LT3478IFE-1

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 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