3518 [Linear]
Full-Featured LED Driver with 2.3A Switch Current; 全功能的LED驱动器，具有2.3A电流开关
| 型号： | 3518 |
| 厂家： | 
Linear |
| 描述： | Full-Featured LED Driver with 2.3A Switch Current |
| 文件： | 总20页 (文件大小：270K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3518
Full-Featured LED Driver
with 2.3A Switch Current
FEATURES
DESCRIPTION
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. Pro-
grammableswitchingfrequencyallowsoptimizationofthe
externalcomponentsforefficiencyorcomponentsize.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.
n
3000:1 True Color PWM™ Dimming Ratio
n
2.3A, 45V Internal Switch
n
100mV High Side Current Sense
Open LED Protection
Adjustable Frequency: 250kHz to 2.5MHz
Wide Input Voltage Range:
n
n
n
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*
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
n
n
n
n
The LT3518 is available in the tiny footprint 16-Lead QFN
(4mm×4mm)andthe16-PinTSSOPpackage.TheLT3518
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.
APPLICATIONS
n
Display Backlighting
n
Automotive and Avionic Lighting
n
Illumination
Scanners
n
TYPICAL APPLICATION
1.5A Buck Mode LED Driver
Efficiency
100
10μF
68mΩ
M1
CTRL = V
REF
PV
IN
24V
90
2.2μF
1.5A
15μH
80
70
ISP
ISN TG
V
IN
V
SW
IN
3.3V
2.2μF
SHDN
LT3518
V
60
50
40
CTRL
PWM
FB
REF
PWM
SS
SYNC
TGEN V
R
T
GND
V
REF
C
16.9k
1MHz
0
20
40
60
80
100
0.1μF
0.1μF
PWM DUTY CYCLE (%)
3518 TA01a
3518 TA01b
3518fb
1
LT3518
ABSOLUTE MAXIMUM RATINGS
V , SHDN, PWM, TGEN (Note 3)..............................40V
IN
(Note 1)
Storage Temperature Range
SW, ISP, ISN, TG........................................................45V
TG Pin Below ISP Pin................................................10V
FB, SYNC, SS, CTRL ...................................................6V
V , R , V .................................................................3V
QFN.................................................... –65°C to 125°C
TSSOP ............................................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
TSSOP .............................................................. 300°C
C
T
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
TOP VIEW
V
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
SW
SW
TG
IN
16 15 14 13
SHDN
SW
SW
1
2
3
4
12 FB
V
REF
11
10
9
V
C
R
T
ISP
ISN
TGEN
FB
17
GND
17
V
CTRL
PWM
IN
SYNC
SS
SHDN
5
6
7
8
PWM
CTRL
V
C
UF PACKAGE
16-LEAD (4mm × 4mm) PLASTIC QFN
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
= 125°C, θ = 36°C/W
JA
JMAX
T
= 150°C, θ = 40°C/W, θ = 10°C/W
JC(PAD)
JMAX
JA
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3518EUF#PBF
LT3518IUF#PBF
LT3518EFE#PBF
LT3518IFE#PBF
LT3518HFE#PBF
TAPE AND REEL
PART MARKING*
3518
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
LT3518EUF#TRPBF
LT3518IUF#TRPBF
LT3518EFE#TRPBF
LT3518IFE#TRPBF
LT3518HFE#TRPBF
16-Lead (4mm × 4mm) Plastic QFN
16-Lead (4mm × 4mm) Plastic QFN
16-Lead Plastic TSSOP
3518
3518FE
3518FE
16-Lead Plastic TSSOP
3518HFE
16-Lead Plastic TSSOP
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/
3518fb
2
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
Minimum V Operating Voltage
CONDITIONS
MIN
TYP
MAX
UNITS
3
V
V
IN
Maximum V Operating Voltage
Continuous Operation (Note 3)
30
96
IN
l
Current Sense Voltage (V – V
)
ISN
V
V
= 2V, V = 24V, V = 1V
100
100
103
mV
mV
ISP
CTRL
CTRL
ISP
C
= 2V, V = 0V, V = 1V
ISP
C
10% Scale Current Sense Voltage (V – V
)
ISN
V
= 100mV, V = 24V, V = 1V
9
mV
ISP
CTRL
ISP
C
Current Sense Voltage Line Regulation
2V < V < 45V
0.03
%/V
ISP
V
IN
Supply Current
PWM > 1.5V, V = 0V
6
4.5
0.1
10
1
mA
mA
μA
C
PWM = 0V
SHDN = 0V
l
Switching Frequency
R = 16.7k
0.85
2.25
220
1.0
2.5
250
1.15
2.7
270
MHz
MHz
kHz
T
R = 4.03k
T
R = 91.5k
T
R Voltage
1
9
V
μA
μA
V
T
Soft-Start Pin Current
SYNC Pull-Down Current (Into the Pin)
SYNC Input Low
SS = 0.5V, Out of Pin
6
12
V
SYNC
= 2V
60
0.4
SYNC Input High
1.5
V
Maximum Duty Cycle
R = 91.5k (250kHz)
95
94
85
97
96
90
74
%
%
%
%
T
SYNC = 300kHz Clock Signal, R = 91.5k
T
l
R = 16.7k (1MHz)
T
R = 4.03k (2.5MHz)
T
Switch Current Limit
2.3
2.8
3.5
A
mV
μA
nA
μS
kΩ
nA
nA
V
Switch V
I
= 1.5A
SW
400
CESAT
Switch Leakage Current
V
= 45V, PWM = 0V
2
SW
CTRL Input Bias Current
Current Out of Pin, V
= 0.1V
20
550
1000
0
100
CTRL
Error Amplifier Transconductance
V Output Impedance
C
V Idle Input Bias Current
C
PWM = 0, V = 1V
–20
20
C
FB Pin Input Bias Current
FB Pin Threshold
Current Out of Pin, V = 0.5V
20
100
1.04
300
FB
l
0.98
1.01
ISP, ISN Idle Input Bias Current
ISP, ISN Full-Scale Input Bias Current
SHDN Voltage High
PWM = 0V
nA
μA
V
ISP Tied to ISN, V = 24V, V
= 2V
20
60
60
ISP
CTRL
1.5
1.5
SHDN Voltage Low
0.4
V
SHDN Pin Bias Current
PWM Input High Voltage
PWM Input Low Voltage
PWM Pin Bias Current
TGEN Input High Voltage
TGEN Input Low Voltage
TGEN Pin Bias Current
100
μA
V
0.4
V
120
μA
V
1.5
0.4
200
2.04
V
TGEN = 5V
100
2
μA
V
l
V
REF
Pin Voltage
I
= –100μA
1.96
REF
3518fb
3
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
Pin Voltage Line Regulation
CONDITIONS
3V < V < 40V
MIN
TYP
MAX
UNITS
%/V
ns
V
REF
0.03
IN
Gate Turn-On Delay
Gate Turn-Off Delay
C
C
V
= 1nF Between ISP and TG
= 1nF Between ISP and TG
200
200
LOAD
LOAD
ns
Top Gate Drive V (V – V
)
TG
= 24V, TGEN = 5V
ISP
7
0
V
V
GS ISP
PWM = 0V
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 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.
Note 3: Absolute maximum voltage at V , 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.
IN
3518fb
4
LT3518
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Current Limit
vs Duty Cycle
VISP – VISN Threshold vs VCTRL
3.0
2.5
120
100
80
60
40
20
0
V
V
V
= 5V
IN
= 24V
ISP
= 1V
C
A
T
= 25°C
2.0
1.5
1.0
0.5
0
T
= 25°C
A
1.0 1.2
(V)
0
0.2 0.4 0.6 0.8
1.4 1.6
0
20
40
60
80
100
V
DUTY CYCLE (%)
CTRL
3518 G01
3518 G02
VISP – VISN Threshold vs
Temperature
Oscillator Frequency vs RT
10000
1000
100
105
104
103
102
101
100
99
T
= 25°C
V
V
= 2V
A
CTRL
IN
= 5V
T
= 25°C
= 1V
A
V
C
98
97
96
95
1
10
100
0
10
20
V
ISP
30
(V)
40
50
R
(kΩ)
T
3518 G03
3518 G04
Switch Current Limit vs
Temperature
Oscillator Frequency vs
Temperature
3.0
2.8
2.6
2.4
2.2
2.0
2.5
2.3
2.1
1.9
1.7
1.5
V
= 5V
V
= 5V
= 6.04k
IN
IN
T
R
–40 –15 –10 35 60 85 110 135 160
–40 –20
0
20 40 60 80 100 120 140 160
TEMPERATURE (°C)
TEMPERATURE (°C)
3518 G05
3518 G06
3518fb
5
LT3518
TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage vs
Temperature
VISP – VISN Threshold vs VISP
105
104
103
102
101
100
99
2.02
2.01
2.00
1.99
V
V
= 2V
V
= 5V
CTRL
IN
IN
= 5V
T
= 25°C
= 1V
A
V
C
98
97
96
1.98
95
–40 –20
0
20 40 60 80 100 120 140 160
TEMPERATURE (°C)
0
10
20
V
30
(V)
40
50
ISP
3518 G08
3518 G07
FB Pin Threshold vs Temperature
Quiescent Current vs VIN
1.04
1.03
1.02
8
7
6
5
4
3
2
1
0
T
= 25°C
V
= 5V
A
C
IN
V
= 0V
1.01
1.00
0.99
0.98
–40 –20
0
20 40 60 80 100 120 140 160
TEMPERATURE (°C)
20
(V)
0
10
30
40
V
IN
3518 G10
3518 G09
PMOS Turn-On
PMOS Turn-Off
5V
5V
0V
PWM
PWM
0V
40V
40V
TG
30V
TG
30V
3518 G11
3518 G12
V
= 40V
200ns/DIV
V
ISP
= 40V
200ns/DIV
ISP
3518fb
6
LT3518
PIN FUNCTIONS
SW: Switch Pin. Minimize trace at this pin to reduce
CTRL: LED Current Adjustment Pin. Sets voltage across
EMI.
sense resistor between ISP and ISN. Connect directly to
V
forfull-scalethresholdof100mV,orusesignalvalues
REF
V : Input Supply Pin. Must be locally bypassed.
IN
betweenGNDand1VtomodulateLEDcurrent.TietheCTRL
pin to the V pin if not used.
SHDN: Shutdown Pin. Tie to 1.5V or higher to enable
device or 0.4V or less to disable device.
REF
V : g Error Amplifier Output Pin. Stabilize the loop with
C
m
V
: Reference Output Pin. This pin can supply up to
REF
100μA.
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.
R : Switching Frequency Adjustment Pin. Set switching
T
frequencyusingaresistortoGND(seeTypicalPerformance
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.
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.
SYNC: Frequency Synchronization Pin. Tie an external
clock signal here. R resistor should be chosen to pro-
T
ISN: Current Sense (–) Pin. The inverting input to the
current sense amplifier.
gram a switching frequency 20% slower than SYNC pulse
frequency.Synchronization(powerswitchturn-on)occurs
a fixed delay after the rising edge of SYNC. Tie the SYNC
pin to ground if this feature is not used.
ISP: Current Sense (+) Pin. The noninverting input to the
current sense amplifier. Also serves as positive rail for
TG pin driver.
SS: Soft-Start Pin. Place a soft-start capacitor here. Leave
the pin open if not in use.
TG: Top Gate Driver Output. An inverted PWM sig-
nal drives series PMOS device between V
and
ISP
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.
(V – 7V). An internal 7V clamp protects the V PMOS
ISP
ISP
gate. Leave TG unconnected if not used.
Ground: Exposed Pad. Solder paddle directly to ground
plane.
3518fb
7
LT3518
BLOCK DIAGRAM
LED ARRAY
C
IN
C
FILT
R
SENSE
PV
IN
ISP
ISN
TG
TGEN
PWM
SW SW
+
–
V
ISP
CURRENT
SENSE
AMPLIFIER
X10
V
– 7V
ISP
SHDN
MOSFET DRIVER
–
+
+
CTRL
A1
A2
ERROR
AMPLIFIER
1V
MAIN SWITCH
DRIVER
+
+
+
–
A3
Q1
MAIN
SWITCH
A4
R
Q
1.01V
S
+
–
FB
PWM
COMPARATOR
+
V
C
A8
GND
–
SS
SYNC
RAMP
GENERATOR
+
V
IN
A5
2.5MHz TO 250kHz
OSCILLATOR
100μA
1V
–
V
REF
V
IN
+
–
10μA 1V
+
+
–
SS
A7
A6
Q2
2V
FREQ
ADJUST
R
T
V
IN
3518 F01
Figure 1. Buck Mode LED Driver
3518fb
8
LT3518
OPERATION
The LT3518 is a constant frequency, current mode regula-
tor with an internal power switch. Operation can be best
understoodbyreferringtotheBlockDiagraminFigure 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.Whicheverfeedbackinputislowertakesprecedence,
and forces the converter into either constant-current or
constant-voltage mode. The LT3518 is designed to transi-
tion cleanly between these two modes of operation. The
through R
. If the error amplifier’s output increases,
SENSE
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 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).
SENSE
.
CTRL
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 V pin. Also, all internal
C
loads on the V pin are disabled so that the charge state
C
of the V pin will be saved on the external compensation
C
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
current sense amplifier senses the voltage across R
SENSE
and provides a pre-gain to amplifier A1. The output of A1
is simply an amplified version of the difference between
the voltage across R
and the lower of V
/10
CTRL
SENSE
or 100mV. In this manner, the error amplifier sets the
correct peak switch current level to regulate the current
C
from discharging.
FILT
3518fb
9
LT3518
APPLICATIONS INFORMATION
Dimming Control
When V
to be:
is higher than 1V, the LED current is clamped
(3)
CTRL
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 aver-
age current. To make this method of current control more
100mV
ILED
=
RSENSE
The LED current programming feature possibly increases
accurate, the switch demand current is stored on the V
C
2V
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
V
REF
45.3k
49.9k
CTRL
5k
PTC
3518 F02
frequency through R input pin or SYNC pin. When using
T
the SYNC function, the SYNC and PWM signals must have
thealignedrisingedgestoachievetheoptimizedhighPWM
dimming ratio. For best current accuracy, the minimum
PWM low or high time should be at least six switching
Figure 2
total dimming range by a factor of ten.
The CTRL pin should not be left open (tie to V
if not
REF
cycles (3μs for f = 2MHz). Maximum PWM period is
SW
used). The CTRL pin can also be used in conjunction with
a PTC thermistor to provide overtemperature protection
for the LED load.
determined by the system and is unlikely to be longer than
12ms.ThemaximumPWMdimmingratio(PWM
)can
) and
RATIO
be calculated from the maximum PWM period (t
MAX
the minimum PWM pulse width (t ) as follows:
MIN
Setting Output Voltage
tMAX
tMIN
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:
PWMRATIO
=
(1)
Example:
= 9ms, t
R1
R2
ꢀ
ꢁ
ꢃ
ꢄ
t
= 3μs (f = 2MHz)
SW
MAX
MIN
VOUT
=
+1 •1.01V
(4)
ꢂ
ꢅ
PWM
= 9ms/3μs = 3000:1
RATIO
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
OUT
R1
LT3518
FB
R2
3518 F03
VCTRL
10 •RSENSE
ILED
=
(2)
Figure 3
3518fb
10
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:
Table 1 provides some recommended inductor vendors.
Table 1. Inductor Manufacturers
VENDOR
Sumida
Toko
PHONE
WEB
(408) 321-9660
(408) 432-8281
(561) 998-4100
(402) 563-6866
www.sumida.com
www.toko.com
www.cooperet.com
www.vishay.com
R1
VOUT
=
•1.01V + VBE(Q1)
(5)
Cooper
Vishay
R2
+
R1
R
SENSE
Input Capacitor Selection
V
OUT
LED
ARRAY
For proper operation, it is necessary to place a bypass
–
LT3518
FB
capacitor to GND close to the V pin of the LT3518. A
IN
1μF or greater capacitor with low ESR should be used. A
R2
ceramic capacitor is usually the best choice.
3518 F04
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
switchisoff. Forbestreliability, thiscapacitorshouldhave
low ESR and ESL and have an adequate ripple current
rating. The RMS input current is:
Figure 4
Inductor Selection
The inductor used with the LT3518 should have a satura-
tion 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:
IIN(RMS) =ILED • (1–D)•D
(8)
where D is the switch duty cycle. A 2.2μF ceramic type
capacitor is usually sufficient.
D
BUCK • tSW(µs) • V – V
(
)
Output Capacitor Selection
IN LED
L µH =
(
)
(6)
ꢀI
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.
VLED
V
DBUCK
=
IN
V
is the voltage across the LED string, V is the input
IN
LED
voltage to the converter, and t is the switching period.
ToachievethesameLEDripplecurrent, therequiredfilter
capacitorvalueislargerintheboostandbuck-boostmode
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 appli-
cations may require a larger capacitor value to support
the LED voltage during PWM transitions.
SW
In the boost configuration, the inductor can be estimated
using the formula:
DBOOST • tSW(µs) • V
IN
L µH =
(
)
(7)
ꢀI
VLED – V
IN
DBOOST
=
VLED
3518fb
11
LT3518
APPLICATIONS INFORMATION
UseonlyceramiccapacitorswithX7R,X5Rorbetterdielec-
tric 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
PART NUMBER
On Semiconductor
MBRS260T3
V (V)
I
(A)
R
AVE
60
40
40
60
2
Diodes Inc.
DFLS140L
1
Zetex
ZLLS2000TA
2.2
1.5
International Rectifier
10MQ060N
Table 2. Ceramic Capacitor Manufacturers
VENDOR
Taiyo Yuden
AVX
PHONE
WEB
Board Layout
(408) 573-4150
(843) 448-9411
(770) 436-1300
(847) 803-6100
www.t-yuden.com
www.avxcorp.com
www.murata.com
www.tdk.com
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,
resistanceinserieswiththeseinputsshouldbeminimized
and matched, otherwise there will be an offset. Finally, the
Murata
TDK
Loop Compensation
The LT3518 uses an internal transconductance error
amplifier whose V output compensates the control loop.
C
The external inductor, output capacitor, and the compen-
sation 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.FortypicalLEDapplications,a10nFcompensation
capacitor at V is adequate, and a series resistor is not
C
bypass capacitor on the V supply to the LT3518 should
IN
required.Acompensationresistormaybeusedtoincrease
be placed as close as possible to the V terminal of the
IN
the slew rate on the V pin to maintain tighter regulation
C
device.
of LED current during fast transients on V or CTRL.
IN
Soft-Start
Diode Selection
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
outputvoltageovershoot.Atypicalvalueforthesoft-start
capacitor is 0.1μF.
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
increaseswiththetemperature,fromtheoutputduringthe
PWM low interval. Therefore, choose the Schottky diode
with sufficiently low leakage current. Table 3 has some
recommended component vendors.
3518fb
12
LT3518
APPLICATIONS INFORMATION
Switching Frequency
to generate a switching frequency 20% lower than the
external clock when external clock is absent.
There are two methods to set the switching frequency of
LT3518. Both methods require a resistor connected at R
In general, a lower switching frequency should be used
where either very high or very low switching duty cycle
operationisrequired,orhighefficiencyisdesired.Selection
of a higher switching frequency will allow use of smaller
value external components and yield a smaller solution
size and profile.
T
pin. Do not leave the R pin open. Also, do not load this pin
T
with a capacitor. A resistor must always be connected for
proper operation. One way to set the frequency is simply
connectinganexternalresistorbetweentheR pinandGND.
T
See Table 4 below or see the Oscillator Frequency vs R
T
graphintheTypicalPerformanceCharacteristicsforresis-
Thermal Considerations
tor values and corresponding switching frequencies.
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
voltagestoensurethatthemaximumjunctiontemperature
is not exceeded. This junction limit is especially important
whenoperatingathighambienttemperatures.TheExposed
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
directlyunderthepackagetospreadouttheheatdissipated
by the LT3518.
Table 4. Switching Frequency vs RT
Switching Frequency (kHz)
R ( kΩ )
T
250
500
90.9
39.2
16.9
9.53
6.04
4.02
1000
1500
2000
2500
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
3518fb
13
LT3518
TYPICAL APPLICATIONS
Buck Mode 1.5A LED Driver
C3
10μF
R
SENSE
M1
68mΩ
1000:1 PWM Dimming at 120Hz
PV
IN
24V
C2
2.2μF
L1
1.5A
15μH
PWM
5V/DIV
D1
ISP
ISN TG
V
IN
V
SW
IN
3.3V
C1
2.2μF
I
SHDN
LED
1A/DIV
LT3518
V
CTRL
PWM
SYNC
FB
REF
I
L
PWM
SS
1A/DIV
3518 TA02b
R
T
PV = 24V
2μs/DIV
IN
TGEN V
V
GND
f
I
= 1MHz
= 1.5A
REF
C4
C
OSC
LED
R
T
C5
0.1μF
16.9k
1MHz
0.1μF
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
R
L1
Efficiency
SENSE
D1
50m7 4.3MH
V
OUT
V
IN
12V
90
80
70
60
50
5V
500mA
C2
2.2MF
R1
549k
ISP TG ISN
SW
FB
V
IN
CTRL
PWM
SHDN
TGEN
C2
10MF
LT3518
GND
SHDN
SYNC
R2
49.9k
V
R
T
REF
V
C
SS
R3
10k
C4
10nF
R
6.04k
2MHz
T
C3
0.1MF
0
100
200
300
(mA)
400
500
I
LOAD
3518 TA03b
3518 TA03a
C1: KEMET C0805C225K4RAC
C2: KEMET C1206C106K4RAC
C3: MURATA GRM21BR71H104KA01B
C4: MURATA GCM033R71A103KA03
D1: ZETEX ZLLS2000TA
L1: TOKO B992AS-4R3N
3518fb
14
LT3518
TYPICAL APPLICATIONS
Buck-Boost Mode LED Driver
L1
4.3μH
D1
V
IN
8V TO 16V
C5
0.22μF
R1
3.92M
SHDN
V
IN
SW
FB
R2
124k
300mA
PWM
PWM
TGEN
ISP
LT3518
R
SENSE
330mΩ
V
REF
ISN
TG
CTRL
SYNC
C1
2.2μF
C2
4.7μF
M1
V
R
SS
T
GND
C
R
T
6.04k
2MHz
C4
0.1μF
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
V
= 10V
PWM
5V/DIV
IN
CTRL = V
REF
80
I
LED
200mA/DIV
70
60
50
40
30
20
I
L1
1A/DIV
3518 TA04b
V
= 10V
500ns/DIV
IN
f
I
= 2MHz
OSC
LED
= 300mA
20
40
60
100
0
80
PWM DUTY CYCLE (%)
3518 TA04c
3518fb
15
LT3518
TYPICAL APPLICATIONS
Buck Mode 1A LED Driver with Open LED Protection and Sync Input
R
SENSE
M1
100mΩ
PV
IN
32V
C2
2.2μF
R1
LED1
1A
49.9k
C3
10μF
Q1
FB
R2
2.00k
LED6
R3
5.62k
L1
10μH
D1
ISP
ISN TG
V
IN
V
SW
IN
3.3V
C1
2.2μF
SHDN
LT3518
V
FB
CTRL
PWM
FB
REF
C1: KEMET C0806C225K4RAC
C2: MURATA GRM31MR71E225KA93
C3: MURATA GRM32DR71E106KA12B
C4, C5: MURATA GRM21BR71H104KA01B
D1: ZETEX ZLLS2000TA
PWM
SS
SYNC
3.3V, 1.2MHz
SYNC
TGEN V
R
T
GND
V
REF
C4
C
R
16.7k
1MHz
T
C5
L1: TOKO B992AS-100M
0.1μF
LEDS: LUXEON III (WHITE)
M1: ZETEX ZXMP6A13GTA
0.1μF
3518 TA05a
M2: PHILIPS PMBT3906
1000:1 PWM Dimming at 120Hz
Efficiency
100
CTRL = V
REF
PWM
5V/DIV
90
I
LED
1A/DIV
80
70
I
L1
1A/DIV
60
50
40
3518 TA05b
PV = 32V
2μs/DIV
IN
f
I
= 1.2MHz
= 1A
OSC
LED
0
20
40
60
80
100
PWM DUTY CYCLE (%)
3518 TA05c
3518fb
16
LT3518
TYPICAL APPLICATIONS
Boost 300mA LED Driver with LED Open Protection
L1
8.2μH
D1
V
IN
8V TO 16V
SHDN V
PWM
SW
ISP
IN
PWM
R
SENSE
TGEN
330mΩ
LT3518
V
ISN
TG
FB
REF
CTRL
SYNC
M1
C1
2.2μF
C2
6.8μF
R1
LED1
1M
V
C
R
SS
GND
C1: KEMET C1206C225K2RAC
C2: TDK C5750X7R1H685M
C3, C4: MURATA GRM21BR71H104KA01B
D1: ZETEX ZLLS2000TA
T
LED2
R
T
300mA
16.9k
1MHz
R2
30.1k
L1: TOKO B992AS-8R2N
C4
0.1μF
C3
0.1μF
LED8
LEDS: LUXEON I (WHITE)
M1: ZETEX ZXMP6A13GTA
3518 TA06a
3000:1 PWM Dimming at 100Hz
Efficiency
100
90
80
70
60
50
40
30
PWM
5V/DIV
I
LED
200mA/DIV
I
L1
1A/DIV
3518 TA06b
V
= 12V
1μs/DIV
IN
f
I
= 1MHz
OSC
LED
= 300mA
V
= 12V
IN
CTRL = V
REF
20
0
20
40
60
80
100
PWM DUTY CYCLE (%)
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)
PACKAGE OUTLINE
0.30 0.05
0.65 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH R = 0.20 TYP
OR 0.35 × 45° CHAMFER
0.75 0.05
R = 0.115
TYP
4.00 0.10
(4 SIDES)
15
16
0.55 0.20
PIN 1
TOP MARK
(NOTE 6)
1
2
2.15 0.10
(4-SIDES)
(UF16) QFN 1004
0.200 REF
0.30 0.05
0.65 BSC
0.00 – 0.05
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
3518fb
18
LT3518
PACKAGE DESCRIPTION
FE Package
16-Lead Plastic TSSOP
(Reference LTC DWG # 05-08-1663)
4.90 – 5.10*
(.193 – .201)
2.74
(.108)
2.74
(.108)
16 1514 13 12 1110
9
6.60 0.10
2.74
(.108)
4.50 0.10
SEE NOTE 4
6.40
(.252)
BSC
2.74
(.108)
0.45 0.05
1.05 0.10
0.65 BSC
5
7
8
1
2
3
4
6
RECOMMENDED SOLDER PAD LAYOUT
1.10
(.0433)
MAX
4.30 – 4.50*
(.169 – .177)
0.25
REF
0° – 8°
0.65
(.0256)
BSC
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
0.05 – 0.15
(.002 – .006)
0.195 – 0.30
FE16 (BA) TSSOP 0204
(.0077 – .0118)
TYP
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
MILLIMETERS
(INCHES)
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
3518fb
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 representa-
tion 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
100
90
C5
10μF
L1
2.4μH
R
SENSE
0.15Ω
D1
V
OUT
V
IN
3V
5.5V
500mA
80
70
60
50
40
30
C2
10μF
C2
R1
L2
2.2μF
221k
2.4μH
V
PWM
SW
FB
IN
CTRL
SYNC
SHDN
TGEN
TG
ISP
ISN
SHDN
LT3518
V
R
T
REF
R2
49.9k
V
GND
SS
C
0
100
200
I
300
(mA)
400
500
R3
10k
C4
10nF
R
T
C3
0.1μF
LOAD
6.04k
2MHz
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
V : 5V to 18V, V
LT1618
Constant Current, 1.4MHz, 1.5A Boost Converter
= 36V, Dimming = Analog/PWM, I < 1μA,
OUT(MAX) SD
IN
MSOP10 Package
LT3003
3-Channel LED Ballaster with PWM Dimming
V : 3V to 48V, Dimming = 3000:1 True Color PWM, I < 5μA,
IN
SD
MSOP10 Package
LT3474
36V, 1A (I ), 2MHz, Step-Down LED Driver
V : 4V to 36V, V
SD
= 13.5V, Dimming = 400:1 True Color PWM,
LED
IN
OUT(MAX)
I
< 1μA, TSSOP16E Package
LT3475
Dual 1.5A (I ), 36V 2MHz Step-Down LED Driver
V : 4V to 36V, V
SD
= 13.5V, Dimming = 3000:1 True Color PWM,
LED
IN
OUT(MAX)
I
< 1μA, TSSOP20E Package
LT3476
Quad Output 1.5A, 36V, 2MHz High Current LED Driver
with 1,000:1 Dimming
V : 2.8V to 16V, V
SD
= 36V, Dimming = 1000:1 True Color PWM,
IN
OUT(MAX)
I
< 10μA, 5mm × 7mm QFN Package
LT3477
3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver
V : 2.5V to 25V, V
= 40V, Dimming = Analog/PWM, I < 1μA,
IN
OUT(MAX)
SD
QFN, TSSOP20E Packages
LT3478/LT3478-1
LT3479
4.5A, 42V, 2.5MHz High Current LED Driver with 3,000:1 V : 2.8V to 36V, V
= 42V, Dimming = 3000:1 True Color PWM,
IN
OUT(MAX)
Dimming
I
< 3μA, TSSOP16E Packages
SD
3A, Full Featured DC/DC Converter with Soft-Start and
Inrush Current Protection
V : 2.5V to 24V, V
= 40V, I = 6.5mA, I < 1μA,
IN
OUT(MAX) Q SD
DFN and TSSOP Packages
V : 2.5V to 24V, V = 36V, Dimming = 1000:1 True Color PWM,
OUT(MAX)
LT3486
Dual 1.3A, 2MHz High Current LED Driver
IN
SD
I
< 1μA, 5mm × 3mm DFN, TSSOP16E
LT3496
Triple Output LED Driver
V : 3V to 40V, V
SD
= 45V, Dimming = 3000:1 True Color PWM,
IN
OUT(MAX)
I
< 10μA, 4mm × 5mm QFN Package
LT3517
Full-Featured LED Driver with 1.5A Switch Current
48V Buck Mode 50mA LED Driver
16 Channel Buck LED Driver Mode
High Current LED Controller
V : 3V to 40V, V
SD
= 45V, Dimming = 5000:1 True Color PWM,
IN
OUT(MAX)
I
< 1μA, 4mm × 4mm QFN and TSSOP Packages
LT3590
V : 4.5V to 55V, Drives Up to 10 LEDs, 200:1 Dimming,
IN
I
SO
= 15mA, 2mm × 2mm DFN SC70
LT3595
V : 4.5V to 45V, Drives Up to 160 LEDs, 5000:1 Dimming,
IN
5mm × 9mm QFN
LTC®3783
V : 3V to 36V, V
SD
= Ext FET, Dimming = 3000:1 True Color PWM,
IN
OUT(MAX)
I
< 20μA, 5mm × 4mm QFN10, TSSOP16E Packages
3518fb
LT 0308 REV B • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
20
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