LT3478/LT3478-1 4.5A Monolithic Drivers with True Color Dimming DESCRI
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LT3478/LT3478-1 4.5A Monolithic Drivers with True Color Dimming DESCRIPTIO
LT®3478/LT3478-1 4.5A step-up DC/DC converters designed drive LEDs with constant current over wide programmable range. Series connection LEDs provides identical currents uniform brightness without need ballast resistors expensive factory calibration. LT3478-1 reduces external component count cost integrating current sense resistor. LT3478 uses external sense resistor extend maximum programmable current beyond also achieve greater accuracy when programming currents. Operating frequency with external resistor from 200kHz 2.25MHz. Unique circuitry allows dimming range 3000:1 while maintaining constant color. LT3478/LT3478-1 ideal high power driver applications such automotive backlights, courtesy lighting heads-up displays. CTRL pins used program maximum current. other CTRL used program reduction maximum current temperature maximize usage improve reliability. Additional features include inrush current protection, programmable open protection programmable soft-start. Each part available 16-pin thermally enhanced TSSOP Package.
True Color PWMDimming Delivers Constant Color with 3000:1 Range Wide Input Voltage Range: 2.8V 4.5A, 60m, Internal Switch Drives LEDs Boost, Buck-Boost Buck Modes Integrated Resistors Inductor Current Sensing Program Current: 100mA 1050mA (LT3478-1) (10mV 105mV)/RSENSE (LT3478) Program Current De-Rating Temperature Separate Inductor Supply Input Inrush Current Protection Programmable Soft-Start Fixed Frequency Operation from 200kHz 2.25MHz Open Protection (Programmable OVP) Accurate Shutdown/UVLO Threshold with Programmable Hysteresis 16-Pin Thermally Enhanced TSSOP Package
APPLICATIO
High Power Driver Automotive Lighting
Lare registered trademarks Linear Technology Corporation. other trademarks property their respective owners. Patents Pending.
TYPICAL APPLICATIO
4.7µF SHDN VREF 45.3k CTRL2 OVPSET 54.9k CTRL1 130k DIMMING CONTROL
Automotive Backlight
10µH RSENSE (LT3478) 10µF EFFICIENCY
Efficiency
ILED 700mA fOSC 500kHz DUTY CYCLE 100%
LT3478-1
700mA LEDs (WHITE) 69.8k
0.1µF
3478 TA01
LEDs LUXEON (WHITE)
3478 TA01b
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LT3478/LT3478-1 ABSOLUTE
(Note
RATI
PACKAGE/ORDER ATIO
VIEW VOUT OVPSET CTRL2 CTRL1 SHDN VREF
.42V VOUT, .42V VIN, SHDN (Note .36V .15V CTRL1, VREF, OVPSET.2V Operating Junction Temperature Range (Notes -40°C 125°C Storage Temperature Range. -65°C 150°C Lead Temperature (Soldering, Sec) 300°C
PACKAGE 16-LEAD PLASTIC TSSOP TJMAX 125°C, 35°C/W EXPOSED (PIN PGND, MUST SOLDERED PCB.
ORDER PART NUMBER LT3478EFE LT3478EFE-1 LT3478IFE LT3478IFE-1
PART MARKING 3478FE 3478FE-1 3478FE 3478FE-1
Order Options Tape Reel: Lead Free: #PBF Lead Free Tape Reel: #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult Marketing parts specified with wider operating temperature ranges.
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. open, VOUT SHDN 2.7V, open, open, CTRL1, CTRL2 1.25V, VREF open, open, 31.6k.
PARAMETER Minimum Operating Voltage Operational Input Voltage Quiescent Current Shutdown Current SHDN Threshold (VSD_µp) SHDN Threshold (VSD_UVLO) SHDN Current VREF Voltage VREF Line Regulation VREF Load Regulation Frequency: fOSC 200kHz Frequency: fOSC 1MHz CONDITIONS (Rising) (Note Switching) SHDN (Micropower) (Switching) SHDN VSD_UVLO 50mV SHDN VSD_UVLO 50mV I(VREF) 0µA, I(VREF) 0µA, 2.7V I(VREF) 100µA (Max) 200k 31.6k
ELECTRICAL CHARACTERISTICS
UNITS
1.213 1.240 0.005 0.18 0.88 1.263 0.015 0.22 1.12
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LT3478/LT3478-1
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. open, VOUT SHDN 2.7V, open, open, CTRL1, CTRL2 1.25V, VREF open, open, 31.6k.
PARAMETER Frequency: fOSC 2.25MHz Line Regulation fOSC Nominal Voltage Maximum Duty Cycle 31.6k 200k 9.09k (Note (Note CTRL1 0.4V, CTRL1 CTRL1 0.4V CTRL1 2.7V 4.5A 42V, OVPSET OVPSET 0.3V CTRL1 VREF, Current CTRL1 700mV, Current CTRL1 350mV, Current CTRL1 100mV, Current CTRL1 700mV, VSENSE VVOUT VLED CTRL1 350mV, VSENSE VVOUT VLED CTRL1 100mV, VSENSE VVOUT VLED CTRL1 100mV, CTRL2 1.25V CTRL2 100mV, CTRL1 1.25V (Current Pin) OVPSET VOUT (Current Pin) I(SS) 20µA Current Pin, 0.5V,
ELECTRICAL CHARACTERISTICS
CONDITIONS 9.09k
2.25 0.05 0.64
UNITS µA/A
31.6k, 2.7V
0.65
Current Current Gain Current Voltage Gain Switch Current Gain Source Current (Out Pin) Sink Current Switching Threshold High Level (VOH) Level (VOL) Inductor Current Limit Switch Current Limit Switch Switch Leakage Current VOUT Overvoltage Protection (OVP) (Rising) Full Scale Current (LT3478-1) 700mA Current (LT3478-1) 350mA Current (LT3478-1) 100mA Current (LT3478-1) CTRL1 700mV, VSENSE (LT3478) CTRL1 350mV, VSENSE (LT3478) CTRL1 100mV, VSENSE (LT3478) CTRL1, Input Currents OVPSET Input Current Switching Threshold Current Mode Current Mode Level (VOL) Reset Threshold High Level (VOH) Soft-Start (SS) Charge Current Soft-Start (SS) Discharge Current
12.3
1010
1050 70.5 35.5 0.15 0.25
1090
Full Scale Current VSENSE (LT3478) CTRL1 VREF, VSENSE VVOUT VLED
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LT3478/LT3478-1 ELECTRICAL CHARACTERISTICS
Note Stresses beyond those listed under Absolute Maximum Ratings cause permanent damage device. Exposure Absolute Maximum Rating condition extended periods affect device reliability lifetime. Note LT3478EFE/LT3478EFE-1 guaranteed meet performance specifications from 125°C junction temperature. Specifications over -40°C 125°C operating junction temperature range assured design, characterization correlation with statistical process controls. LT3478IFE/LT3478IFE-1 guaranteed over full -40°C 125°C operating junction temperature range. Note This includes over-temperature protection that intended protect device during momentary overload conditions. Junction temperature will exceed 125°C when over-temperature protection active. Continuous operation above specified maximum operating junction temperature impair device reliability. Note maximum operating ambient temperature, "Thermal Calculations" section Applications Information section. Note maximum operational voltage limited thermal efficiency considerations. Power switch base current delivered from should therefore driven from lowest available power supply system. "Thermal Calculations" Applications Information section. Note LT3478, parameter scales (RSENSE/0.1).
TYPICAL PERFOR CHARACTERISTICS
Current CTRL1
1400 25°C CTRL2 VREF (FOR LT3478 SCALE 0.1/RSENSE) CURRENT (mA) 1400
CURRENT (mA)
LT3478-1
LT3478-1
CURRENT (mA)
1050
VREF 0.35 0.70 CTRL1 1.05 1.40
3478
CTRL1 Current Temperature
CTRL1 CURRENT (-1) (nA) CTRL1 0.1V SWITCH (SAT) (mV)
CURRENT LIMIT
CTRL1 0.35V CTRL2 VREF CTRL1 CTRL2 PINS INTERCHANGEABLE
CTRL1 0.7V
CTRL1 0.9V JUNCTION TEMPERATURE (°C)
Current Temperature
(FOR LT3478 SCALE 0.1/RSENSE) ILED 1050mA, CTRL1 CTRL2 VREF 1050 1000
Current Duty Cycle Wide Dimming Range (3000:1)
25°C LEDS 500mA FREQ 100Hz CTRL1 0.5V CTRL2 VREF FOSC 1.6MHz 2.2µH
ILED 100mA, CTRL1 100mV, CTRL2 VREF JUNCTION TEMPERATURE (°C)
0.01
DUTY CYCLE
3478
3478
Switch (SAT) Switch Current
25°C
Switch Inductor Peak Current Limits Temperature
SWITCH INDUCTOR
SWITCH CURRENT
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JUNCTION TEMPERATURE (°C)
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LT3478/LT3478-1 TYPICAL PERFOR CHARACTERISTICS
VREF Temperature
1.28 1.60
1.50 SHDN
SHDN CURRENT (µA)
1.26
VREF
1.24
1.22
1.20
1.18 JUNCTION TEMPERATURE (°C)
Shutdown Current Temperature
SHDN CURRENT (mA)
2.8V JUNCTION TEMPERATURE (°C)
CURRENT (mA)
CURRENT (µA)
Shutdown Currents Temperature
SWITCH PEAK CURRENT LIMIT SHDN
CURRENT (µA)
I(VS PIN) PIN)
JUNCTION TEMPERATURE (°C)
SHDN Threshold Temperature
SHDN (Hysteresis) Current Temperature
JUST BEFORE PART TURNS
1.40
1.30
1.20 JUNCTION TEMPERATURE (°C)
AFTER PART TURNS JUNCTION TEMPERATURE (°C)
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3478
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Quiescent Current
Quiescent Current Temperature
25°C
3478
2.8V JUNCTION TEMPERATURE (°C)
3478
3478
Switch Peak Current Limit Duty Cycle
I(SW PIN)
25°C DUTY CYCLE
3478
3478
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LT3478/LT3478-1 TYPICAL PERFOR CHARACTERISTICS
Switching Frequency
10000 25°C SWITCHING FREQUENCY (MHz) 1.20 1.15 SWITCHING FREQUENCY (kHz) 1.10 1.05 1.00 0.95 0.90 0.85
3478
VOUT CLAMP
1000
Charge Current Temperature
CURRENT (µA) (OUT PIN) JUNCTION TEMPERATURE (°C)
Switching Frequency Temperature
31.6k 43.0 42.5 42.0 41.5 41.0 40.5 40.0 39.5
Open-Circuit Output Clamp Voltage Temperature
OVPSET
1000
0.80 JUNCTION TEMPERATURE (°C)
39.0 JUNCTION TEMPERATURE (°C)
3478
3478
Active Clamp Voltages Temperature
CLAMP
ACTIVE THRESHOLD
JUNCTION TEMPERATURE (°C)
3478
3478
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LT3478/LT3478-1 CTIO
(Pins Switch Pin. Collector internal power switch. Both pins fused together inside Connect inductor diode here minimize metal trace area connected this minimize EMI. (Pin Input Supply. Must locally bypassed with capacitor ground. (Pin Inductor Supply. Must locally bypassed with capacitor ground. shorted only supply available (see (Pin function). (Pin Inductor Pin. internal resistor between pins monitors inductor current protect against inrush current. Exceeding immediately turns internal power switch discharges soft-start pin. Input current monitoring disabled connecting inductor power supply directly leaving open (requires local bypass capacitor pin; pin). VOUT (Pin Output voltage converter. Connect capacitor from this ground. Internal circuitry monitors VOUT protection against open faults. (Pin Connect string from this ground. internal (LT3478-1)/external (LT3478) resistor between VOUT pins senses current accurate control. OVPSET (Pin Programs VOUT overvoltage protection level (OVP) protect against open faults. (OVPSET 41)V. OVPSET range 0.3V range typically 12.3V 41V. (Pin Output transconductance error amplifier compensation converter regulation loop. VREF (Pin 10): Bandgap Voltage Reference. This supply 100µA. used program CTRL1, CTRL2, OVPSET voltages using resistor dividers ground. SHDN (Pin 11): SHDN accurate 1.4V threshold used program undervoltage lockout (UVLO) threshold system input supply using resistor divider from supply ground. 10µA current hysteresis allows programming undervoltage lockout (UVLO) hysteresis. SHDN above 1.4V turns part removes 10µA sink current from pin. SHDN reduces current 3µA. SHDN directly connected VIN. left open circuit part will turned off. CTRL1 (Pin 12): CTRL1 voltage used program maximum current (CTRL2 VREF). CTRL1 voltage resistor divider from VREF external voltage source. Maximum current given (LT3478-1) Current Min(CTRL1, 1.05) Amps (LT3478) Current Min(CTRL, 1.05) Amps RSENSE (linear 0.1V CTRL1< 0.95V CTRL2 VREF) maximum current, short CTRL1 CTRL2 pins VREF. CTRL2 (Pin 13): CTRL2 available programming decrease current versus temperature (setting temperature breakpoint slope). This feature allows output LED(s) programmed maximum allowable current without damage higher temperatures. This maximizes usage increases reliability. CTRL2 voltage with negative temperature coefficient created using external resistor divider from VREF with temperature dependant resistance. used, CTRL2 should tied VREF. (Pin 14): Input dimming control. Above allows converter switching below disables switching with level maintained. With external MOSFET placed series with ground side string, signal driving MOSFET gate provides accurate dimming control. signal driven from 15V. unused, should connected VREF. (Pin 15): resistor ground programs switching frequency between 200kHz 2.25MHz. (Pin 16): Soft-Start Pin. Placing capacitor here programs soft-start timing limit inductor inrush current during start-up converter. When inductor current
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LT3478/LT3478-1 CTIO
exceeds VOUT exceeds OVP, internal soft-start latch set, power immediately turned discharged. soft-start latch also and/or SHDN meet their turn thresholds. only recharges when faults removed been discharged below 0.25V. Exposed (Pin 17): ground converter. package Exposed underneath which best path heat package. should soldered continuous copper ground plane under device reduce temperature increase power capability LT3478/LT3478-1.
BLOCK DIAGRA
SHDN 10µA
1.4V
1.24V
VREF
CTRL1 CTRL2
1.05V
1000
OVERVOLTAGE DETECT CIRCUIT OVPSET EXPOSED (GND)
3478
Figure
OVERVOLTAGE DETECT
VOUT
9.5m
OVPSET
57mV
UVLO
INRUSH CURRENT PROTECTION
SOFT-START
RSENSE (INTERNAL LT3478-1)
RSENSE (EXTERNAL LT3478)
DETECT
SLOPE COMP
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LT3478/LT3478-1 OPERATIO
LT3478/LT3478-1 high powered drivers with 42V, 4.5A internal switch ability drive LEDs with 1050mA LT3478-1 105mV/RSENSE LT3478. LT3478/LT3478-1 work similarly conventional current mode boost converter current (instead output voltage) feedback control loop. Block Diagram Figure shows major functions LT3478/LT3478-1. part turn must exceed 2.8V SHDN must exceed 1.4V. SHDN threshold allows programming undervoltage lockout (UVLO) threshold system input supply using simple resistor divider. 10µA current flows into SHDN before part turn removed after part turn This current hysteresis allows programming hysteresis UVLO threshold. "Shutdown Programming Undervoltage Lockout" Applications Information Section. micropower shutdown SHDN reduces supply current approximately 3µA. Each driver current mode step-up switching regulator. regulation point achieved when boosted output voltage VOUT across output LED(s) high enough create current LED(s) equal programmed current. sense resistor connected series with LED(s) provides feedback current converter loop. basic loop uses pulse from internal oscillator flip-flop turn internal power switch connected between switch pin, ground. Current increases external inductor until switch current limit exceeded until oscillator reaches maximum duty cycle. switch then turned off, causing inductor current lift turn external Schottky diode connected output. Inductor current flows Schottky diode charging output capacitor. switch turned back next reset cycle internal oscillator. During normal operation
voltage controls peak switch current limit hence inductor current available output LED(s). with current mode converters, slope compensation added control path ensure stability. CTRL1 used program maximum current CTRL2 used program decrease current versus temperature maximum reliability utilization LED(s). CTRL2 voltage with negative temperature coefficient created using external resistor divider from VREF with temperature dependant resistance. Unused CTRL2 tied VREF. True Color dimming, LT3478/LT3478-1 provide 3000:1 wide dimming range allowing duty cycle (connected external N-channel MOSFET series with LED(s)) reduced from 100% 0.033% frequency 100Hz. Dimming duty cycle, allows constant color maintained over entire dimming range. robust operation, LT3478/LT3478-1 monitor system performance following faults SHDN voltages and/or inductor current high and/or boosted output voltage high. detection these faults, LT3478/LT3478-1 stop switching immediately soft-start latch discharging (see Timing Diagram Figure 11). faults detected internally require external components. When faults longer exist, internal 12µA supply charges with timing programmed using single external capacitor. gradual ramp voltage limits switch current during startup. optimum component sizing, duty cycle range efficiency LT3478/LT3478-1 allow separate inductor supply switching frequency programmed from 200kHz 2.25MHz using resistor from ground. advantages these options covered Applications Informations section.
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LT3478/LT3478-1 APPLICATIO ATIO
Inductor Selection Several inductors that work well with LT3478/LT3478-1 listed Table However, there many other manufacturers inductors that used. Consult each manufacturer more detailed information their entire range parts. Ferrite cores should used obtain best efficiency. Choose inductor that handle necessary peak current without saturating. Also ensure that inductor (copper-wire resistance) minimize power losses. Values between 4.7µH 22µH will suffice most applications. Inductor manufacturers specify maximum current rating current where inductance falls given percentage nominal value. inductor pass current greater than rated value without damaging Aggressive designs where board space precious will exceed maximum current rating inductor save space. Consult each manufacturer determine maximum inductor current measured much more current inductor reliably conduct.
Table Suggested Inductors
MANUFACTURER PART NUMBER CDRH104R-100NC CDRH103RNP-4R7NC-B CDRH124R-100MC CDRH104R-5R2NC FDV0630-4R7M UP4B-220 INDUCTANCE (µH) (mm) 10.5 10.3 10.5 10.3 12.3 12.3 10.5 10.3 MANUFACTURER Sumida www.sumida.com Toko www.toko.com Cooper www.cooperet.com
Table Ceramic Capacitor Manufacturers
MANUFACTURER Taiyo Yuden Murata PHONE NUMBER (408) 573-4150 (803) 448-9411 (714) 852-2001 www.t-yuden.com www.avxcorp.com www.murata.com
Table Suggested Diodes
MANUFACTURER PART NUMBER UPS340 B520C B530C B340A B540C PDS560 CURRENT REVERSE VOLTAGE Microsemi www.microsemi.com Diodes, Inc. www.diodes.com
Capacitor Selection (equivalent series resistance) ceramic capacitors should used output minimize output ripple voltage. only dielectrics, these materials retain their capacitance over wider voltage temperature ranges than other dielectrics. 4.7µF 10µF output capacitor sufficient most high output current designs. Some suggested manufacturers listed Table Diode Selection Schottky diodes, with their forward voltage drop fast switching speed, ideal LT3478/LT3478-1 applications. Table lists several Schottky diodes that work well. diode's average current rating must exceed application's average output current. diode's maximum reverse voltage must exceed application's output voltage. 4.5A diode sufficient most designs. dimming applications, aware reverse leakage current diode. Lower leakage current will drain output capacitor less, allowing higher dimming range. companies below offer Schottky diodes with high voltage current ratings.
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LT3478/LT3478-1 APPLICATIO ATIO
Shutdown Programming Undervoltage Lockout LT3478/LT3478-1 have accurate 1.4V shutdown threshold SHDN pin. This threshold used conjunction with resistor divider from system input supply define accurate undervoltage lockout (UVLO) threshold system (Figure SHDN current hysteresis allows programming hysteresis voltage this UVLO threshold. Just before part turn 10µA flows into SHDN pin. After part turn flows from SHDN pin. Calculation on/off thresholds system input supply using LT3478/LT3478-1 SHDN made follows: VSUPPLY R1/R2)] VSUPPLY VSUPPLY (10µA open drain transistor added resistor divider network SHDN independently control turn LT3478/LT3478-1.
VSUPPLY SHDN
1.4V
SWITCHING FREQUENCY (kHz)
10µA
Figure Programming Undervoltage Lockout (UVLO) with Hysteresis
With SHDN connected directly pin, internal undervoltage lockout threshold exists (2.8V max). This prevents converter from operating erratic mode when supply voltage low. LT3478/LT3478-1 provide soft-start function when recovering from such faults SHDN <1.4V and/or <2.8V. details Applications Information section "Soft-Start".
Programming Switching Frequency switching frequency programmed using external resistor (RT) connected between ground. internal free-running oscillator programmable between 200kHz 2.25MHz. Table shows typical values required range switching frequencies. Selecting optimum switching frequency depends several factors. Inductor size reduced with higher frequency efficiency drops higher switching losses. addition, some applications require very high duty cycles drive large number LEDs from supply. switching frequency allows greater operational duty cycle hence greater number LEDs driven. each case switching frequency tailored provide optimum solution. When programming switching frequency total power losses within should considered. "Thermal Calculations" Applications Information section.
10000 25°C 1000
3478
3478
1000
Figure Switching Frequency Resistor Value Table Switching Frequencies Values
SWITCHING FREQUENCY (MHz) 2.25 9.09 31.6
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LT3478/LT3478-1 APPLICATIO ATIO
Programming Maximum current Maximum current programmed using CTRL1 with CTRL2 tied VREF (see Figures maximum allowed current defined (LT3478-1) Current Min(CTRL1, 1.05) Amps (LT3478) Current Min(CTRL1, 1.05) Amps RSENSE current CTRL1 linear approximately 0.1V CTRL1 0.95V maximum possible current, connect CTRL1 CTRL2 VREF pin.
1400 25°C CTRL2 VREF (FOR LT3478 SCALE 0.1/RSENSE) FORWARD CURRENT (mA) AMBIENT TEMPERATURE (°C) EXAMPLE LT3478-1 PROGRAMMED CURRENT DERATING CURVE LUXEON EMITTER CURRENT DERATING CURVE
1050 CURRENT (mA)
LT3478-1
VREF 0.35 0.70 CTRL1 1.05 1.40
3478
Figure Current CTRL1 Voltage
LT3478/LT3478-1 VREF CTRL2 CTRL1
3478
(LT3478) RSENSE
VOUT
Figure Programming Current
Programming Current Derating Temperature useful feature LT3478/LT3478-1 ability program derating curve maximum current versus temperature. data sheets provide curves
maximum allowed current versus temperature warn against exceeding this current limit damaging (Figure
Luxeon (Maximum) LT3478-1 (Programmed) Current Derating Curves Temperature
LUXEON EMITTER (GREEN, CYAN, BLUE, ROYAL BLUE) 20°C/W
3478
Figure Current Derating Curve Ambient Temperature
Without ability back current temperature increases, many drivers limited driving LED(s) only less their maximum rated currents. This limitation requires more LEDs obtain intended brightness application. LT3478/LT3478-1 allow output LED(s) programmed maximum allowable current while still protecting LED(s) from excessive currents high temperature. This achieved programming voltage CTRL2 with negative temperature coefficient using resistor divider with temperature dependent resistance (Figures CTRL2 voltage programmed higher than CTRL1 voltage. This allows initial current defined CTRL1. temperature increases, CTRL2 voltage will fall below CTRL1 voltage causing currents controlled CTRL2 voltage. choice resistor ratios temperature dependent resistance divider CTRL2 will define current curve breakpoint slope versus temperature (Figure variety resistor networks resistors with different temperature coefficients used programming
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LT3478/LT3478-1 APPLICATIO ATIO
CTRL2 achieve desired CTRL2 curve temperature. current derating curve shown Figure uses resistor network shown option Figure
VREF LT3478/LT3478-1 CTRL2 CTRL1
OPTION
RNTC
RNTC
RNTC
RNTC
3478
Figure Programming Current Derating Curve Temperature (RNTC Located LEDs PCB)
1100 CTRL1, CTRL2 VOLTAGES (mV) 1000 CURRENT MINIMUM CTRL1, CTRL2 OPTION AMBIENT TEMPERATURE (°C) CTRL2 CTRL1
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Figure CTRL1, Programmed Voltages Temperature
Table shows list manufacturers/distributors resistors. There several other manufacturers available chosen supplier should contacted more detailed information. resistor indicate temperature only effective resistor connected close possible LED(s). derating curves shown manufacturers listed ambient temperature. resistor should submitted same ambient temperature LED(s). Since temperature dependency resistor nonlinear over wide range temperatures important
obtain resistor's exact values over temperature from manufacturer. Hand calculations CTRL2 voltage then performed each given temperature resulting CTRL2 curve plotted versus temperature. Several iterations resistor value calculations required achieve desired breakpoint slope current derating curve.
Table Resistor Manufacturers/Distributors
MANUFACTURER Murata Electronics North America Corporation Digi-key
www.murata.com www.tdk.com www.digikey.com
calculation CTRL2 voltage various temperatures gives downward slope that strong, alternative resistor networks chosen Figure which temperature independent resistance reduce effects resistor over temperature. Murata Electronics provides selection resistors with complete data over wide range temperatures. addition, software tool available which allows user select from different resistor networks resistor values then simulate exact output voltage curve (CTRL2 behavior) over temperature. Referred `Murata Chip Thermistor Output Voltage Simulator', users onto www.murata.com/designlib download software followed instructions creating output voltage VOUT (CTRL2) from specified supply (VREF). time during selection circuit parameters user access data chosen resistor clicking link Murata catalog. following example uses hand calculations derive resistor values required CTRL1 CTRL2 voltages achieve given current derating curve. resistor values obtained using Murata simulation tool also provided were used create derating curve shown Figure simulation tool illustrates non-linear nature resistor temperature coefficient temperatures exceeding 50°C ambient. addition, resistor divider technique using resistor derive CTRL2 voltage inherently flattening characteristic (reduced downward slope) higher temperatures. avoid current exceeding maximum
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LT3478/LT3478-1 APPLICATIO ATIO
allowed level higher temperatures, CTRL2 voltage curve require greater downward slope between 25°C 50°C compensate that loss slope higher temperatures. Example: Calculate resistor values required generating CTRL1 CTRL2 from VREF based following requirements: ILED 700mA 25°C ILED derating curve breakpoint occurs 25°C ILED derating curve slope -200mA/25°C between 25°C 50°C ambient temperature Step1: Choose CTRL1 700mV ILED 700mA CTRL1 VREF/(1 R2/R1) [(VREF/CTRL1) VREF 1.24V choosing 22.1k, 22.1k [(1.24/0.7) (choose 16.9k) CTRL1 1.24/(1 (16.9/22.1)) CTRL1 703mV (ILED 703mA) Step Choose resistor network option (Figure CTRL2 CTRL1 25°C breakpoint start with 16.9k, RNTC (closest value available) CTRL2 701mV (ILED Min(CTRL1, CTRL2) 701mA) Step Calculate CTRL2 slope between 25°C 50°C CTRL2 1.24/(1 R4/RNTC (T)) 25°C, CTRL2 701mV 50°C, RNTC RNTC (TO).ex, [(1/(T 273) 1/298)] B-constant; linear over 25°C 50°C temperature range) RNTC B-constant 3950 50°C 3950 [(1/323) 1/298] -1.026
RNTC (50°C) RNTC (25°C).e-1.026 RNTC (50°C) 0.358 RNTC (50°C) 7.9k CTRL2(50°C) 1.24/(1 16.9/7.9) 395mV CTRL2 slope (25°C 50°C) [CTRL2(50°C) CTRL2(25°C)]/25°C (395 701)/25 -306mV/25°C ILED slope -306mA/25°C required ILED slope -200mA/25°C. reduce slope CTRL2 versus temperature easier keep exact same resistor value B-constant (there limited choices) simply adjust type resistor network used CTRL2 pin. changing resistor network option possible place temperature independent resistor series with RNTC reduce effects RNTC CTRL2 voltage over temperature. Step Calculate resistor value required resistor network option (Figure provide ILED slope -200mA/25°C between 25°C 50°C ambient temperature. CTRL2 (25°C) 0.7V 1.24/(1 (R4/(RNTC(25°C)+ RY)) 0.77 (RNTC(25°C) -200mA/25°C slope CTRL2(50°C) CTRL2(50°C) 0.5V 1.24/(1 (R4/(RNTC RY)) 1.48 (RNTC(50°C) Equating knowing RNTC(25°C) RNTC(50°C) 7.9k gives, 0.77 (22k 1.48 (7.9k 0.77 11.7 1.48 (17k 11.7k)/(1.48 0.77) 7.5k
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LT3478/LT3478-1 APPLICATIO ATIO
value solved using equation where, 0.77 (RNTC(25°C) 0.77 (22k 7.5k) 22.7k (choose 22.6k) ILED slope calculated from, ILED slope [CTRL2(50°C) CTRL2(25°C)]/25°C where CTRL2 (50°C) 1.24/(1 22.6/(7.9 7.5)) 503mV CTRL2 (25°C) 1.24/(1 39.2/(22 28.7)) 699mV giving ILED slope (from 25°C 50°C) 503mV 699mV/25°C -196mV/25°C ILED slope -196mA/25°C Using Murata simulation tool resistor network values above example shows CTRL2 voltage curve that flattens temperatures approach 100°C ambient. final resistor network chosen derating curve Figure used option network with 19.3k, RNTC (NCP15XW223J0SRC) 3.01k. Although CTRL2 downward slope greater than -200mA/25°C initially, slope required avoid exceeding maximum allowed currents high ambient temperatures (see Figure Dimming Many applications require accurate control brightness LED(s). addition, being able maintain constant color over entire dimming range just critical. constant color dimming, LT3478/LT3478-1 provide special internal circuitry allow 3000:1 wide dimming range. With N-channel MOSFET connected between LED(s) ground signal connected gate MOSFET (Figure possible control brightness LED(s) based signal duty cycle only. This form dimming superior dimming control using analog input voltage (reducing CTRL1 voltage) because allows constant color maintained during dimming. maximum current
SHDN VREF CTRL2 CTRL1 OVPSET LT3478/ LT3478-1 (LT3478) RSENSE VOUT
output LED(s) programmed given brightness/color "chopped" over duty cycle range (Figure from 100% 0.033%.
COUT DIMMING CONTROL
3478
Figure Dimming Control Using LT3478/LT3478-1
TPWM TONPWM
1/fPWM)
INDUCTOR CURRENT
CURRENT
ILED
3478
Figure Dimming Waveforms Using LT3478/LT3478-1
Some general guidelines Current Dimming using (see Figure 10): Dimming Ratio (PDR) 1/(PWM duty cycle) 1/(TONPWM fPWM) Lower fPWM allows higher Dimming Ratios (use minimum fPWM 100Hz avoid visible flicker maximize PDR) Higher fOSC value improves (allows lower TONPWM) will reduce efficiency increase internal heating. general, minimum operational TONPWM (1/fOSC). Lower inductor value improves
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LT3478/LT3478-1 APPLICATIO ATIO
Higher output capacitor value improves Choose schottky diode (D2, Figure minimum reverse leakage Typical Performance Characteristics graph "LED Current Duty Cycle". Soft-Start limit inrush current output voltage overshoot during startup/recovery from fault condition, LT3478/ LT3478-1 provide soft-start used program switch current ramp timing using capacitor ground. LT3478/LT3478-1 monitor system parameters following faults: <2.8V, SHDN <1.4, inductor current boosted output voltage >OVP. detection these faults, LT3478/LT3478-1 stop switching immediately soft-start latch causing discharged (see Timing Diagram Figure 11). When faults longer exist been discharged least 0.25V, soft-start latch reset internal 12µA supply charges pin. gradual ramp voltage equivalent ramp switch current limit until exceeds ramp rate given VSS/t 12µA/CSS
FAULTS TRIGGERING SOFT-START LATCH WITH TURNED IMMEDIATELY: 2.8V SHDN 1.4V VOUT I(INDUCTOR)
0.65V (ACTIVE THRESHOLD) 0.25V (RESET THRESHOLD) 0.15V SOFT-START LATCH RESET: SOFT-START LATCH SET: 0.25V 2.8V SHDN 1.4V VOUT I(INDUCTOR)
Figure LT3478 Fault Detection Timing Diagram
limit inductor current overshoot <0.5A when charges past level required loop control, capacitor should chosen using following formula: CSS(MIN) (7.35 0.6(ILED VOUT/VS)) Example: VOUT 16V, ILED 1.05A, 0.1µF, CSS(MIN) 0.1µF (7.35 0.6(1.05 16/8)) 0.612µF (choose 0.68µF). High Inductor Current "Inrush" Protection LT3478/LT3478-1 provide integrated resistor between pins monitor inductor current (Figure During startup "hotplugging" inductor supply, possible inductor currents exceed maximum switch current limit. When inductor current exceeds LT3478/LT3478-1 protect internal power switch turning triggering soft-start latch. This protection prevents switch from repetitively turning during excessive inductor currents delaying switching until fault been removed. defeat inductor current sensing inductor supply should connected left open. details Applications Information section "Soft-Start". Open Circuit Protection Maximum Dimming Ratios LT3478/LT3478-1 drivers provide optimum protection from open faults clamping converter output programmable overvoltage protection level (OVP). addition, programmable feature draws zero current from output during allow higher dimming ratios. This provides advantage over other driver applications which connect resistor divider directly from VOUT. open fault occurs when connection LED(s) becomes broken LED(s) fails open. driver using step-up switching regulator, open circuit fault cause converter output exceed voltage capabilities regulator's power switch, causing permanent damage. When VOUT exceeds OVP,
3478
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LT3478/LT3478-1 APPLICATIO ATIO
LT3478/LT3478-1 immediately stop switching, soft-start latch discharged. latch only reset when VOUT falls below been discharged below 0.25V (Figure 11). LED(s) simply open circuit reconnected, however, used protect switch might high reconnected LED(s). LT3478/LT3478-1 therefore allow programmable protect both driver switch LED(s). (The minimum allowable normal operation given string depends number LEDs their maximum forward voltage ratings.) programmed using OVPSET (front page), given (OVPSET 41)V where programmable range OVPSET 0.3V resulting range 12.3V 41V. OVPSET programmed with single resistor tapping resistor divider from VREF used program CTRL1. both CTRL1 CTRL2 connected directly VREF (maximum current setting) then OVPSET requires simple resistor divider from VREF. Thermal Calculations maximize output power capability application without exceeding LT3478/LT3478-1 125°C maximum operational junction temperature, useful able calculate power dissipation within power dissipation within comes from four main sources: switch loss, switch loss, Inductor current sensing input quiescent current. These formulas assume boost converter architecture, continuous mode operation dimming. Switch loss PSW(DC) (RSW IL(AVE)2 switch resistance 0.07 125°C) IL(AVE) POUT/( POUT VOUT ILED converter efficiency POUT/(POUT PLOSS)
inductor supply input switch duty cycle (VOUT VS)/(VOUT VSAT) forward voltage drop external Schottky diode VSAT IL(AVE) Switch loss PSW(AC) tEFF(1/2)IL(AVE)(VOUT VF)(FOSC) tEFF effective switch current switch voltage overlap time during turn turn (tISW tVSW) tISW ISWITCH rise/fall time IL(AVE) tVSW fall/rise time (VOUT 0.7ns fOSC switching frequency Current sensing loss PSENSE PSENSE(IL) PSENSE(ILED) PSENSE(IL) IL(AVE)2 9.5m PSENSE(ILED) ILED2 Input quiescent loss where (6.2mA (100mA Example (Using LT3478-1): ILED 700mA, VOUT 24.5V LEDs), 0.5V fOSC 0.2Mhz, 0.89 (initial assumption) IL(AVE) (24.5 0.7)/(0.89 2.41A (24.5 8)/(24.5 0.17) 0.684 TEFF ((2.41 2)ns (24.5 0.5) 0.7)ns 45ns Total Power Dissipation: PSW(DC) PSW(AC) PSENSE PSW(DC) 0.07 (2.41)2 0.684 0.278W PSW(AC) 45ns 2.41 0.2MHz 0.271W PSENSE ((2.41)2 0.0095) ((0.7)2 0.1) 0.104W (6.2mA (100mA 0.684)) 0.597W 0.278 0.271 0.104 0.597 1.25W
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LT3478/LT3478-1 APPLICATIO ATIO
Local heating from nearby inductor Schottky diode will also final junction temperature Based empirical measurements, effect diode inductor heating LT3478-1 junction temperature approximated (LT3478-1) 5°C/W (PDIODE PINDUCTOR) PDIODE IL(AVE) 0.316 0.5V IL(AVE) 2.41 PDIODE 0.316 2.41 0.381W PINDUCTOR IL(AVE)2 inductor resistance (assume 0.05) PINDUCTOR (2.41)2 0.05 0.29W LT3478/LT3478-1 thermally enhanced package. With proper soldering Exposed underside package combined with full copper plane underneath device, thermal resistance (JA) will about 35°C/W. ambient temperature 70°C, junction temperature LT3478-1 example application described above, calculated (LT3478-1) JA(PTOT) 5(PDIODE PINDUCTOR) 35(1.25) 5(0.671) 118°C above example, efficiency initially assumed 0.89. lower efficiency converter will increase IL(AVE) hence increase calculated value calculated POUT/(POUT PLOSS) POUT VOUT ILED 17.15W PLOSS (estimated) PDIODE PINDUCTOR 1.92W 17.15/(17.15 1.92)
application built, inductor current measured value junction temperature estimated. Ideally thermal measurement should made achieve greatest accuracy Note: junction temperature reduced lower supply available separate from inductor supply above example, driving from available source (instead reduces input quiescent losses item(4) from 0.597W 0.224W, resulting reduction from 118°C 105°C. Layout Considerations with switching regulators, careful attention must given layout component placement achieve optimal thermal,electrical noise performance (Figure 12). exposed LT3478/LT3478-1 (Pin only connection exposed should soldered continuous copper ground plane underneath device reduce temperature maximize power capability ground path resistor capacitor should taken from nearby analog ground connection exposed (near separate from power ground connection exposed (near 16). bypass capacitor should placed close possible analog ground connection. voltage rise fall times designed short possible maximum efficiency. reduce effects both radiated conducted noise, area trace should kept small possible. ground plane under switching regulator minimize interplane coupling. schottky diode output capacitor should placed close possible node minimize this high frequency switching path. minimize current sensing errors LT3478, terminals external sense resistor RSENSE should tracked VOUT pins separate from high current paths.
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LT3478/LT3478-1 APPLICATIO ATIO
CVIN VOUT SCHOTTKY DIODE OUTPUT CAPACITOR
INDUCTOR RSENSE (LT3478 ONLY)
Figure Recommended Layout LT3478/LT3478-1 (Boost Configuration)
TYPICAL APPLICATIO
15W, LEDs 700mA, Boost Driver
10µH 4.7µF 10µF 5V/DIV LT3478-1 700mA INDUCTOR CURRENT 1A/DIV ILED 0.5A/DIV 0.1µF 69.8k 2µs/DIV DIMMING RATIO 1000:1 (SEE EFFICIENCY PAGE
3478 TA02b
SHDN VREF
45.3k 54.9k
CTRL2 OVPSET
CTRL1 130k CDRH104R-100NC PDS560 Si2318DS LEDs: LUXEON (WHITE) 3.3V 100Hz DIMMING RATIO 1000:1
fOSC 500kHz
(CONNECT MULTIPLE GROUND PLANES THROUGH VIAS UNDERNEATH LT3478/LT3478-1 VOUT OVPSET POWER SOLDER EXPOSED (PIN ENTIRE COPPER GROUND PLANE UNDERNEATH DEVICE CTRL2 CTRL1 SHDN EXPOSED VREF ANALOG BYPASS
3252
LT3478-1 Dimming Waveforms
fPWM 100Hz
3478 TA02a
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LT3478/LT3478-1 TYPICAL APPLICATIO
17W, LEDs 350mA, Boost Driver plus LT3003
4.7µF 3.3µF
3.3V
SHDN VREF
CTRL1 OVPSET
EFFICIENCY
CTRL2
100k
CDRH104R-5R2 PDS560 LEDs: LUXEON (WHITE)
3.3V VOUT 100Hz DIMMING RATIO 3000:1
16W, LEDs 350mA, Buck-Boost Mode Driver plus LT3003
4.7µF 3.3µF
SHDN VREF
EFFICIENCY
CTRL2 CTRL1 OVPSET
100k
CDRH105R-8R2 PDS560 7.5V ZENER LEDs: LUXEON (WHITE) 3.3V 100Hz
DIMMING RATIO 200:1
3478 TA04a
5.2µH
VOUT 3.3µF
Efficiency Input
3.3V ILED 350mA fOSC 1MHz DUTY CYCLE 100%
LT3478-1 1.05A
0.1µF
31.6k
LEDs SERIES CHANNELS) LUXEON (WHITE) LED1 VMAX LED2 LT3003 LED3
3478 TA03b
fOSC 1MHz
3478 TA03a
8.2µH
VOUT 10µF
Efficiency Input
ILED 350mA fOSC 500kHz DUTY CYCLE 100%
LT3478-1
1.05A
0.1µF
69.8k
LEDs SERIES CHANNELS) LUXEON (WHITE)
3478 TA04b
fOSC 500kHz VOUT LED1 VMAX LED2 LT3003 LED3
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LT3478/LT3478-1 TYPICAL APPLICATIO
Buck-Boost Mode Driver
3.8V 6.5V NiMH 10µF SHDN VREF 100k CTRL2 CTRL1 OVPSET CDRH105R-6R8 B320 Si2302ADS Si2315BDS LED: LUXEON (WHITE) 3.3V 1kHz DIMMING RATIO 200:1 69.8k SINGLE LUXEON (WHITE)
3478 TA06b
LT3478-1
EFFICIENCY
6.8µH
4.7µF
Efficiency
ILED fOSC 500kHz DUTY CYCLE 100%
0.1µF
fOSC 500kHz
3478 TA06a
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LT3478/LT3478-1 TYPICAL APPLICATIO
24W, LEDs 1.5A, Buck Mode Driver
PVIN 3.3µF RSENSE 0.068 1.5A
TYPICAL EFFICIENCY CONDITIONS/COMPONENTS SHOWN (PWM DUTY CYCLE 100%, =25°C)
3.3V
CDRH105R-100 PDS560 2N7002 Si2319DS LEDs: LXK2 (WHITE)
LEDs 10µF
10µH
4.7µF
SHDN LT3478 DIMMING RATIO 3000:1 3.3V 69.8k 100Hz
VREF CTRL2 CTRL1 OVPSET 100k
0.1µF fOSC 500kHz
3478 TA07a
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LT3478/LT3478-1 PACKAGE DESCRIPTIO
Package 16-Lead Plastic TSSOP (4.4mm)
(Reference 05-08-1663)
Exposed Variation
3.58 (.141) 4.90 5.10* (.193 .201) 3.58 (.141) 1514 1110 6.60 ±0.10 4.50 ±0.10
NOTE
2.94 (.116) 0.45 ±0.05 1.05 ±0.10 0.65
6.40 2.94 (.252) (.116)
RECOMMENDED SOLDER LAYOUT
1.10 (.0433)
4.30 4.50* (.169 .177)
0.25
0.09 0.20 (.0035 .0079)
0.50 0.75 (.020 .030)
0.65 (.0256)
NOTE: CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS DIMENSIONS (INCHES) DRAWING SCALE
0.195 0.30 (.0077 .0118)
0.05 0.15 (.002 .006)
FE16 (BC) TSSOP 0204
RECOMMENDED MINIMUM METAL SIZE EXPOSED ATTACHMENT *DIMENSIONS INCLUDE MOLD FLASH. MOLD FLASH SHALL EXCEED 0.150mm (.006") SIDE
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Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
LT3478/LT3478-1 TYPICAL APPLICATIO
4.7µF 3.3µF
3.3V
SHDN VREF
250mA
EFFICIENCY
8.25k
CTRL2 CTRL1 OVPSET
CDRH6D28 ZLLS1000 Si2318DS LEDs: LUXEON (WHITE) 3.3V 100Hz
DIMMING RATIO 1000:1
RELATED PARTS
PART NUMBER DESCRIPTION LT1618 LT3003 LT3474 LT3475 LT3476 LT3477 LT3479 LT3486 LTC3783 Constant Current, 1.4MHz, 1.5A Boost Converter with Analog/PWM Dimming Three Channel Ballaster with 3,000:1 True Color Dimming 36V, (ILED), 2MHz,Step-Down Driver with 400:1 True Color Dimming Dual 1.5A(ILED), 36V, 2MHz,Step-Down Driver 3,000:1 True Color Dimming Quad Output 1.5A, 2MHz High Current Driver with 1,000:1 True Color Dimming 42V, 3.5MHz Boost, Buck-Boost, Buck Driver with Analog/ Dimming 3.5MHz Full Featured DC/DC Converter with Soft-Start Inrush Current Protection Analog/PWM Dimming Dual 1.3A 2MHz High Current Driver with 1,000:1 True Color Dimming High Current Controller with 3,000:1 True Color Dimming COMMENTS VIN: 18V, VOUT(MAX) 36V, <1µA, MS10 Package VIN: 48V, <5µA, MSOP10 Package VIN: 36V, VOUT(MAX) 13.5V, <1µA, TSSOP16E Package VIN: 36V, VOUT(MAX) 13.5V, <1µA, TSSOP20E Package VIN: 2.8V 16V, VOUT(MAX) 36V, <10µA, Package VIN: 2.5V 25V, VOUT(MAX) 40V, <1µA, QFN, TSSOP20E Packages VIN: 2.5V 24V, VOUT(MAX) 40V, <1µA, DFN, TSSOP16E Packages VIN: 2.5V 24V, VOUT(MAX) 36V, <1µA, DFN, TSSOP16E Packages VIN: 36V, VOUT(MAX) FET, <20µA, DFN, TSSOP16E Packages
Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
1630 McCarthy Blvd., Milpitas, 95035-7417
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2007
LEDs 250mA, Boost Driver
10µH 3.3µF RSENSE 0.42 LT3478
Efficiency Input
3.3V ILED 250mA fOSC 2MHz DUTY CYCLE 100%
0.1µF
LEDs LUXEON (WHITE)
3478 TA05b
fOSC 2MHz
3478 TA05a
34781f 0107 PRINTED
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