LT3754 16-Channel × 50mA LED Driver DESCRIPTION FEATURES n n n n n n n n n n n n n n Up to 45V of LEDs × 50mA, 16-Channel LED Driver Wide Input Range : 6V to 40V ±2.8% LED Current Matching at 20mA (Typ ±0.7%) Up to 3000:1 True Color PWM™ Dimming Range Single Resistor Sets LED Current (10mA to 50mA) LED Current Regulated Even for PVIN > VOUT Output Adapts to LED VF for Optimum Efficiency Fault Flag + Protection for Open LED Strings Protection for LED Pin to VOUT Short Parallel Channels for Higher LED Current Programmable LED Current Derating vs Temperature Accurate Undervoltage Lockout Threshold with Programmable Hysteresis Programmable Frequency (100kHz to 1MHz) Synchronizable to an External Clock The LT®3754 is a 16-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 ±2.8% current matching. Channels follow a master programmable current to allow between 10mA to 50mA 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. The LT3754 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 LT3754 is available in a thermally enhanced 5mm × 5mm 32-pin QFN Package. APPLICATIONS n 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. TYPICAL APPLICATION Worst-Case Channels LED Current Matching (Normalized to 16-channel Average) 92% Efficient, 36W Backlight LED Driver 4.7μF VIN 10V 10μH 4.7μF VIN INTVCC 499k 5s 2.2μF GATE 4.7μF •••• SENSE SHDN/UVLO 0.015Ω 40.2k CTRL VOUT LED1 LED2 • • • • LED15 LED16 REF 20k TSET 30.9k • • • • • • • • • • LT3754 PWM • 16 CHANNELS • • • 100k 11k 0.8 UP TO 45V OF LEDs PER STRING VIN LED CURRENT MATCHING (%) PVIN 24V 0.4 0.0 –0.4 RISET = 14.7k (I(LED) = 20mA) –0.8 0 25 50 75 100 –50 –25 JUNCTION TEMERATURE (°C) 3754 TA01 125 3754 TA01 FAULT OVPSET GND RT ISET VC SYNC 20k 39.2k 5.76k 10k 2.2nF 3754f 1 LT3754 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) RT VC OVPSET PWM CTRL TSET REF ISET TOP VIEW 32 31 30 29 28 27 26 25 LED1 1 24 LED16 LED2 2 23 LED15 LED3 3 22 LED14 LED4 4 LED5 5 LED6 6 19 LED11 LED7 7 18 LED10 LED8 8 21 LED13 33 20 LED12 17 LED9 VOUT SYNC FAULT SHDN/UVLO VIN INTVCC GATE 9 10 11 12 13 14 15 16 SENSE VOUT, LED1-16 .........................................................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 UH PACKAGE 32-LEAD (5mm s 5mm) PLASTIC QFN TJMAX = 125°C, θJA = 34°C/W, θJC = 3°C/W EXPOSED PAD (PIN 33) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3754EUH#PBF LT3754EUH#TRPBF 3754 32-Lead (5mm × 5mm) Plastic QFN –40°C to 125°C LT3754IUH#PBF LT3754IUH#TRPBF 3754 32-Lead (5mm × 5mm) Plastic QFN –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/ 3754f 2 LT3754 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 14 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) LED1–16 = 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) l INTVCC Current Limit 44 7 3.4 V 57 mA OVP/ LED ERROR AMPLIFIERS Transconductance (OVP) ΔIVC = ±2.5μA Voltage Gain (OVP) Transconductance (LED) ΔIVC = ±2.5μA 4 μmhos 5 V/V 33 μmhos 3754f 3 LT3754 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 μS 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 l SENSE Current Limit Threshold Current Mode Gain 46 ΔV(VC)/ΔV(SENSE) μA 60 6 l SENSE Over Current Limit Threshold 52 90 100 mV V/V 110 mV 20.2 21.11 mA ±0.7 ±2.8 % 50.1 52.35 mA LED CURRENT / CONTROL ISET Pin Voltage CTRL = 1.5V LEDx Current (20mA) (RISET = 14.7k) VLEDx = 1V, CTRL = 1.5V LEDx Current Matching (20mA) (RISET = 14.7k) VLEDx = 1V, CTRL = 1.5V LEDx Current (50mA) (RISET = 5.76k) VLEDx = 1V, CTRL = 0.04V 1.00 19.29 l 47.85 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 0.8 200 200 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 6 6 V LED Open Detection Threshold VOUT = 12V 0.5 V 0.3 0.6 mA 3754f 4 LT3754 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 LT3754E 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 0.8 LED Current vs Junction Temperature LED Current vs CTRL Pin Voltage 55 50 45 LED CURRENT (mA) 20.50 LED CURRENT (mA) LED CURRENT MATCHING (%) RISET = 14.7k 0.0 20.00 19.50 –0.4 nA TA = 25°C, unless otherwise noted. 21.00 0.4 200 LT3754I 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 16-channel Average) 40 40 35 RISET = 5.76k 7.32k 9.76k 14.7k 29.4k 30 25 20 15 10 RISET = 14.7k (I(LED) = 20mA) –0.8 0 25 100 –50 –25 50 75 JUNCTION TEMERATURE (°C) 5 125 3754 G01 19.00 –50 25 75 100 –25 0 50 JUNCTION TEMERATURE (°C) 125 3754 G02 0 0.00 0.25 0.50 0.75 1.00 CTRL (V) 1.25 1.50 3754 G03 3754f 5 LT3754 TYPICAL PERFORMANCE CHARACTERISTICS LED Current Waveforms 3000:1 PWM Dimming (100Hz) TA = 25°C, unless otherwise noted. SHDN/UVLO Threshold vs Junction Temperature VREF vs Junction Temperature 1.525 1.525 SHDN/UVLO PIN VOLTAGE (V) I(LEDx) 20mA/DIV 1.505 VREF VOLTAGE (V) 1.505 I(L) 1A/DIV 1.485 1.485 PWM 10V/DIV 1.465 1.465 5μs/DIV 3754 G04 1.445 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 1.445 –50 125 –25 0 25 50 75 100 JUNCTION TEMERATURE (°C) 3754 G05 SHDN/UVLO Pin (Hysteresis) Current vs Junction Temperature VIN Shutdown Current vs Junction Temperature 5 2.70 VIN Quiescent Current vs VIN 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) PWM = 1.5V, NO SWITCHING, V(LED1-16) = 1.2V, CTRL = 0.1V 8 6 4 PWM = 0V, CTRL = 0.1V 2 0 –50 125 RISET = 14.7k 10 4 VIN CURRENT (μA) SHDN/UVLO PIN CURRENT (μA) 2.80 0 25 50 75 100 –25 JUNCTION TEMPERATURE (°C) 3754 G07 0 125 0 5 10 15 35 40 VC High Clamp, Active and Low Clamp Levels vs Junction Temperature 2.5 1100 VC HIGH CLAMP 10 PWM = 1.5V, NO SWITCHING, V(LED1-16) = 1.2V, CTRL = 0.1V 5 PWM = 0V, CTRL = 0.1V 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3754 G10 2.0 1050 VC PIN VOLTAGE (V) SWITCHING FREQUENCY (kHz) VIN = 6V, RISET = 14.7k, CTRL = 0.1V VIN CURRENT (mA) 30 3754 G09 Switching Frequency vs Junction Temperature 15 0 –50 –25 20 25 VIN (V) 3754 G08 VIN Quiescent Current vs Junction Temperature 125 3754 G06 1000 RT = 39.2k 950 900 –50 –25 1.5 VC ACTIVE (SWITCHING) 1.0 VC LOW CLAMP 0.5 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3754 G11 0.0 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 125 3754 G12 3754f 6 LT3754 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) INTVCC (V) 5.5 5.00 ILOAD = 40mA ILOAD = 10mA ILOAD = 20mA ILOAD = 30mA ILOAD = 40mA 6.7 VIN = 8V, PWM = 0V 6.6 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 4.5 –50 125 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 5 4 INTVCC UVLO(+) 3 INTVCC UVLO(–) 2 –50 –25 125 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 3754 G14 3754 G13 INTVCC Current Limit vs Junction Temperature 3754 G15 SENSE Threshold vs Junction Temperature 60 125 Overvoltage Protection (OVP) Level vs Junction Temperature 60.0 70 57.5 60 55 50 45 55.0 50 50.0 INDUCTOR PEAK CURRENT THRESHOLD (CYCLE-BY-CYCLE) 47.5 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 30 OVPSET = 0.22V 10 40.0 –50 125 40 20 45.0 42.5 40 –50 –25 OVPSET = 1.0V 52.5 OVP (V) SENSE PIN VOLTAGE (mV) INTVCC CURRENT (mA) VIN = 6V, INTVCC = 0V –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 3754 G16 0 –50 125 3754 G17 VOUT − V(LEDx) Short Threshold vs Junction Temperature GATE Rise/Fall Times vs GATE Capacitance 120 250 VIN = 8V, INTVCC = 7V, RT = 523k CGATE = 3300pF VOUT = 12V MINIMUM ON-TIME 80 TIME (ns) 200 VOUT = 60V 6.00 5.75 100 225 6.50 TIME (ns) SHORT THRESHOLD (V) 6.75 125 3754 G18 Minimum ON and OFF Times vs Junction Temperature 7.00 6.25 0 25 50 75 100 –25 JUNCTION TEMPERATURE (°C) 175 MINIMUM OFF-TIME FALL TIME 60 150 40 125 20 RISE TIME 5.50 5.25 5.00 –50 0 25 50 75 100 –25 JUNCTION TEMPERATURE (°C) 125 3754 G18 100 –50 0 25 50 75 100 –25 JUNCTION TEMPERATURE (°C) 125 3754 G20 0 0 5 10 CL (nF) 15 20 3754 G21 3754f 7 LT3754 PIN FUNCTIONS LEDx (Pin 1-8,17-24): 16 LED Driver Outputs. Each output contains an open collector constant current sink. LED currents are programmable from 10mA to 50mA using a single resistor at the ISET pin. Channel matching is ±2% with an absolute current accuracy of ±3%. 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). SENSE (Pin 9): The Current Sense Input for the Control Loop. Connect this pin to the sense resistor in the source of the external power MOSFET. GATE (Pin 10): Drives the gate of an N-channel MOSFET from 0V to INTVCC. INTVCC (Pin 11): 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 GND. VIN (Pin 12): Input Supply Pin. Must be locally bypassed with a 1μF capacitor to ground. SHDN/UVLO (Pin 13): 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 ground. 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 14): Active low if any or all LED strings have an open fault or if any/all LED pins have been shorted to VOUT. If fault(s) removed, FAULT flag returns high. Fault status is only updated during PWM high state and latched during PWM low. SYNC (Pin 15): 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. RT (Pin 25): A resistor to ground 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 ground. 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. OVPSET (Pin 28): Programs maximum allowed VOUT regulation level if all LEDs are open circuit. CTRL (Pin 29): CTRL pin voltage below 1V controls maximum 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 with temperature dependent resistance. TSET (Pin 30): Programs LT3754 junction temperature breakpoint past which LED current will begin to derate. VREF (Pin 31): 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 ground. ISET (Pin 32): Resistor to Ground Programs LED pin current. See Table 6 in the Applications Information Section. Exposed Pad (Pin 33): GND. The ground for the IC and the converter. The package has an exposed pad (Pin 33) underneath the IC which is the best path for heat out of the package. Pin 33 should be soldered to a continuous copper ground plane under the device to reduce die temperature and increase the power capability of the LT3754. VOUT (Pin16): Boosted Output Voltage of the Converter. Connect a capacitor from this pin to ground. Connect the anode of each LED (string) to VOUT. 3754f 8 LT3754 BLOCK DIAGRAM 13 12 SHDN/UVLO 600k 1.476V 11 VIN + – INTVCC 7V(REGULATED) UVLO() = 3.8V, UVLO() = 3.4V R S VC – + GATE Q SYNC EN – + OSC SLOPE 10 15 RT 25 + – 1.485V + – 4.2V() 3.7V() 29 + – PWM 100mV OVER CURRENT HICCUP__MODE 6V SENSE VOUT LEDx 1-8, 17-24 EN FAULT SOFT START VREF EN 9 16 14 LED LOGIC 1V CTRL SS + + + – LED CURRENT CONTROL CHANNEL X 1.1V – + PWM OVERVOLTAGE AMP LED AMP – + VPTAT 30 TSET 32 ISET 33 EXPOSED PAD (GND) 26 VC 56R + – 31 52mV PEAK CURRENT EN INTVCC_UV VIN_UV SHDN_UV 27 + – REF 1.485V R 28 OVPSET 3754 BD Figure 1. LT3754 Block Diagram OPERATION The operation of the LT3754 is best understood by referring to the typical application circuit on the front page and the Block Diagram in Figure 1. The LT3754 drives 16 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 10mA to 50mA using a single resistor at the ISET pin. LED channels can be paralleled to achieve higher LED currents. For applications requiring less than 16 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 LT3754. It can be seen that two external supplies, VREF and INTVCC, are generated by the LT3754. 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 3754f 9 LT3754 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 LT3754 only allows MOSFET turn-on approximately every 2ms. This hiccup mode significantly reduces the power rating required for the MOSFET. The LT3754 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 LT3754 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 LT3754 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 LT3754 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 LT3754, when entering these faults, discharges an internal soft start node and prevents switching at the GATE pin. When exiting these faults the LT3754 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 LT3754 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 LT3754 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 LT3754 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. 3754f 10 LT3754 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 LT3754 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 44mA 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 LT3754 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 where Qg is the gate charge (at VGS = INTVCC) specified for the MOSFET and fosc is the switching frequency of the LT3754 boost converter. It is possible to drive the INTVCC pin from a variety of external sources in order to remove power dissipation from the LT3754 and/or to remove the INTVCC current limitation of 44mA. 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. 1 1 • •V VOUT fOSC IN VIN L= V 0.5 • OUT • ILEDx • 16 VIN 1− where: VOUT = (N • VF) + 1V (N = number of LEDs per string), VF = LED forward voltage drop, ILEDx = LED current per string Example: For a 12W LED driver application requiring 16 strings of 10 LEDs each driven with 20mA, and choosing VIN = 12V, VOUT = (3.75V • 10) + 1V = 38.5V, ILEDx = 20mA and fOSC = 1MHz the value for L is calculated as 1 1 )• • 12V 3.2 106 L= = 16.5μH 0.5 • 3.2 • 20mA • 16 (1− 3754f 11 LT3754 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 LT3754 boost converter will supply the transient input current of the power inductor. Values between 2.2μF and 10μF will work well for the LT3754. 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 LT3754 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 LT3754 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 ≤ 44mA 3754f 12 LT3754 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. Table 4. MOSFET Manufacturers MANUFACTURER PHONE NUMBER WEB Vishay Siliconix 402-563-6866 www.vishay.com International Rectifier 310-252-7105 www.irf.com Fairchild 972-910-8000 www.fairchildsemi.com Power MOSFET: Current Sense Resistor The LT3754 current mode boost converter controls peak current in the inductor by controlling peak MOSFET current in each switching cycle. The LT3754 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 ) where ⎛⎛ 1 ⎞ ⎞ ⎛ 0.5 ⎞ IL(PEAK ) = ⎜⎜ ⎟ • 16 • ILEDx ⎟ • ⎜1+ ⎟ 2 ⎠ ⎝⎝ 1− D ⎠ ⎠ ⎝ ⎛V ⎞ OUT(MAX ) ⎜ ⎟, D = MOS SFET duty cycle = ⎜ ⎟ V ⎝ IN(MIN) ⎠ ( ) VOUT(MAX) = N • VF(MAX ) + 1V N = number of LEDs in each string, VF(MAX ) = max imum LED forward voltage drop, VIN(MIN) = min imum input voltage to the inductor, 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 50mV sense threshold, ILEDx, RS and circuit efficiency. Example: For a 12W LED driver application requiring 16 strings of 10 LEDs each driven with 20mA, and choosing VIN(MIN) = 8V, VOUT(MAX) = (4V • 10)+1V = 41V and ILEDx = 20mA, the value for RS is chosen as: 52mV • 0.7 52mV • 0.7 ≤ ⎞ ⎛ 41 IL(PEAK ) ⎜ • 16 • 0.02⎟ • (1+ 0.25) ⎝8 ⎠ 52mV • 0.7 ≤ ≤ 17.7 mΩ 2.05 RS ≤ 3754f 13 LT3754 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 LT3754 has an overcurrent comparator which triggers soft start and turns off the MOSFET driver for currents exceeding, ID(OVERCURRENT ) = 100mV RS In this fault mode the LT3754 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 LT3754 provides a soft start function. The LT3754 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 INTV CC voltages too low or MOSFET current too high (see the timing diagram in Figure 2). When exiting these faults the LT3754 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 LT3754 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 r 4.2V, SHDN > 1.476V, INTVCC > 3.8V, IDSS (EXTERNAL MOSFET) < 100mV/RS, PWM > 1.4V (FOR AT LEAST 200ns) 3754 F02 Figure 2. LT3754 Fault Detection and Soft Start Timing for VC Pin and Internal SS Node 3754f 14 LT3754 APPLICATIONS INFORMATION the soft start of VC pin to begin. This feature ensures that during startup of the LT3754 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. 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 LT3754 SHDN/UVLO pin can be made as follows : Shutdown and Programming Undervoltage Lockout The switching frequency of the LT3754 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. The LT3754 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/UVLO ⎛ R1 ⎞ VSUPPLY OFF = 1.476 ⎜1+ ⎟ ⎝ R2 ⎠ 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 LT3754. Programming Switching Frequency 1000 900 R1 13 SHDN/UVLO SWITCHING FREQUENCY (kHz) VSUPPLY – 600k R2 OFF ON 1.476V + 800 700 600 500 400 300 200 100 3754 F03 Figure 3. Programming Undervoltage Lockout (UVLO) with Hysteresis 0 100 200 300 400 RT (kΩ) 500 600 3754 F04 Figure 4. Switching Frequency vs RT 3754f 15 LT3754 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 LT3754 LED driver for a given application, the duty cycle requirements should be considered and compared to the minimum/maximum achievable duty cycles for the LT3754 GATE pin. If required, the LT3754 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) 10 29.4 20 14.7 30 9.76 40 7.32 50 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 LT3754 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 LT3754 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 ) ≈ 295 ( ) A (CTRL > 1.1V ) RISET See Table 6 for resistor values and corresponding programmed LED. The typical values for LT3754 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. 3754f 16 LT3754 APPLICATIONS INFORMATION Analog Dimming The LT3754 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: I (LEDX ) ≈ CTRL • 295 (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. PWM Dimming 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 LT3754 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 turn LED currents on/off as quickly as possible. For PWM low, the LT3754 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. TPWM TON(PWM) (= 1/fPWM) PWM INDUCTOR CURRENT LED CURRENT MAX ILED 3754 F05 Figure 5. PWM Dimming Waveforms 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) (4) Lower inductor value improves PDR (5) Higher output capacitor value improves PDR (6) Choose the Schottky diode for the LT3754 boost converter for minimum reverse leakage current. See “LED Current vs PWM Duty Cycle” in the Typical Performance Characteristics section. 3754f 17 LT3754 APPLICATIONS INFORMATION Programming LED Current Derating (Breakpoint and Slope) versus LED Ambient Temperature (CTRL Pin) LED datasheets provide curves of maximum allowed LED current versus ambient temperature to warn against damaging of the LED (Figure 6). The LT3754 LED driver improves the utilization and reliability of the LED(s) by allowing 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. The LT3754 allows the temperature breakpoint and the slope of LED current versus ambient temperature to be programmed using a simple resistor network shown in Figure 7. Without the ability to back off LED current as the ambient temperature of the LED(s) increases, many LED drivers are limited to driving the LED(s) at only 50% or less of their maximum rated currents. This limitation requires more LEDs to obtain the intended brightness for the application. The LT3754 allows the LED(s) to be programmed for maximum allowable current while still protecting the LED(s) from excessive currents at high temperature. This is achieved by programming a voltage at the 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 behaviour versus temperature. The current derating curve in Figure 6 uses the resistor network shown in option C 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 nonlinear 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 - FORWARD CURRENT (mA) 250 31 225 REF R1 RESISTOR OPTION A 29 LT3754 CTRL 200 175 150 OPTION A TO D R2 LT3754 PROGRAMMED LED CURRENT DERATING CURVE RY RY 125 100 –50 RNTC –25 0 25 50 75 TA-TEMPERATURE (°C) 100 RNTC RX RNTC RNTC RX 125 3754 F06 A B C D 3754 F07 Figure 6. LED Current Derating vs LED Ambient Temperature Figure 7. Programming LED Current Derating Curve vs Ambient Temperature (RNTC Located on LED PCB) 3754f 18 LT3754 APPLICATIONS INFORMATION Using the TSET Pin for Thermal Protection 1.50 CTRL VOLTAGE (V) 1.25 1.00 RESISTOR OPTION A 0.75 0.50 0.25 0 10 20 30 40 50 60 70 TA - AMBIENT TEMPERATURE (°C) 80 3754 F08 Figure 8. Programmed CTRL Voltage vs Temperature 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’, users can log onto www.murata.com/designlib and download the software followed by instructions for creating an output voltage ‘VOUT’ (LT3754 CTRL pin voltage) from a specified VCC supply (LT3754 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. The LT3754 contains a special programmable thermal regulation loop that limits the internal junction temperature of the part. Since the LT3754 topology consists of a single boost controller with sixteen 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 LT3754 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 LT3754 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. While this feature is intended to directly protect the LT3754, it can also be used to derate the LED current at high temperatures. Since there is a direct relationship between the LED temperature and LT3754 junction temperature, the TSET function also provides some LED current derating at high temperatures. 3754f 19 LT3754 APPLICATIONS INFORMATION 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. 31 R2 If every LED string fails open or the multiple string LED display becomes disconnected the LT3754 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: TSET R1 3754 F09 Figure 9. Programming the TSET Pin 950 OVP(RECOMMENDED) = 1.2 • ((N • VF) + 1V) 900 VTSET THRESHOLD (mV) The LT3754 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 VREF LT3754 30 Programming Overvoltage Protection (OVP) level 850 where: 800 N = number of LEDs in each string, VPTAT 750 VF = maximum LED forward voltage drop 700 650 and the scaling factor of 1.2 accounts for variation in the generation of OVP from OVPSET pin voltage and startup logic requirements. 600 550 500 0 25 50 75 100 125 JUNCTION TEMPERATURE (°C) 150 3598 F10 Figure 10. Programing the TSET Pin Threshold Table 7. Resistor Values to Program Maximum IC Junction Temperature (VREF (Typical) = 1.485V) TJ (°C) R1 (k) R2 (k) TSET (V) 100 24.9 20 0.824 115 28.0 20 0.866 130 30.9 20 0.902 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 OV VP = 49.2V, OVPSET = 49.2 = 0.863V 57 The OVPSET pin voltage can be generated using a resistor divider from the REF pin. 3754f 20 LT3754 APPLICATIONS INFORMATION LED Open Circuit and PWM Dimming Ratios The LT3754 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 LT3754 ignores low LED pin voltages until VOUT reaches 90% of its maximum allowed OVP level. Once this condition is met, the LT3754 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 LT3754 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 datasheet 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 datasheet 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 the 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. Thermal Considerations The internal power dissipation of the LT3754 comes from 3 main sources: VIN quiescent current (IQ total), VIN current for GATE switching (IGATE) and the LT3754 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 LT3754. A low Qg MOSFET should always be used when operating the LT3754 from high VIN voltages. The internal junction temperature of the LT3754 can be estimated as: TJ = TA + [VIN • (IQTOTAL + (fOSC • Qg)) + (16 • I(LEDX) • 1.1V)] • θJA where, TA is the ambient temperature for the LT3754 IQTOTAL represents the VIN quiescent current for the LT3754 (not switching, PWM = 1.5V and CTRL = 0.1V) - illustrated in the Typical Characteristics Graphs – plus the base currents of active channels (typically 16 • I(LED)/75). θJA is the thermal resistance of the package (34°C/W for the 5mm × 5mm QFN package). 3754f 21 LT3754 APPLICATIONS INFORMATION Example : For a 12W LED driver application requiring 16 strings of 10 LEDs each driven with 20mA, VIN = 24V, fOSC = 1MHz, Qg (at 7V VGS) = 15nC, I(LEDX) = 20mA, and an 85°C ambient temperature for the LT3754 IC, the LT3754 junction temperature can be approximated as: TJ = 85°C + [24 • (9.5mA + (16 • 20mA/75) + (1MHz • 15nC)) + (16 •20mA • 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 LT3754. 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 LT3754 is the only ground connection for the IC. The exposed pad should be soldered to a continuous copper ground plane underneath the device to reduce die temperature and maximize the power capability of the IC. An analog ground 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 near these pins. 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 near the GATE pin. This area of copper 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 analog ground. 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. 3754f 22 40.2k 499k VIN 30.9k 20k 39.2k OVPSET RT TSET 5.76k ISET LT3754 VIN L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS360 20k 11k REF CTRL PWM SYNC GND SHDN/UVLO FAULT INTVCC 4.7μF 25V 4.7μF 50V 100k 4.7μF 10V ANALOG DIMMING PWM DIMMING VIN 10V PVIN 24V VC GATE 2.2nF 10k LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 LED16 VOUT SENSE L1 10μH 0.015Ω M1 D1 5 s 2.2μF 100V • • • • • • • • • • • • • • • • • • • • • PWM 10V/DIV I(L) 1A/DIV 5μs/DIV LED Current Waveforms 3000:1 PWM Dimming (100Hz) I(LEDx) 20mA/DIV • • • 3754 G04 • • • • • • UP TO 45V OF LEDs PER STRING 92% Efficient, 36W LED Driver, 1MHz Boost, 16 Strings, 50mA Per String • • • • • • • • • • • • • • • 3754 TA03 • • • LT3754 TYPICAL APPLICATIONS 3754f 23 24 110k 1M VIN 10V TO 14V 30.9k 20k 20k 19.1k 100k 39.2k OVPSET RT TSET REF PWM SYNC GND CTRL SHDN/UVLO FAULT 9.76k ISET LT3754 VIN L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7308DN D1: DIODES, INC. DFLS160 232k 1M 4.7μF 10V INTVCC 4.7μF 25V VC GATE 2.2nF 10k LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 LED16 VOUT SENSE L1 10μH 0.02Ω M1 D1 4 s 2.2μF 50V • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • UP TO 36V OF LEDs PER STRING 17W LED Driver, 1MHz Boost, 16 Strings, 1MHz Per String • • • • • • • • • • • • • • • 3754 TA04 • • • LT3754 TYPICAL APPLICATIONS 3754f LT3754 TYPICAL APPLICATIONS 31W LED Driver, 400kHz Boost, 3 Strings, 250mA Per String L1 10μH VIN 8V TO 36V D1 10 s 2.2μF 100V 4.7μF 50V VIN 4.7μF 10V INTVCC GATE M1 SENSE 0.007Ω 100k 1M UP TO 42V OF LEDs PER STRING LT3754 FAULT • • • • • • • • • VOUT SHDN/UVLO LED16 232k GND LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 SYNC PWM PWM DIMMING ANALOG DIMMING CTRL REF 20k TSET 15k OVPSET RT 30.9k 23.2k 115k ISET VC 5.76k 3754 TA06 5.1k 4.7nF L1: COOPER BUSSMANN HC9-100-R M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS560 3754f 25 LT3754 TYPICAL APPLICATIONS 14W LED Driver, 700kHz Boost, 4 Strings, 80mA Per String L1 15μH 12VIN 10V TO 14V D1 UP TO 45V OF LEDs PER STRING 4.7μF 25V VIN 4.7μF 10V INTVCC GATE M1 SENSE VIN 0.02Ω 1M LT3754 100k FAULT GND SYNC PWM PWM DIMMING ANALOG DIMMING CTRL REF 20k TSET 11k OVPSET RT 20k 60.4k • • • • • • • • • • • • VOUT SHDN/UVLO 30.9k 5 s 2.2μF 100V ISET 14.7k VC LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 LED16 3754 TA06 7.5k 4.7nF L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7308DN D1: DIODES, INC. DFLS160 3754f 26 LT3754 PACKAGE DESCRIPTION DH Package 32-Lead Plastic QFN (5mm × 5mm) (Reference LTC DWG # 05-08-1693) 0.70 p0.05 5.50 p0.05 4.10 p0.05 3.45 p 0.05 3.50 REF (4 SIDES) 3.45 p 0.05 PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD 0.75 p 0.05 R = 0.05 TYP 0.00 – 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 s 45° CHAMFER R = 0.115 TYP 31 32 0.40 p 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 3.50 REF (4-SIDES) 3.45 p 0.10 3.45 p 0.10 (UH32) QFN 0406 REV D 0.200 REF NOTE: 1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE M0-220 VARIATION WHHD-(X) (TO BE APPROVED) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 0.25 p 0.05 0.50 BSC 3754f 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. 27 LT3754 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3755/LT3755-1 High Side 40V, 1MHz LED Controller with True Color 3,000:1 PWM Dimming VIN(MIN) = 4.5V, VIN(MAX) = 40V, VOUT(MAX) = 60V, 3,000:1 True Color PWM Dimming, ISD = <1μA, 3mm × 3mm QFN-16 MSOP-16E LT3756/LT3756-1 High Side 100V, 1MHz LED Controller with VIN(MIN) = 6.0V, VIN(MAX) = 100V, VOUT(MAX) = 100V, 3,000:1 True Color PWM Dimming, ISD = <1μA, 3mm × 3mm QFN-16 MSOP-16E True Color 3,000:1 PWM Dimming LT3598 44V, 1.5A, 2.5MHz Boost 6-Channel 20mA VIN(MIN) = 3V, VIN(MAX) = 30V(40VMAX), VOUT(MAX) = 44V, 1,000:1 True Color PWM Dimming, ISD = <1μA, 4mm × 4mm QFN-24 LED Driver LT3599 44V, 2A, 2.5MHz Boost 4-Channel 100mA LED Driver VIN(MIN) = 3V, VIN(MAX) = 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(MIN) = 4.5V, VIN(MAX) = 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(MIN) = 3.0V, VIN(MAX) = 36V, VOUT(MAX) = 40V, 3,000:1 True Color PWM Dimming, ISD = <20μA, 4mm × 5mm DFN-16 TSSOP-16E LT3517 1.3A, 2.5MHz High Current LED Driver with 3,000:1 Dimming VIN(MIN) = 3.0V, VIN(MAX) = 30V, VOUT(MAX) = 45, 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(MIN) = 3.0V, VIN(MAX) = 30V, VOUT(MAX) = 45, 3,000:1 True Color PWM Dimming, ISD = <1μA, 4mm × 4mm QFN-16 LT3486 Dual 1.3A, 2MHz High Current LED Driver VIN(MIN) = 2.5V, VIN(MAX) = 24V, 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 LT3496 VIN(MIN) = 2.8V, VIN(MAX) = 36V, VOUT(MAX) = 40V, 1,000:1 True Color PWM Dimming, ISD = <10μA, 5mm × 7mm QFN-10 Triple Output 750mA, 2.1 MHz High VIN(MIN) = 3.0V, VIN(MAX) = 30V, VOUT(MAX) = 40V, 3,000:1 True Color PWM Dimming, Current LED Driver with 3,000:1 Dimming ISD = <1μA, 4mm × 5mm QFN-28 LT3474/LT3474-1 36V, 1A (ILED), 2MHz, Step-Down LED Driver VIN(MIN) = 4.0V, VIN(MAX) = 36V, VOUT(MAX) = 13.5V, 400:1 True Color PWM Dimming, ISD = <1μA, TSSOP16E LT3475/LT3475-1 Dual 1.5A(ILED), 36V, 2MHz, Step-Down LED Driver VIN(MIN) = 4.0V, VIN(MAX) = 36V, VOUT(MAX) = 13.5V, 3,000:1 True Color PWM Dimming, ISD = <1μA, TSSOP20E LT3476 VIN(MIN) = 2.8V, VIN(MAX) = 16V, VOUT(MAX) = 36V, 1,000:1 True Color PWM Dimming, ISD = <10μA, 5mm × 7mm QFN-10 Quad Output 1.5A, 2MHz High Current LED Driver with 1,000:1 Dimming 3754f 28 Linear Technology Corporation LT 0809 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009