LT3518 Full-Featured LED Driver with 2.3A Switch Current FEATURES DESCRIPTION n The LT®3518 is a current mode DC/DC converter with an internal 2.3A, 45V switch specifically designed to drive LEDs. The LT3518 operates as a LED driver in boost, buck mode and buck-boost mode. It combines a traditional voltage loop and a unique current loop to operate as a constant-current source or constant-voltage source. Programmable switching frequency allows optimization of the external components for efficiency or component size. The switching frequency of the LT3518 can be synchronized to an external clock signal. The LED current is externally programmable with a 100mV sense resistor. The external PWM input provides 3000:1 LED dimming. The CTRL pin provides further 10:1 dimming ratio. n n n n n n n n n n n 3000:1 True Color PWM™ Dimming Ratio 2.3A, 45V Internal Switch 100mV High Side Current Sense Open LED Protection Adjustable Frequency: 250kHz to 2.5MHz Wide Input Voltage Range: Operation from 3V to 30V Transient Protection to 40V Operates in Boost, Buck Mode and Buck-Boost Mode Gate Driver for PMOS LED Disconnect* Constant-Current and Constant-Voltage Regulation CTRL Pin Provides 10:1 Analog Dimming Low Shutdown Current: <1μA Available in (4mm × 4mm) 16-Lead QFN and 16-Pin TSSOP Packages APPLICATIONS n n n n Display Backlighting Automotive and Avionic Lighting Illumination Scanners The LT3518 is available in the tiny footprint 16-Lead QFN (4mm × 4mm) and the 16-Pin TSSOP package. The LT3518 provides a complete solution for both constant-voltage and constant-current applications. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation. Patent Pending. All other trademarks are the property of their respective owners. *Patents Pending. TYPICAL APPLICATION 1.5A Buck Mode LED Driver Efficiency PVIN 24V 68mΩ 100 10μF M1 CTRL = VREF 90 2.2μF 1.5A 15μH ISP ISN TG EFFICIENCY (%) VIN 3.3V 2.2μF SW VIN SHDN LT3518 VREF CTRL FB PWM PWM SS 70 60 50 RT SYNC TGEN VREF VC GND 0.1μF 80 16.9k 1MHz 40 0.1μF 3518 TA01a 0 20 40 60 80 PWM DUTY CYCLE (%) 100 3518 TA01b 3518fb 1 LT3518 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, SHDN, PWM, TGEN (Note 3)..............................40V SW, ISP, ISN, TG ........................................................45V TG Pin Below ISP Pin ................................................10V FB, SYNC, SS, CTRL ...................................................6V VC, RT, VREF .................................................................3V Operating Junction Temperature Range (Notes 2, 4) LT3518E ............................................. –40°C to 125°C LT3518I .............................................. –40°C to 125°C LT3518H ............................................ –40°C to 150°C Storage Temperature Range QFN.................................................... –65°C to 125°C TSSOP ............................................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) TSSOP .............................................................. 300°C PIN CONFIGURATION TOP VIEW 16 SW 2 15 SW 14 TG ISN 1 SHDN ISP VIN 16 15 14 13 TG TGEN TOP VIEW SW 1 12 FB VREF 3 SW 2 11 VC RT 4 SYNC 5 SS 6 11 TGEN PWM 7 10 FB CTRL 8 9 17 VIN 3 10 CTRL SHDN 4 5 6 7 8 VREF RT SYNC SS 9 PWM UF PACKAGE 16-LEAD (4mm × 4mm) PLASTIC QFN 17 GND 13 ISP 12 ISN VC FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 36°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB TJMAX = 150°C, θJA = 40°C/W, θJC(PAD) = 10°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3518EUF#PBF LT3518EUF#TRPBF 3518 16-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C LT3518IUF#PBF LT3518IUF#TRPBF 3518 16-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C LT3518EFE#PBF LT3518EFE#TRPBF 3518FE 16-Lead Plastic TSSOP –40°C to 125°C LT3518IFE#PBF LT3518IFE#TRPBF 3518FE 16-Lead Plastic TSSOP –40°C to 125°C LT3518HFE#PBF LT3518HFE#TRPBF 3518HFE 16-Lead Plastic TSSOP –40°C to 150°C Consult LTC Marketing for parts 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 CONDITIONS MIN TYP Minimum VIN Operating Voltage MAX 3 V Maximum VIN Operating Voltage Continuous Operation (Note 3) Current Sense Voltage (VISP – VISN) VCTRL = 2V, VISP = 24V, VC = 1V VCTRL = 2V, VISP = 0V, VC = 1V 10% Scale Current Sense Voltage (VISP – VISN) VCTRL = 100mV, VISP = 24V, VC = 1V 9 mV Current Sense Voltage Line Regulation 2V < VISP < 45V 0.03 %/V VIN Supply Current PWM > 1.5V, VC = 0V PWM = 0V SHDN = 0V 6 4.5 0.1 10 1 mA mA μA 1.0 2.5 250 1.15 2.7 270 MHz MHz kHz Switching Frequency RT = 16.7k RT = 4.03k RT = 91.5k 30 UNITS l l 96 0.85 2.25 220 RT Voltage V 100 100 1 Soft-Start Pin Current SS = 0.5V, Out of Pin SYNC Pull-Down Current (Into the Pin) VSYNC = 2V 6 9 12 1.5 RT = 91.5k (250kHz) SYNC = 300kHz Clock Signal, RT = 91.5k RT = 16.7k (1MHz) RT = 4.03k (2.5MHz) l Switch Current Limit Switch VCESAT ISW = 1.5A Switch Leakage Current VSW = 45V, PWM = 0V CTRL Input Bias Current Current Out of Pin, VCTRL = 0.1V μA μA 0.4 SYNC Input High mV mV V 60 SYNC Input Low Maximum Duty Cycle 103 V V 95 94 85 97 96 90 74 2.3 2.8 % % % % 3.5 400 20 A mV 2 μA 100 nA Error Amplifier Transconductance 550 μS VC Output Impedance 1000 kΩ VC Idle Input Bias Current PWM = 0, VC = 1V FB Pin Input Bias Current Current Out of Pin, VFB = 0.5V –20 l FB Pin Threshold ISP , ISN Idle Input Bias Current PWM = 0V ISP , ISN Full-Scale Input Bias Current ISP Tied to ISN, VISP = 24V, VCTRL = 2V 0.98 0 20 nA 20 100 nA 1.01 1.04 V 300 20 SHDN Voltage High 1.5 V SHDN Voltage Low SHDN Pin Bias Current 60 PWM Input High Voltage 0.4 V 100 μA 1.5 V PWM Input Low Voltage PWM Pin Bias Current 60 TGEN Input High Voltage 0.4 V 120 μA 1.5 V TGEN Input Low Voltage TGEN Pin Bias Current TGEN = 5V VREF Pin Voltage IREF = –100μA l 1.96 nA μA 0.4 V 100 200 μA 2 2.04 V 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 CONDITIONS VREF Pin Voltage Line Regulation 3V < VIN < 40V Gate Turn-On Delay CLOAD = 1nF Between ISP and TG 200 ns Gate Turn-Off Delay CLOAD = 1nF Between ISP and TG 200 ns Top Gate Drive VGS (VISP – VTG) VISP = 24V, TGEN = 5V PWM = 0V Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3518E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3518I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3518H is guaranteed over the full –40°C to 150°C operating junction temperature range. Operating lifetime is derated at junction temperatures greater than 125°C. MIN TYP 7 0 MAX UNITS 0.03 %/V 0.3 V V Note 3: Absolute maximum voltage at VIN, SHDN, PWM and TGEN pins is 40V for nonrepetitive 1 second transients and 30V for continuous operation. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. 3518fb 4 LT3518 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit vs Duty Cycle VISP – VISN Threshold vs VCTRL 3.0 VIN = 5V VISP = 24V 100 VC = 1V TA = 25°C 2.5 CURRENT LIMIT (A) VISP – VISN THRESHOLD (mV) 120 80 60 40 2.0 1.5 1.0 20 0.5 0 0 0 0.2 0.4 0.6 0.8 1.0 VCTRL (V) 1.2 1.4 1.6 TA = 25°C 0 20 40 60 DUTY CYCLE (%) 80 3518 G01 3518 G02 VISP – VISN Threshold vs Temperature Oscillator Frequency vs RT 10000 100 105 TA = 25°C VCTRL = 2V VIN = 5V TA = 25°C VC = 1V VISP – VISN THRESHOLD (mV) OSCILLATOR FREQUENCY (kHz) 104 1000 103 102 101 100 99 98 97 96 100 10 RT (kΩ) 1 95 100 0 10 30 20 VISP (V) 3518 G03 Oscillator Frequency vs Temperature 2.5 OSCILLATOR FREQUENCY (MHz) VIN = 5V 2.8 CURRENT LIMIT (A) 50 3518 G04 Switch Current Limit vs Temperature 3.0 40 2.6 2.4 2.2 2.0 –40 –15 –10 35 60 85 110 135 160 TEMPERATURE (°C) 3518 G05 VIN = 5V RT = 6.04k 2.3 2.1 1.9 1.7 1.5 –40 –20 0 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 3518 G06 3518fb 5 LT3518 TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Temperature VISP – VISN Threshold vs VISP 105 104 103 VIN = 5V 2.01 102 101 VREF (V) VISP – VISN THRESHOLD (mV) 2.02 VCTRL = 2V VIN = 5V TA = 25°C VC = 1V 100 99 98 2.00 1.99 97 96 95 10 0 30 20 VISP (V) 40 1.98 –40 –20 0 50 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 3518 G08 3518 G07 FB Pin Threshold vs Temperature Quiescent Current vs VIN 1.04 8 TA = 25°C 7 VC = 0V 1.03 FB PIN THRESHOLD (V) 6 VIN CURRENT (mA) VIN = 5V 5 4 3 2 1.02 1.01 1.00 0.99 1 0 0 20 10 30 40 0.98 –40 –20 0 VIN (V) 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 3518 G10 3518 G09 PMOS Turn-On PMOS Turn-Off 5V 5V PWM PWM 0V 0V 40V 40V TG TG 30V 30V VISP = 40V 200ns/DIV 3518 G11 VISP = 40V 200ns/DIV 3518 G12 3518fb 6 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 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 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. 3518fb 7 LT3518 BLOCK DIAGRAM LED ARRAY CIN CFILT RSENSE PVIN ISP + ISN TG – TGEN PWM SW SW VISP CURRENT SENSE AMPLIFIER X10 VISP – 7V SHDN – + + CTRL 1V MOSFET DRIVER A1 ERROR AMPLIFIER + + A3 + + 1.01V A4 R S Q1 MAIN SWITCH Q – A2 FB MAIN SWITCH DRIVER PWM COMPARATOR – + VC A8 RAMP GENERATOR + VIN A5 1V GND – SS SYNC 2.5MHz TO 250kHz OSCILLATOR – 100μA VREF VIN 10μA SS RT 1V + + – + A6 A7 Q2 2V FREQ ADJUST – VIN 3518 F01 Figure 1. Buck Mode LED Driver 3518fb 8 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. 3518fb 9 LT3518 APPLICATIONS INFORMATION Dimming Control There are two methods to control the current source for dimming using the LT3518. The first method uses the PWM pin to modulate the current source between zero and full current to achieve a precisely programmed average current. To make this method of current control more accurate, the switch demand current is stored on the VC node during the quiescent phase. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch is used in the LED current path to prevent the output capacitor from discharging in the PWM signal low phase. The minimum PWM on or off time will depend on the choice of operating frequency through RT input pin or SYNC pin. When using the SYNC function, the SYNC and PWM signals must have the aligned rising edges to achieve the optimized high PWM dimming ratio. For best current accuracy, the minimum PWM low or high time should be at least six switching cycles (3μs for fSW = 2MHz). Maximum PWM period is determined by the system and is unlikely to be longer than 12ms. The maximum PWM dimming ratio (PWMRATIO) can be calculated from the maximum PWM period (tMAX) and the minimum PWM pulse width (tMIN) as follows: t PWMRATIO = MAX tMIN (1) Example: PWMRATIO = 9ms/3μs = 3000:1 The second method of dimming control uses the CTRL pin to linearly adjust the current sense threshold during the PWM high state. When the CTRL pin voltage is less than 1V, the LED current is: VCTRL 10 • RSENSE ILED = 100mV RSENSE (3) The LED current programming feature possibly increases 2V VREF 45.3k 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: R1 VOUT = + 1 • 1.01V R2 tMAX = 9ms, tMIN = 3μs (fSW = 2MHz) ILED = When VCTRL is higher than 1V, the LED current is clamped to be: (4) VOUT LT3518 R1 FB R2 3518 F03 (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: R1 VOUT = • 1.01V + VBE(Q1) R2 (5) Table 1 provides some recommended inductor vendors. Table 1. Inductor Manufacturers VENDOR PHONE WEB Sumida (408) 321-9660 www.sumida.com Toko (408) 432-8281 www.toko.com Cooper (561) 998-4100 www.cooperet.com Vishay (402) 563-6866 www.vishay.com + R1 RSENSE VOUT – LT3518 Input Capacitor Selection LED ARRAY For proper operation, it is necessary to place a bypass capacitor to GND close to the VIN pin of the LT3518. A 1μF or greater capacitor with low ESR should be used. A ceramic capacitor is usually the best choice. FB R2 3518 F04 Figure 4 Inductor Selection The inductor used with the LT3518 should have a saturation current rating of 2A or greater. For buck mode LED drivers, the inductor value should be chosen to give a ripple current “ΔI” of ~30% to 40% of the LED current. In the buck mode, the inductor value can be estimated using the formula: DBUCK • t SW (µs) • ( VIN – VLED ) I V DBUCK = LED VIN L (µH) = (6) VLED is the voltage across the LED string, VIN is the input voltage to the converter, and tSW is the switching period. In the boost configuration, the inductor can be estimated using the formula: DBOOST • t SW (µs) • VIN I V –V DBOOST = LED IN VLED L (µH) = (7) 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) where D is the switch duty cycle. A 2.2μF 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 1μF ceramic capacitor is usually sufficient. For the LED boost and buck-boost mode applications, a 2.2μF ceramic capacitor is usually sufficient. Very high performance PWM dimming applications may require a larger capacitor value to support the LED voltage during PWM transitions. 3518fb 11 LT3518 APPLICATIONS INFORMATION Use only ceramic capacitors with X7R, X5R or better dielectric as they are best for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. Always check that the voltage rating of the capacitor is sufficient. Table 2 shows some recommended capacitor vendors. Table 3. Schottky Diodes Table 2. Ceramic Capacitor Manufacturers 10MQ060N 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 PART NUMBER VR (V) IAVE (A) 60 2 40 1 40 2.2 60 1.5 On Semiconductor MBRS260T3 Diodes Inc. DFLS140L Zetex ZLLS2000TA International Rectifier Board Layout 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. 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. Diode Selection Soft-Start 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. For many applications, it is necessary to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. A typical value for the soft-start capacitor is 0.1μF. Loop Compensation 3518fb 12 LT3518 APPLICATIONS INFORMATION Switching Frequency There are two methods to set the switching frequency of LT3518. Both methods require a resistor connected at RT pin. Do not leave the RT pin open. Also, do not load this pin with a capacitor. A resistor must always be connected for proper operation. One way to set the frequency is simply connecting an external resistor between the RT pin and GND. See Table 4 below or see the Oscillator Frequency vs RT graph in the Typical Performance Characteristics for resistor values and corresponding switching frequencies. Table 4. Switching Frequency vs RT Switching Frequency (kHz) RT ( kΩ ) 250 90.9 500 39.2 1000 16.9 1500 9.53 2000 6.04 2500 4.02 The other way is to make the LT3518 synchronize with an external clock via SYNC pin. For proper operation, a resistor should be connected at the RT pin and be able to generate a switching frequency 20% lower than the external clock when external clock is absent. In general, a lower switching frequency should be used where either very high or very low switching duty cycle operation is required, or high efficiency is desired. Selection of a higher switching frequency will allow use of smaller value external components and yield a smaller solution size and profile. Thermal Considerations The LT3518 is rated to a maximum input voltage of 30V for continuous operation, and 40V for nonrepetitive one second transients. Careful attention must be paid to the internal power dissipation of the LT3518 at higher input voltages to ensure that the maximum junction temperature is not exceeded. This junction limit is especially important when operating at high ambient temperatures. The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should then be connected to an internal copper ground plane with thermal vias placed directly under the package to spread out the heat dissipated by the LT3518. 3518fb 13 LT3518 TYPICAL APPLICATIONS Buck Mode 1.5A LED Driver RSENSE 68mΩ PVIN 24V VIN 3.3V C3 10μF M1 C2 2.2μF 1.5A ISP 1000:1 PWM Dimming at 120Hz L1 15μH PWM 5V/DIV D1 ISN TG SW VIN C1 2.2μF SHDN VREF CTRL FB PWM PWM SS ILED 1A/DIV LT3518 IL 1A/DIV RT SYNC TGEN VREF VC GND RT 16.9k 1MHz C4 0.1μF PVIN = 24V fOSC = 1MHz ILED = 1.5A C5 0.1μF 3518 TA02b 2μs/DIV 3518 TA02a C1: KEMET C0805C225K4RAC C2: MURATA GRM31MR71E225KA93 C3: MURATA GRM32DR71E106KA12B C4, C5: MURATA GRM21BR71H104KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-150M LEDS: LUXEON K2 (WHITE) M1: ZETEX ZXMP6A13GTA 500mA, 5V to 12V Boost Converter with Accurate Input Current Limit RSENSE L1 50m7 4.3MH VIN 5V Efficiency D1 VOUT 12V 500mA C2 2.2MF R1 549k 80 SW FB CTRL PWM SHDN LT3518 C2 10MF SYNC TGEN VREF VC R2 49.9k RT GND R3 10k C4 10nF EFFICIENCY (%) ISP TG ISN VIN SHDN 90 70 60 SS C3 0.1MF RT 6.04k 2MHz 50 3518 TA03a 0 100 300 200 ILOAD (mA) 400 500 3518 TA03b C1: KEMET C0805C225K4RAC C2: KEMET C1206C106K4RAC C3: MURATA GRM21BR71H104KA01B C4: MURATA GCM033R71A103KA03 D1: ZETEX ZLLS2000TA L1: TOKO B992AS-4R3N 3518fb 14 LT3518 TYPICAL APPLICATIONS Buck-Boost Mode LED Driver L1 4.3μH VIN 8V TO 16V SHDN VIN D1 C5 0.22μF R1 3.92M SW FB R2 124k PWM PWM TGEN ISP LT3518 RSENSE 330mΩ VREF ISN CTRL C1 2.2μF TG C4 0.1μF RT SS C2 4.7μF M1 SYNC VC 300mA GND RT 6.04k 2MHz C3 0.1μF 3518 TA04a C1: KEMET C0806C225K4RAC C2: KEMET C1206C475K3RAC C3, C4: MURATA GRM21BR71H104KA01B C5: MURATA GRM21BR71H224KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-4R3N LEDS: LUXEON I (WHITE) M1: ZETEX ZXMP6A13GTA 3000:1 PWM Dimming at 120Hz Efficiency 90 PWM 5V/DIV VIN = 10V CTRL = VREF 80 ILED 200mA/DIV EFFICIENCY (%) 70 IL1 1A/DIV VIN = 10V fOSC = 2MHz ILED = 300mA 500ns/DIV 3518 TA04b 60 50 40 30 20 0 20 40 60 80 100 PWM DUTY CYCLE (%) 3518 TA04c 3518fb 15 LT3518 TYPICAL APPLICATIONS Buck Mode 1A LED Driver with Open LED Protection and Sync Input RSENSE 100mΩ PVIN 32V M1 C2 2.2μF R1 49.9k LED1 1A FB R2 2.00k LED6 R3 5.62k VIN 3.3V ISP ISN C3 10μF Q1 L1 10μH TG D1 SW VIN C1 2.2μF SHDN VREF CTRL FB PWM PWM SS SYNC 3.3V, 1.2MHz RT SYNC TGEN VREF VC GND LT3518 C4 0.1μF FB RT 16.7k 1MHz C5 0.1μF 3518 TA05a C1: KEMET C0806C225K4RAC C2: MURATA GRM31MR71E225KA93 C3: MURATA GRM32DR71E106KA12B C4, C5: MURATA GRM21BR71H104KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-100M LEDS: LUXEON III (WHITE) M1: ZETEX ZXMP6A13GTA M2: PHILIPS PMBT3906 1000:1 PWM Dimming at 120Hz Efficiency 100 CTRL = VREF PWM 5V/DIV 90 EFFICIENCY (%) ILED 1A/DIV IL1 1A/DIV PVIN = 32V fOSC = 1.2MHz ILED = 1A 2μs/DIV 3518 TA05b 80 70 60 50 40 0 20 40 60 80 PWM DUTY CYCLE (%) 100 3518 TA05c 3518fb 16 LT3518 TYPICAL APPLICATIONS Boost 300mA LED Driver with LED Open Protection L1 8.2μH VIN 8V TO 16V SHDN VIN PWM PWM D1 SW ISP RSENSE 330mΩ TGEN LT3518 VREF C1 2.2μF ISN CTRL TG SYNC FB RT SS VC GND R1 1M C2 6.8μF LED1 LED2 RT 16.9k 1MHz C4 0.1μF M1 300mA C3 0.1μF R2 30.1k LED8 C1: KEMET C1206C225K2RAC C2: TDK C5750X7R1H685M C3, C4: MURATA GRM21BR71H104KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-8R2N LEDS: LUXEON I (WHITE) M1: ZETEX ZXMP6A13GTA 3518 TA06a 3000:1 PWM Dimming at 100Hz Efficiency 100 PWM 5V/DIV 90 ILED 200mA/DIV EFFICIENCY (%) 80 IL1 1A/DIV VIN = 12V fOSC = 1MHz ILED = 300mA 1μs/DIV 70 60 50 3518 TA06b 40 30 20 VIN = 12V CTRL = VREF 0 20 60 80 40 PWM DUTY CYCLE (%) 100 3518 TA06c 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 2.15 ± 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) 0.75 ± 0.05 R = 0.115 TYP 15 PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER 16 0.55 ± 0.20 PIN 1 TOP MARK (NOTE 6) 1 2.15 ± 0.10 (4-SIDES) 2 (UF16) QFN 1004 0.200 REF 0.00 – 0.05 0.30 ± 0.05 0.65 BSC NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 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 6.60 ±0.10 9 2.74 (.108) 4.50 ±0.10 2.74 6.40 (.108) (.252) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE 0.25 REF 1.10 (.0433) MAX 0° – 8° 0.65 (.0256) BSC 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 Efficiency 5.5V SEPIC Converter with Short-Circuit Protection C2 2.2μF C2 10μF L2 2.4μH VIN CTRL PWM SHDN 90 80 R1 221k TG ISN VREF R2 49.9k SS R3 10k C4 10nF C3 0.1μF 60 40 RT GND 70 50 ISP LT3518 TGEN VC VOUT 5.5V 500mA SW FB SYNC SHDN RSENSE 0.15Ω D1 EFFICIENCY (%) L1 2.4μH VIN 3V 100 C5 10μF 30 0 RT 6.04k 2MHz 100 200 300 400 500 ILOAD (mA) 3518 TA07b 3518 TA07a C1: KEMET C0805C225K4RAC C2, C5: KEMET C1206C106K4RAC C3: MURATA GRM21BR71H104KA01B C4: MURATA GCM033R71A103KA03 D1: ZETEX ZLLS2000TA L1, L2: TOKO 962BS-2R4M RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant Current, 1.4MHz, 1.5A Boost Converter VIN: 5V to 18V, VOUT(MAX) = 36V, Dimming = Analog/PWM, ISD < 1μA, MSOP10 Package LT3003 3-Channel LED Ballaster with PWM Dimming VIN: 3V to 48V, Dimming = 3000:1 True Color PWM, ISD < 5μA, MSOP10 Package LT3474 36V, 1A (ILED), 2MHz, Step-Down LED Driver VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 400:1 True Color PWM, ISD < 1μA, TSSOP16E Package LT3475 Dual 1.5A (ILED), 36V 2MHz Step-Down LED Driver VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 3000:1 True Color PWM, ISD < 1μA, TSSOP20E Package LT3476 Quad Output 1.5A, 36V, 2MHz High Current LED Driver with 1,000:1 Dimming VIN: 2.8V to 16V, VOUT(MAX) = 36V, Dimming = 1000:1 True Color PWM, ISD < 10μA, 5mm × 7mm QFN Package LT3477 3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver VIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1μA, QFN, TSSOP20E Packages LT3478/LT3478-1 4.5A, 42V, 2.5MHz High Current LED Driver with 3,000:1 Dimming VIN: 2.8V to 36V, VOUT(MAX) = 42V, Dimming = 3000:1 True Color PWM, ISD < 3μA, TSSOP16E Packages LT3479 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD < 1μA, DFN and TSSOP Packages LT3486 Dual 1.3A, 2MHz High Current LED Driver VIN: 2.5V to 24V, VOUT(MAX) = 36V, Dimming = 1000:1 True Color PWM, ISD < 1μA, 5mm × 3mm DFN, TSSOP16E LT3496 Triple Output LED Driver VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 3000:1 True Color PWM, ISD < 10μA, 4mm × 5mm QFN Package LT3517 Full-Featured LED Driver with 1.5A Switch Current VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 5000:1 True Color PWM, ISD < 1μA, 4mm × 4mm QFN and TSSOP Packages LT3590 48V Buck Mode 50mA LED Driver VIN: 4.5V to 55V, Drives Up to 10 LEDs, 200:1 Dimming, ISO = 15mA, 2mm × 2mm DFN SC70 LT3595 16 Channel Buck LED Driver Mode VIN: 4.5V to 45V, Drives Up to 160 LEDs, 5000:1 Dimming, 5mm × 9mm QFN LTC®3783 High Current LED Controller VIN: 3V to 36V, VOUT(MAX) = Ext FET, Dimming = 3000:1 True Color PWM, ISD < 20μA, 5mm × 4mm QFN10, TSSOP16E Packages 3518fb 20 Linear Technology Corporation LT 0308 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007