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LT3518 Full-Featured LED Driver with 2.3A Switch Current FEATURES
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DESCRIPTION
The LT(R)3518 is a current mode DC/DC converter with an internal 2.3A, 45V switch specifically designed to drive LEDs. The LT3518 operates as a LED driver in boost, buck mode and buck-boost mode. It combines a traditional voltage loop and a unique current loop to operate as a constant-current source or constant-voltage source. Programmable switching frequency allows optimization of the external components for efficiency or component size. The switching frequency of the LT3518 can be synchronized to an external clock signal. The LED current is externally programmable with a 100mV sense resistor. The external PWM input provides 3000:1 LED dimming. The CTRL pin provides further 10:1 dimming ratio. The LT3518 is available in the tiny footprint 16-Lead QFN (4mm x 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.
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3000:1 True Color PWMTM Dimming Ratio 2.3A, 45V Internal Switch 100mV High Side Current Sense Open LED Protection Adjustable Frequency: 250kHz to 2.5MHz Wide Input Voltage Range: Operation from 3V to 30V Transient Protection to 40V Operates in Boost, Buck Mode and Buck-Boost Mode Gate Driver for PMOS LED Disconnect* Constant-Current and Constant-Voltage Regulation CTRL Pin Provides 10:1 Analog Dimming Low Shutdown Current: <1A Available in (4mm x 4mm) 16-Lead QFN and 16-Pin TSSOP Packages
APPLICATIONS
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Display Backlighting Automotive and Avionic Lighting Illumination Scanners
TYPICAL APPLICATION
1.5A Buck Mode LED Driver Efficiency
PVIN 24V 68m 2.2F ISP VIN SHDN LT3518 VREF PWM CTRL PWM FB SS 50 40 0.1F
3518 TA01a
M1
10F
100 CTRL = VREF 90
1.5A 15H EFFICIENCY (%) 16.9k 1MHz ISN TG SW 80 70 60
VIN 3.3V 2.2F
RT SYNC TGEN VREF VC GND 0.1F
0
20
40 60 80 PWM DUTY CYCLE (%)
100
3518 TA01b
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LT3518 ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, SHDN, PWM, TGEN (Note 3)..............................40V SW, ISP ISN, TG ........................................................45V , TG Pin Below ISP Pin ................................................10V FB, SYNC, SS, CTRL ...................................................6V VC, RT, VREF .................................................................3V Operating Junction Temperature Range (Notes 2, 4) LT3518E ............................................. -40C to 125C LT3518I .............................................. -40C to 125C LT3518H ............................................ -40C to 150C
Storage Temperature Range QFN.................................................... -65C to 125C TSSOP ............................................... -65C to 150C Lead Temperature (Soldering, 10 sec) TSSOP .............................................................. 300C
PIN CONFIGURATION
TOP VIEW TGEN ISN ISP TG VIN SHDN 12 FB 17 11 VC 10 CTRL 9 5 VREF 6 RT 7 SYNC 8 SS PWM VREF RT SYNC SS PWM CTRL 1 2 3 4 5 6 7 8 17 GND TOP VIEW 16 SW 15 SW 14 TG 13 ISP 12 ISN 11 TGEN 10 FB 9 VC 16 15 14 13 SW 1 SW 2 VIN 3 SHDN 4
UF PACKAGE 16-LEAD (4mm x 4mm) PLASTIC QFN TJMAX = 125C, JA = 36C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 150C, JA = 40C/W, JC(PAD) = 10C/W
ORDER INFORMATION
LEAD FREE FINISH LT3518EUF#PBF LT3518IUF#PBF LT3518EFE#PBF LT3518IFE#PBF LT3518HFE#PBF TAPE AND REEL LT3518EUF#TRPBF LT3518IUF#TRPBF LT3518EFE#TRPBF LT3518IFE#TRPBF LT3518HFE#TRPBF PART MARKING* 3518 3518 3518FE 3518FE 3518HFE PACKAGE DESCRIPTION 16-Lead (4mm x 4mm) Plastic QFN 16-Lead (4mm x 4mm) Plastic QFN 16-Lead Plastic TSSOP 16-Lead Plastic TSSOP 16-Lead Plastic TSSOP TEMPERATURE RANGE -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 150C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LT3518 ELECTRICAL CHARACTERISTICS
PARAMETER Minimum VIN Operating Voltage Maximum VIN Operating Voltage Current Sense Voltage (VISP - VISN) 10% Scale Current Sense Voltage (VISP - VISN) Current Sense Voltage Line Regulation VIN Supply Current Continuous Operation (Note 3) VCTRL = 2V, VISP = 24V, VC = 1V VCTRL = 2V, VISP = 0V, VC = 1V VCTRL = 100mV, VISP = 24V, VC = 1V 2V < VISP < 45V PWM > 1.5V, VC = 0V PWM = 0V SHDN = 0V RT = 16.7k RT = 4.03k RT = 91.5k SS = 0.5V, Out of Pin VSYNC = 2V 1.5 RT = 91.5k (250kHz) SYNC = 300kHz Clock Signal, RT = 91.5k RT = 16.7k (1MHz) RT = 4.03k (2.5MHz) ISW = 1.5A VSW = 45V, PWM = 0V Current Out of Pin, VCTRL = 0.1V 20 550 1000 PWM = 0, VC = 1V Current Out of Pin, VFB = 0.5V
l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.
CONDITIONS MIN 30 96 100 100 9 0.03 6 4.5 0.1 0.85 2.25 220 6 1.0 2.5 250 1 9 60 0.4 95 94 85 2.3 97 96 90 74 2.8 400 2 100 3.5 12 10 1 1.15 2.7 270 103 TYP MAX 3 UNITS V V mV mV mV %/V mA mA A MHz MHz kHz V A A V V % % % % A mV A nA S k 20 100 1.04 300 20 1.5 0.4 60 1.5 0.4 60 1.5 0.4 TGEN = 5V IREF = -100A
l
Switching Frequency
RT Voltage Soft-Start Pin Current SYNC Pull-Down Current (Into the Pin) SYNC Input Low SYNC Input High Maximum Duty Cycle
l
Switch Current Limit Switch VCESAT Switch Leakage Current CTRL Input Bias Current Error Amplifier Transconductance VC Output Impedance VC Idle Input Bias Current FB Pin Input Bias Current FB Pin Threshold ISP , ISN Idle Input Bias Current ISP , ISN Full-Scale Input Bias Current SHDN Voltage High SHDN Voltage Low 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 VREF Pin Voltage PWM = 0V ISP Tied to ISN, VISP = 24V, VCTRL = 2V
-20 0.98
0 20 1.01
nA nA V nA A V V A V V A V V A V
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120
100 1.96 2
200 2.04
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LT3518 ELECTRICAL CHARACTERISTICS
PARAMETER VREF Pin Voltage Line Regulation Gate Turn-On Delay Gate Turn-Off Delay Top Gate Drive VGS (VISP - VTG)
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.
CONDITIONS 3V < VIN < 40V CLOAD = 1nF Between ISP and TG CLOAD = 1nF Between ISP and TG VISP = 24V, TGEN = 5V PWM = 0V 200 200 7 0 0.3 MIN TYP MAX 0.03 UNITS %/V ns ns 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 specifications from 0C to 125C junction temperature. Specifications over the -40C to 125C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3518I is guaranteed over the full -40C to 125C operating junction temperature range. The LT3518H is guaranteed over the full -40C to 150C operating junction temperature range. Operating lifetime is derated at junction temperatures greater than 125C.
Note 3: Absolute maximum voltage at VIN, SHDN, PWM and TGEN pins is 40V for nonrepetitive 1 second transients and 30V for continuous operation. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.
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LT3518 TYPICAL PERFORMANCE CHARACTERISTICS
VISP - VISN Threshold vs VCTRL
120 VIN = 5V VISP = 24V 100 VC = 1V TA = 25C 80 60 40 20 0 0 0.2 0.4 CURRENT LIMIT (A) 3.0 2.5 2.0 1.5 1.0 0.5 0 0 TA = 25C 20 40 60 DUTY CYCLE (%) 80 100
3518 G02
Switch Current Limit vs Duty Cycle
VISP - VISN THRESHOLD (mV)
0.6 0.8 1.0 VCTRL (V)
1.2
1.4
1.6
3518 G01
Oscillator Frequency vs RT
10000 OSCILLATOR FREQUENCY (kHz) TA = 25C VISP - VISN THRESHOLD (mV) 105 104 103 102 101 100 99 98 97 96 100 1 10 RT (k) 100
3518 G03
VISP - VISN Threshold vs Temperature
VCTRL = 2V VIN = 5V TA = 25C VC = 1V
1000
95
0
10
30 20 VISP (V)
40
50
3518 G04
Switch Current Limit vs Temperature
3.0 VIN = 5V OSCILLATOR FREQUENCY (MHz) 2.5
Oscillator Frequency vs Temperature
VIN = 5V RT = 6.04k
2.8 CURRENT LIMIT (A)
2.3
2.6
2.1
2.4
1.9
2.2
1.7
2.0 -40 -15 -10 35 60 85 110 135 160 TEMPERATURE (C)
3518 G05
1.5 -40 -20 0
20 40 60 80 100 120 140 160 TEMPERATURE (C)
3518 G06
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LT3518 TYPICAL PERFORMANCE CHARACTERISTICS
VISP - VISN Threshold vs VISP
105 104 VISP - VISN THRESHOLD (mV) 103 102 VREF (V) 101 100 99 98 97 96 95 0 10 30 20 VISP (V) 40 50
3518 G07
Reference Voltage vs Temperature
2.02 VIN = 5V
VCTRL = 2V VIN = 5V TA = 25C VC = 1V
2.01
2.00
1.99
1.98 -40 -20 0
20 40 60 80 100 120 140 160 TEMPERATURE (C)
3518 G08
Quiescent Current vs VIN
8 TA = 25C 7 VC = 0V FB PIN THRESHOLD (V) VIN CURRENT (mA) 6 5 4 3 2 1 0 0 10 20 VIN (V)
3518 G09
FB Pin Threshold vs Temperature
1.04 1.03 1.02 1.01 1.00 0.99 0.98 -40 -20 0 VIN = 5V
30
40
20 40 60 80 100 120 140 160 TEMPERATURE (C)
3518 G10
PMOS Turn-On
PMOS Turn-Off
5V PWM 0V
5V PWM 0V
40V TG 30V
40V TG 30V
VISP = 40V
200ns/DIV
3518 G11
VISP = 40V
200ns/DIV
3518 G12
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LT3518 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 100A. 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 program a switching frequency 20% slower than SYNC pulse frequency. Synchronization (power switch turn-on) occurs a fixed delay after the rising edge of SYNC. Tie the SYNC pin to ground if this feature is not used. SS: Soft-Start Pin. Place a soft-start capacitor here. Leave the pin open if not in use. PWM: Pulse Width Modulated Input Pin. Signal low turns off channel, disables the main switch and makes the TG pin high. Tie the PWM pin to SHDN pin if not used. There is an equivalent 50k resistor from PWM pin to ground internally. 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 Amplifier 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 amplifier. ISP: Current Sense (+) Pin. The noninverting input to the current sense amplifier. Also serves as positive rail for TG pin driver. TG: Top Gate Driver Output. An inverted PWM signal drives series PMOS device between VISP and (VISP - 7V). An internal 7V clamp protects the VISP PMOS gate. Leave TG unconnected if not used. Ground: Exposed Pad. Solder paddle directly to ground plane.
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LT3518 BLOCK DIAGRAM
CIN RSENSE PVIN LED ARRAY CFILT
ISP
ISN
TG VISP CURRENT SENSE AMPLIFIER VISP - 7V MOSFET DRIVER
TGEN
PWM
SW
SW
+
X10
-
SHDN CTRL 1V
1.01V FB VC SYNC
1V VIN
SS
RT
8
+ + - + -
A1
+
A3
ERROR AMPLIFIER
+
A4 R S Q
MAIN SWITCH DRIVER Q1 MAIN SWITCH
+
A2
-
PWM COMPARATOR A8 SS
+ -
GND
RAMP GENERATOR VIN 2.5MHz TO 250kHz OSCILLATOR 100A VREF
+
A5
- + + -
10A
1V
+
A6 FREQ ADJUST VIN
3518 F01
Q2 2V
A7
-
Figure 1. Buck Mode LED Driver
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LT3518 OPERATION
The LT3518 is a constant frequency, current mode regulator with an internal power switch. Operation can be best understood by referring to the Block Diagram in Figure 1. At the start of each oscillator cycle, the SR latch is set, which turns on the Q1 power switch. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A4. When this voltage exceeds the level at the negative input of A4, the SR latch is reset, turning off the power switch. The level at the negative input of A4 is set by the error amplifier A3. A3 has two inputs, one from the voltage feedback loop and the other one from the current loop. Whichever feedback input is lower takes precedence, and forces the converter into either constant-current or constant-voltage mode. The LT3518 is designed to transition cleanly between these two modes of operation. The current sense amplifier senses the voltage across RSENSE and provides a pre-gain to amplifier A1. The output of A1 is simply an amplified version of the difference between the voltage across RSENSE and the lower of VCTRL/10 or 100mV. In this manner, the error amplifier sets the correct peak switch current level to regulate the current through RSENSE. If the error amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered. The current regulated in RSENSE can be adjusted by changing the input voltage VCTRL. The current sense amplifier provides rail-to-rail current sense operation. The FB voltage loop is implemented by the amplifier A2. When the voltage loop dominates, the error amplifier and the amplifier A2 regulate the FB pin to 1.01V (constant-voltage mode). Dimming of the LED array is accomplished by pulsing the LED current using the PWM pin. When the PWM pin is low, switching is disabled and the error amplifier is turned off so that it does not drive the VC pin. Also, all internal loads on the VC pin are disabled so that the charge state of the VC pin will be saved on the external compensation capacitor. This feature reduces transient recovery time. When the PWM input again transitions high, the demand current for the switch returns to the value just before PWM last transitioned low. To further reduce transient recovery time, an external PMOS is used to disconnect the LED array current loop when PWM is low, stopping CFILT from discharging.
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LT3518 APPLICATIONS INFORMATION
Dimming Control There are two methods to control the current source for dimming using the LT3518. The first method uses the PWM pin to modulate the current source between zero and full current to achieve a precisely programmed average current. To make this method of current control more accurate, the switch demand current is stored on the VC node during the quiescent phase. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch is used in the LED current path to prevent the output capacitor from discharging in the PWM signal low phase. The minimum PWM on or off time will depend on the choice of operating frequency through RT input pin or SYNC pin. When using the SYNC function, the SYNC and PWM signals must have the aligned rising edges to achieve the optimized high PWM dimming ratio. For best current accuracy, the minimum PWM low or high time should be at least six switching cycles (3s 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: t PWMRATIO = MAX tMIN Example: tMAX = 9ms, tMIN = 3s (fSW = 2MHz) PWMRATIO = 9ms/3s = 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 3
When VCTRL is higher than 1V, the LED current is clamped to be: ILED = 100mV RSENSE (3)
The LED current programming feature possibly increases
VREF 45.3k 2V 49.9k CTRL
5k PTC
3518 F02
Figure 2
total dimming range by a factor of ten. 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 + 1 * 1.01V R2
VOUT LT3518 FB R2
3518 F03
(1)
(4)
R1
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LT3518 APPLICATIONS INFORMATION
For a buck or a buck-boost configuration, the output voltage is typically level-shifted to a signal with respect to GND as illustrated in the Figure 4. The output can be expressed as: R1 VOUT = * 1.01V + VBE(Q1) R2
+
R1 VOUT RSENSE LED ARRAY
Table 1 provides some recommended inductor vendors.
Table 1. Inductor Manufacturers
VENDOR Sumida Toko Cooper Vishay PHONE (408) 321-9660 (408) 432-8281 (561) 998-4100 (402) 563-6866 WEB www.sumida.com www.toko.com www.cooperet.com www.vishay.com
(5)
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 1F or greater capacitor with low ESR should be used. A ceramic capacitor is usually the best choice. In the buck mode configuration, the capacitor at the input to the power converter has large pulsed currents due to the current returned though the Schottky diode when the switch is off. For best reliability, this capacitor should have low ESR and ESL and have an adequate ripple current rating. The RMS input current is: IIN(RMS) =ILED * (1- D) * D (8)
LT3518 FB R2
3518 F04
-
Figure 4
Inductor Selection The inductor used with the LT3518 should have a saturation current rating of 2A or greater. For buck mode LED drivers, the inductor value should be chosen to give a ripple current "I" of ~30% to 40% of the LED current. In the buck mode, the inductor value can be estimated using the formula: DBUCK * t SW (s) * ( VIN - VLED ) I V DBUCK = LED VIN L (H) = (6)
where D is the switch duty cycle. A 2.2F ceramic type capacitor is usually sufficient. Output Capacitor Selection The selection of output capacitor depends on the load and converter configuration, i.e., step-up or step-down. For LED applications, the equivalent resistance of the LED is typically low, and the output filter capacitor should be sized to attenuate the current ripple. To achieve the same LED ripple current, the required filter capacitor value is larger in the boost and buck-boost mode applications than that in the buck mode applications. For LED buck mode applications, a 1F ceramic capacitor is usually sufficient. For the LED boost and buck-boost mode applications, a 2.2F ceramic capacitor is usually sufficient. Very high performance PWM dimming applications may require a larger capacitor value to support the LED voltage during PWM transitions.
VLED is the voltage across the LED string, VIN is the input voltage to the converter, and tSW is the switching period. In the boost configuration, the inductor can be estimated using the formula: DBOOST * t SW (s) * VIN I V -V DBOOST = LED IN VLED L (H) = (7)
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LT3518 APPLICATIONS INFORMATION
Use only ceramic capacitors with X7R, X5R or better dielectric as they are best for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. Always check that the voltage rating of the capacitor is sufficient. Table 2 shows some recommended capacitor vendors.
Table 2. Ceramic Capacitor Manufacturers
VENDOR Taiyo Yuden AVX Murata TDK PHONE (408) 573-4150 (843) 448-9411 (770) 436-1300 (847) 803-6100 WEB www.t-yuden.com www.avxcorp.com www.murata.com www.tdk.com
Table 3. Schottky Diodes
PART NUMBER On Semiconductor MBRS260T3 Diodes Inc. DFLS140L Zetex ZLLS2000TA International Rectifier 10MQ060N 60 1.5 40 2.2 40 1 60 2 VR (V) IAVE (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 VIN supply to the LT3518 should be placed as close as possible to the VIN terminal of the device. Soft-Start For many applications, it is necessary to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. A typical value for the soft-start capacitor is 0.1F .
Loop Compensation The LT3518 uses an internal transconductance error amplifier whose VC output compensates the control loop. The external inductor, output capacitor, and the compensation resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop stability. For typical LED applications, a 10nF compensation capacitor at VC is adequate, and a series resistor is not required. A compensation resistor may be used to increase the slew rate on the VC pin to maintain tighter regulation of LED current during fast transients on VIN or CTRL. Diode Selection The Schottky diode conducts current during the interval when the switch is turned off. Select a diode rated for the maximum SW voltage. If using the PWM feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval. Therefore, choose the Schottky diode with sufficiently low leakage current. Table 3 has some recommended component vendors.
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LT3518 APPLICATIONS INFORMATION
Switching Frequency There are two methods to set the switching frequency of LT3518. Both methods require a resistor connected at RT pin. Do not leave the RT pin open. Also, do not load this pin with a capacitor. A resistor must always be connected for proper operation. One way to set the frequency is simply connecting an external resistor between the RT pin and GND. See Table 4 below or see the Oscillator Frequency vs RT graph in the Typical Performance Characteristics for resistor values and corresponding switching frequencies.
Table 4. Switching Frequency vs RT
Switching Frequency (kHz) 250 500 1000 1500 2000 2500 RT ( k ) 90.9 39.2 16.9 9.53 6.04 4.02
to generate a switching frequency 20% lower than the external clock when external clock is absent. In general, a lower switching frequency should be used where either very high or very low switching duty cycle operation is required, or high efficiency is desired. Selection of a higher switching frequency will allow use of smaller value external components and yield a smaller solution size and profile. Thermal Considerations The LT3518 is rated to a maximum input voltage of 30V for continuous operation, and 40V for nonrepetitive one second transients. Careful attention must be paid to the internal power dissipation of the LT3518 at higher input voltages to ensure that the maximum junction temperature is not exceeded. This junction limit is especially important when operating at high ambient temperatures. The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should then be connected to an internal copper ground plane with thermal vias placed directly under the package to spread out the heat dissipated by the LT3518.
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
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LT3518 TYPICAL APPLICATIONS
Buck Mode 1.5A LED Driver
RSENSE 68m C2 2.2F ISP VIN C1 2.2F VREF PWM SHDN LT3518 CTRL PWM FB SS IL 1A/DIV PVIN = 24V fOSC = 1MHz ILED = 1.5A 2s/DIV
3518 TA02b
PVIN 24V
M1 1.5A
C3 10F L1 15H D1 SW ILED 1A/DIV
1000:1 PWM Dimming at 120Hz
PWM 5V/DIV
VIN 3.3V
ISN TG
RT SYNC TGEN VREF VC GND C4 0.1F 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 RT 16.9k 1MHz C5 0.1F
3518 TA02a
500mA, 5V to 12V Boost Converter with Accurate Input Current Limit
RSENSE L1 50m 4.3 H
D1
Efficiency
VOUT 12V 500mA R1 549k EFFICIENCY (%) 90
VIN 5V C2 2.2 F
ISP TG ISN VIN CTRL PWM SHDN SHDN TGEN VREF VC R3 10k C4 10nF C1: KEMET C0805C225K4RAC C2: KEMET C1206C106K4RAC C3: MURATA GRM21BR71H104KA01B C4: MURATA GCM033R71A103KA03 D1: ZETEX ZLLS2000TA L1: TOKO B992AS-4R3N GND LT3518
SW FB C2 10 F R2 49.9k
80
70
SYNC RT SS C3 0.1 F RT 6.04k 2MHz
3518 TA03a
60
50
0
100
300 200 ILOAD (mA)
400
500
3518 TA03b
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LT3518 TYPICAL APPLICATIONS
Buck-Boost Mode LED Driver
L1 4.3H D1 R1 3.92M FB R2 124k PWM PWM TGEN VREF C1 2.2F CTRL SYNC VC RT 6.04k 2MHz C3 0.1F
3518 TA04a
VIN 8V TO 16V SHDN VIN
SW
C5 0.22F
300mA
ISP LT3518 ISN TG M1 C2 4.7F RSENSE 330m
RT SS
GND
C4 0.1F
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
90 PWM 5V/DIV ILED 200mA/DIV EFFICIENCY (%) 80 70 60 50 40 30 20 0 20 VIN = 10V CTRL = VREF
Efficiency
IL1 1A/DIV VIN = 10V fOSC = 2MHz ILED = 300mA 500ns/DIV
3518 TA04b
40
60
80
100
3518 TA04c
PWM DUTY CYCLE (%)
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15
LT3518 TYPICAL APPLICATIONS
Buck Mode 1A LED Driver with Open LED Protection and Sync Input
RSENSE 100m C2 2.2F
PVIN 32V
M1 LED1 1A LED6 R3 5.62k L1 10H SW R1 49.9k Q1 FB R2 2.00k C3 10F
VIN 3.3V
ISP VIN C1 2.2F VREF PWM SYNC 3.3V, 1.2MHz SHDN
ISN
TG
D1
LT3518 CTRL PWM FB SS 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
RT SYNC TGEN VREF VC GND C4 0.1F RT 16.7k 1MHz C5 0.1F
3518 TA05a
1000:1 PWM Dimming at 120Hz
100 PWM 5V/DIV ILED 1A/DIV IL1 1A/DIV CTRL = VREF 90 EFFICIENCY (%) 80 70 60 50 40 0 20
Efficiency
PVIN = 32V fOSC = 1.2MHz ILED = 1A
2s/DIV
3518 TA05b
40 60 80 PWM DUTY CYCLE (%)
100
3518 TA05c
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16
LT3518 TYPICAL APPLICATIONS
Boost 300mA LED Driver with LED Open Protection
L1 8.2H
D1
VIN 8V TO 16V SHDN VIN PWM TGEN VREF C1 2.2F CTRL SYNC VC RT 16.9k 1MHz C4 0.1F
PWM
SW ISP RSENSE 330m
LT3518
ISN TG FB R1 1M 300mA C3 0.1F R2 30.1k M1 LED1 LED2 C1: KEMET C1206C225K2RAC C2: TDK C5750X7R1H685M C3, C4: MURATA GRM21BR71H104KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-8R2N LEDS: LUXEON I (WHITE) M1: ZETEX ZXMP6A13GTA C2 6.8F
RT SS
GND
LED8
3518 TA06a
3000:1 PWM Dimming at 100Hz
PWM 5V/DIV ILED 200mA/DIV 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 0 20
Efficiency
IL1 1A/DIV VIN = 12V fOSC = 1MHz ILED = 300mA 1s/DIV
3518 TA06b
VIN = 12V CTRL = VREF 60 80 40 PWM DUTY CYCLE (%) 100
3518 TA06c
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17
LT3518 PACKAGE DESCRIPTION
UF Package 16-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1692)
0.72 0.05
4.35 0.05 2.15 0.05 2.90 0.05 (4 SIDES)
PACKAGE OUTLINE 0.30 0.05 0.65 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW--EXPOSED PAD 4.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 2.15 0.10 (4-SIDES) 0.75 0.05 R = 0.115 TYP PIN 1 NOTCH R = 0.20 TYP OR 0.35 x 45 CHAMFER
15
16 0.55 0.20 1 2
(UF16) QFN 1004
0.200 REF 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
0.30 0.05 0.65 BSC
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) 16 1514 13 12 1110 6.60 0.10 4.50 0.10
SEE NOTE 4
2.74 (.108)
9
2.74 (.108) 0.45 0.05 1.05 0.10 0.65 BSC 2.74 6.40 (.108) (.252) BSC
RECOMMENDED SOLDER PAD LAYOUT
12345678 1.10 (.0433) MAX
0 - 8
4.30 - 4.50* (.169 - .177)
0.25 REF
0.09 - 0.20 (.0035 - .0079)
0.50 - 0.75 (.020 - .030)
0.65 (.0256) BSC
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE
0.195 - 0.30 (.0077 - .0118) TYP
0.05 - 0.15 (.002 - .006)
FE16 (BA) TSSOP 0204
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
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT3518 TYPICAL APPLICATION
5.5V SEPIC Converter with Short-Circuit Protection
VIN 3V C2 2.2F VIN CTRL SYNC SHDN SHDN TGEN VREF VC R3 10k C4 10nF C1: KEMET C0805C225K4RAC C2, C5: KEMET C1206C106K4RAC C3: MURATA GRM21BR71H104KA01B C4: MURATA GCM033R71A103KA03 D1: ZETEX ZLLS2000TA L1, L2: TOKO 962BS-2R4M GND SS C3 0.1F RT 6.04k 2MHz
3518 TA07a
Efficiency
100 VOUT 5.5V 500mA EFFICIENCY (%) R1 221k 90 80 70 60 50 40 R2 49.9k 30 0 100 200 300 400 500
3518 TA07b
L1 2.4H
C5 10F
D1
RSENSE 0.15
L2 2.4H PWM SW FB TG LT3518 ISP ISN RT
C2 10F
ILOAD (mA)
RELATED PARTS
PART NUMBER LT1618 LT3003 LT3474 LT3475 LT3476 LT3477 LT3478/LT3478-1 LT3479 LT3486 LT3496 LT3517 LT3590 LT3595 LTC(R)3783 DESCRIPTION Constant Current, 1.4MHz, 1.5A Boost Converter 3-Channel LED Ballaster with PWM Dimming 36V, 1A (ILED), 2MHz, Step-Down LED Driver Dual 1.5A (ILED), 36V 2MHz Step-Down LED Driver Quad Output 1.5A, 36V, 2MHz High Current LED Driver with 1,000:1 Dimming 3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver 4.5A, 42V, 2.5MHz High Current LED Driver with 3,000:1 Dimming 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection Dual 1.3A, 2MHz High Current LED Driver Triple Output LED Driver 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 COMMENTS VIN: 5V to 18V, VOUT(MAX) = 36V, Dimming = Analog/PWM, ISD < 1A, MSOP10 Package VIN: 3V to 48V, Dimming = 3000:1 True Color PWM, ISD < 5A, MSOP10 Package VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 400:1 True Color PWM, ISD < 1A, TSSOP16E Package VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 3000:1 True Color PWM, ISD < 1A, TSSOP20E Package VIN: 2.8V to 16V, VOUT(MAX) = 36V, Dimming = 1000:1 True Color PWM, ISD < 10A, 5mm x 7mm QFN Package VIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1A, QFN, TSSOP20E Packages VIN: 2.8V to 36V, VOUT(MAX) = 42V, Dimming = 3000:1 True Color PWM, ISD < 3A, TSSOP16E Packages VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD < 1A, DFN and TSSOP Packages VIN: 2.5V to 24V, VOUT(MAX) = 36V, Dimming = 1000:1 True Color PWM, ISD < 1A, 5mm x 3mm DFN, TSSOP16E VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 3000:1 True Color PWM, ISD < 10A, 4mm x 5mm QFN Package VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 5000:1 True Color PWM, ISD < 1A, 4mm x 4mm QFN and TSSOP Packages VIN: 4.5V to 55V, Drives Up to 10 LEDs, 200:1 Dimming, ISO = 15mA, 2mm x 2mm DFN SC70 VIN: 4.5V to 45V, Drives Up to 160 LEDs, 5000:1 Dimming, 5mm x 9mm QFN VIN: 3V to 36V, VOUT(MAX) = Ext FET, Dimming = 3000:1 True Color PWM, ISD < 20A, 5mm x 4mm QFN10, TSSOP16E Packages
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20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0308 REV B * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2007

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