
SHDN
PWM
TGEN
V
REF
CTRL
SYNC
V
C
16.9k
1MHz
PWM
C1
2.2μF
D1
R1
1M
300mA
R2
30.1k
R
SENSE
330mΩ
M1
C2
6.8μF
LED1
C1: KEMET C1206C225K2RAC
LED2
C2: TDK C5750X7R1H685M
C3, C4: MURATA GRM21BR71H104KA01B
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-8R2N
LED8
LEDS: LUXEON I (WHITE)
M1: ZETEX ZXMP6A13GTA
3518 TA06a
3000:1 PWM Dimming at 100Hz Effi ciency
100
90
80
70
I
L1
3518 TA06b
f
I
IN
OSC
LED
= 12V
= 1MHz
= 300mA
1μs/DIVV
60
50
EFFICIENCY (%)
40
30
20
0
20
60
40
PWM DUTY CYCLE (%)
VIN = 12V
CTRL = V
80
REF
100
3518 TA06c
3518fb
17
LT3518
PACKAGE DESCRIPTION
UF Package
16-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1692)
0.72 ±0.05
4.35 ± 0.05
2.90 ± 0.05
2.15 ± 0.05
(4 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 ± 0.10
(4 SIDES)
PIN 1
TOP MARK
(NOTE 6)
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
0.30 ±0.05
0.65 BSC
PACKAGE OUTLINE
0.75 ± 0.05
2.15 ± 0.10
(4-SIDES)
0.200 REF
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
R = 0.115
TYP
1615
PIN 1 NOTCH R = 0.20 TYP
OR 0.35 × 45° CHAMFER
0.55 ± 0.20
1
2
(UF16) QFN 1004
0.30 ± 0.05
0.65 BSC
18
3518fb
PACKAGE DESCRIPTION
2.74
(.108)
FE Package
16-Lead Plastic TSSOP
(Reference LTC DWG # 05-08-1663)
16 1514 13 12 11
4.90 – 5.10*
(.193 – .201)
2.74
(.108)
LT3518
10 9
6.60 ±0.10
4.50 ±0.10
RECOMMENDED SOLDER PAD LAYOUT
0.09 – 0.20
(.0035 – .0079)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
SEE NOTE 4
0.65 BSC
4.30 – 4.50*
(.169 – .177)
0.50 – 0.75
(.020 – .030)
MILLIMETERS
(INCHES)
(.108)
0.45 ±0.05
2.74
1.05 ±0.10
1345678
2
0.25
REF
0° – 8°
0.65
(.0256)
BSC
0.195 – 0.30
(.0077 – .0118)
TYP
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
2.74
(.108)
1.10
(.0433)
MAX
0.05 – 0.15
(.002 – .006)
FE16 (BA) TSSOP 0204
6.40
(.252)
BSC
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.
3518fb
19
LT3518
TYPICAL APPLICATION
5.5V SEPIC Converter with Short-Circuit Protection
C5
10μF
SW
FB
TG
ISP
ISN
R
T
C3
0.1μF
2.4μH
R
T
6.04k
2MHz
D1
L2
V
IN
3V
C2
2.2μF
SHDN
C4
10nF
C1: KEMET C0805C225K4RAC
C2, C5: KEMET C1206C106K4RAC
C3: MURATA GRM21BR71H104KA01B
C4: MURATA GCM033R71A103KA03
D1: ZETEX ZLLS2000TA
L1, L2: TOKO 962BS-2R4M
PWM
V
IN
CTRL
SYNC
SHDN
LT3518
TGEN
V
REF
VCGND SS
R3
10k
L1
2.4μH
C2
10μF
R
SENSE
0.15Ω
3518 TA07a
R1
221k
R2
49.9k
V
OUT
5.5V
500mA
100
90
80
70
60
EFFICIENCY (%)
50
40
30
0
Effi ciency
100 200 300 500
I
LOAD
400
(mA)
3518 TA07b
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LT1618 Constant Current, 1.4MHz, 1.5A Boost Converter V
: 5V to 18V, V
IN
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 (I
LT3475 Dual 1.5A (I
LT3476 Quad Output 1.5A, 36V, 2MHz High Current LED Driver
with 1,000:1 Dimming
LT3477 3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver V
), 2MHz, Step-Down LED Driver VIN: 4V to 36V, V
LED
), 36V 2MHz Step-Down LED Driver VIN: 4V to 36V, V
LED
I
< 1μA, TSSOP16E Package
SD
I
< 1μA, TSSOP20E Package
SD
: 2.8V to 16V, V
V
IN
I
< 10μA, 5mm × 7mm QFN Package
SD
: 2.5V to 25V, V
IN
QFN, TSSOP20E Packages
LT3478/LT3478-1 4.5A, 42V, 2.5MHz High Current LED Driver with 3,000:1
Dimming
LT3479 3A, Full Featured DC/DC Converter with Soft-Start and
Inrush Current Protection
VIN: 2.8V to 36V, V
I
< 3μA, TSSOP16E Packages
SD
: 2.5V to 24V, V
V
IN
DFN and TSSOP Packages
LT3486 Dual 1.3A, 2MHz High Current LED Driver VIN: 2.5V to 24V, V
I
< 1μA, 5mm × 3mm DFN, TSSOP16E
SD
LT3496 Triple Output LED Driver V
: 3V to 40V, V
IN
I
< 10μA, 4mm × 5mm QFN Package
SD
LT3517 Full-Featured LED Driver with 1.5A Switch Current VIN: 3V to 40V, V
ISD < 1μA, 4mm × 4mm QFN and TSSOP Packages
LT3590 48V Buck Mode 50mA LED Driver V
LT3595 16 Channel Buck LED Driver Mode V
: 4.5V to 55V, Drives Up to 10 LEDs, 200:1 Dimming,
IN
I
= 15mA, 2mm × 2mm DFN SC70
SO
: 4.5V to 45V, Drives Up to 160 LEDs, 5000:1 Dimming,
IN
5mm × 9mm QFN
LT C®3783 High Current LED Controller VIN: 3V to 36V, V
I
< 20μA, 5mm × 4mm QFN10, TSSOP16E Packages
SD
= 36V, Dimming = Analog/PWM, ISD < 1μA,
OUT(MAX)
= 13.5V, Dimming = 400:1 True Color PWM,
OUT(MAX)
= 13.5V, Dimming = 3000:1 True Color PWM,
OUT(MAX)
= 36V, Dimming = 1000:1 True Color PWM,
OUT(MAX)
= 40V, Dimming = Analog/PWM, ISD < 1μA,
OUT(MAX)
= 42V, Dimming = 3000:1 True Color PWM,
OUT(MAX)
= 40V, IQ = 6.5mA, ISD < 1μA,
OUT(MAX)
= 36V, Dimming = 1000:1 True Color PWM,
OUT(MAX)
= 45V, Dimming = 3000:1 True Color PWM,
OUT(MAX)
= 45V, Dimming = 5000:1 True Color PWM,
OUT(MAX)
= Ext FET, Dimming = 3000:1 True Color PWM,
OUT(MAX)
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
3518fb
LT 0308 REV B • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 2007
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