LT3517 Full-Featured LED Driver with 1.5A Switch Current FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION 5000:1 True Color PWMTM Dimming Ratio 1.5A, 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 ■ ■ ■ ■ Display Backlighting Automotive and Avionic Lighting Illumination Scanners The LT®3517 is a current mode DC/DC converter with an internal 1.5A, 45V switch specifically designed to drive LEDs. The LT3517 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 LT3517 can be synchronized to an external clock signal. The LED current is externally programmable with a 100mV sense resistor. The external PWM input provides up to 5000:1 LED dimming. The CTRL pin provides further 10:1 dimming ratio. The LT3517 is available in the tiny footprint 16-Lead QFN (4mm × 4mm) and the16-Pin TSSOP packages. The LT3517 provides a complete solution for both constant-voltage and constant-current applications. , 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 Efficiency 1A Buck Mode LED Driver 100 CTRL = VREF 90 2.2μF 1A 15μH ISP ISN TG VIN 3.3V 2.2μF SHDN VREF CTRL FB PWM PWM SS SW VIN LT3517 80 70 60 50 RT SYNC TGEN VREF VC GND 0.1μF EFFICIENCY (%) PVIN 24V 10μF 100mΩ 40 0 16.9k 1MHz 0.1μF 20 40 60 80 PWM DUTY CYCLE (%) 100 3517 TA01b 3517 TA01a 3517fa 1 LT3517 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) LT3517E ..............................................–40ºC to 125ºC LT3517I ...............................................–40ºC to 125ºC LT3517H .............................................–40ºC to 150ºC Storage Temperature Range QFN.....................................................–65ºC to 125ºC TSSOP ................................................–65ºC to 150ºC Lead Temperature (Soldering, 10 sec) TSSOP ..............................................................300ºC PIN CONFIGURATION TOP VIEW 3 14 TG SW 1 RT 4 13 ISP SW 2 SYNC 5 12 ISN VIN 3 9 VC 9 5 6 7 8 SS 10 FB 8 10 CTRL SYNC 7 CTRL 11 VC 17 SHDN 4 11 TGEN PWM 12 FB RT 6 16 15 14 13 VREF SS 17 GND TGEN 15 SW VREF ISN 16 SW 2 TG VIN SHDN ISP TOP VIEW 1 PWM UF PACKAGE 16-LEAD (4mm × 4mm) PLASTIC QFN 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, QJA = 40°C/W, QJC(PAD) = 10°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3517EUF#PBF LT3517EUF#TRPBF 3517 16-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C LT3517IUF#PBF LT3517IUF#TRPBF 3517 16-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C LT3517EFE#PBF LT3517EFE#TRPBF 3517FE 16-Lead Plastic TSSOP –40°C to 125°C LT3517IFE#PBF LT3517IFE#TRPBF 3517FE 16-Lead Plastic TSSOP –40°C to 125°C LT3517HFE#PBF LT3517HFE#TRPBF 3517HFE 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/ 3517fa 2 LT3517 ELECTRICAL CHARACTERISTICS The ● 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 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 Current Sense Voltage Line Regulation 30 ● 96 UNITS V V 100 100 103 mV mV 9 mV 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 Switching Frequency RT = 16.7k RT = 4.03k RT = 91.5k 1.0 2.5 250 1.15 2.7 270 MHz MHz kHz ● 0.85 2.25 220 RT Voltage 1 Soft-Start Pin Current SS = 0.5V, Out of Pin SYNC Pull-Down Current (Into the Pin) VSYNC = 2V 6 9 12 60 SYNC Input Low 1.5 RT = 91.5k (250kHz) SYNC = 300kHz Clock Signal, RT = 91.5k RT = 16.7k (1MHz) RT = 4.03k (2.5MHz) ● Switch Current Limit Switch VCESAT ISW = 1A 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 Maximum Duty Cycle V V V 95 94 85 97 96 90 74 1.5 1.9 % % % % 2.3 300 mV 2 20 A 100 μA nA Error Amplifier Transconductance 550 μS VC Output Impedance 1000 kΩ VC Idle Input Bias Current PWM = 0V, VC = 1V FB Pin Input Bias Current Current Out of Pin, VFB = 0.5V ● 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 SHDN Voltage High –20 0.98 0 20 nA 20 100 nA 1.01 1.04 V 300 nA 20 1.5 V SHDN Voltage Low SHDN Pin Bias Current PWM Input High Voltage 60 0.4 V 100 μA 1.5 V PWM Input Low Voltage PWM Pin Bias Current μA 60 0.4 V 120 μA 3517fa 3 LT3517 ELECTRICAL CHARACTERISTICS The ● 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 TGEN Input High Voltage TYP MAX UNITS 1.5 V TGEN Input Low Voltage 0.4 V 100 200 μA 2 2.04 V 0.03 %/V TGEN Pin Bias Current TGEN = 5V VREF Pin Voltage IREF = –100μA 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 LT3517E 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 LT3517I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3517H is guaranteed over the full –40ºC to 150ºC operating junction temperature range. Operating lifetime is derated at junction temperatures greater than 125ºC. ● 1.96 7 0 V V 0.3 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. TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit vs Duty Cycle VISP – VISN Threshold vs VCTRL SWITCH CURRENT LIMIT (A) VIN = 5V VISP = 24V 100 VC = 1V TA = 25°C 80 60 40 10000 1.5 1.0 1000 0.5 20 TA = 25°C 0 0 0 0.2 0.4 0.6 0.8 1.0 VCTRL (V) 1.2 1.4 1.6 3518 G01 TA = 25°C OSCILLATOR FREQUENCY (kHz) 120 VISP – VISN THRESHOLD (mV) Oscillator Frequency vs RT 2.0 0 20 40 60 DUTY CYCLE (%) 80 100 3517 G02 100 1 10 RT (kΩ) 100 3518 G03 3517fa 4 LT3517 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit vs Temperature 2.5 VIN = 5V 103 VISP = 24V VC = 1V 102 VCTRL = 2V SWITCH CURRENT LIMIT (A) VISP – VISN THRESHOLD (mV) 104 101 100 99 98 Oscillator Frequency vs Temperature 2.5 VIN = 5V OSCILLATOR FREQUENCY (MHz) VISP – VISN Threshold vs Temperature 2.0 1.5 1.0 0.5 VIN = 5V RT = 6.04k 2.3 2.1 1.9 1.7 97 96 –40 –20 0 0 –40 –20 0 20 40 60 80 100 120 140 160 TEMPERATURE (°C) Quiescent Current vs VIN 8 TA = 25°C 7 VC = 0V VIN = 5V 2.01 101 100 99 98 6 VIN CURRENT (mA) 102 VREF (V) VISP – VISN THRESHOLD (mV) 2.02 VCTRL = 2V VIN = 5V TA = 25°C VC = 1V 103 3517 G06 Reference Voltage vs Temperature VISP – VISN Threshold vs VISP 104 2.00 5 4 3 1.99 2 97 1 96 95 0 10 30 20 VISP (V) 40 50 1.98 –40 –20 0 0 20 10 30 40 VIN (V) 3517 G08 3517 G09 PMOS Turn-On FB Pin Threshold vs Temperature PMOS Turn-Off VIN = 5V 1.03 FB PIN THRESHOLD (V) 0 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 3517 G07 1.04 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 3518 G05 3517 G04 105 1.5 –40 –20 0 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 5V 5V PWM PWM 1.02 0V 0V 1.01 40V 40V 1.00 TG TG 30V 30V 0.99 0.98 –40 –20 0 20 40 60 80 100 120 140 160 TEMPERATURE (°C) VISP = 40V 200ns/DIV 3517 G11 VISP = 40V 200ns/DIV 3517 G11 3517 G10 3517fa 5 LT3517 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 VREF pin or 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 PMOS gate. Leave TG unconnected if not used. Ground: Exposed Pad. Solder paddle directly to ground plane. 3517fa 6 LT3517 BLOCK DIAGRAM LED ARRAY CIN CFILT RSENSE PVIN ISP + ISN TG – TGEN SW PWM VISP CURRENT SENSE AMPLIFIER X10 VISP – 7V SHDN – + + CTRL 1V MOSFET DRIVER A1 ERROR AMPLIFIER + + A3 + + 1.01V A4 R S Q1 POWER 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 3517 F01 Figure 1. Buck Mode LED Driver 3517fa 7 LT3517 OPERATION The LT3517 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 LT3517 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. APPLICATIONS INFORMATION Dimming Control There are two methods to control the current source for dimming using the LT3517. 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 four switching cycles (2μs 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: tMAX = 10ms, tMIN = 2μs (fSW = 2MHz) PWMRATIO = 10ms/2μs = 5000:1 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 3517fa 8 LT3517 APPLICATIONS INFORMATION than 1V, the LED current is: ILED = VCTRL 10 • RSENSE (2) When VCTRL is higher than 1V, the LED current is clamped to be: ILED = 100mV RSENSE 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: VOUT = R1 • 1.01V + VBE(Q2) R2 (3) + R1 RSENSE VOUT The LED current programming feature possibly increases total dimming range by a factor of ten. LT3517 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. – Q2 LED ARRAY FB R2 3517 F04 Figure 4 Inductor Selection 2V VREF (5) 45.3k The inductor used with the LT3517 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: 49.9k CTRL 5k PTC 3517 F02 Figure 2 L (µH) = 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 ⎠ (4) VOUT LT3517 R1 DBUCK • t SW (µs) • (VIN – VLED) ΔI DBUCK = (6) VLED VIN 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: L (µH) = DBOOST • t SW (µs) • VIN ΔI (7) FB R2 3517 F03 DBOOST = VLED – VIN VLED Figure 3 3517fa 9 LT3517 APPLICATIONS INFORMATION 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 LT3517. A 1μF 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) 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. 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 PHONE WEB Taiyo Yuden (408) 573-4150 www.t-yuden.com AVX (843) 448-9411 www.avxcorp.com where D is the switch duty cycle. A 2.2μF ceramic type capacitor is usually sufficient. Murata (770) 436-1300 www.murata.com TDK (847) 803-6100 www.tdk.com Output Capacitor Selection Loop Compensation 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. The LT3517 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 3517fa 10 LT3517 APPLICATIONS INFORMATION 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. Table 3. Schottky Diodes PART NUMBER VR (V) IAVE (A) 60 1 40 1 60 1.5 Diodes Inc. DFLS160 Zetex ZLLS1000TA International Rectifier 10MQ060N Board Layout The high speed operation of the LT3517 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 LT3517 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 0.1μF. 3517fa 11 LT3517 APPLICATIONS INFORMATION Switching Frequency There are two methods to set the switching frequency of LT3517. 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 LT3517 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 LT3517 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 LT3517 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 LT3517. 3517fa 12 LT3517 TYPICAL APPLICATIONS Buck Mode 1A LED Driver RSENSE 100mΩ PVIN 24V VIN 3.3V C2 2.2μF L1 15μH 1A ISP VIN 2000:1 PWM Dimming at 120Hz C3 10μF M1 PWM 5V/DIV D1 ISN TG SW ILED 1A/DIV IL 1A/DIV C1 2.2μF SHDN VREF CTRL FB PWM PWM SS LT3517 RT 16.9k 1MHz C4 0.1μF 3517 TA02b 1μs/DIV RT SYNC TGEN VREF VC GND PVIN = 24V fOSC = 1MHz ILED = 1A C5 0.1μF 3517 TA02a C1: KEMET C0805C225K4RAC C2: MURATA GRM31MR71E225KA93 C3: MURATA GRM32DR71E106KA12B C4, C5: MURATA GRM21BR71H104KA01B D1: DIODES DFLS160 L1: TOKO B992AS-150M LEDS: LUXEON K2 (WHITE) M1: ZETEX ZXMP6A13FTA 350mA, 5V to 12V Boost Converter with Accurate Input Current Limit RSENSE L1 100mΩ 6.8μH VIN 5V D1 C1 2.2μF 90 R1 549k 80 SW FB CTRL PWM SHDN LT3517 C2 10μF SYNC TGEN VREF VC R2 49.9k RT GND R3 10k C4 10nF EFFICIENCY (%) ISP TG ISN VIN SHDN Efficiency VOUT 12V 350mA 70 60 SS C3 0.1μF RT 6.04k 2MHz 50 50 3517 TA03a 100 150 200 250 ILOAD(mA) 300 350 3517 TA03b C1: KEMET C0805C225K4RAC C2: KEMET C1206C106K4RAC C3: MURATA GRM21BR71H104KA01B C4: MURATA GCM033R71A103KA03 D1: ZETEX ZLLS1000TA L1: TOKO B992AS-6R8N 3517fa 13 LT3517 TYPICAL APPLICATIONS Buck-Boost Mode LED Driver L1 6.8μH VIN 8V TO 16V SHDN VIN D1 C5 0.22μF R1 3.92M SW FB R2 124k PWM 300mA PWM TGEN LT3517 ISP RSENSE 330mΩ VREF C1 2.2μF CTRL ISN TG M1 C2 4.7μF SYNC VC C4 0.1μF RT SS GND RT 6.04k 2MHz 3517 TA04a C3 0.1μF C1: KEMET C0806C225K4RAC C2: KEMET C1206C475K3RAC C3, C4: MURATA GRM21BR71H104KA01B C5: MURATA GRM21BR71H224KA01B D1: DIODE DFLS160 L1: TOKO B992AS-6R8N LEDS: LUXEON I (WHITE) M1: ZETEX ZXMP6A13FTA Efficiency 5000:1 PWM Dimming at 100Hz 90 VIN = 12V CTRL = VREF 80 PWM 5V/DIV 70 EFFICIENCY (%) ILED 200mA/DIV IL 1A/DIV 60 50 40 500ns/DIV VIN = 12V fOSC = 2MHz ILED = 300mA 3517 TA04b 30 20 0 20 40 60 80 PWM DUTY CYCLE (%) 100 3517 TA04c 3517fa 14 LT3517 TYPICAL APPLICATIONS Low Side Current Sensing Load Dump Protected Buck-Boost Mode LED Driver L1 6.8μH VIN 8V TO 16V SHDN VIN D1 C5 0.22μF R1 3.92M SW FB R2 124k PWM PWM TGEN LT3517 ISP VREF RSENSE 330mΩ CTRL C1 2.2μF C2 4.7μF ISN SYNC TG RT SS VC R3 10k C4 33nF GND M1 300mA RT 6.04k 2MHz C3 0.1μF 3517 TA05a D1: DIODES DFLS160 L1: TOKO B992AS-6R8N C1: KEMET C0806C225K4RAC C2: KEMET C1206C475K3RAC C3: MURATA GRM21BR71H104KA01B C4: MURATA GRM219R71H333KAQ01B C5: MURATA GRM21BR71H224KA01B M1: ZETEX ZXMP6A13FTA LEDs: LUXEON I (WHITE) Efficiency 5000:1 PWM Dimming at 100Hz 90 VIN = 12V CTRL = VREF 80 PWM 5V/DIV 70 EFFICIENCY (%) ILED 200mA/DIV IL 1A/DIV 60 50 40 500ns/DIV 3517 TA05b 30 VIN = 12V fOSC = 2MHz ILED = 300mA 20 0 Load Dump Response 20 40 60 80 PWM DUTY CYCLE (%) 100 3517 TA05c VISP 10V/DIV VIN 10V/DIV VISP 15V TO 40V VIN 15V TO 40V VISP REF GND ILED 200mA/DIV VIN REF GND IL 1A/DIV 5ms/DIV 3517 TA05d VIN Raises From 15V to 40V in 5ms. 3517fa 15 LT3517 TYPICAL APPLICATIONS Boost 100mA LED Driver with LED Open Protection L1 22μH VIN 8V TO 16V SHDN VIN PWM PWM D1 SW ISP RSENSE 1Ω TGEN LT3517 VREF C1 2.2μF ISN CTRL TG SYNC FB RT SS VC GND R1 1M C2 2.2μF LED1 LED2 RT 16.9k 1MHz C4 0.1μF M1 100mA C3 0.1μF R2 30.1k LED10 C1, C2: KEMET C1206C225K2RAC C3, C4: MURATA GRM21BR71H104KA01B D1: DIODES DFLS160 L1: COILCRAFT DS3316P-223 LEDS: CREE XLAMP 7090 M1: ZETEX ZXMP6A13FTA 3517 TA06a Efficiency 3000:1 PWM Dimming at 100Hz 90 80 PWM 5V/DIV EFFICIENCY (%) 70 ILED 100mA/DIV IL 500mA/DIV VIN = 12V CTRL = VREF 60 50 40 1μs/DIV VIN = 12V fOSC = 1MHz ILED = 100mA 3517 TA06b 30 20 0 20 40 60 80 PWM DUTY CYCLE (%) 100 3517 TA06c 3517fa 16 LT3517 TYPICAL APPLICATION 5.5V SEPIC Converter with Short-Circuit Protection Efficiency C5 10μF L1 4.3μH VIN 3V C2 2.2μF L2 4.3μH PWM SHDN TG ISP LT3517 TGEN ISN VREF RT VC GND R3 10k C4 10nF R2 49.9k SS C3 0.1μF 90 R1 221k SW FB SYNC SHDN C2 10μF VOUT 5.5V 350mA 80 EFFICIENCY (%) VIN CTRL RSENSE 0.22Ω D1 RT 6.04k 2MHz 70 60 50 40 30 0 50 100 150 200 ILOAD (mA) 250 300 350 3517 TA07a C1: KEMET C0805C225K4RAC C2, C5: KEMET C1206C106K4RAC C3: MURATA GRM21BR71H104KA01B C4: MURATA GCM033R71A103KA03 D1: ZETEX ZLLS1000TA L1, L2: TOKO B992AS-4R3N 3517 TA07b 3517fa 17 LT3517 PACKAGE DESCRIPTION UF Package 16-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1692) 0.72 p0.05 4.35 p 0.05 2.15 p 0.05 2.90 p 0.05 (4 SIDES) NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (W 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 A 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 PACKAGE OUTLINE 0.30 p0.05 0.65 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW—EXPOSED PAD 4.00 p 0.10 (4 SIDES) 0.75 p 0.05 R = 0.115 TYP 15 PIN 1 NOTCH R = 0.20 TYP OR 0.35 s 45o CHAMFER 16 0.55 p 0.20 PIN 1 TOP MARK (NOTE 6) 1 2.15 p 0.10 (4-SIDES) 2 (UF16) QFN 1004 0.200 REF 0.00 – 0.05 0.30 p 0.05 0.65 BSC 3517fa 18 LT3517 PACKAGE DESCRIPTION FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation BA 4.90 – 5.10* (.193 – .201) 2.74 (.108) 2.74 (.108) 16 1514 13 12 1110 6.60 p0.10 9 2.74 (.108) 4.50 p0.10 2.74 6.40 (.108) (.252) BSC SEE NOTE 4 0.45 p0.05 1.05 p0.10 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 3517fa 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 LT3517 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 = 3,000:1 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 = 3,000:1 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 = 1,000:1 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 = 3,000:1 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 = 1,000:1 True Color PWM, ISD < 1μA, 5mm × 3mm DFN, TSSOP16E LT3496 Triple Output LED Driver VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 3,000:1 True Color PWM, ISD < 10μA, 4mm × 5mm QFN Package LT3518 Full-Featured LED Driver with 2.3A Switch Current VIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 3,000:1 True Color PWM, ISD < 10μA, 4mm × 4mm QFN Package 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, 5,000:1 Dimming, 5mm × 9mm QFN LTC®3783 High Current LED Controller VIN: 3V to 36V, VOUT(MAX) = Ext FET, Dimming = 3,000:1 True Color PWM, ISD < 20μA, 5mm × 4mm QFN10, TSSOP16E Packages 3517fa 20 Linear Technology Corporation LT 0308 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007