LT3518EUF#PBF - LINEAR TECHNOLOGY

LT3518

3518fe

FEATURES

APPLICATIONS

DESCRIPTION

Full-Featured LED Driver

with 2.3A Switch Current

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. Pro-

grammable 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.

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

n Display Backlighting

n Automotive and Avionic Lighting

n Illumination

n Scanners

1.5A Buck Mode LED Driver Efﬁ ciency

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.

1.5A

10µF

15µH

2.2µF

0.1µF

68m M1

2.2µF

VREF

ISP

TGEN GNDVREF VC

16.9k

1MHz

3518 TA01a

0.1µF

VIN

SHDN

CTRL

PWM

SYNC

PVIN

24V

VIN

3.3V

ISN

LT3518

PWM

PWM DUTY CYCLE (%)

EFFICIENCY (%)

100

20 40 60 80

3518 TA01b

100

CTRL = VREF

TYPICAL APPLICATION

LT3518

3518fe

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

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

(Note 1)

16 15 14 13

5 6 7 8

TOP VIEW

UF PACKAGE

16-LEAD (4mm = 4mm) PLASTIC QFN

1SW

VIN

SHDN

CTRL

PWM

ISP

ISN

TGEN

VREF

SYNC

TJMAX = 125°C, θJA = 36°C/W

EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB

FE PACKAGE

16-LEAD PLASTIC TSSOP

TOP VIEW

VIN

SHDN

VREF

SYNC

PWM

CTRL

ISP

ISN

TGEN

GND

TJMAX = 150°C, θJA = 40°C/W, θJC(PAD) = 10°C/W

PIN CONFIGURATION

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 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/

Storage Temperature Range

QFN ....................................................–65°C to 150°C

TSSOP ...............................................–65°C to 150°C

Lead Temperature (Soldering, 10 sec)

TSSOP .............................................................. 300°C

LT3518

3518fe

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at TA = 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum VIN Operating Voltage 3V

Maximum VIN Operating Voltage Continuous Operation (Note 3) 30 V

Current Sense Voltage (VISP – VISN)V

CTRL = 2V, VISP = 24V, VC = 1V

VCTRL = 2V, VISP = 0V, VC = 1V

l96 100

100

103 mV

10% Scale Current Sense Voltage (VISP – VISN)V

CTRL = 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

4.5

0.1

µA

Switching Frequency RT = 16.7k

RT = 4.03k

RT = 91.5k

l0.85

2.25

220

1.0

2.5

250

1.15

2.7

270

MHz

kHz

RT Voltage 1V

Soft-Start Pin Current SS = 0.5V, Out of Pin 6 9 12 µA

SYNC Pull-Down Current (Into the Pin) VSYNC = 2V 60 µA

SYNC Input Low 0.4 V

SYNC Input High 1.5 V

Maximum Duty Cycle RT = 91.5k (250kHz)

SYNC = 300kHz Clock Signal, RT = 91.5k

RT = 16.7k (1MHz)

RT = 4.03k (2.5MHz)

Switch Current Limit 2.3 2.8 3.5 A

Switch VCESAT ISW = 1.5A 400 mV

Switch Leakage Current VSW = 45V, PWM = 0V 2 µA

CTRL Input Bias Current Current Out of Pin, VCTRL = 0.1V 20 100 nA

Error Ampliﬁ er Transconductance 550 µS

VC Output Impedance 1000 k

VC Idle Input Bias Current PWM = 0, VC = 1V –20 0 20 nA

FB Pin Input Bias Current Current Out of Pin, VFB = 0.5V 20 100 nA

FB Pin Threshold l0.98 1.01 1.04 V

ISP , ISN Idle Input Bias Current PWM = 0V 300 nA

ISP , ISN Full-Scale Input Bias Current ISP Tied to ISN, VISP = 24V, VCTRL = 2V 20 µA

SHDN Voltage High l1.2 V

SHDN Voltage Low –40°C ≤ TJ ≤ 125°C

125°C < TJ ≤ 150°C

0.45

0.40

SHDN Pin Bias Current 60 100 µA

PWM Input High Voltage l1.2 V

PWM Input Low Voltage –40°C ≤ TJ ≤ 125°C

125°C < TJ ≤ 150°C

0.45

0.40

PWM Pin Bias Current 60 120 µA

LT3518

3518fe

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at TA = 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.

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.

PARAMETER CONDITIONS MIN TYP MAX UNITS

TGEN Input High Voltage 1.5 V

TGEN Input Low Voltage 0.4 V

TGEN Pin Bias Current TGEN = 5V 100 200 µA

VREF Pin Voltage IREF = –100µA l1.96 2 2.04 V

VREF Pin Voltage Line Regulation 3V < VIN < 40V 0.03 %/V

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)V

ISP = 24V, TGEN = 5V

PWM = 0V

0 0.3

LT3518

3518fe

TYPICAL PERFORMANCE CHARACTERISTICS

VISP – VISN Threshold vs VCTRL

Switch Current Limit

vs Duty Cycle Oscillator Frequency vs RT

VISP – VISN Threshold

vs Temperature

Switch Current Limit

vs Temperature

Oscillator Frequency

vs Temperature

VCTRL (V)

VISP – VISN THRESHOLD (mV)

0.4 0.8 1.2 1.6

3518 G01

100

120

0.2 0.6 1.0 1.4

VIN = 5V

VISP = 24V

VC = 1V

TA = 25°C

DUTY CYCLE (%)

CURRENT LIMIT (A)

0.5

1.0

1.5

2.0

2.5

3.0

20 40 60 80

3518 G02

100

TA = 25°C

RT (kΩ)

100

OSCILLATOR FREQUENCY (kHz)

1000

10000

10 100

3518 G03

TA = 25°C

TEMPERATURE (°C)

–40

VISP – VISN THRESHOLD (mV)

103

3518 G04

100

–20 0 40

104

102

101

60 80 100 120 140 160

VIN = 5V

VISP = 24V

VC = 1V

VCTRL = 2V

TEMPERATURE (°C)

–40 –15

2.0

CURRENT LIMIT (A)

2.4

3.0

–10 60 85

3518 G05

2.2

2.8

2.6

35 110 135 160

VIN = 5V

TEMPERATURE (°C)

–40 –20

1.5

OSCILLATOR FREQUENCY (MHz)

1.9

2.5

040 60

3518 G06

1.7

2.3

2.1

20 80 160100 120 140

VIN = 5V

RT = 6.04k

Reference Voltage

vs Temperature

VISP – VISN Threshold vs VISP

VISP (V)

VISP – VISN THRESHOLD (mV)

101

103

105

3518 G07

100

102

104

95 10 20 30 50

VCTRL = 2V

VIN = 5V

TA = 25°C

VC = 1V

TEMPERATURE (°C)

–40

1.98

VREF (V)

1.99

2.00

2.01

2.02

–20 0 20 40

3518 G08

60 80 100 120 140 160

VIN = 5V

Quiescent Current vs VIN

VIN (V)

VIN CURRENT (mA)

3518 G09

010 20 30

TA = 25°C

VC = 0V

LT3518

3518fe

TYPICAL PERFORMANCE CHARACTERISTICS

FB Pin Threshold vs Temperature PMOS Turn-On PMOS Turn-Off

PIN FUNCTIONS

SW: Switch Pin. Minimize trace at this pin to reduce EMI.

VIN: Input Supply Pin. Must be locally bypassed.

SHDN: Shutdown Pin. Tie to 1.5V or higher to enable

device or 0.4V or less to disable device.

VREF: Reference Output Pin. This pin can supply up to

100µA.

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.

SYNC: Frequency Synchronization Pin. Tie an external

clock signal here. RT resistor should be chosen to pro-

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.

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.

VC: gm Error Ampliﬁ er Output Pin. Stabilize the loop with

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 sig-

nal 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.

TEMPERATURE (°C)

–40

0.98

FB PIN THRESHOLD (V)

1.00

0.99

1.01

1.03

1.02

1.04

–20 0 20 40

3518 G10

60 80 140 160100 120

VIN = 5V

PWM

40V

30V

200ns/DIVVISP = 40V 3518 G11

PWM

40V

30V

200ns/DIVVISP = 40V 3518 G12

LT3518

3518fe

BLOCK DIAGRAM

Figure 1. Buck Mode LED Driver

CTRL

1.01V

10µA

VIN

–

–+

X10

RSENSE

ISP ISN

PVIN

PWM

LED ARRAY

–

3518 F01

MAIN SWITCH

DRIVER

ERROR

AMPLIFIER

CURRENT

SENSE

AMPLIFIER

PWM

COMPARATOR

MAIN

SWITCH

VISP

VISP – 7V

MOSFET DRIVER

TGEN

CFILT

SW SW

GND

CIN

SHDN

SYNC

VIN

FREQ

ADJUST

VREF

RAMP

GENERATOR

2.5MHz TO 250kHz

OSCILLATOR

100µA

VIN

LT3518

3518fe

OPERATION

The LT3518 is a constant frequency, current mode regula-

tor 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 transi-

tion cleanly between these two modes of operation. The

current sense ampliﬁ er senses the voltage across RSENSE

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 RSENSE and the lower of VCTRL/10

or 100mV. In this manner, the error ampliﬁ er sets the

correct peak switch current level to regulate the current

through RSENSE. If the error ampliﬁ er’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 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 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.

LT3518

3518fe

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 aver-

age 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 us-

ing 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:

MAX = 9ms, tMIN = 3µs (fSW = 2MHz)

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:

ILED =VCTRL

10 •RSENSE

(2)

Figure 2

Figure 3

APPLICATIONS INFORMATION

When VCTRL is higher than 1V, the LED current is clamped

to be:

ILED =100mV

RSENSE

(3)

The LED current programming feature possibly increases

total dimming range by a factor of ten.

49.9k

3518 F02

45.3k

VREF

PTC

CTRL

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:

VOUT =R1

R2 +1

⎛

⎝

⎜⎞

⎠

⎟•1.01V

(4)

LT3518

VOUT

3518 F03

LT3518

3518fe

APPLICATIONS INFORMATION

Figure 4

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:

VOUT =R1

R2 •1.01V +VBE(Q1)

(5)

LT3518

–

LED

ARRAY

VOUT

3518 F04

RSENSE

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:

LµH

()

=DBUCK •t

SW(µs) • VIN –V

LED

()

ΔI

DBUCK =VLED

(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 conﬁ guration, the inductor can be estimated

using the formula:

LµH

()

=DBOOST •t

SW(µs) • VIN

ΔI

DBOOST =VLED –V

VLED

(7)

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 VIN pin of the LT3518. A

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:

IIN(RMS) =ILED •(1–D)•D

(8)

where D is the switch duty cycle. A 2.2µF ceramic type

capacitor is usually sufﬁ cient.

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 appli-

cations may require a larger capacitor value to support

the LED voltage during PWM transitions.

LT3518

3518fe

APPLICATIONS INFORMATION

Use only ceramic capacitors with X7R, X5R or better dielec-

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 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 ampli-

ﬁ er whose VC output compensates the control loop. The

external inductor, output capacitor, and the compensa-

tion 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 sufﬁ ciently low leakage current. Table 3 has some

recommended component vendors.

Table 3. Schottky Diodes

PART NUMBER VR (V) IAVE (A)

On Semiconductor

MBRS260T3 60 2

Diodes Inc.

DFLS140L 40 1

Zetex

ZLLS2000TA 40 2.2

International Rectiﬁ er

10MQ060N 60 1.5

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

signiﬁ cantly reduces the start-up current spike and

output voltage overshoot. A typical value for the soft-start

capacitor is 0.1µF.

LT3518

3518fe

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 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.

LT3518

3518fe

TYPICAL APPLICATIONS

Buck Mode 1.5A LED Driver

1000:1 PWM Dimming at 120Hz

500mA, 5V to 12V Boost Converter with Accurate Input Current Limit

Efﬁ ciency

1.5A

10µF

15µH

2.2µF

0.1µF

RSENSE

68m M1

2.2µF

ISP

TGEN GNDVREF VC

16.9k

1MHz

3518 TA02a

0.1µF

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

VIN

SHDN

CTRL

PWM

SYNC

PVIN

24V

VIN

3.3V

ISN

LT3518

PWM

VREF

PWM

5V/DIV

ILED

1A/DIV

2µs/DIVPVIN = 24V

fOSC = 1MHz

ILED = 1.5A

3518 TA02b

VC GND SS

VIN

CTRL

PWM

SHDN

TGEN

VREF

VOUT

12V

500mA

VIN

SYNC

549k

10µF

0.1µF

10nF

SHDN

ISN

LT3518

TGISP

49.9k

6.04k

2MHz

10k

3518 TA03a

2.2µF

4.3µH

RSENSE

50m1

C1: KEMET C0805C225K4RAC

C2: KEMET C1206C106K4RAC

C3: MURATA GRM21BR71H104KA01B

C4: MURATA GCM033R71A103KA03

D1: ZETEX ZLLS2000TA

L1: TOKO B992AS-4R3N

ILOAD (mA)

EFFICIENCY (%)

400

3518 TA03b

50 100 200 300 500

LT3518

3518fe

TYPICAL APPLICATIONS

3000:1 PWM Dimming at 120Hz Efﬁ ciency

Buck-Boost Mode LED Driver

4.3µH

RSENSE

330m

SHDN

VC GNDRT SS

6.04k

2MHz

3518 TA04a

0.1µF

4.7µF

3.92M

124k

0.22µF

2.2µF

0.1µF

PWM ISP

ISN

TGEN

VREF

CTRL

SYNC

300mA

VIN

8V TO 16V

VIN

LT3518

PWM

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

PWM

5V/DIV

ILED

200mA/DIV

IL1

1A/DIV

500ns/DIVVIN = 10V

fOSC = 2MHz

ILED = 300mA

3518 TA04b

PWM DUTY CYCLE (%)

3518 TA04c

20 40 60 100

EFFICIENCY (%)

VIN = 10V

CTRL = VREF

LT3518

3518fe

TYPICAL APPLICATIONS

1000:1 PWM Dimming at 120Hz Efﬁ ciency

Buck Mode 1A LED Driver with Open LED Protection and Sync Input

LED1

LED6

10µF

49.9k

2.00k

5.62k

10µH

2.2µF

0.1µF

RSENSE

100m M1

2.2µF

VREF

ISP

TGEN GNDVREF VC

16.9k

1MHz

3518 TA05a

0.1µF

VIN

SHDN

CTRL

PWM

SYNC

PVIN

32V

VIN

3.3V

ISN

LT3518

PWM

SYNC

3.3V, 1.2MHz

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

PWM

5V/DIV

ILED

1A/DIV

IL1

1A/DIV

2µs/DIVPVIN = 32V

fOSC = 1.2MHz

ILED = 1A

3518 TA05b

PWM DUTY CYCLE (%)

EFFICIENCY (%)

100

20 40 60 80

3518 TA05c

100

CTRL = VREF

LT3518

3518fe

TYPICAL APPLICATIONS

Boost 300mA LED Driver with LED Open Protection

3000:1 PWM Dimming at 100Hz Efﬁ ciency

LED1

LED2

LED8

8.2µH

RSENSE

330mΩ

SHDN

VC GNDRT SS

16.9k

1MHz

3518 TA06a

0.1µF

6.8µF

2.2µF

0.1µF

PWM ISP

ISN

TGEN

VREF

CTRL

SYNC

300mA

VIN

8V TO 16V

VIN

LT3518

PWM

C1: KEMET C1206C225K2RAC

C2: TDK C5750X7R1H685M

C3, C4: MURATA GRM21BR71H104KA01B

D1: ZETEX ZLLS2000TA

L1: TOKO B992AS-8R2N

LEDS: LUXEON I (WHITE)

M1: ZETEX ZXMP6A13GTA

30.1k

PWM

5V/DIV

ILED

200mA/DIV

IL1

1A/DIV

1µs/DIVVIN = 12V

fOSC = 1MHz

ILED = 300mA

3518 TA06b

PWM DUTY CYCLE (%)

EFFICIENCY (%)

100

3518 TA06c

20 20 40 60 100

VIN = 12V

CTRL = VREF

LT3518

3518fe

PACKAGE DESCRIPTION

4.00 t0.10

(4 SIDES)

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

PIN 1

TOP MARK

(NOTE 6)

0.55 t0.20

1615

BOTTOM VIEW—EXPOSED PAD

2.15 t0.10

(4-SIDES)

0.75 t0.05 R = 0.115

TYP

0.30 t0.05

0.65 BSC

0.200 REF

0.00 – 0.05

(UF16) QFN 10-04

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

0.72 t0.05

0.30 t0.05

0.65 BSC

2.15 t0.05

(4 SIDES)

2.90 t0.05

4.35 t0.05

PACKAGE OUTLINE

PIN 1 NOTCH R = 0.20 TYP

OR 0.35 w 45s CHAMFER

UF Package

16-Lead Plastic QFN (4mm w 4mm)

(Reference LTC DWG # 05-08-1692 Rev Ø)

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LT3518

3518fe

PACKAGE DESCRIPTION

FE16 (BA) TSSOP REV I 0211

0.09 – 0.20

(.0035 – .0079)

0s – 8s

0.25

REF

0.50 – 0.75

(.020 – .030)

4.30 – 4.50*

(.169 – .177)

134

5678

10 9

4.90 – 5.10*

(.193 – .201)

16 1514 13 12 11

1.10

(.0433)

MAX

0.05 – 0.15

(.002 – .006)

0.65

(.0256)

BSC

2.74

(.108)

2.74

(.108)

0.195 – 0.30

(.0077 – .0118)

TYP

MILLIMETERS

(INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.150mm (.006") PER SIDE

NOTE:

1. CONTROLLING DIMENSION: MILLIMETERS

2. DIMENSIONS ARE IN

RECOMMENDED SOLDER PAD LAYOUT

3. DRAWING NOT TO SCALE

0.45 t0.05

0.65 BSC

4.50 t0.10

6.60 t0.10

1.05 t0.10

2.74

(.108)

2.74

(.108)

SEE NOTE 4

4. RECOMMENDED MINIMUM PCB METAL SIZE

FOR EXPOSED PAD ATTACHMENT

6.40

(.252)

BSC

FE Package

16-Lead Plastic TSSOP (4.4mm)

(Reference LTC DWG # 05-08-1663 Rev I)

Exposed Pad Variation BA

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LT3518

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 representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER

D 01/11 Revised Electrical Characteristics 3

E 08/12 Clariﬁ ed Abs Max Table, Pin Conﬁ guration, and Order Information

Clariﬁ ed Electrical Speciﬁ cation Table

Clariﬁ ed Pin Functions

Clariﬁ ed Typical Application

(Revision history begins at Rev D)

LT3518

3518fe

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com

LT 0812 REV E • PRINTED IN USA

TYPICAL APPLICATION

5.5V SEPIC Converter with Short-Circuit Protection Efﬁ ciency

VC GND SS

CTRL

SYNC

SHDN

TGEN

VREF

ISP

ISN

VOUT

5.5V

500mA

VIN

221k

10µF

0.1µF

10nF

SHDN

PWM

LT3518

VIN

49.9k

6.04k

2MHz

10k

3518 TA07a

2.2µF

2.4µH

RSENSE

0.15

C1: KEMET C0805C225K4RAC

C2, C5: KEMET C1206C106K4RAC

C3: MURATA GRM21BR71H104KA01B

C4: MURATA GCM033R71A103KA03

D1: ZETEX ZLLS2000TA

L1, L2: TOKO 962BS-2R4M

ILOAD (mA)

100

400

3518 TA07b

100 200 300 500

EFFICIENCY (%)

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

LT C

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

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