LT3760 8-Channel × 100mA LED Driver Description Features The LT®3760 is an 8-channel LED driver with a step-up DC/DC controller capable of driving up to 45V of LEDs. Each channel contains an accurate current sink with ±2% current matching. Channels follow a master programmable current to allow between 20mA to 100mA of LED current per string. Channels can be paralleled for higher LED current. Output voltage adapts to variations in LED VF for optimum efficiency and open LED faults do not affect the operation of connected LED strings. Up to 45V of LEDs × 100mA, 8-Channel LED Driver n Wide Input Range : 6V to 40V (4.5V to 13V, VIN Connected to INTVCC) n ±2% LED Current Matching at 40mA (Typ ±0.7%) n Up to 3000:1 True Color PWM™ Dimming Range n Single Resistor Sets LED Current (20mA to 100mA) n LED Current Regulated Even for PV > V IN OUT n Output Adapts to LED V for Optimum Efficiency F n Fault Flag + Protection for Open LED Strings n Protection for LED Pin to V OUT Short n Parallel Channels for Higher LED Current n Programmable LED Current Derating vs Temperature n Accurate Undervoltage Lockout Threshold with Programmable Hysteresis n Programmable Frequency (100kHz to 1MHz) n Synchronizable to an External Clock n The LT3760 allows a PWM dimming range up to 3000:1 and an analog dimming range up to 25:1. Operating frequency can be programmed from 100kHz up to 1MHz using a single resistor or synchronized to an external clock. Additional features include: programmable maximum VOUT for open LED protection, a fault flag for open LED, programmable LED current derating vs temperature, micropower shutdown and internal soft-start. The LT3760 is available in a thermally enhanced 28-pin TSSOP package. Applications Automotive, Notebook and TV Monitor Backlighting L, LT, LTC and LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 7199560, 7321203. n Typical Application Worst-Case Channels LED Current Matching (Normalized to 8-Channel Average) 92% Efficient, 36W Backlight LED Driver 4.7µF VIN 8V TO 14V 10µH 4.7µF 499k 4.7µF VIN INTVCC 5× 2.2µF GATE •••• SENSE SHDN/UVLO 0.015Ω 40.2k VOUT PWM LED1 LED2 • • • • LED7 LED8 LT3760 REF 20k TSET 30.9k • • • PGND CTRL 11k 20k OVPSET GND RT FAULT ISET 39.2k 1MHz 5.76k VC 0.8 UP TO 45V OF LEDs PER STRING • • • • • • • LED CURRENT MATCHING (%) PVIN 20V TO 36V 0.4 0.0 –0.4 RISET = 14.7k (I(LED) = 40mA) –0.8 0 25 100 –50 –25 50 75 JUNCTION TEMERATURE (°C) • • 8 CHANNELS × 100mA • • 125 3760 TA01 100k 3760 TA01 VIN SYNC 10k 2.2nF 3760fc 1 LT3760 Absolute Maximum Ratings Pin Configuration (Note 1) TOP VIEW VOUT, LED1-8.............................................................60V VIN, SHDN/UVLO, FAULT............................................40V INTVCC....................................................................... 13V INTVCC above VIN................................................... +0.3V PWM, CTRL, SYNC......................................................6V VC................................................................................3V VREF , RT, ISET, TSET, OVPSET........................................2V SENSE.......................................................................0.4V Operating Junction Temperature Range (Notes 2,3)..............................................-40°C to 125°C Storage Temperature Range...................-65°C to 150°C Lead Temperature (Soldering, 10 sec).................... 300°C CTRL 1 28 OVPSET TSET 2 27 PWM VREF 3 26 VC ISET 4 25 RT NC 5 24 GND LED1 6 23 LED8 LED2 7 LED3 8 LED4 9 20 LED5 PGND 10 19 PGND SENSE 11 18 VOUT GATE 12 17 SYNC INTVCC 13 VIN 14 29 PGND 22 LED7 21 LED6 16 FAULT 15 SHDN/UVLO FE PACKAGE 28-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 28°C/W, θJC = 10°C/W EXPOSED PAD (PIN 29) IS PGND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3760EFE#PBF LT3760EFE#TRPBF LT3760FE 28-Lead Plastic TSSOP –40°C to 125°C LT3760IFE#PBF LT3760IFE#TRPBF LT3760FE 28-Lead Plastic TSSOP –40°C to 125°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/ 3760fc 2 LT3760 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = VOUT = 6V, RISET = 14.7k unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS 4.2 5.5 4.5 6.0 V V 13 40 V V INPUT BIAS, REFERENCE Minimum Operational VIN (To Allow GATE Switching) VC = 1.5V VIN = INTVCC (Shorted) VIN ≠ INTVCC l l Operational VIN VIN = INTVCC (Shorted) VIN ≠ INTVCC VIN Quiescent Current CTRL = 0.1V, PWM = 0V CTRL = 0.1V, PWM = 1.5V, (Not Switching) LED1–8 = 1.2V 4.2 9.5 5.7 12 mA mA VIN Shutdown Current (VIN ≠ INTVCC) (Not Shorted) SHDN/UVLO = 0V, VIN =6V SHDN/UVLO = 0V, VIN = 40V 0.1 2 10 µA µA VIN Shutdown Current (VIN = INTVCC (Shorted)) SHDN/UVLO = 0V, VIN = INTVCC = 4.5V SHDN/UVLO = 0V, VIN = INTVCC = 13V 10 20 20 40 µA µA SHDN/UVLO Threshold (Micropower) (Falling) (VSD) IVIN < 20µA SHDN/UVLO Threshold (UVLO) (Falling) (Stop Switching) (VUV) 4.5 6 l 0.3 0.7 l 1.414 1.476 1.538 V V SHDN/UVLO Pin Current SHDN/UVLO = VUV - 50mV SHDN/UVLO = VUV + 50mV l 1.6 2.4 0 3.2 µA µA VREF Voltage IVREF = 0µA l 1.450 1.485 1.524 V VREF Line Regulation IVREF = 0µA, 6V < VIN < 40V 0.01 0.05 %/V VREF Load Regulation 0 < IVREF < 150µA (Max) 2 mV OSCILLATOR Frequency: fOSC (100kHz) RT = 523k l 92 101 112 kHz Frequency: fOSC (1MHz) RT = 39.2k l 0.90 1 1.10 MHz fOSC (1MHz) Line Regulation RT = 39.2k, 6V < VIN < 40V 0.1 0.2 %/V 250 250 nS nS 2.2 V RT Pin Voltage RT = 39.2k 1.6 Minimum Off-Time Minimum On-Time (Note 5) (Note 5) 170 190 SYNC Input High Threshold SYNC Input Low Threshold V 0.6 V SYNC Input Current SYNC = 0V SYNC = 5V 0 25 µA µA SYNC Frequency Range RT = 523k RT = 39.2k 0.12 1.2 INTVCC Regulation Voltage VIN = 12V 6.65 Dropout (VIN - INTVCC) IINTVCC = 10mA 250 mV INTVCC UVLO (+) (Start Switching) 3.8 V INTVCC UVLO (-) (Stop Switching) 1.5 1.5 MHz MHz 7.35 V LINEAR REGULATOR (INTVCC) INTVCC Current Limit l 40 7 3.4 V 57 mA 4 µmhos 5 V/V 33 µmhos OVP/LED ERROR AMPLIFIERS Transconductance (OVP) ∆IVC = ±2.5µA Voltage Gain (OVP) Transconductance (LED) ∆IVC = ±2.5µA 3760fc 3 LT3760 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = VOUT = 6V, RISET = 14.7k unless otherwise noted. PARAMETER CONDITIONS MIN Voltage Gain (LED) TYP MAX UNITS 45 V/V VC Source Current (Out of Pin) VC = 1.5V, VLEDx = 0.8V, OVPSET = 1.5V 10 µA VC Sink Current (OVP) VC = 1.5V, VLEDx = 0.8V, OVPSET = 0V 15 µA VC Sink Current (LED) VC = 1.5V, VLEDx = 1.2V, OVPSET = 1.5V 9 µA VC Output High (clamp) (VCOH) 2.3 V VC Output Low (clamp) (VCOL) 0.8 V VC Switching Threshold (VCSW) 1.1 V SENSE AMP SENSE Input Current (Out of Pin) 65 SENSE Current Limit Threshold Current Mode Gain l 44 µA 60 6 ∆V(VC)/∆V(SENSE) SENSE Over Current Limit Threshold 52 l 90 100 mV V/V 110 mV 40.1 41.9 mA ±0.7 ±2 % 100.7 105.9 mA LED CURRENT / CONTROL ISET Pin Voltage CTRL = 1.5V LEDx Current (40mA) (RISET = 14.7k) VLEDx = 1V, CTRL = 1.5V LEDx Current Matching (40mA) (RISET = 14.7k) VLEDx = 1V, CTRL = 1.5V LEDx Current (100mA) (RISET = 5.76k) VLEDx = 1V, CTRL = 1.5V 1.00 38.3 l 95.5 V LED Pin Regulation Voltage 1.1 V TSET Threshold 630 mV ANALOG DIMMING CTRL Input Current (Out of Pin) CTRL = 1V CTRL = 0.04V 40 50 LEDx Current (Dimming 25:1) VLEDx = 1V, CTRL = 0.04V 1.6 200 200 nA nA mA PWM DIMMING PWM Input Low Threshold 0.7 PWM Input High Threshold 1 1.1 V 1.4 V PWM Input Current PWM = 1.5V PWM = 6V 6 24 µA µA VOUT Pin Current in PWM Mode V(VOUT) = 60V PWM = 1.5V, VLEDx = 1V PWM = 0V, VLEDx = 1V 370 20 µA µA LEDx Leakage Current (PWM = 0V) VLEDx = 1V, VOUT = 12V VLEDx = 50V, VOUT = 60V 0.1 0.1 1 2 µA µA FAULT DIAGNOSTICS FAULT Output Sink Current LED1 = Open, VFAULT = 0.3V LEDx Short Threshold (VSH) (VOUT – VLEDx) VOUT = 12V VOUT = 60V 0.3 0.6 6 6 mA V LED Open Detection Threshold VOUT = 12V 0.5 V 3760fc 4 LT3760 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = VOUT = 6V, RISET =14.7k unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS GATE DRIVER GATE Driver Output Rise Time VIN = 12V, CL = 3300pF (Note 4) 30 nS GATE Driver Output Fall Time VIN = 12V, CL = 3300pF (Note 4) 30 nS GATE Output Low IGATE= 0µA GATE Output High INTVCC = VIN = 7V IGATE = 0µA 6.95 V VOUT Over Voltage Protection (OVP) Regulation Voltage OVPSET = 0.22V OVPSET = 1V 12.5 57 V V OVPSET Input Current (Out of Pin) OVPSET = 0.22V, VOUT =12V 0.1 V OUTPUT VOLTAGE 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 LT3760E 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 LED Current vs CTRL Pin Voltage 110 RISET = 14.7k 100 90 0.4 0.0 –0.4 LED CURRENT (mA) 41 LED CURRENT (mA) LED CURRENT MATCHING (%) 42 nA TA = 25°C, unless otherwise noted. LED Current vs Junction Temperature 0.8 200 LT3760I is guaranteed to meet performance specifications from -40°C to 125°C junction temperature. Note 3: For Maximum Operating Ambient Temperature, see Thermal Calculations in the Applications Information section. Note 4: GATE rise and fall times are measured between 10% and 90% of INTVCC voltage. Note 5: See Duty Cycle Considerations in the Applications Information. Typical Performance Characteristics Worst-Case Channels LED Current Matching (Normalized to 8-Channel Average) 40 40 39 80 70 RISET = 5.76k 7.32k 9.76k 14.7k 29.4k 60 50 40 30 20 10 RISET = 14.7k (I(LED) = 40mA) –0.8 –50 –25 0 25 100 50 75 JUNCTION TEMERATURE (°C) 125 3760 G01 38 –50 –25 0 50 25 75 100 JUNCTION TEMERATURE (°C) 125 3760 G02 0 0.00 0.25 0.50 0.75 1.00 CTRL (V) 1.25 1.50 3760 G03 3760fc 5 LT3760 Typical Performance Characteristics 1.525 (FRONT COVER APPLICATION) I(LEDx) 40mA/DIV SHDN/UVLO Threshold vs Junction Temperature VREF vs Junction Temperature 1.525 SHDN/UVLO PIN VOLTAGE (V) LED Current Waveforms 3000:1 PWM Dimming (100Hz) TA = 25°C, unless otherwise noted. 1.505 VREF VOLTAGE (V) 1.505 1.485 1.485 I(L1) 1A/DIV PWM 10V/DIV 1.465 1.465 3760 G04 5µs/DIV 1.445 –50 0 25 50 75 100 –25 JUNCTION TEMPERATURE (°C) 1.445 –50 125 –25 0 25 50 75 100 JUNCTION TEMERATURE (°C) 3760 G05 SHDN/UVLO Pin (Hysteresis) Current vs Junction Temperature VIN Shutdown Current vs Junction Temperature 5 2.70 12 VIN = 6V, SHDN/UVLO = 0V 2.50 2.40 VIN CURRENT (mA) 2.60 3 2 1 2.30 2.20 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) RISET = 14.7k PWM = 1.5V, NO SWITCHING, V(LED1-8) = 1.2V, CTRL = 0.1V 8 6 4 PWM = 0V, CTRL = 0.1V 2 0 –50 125 VIN Quiescent Current vs VIN 10 4 VIN CURRENT (µA) SHDN/UVLO PIN CURRENT (µA) 2.80 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 3760 G07 0 125 0 5 10 15 3760 G08 VIN Quiescent Current vs Junction Temperature 2.5 1100 5 PWM = 0V, CTRL = 0.1V 0 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3760 G10 1000 RT = 39.2k 950 900 –50 –25 40 35 3760 G09 2.0 1050 VC PIN VOLTAGE (V) SWITCHING FREQUENCY (kHz) VIN CURRENT (mA) PWM = 1.5V, NO SWITCHING, V(LED1-8) = 1.2V, CTRL = 0.1V 30 VC HIGH CLAMP VIN = 6V, RISET = 14.7k, CTRL = 0.1V 10 20 25 VIN (V) VC High Clamp, Active and Low Clamp Levels vs Junction Temperature Switching Frequency vs Junction Temperature 15 125 3760 G06 1.5 VC ACTIVE (SWITCHING) 1.0 VC LOW CLAMP 0.5 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3760 G11 0.0 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3760 G12 3760fc 6 LT3760 Typical Performance Characteristics INTVCC vs Current, Junction Temperature 7.0 TA = 25°C, unless otherwise noted. INTVCC vs Current, Junction Temperature 6.0 ILOAD = 10mA, 20mA, 30mA INTVCC, UVLO(+), UVLO(–) vs Junction Temperature 8 VIN = 6V, PWM = 0V VIN = 12V INTVCC (REGULATED) 7 6.9 6 6.8 INTVCC (V) INTVCC (V) 5.00 ILOAD = 40mA 6.7 VIN = 8V, PWM = 0V 6.6 0 25 50 75 100 –50 –25 JUNCTION TEMPERATURE (°C) 4.5 –50 125 ILOAD = 10mA ILOAD = 20mA ILOAD = 30mA ILOAD = 40mA –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) INTVCC Current Limit vs Junction Temperature SENSE PIN VOLTAGE (mV) INTVCC CURRENT (mA) 55 50 45 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 57.5 60 55.0 50.0 INDUCTOR PEAK CURRENT THRESHOLD (CYCLE-BY-CYCLE) 47.5 40 30 20 45.0 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 0 –50 125 0 25 50 75 100 –25 JUNCTION TEMPERATURE (°C) 120 225 100 200 TIME (ns) GATE Rise/Fall Times vs GATE Capacitance 250 6.50 125 3760 G18 MINIMUM ON-TIME VIN = 8V INTVCC = 7V 80 TIME (ns) 6.75 VOUT = 12V OVPSET = 0.22V 10 Minimum ON and OFF Times vs Junction Temperature 6.00 OVPSET = 1.0V 3760 G17 7.00 125 50 52.5 40.0 –50 125 VOUT = 60V 0 25 50 75 100 JUNCTION TEMPERATURE (°C) Overvoltage Protection (OVP) Level vs Junction Temperature 70 VOUT - V(LEDx) Short Threshold vs Junction Temperature SHORT THRESHOLD (V) INTVCC UVLO(–) 60.0 3760 G16 5.75 3 3760 G15 42.5 6.25 INTVCC UVLO(+) SENSE Threshold vs Junction Temperature VIN = 6V, INTVCC = 0V 40 –50 –25 4 3760 G14 3760 G13 60 5 2 –50 –25 125 OVP (V) INTVCC (V) 5.5 175 MINIMUM OFF-TIME FALL TIME 60 150 40 125 20 RISE TIME 5.50 5.25 5.00 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3760 G18 100 –50 0 25 50 75 100 –25 JUNCTION TEMPERATURE (°C) 125 3760 G20 0 0 5 10 CGATE (nF) 15 20 3760 G21 3760fc 7 LT3760 Pin Functions CTRL (Pin 1): CTRL pin voltage below 1V controls LED current. CTRL voltage can be set by a resistor divider from VIN, VREF or an external voltage source. LED current derating versus temperature is achievable if the voltage programmed at the CTRL pin has a negative temperature coefficient using an external resistor divider from VREF pin to GND with temperature dependent resistance. TSET (Pin 2): Programs LT3760 junction temperature breakpoint past which LED current will begin to derate. Program using a resistor divider from VREF to GND. VREF (Pin 3): 1.485V Reference Output Pin. This pin can supply up to 150µA. Can be used to program CTRL, TSET and OVPSET pin voltages using resistor dividers to GND. ISET (Pin 4): Resistor to GND Programs LED pin current. See Table 6 in the Applications Information Section. NC (Pin 5): No Connect. Okay to leave open or to connect to GND. LEDx (Pins 6 to 9, 20 to 23): 8 LED Driver Outputs. Each output contains an open collector constant current sink. LED currents are programmable from 20mA to 100mA using a single resistor at the ISET pin. Connect the cathode of each LED string to an LED pin. Connect the anode of each LED string to VOUT. Channels can be paralleled for greater LED current or individually disabled (connect LED to VOUT). PGND (Pins 10, 19, Exposed Pad Pin 29): Power grounds for the IC and the converter. The package has an exposed pad (Pin 29) underneath the IC which is the best path for heat out of the package. Pin 29 should be soldered to a continuous copper ground plane under the device to reduce die temperature and increase the power capability of the LT3760. SENSE (Pin 11): The Current Sense Input for the Control Loop. Connect this pin to the sense resistor in the source of the external power MOSFET. VIN (Pin 14): Input Supply Pin. Must be locally bypassed with a 1µF capacitor to PGND. SHDN/UVLO (Pin 15): The SHDN/UVLO pin has an accurate 1.476V threshold and can be used to program an under voltage lockout (UVLO) threshold for system input supply using a resistor divider from supply to GND. A 2.4µA pin current hysteresis allows programming of UVLO hysteresis. SHDN/UVLO above 1.476V turns the part on and removes a 2.4µA sink current from the pin. SHDN/UVLO < 0.7V reduces VIN current < 20µA. If the shutdown function is not required, it should be forced above 1.476V or connected directly to VIN. FAULT (Pin 16): Active low if any or all LED strings have an open fault. If fault(s) removed, FAULT flag returns high. Fault status is only updated during PWM high state and latched during PWM low. SYNC (Pin 17): Allows synchronization of boost converter switching frequency to an external clock. RT resistor should be programmed for fOSC 20% below SYNC frequency. If unused, connect to GND. VOUT (Pin 18): Boosted Output Voltage of the Converter. Connect a capacitor from this pin to PGND. Connect the anode of each LED (string) to VOUT. GND (Pin 24): Signal Ground. RT (Pin 25): A resistor to GND programs switching frequency fOSC between 0.1MHz and 1MHz. VC (Pin 26): Output of Both Transconductance Error Amplifiers for the Converter Regulation Loop. The most commonly used gm error amplifier (LED) regulates VOUT to ensure no LED pin falls below 1V. The other gm error amplifier (OVP) is activated if all LEDs fail open and a regulated maximum VOUT is required. Connect a resistor and capacitor in series from the VC pin to GND. GATE (Pin 12): Drives the gate of an N-channel MOSFET from 0V to INTVCC. PWM (Pin 27): Input Pin for PWM Dimming Control. Above 1.4V allows converter switching and below 0.7V disables switching. The PWM signal can be driven from 0V to 6V. If unused, connect to VREF. INTVCC (Pin 13): A 7V LDO supply generated from VIN and used to power the GATE driver and some control circuitry. Must be bypassed with a 4.7µF capacitor to PGND. OVPSET (Pin 28): Programs maximum allowed VOUT regulation level if all LEDs are open circuit. Program using a resistor divider from VREF to GND. 3760fc 8 LT3760 Block Diagram 1.476V INTVCC VC – + 3 1 R S + – 4.2V(+) 3.7V(−) – + OSC SLOPE PWM EN 52mV PEAK CURRENT 6V LED CURRENT CONTROL EN 1.1V – + 18 16 PWM OVERVOLTAGE AMP – + TSET 11 VOUT SS + + + – LED AMP 2 25 SENSE FAULT SOFT START CHANNEL X VPTAT 17 LEDx 6 TO 9, 20 TO 23 LED LOGIC 1V 100mV OVER CURRENT HICCUP__MODE VREF CTRL 12 SYNC RT EN + – GATE Q EN 1.485V INTVCC_UV VIN_UV SHDN_UV 27 UVLO(+) = 3.8V, UVLO(−) = 3.4V + – REF 1.485V 7V(REGULATED) 4 ISET 24 GND PGND (10, 19, EXPOSED PAD (29)) 26 VC 56R + – 600k + – 13 VIN + – 15 14 SHDN/UVLO R 28 OVPSET 3760 BD Figure 1. LT3760 Block Diagram Operation The operation of the LT3760 is best understood by referring to the typical application circuit on the front page and the Block Diagram in Figure 1. The LT3760 drives 8 strings of LEDs by using a constant switching frequency, current mode boost controller to generate a single output voltage VOUT for the top (anode) of all LED strings. LED string current is generated and controlled by connection of the bottom LED in each string (cathode) to a current source contained in each corresponding LED pin. Each LED pin contains an accurate current sink to ground, programmable between 20mA to 100mA using a single resistor at the ISET pin. LED channels can be paralleled to achieve higher LED currents. For applications requiring less than 8 strings of LEDs, channels can be paralleled or disabled (connect LED pin to VOUT before startup). For optimum efficiency, VOUT regulates to the lowest possible voltage allowable to maintain regulated current in each LED string. Any OPEN LED fault is indicated by the FAULT pin driven low without effecting the operation of the connected LED strings. The Block Diagram in Figure 1 illustrates the key functions of the LT3760. It can be seen that two external supplies, VREF and INTVCC, are generated by the LT3760. The VREF pin provides a precision 1.485V output for use with external resistors to program the CTRL, OVPSET and TSET input pins. The INTVCC pin provides a regulated 7V output to supply the gate driver for the boost controller GATE pin. An accurate 1.476V threshold on the SHDN/UVLO pin combined with a SHDN/UVLO pin current hysteresis allows a programmable resistor divider from VIN to SHDN/UVLO 3760fc 9 LT3760 Operation to define the turn on/off voltages for VIN. SHDN/UVLO pin current switches from 2.4µA to 0µA when SHDN/UVLO pin voltage exceeds 1.476V. the LT3760 only allows MOSFET turn-on approximately every 2ms. This hiccup mode significantly reduces the power rating required for the MOSFET. The LT3760 constant switching frequency is programmable from 100kHz up to 1MHz using a single resistor at the RT pin to ground. A SYNC pin is also provided to allow an external clock to define the converter switching frequency. The GATE output provides a ±0.8A peak gate drive for an external N-channel power MOSFET to generate a boosted output voltage VOUT using a single inductor, Schottky diode and output capacitor. With LED strings connected from VOUT to every LED pin, the lowest voltage on each LED pin is monitored and compared to an internal 1V reference. VOUT is regulated to ensure the lowest LED pin voltage of any connected LED string is maintained at 1V. If any of the LED strings are open, the LT3760 will ignore the open LED pin. If all of the LED strings are open VOUT charges up until a user programmable OVP (overvoltage protection) level is reached. This programmable OVP level allows the user to protect against LED damage when the LED strings are opened and then reconnected. LED current programming and dimming can be achieved using the ISET, CTRL and PWM pins. A single resistor at the ISET pin programs LED current. Analog dimming of LED brightness is achieved using the CTRL pin below 1V. PWM dimming of LED brightness is achieved by controlling the duty cycle of the PWM pin. Since the LT3760 boost controller uses a current mode topology, the VC pin voltage determines the peak current in the inductor of the converter and hence the duty cycle of the GATE switching waveform. The basic loop uses a pulse from an internal oscillator to set an RS flip-flop and turn on the external power MOSFET. Current increases in the MOSFET and inductor until the VC commanded peak switch current is exceeded and the MOSFET is then turned off. Inductor current is sensed during the GATE on period by a sense resistor RS in the source of the external N-channel power MOSFET. As with all current mode converters, slope compensation is added to the control path to ensure stability for duty cycles above 50%. Any over current fault condition in the MOSFET turns off the MOSFET and triggers soft start internally. In this fault mode For robust operation the LT3760 monitors system conditions and performs soft start for startup after any of the following faults: VIN, SHDN or INTVCC voltages too low or MOSFET current too high. The LT3760, when entering these faults, discharges an internal soft start node and prevents switching at the GATE pin. When exiting these faults the LT3760 ramps up an internal soft start node to control VC pin voltage rise and hence control MOSFET peak switch current rise. In addition the soft start period gradually ramps up switching frequency from approximately 33% to 100% of full scale. The LT3760 monitors each LED pin voltage. If the LED string has an open fault (V(LEDX)<0.5V) the FAULT flag is pulled low. For LED protection, the LT3760 CTRL pin allows an LED current derating curve to be programmed versus the ambient temperature of the LED strings. An NTC resistor placed close to the LEDs decreases CTRL pin voltage and hence decreases LED current as LED ambient temperature increases. The LT3760 also allows it’s own junction temperature to be monitored and regulated by derating LED currents when a junction temperature programmed by the TSET pin is exceeded. 3760fc 10 LT3760 Applications Information INTVCC Regulator Bypassing and Operation Inductor The INTVCC pin is the output of an internal linear regulator driven from VIN and is the supply for the LT3760 gate driver. The INTVCC pin should be bypassed with a 10V rated 4.7µF low ESR, X7R or X5R ceramic capacitor to ensure stability and to provide enough charge for the gate driver. For high enough VIN levels the INTVCC pin provides a regulated 7V supply. Make sure INTVCC voltage does not exceed the VGS rating of the external MOSFET driven by the GATE pin. For low VIN levels the INTVCC level will depend on VIN and the voltage drop of the regulator. The INTVCC regulator has an undervoltage lockout which prevents gate driver switching until INTVCC reaches 3.8V and maintains switching until INTVCC falls below 3.4V. This feature prevents excessive power dissipation in the external MOSFET by ensuring a minimum gate drive level to keep RDS(ON) low. The INTVCC regulator has a current limit of 40mA to limit power dissipation inside the I.C. This current limit should be considered when choosing the N‑channel power MOSFET and the switching frequency. The average current load on the INTVCC pin due to the LT3760 gate driver can be calculated as: A list of inductor manufacturers is given 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 2.2µH and 33µH will suffice for most applications. The typical inductor value required for a given application (assuming 50% inductor ripple current for example) can be calculated as: IINTVCC = Qg • fOSC (N = number of LEDs per string), where Qg is the gate charge (at VGS = INTVCC) specified for the MOSFET and fosc is the switching frequency of the LT3760 boost converter. It is possible to drive the INTVCC pin from a variety of external sources in order to remove power dissipation from the LT3760 and/or to remove the INTVCC current limitation of 40mA. An external supply for INTVCC should never exceed the VIN pin voltage or the maximum INTVCC pin rating of 13V. If INTVCC is shorted to the VIN pin, VIN operational range is 4.5V to 13V. VF = LED forward voltage drop, 1 • 1 •V VOUT fOSC IN VIN L= V 0.5 • OUT • ILEDx • 8 VIN 1- where: VOUT = (N • VF) + 1V ILEDx = LED current per string Example: For a 12W LED driver application requiring 8 strings of 10 LEDs each driven with 40mA, and choosing VIN = 12V, VOUT = (3.75V • 10) + 1V = 38.5V, ILEDx = 40mA and fOSC = 1MHz the value for L is calculated as 1 1 ) • 6 • 12V 3.2 10 L= = 16.5µH 0.5 • 3.2 • 40mA • 8 (1- 3760fc 11 LT3760 Applications Information Schottky Rectifier Table 1. Inductor Manufacturers Manufacturer PHONE NUMBER WEB Sumida 408-321-9660 www.sumida.com Würth Elektronik 605-886-4385 www.we-online.com Vishay 402-563-6866 www.vishay.com Coilcraft 847-639-6400 www.coilcraft.com Coiltronics 561-998-4100 www.cooperet.com Input Capacitor The input capacitor of the LT3760 boost converter will supply the transient input current of the power inductor. Values between 2.2µF and 10µF will work well for the LT3760. Use only X5R or X7R ceramic capacitors to minimize variation over voltage and temperature. If inductor input voltage is required to operate near the minimum allowed operational VIN for the I.C., a larger capacitor value may be required. This is to prevent excessive input voltage ripple causing dips below the minimum operating input voltage. Output Capacitor Low ESR ceramic capacitors should be used at the LT3760 converter output to minimize output ripple voltage. Use only X5R or X7R dielectrics as these materials retain their capacitance over wider voltage and temperature ranges than other dielectrics. The output capacitance requirements for several LED driver application circuits are shown in the Applications Information section for various ILED, VIN, VOUT, L and fOSC values. Some suggested capacitor manufacturers are listed in Table 2. Table 2. Ceramic Capacitor Manufacturers Manufacturer PHONE NUMBER WEB TDK 516-535-2600 www.tdk.com Kemet 408-986-0424 www.kemet.com Murata 814-237-1431 www.murata.com Taiyo Yuden 408-573-4150 t-yuden.com AVX 843-448-9411 www.avxcorp.com The external diode for the LT3760 boost converter must be a Schottky diode, with low forward voltage drop and fast switching speed. Table 3 lists several Schottky manufacturers. The diodes average current rating must exceed the application’s average output current. The diode’s maximum reverse voltage must exceed the maximum output voltage of the application. For PWM dimming applications be aware of the reverse leakage of the Schottky diode. Lower leakage current will drain the output capacitor less during PWM low periods, allowing for higher PWM dimming ratios. The companies below offer Schottky diodes with high voltage and current ratings. Table 3. Schottky Rectifier Manufacturers Manufacturer PHONE NUMBER WEB Diodes, Inc. 805-446-4800 www.microsemi.com On Semiconductor 888-743-7826 www.onsemi.com Zetex 631-360-2222 www.zetex.com Vishay Siliconix 402-563-6866 www.vishay.com Power MOSFET Selection Several MOSFET vendors are listed in Table 4. Consult the factory applications department for other recommended MOSFETs. The power MOSFET selected should have a VDS rating which exceeds the maximum Overvoltage Protection (OVP) level programmed for the application. (See “Programming OVP level” in the Applications Information section). The MOSFET should also have a low enough total gate charge Qg (at 7V VGS) and a low enough switching frequency (fOSC) to not exceed the INTVCC regulator current limit, where loading on INTVCC pin due to gate switching should obey, IGATE = Qg • fOSC ≤ 40mA 3760fc 12 LT3760 Applications Information In addition, the current drive required for GATE switching should also be kept low in the case of high VIN voltages (see “Thermal Considerations” in the Applications Information section). The RDS(ON) of the MOSFET will determine d.c. power losses but will usually be less significant compared to switching losses. Be aware of the power dissipation within the MOSFET by calculating d.c. and switching losses and deciding if the thermal resistance of the MOSFET package causes the junction temperature to exceed maximum ratings. where IL(PEAK) = 1 0.5 • 8 •ILEDx • 1+ 1− D 2 D = MOSFET duty cycle = 1− ( ) VIN(MIN) VOUT(MAX) , VOUT(MAX) = N • VF(MAX) + 1V N = number of LEDs in each string, VF(MAX) = maximum LED forward voltage drop, Table 4. MOSFET Manufacturers Manufacturer PHONE NUMBER WEB VIN(MIN) = minimum input voltage to the inductor, Vishay Siliconix 402-563-6866 www.vishay.com International Rectifier 310-252-7105 www.irf.com I LED = current in each LED pin, Fairchild 972-910-8000 www.fairchildsemi.com Power MOSFET: Current Sense Resistor The LT3760 current mode boost converter controls peak current in the inductor by controlling peak MOSFET current in each switching cycle. The LT3760 monitors current in the external N-channel power MOSFET by sensing the voltage across a sense resistor (RS) connected between the source of the FET and the power ground in the application. The length of these tracks should be minimized and a Kelvin sense should be taken from the top of RS to the sense pin. A 52mV sense pin threshold combined with the value of RS sets the maximum cycle-by-cycle peak MOSFET current. The low 52mV threshold improves efficiency and determines the value for RS given by: RS ≤ 52mV • 0.7 IL(PEAK) and the 0.5 term represents an inductor peak-to-peak ripple current of 50% of average inductor current. The scale factor of • 0.7 ensures the boost converter can meet the peak inductor requirements of the loop by accounting for the combined errors of the 52mV sense threshold, ILEDx, RS and circuit efficiency. Example: For a 12W LED driver application requiring 8 strings of 10 LEDs each driven with 40mA, and choosing VIN(MIN) = 8V, VOUT(MAX) = (4V • 10)+1V = 41V and ILEDx = 40mA, the value for RS is chosen as: 52mV • 0.7 52mV • 0.7 ≤ 41 IL(PEAK) • 8 • 0.04 • (1+ 0.25) 8 52mV • 0.7 ≤ ≤ 17.7 mΩ 2.05 RS ≤ 3760fc 13 LT3760 Applications Information The power rating of RS should be selected to exceed the I2R losses in the resistor. The peak inductor current should be recalculated for the chosen RS value to ensure the chosen inductor will not saturate. Power MOSFET: Overcurrent and Hiccup Mode For severe external faults which may cause the external MOSFET to reach currents greater than the peak current defined by RS and the 52mV sense pin threshold described above, the LT3760 has an overcurrent comparator which triggers soft start and turns off the MOSFET driver for currents exceeding, I(OVERCURRENT ) = 100mV RS In this fault mode the LT3760 only allows MOSFET turn on for approximately 100ns every 2ms. This hiccup mode significantly reduces the power rating required for the MOSFET. Soft Start To limit inductor inrush current and output voltage during startup or recovery from a fault condition, the LT3760 provides a soft start function. The LT3760 when entering these faults will discharge an internal soft start node and prevent switching at the GATE pin for any of the following faults: VIN, SHDN/UVLO or INTVCC voltages too low or MOSFET current too high (see the timing diagram in Figure 2). When exiting these faults the LT3760 ramps up an internal soft start node at approximately 0.5V/ms to control VC pin voltage rise and hence control MOSFET switch current rise. In addition the soft start period gradually ramps up switching frequency from approximately 33% to 100% of full scale. The conditions required to exit all faults and allow a soft start ramp of the VC pin are listed in Figure 2. An added feature of the LT3760 is that it waits for the first PWM pin active high (minimum 200ns pulse width) before it allows GATE VC VC MIN CLAMP 0.5V/ms 0.4V + VBE (VC SWITCHING THRESHOLD) 0.1V + VBE SS (INTERNAL) 0.5V/ms 0.4V 0.1V ANY OF THE FOLLOWING FAULTS TRIGGERS SOFT START LATCH WITH GATE TURNED OFF IMMEDIATELY: VIN < 3.7V, SHDN < 1.476V, INTVCC < 3.4V IDSS (EXTERNAL MOSFET) > 100mV/RS SOFT-START LATCH SET: SOFT-START LATCH RESET REQUIRES ALL CONDITIONS SATISFIED: SS (INTERNAL) < 0.2V, VIN ≥ 4.2V, SHDN > 1.476V, INTVCC > 3.8V, IDSS (EXTERNAL MOSFET) < 100mV/RS, PWM > 1.4V (FOR AT LEAST 200ns) 3760 F02 Figure 2. LT3760 Fault Detection and Soft Start Timing for VC Pin and Internal SS Node 3760fc 14 LT3760 Applications Information the soft start of VC pin to begin. This feature ensures that during startup of the LT3760 the soft start ramp has not timed out before PWM is asserted high. Without this ‘wait for PWM high’ feature, systems which apply PWM after VIN and SHDN/UVLO are valid, can potentially turn on without soft start and experience high inductor currents during wake up of the converter’s output voltage. It is important to note that when PWM subsequently goes low, the soft start ramp is not held at its present voltage but continues to ramp upwards. If the soft start ramp voltage was held every time PWM goes low, this would cause very slow startup of LED displays for applications using very high PWM Dimming ratios. Shutdown and Programming Undervoltage Lockout The LT3760 has an accurate 1.476V shutdown threshold at the SHDN/UVLO 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 3). An internal hysteresis current at the SHDN/UVLO pin allows programming of hysteresis voltage for this UVLO threshold. Just before part turn on, an internal 2.4µA flows from the SHDN/ OFF ON VSUPPLY ON = VSUPPLY OFF + (2.4µA • R1) An open drain transistor can be added to the resistor divider network at the SHDN/UVLO pin to independently control the turn off of the LT3760. Programming Switching Frequency The switching frequency of the LT3760 boost converter can be programmed between 100kHz and 1MHz using a single resistor (RT) connected from the RT pin to ground (Figure 4). Connect the RT resistor as close as possible to the RT pin to minimize noise pick up and stray capacitance (see “Circuit Layout Considerations” in the Applications Information section). Table 5 shows the typical RT values required for a range of frequencies. 1000 1.476V SWITCHING FREQUENCY (kHz) 15 SHDN/UVLO 600k R2 R1 VSUPPLY OFF = 1.476 1+ R2 900 VSUPPLY R1 UVLO pin. After part turn on, 0µA flows from the SHDN/ UVLO pin. Calculation of the turn on/off thresholds for a system input supply using the LT3760 SHDN/UVLO pin can be made as follows : – + 800 700 600 500 400 300 200 100 3760 F03 Figure 3. Programming Undervoltage Lockout (UVLO) with Hysteresis 0 100 200 300 400 RT (kΩ) 500 600 3760 F04 Figure 4. Switching Frequency vs RT 3760fc 15 LT3760 Applications Information Selecting the optimum frequency depends on several factors. Higher frequency allows reduction of inductor size but efficiency drops due to higher switching losses. Lower frequency allows higher operational duty cycles to drive a larger number of LEDs per string from a low input supply but require larger magnetics. In each application the switching frequency can be tailored to provide the optimum solution. Table 5. Switching Frequency vs. RT (1% resistors) SWITCHING FREQUENCY (kHz) RT (kΩ) 100 523 200 249 300 158 400 115 500 90.9 600 73.2 700 60.4 800 51.1 900 44.2 1000 39.2 Duty Cycle Considerations When designing the LT3760 LED driver for a given application, the duty cycle requirements should be considered and compared to the minimum/maximum achievable duty cycles for the LT3760 GATE pin. If required, the LT3760 switching frequency can be programmed to a lower value to meet the duty cycle requirements. In general, the minimum/maximum GATE duty cycles required for a particular application are given by: MIN Duty Cycle = GATE Minimum On-Time • Switching Frequency fOSC MAX Duty Cycle = 1 – (GATE Minimum Off-Time • Switching Frequency fOSC) Table 6. LED Current vs. RISET (1% resistors) LED CURRENT PER CHANNEL (mA) RISET (kΩ) 20 29.4 40 14.7 60 9.76 80 7.32 100 5.76 An extra 50ns should be added to these tested timings to account for errors in the rise/fall times of the GATE and DRAIN of the external MOSFET and the d.c. resistance of the external MOSFET and inductor. Synchronizing to an External Clock The SYNC pin allows the LT3760 oscillator to be synchronized to an external clock. The SYNC pin can be driven from a logic level output, requiring less than 0.6V for a logic level low and greater than 2.2V for a logic level high. SYNC pin high or low periods should exists for at least 100ns. If unused, the SYNC pin should be tied to ground. To avoid loss of slope compensation during synchronization, the free running oscillator frequency (fOSC) of the LT3760 should be programmed to 80% of the external clock frequency. Programming LED Current The current source to ground at each LED pin is programmed using a single resistor RISET connected from the ISET pin to ground according to the following equation: I (LEDX ) ≈ 590 ( ) A (CTRL > 1.1V ) RISET See Table 6 for resistor values and corresponding programmed LED. The typical values for LT3760 GATE pin minimum on and off times versus temperature are shown in the Typical Performance Characteristics. The range of GATE pin minimum on time and off times are given in the electrical specifications. 3760fc 16 LT3760 Applications Information Analog Dimming The LT3760 allows for LED dimming (brightness reduction) by analog dimming or by PWM dimming. Analog dimming uses the CTRL pin voltage below 1V to reduce LED brightness by reducing LED current. For CTRL pin voltage below 1V, the current in each LED pin is given by: 590 I (LEDX ) ≈ CTRL • (0.04 < CTRL < 1V ) RISET For CTRL pin voltages below 40mV (greater than 25:1 dimming) the LED current will approach zero current. The CTRL pin voltage can be derived from a resistor divider from VREF pin to ground or generated from an external source. If analog dimming is not required, the pin can be directly connected to the VREF pin. The only drawback of analog dimming is that reducing LED current to reduce the brightness of the LED also changes the perceived color of the LED. turn LED currents on/off as quickly as possible. For PWM low, the LT3760 turns off the boost converter, turns off all LED channel currents and disconnects the VC pin and internal VOUT resistor divider connected to the OVP error amplifier. This allows the part to quickly return to the last state of operation when the PWM pin is returned high. Some general guidelines for LED current dimming using the PWM pin (see Figure 5): (1) PWM Dimming Ratio (PDR) = 1/(PWM Duty Cycle) = 1/TON(PWM) • fPWM (2) Lower PWM frequency (fPWM) allows higher PWM dimming ratios (Typically choose 100Hz to maximize PDR and to avoid visible flicker which can occur for display systems with refresh rates at frequencies below 80Hz) (3) Higher fOSC value improves PDR (allows lower TON(PWM)) but will reduce efficiency and increase internal heating. In general, minimum operational TON(PWM) = 3 • (1/fOSC) PWM Dimming (4) Lower inductor value improves PDR Many 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 LT3760 provides a PWM pin and special internal circuitry to achieve up to a 3000:1 wide PWM dimming range. This is achieved by operating the LED at it’s programmed current and then controlling the on time of that LED current. The duty cycle of the PWM pin controls the on time of each LED pin current source (Figure 5). For maximum PWM dimming ratios (low PWM duty cycles) it is important to be able to (5) Higher output capacitor value improves PDR (6) Choose the Schottky diode for the LT3760 boost converter for minimum reverse leakage current. Programming LED Current Derating (Breakpoint and Slope) versus LED Ambient Temperature (CTRL Pin) LED data sheets provide curves of maximum allowed LED current versus ambient temperature to warn against damaging of the LED (Figure 6). The LT3760 LED driver improves the utilization and reliability of the LED(s) by al90 TON(PWM) IF - FORWARD CURRENT (mA) 80 TPWM (= 1/fPWM) PWM INDUCTOR CURRENT RESISTOR OPTION A 70 60 50 LT3760 PROGRAMMED LED CURRENT DERATING CURVE 40 30 20 10 LED CURRENT MAX ILED 0 3760 F05 Figure 5. PWM Dimming Waveforms 0 10 20 30 40 50 60 TA-TEMPERATURE (°C) 70 80 3760 F06 Figure 6. LED Current Derating vs LED Ambient Temperature 3760fc 17 LT3760 Applications Information lowing the programming of an LED current derating curve versus the ambient temperature of the LED(s). Without the ability to back off LED currents as temperature increases, many LED drivers are limited to driving the LED(s) at 50% or less of their maximum rated currents. This limitation requires more LEDs to obtain the intended brightness for the application. The LT3760 allows the LED(s) to be programmed for maximum allowable current while still protecting the LED(s) from excessive currents at high temperature. The temperature breakpoint and the slope of LED current versus ambient temperature can be programmed using a simple resistor network shown in Figure 7. This is achieved by programming a voltage at the CTRL pin with a negative temperature coefficient using a resistor divider with temperature dependent resistance (Figures 7 and 8). A variety of resistor networks and NTC resistors with different temperature coefficients can be used to achieve the desired CTRL pin voltage behavior versus temperature. The current derating curve in Figure 6 uses the resistor network shown in option A of Figure 7. Table 7 shows a list of NTC resistor manufacturers/ distributors. There are several other manufacturers available and the chosen supplier should be contacted for more detailed information. To use an NTC resistor to monitor the ambient temperature of the LED(s) it should be placed as close as possible to the LED(s). Since the temperature dependency of an NTC resistor can be non-linear over a wide range of temperatures it is important to obtain a resistor’s exact values over temperature from the manufacturer. Hand calculations of CTRL voltage can then be performed at each given temperature and the resulting CTRL voltage plotted versus temperature. Table 7. NTC Resistor Manufacturers Manufacturer WEB Murata Electronics North America www.murata.com TDK Corporation www.tdk.com Digi-key www.digikey.com If calculation of CTRL 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 (CTRL pin behavior) over temperature. Referred to on the website as the ‘Murata Chip NTC Thermistor Output Voltage Simulator’, 1.50 R1 1 VREF 1.25 LT3760 CTRL VOLTAGE (V) 3 CTRL OPTION A TO D R2 RY RNTC RNTC RX RNTC RESISTOR OPTION A 0.75 0.50 RY RNTC 1.00 RX 0.25 0 10 20 30 40 50 60 70 TA - AMBIENT TEMPERATURE (°C) 80 3760 F08 A B C D 3760 F07 Figure 7. Programming LED Current Derating Curve vs Ambient Temperature (RNTC Located on LED PCB) Figure 8. Programmed CTRL Voltage vs Temperature 3760fc 18 LT3760 Applications Information users can log onto www.murata.com/designlib and download the software followed by instructions for creating an output voltage ‘VOUT’ (LT3760 CTRL pin voltage) from a specified VCC supply (LT3760 VREF pin voltage). At any time during selection of circuit parameters the user can access data on the chosen NTC resistor by clicking on the link to the Murata catalog. For a detailed example of hand calculations using an NTC type resistor divider to program CTRL pin voltage, read the LT3478 LED driver data sheet section Programming LED Current Derating vs Temperature under Applications Information. Using the TSET Pin for Thermal Protection While this feature is intended to directly protect the LT3760, it can also be used to derate the LED current at high temperatures. Since there is a direct relationship between the LED temperature and LT3760 junction temperature, the TSET function also provides some LED current derating at high temperatures. Two external resistors program the maximum IC junction temperature using a resistor divider from the VREF pin, as shown in Figure 9. Choose the ratio of R1 and R2 for the desired junction temperature. Figure 10 shows the relationship of TSET voltage to junction temperature, and Table 8 shows commonly used values for R1 and R2. The LT3760 contains a special programmable thermal regulation loop that limits the internal junction temperature of the part. Since the LT3760 topology consists of a single boost controller with eight linear current sources, any LED string voltage mismatch will cause additional power to be dissipated in the package. This topology provides excellent current matching between LED strings and allows a single power stage to drive a large number of LEDs, but at the price of additional power dissipation inside the part (which means a higher junction temperature). Being able to limit the maximum junction temperature allows the benefits of this topology to be fully realized. This thermal regulation feature provides important protection at high ambient temperatures, and allows a given application to be optimized for typical, not worst-case, ambient temperatures with the assurance that the LT3760 will automatically protect itself and the LED strings under worst-case conditions. The operation of the thermal loop is simple. As the ambient temperature increases, so does the internal junction temperature of the part. Once the programmed maximum junction temperature is reached, the LT3760 begins to linearly reduce the LED current, as needed, to try and maintain this temperature. This can only be achieved when the ambient temperature stays below the desired maximum junction temperature. If the ambient temperature continues to rise past the programmed maximum junction temperature, the LEDs current will be reduced to approximately 5% of the full LED current. 3 R2 VREF LT3760 2 TSET R1 3760 F09 Figure 9. Programming the TSET Pin 950 VTSET THRESHOLD (mV) 900 850 800 VPTAT 750 700 650 600 550 500 0 25 50 75 100 125 JUNCTION TEMPERATURE (°C) 150 3760 F10 Figure 10. Programing the TSET Pin Threshold Table 8. Resistor Values to Program Maximum IC Junction Temperature (VREF (Typical) = 1.485V) TJ (°C) R1 (kΩ) R2 (kΩ) TSET (V) 100 24.9 20 0.824 115 28.0 20 0.866 130 30.9 20 0.902 3760fc 19 LT3760 Applications Information Programming Overvoltage Protection (OVP) Level The LT3760 LED driver provides optimum protection to the LEDs and the external MOSFET by providing a programmable maximum regulated output voltage limit using the OVPSET pin. The Overvoltage Protection (OVP) level is programmed as: OVP(maximum regulated VOUT) = 57 • OVPSET If every LED string fails open or the multiple string LED display becomes disconnected the LT3760 LED driver loop regulates to the programmed OVP level. The OVP level should be programmed to a level high enough to regulate the LED strings but low enough to prevent damage to the power switch and to minimize the voltage across the LED pins upon reconnection of the LED strings. Recommended OVP level is given by: OVP(RECOMMENDED) = 1.2 • ((N • VF) + 1V) where: N = number of LEDs in each string, VF = maximum LED forward voltage drop and the scaling factor of 1.2 accounts for variation in the generation of OVP from OVPSET pin voltage and startup logic requirements. Example: For a converter operating with 10 LEDs per string at a maximum forward voltage of 4V per LED, the OVP level should be programmed to: OVP(RECOMMENDED) = 1.2 • ((10 • 4) + 1V ) = 49.2V For OVP = 49.2V, OVPSET = 49.2 = 0.863V 57 The OVPSET pin voltage can be generated using a resistor divider from the REF pin. LED Open Circuit and PWM Dimming Ratios The LT3760 monitors each LED pin voltage to determine if the LED string has an open fault (LED pin voltage < 0.5V). If an open LED fault occurs, the FAULT flag is pulled low. To avoid false detection of faults during the initial converter startup when VOUT is low, the LT3760 ignores low LED pin voltages until VOUT reaches 90% of its maximum allowed OVP level. Once this condition is met, the LT3760 monitors all LED pins for open LED faults. To avoid false detection of faults during PWM dimming edges (where LED pins can possibly ring and trip fault detection levels) the LT3760 only monitors/updates fault conditions during PWM high (and only after a blank duration of 2µs following each PWM rising edge). LED Short Circuit A short circuit fault between the positive terminal of an LED string (VOUT) and the negative terminal of the LED string (LEDx pin) causes the channel to be disabled in order to protect the internal current source. A resistive short is allowed as long as (VOUT -VLEDx) < 6V. Loop Compensation Be sure to check the stability of the loop with the LEDs connected (LED regulation loop) and disconnected (Overvoltage Protection (OVP) regulation loop). Various application circuits are shown in the data sheet which cover a range of VIN, VOUT, fOSC, output power and inductor current ripple values. For application requirements which deviate from the circuits shown in the data sheet be sure to check the stability of the final application over the full VIN range, LED current range (if analog dimming) and temperature range. Be aware that if the VC pin components represent a dominant pole for the converter loop and they have been adjusted to achieve stability, the VC pin might move more slowly during load transient conditions such as an all-LEDs-open fault. A slower moving VC pin will add to VOUT overshoot during an all-LEDs-open fault. An alternative compensation approach is to place the dominant pole of the converter loop at the output. This requires an increased output capacitor value but will allow a much reduced Vc capacitor. The combination will allow VC to move more quickly and VOUT to move more slowly resulting in less overshoot during an all-LEDs-open fault. 3760fc 20 LT3760 Applications Information Thermal Considerations The internal power dissipation of the LT3760 comes from 3 main sources: VIN quiescent current (IQ total), VIN current for GATE switching (IGATE) and the LT3760 LED current sources. Since the maximum operational VIN voltage is 40V, care should be taken when selecting the switching frequency and type of external power MOSFET since the current required from VIN for GATE switching is given by, IGATE = fOSC • Qg where Qg is the gate charge (at VGS = INTVCC) specified for the MOSFET and fOSC is the programmed switching frequency for the LT3760. A low Qg MOSFET should always be used when operating the LT3760 from high VIN voltages. The internal junction temperature of the LT3760 can be estimated as: TJ = TA + [VIN • (IQTOTAL + (fOSC • Qg)) + (8 • I(LEDX) • 1.1V)] • θJA where, TA is the ambient temperature for the LT3760 IQTOTAL represents the VIN quiescent current for the LT3760 (not switching, PWM = 1.5V and CTRL = 0.1V) - illustrated in the Typical Characteristics Graphs – plus the base currents of active channels (typically 8 • I(LED)/75). θJA is the thermal resistance of the package (28°C/W for the 28-pin TSSOP package). Example : For a 12W LED driver application requiring 8 strings of 10 LEDs each driven with 40mA, VIN = 24V, fOSC = 1MHz, Qg (at 7V VGS) = 15nC, I(LEDX) = 40mA, and an 85°C ambient temperature for the LT3760 IC, the LT3760 junction temperature can be approximated as: TJ = 85°C + [24 • (9.5mA + (8 • 40mA/75) + (1MHz • 15nC)) + (8 • 40mA • 1.1V)] • 34 = 85°C + [(24 • 28.8mA) + (320mA • 1.1V)] • 34 = 85°C + (0.691W + 0.35W) • 34 = 85°C + 35°C TJ = 120°C The exposed pad on the bottom of the package must be soldered to the ground plane. The ground plane should be connected to an internal copper ground plane with vias placed directly under the package to spread out the heat generated by the LT3760. Circuit Layout Considerations As with all switching regulators, careful attention must be given to PCB layout and component placement to achieve optimal thermal, electrical and noise performance. The exposed pad of the LT3760 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 signal ground (GND, pin 24) is down bonded to the exposed pad near the RT and VC pins. ISET, RT and VC components should be connected to an area of ground copper connected to pin 24. The OVPSET track should be kept away from fast moving signals and not loaded with an external capacitor. GATE pin turn off currents escape through a downbond to the exposed pad and exit the PGND, pin 10. This area of copper and pin 10 should be the power ground (PGND) connection for the inductor input capacitor, INTVCC capacitor and output capacitor. A separate bypass capacitor for the VIN pin of the IC may be required close the VIN pin and connected to the copper area associated with signal ground, pin 24. To minimize MOSFET peak current sensing errors the sense resistor (RS) should have Kelvin connections to the SENSE pin and the power ground copper area near the pin. The MOSFET drain 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 copper trace for the MOSFET drain 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 drain node to minimize this high switching frequency path. 3760fc 21 LT3760 Typical Applications 92% Efficient, 36W LED Driver, 1MHz Boost, 8 Strings, 100mA Per String PVIN 20V TO 36V 4.7µF 50V VIN 8V TO 14V L1 10µH D1 UP TO 45V OF LEDs PER STRING 4.7µF 25V VIN 4.7µF 10V INTVCC GATE M1 SENSE VIN 499k 2.2µF 100V ×5 100k • • • 0.015Ω LT3760 • • • • • • • • • • • • • • • • • • • • • PGND FAULT VOUT SHDN/UVLO 40.2k GND LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 SYNC PWM PWM DIMMING ANALOG DIMMING CTRL 3760 TA03 VREF 20k LED Current Waveforms 3000:1 PWM Dimming (100Hz) TSET 30.9k 11k OVPSET RT 20k 39.2k ISET 5.76k VC 10k (FRONT COVER APPLICATION) I(LEDx) 40mA/DIV 2.2nF L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS360 I(L1) 1A/DIV PWM 10V/DIV 5µs/DIV 3760 G04 3760fc 22 LT3760 Typical Applications 28W LED Driver, 750kHz Boost, 8 Strings, 80mA Per String L1 10µH VIN 11V TO 18V D1 UP TO 44V OF LEDs PER STRING 4.7µF 25V VIN 4.7µF 10V INTVCC GATE M1 SENSE 0.0125Ω 1M LT3760 100k • • • • • • • • • • • • • • • • • • • • • VOUT SHDN/UVLO ANALOG DIMMING • • • PGND FAULT SHDN 2.2µF 100V ×7 CTRL GND LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 SYNC PWM DIMMING PWM 3760 TA04 VREF 20k TSET 10k OVPSET RT 30.9k 16.9k 56.2k ISET 7.32k VC 10k 4.7nF L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7308DN D1: DIODES, INC. DFLS160 3760fc 23 LT3760 Typical Applications 15W LED Driver, 750kHz Boost, 8 Strings, 55mA Per String L1 7.3µH VIN 8V TO 21V D1 UP TO 34V OF LEDs PER STRING 4.7µF 50V VIN 4.7µF 10V INTVCC GATE M1 2.2µF 50V ×5 SENSE 0.015Ω 1M LT3760 100k • • • • • • • • • • • • • • • • • • • • • • • • PGND FAULT SHDN VOUT SHDN/UVLO ANALOG DIMMING CTRL GND LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 SYNC PWM DIMMING* PWM 3760 TA05 VREF 20k TSET 10k OVPSET RT 30.9k 10k 56.2k ISET 10.7k VC 5.1k 4.7nF L1: SUMIDA CDRH8D28 M1: VISHAY SILICONIX Si7308DN D1: DIODES, INC. DFLS160 *MAXIMUM PWM DIMMING RATIO: (a) fPWM = 20kHz = 20:1 (VIN > 10V) = 5:1 (VIN = 8V) (b) fPWM = 100Hz = 3000:1 (VIN > 10V) = 750:1 (VIN = 8V) 3760fc 24 LT3760 Typical Applications 29W LED Driver, 400kHz Boost, 2 Strings, 350mA Per String L1 10µH VIN 8V TO 36V D1 UP TO 42V OF LEDs PER STRING 4.7µF 50V VIN 4.7µF 10V INTVCC GATE M1 SENSE 0.007Ω 100k 1M LT3760 PGND 2.2µF 100V ×10 • • • • • • FAULT VOUT SHDN/UVLO 232k GND LED1 LED2 LED3 LED4 SYNC PWM PWM DIMMING ANALOG DIMMING LED5 LED6 LED7 LED8 CTRL VREF 3760 TA06 20k TSET 15k OVPSET RT 30.9k 23.2k 115k ISET VC 6.65k 5.1k 4.7nF L1: COOPER BUSSMANN HC9-100-R M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS560 3760fc 25 LT3760 Typical Applications 25W LED Driver, 400kHz Boost, 3 Strings, 200mA Per String L1 10µH VIN 8V TO 36V D1 UP TO 42V OF LEDs PER STRING 4.7µF 50V VIN 4.7µF 10V INTVCC GATE M1 SENSE 0.007Ω 100k 1M LT3754 • • • • • • VOUT SHDN/UVLO LED8 LED7 232k GND LED1 LED2 SYNC PWM ANALOG DIMMING • • • PGND FAULT PWM DIMMING 2.2µF 100V ×10 LED3 LED4 CTRL VREF LED5 LED6 20k TSET 3760 TA07 15k OVPSET RT 30.9k 23.2k 115k ISET VC 5.76k 5.1k 4.7nF L1: COOPER BUSSMANN HC9-100-R M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS560 3760fc 26 LT3760 Typical Applications 29W LED Driver, 700kHz Boost, 4 Strings, 160mA Per String L1 15µH VIN 10V TO 18V D1 UP TO 45V OF LEDs PER STRING 4.7µF 25V VIN 4.7µF 10V INTVCC GATE SENSE VIN 1M M1 0.02Ω LT3760 100k PGND FAULT GND • • • • • • LED3 LED4 PWM ANALOG DIMMING • • • LED1 LED2 SYNC PWM DIMMING • • • VOUT SHDN/UVLO SHDN 2.2µF 100V ×5 CTRL LED5 LED6 VREF 20k LED7 LED8 TSET 11k OVPSET RT 30.9k 20k 60.4k ISET 7.32k 3760 TA08 VC 7.5k 4.7nF L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7308DN D1: DIODES, INC. DFLS160 3760fc 27 LT3760 Typical Applications 14W LED Driver, 700kHz Boost, 4 Strings, 80mA Per String (For Machine Vision Systems with Very Long Off-Times) L1 15µH VIN 10V TO 28V D1 4.7µF 10V INTVCC GATE 0.02Ω LT3760 100k PGND FAULT • • • • • • • • • CTRL LED5 LED6 VREF 20k LED7 LED8 TSET 20.5k OVPSET RT 30.9k • • • LED3 LED4 PWM ANALOG DIMMING 2.2µF 100V ×5 LED1 LED2 SYNC PWM DIMMING 5V/0V ON/OFF VOUT SHDN/UVLO GND 140k M1 SENSE SHDN 10k Q1 1k VIN 1M UP TO 45V OF LEDs PER STRING M2 4.7µF 50V VIN M3 1.5k 20k 60.4k ISET 7.32k 3760 TA09 VC 10k 820pF 200pF L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7308DN M2: VISHAY SILICONIX Si2309DS M3: VISHAY SILICONIX Si2312DS Q1: MMBTA42 D1: DIODES, INC. DFLS160 3760fc 28 LT3760 Typical Applications 13W LED Driver, 1MHz SEPIC, 8 Strings, 100mA Per String (Survives VOUT Short to PGND) PVIN 10V TO 32V 4.7µF 50V 4.7µF 25V L1B 15µH VIN 4.7µF 10V INTVCC GATE D1 VOUT UP TO 16V OF LEDs PER STRING 499k M1 • 4.7µF 25V ×4 SENSE VIN 1M 2.2µF 50V ×2 • VIN 8V TO 14V L1A 15µH 0.015Ω LT3760 100k PGND FAULT VOUT SHDN/UVLO 100k CTRL GND 110k LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 SYNC PWM PWM DIMMING ANALOG DIMMING CTRL 3760 TA10 VREF 20k TSET 30.9k 20k OVPSET RT 6.34k 39.2k ISET 5.76k VC 7.5k 4.7nF L1A, L1B: 15µH COUPLED INDUCTOR DRQ125 M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS560 3760fc 29 LT3760 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 2726 25 24 23 22 21 20 19 18 1716 15 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.65 BSC 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 2. DIMENSIONS ARE IN MILLIMETERS (INCHES) 3. DRAWING NOT TO SCALE 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.25 REF 1.20 (.047) MAX 0° – 8° 0.65 (.0256) BSC 0.195 – 0.30 (.0077 – .0118) TYP 0.05 – 0.15 (.002 – .006) FE28 (EB) 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 3760fc 30 LT3760 Revision History REV DATE DESCRIPTION A 1/11 Revised FAULT pin description. 8 B 10/11 Updated Features section. 1 Updated equation in “Power MOSFET: Current Sense Resistor” section. 13 Updated “Programming LED Current Derating (Breakpoint and Slope) versus LED Ambient Temperature (CTRL Pin)” section. 18 Corrected the inductor value formula. 11 C 3/12 PAGE NUMBER 3760fc 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. 31 LT3760 Related Parts PART NUMBER DESCRIPTION COMMENTS LT3755/LT37551/ LT3755-2 High Side 40V, 1MHz LED Controller with True Color 3,000:1 PWM Dimming VIN = 4.5V to 40V, VOUT(MAX) = 75V, 3,000:1 True Color PWM Dimming, ISD = <1µA, 3mm × 3mm QFN-16 MSOP-16E LT3756/LT37561/ LT3756-2 High Side 100V, 1MHz LED Controller with True Color 3,000:1 PWM Dimming VIN = 6V to 100V, VOUT(MAX) = 100V, 3,000:1 True Color PWM Dimming, ISD = <1µA, 3mm × 3mm QFN-16 MSOP-16E LT3598 44V, 1.5A, 2.5MHz Boost 6-Channel 20mA LED Driver VIN = 3V to 30V (40VMAX), VOUT(MAX) = 44V, 1,000:1 True Color PWM Dimming, ISD = <1µA, 4mm × 4mm QFN-24 LT3599 44V, 2A, 2.5MHz Boost 4-Channel 100mA LED Driver VIN = 3V to 30V (40VMAX), VOUT(MAX) = 44V, 1,000:1 True Color PWM Dimming, ISD = <1µA, 4mm × 4mm QFN-24 LT3595 45V, 2.5MHz 16-Channel Full Featured LED Driver VIN = 4.5V to 45V, VOUT(MAX) = 45V, 5,000:1 True Color PWM Dimming, ISD = <1µA, 5mm × 9mm QFN-56 LTC3783 High Side 36V, 1MHz LED Controller with True Color 3,000:1 PWM Dimming VIN = 3V to 36V, VOUT(MAX) = 40V, 3,000:1 True Color PWM Dimming, ISD = <20µA, 4mm × 5mm DFN-16 TSSOP-16E LT3517 1.5A, 2.5MHz High Current LED Driver with 3,000:1 Dimming VIN = 3V to 30V, VOUT(MAX) = 45V, 3,000:1 True Color PWM Dimming, ISD = <1µA, 4mm × 4mm QFN-16 LT3518 2.3A, 2.5MHz High Current LED Driver with 3,000:1 Dimming VIN = 3V to 30V, VOUT(MAX) = 45V, 3,000:1 True Color PWM Dimming, ISD = <1µA, 4mm × 4mm QFN-16 LT3519/LT35191/ LT3519-2 750mA, 2.2MHz High Current LED Driver VIN = 3V to 30V, VOUT(MAX) = 45V, 3,000:1 True Color PWM Dimming, ISD = <1µA, MSOP-16E LT3486 Dual 1.3A, 2MHz High Current LED Driver VIN = 3V to 40V, VOUT(MAX) = 36V, 1,000:1 True Color PWM Dimming, ISD = <1µA, 5mm × 3mm DFN, TSSOP-16E LT3478/LT3478-1 4.5A, 2MHz High Current LED Driver with 3,000:1 Dimming VIN = 2.8V to 36V, VOUT(MAX) = 60V, 3,000:1 True Color PWM Dimming, ISD = <10µA, 5mm × 7mm QFN-10 LT3496 VIN = 3V to 30V, VOUT(MAX) = 40V, 3,000:1 True Color PWM Dimming, ISD = <1µA, 4mm × 5mm QFN-28 Triple Output 750mA, 2.1 MHz High Current LED Driver with 3,000:1 Dimming LT3474/LT3474-1 36V, 1A (ILED), 2MHz, Step-Down LED Driver VIN = 4V to 36V, VOUT(MAX) = 13.5V, 400:1 True Color PWM Dimming, ISD = <1µA, TSSOP-16E LT3475/LT3475-1 Dual 1.5A(ILED), 36V, 2MHz, Step-Down LED Driver VIN = 4V to 36V, VOUT(MAX) = 13.5V, 3,000:1 True Color PWM Dimming, ISD = <1µA, TSSOP-20E LT3476 Quad Output 1.5A, 2MHz High Current LED Driver with 1,000:1 Dimming VIN = 2.8V to 16V, VOUT(MAX) = 36V, 1,000:1 True Color PWM Dimming, ISD = <10µA, 5mm × 7mm QFN-10 LT3754 16-Channel × 50mA LED Driver VIN = 6V to 40V, VOUT(MAX) = 60V, 3,000:1 True Color PWM Dimming, ISD = <2µA, 5mm × 5mm QFN-32 3760fc 32 Linear Technology Corporation LT 0312 REV C • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2009