LT3478/LT3478-1 4.5A Monolithic LED Drivers with True Color PWM Dimming DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ True Color PWM™ Dimming Delivers Constant LED Color with Up to 3000:1 Range Wide Input Voltage Range: 2.8V to 36V 4.5A, 60mΩ, 42V Internal Switch Drives LEDs in Boost, Buck-Boost or Buck Modes Integrated Resistors for Inductor and LED Current Sensing Program LED Current: 100mA to 1050mA (LT3478-1) (10mV to 105mV)/RSENSE (LT3478) Program LED Current De-Rating vs Temperature Separate Inductor Supply Input Inrush Current Protection Programmable Soft-Start Fixed Frequency Operation from 200kHz to 2.25MHz Open LED Protection (Programmable OVP) Accurate Shutdown/UVLO Threshold with Programmable Hysteresis 16-Pin Thermally Enhanced TSSOP Package U APPLICATIO S ■ ■ High Power LED Driver Automotive Lighting The LT®3478/LT3478-1 are 4.5A step-up DC/DC converters designed to drive LEDs with a constant current over a wide programmable range. Series connection of the LEDs provides identical LED currents for uniform brightness without the need for ballast resistors and expensive factory calibration. The LT3478-1 reduces external component count and cost by integrating the LED current sense resistor. The LT3478 uses an external sense resistor to extend the maximum programmable LED current beyond 1A and also to achieve greater accuracy when programming low LED currents. Operating frequency can be set with an external resistor from 200kHz up to 2.25MHz. Unique circuitry allows a PWM dimming range up to 3000:1 while maintaining constant LED color. The LT3478/LT3478-1 are ideal for high power LED driver applications such as automotive TFT LCD backlights, courtesy lighting and heads-up displays. One of two CTRL pins can be used to program maximum LED current. The other CTRL pin can be used to program a reduction in maximum LED current vs temperature to maximize LED usage and improve reliability. Additional features include inrush current protection, programmable open LED protection and programmable soft-start. Each part is available in a 16-pin thermally enhanced TSSOP Package. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Patents Pending. U TYPICAL APPLICATIO Automotive TFT LCD Backlight VIN 8V TO 16V VIN VS L 10µF SW OUT VREF 45.3k CTRL2 LT3478-1 0.1Ω RSENSE (LT3478) LED OVPSET 54.9k 90 85 CTRL1 130k ILED = 700mA fOSC = 500kHz PWM DUTY CYCLE = 100% 95 SHDN EFFICIENCY (%) 4.7µF Efficiency vs VIN 100 10µH PWM 1µF SS 700mA 15W 6 LEDs (WHITE) RT VC 0.1µF 69.8k PWM DIMMING CONTROL 6 LEDs LUXEON III (WHITE) 80 8 10 12 VIN (V) 14 16 3478 TA01b 3478 TA01 34781f 1 LT3478/LT3478-1 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) SW ............................................................................42V VOUT, LED ..................................................................42V VIN, VS, VL, ⎯S⎯H⎯D⎯N (Note 5) .......................................36V PWM .........................................................................15V CTRL1, 2 .....................................................................6V SS, RT, VC, VREF, OVPSET............................................2V Operating Junction Temperature Range (Notes 2, 3, 4).................................... –40°C to 125°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 Sec) .................. 300°C TOP VIEW SW 1 16 SS SW 2 15 RT VIN 3 14 PWM VS 4 L 5 12 CTRL1 VOUT 6 11 SHDN LED 7 10 VREF OVPSET 8 9 17 13 CTRL2 VC FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 35°C/W EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB. ORDER PART NUMBER FE PART MARKING LT3478EFE LT3478EFE-1 LT3478IFE LT3478IFE-1 3478FE 3478FE-1 3478FE 3478FE-1 Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. SW = open, VIN = VS = L = VOUT = ⎯S⎯H⎯D⎯N = 2.7V, LED = open, SS = open, PWM = CTRL1, CTRL2 = 1.25V, VREF = open, VC = open, RT = 31.6k. PARAMETER CONDITIONS Minimum Operating Voltage (Rising) Operational Input Voltage VS VIN (Note 5) VIN Quiescent Current VC = 0V (No Switching) VIN Shutdown Current ⎯S⎯H⎯D⎯N = 0V ⎯S⎯H⎯D⎯N Pin Threshold (VSD_µp) (Micropower) ⎯S⎯H⎯D⎯N Pin Threshold (VSD_UVLO) (Switching) ⎯S⎯H⎯D⎯N Pin Current ⎯S⎯H⎯D⎯N = VSD_UVLO – 50mV ⎯S⎯H⎯D⎯N = VSD_UVLO + 50mV VREF Voltage I(VREF) = 0µA, VC = 0V VREF Line Regulation I(VREF) = 0µA, 2.7V < VIN < 36V VREF Load Regulation 0 < I(VREF) < 100µA (Max) Frequency: fOSC 200kHz RT = 200k Frequency: fOSC 1MHz RT = 31.6k MIN ● TYP MAX 2.4 2.8 V 36 36 V V 2.8 2.8 6.1 ● ● mA 3 6 µA 0.1 0.4 0.7 V 1.3 1.4 1.5 V 8 10 0 12 µA µA 1.213 0.18 ● UNITS 0.88 1.240 1.263 V 0.005 0.015 %/V 8 12 mV 0.2 0.22 MHz 1.12 MHz 34781f 2 LT3478/LT3478-1 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. SW = open, VIN = VS = L = VOUT = ⎯S⎯H⎯D⎯N = 2.7V, LED = open, SS = open, PWM = CTRL1, CTRL2 = 1.25V, VREF = open, VC = open, RT = 31.6k. PARAMETER CONDITIONS Frequency: fOSC 2.25MHz RT = 9.09k Line Regulation fOSC RT = 31.6k, 2.7V < VIN < 36V MIN TYP MAX 2 2.25 2.6 MHz 0.05 0.2 %/V Nominal RT Pin Voltage UNITS 0.64 V 88 97 73 % % % (Note 6) 770 µA/A (Note 6) 400 V/A 13 A/V Maximum Duty Cycle RT = 31.6k RT = 200k RT = 9.09k LED Current to VC Current Gain LED Current to VC Voltage Gain ● 80 VC to Switch Current Gain VC Source Current (Out of Pin) CTRL1 = 0.4V, VC = 1V 40 µA VC Sink Current CTRL1 = 0V, VC = 1V 40 µA VC Switching Threshold 0.65 V VC High Level (VOH) CTRL1 = 0.4V 1.5 V VC Low Level (VOL) CTRL1 = 0V 0.2 V Inductor Current Limit 2.7V < VS < 36V Switch Current Limit Switch VCE SAT ISW = 4.5A Switch Leakage Current SW = 42V, VC = 0V VOUT Overvoltage Protection (OVP) (Rising) OVPSET = 1V OVPSET = 0.3V Full Scale LED Current (LT3478-1) CTRL1 = VREF, Current Out of LED Pin 700mA LED Current (LT3478-1) CTRL1 = 700mV, Current Out of LED Pin 350mA LED Current (LT3478-1) 100mA LED Current (LT3478-1) ● 4.5 ● 4.5 6 6.8 6.3 7.5 A A 270 mV 1 µA 41 12.3 V V 1010 1050 1090 mA 655 700 730 mA CTRL1 = 350mV, Current Out of LED Pin 325 350 375 mA CTRL1 = 100mV, Current Out of LED Pin 70 100 130 mA 101 105 109 mV 67 70.5 74 mV ● Full Scale LED Current VSENSE (LT3478) CTRL1 = VREF, VSENSE = VVOUT – VLED ● CTRL1 = 700mV, VSENSE (LT3478) CTRL1 = 700mV, VSENSE = VVOUT – VLED CTRL1 = 350mV, VSENSE (LT3478) CTRL1 = 350mV, VSENSE = VVOUT – VLED 33 35.5 38 mV CTRL1 = 100mV, VSENSE (LT3478) CTRL1 = 100mV, VSENSE = VVOUT – VLED 7 10 13 mV CTRL1, 2 Input Currents CTRL1 = 100mV, CTRL2 = 1.25V or CTRL2 = 100mV, CTRL1 = 1.25V (Current Out of Pin) 40 nA OVPSET Input Current OVPSET = 1V, VOUT = 41V (Current Out of Pin) 200 nA PWM Switching Threshold 0.8 1 1.2 V VC Pin Current in PWM Mode VC = 1V, PWM = 0 1 50 nA OUT Pin Current in PWM Mode PWM = 0 1 100 nA SS Low Level (VOL) I(SS) = 20µA 0.15 V SS Reset Threshold VC = 0V 0.25 V SS High Level (VOH) VC = 0V 1.5 V Soft-Start (SS) Pin Charge Current SS = 1V, Current Out of Pin, VC = 0V 12 µA Soft-Start (SS) Pin Discharge Current SS = 0.5V, VC = 0V 350 µA 34781f 3 LT3478/LT3478-1 ELECTRICAL CHARACTERISTICS 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 LT3478EFE/LT3478EFE-1 are 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 LT3478IFE/LT3478IFE-1 are guaranteed over the full –40°C to 125°C operating junction temperature range. Note 3: This IC includes over-temperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when over-temperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 4: For maximum operating ambient temperature, see the “Thermal Calculations” section in the Applications Information section. Note 5: The maximum operational voltage for VIN is limited by thermal and efficiency considerations. Power switch base current is delivered from VIN and should therefore be driven from the lowest available power supply in the system. See “Thermal Calculations” in the Applications Information section. Note 6: For LT3478, parameter scales • (RSENSE/0.1Ω). U W TYPICAL PERFOR A CE CHARACTERISTICS LED Current vs CTRL1 1400 LED Current vs Temperature 1400 TA = 25°C CTRL2 = VREF (FOR LT3478 SCALE BY 0.1Ω/RSENSE) LT3478-1 700 LED CURRENT (mA) 1050 LED CURRENT (mA) LED CURRENT (mA) 1000 (FOR LT3478 SCALE BY 0.1Ω/RSENSE) ILED = 1050mA, CTRL1 = CTRL2 = VREF 1050 LT3478-1 700 ILED = 100mA, CTRL1 = 100mV, CTRL2 = VREF VREF 0 –50 –25 50 75 100 0 25 JUNCTION TEMPERATURE (°C) 0 0 0.35 0.70 CTRL1 (V) 1.05 1.40 240 50 CTRL1 = 0.1V CTRL1 = 0.7V CTRL1 = 0.9V 0 –50 –25 50 75 100 0 25 JUNCTION TEMPERATURE (°C) 7.0 TA = 25°C 6.5 CURRENT LIMIT (A) SWITCH VCE (SAT) (mV) CTRL1 = 0.35V 180 120 60 3478 G04 6.0 SWITCH INDUCTOR 5.5 5.0 0 125 100 Switch and Inductor Peak Current Limits vs Temperature 210 30 0.1 1 10 PWM DUTY CYCLE (%) 3478 G03 Switch VCE (SAT) vs Switch Current CTRL1 Pin Current vs Temperature 20 CTRL2 = VREF CTRL1 AND CTRL2 PINS INTERCHANGEABLE 10 0 0.01 125 3478 G02 3478 G01 40 TA = 25°C VIN = VS = 12V 6 LEDS AT 500mA PWM FREQ = 100Hz 100 CTRL1 = 0.5V CTRL2 = VREF FOSC = 1.6MHz L = 2.2µH 10 1 350 350 CTRL1 PIN CURRENT X (–1) (nA) LED Current vs PWM Duty Cycle Wide PWM Dimming Range (3000:1) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 SWITCH CURRENT (A) 3478 G05 4.5 –50 –25 50 75 100 0 25 JUNCTION TEMPERATURE (°C) 125 3478 G06 34781f 4 LT3478/LT3478-1 U W TYPICAL PERFOR A CE CHARACTERISTICS 1.60 1.50 1.24 SHDN (V) VREF (V) 1.26 15 SHDN PIN CURRENT (µA) 1.28 1.22 1.40 1.30 1.20 1.18 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 1.20 –50 –25 50 75 100 0 25 JUNCTION TEMPERATURE (°C) 3478 G07 JUST BEFORE PART TURNS ON 10 5 AFTER PART TURNS ON 0 –50 –25 50 75 100 0 25 JUNCTION TEMPERATURE (°C) 125 3478 G08 VIN Shutdown Current vs Temperature 50 ⎯S⎯H⎯D⎯N Pin (Hysteresis) Current vs Temperature ⎯S⎯H⎯D⎯N Threshold vs Temperature VREF vs Temperature 3478 G09 VIN Quiescent Current vs Temperature VIN Quiescent Current vs VIN SHDN = 0V 125 14 14 12 12 10 10 20 VIN = 36V VIN = 20V VIN CURRENT (mA) VIN CURRENT (mA) 30 8 6 4 8 6 4 10 2 VIN = 2.8V 0 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 0 125 0 3 6 9 12 15 18 21 24 27 30 33 36 VIN (V) VIN = 2.8V V = 0V 0 C –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 3478 G11 VS, L, SW Shutdown Currents vs Temperature 125 3478 G12 Switch Peak Current Limit vs Duty Cycle 7 SHDN = 0V VS = L = SW = 36V SWITCH PEAK CURRENT LIMIT (A) 4 2 TA= 25°C VC = 0V 3478 G10 PIN CURRENT (µA) VIN CURRENT (µA) 40 2 I(VS PIN) = I(L PIN) I(SW PIN) 0 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 6 5 4 3 2 1 0 125 3478 G18 TA= 25°C 0 20 40 60 DUTY CYCLE (%) 80 100 3478 G19 34781f 5 LT3478/LT3478-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Switching Frequency vs Temperature Switching Frequency vs RT 1.20 1000 100 1 10 100 1000 RT (kΩ) 43.0 RT = 31.6k 1.15 42.5 1.10 42.0 VOUT CLAMP (V) TA = 25°C SWITCHING FREQUENCY (MHz) SWITCHING FREQUENCY (kHz) 10000 Open-Circuit Output Clamp Voltage vs Temperature 1.05 1.00 0.95 41.0 40.5 40.0 0.85 39.5 3478 G13 125 39.0 0 25 –50 –25 50 75 100 JUNCTION TEMPERATURE (°C) 3478 G14 125 3478 G15 VC Pin Active and Clamp Voltages vs Temperature SS Pin Charge Current vs Temperature 14 1.8 1.5 VC CLAMP 13 1.2 VC (V) SS PIN CURRENT (µA) (OUT OF PIN) 41.5 0.90 0.80 0 25 –50 –25 50 75 100 JUNCTION TEMPERATURE (°C) OVPSET = 1V 12 0.9 0.6 VC ACTIVE THRESHOLD 11 0.3 10 –50 –25 50 75 100 0 25 JUNCTION TEMPERATURE (°C) 125 3478 G16 0 0 25 –50 –25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3478 G17 34781f 6 LT3478/LT3478-1 U U U PI FU CTIO S SW (Pins 1, 2): Switch Pin. Collector of the internal NPN power switch. Both pins are fused together inside the IC. Connect the inductor and diode here and minimize the metal trace area connected to this pin to minimize EMI. VIN (Pin 3): Input Supply. Must be locally bypassed with a capacitor to ground. VS (Pin 4): Inductor Supply. Must be locally bypassed with a capacitor to ground. Can be shorted to VIN if only one supply is available (see L (Pin 5) function). L (Pin 5): Inductor Pin. An internal resistor between VS and L pins monitors inductor current to protect against inrush current. Exceeding 6A immediately turns off the internal NPN power switch and discharges the soft-start pin. Input current monitoring can be disabled by connecting the inductor power supply directly to the L pin and leaving the VS pin open (requires local bypass capacitor to GND on L pin; not VS pin). VOUT (Pin 6): Output voltage of the converter. Connect a capacitor from this pin to ground. Internal circuitry monitors VOUT for protection against open LED faults. LED (Pin 7): Connect the LED string from this pin to ground. An internal (LT3478-1)/external (LT3478) resistor between the VOUT and LED pins senses LED current for accurate control. OVPSET (Pin 8): Programs VOUT overvoltage protection level (OVP) to protect against open LED faults. OVP = (OVPSET • 41)V. OVPSET range is 0.3V to 1V for an OVP range of typically 12.3V to 41V. VC (Pin 9): Output of the transconductance error amplifier and compensation pin for the converter regulation loop. VREF (Pin 10): Bandgap Voltage Reference. This pin can supply up to 100µA. Can be used to program CTRL1, CTRL2, OVPSET pin voltages using resistor dividers to ground. ⎯ ⎯H⎯D⎯N (Pin 11): The ⎯S⎯H⎯D⎯N pin has an accurate 1.4V S threshold and can be used to program an undervoltage lockout (UVLO) threshold for system input supply using a resistor divider from supply to ground. A 10µA pin current hysteresis allows programming of undervoltage lockout (UVLO) hysteresis. ⎯S⎯H⎯D⎯N above 1.4V turns the part on and removes a 10µA sink current from the pin. ⎯S⎯H⎯D⎯N = 0V ⎯ H ⎯ D ⎯ N ⎯ can be directly connected reduces VIN current < 3µA. S to VIN. If left open circuit the part will be turned off. CTRL1 (Pin 12): CTRL1 pin voltage is used to program maximum LED current (CTRL2 = VREF). CTRL1 voltage can be set by a resistor divider from VREF or an external voltage source. Maximum LED current is given by: (LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps (LT3478) Max LED Current = 0.1 Min(CTRL, 1.05) • Amps RSENSE (linear for 0.1V < CTRL1< 0.95V ; CTRL2 = VREF) For maximum LED current, short CTRL1 and CTRL2 pins to VREF. CTRL2 (Pin 13): The CTRL2 pin is available for programming a decrease in LED current versus temperature (setting temperature breakpoint and slope). This feature allows the output LED(s) to be programmed for maximum allowable current without damage at higher temperatures. This maximizes LED usage and increases reliability. A CTRL2 voltage with negative temperature coefficient is created using an external resistor divider from VREF with temperature dependant resistance. If not used, CTRL2 should be tied to VREF. PWM (Pin 14): Input pin for PWM dimming control. Above 1V allows converter switching and below 1V disables switching with VC pin level maintained. With an external MOSFET placed in series with the ground side of the LED string, a PWM signal driving the PWM pin and MOSFET gate provides accurate dimming control. The PWM signal can be driven from 0V to 15V. If unused, the pin should be connected to VREF. RT (Pin 15): A resistor to ground programs switching frequency between 200kHz and 2.25MHz. SS (Pin 16): Soft-Start Pin. Placing a capacitor here programs soft-start timing to limit inductor inrush current during start-up due to the converter. When inductor current 34781f 7 LT3478/LT3478-1 U U U PI FU CTIO S exceeds 6A or VOUT exceeds OVP, an internal soft-start latch is set, the power NPN is immediately turned off and the SS pin is discharged. The soft-start latch is also set if VIN and/or ⎯S⎯H⎯D⎯N do not meet their turn on thresholds. The SS pin only recharges when all faults are removed and the pin has been discharged below 0.25V. Exposed Pad (Pin 17): The ground for the IC and the converter. The FE package has an Exposed Pad underneath the IC which is the best path for heat out of the package. Pin 17 should be soldered to a continuous copper ground plane under the device to reduce die temperature and increase the power capability of the LT3478/LT3478-1. W BLOCK DIAGRA SHDN L VS 11 4 SS 5 10µA 9.5mΩ + – + 1.4V VIN REF 1.24V 3 1, 2 VOUT VC – UVLO SW 16 6 OVERVOLTAGE DETECT – 57mV OVPSET INRUSH CURRENT PROTECTION + 100Ω RSENSE 0.1Ω (INTERNAL FOR LT3478-1) SOFT-START RSENSE (EXTERNAL FOR LT3478) LED 7 PWM DETECT VREF 10 OSC S Q Q1 R LED 1.05V + + + – 13 GM LED – + CTRL2 LED SLOPE COMP Q2 – 12 LED + – PWM CTRL1 Σ 1V PWM 14 + + 1000Ω RS – TO OVERVOLTAGE DETECT CIRCUIT 8 15 OVPSET 17 EXPOSED PAD (GND) RT 9 3478 F01 VC Figure 1 34781f 8 LT3478/LT3478-1 U OPERATIO The LT3478/LT3478-1 are high powered LED drivers with a 42V, 4.5A internal switch and the ability to drive LEDs with up to 1050mA for LT3478-1 and up to 105mV/RSENSE for LT3478. the VC voltage controls the peak switch current limit and hence the inductor current available to the output LED(s). As with all current mode converters, slope compensation is added to the control path to ensure stability. The LT3478/LT3478-1 work similarly to a conventional current mode boost converter but use LED current (instead of output voltage) as feedback for the control loop. The Block Diagram in Figure 1 shows the major functions of the LT3478/LT3478-1. The CTRL1 pin is used to program maximum LED current via Q2. The CTRL2 pin can be used to program a decrease in LED current versus temperature for maximum reliability and utilization of the LED(s). A CTRL2 voltage with negative temperature coefficient can be created using an external resistor divider from VREF with temperature dependant resistance. Unused CTRL2 is tied to VREF. For the part to turn on, the VIN pin must exceed 2.8V and the ⎯S⎯H⎯D⎯N pin must exceed 1.4V. The ⎯S⎯H⎯D⎯N pin threshold allows programming of an undervoltage lockout (UVLO) threshold for the system input supply using a simple resistor divider. A 10µA current flows into the ⎯S⎯H⎯D⎯N pin before part turn on and is removed after part turn on. This current hysteresis allows programming of hysteresis for the UVLO threshold. See “Shutdown Pin and Programming Undervoltage Lockout” in the Applications Information Section. For micropower shutdown the ⎯S⎯H⎯D⎯N pin at 0V reduces VIN supply current to approximately 3µA. Each LED driver is a current mode step-up switching regulator. A regulation point is achieved when the boosted output voltage VOUT across the output LED(s) is high enough to create current in the LED(s) equal to the programmed LED current. A sense resistor connected in series with the LED(s) provides feedback of LED current to the converter loop. The basic loop uses a pulse from an internal oscillator to set the RS flip-flop and turn on the internal power NPN switch Q1 connected between the switch pin, SW, and ground. Current increases in the external inductor until switch current limit is exceeded or until the oscillator reaches its maximum duty cycle. The switch is then turned off, causing inductor current to lift the SW pin and turn on an external Schottky diode connected to the output. Inductor current flows via the Schottky diode charging the output capacitor. The switch is turned back on at the next reset cycle of the internal oscillator. During normal operation For True Color PWM dimming, the LT3478/LT3478-1 provide up to a 3000:1 wide PWM dimming range by allowing the duty cycle of the PWM pin (connected to the IC and an external N-channel MOSFET in series with the LED(s)) to be reduced from 100% to as low as 0.033% for a PWM frequency of 100Hz. Dimming by PWM duty cycle, allows for constant LED color to be maintained over the entire dimming range. For robust operation, the LT3478/LT3478-1 monitor system performance for any of the following faults : VIN or ⎯S⎯H⎯D⎯N pin voltages too low and/or inductor current too high and/or boosted output voltage too high. On detection of any of these faults, the LT3478/LT3478-1 stop switching immediately and a soft-start latch is set discharging the SS pin (see Timing Diagram for SS pin in Figure 11). All faults are detected internally and do not require external components. When all faults no longer exist, an internal 12µA supply charges the SS pin with a timing programmed using a single external capacitor. A gradual ramp up of SS pin voltage limits switch current during startup. For optimum component sizing, duty cycle range and efficiency the LT3478/LT3478-1 allow for a separate inductor supply VS and for switching frequency to be programmed from 200kHz up to 2.25MHz using a resistor from the RT pin to ground. The advantages of these options are covered in the Applications Informations section. 34781f 9 LT3478/LT3478-1 U W U U APPLICATIO S I FOR ATIO Inductor Selection Capacitor Selection Several inductors that work well with the LT3478/LT3478-1 are listed in Table 1. However, there are many other manufacturers and inductors that can be used. Consult each manufacturer for more detailed information and their entire range of parts. Ferrite cores should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating. Also ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. Values between 4.7µH and 22µH will suffice for most applications. Low ESR (equivalent series resistance) ceramic capacitors should be used at the output to minimize the output ripple voltage. Use only X5R or X7R dielectrics, as these materials retain their capacitance over wider voltage and temperature ranges than other dielectrics. A 4.7µF to 10µF output capacitor is sufficient for most high output current designs. Some suggested manufacturers are listed in Table 2. Inductor manufacturers specify the maximum current rating as the current where inductance falls by a given percentage of its nominal value. An inductor can pass a current greater than its rated value without damaging it. Aggressive designs where board space is precious will exceed the maximum current rating of the inductor to save space. Consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct. Schottky diodes, with their low forward voltage drop and fast switching speed, are ideal for LT3478/LT3478-1 applications. Table 3 lists several Schottky diodes that work well. The diode’s average current rating must exceed the application’s average output current. The diode’s maximum reverse voltage must exceed the application’s output voltage. A 4.5A diode is sufficient for most designs. For PWM dimming applications, be aware of the reverse leakage current of the diode. Lower leakage current will drain the output capacitor less, allowing for higher dimming range. The companies below offer Schottky diodes with high voltage and current ratings. Diode Selection Table 1. Suggested Inductors MANUFACTURER PART NUMBER CDRH104R-100NC CDRH103RNP-4R7NC-B CDRH124R-100MC CDRH104R-5R2NC FDV0630-4R7M IDC (A) 3.8 4 4.5 5.5 4.2 INDUCTANCE (µH) 10 4.7 10 5.2 4.7 MAX DCR (mΩ) 35 30 28 22 49 L × W × H (mm) 10.5 × 10.3 × 4.0 10.5 × 10.3 × 3.1 12.3 × 12.3 × 4.5 10.5 × 10.3 × 4.0 7.0 × 7.7 × 3.0 UP4B-220 7.6 22 34 22 × 15 × 7.9 MANUFACTURER Sumida www.sumida.com Toko www.toko.com Cooper www.cooperet.com Table 2. Ceramic Capacitor Manufacturers MANUFACTURER Taiyo Yuden AVX Murata PHONE NUMBER (408) 573-4150 (803) 448-9411 (714) 852-2001 WEB www.t-yuden.com www.avxcorp.com www.murata.com Table 3. Suggested Diodes MANUFACTURER PART NUMBER UPS340 MAX CURRENT (A) 3 MAX REVERSE VOLTAGE 40 B520C B530C B340A B540C PDS560 5 5 3 5 5 30 30 40 40 60 WEB Microsemi www.microsemi.com Diodes, Inc. www.diodes.com 34781f 10 LT3478/LT3478-1 U U W U APPLICATIO S I FOR ATIO Shutdown and Programming Undervoltage Lockout Programming Switching Frequency The LT3478/LT3478-1 have an accurate 1.4V shutdown threshold at the ⎯S⎯H⎯D⎯N pin. This threshold can be used in conjunction with a resistor divider from the system input supply to define an accurate undervoltage lockout (UVLO) threshold for the system (Figure 2). ⎯S⎯H⎯D⎯N pin current hysteresis allows programming of hysteresis voltage for this UVLO threshold. Just before part turn on, 10µA flows into the ⎯S⎯H⎯D⎯N pin. After part turn on, 0µA flows from the ⎯ H ⎯ D ⎯ N ⎯ pin. Calculation of the on/off thresholds for a system S input supply using the LT3478/LT3478-1 ⎯S⎯H⎯D⎯N pin can be made as follows: The switching frequency is programmed using an external resistor (RT) connected between the RT pin and ground. The internal free-running oscillator is programmable between 200kHz and 2.25MHz. Table 4 shows the typical RT values required for a range of switching frequencies. VSUPPLY OFF = 1.4 [1 + R1/R2)] VSUPPLY ON = VSUPPLY OFF + (10µA • R1) An open drain transistor can be added to the resistor divider network at the ⎯S⎯H⎯D⎯N pin to independently control the turn off of the LT3478/LT3478-1. Selecting the optimum switching frequency depends on several factors. Inductor size is reduced with higher frequency but efficiency drops due to higher switching losses. In addition, some applications require very high duty cycles to drive a large number of LEDs from a low supply. Low switching frequency allows a greater operational duty cycle and hence a greater number of LEDs to be driven. In each case the switching frequency can be tailored to provide the optimum solution. When programming the switching frequency the total power losses within the IC should be considered. See “Thermal Calculations” in the Applications Information section. VSUPPLY 10000 11 R2 SHDN SWITCHING FREQUENCY (kHz) R1 – 1.4V OFF ON + 10µA TA = 25°C 1000 3478 F02 100 Figure 2. Programming Undervoltage Lockout (UVLO) with Hysteresis With the ⎯S⎯H⎯D⎯N pin connected directly to the VIN pin, an internal undervoltage lockout threshold exists for the VIN pin (2.8V max). This prevents the converter from operating in an erratic mode when supply voltage is too low. The LT3478/LT3478-1 provide a soft-start function when recovering from such faults as ⎯S⎯H⎯D⎯N <1.4V and/or VIN <2.8V. See details in the Applications Information section “Soft-Start”. 1 10 100 1000 RT (kΩ) 3478 F03 Figure 3. Switching Frequency vs RT Resistor Value Table 4. Switching Frequencies vs RT Values SWITCHING FREQUENCY (MHz) RT (kΩ) 2.25 9.09 1 31.6 0.2 200 34781f 11 LT3478/LT3478-1 U U W U APPLICATIO S I FOR ATIO Programming Maximum LED current Maximum LED current can be programmed using the CTRL1 pin with CTRL2 tied to the VREF pin (see Figures 4 and 5). The maximum allowed LED current is defined as: maximum allowed LED current versus temperature to warn against exceeding this current limit and damaging the LED (Figure 6). Luxeon V (Maximum) and LT3478-1 (Programmed) Current Derating Curves vs Temperature (LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps (LT3478) Max LED Current = 0.1 Min(CTRL1, 1.05) • Amps RSENSE 900 LED current vs CTRL1 is linear for approximately 0.1V < CTRL1 < 0.95V For maximum possible LED current, connect CTRL1 and CTRL2 to the VREF pin. If FORWARD CURRENT (mA) 800 700 LUXEON V EMITTER CURRENT DERATING CURVE 600 500 EXAMPLE LT3478-1 PROGRAMMED LED CURRENT DERATING CURVE 400 300 200 100 0 0 1400 TA = 25°C CTRL2 = VREF (FOR LT3478 SCALE BY 0.1Ω/RSENSE) LED CURRENT (mA) 1050 LUXEON V EMITTER (GREEN, CYAN, BLUE, ROYAL BLUE) θJA = 20°C/W 350 VREF 0 0.35 0.70 CTRL1 (V) 1.05 1.40 3478 F04 Figure 4. LED Current vs CTRL1 Voltage LT3478/LT3478-1 10 R2 13 12 R1 VREF (LT3478) VOUT RSENSE CTRL2 CTRL1 100 3478 F06 Figure 6. LED Current Derating Curve vs Ambient Temperature LT3478-1 700 0 50 75 25 TA AMBIENT TEMPERATURE (°C) LED 3478 F05 Figure 5. Programming LED Current Programming LED Current Derating vs Temperature A useful feature of the LT3478/LT3478-1 is the ability to program a derating curve for maximum LED current versus temperature. LED data sheets provide curves of Without the ability to back off LED current as temperature increases, many LED drivers are limited to driving the LED(s) at only 50% or less of their maximum rated currents. This limitation requires more LEDs to obtain the intended brightness for the application. The LT3478/LT3478-1 allow the output LED(s) to be programmed for maximum allowable current while still protecting the LED(s) from excessive currents at high temperature. This is achieved by programming a voltage at the CTRL2 pin with a negative temperature coefficient using a resistor divider with temperature dependent resistance (Figures 7 and 8). CTRL2 voltage is programmed higher than CTRL1 voltage. This allows initial LED current to be defined by CTRL1. As temperature increases, CTRL2 voltage will fall below CTRL1 voltage causing LED currents to be controlled by CTRL2 pin voltage. The choice of resistor ratios and use of temperature dependent resistance in the divider for the CTRL2 pin will define the LED current curve breakpoint and slope versus temperature (Figure 8). A variety of resistor networks and NTC resistors with different temperature coefficients can be used for programming 34781f 12 LT3478/LT3478-1 U U W U APPLICATIO S I FOR ATIO CTRL2 to achieve the desired CTRL2 curve vs temperature. The current derating curve shown in Figure 6 uses the resistor network shown in option C of Figure 7. 10 R2 R4 13 12 R1 VREF LT3478/LT3478-1 CTRL2 CTRL1 Table 5. NTC Resistor Manufacturers/Distributors OPTION A TO D R3 MANUFACTURER RY RNTC RNTC A RX RNTC B RY RNTC C D Figure 7. Programming LED Current Derating Curve vs Temperature (RNTC Located on LEDs PCB) CTRL1, CTRL2 PIN VOLTAGES (mV) 1100 1000 900 700 CTRL1 600 500 400 CTRL2 300 200 LED CURRENT = MINIMUM 100 OF CTRL1, CTRL2 R3 = OPTION C 0 0 25 50 75 TA AMBIENT TEMPERATURE (°C) Murata Electronics North America www.murata.com TDK Corporation www.tdk.com Digi-key www.digikey.com RX 3478 F07 800 to obtain a resistor’s exact values over temperature from the manufacturer. Hand calculations of CTRL2 voltage can then be performed at each given temperature and the resulting CTRL2 curve plotted versus temperature. Several iterations of resistor value calculations may be required to achieve the desired breakpoint and slope of the LED current derating curve. 100 3478 F08 Figure 8. CTRL1, 2 Programmed Voltages vs Temperature Table 5 shows a list of manufacturers/distributors of NTC resistors. There are several other manufacturers available and the chosen supplier should be contacted for more detailed information. To use an NTC resistor to indicate LED temperature it is only effective if the resistor is connected as close as possible to the LED(s). LED derating curves shown by manufacturers are listed for ambient temperature. The NTC resistor should be submitted to the same ambient temperature as the LED(s). Since the temperature dependency of an NTC resistor can be nonlinear over a wide range of temperatures it is important If calculation of CTRL2 voltage at various temperatures gives a downward slope that is too strong, alternative resistor networks can be chosen (B, C, D in Figure 7) which use temperature independent resistance to reduce the effects of the NTC resistor over temperature. Murata Electronics provides a selection of NTC resistors with complete data over a wide range of temperatures. In addition, a software tool is available which allows the user to select from different resistor networks and NTC resistor values and then simulate the exact output voltage curve (CTRL2 behavior) over temperature. Referred to as the ‘Murata Chip NTC Thermistor Output Voltage Simulator’, users can log onto www.murata.com/designlib and download the software followed by instructions for creating an output voltage VOUT (CTRL2) from a specified VCC supply (VREF). At any time during selection of circuit parameters the user can access data on the chosen NTC resistor by clicking on a link to the Murata catalog. The following example uses hand calculations to derive the resistor values required for CTRL1 and CTRL2 pin voltages to achieve a given LED current derating curve. The resistor values obtained using the Murata simulation tool are also provided and were used to create the derating curve shown in Figure 6. The simulation tool illustrates the non-linear nature of the NTC resistor temperature coefficient at temperatures exceeding 50°C ambient. In addition, the resistor divider technique using an NTC resistor to derive CTRL2 voltage inherently has a flattening characteristic (reduced downward slope) at higher temperatures. To avoid LED current exceeding a maximum 34781f 13 LT3478/LT3478-1 U W U U APPLICATIO S I FOR ATIO allowed level at higher temperatures, the CTRL2 voltage curve may require a greater downward slope between 25°C and 50°C to compensate for that loss of slope at higher temperatures. RNTC (50°C) = RNTC (25°C).e–1.026 RNTC (50°C) = 22k • 0.358 RNTC (50°C) = 7.9k CTRL2(50°C) = 1.24/(1 + 16.9/7.9) = 395mV Example: Calculate the resistor values required for generating CTRL1 and CTRL2 from VREF based on the following requirements: CTRL2 slope (25°C to 50°C) = [CTRL2(50°C) – CTRL2(25°C)]/25°C (a) ILED = 700mA at 25°C = (395 – 701)/25 (b) ILED derating curve breakpoint occurs at 25°C = –306mV/25°C (c) ILED derating curve has a slope of –200mA/25°C between 25°C and 50°C ambient temperature Step1: Choose CTRL1 = 700mV for ILED = 700mA CTRL1 = VREF/(1 + R2/R1) R2 = R1 • [(VREF/CTRL1) – 1] For VREF = 1.24V and choosing R1 = 22.1k, R2 = 22.1k [(1.24/0.7) – 1] R2 = 17k (choose 16.9k) CTRL1 = 1.24/(1 + (16.9/22.1)) CTRL1 = 703mV (ILED = 703mA) Step 2: Choose resistor network option A (Figure 7) and CTRL2 = CTRL1 for 25°C breakpoint start with R4 = R2 = 16.9k, RNTC = 22k (closest value available) CTRL2 = 701mV (ILED = Min(CTRL1, CTRL2) • 1A = 701mA) Step 3: Calculate CTRL2 slope between 25°C and 50°C CTRL2 (T) = 1.24/(1 + R4/RNTC (T)) at T = TO = 25°C, CTRL2 = 701mV at T = 50°C, RNTC (T) = RNTC (TO).ex, x = B [(1/(T + 273) – 1/298)] (B = B-constant; linear over the 25°C to 50°C temperature range) ILED slope = –306mA/25°C The required ILED slope is –200mA/25°C. To reduce the slope of CTRL2 versus temperature it is easier to keep the exact same NTC resistor value and B-constant (there are limited choices) and simply adjust R4 and the type of resistor network used for the CTRL2 pin. By changing the resistor network to option C it is possible to place a temperature independent resistor in series with RNTC to reduce the effects of RNTC on the CTRL2 pin voltage over temperature. Step 4: Calculate the resistor value required for RY in resistor network option (c) (Figure 7) to provide an ILED slope of –200mA/25°C between 25°C and 50°C ambient temperature. CTRL2 (25°C) = 0.7V = 1.24/(1 + (R4/(RNTC(25°C)+ RY)) R4 = 0.77 (RNTC(25°C) + RY) (a) for –200mA/25°C slope ≥ CTRL2(50°C) = 0.7 – 0.2 = 0.5 CTRL2(50°C) = 0.5V = 1.24/(1 + (R4/(RNTC + RY)) R4 = 1.48 (RNTC(50°C) + RY) (b) Equating (a) = (b) and knowing RNTC(25°C) = 22k and RNTC(50°C) = 7.9k gives, 0.77 (22k + RY) = 1.48 (7.9k + RY) 17k + 0.77 RY = 11.7 k + 1.48 RY For RNTC B-constant = 3950 and T = 50°C RY = (17k – 11.7k)/(1.48 – 0.77) x = 3950 [(1/323) – 1/298] = –1.026 RY = 7.5k 34781f 14 LT3478/LT3478-1 U W U U APPLICATIO S I FOR ATIO The value for R4 can now be solved using equation (a) where, R4 = 0.77 (RNTC(25°C) + RY) = 0.77 (22k + 7.5k) for the output LED(s) is programmed for a given brightness/color and “chopped” over a PWM duty cycle range (Figure 10) from 100% to as low as 0.033%. D2 R4 = 22.7k (choose 22.6k) ILED slope can now be calculated from, VIN ILED slope = [CTRL2(50°C) – CTRL2(25°C)]/25°C SHDN where CTRL2 (50°C) = 1.24/(1 + 22.6/(7.9 + 7.5)) = 503mV VREF CTRL2 RT = 503mV – 699mV/25°C VOUT (LT3478) LT3478/ LT3478-1 RSENSE LED VC D1 PWM PWM DIMMING CONTROL = –196mV/25°C => ILED slope = –196mA/25°C Many LED applications require an accurate control of the brightness of the LED(s). In addition, being able to maintain a constant color over the entire dimming range can be just as critical. For constant color LED dimming, the LT3478/LT3478-1 provide a PWM pin and special internal circuitry to allow up to a 3000:1 wide PWM dimming range. With an N-channel MOSFET connected between the LED(s) and ground and a PWM signal connected to the gate of the MOSFET and the PWM pin (Figure 9), it is possible to control the brightness of the LED(s) based on PWM signal duty cycle only. This form of dimming is superior to dimming control using an analog input voltage (reducing CTRL1 voltage) because it allows constant color to be maintained during dimming. The maximum current COUT SW OVPSET giving ILED slope (from 25°C to 50°C) PWM Dimming L CTRL1 and CTRL2 (25°C) = 1.24/(1 + 39.2/(22 + 28.7)) = 699mV Using the Murata simulation tool for the resistor network and values in the above example shows a CTRL2 voltage curve that flattens out as temperatures approach 100°C ambient. The final resistor network chosen for the derating curve in Figure 6 used option C network with R4 = 19.3k, RNTC = 22k (NCP15XW223J0SRC) and RY = 3.01k. Although the CTRL2 downward slope is greater than –200mA/25°C initially, the slope is required to avoid exceeding maximum allowed LED currents at high ambient temperatures (see Figure 6). VS 3478 F09 Figure 9. PWM Dimming Control Using the LT3478/LT3478-1 TPWM TONPWM (= 1/fPWM) PWM INDUCTOR CURRENT LED CURRENT MAX ILED 3478 F10 Figure 10. PWM Dimming Waveforms Using the LT3478/LT3478-1 Some general guidelines for LED Current Dimming using the PWM pin (see Figure 10): (1) PWM Dimming Ratio (PDR) = 1/(PWM duty cycle) = 1/(TONPWM • fPWM) (2) Lower fPWM allows higher PWM Dimming Ratios (use minimum fPWM = 100Hz to avoid visible flicker and to maximize PDR) (3) Higher fOSC value improves PDR (allows lower TONPWM) but will reduce efficiency and increase internal heating. In general, minimum operational TONPWM = 3 • (1/fOSC). (4) Lower inductor value improves PDR 34781f 15 LT3478/LT3478-1 U W U U APPLICATIO S I FOR ATIO (5) Higher output capacitor value improves PDR (6) Choose the schottky diode (D2, Figure 9) for minimum reverse leakage See Typical Performance Characteristics graph “LED Current vs PWM Duty Cycle”. Soft-Start To limit inrush current and output voltage overshoot during startup/recovery from a fault condition, the LT3478/ LT3478-1 provide a soft-start pin SS. The SS pin is used to program switch current ramp up timing using a capacitor to ground. The LT3478/LT3478-1 monitor system parameters for the following faults: VIN <2.8V, ⎯S⎯H⎯D⎯N <1.4, inductor current >6A and boosted output voltage >OVP. On detection of any of these faults, the LT3478/LT3478-1 stop switching immediately and a soft-start latch is set causing the SS pin to be discharged (see Timing Diagram for the SS pin in Figure 11). When all faults no longer exist and the SS pin has been discharged to at least 0.25V, the soft-start latch is reset and an internal 12µA supply charges the SS pin. A gradual ramp up of SS pin voltage is equivalent to a ramp up of switch current limit until SS exceeds VC. The ramp rate of the SS pin is given by: ΔVSS/Δt = 12µA/CSS SW SS FAULTS TRIGGERING SOFT-START LATCH WITH SW TURNED OFF IMMEDIATELY: VIN < 2.8V OR SHDN < 1.4V OR VOUT > OVP OR I(INDUCTOR) > 6A 0.65V (ACTIVE THRESHOLD) 0.25V (RESET THRESHOLD) 0.15V SOFT-START LATCH RESET: SOFT-START LATCH SET: SS < 0.25V AND VIN > 2.8V AND SHDN > 1.4V AND VOUT < OVP AND I(INDUCTOR) < 6A 3478 F11 Figure 11. LT3478 Fault Detection and SS Pin Timing Diagram To limit inductor current overshoot to <0.5A when SS charges past the VC level required for loop control, the CSS capacitor should be chosen using the following formula: CSS(MIN) = CC (7.35 – 0.6(ILED • VOUT/VS)) Example: VS = 8V, VOUT = 16V, ILED = 1.05A, CC = 0.1µF, CSS(MIN) = 0.1µF (7.35 – 0.6(1.05 • 16/8)) = 0.612µF (choose 0.68µF). High Inductor Current “Inrush” Protection The LT3478/LT3478-1 provide an integrated resistor between the VS and L pins to monitor inductor current (Figure 1). During startup or “hotplugging” of the inductor supply, it is possible for inductor currents to exceed the maximum switch current limit. When inductor current exceeds 6A, the LT3478/LT3478-1 protect the internal power switch by turning it off and triggering a soft-start latch. This protection prevents the switch from repetitively turning on during excessive inductor currents by delaying switching until the fault has been removed. To defeat inductor current sensing the inductor supply should be connected to the L pin and the VS pin left open. See details in the Applications Information section “Soft-Start”. LED Open Circuit Protection and Maximum PWM Dimming Ratios The LT3478/LT3478-1 LED drivers provide optimum protection from open LED faults by clamping the converter output to a programmable overvoltage protection level (OVP). In addition, the programmable OVP feature draws zero current from the output during PWM = 0 to allow higher PWM dimming ratios. This provides an advantage over other LED driver applications which connect a resistor divider directly from VOUT. An open LED fault occurs when the connection to the LED(s) becomes broken or the LED(s) fails open. For an LED driver using a step-up switching regulator, an open circuit LED fault can cause the converter output to exceed the voltage capabilities of the regulator’s power switch, causing permanent damage. When VOUT exceeds OVP, the 34781f 16 LT3478/LT3478-1 U W U U APPLICATIO S I FOR ATIO LT3478/LT3478-1 immediately stop switching, a soft-start latch is set and the SS pin is discharged. The SS latch can only be reset when VOUT falls below OVP and the SS pin has been discharged below 0.25V (Figure 11). If the LED(s) simply go open circuit and are reconnected, however, the OVP used to protect the switch might be too high for the reconnected LED(s). The LT3478/LT3478-1 therefore allow OVP to be programmable to protect both the LED driver switch and the LED(s). (The minimum allowable OVP for normal operation for a given LED string depends on the number of LEDs and their maximum forward voltage ratings.) OVP is programmed using the OVPSET pin (front page), given by, OVP = (OVPSET • 41)V where the programmable range for the OVPSET pin is 0.3V to 1V resulting in an OVP range of 12.3V to 41V. The OVPSET pin can be programmed with a single resistor by tapping off of the resistor divider from VREF used to program CTRL1. If both CTRL1 and CTRL2 are connected directly to VREF (maximum LED current setting) then OVPSET requires a simple 2 resistor divider from VREF. Thermal Calculations VS = inductor supply input D = switch duty cycle = (VOUT + VF – VS)/(VOUT + VF – VSAT) VF = forward voltage drop of external Schottky diode VSAT = IL(AVE) • RSW (2) Switch AC loss = PSW(AC) = tEFF(1/2)IL(AVE)(VOUT + VF)(FOSC) tEFF = effective switch current and switch VCE voltage overlap time during turn on and turn off = 2 • (tISW + tVSW) tISW = ISWITCH rise/fall time = IL(AVE) • 2ns tVSW = SW fall/rise time = (VOUT + VF) • 0.7ns fOSC = switching frequency (3) Current sensing loss = PSENSE = PSENSE(IL) + PSENSE(ILED) PSENSE(IL) = IL(AVE)2 • 9.5mΩ PSENSE(ILED) = ILED2 • 0.1Ω (4) Input quiescent loss = PQ = VIN • IQ where IQ = (6.2mA + (100mA • D)) To maximize output power capability in an application without exceeding the LT3478/LT3478-1 125°C maximum operational junction temperature, it is useful to be able to calculate power dissipation within the IC. The power dissipation within the IC comes from four main sources: switch DC loss, switch AC loss, Inductor and LED current sensing and input quiescent current. These formulas assume a boost converter architecture, continuous mode operation and no PWM dimming. Example (Using LT3478-1): (1) Switch DC loss = PSW(DC) Total Power Dissipation: = (RSW • IL(AVE)2 • D) For VIN = VS = 8V, ILED = 700mA, VOUT = 24.5V (7 LEDs), VF = 0.5V and fOSC = 0.2Mhz, η = 0.89 (initial assumption) IL(AVE) = (24.5 • 0.7)/(0.89 • 8) = 2.41A D = (24.5 + 0.5 – 8)/(24.5 + 0.5 – 0.17) = 0.684 TEFF = 2 • ((2.41 • 2)ns + (24.5 + 0.5) • 0.7)ns = 45ns PIC = PSW(DC) + PSW(AC) + PSENSE + PQ RSW = switch resistance = 0.07Ω (at TJ = 125°C) PSW(DC) = 0.07 • (2.41)2 • 0.684 = 0.278W IL(AVE) = POUT/(η • VS) PSW(AC) = 45ns • 0.5 • 2.41 • 25 • 0.2MHz = 0.271W POUT = VOUT • ILED PSENSE = ((2.41)2 • 0.0095) + ((0.7)2 • 0.1) = 0.104W η = converter efficiency = POUT/(POUT + PLOSS) PQ = 8 • (6.2mA + (100mA • 0.684)) = 0.597W PIC = 0.278 + 0.271 + 0.104 + 0.597 = 1.25W 34781f 17 LT3478/LT3478-1 U W U U APPLICATIO S I FOR ATIO Local heating from the nearby inductor and Schottky diode will also add to the final junction temperature of the IC. Based on empirical measurements, the effect of diode and inductor heating on the LT3478-1 junction temperature can be approximated as: If an application is built, the inductor current can be measured and a new value for junction temperature estimated. Ideally a thermal measurement should be made to achieve the greatest accuracy for TJ. VF = 0.5V Note: The junction temperature of the IC can be reduced if a lower VIN supply is available – separate from the inductor supply VS. In the above example, driving VIN from an available 3V source (instead of VS = 8V) reduces input quiescent losses in item(4) from 0.597W to 0.224W, resulting in a reduction of TJ from 118°C to 105°C. IL(AVE) = 2.41 Layout Considerations PDIODE = 0.316 • 0.5 • 2.41 = 0.381W As with all switching regulators, careful attention must be given to PCB layout and component placement to achieve optimal thermal,electrical and noise performance (Figure 12). The exposed pad of the LT3478/LT3478-1 (Pin 17) is the only GND connection for the IC. The exposed pad should be soldered to a continuous copper ground plane underneath the device to reduce die temperature and maximize the power capability of the IC. The ground path for the RT resistor and VC capacitor should be taken from nearby the analog ground connection to the exposed pad (near Pin 9) separate from the power ground connection to the exposed pad (near Pin 16). The bypass capacitor for VIN should be placed as close as possible to the VIN pin and the analog ground connection. SW pin voltage rise and fall times are designed to be as short as possible for maximum efficiency. To reduce the effects of both radiated and conducted noise, the area of the SW trace should be kept as small as possible. Use a ground plane under the switching regulator to minimize interplane coupling. The schottky diode and output capacitor should be placed as close as possible to the SW node to minimize this high frequency switching path. To minimize LED current sensing errors for the LT3478, the terminals of the external sense resistor RSENSE should be tracked to the VOUT and LED pins separate from any high current paths. ΔTJ (LT3478-1) = 5°C/W • (PDIODE + PINDUCTOR) PDIODE = (1 – D) • VF • IL(AVE) 1 – D = 0.316 PINDUCTOR = IL(AVE)2 • DCR DCR = inductor DC resistance (assume 0.05Ω) PINDUCTOR = (2.41)2 • 0.05 = 0.29W The LT3478/LT3478-1 use a thermally enhanced FE package. With proper soldering to the Exposed Pad on the underside of the package combined with a full copper plane underneath the device, thermal resistance (θJA) will be about 35°C/W. For an ambient temperature of TA = 70°C, the junction temperature of the LT3478-1 for the example application described above, can be calculated as: TJ (LT3478-1) = TA + θJA(PTOT) + 5(PDIODE + PINDUCTOR) = 70 + 35(1.25) + 5(0.671) = 70 + 44 + 4 = 118°C In the above example, efficiency was initially assumed to be η = 0.89. A lower efficiency (η) for the converter will increase IL(AVE) and hence increase the calculated value for TJ. η can be calculated as: η = POUT/(POUT + PLOSS) POUT = VOUT • ILED = 17.15W PLOSS (estimated) = PIC + PDIODE + PINDUCTOR = 1.92W η = 17.15/(17.15 + 1.92) = 0.9 34781f 18 LT3478/LT3478-1 U U W U APPLICATIO S I FOR ATIO (CONNECT MULTIPLE GROUND PLANES THROUGH VIAS UNDERNEATH THE IC) VS CVS VOUT VIN CVIN OUTPUT CAPACITOR SCHOTTKY DIODE SOLDER THE EXPOSED PAD (PIN 17) TO THE ENTIRE COPPER GROUND PLANE UNDERNEATH THE DEVICE LT3478/LT3478-1 SW INDUCTOR L RSENSE (LT3478 ONLY) SW 1 16 SS CSS SW 2 15 RT RT VIN 3 14 PWM R VS 4 13 CTRL2 R L 5 12 CTRL1 R VOUT 6 11 SHDN R LED OVPSET R POWER GND GND EXPOSED PAD 7 10 VREF PIN 17 8 ANALOG GND 9 VC C R CF RC VIN BYPASS CAP CC 3252 F08 Figure 12. Recommended Layout for LT3478/LT3478-1 (Boost Configuration) U TYPICAL APPLICATIO S 15W, 6 LEDs at 700mA, Boost LED Driver L1 10µH VIN 8V TO 16V C1 4.7µF 25V VIN VS L D1 SHDN OUT PWM 5V/DIV VREF R1 45.3k CTRL2 LT3478-1 700mA LED R4 54.9k ILED 0.5A/DIV CTRL1 PWM SS CSS 1µF L1: CDRH104R-100NC D1: PDS560 Q1: Si2318DS LEDs: LUXEON III (WHITE) fPWM = 100Hz INDUCTOR CURRENT 1A/DIV OVPSET R2 130k LT3478-1 PWM Dimming Waveforms C2 10µF 25V SW VC RT CC 0.1µF 2µs/DIV PWM DIMMING RATIO = 1000:1 (SEE EFFICIENCY ON PAGE 1) RT 69.8k 3478 TA02b fOSC = 500kHz 3.3V 0V 100Hz Q1 PWM DIMMING RATIO = 1000:1 R3 10k 3478 TA02a 34781f 19 LT3478/LT3478-1 U TYPICAL APPLICATIO S 17W, 15 LEDs at 350mA, Boost LED Driver plus LT3003 VS 8V TO 14V VIN 3.3V C1 4.7µF 16V L1 5.2µH C3 3.3µF 10V VIN VS D1 L VOUT SW SHDN Efficiency vs Input VS C2 3.3µF 25V 90 OUT VREF 85 LT3478-1 1.05A LED EFFICIENCY (%) CTRL2 R1 24k CTRL1 OVPSET R2 100k PWM SS VC RT CSS 1µF L1: CDRH104R-5R2 D1: PDS560 LEDs: LUXEON I (WHITE) VIN = 3.3V ILED = 350mA fOSC = 1MHz PWM DUTY CYCLE = 100% 80 75 VC RT 31.6k CC 0.1µF 15 LEDs (5 SERIES x 3 CHANNELS) LUXEON I (WHITE) 70 fOSC = 1MHz 8 10 12 14 VS (V) 3.3V 0V LED1 VMAX VOUT 100Hz LT3003 VIN VIN PWM LED2 PWM OT1 OT2 GND 3478 TA03b LED3 VEE VC DIMMING RATIO = 3000:1 3478 TA03a 16W, 12 LEDs at 350mA, Buck-Boost Mode LED Driver plus LT3003 VS 12V TO 16V VIN 5V C1 4.7µF 25V L1 8.2µH C3 3.3µF 10V VIN VS L D1 VOUT SW SHDN Efficiency vs Input VS C2 10µF 50V 90 85 OUT VREF 80 LT3478-1 1.05A LED EFFICIENCY (%) CTRL2 R1 24k CTRL1 OVPSET R2 100k PWM RT 69.8k CC 0.1µF 100Hz PWM 70 65 12 LEDs (4 SERIES x 3 CHANNELS) LUXEON I (WHITE) 55 50 fOSC = 500kHz D2 75 60 VC C4 1µF 3.3V RT VC CSS 1µF L1: CDRH105R-8R2 D1: PDS560 D2: 7.5V ZENER LEDs: LUXEON I (WHITE) 0V SS VIN = 5V ILED = 350mA fOSC = 500kHz PWM DUTY CYCLE = 100% 12 VOUT LED1 VMAX VIN PWM LED2 LT3003 VEE 14 VS (V) 15 16 3478 TA04b LED3 OT1 OT2 GND 13 VC DIMMING RATIO = 200:1 3478 TA04a 34781f 20 LT3478/LT3478-1 U TYPICAL APPLICATIO S 4W, 1 LED at 1A, Buck-Boost Mode LED Driver VIN 3.8V TO 6.5V NiMH 4× C1 10µF 10V L1 6.8µH VIN ON OFF VS L D1 Efficiency vs VIN C2 4.7µF 16V SW SHDN 80 OUT ILED = 1A fOSC = 500kHz 75 PWM DUTY CYCLE = 100% CTRL2 R1 100k LT3478-1 Q2 1A LED CTRL1 R4 510Ω OVPSET R2 L1: CDRH105R-6R8 34k D1: B320 Q1: Si2302ADS Q2: Si2315BDS LED: LUXEON III (WHITE) PWM SS CSS 1µF 3.3V 0V VC CC 0.1µF 65 60 55 RT 69.8k R5 510Ω SINGLE LED LUXEON III (WHITE) 50 3 Q1 PWM DIMMING RATIO = 200:1 70 RT fOSC = 500kHz 1kHz EFFICIENCY (%) VREF 4 5 VIN (V) 6 7 3478 TA06b R3 10k 3478 TA06a 34781f 21 LT3478/LT3478-1 U TYPICAL APPLICATIO S 24W, 4 LEDs at 1.5A, Buck Mode LED Driver PVIN 32V C1 3.3µF 50V RSENSE 0.068Ω 1.5A 4 LEDs R4 365Ω TYPICAL EFFICIENCY = 90% FOR CONDITIONS/COMPONENTS SHOWN (PWM DUTY CYCLE = 100%, TA =25°C) C3 10µF 25V Q2 L1 10µH VIN 3.3V C2 4.7µF 10V D1 VIN VS L OUT LED SW SHDN L1: CDRH105R-100 D1: PDS560 Q1: 2N7002 Q2: Si2319DS LEDs: LXK2 (WHITE) Q1 PWM R3 10k VREF R1 24k R5 510Ω LT3478 CTRL2 PWM CTRL1 DIMMING RATIO = 3000:1 OVPSET 3.3V R2 100k SS CSS 1µF RT VC CC 0.1µF fOSC = 500kHz 0V 100Hz RT 69.8k 3478 TA07a 34781f 22 LT3478/LT3478-1 U PACKAGE DESCRIPTIO FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 3.58 (.141) 16 1514 13 12 1110 6.60 ±0.10 9 2.94 (.116) 4.50 ±0.10 6.40 2.94 (.252) (.116) 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 (BC) 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 34781f 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. 23 LT3478/LT3478-1 U TYPICAL APPLICATIO 6W, 6 LEDs at 250mA, Boost LED Driver VIN 3.3V C1 4.7µF 25V L1 10µH C3 3.3µF 10V VIN VS L D1 SW SHDN OUT 100 VIN = 3.3V ILED = 250mA 95 fOSC = 2MHz PWM DUTY CYCLE = 100% 90 RSENSE 0.42Ω VREF CTRL2 R1 8.25k Efficiency vs Input VS C2 3.3µF 25V LT3478 EFFICIENCY (%) VS 8V TO 16V 250mA LED CTRL1 OVPSET R2 10k PWM SS VC 80 75 70 CSS 1µF L1: CDRH6D28 D1: ZLLS1000 Q1: Si2318DS LEDs: LUXEON I (WHITE) RT 85 RT 10k CC 0.1µF 65 6 LEDs = LUXEON I (WHITE) 60 fOSC = 2MHz 8 10 12 VS (V) 14 3.3V 0V 100Hz 16 3478 TA05b Q1 PWM DIMMING RATIO = 1000:1 R3 10k 3478 TA05a RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant Current, 1.4MHz, 1.5A Boost Converter with Analog/PWM Dimming VIN: 5V to 18V, VOUT(MAX) = 36V, ISD <1µA, MS10 Package LT3003 Three Channel LED Ballaster with 3,000:1 True Color PWM Dimming VIN: 3V to 48V, ISD <5µA, MSOP10 Package LT3474 36V, 1A (ILED), 2MHz,Step-Down LED Driver with 400:1 True Color PWM Dimming VIN: 4V to 36V, VOUT(MAX) = 13.5V, ISD <1µA, TSSOP16E Package LT3475 Dual 1.5A(ILED), 36V, 2MHz,Step-Down LED Driver 3,000:1 True Color PWM Dimming VIN: 4V to 36V, VOUT(MAX) = 13.5V, ISD <1µA, TSSOP20E Package LT3476 Quad Output 1.5A, 2MHz High Current LED Driver with 1,000:1 True Color PWM Dimming VIN: 2.8V to 16V, VOUT(MAX) = 36V, ISD <10µA, 5mm × 7mm QFN Package LT3477 42V, 3A, 3.5MHz Boost, Buck-Boost, Buck LED Driver with Analog/ PWM Dimming VIN: 2.5V to 25V, VOUT(MAX) = 40V, ISD <1µA, QFN, TSSOP20E Packages LT3479 3A, 3.5MHz Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection and Analog/PWM Dimming VIN: 2.5V to 24V, VOUT(MAX) = 40V, ISD <1µA, 4mm × 3mm DFN, TSSOP16E Packages LT3486 Dual 1.3A , 2MHz High Current LED Driver with 1,000:1 True Color PWM Dimming VIN: 2.5V to 24V, VOUT(MAX) = 36V, ISD <1µA, 5mm × 3mm DFN, TSSOP16E Packages LTC3783 High Current LED Controller with 3,000:1 True Color PWM Dimming VIN: 3V to 36V, VOUT(MAX) = Ext FET, ISD <20µA, 5mm × 4mm DFN, TSSOP16E Packages 34781f 24 Linear Technology Corporation LT 0107 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007