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FEATURES

LT3478/LT3478-1 4.5A Monolithic LED Drivers with True Color PWM Dimming DESCRIPTIO
The LT(R)3478/LT3478-1 are 4.5A step-up DC/DC converters designed to drive LEDs with a constant current over a wide programmable range. Series connection of the LEDs provides identical LED currents for uniform brightness without the need for ballast resistors and expensive factory calibration. The LT3478-1 reduces external component count and cost by integrating the LED current sense resistor. The LT3478 uses an external sense resistor to extend the maximum programmable LED current beyond 1A and also to achieve greater accuracy when programming low LED currents. Operating frequency can be set with an external resistor from 200kHz up to 2.25MHz. Unique circuitry allows a PWM dimming range up to 3000:1 while maintaining constant LED color. The LT3478/LT3478-1 are ideal for high power LED driver applications such as automotive TFT LCD backlights, courtesy lighting and heads-up displays. One of two CTRL pins can be used to program maximum LED current. The other CTRL pin can be used to program a reduction in maximum LED current vs temperature to maximize LED usage and improve reliability. Additional features include inrush current protection, programmable open LED protection and programmable soft-start. Each part is available in a 16-pin thermally enhanced TSSOP Package.

True Color PWMTM Dimming Delivers Constant LED Color with Up to 3000:1 Range Wide Input Voltage Range: 2.8V to 36V 4.5A, 60m, 42V Internal Switch Drives LEDs in Boost, Buck-Boost or Buck Modes Integrated Resistors for Inductor and LED Current Sensing Program LED Current: 100mA to 1050mA (LT3478-1) (10mV to 105mV)/RSENSE (LT3478) Program LED Current De-Rating vs Temperature Separate Inductor Supply Input Inrush Current Protection Programmable Soft-Start Fixed Frequency Operation from 200kHz to 2.25MHz Open LED Protection (Programmable OVP) Accurate Shutdown/UVLO Threshold with Programmable Hysteresis 16-Pin Thermally Enhanced TSSOP Package
APPLICATIO S

High Power LED Driver Automotive Lighting
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Patents Pending.
TYPICAL APPLICATIO
VIN 8V TO 16V 4.7F VIN SHDN VREF 45.3k CTRL2 OVPSET 54.9k CTRL1 130k PWM 1F PWM DIMMING CONTROL SS VS
Automotive TFT LCD Backlight
10H L SW OUT 0.1 RSENSE (LT3478) 10F 95 EFFICIENCY (%) 100
Efficiency vs VIN
ILED = 700mA fOSC = 500kHz PWM DUTY CYCLE = 100%
LT3478-1
LED
90
85 700mA 15W 6 LEDs (WHITE) 69.8k
VC 0.1F
RT
80 8 10
3478 TA01
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6 LEDs LUXEON III (WHITE) 12 VIN (V) 14 16
3478 TA01b
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LT3478/LT3478-1 ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SW SW VIN VS L VOUT LED OVPSET 1 2 3 4 5 6 7 8 17 16 SS 15 RT 14 PWM 13 CTRL2 12 CTRL1 11 SHDN 10 VREF 9 VC
SW ............................................................................42V VOUT, LED ..................................................................42V VIN, VS, VL, SHDN (Note 5) .......................................36V PWM .........................................................................15V CTRL1, 2 .....................................................................6V SS, RT, VC, VREF, OVPSET............................................2V Operating Junction Temperature Range (Notes 2, 3, 4).................................... -40C to 125C Storage Temperature Range................... -65C to 150C Lead Temperature (Soldering, 10 Sec) .................. 300C
FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 125C, JA = 35C/W EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB.
ORDER PART NUMBER LT3478EFE LT3478EFE-1 LT3478IFE LT3478IFE-1
FE PART MARKING 3478FE 3478FE-1 3478FE 3478FE-1
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. SW = open, VIN = VS = L = VOUT = SHDN = 2.7V, LED = open, SS = open, PWM = CTRL1, CTRL2 = 1.25V, VREF = open, VC = open, RT = 31.6k.
PARAMETER Minimum Operating Voltage Operational Input Voltage VIN Quiescent Current VIN Shutdown Current SHDN Pin Threshold (VSD_p) SHDN Pin Threshold (VSD_UVLO) SHDN Pin Current VREF Voltage VREF Line Regulation VREF Load Regulation Frequency: fOSC 200kHz Frequency: fOSC 1MHz CONDITIONS (Rising) VS VIN (Note 5) VC = 0V (No Switching) SHDN = 0V (Micropower) (Switching) SHDN = VSD_UVLO - 50mV SHDN = VSD_UVLO + 50mV I(VREF) = 0A, VC = 0V I(VREF) = 0A, 2.7V < VIN < 36V 0 < I(VREF) < 100A (Max) RT = 200k RT = 31.6k

ELECTRICAL CHARACTERISTICS
MIN 2.8 2.8
TYP 2.4
MAX 2.8 36 36
UNITS V V V mA A V V A A V %/V mV MHz MHz
6.1 3 0.1 1.3 8 1.213 0.4 1.4 10 0 1.240 0.005 8 0.18 0.88 0.2 6 0.7 1.5 12 1.263 0.015 12 0.22 1.12
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LT3478/LT3478-1
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. SW = open, VIN = VS = L = VOUT = SHDN = 2.7V, LED = open, SS = open, PWM = CTRL1, CTRL2 = 1.25V, VREF = open, VC = open, RT = 31.6k.
PARAMETER Frequency: fOSC 2.25MHz Line Regulation fOSC Nominal RT Pin Voltage Maximum Duty Cycle RT = 31.6k RT = 200k RT = 9.09k (Note 6) (Note 6) CTRL1 = 0.4V, VC = 1V CTRL1 = 0V, VC = 1V CTRL1 = 0.4V CTRL1 = 0V 2.7V < VS < 36V ISW = 4.5A SW = 42V, VC = 0V OVPSET = 1V OVPSET = 0.3V CTRL1 = VREF, Current Out of LED Pin CTRL1 = 700mV, Current Out of LED Pin CTRL1 = 350mV, Current Out of LED Pin CTRL1 = 100mV, Current Out of LED Pin CTRL1 = 700mV, VSENSE = VVOUT - VLED CTRL1 = 350mV, VSENSE = VVOUT - VLED CTRL1 = 100mV, VSENSE = VVOUT - VLED CTRL1 = 100mV, CTRL2 = 1.25V or CTRL2 = 100mV, CTRL1 = 1.25V (Current Out of Pin) OVPSET = 1V, VOUT = 41V (Current Out of Pin) 0.8 VC = 1V, PWM = 0 PWM = 0 I(SS) = 20A VC = 0V VC = 0V SS = 1V, Current Out of Pin, VC = 0V SS = 0.5V, VC = 0V

ELECTRICAL CHARACTERISTICS
CONDITIONS RT = 9.09k
MIN 2
TYP 2.25 0.05 0.64
MAX 2.6 0.2
UNITS MHz %/V V % % % A/A V/A A/V A A V V V
RT = 31.6k, 2.7V < VIN < 36V 80
88 97 73 770 400 13 40 40 0.65 1.5 0.2
LED Current to VC Current Gain LED Current to VC Voltage Gain VC to Switch Current Gain VC Source Current (Out of Pin) VC Sink Current VC Switching Threshold VC High Level (VOH) VC Low Level (VOL) Inductor Current Limit Switch Current Limit Switch VCE SAT Switch Leakage Current VOUT Overvoltage Protection (OVP) (Rising) Full Scale LED Current (LT3478-1) 700mA LED Current (LT3478-1) 350mA LED Current (LT3478-1) 100mA LED Current (LT3478-1) CTRL1 = 700mV, VSENSE (LT3478) CTRL1 = 350mV, VSENSE (LT3478) CTRL1 = 100mV, VSENSE (LT3478) CTRL1, 2 Input Currents OVPSET Input Current PWM Switching Threshold VC Pin Current in PWM Mode OUT Pin Current in PWM Mode SS Low Level (VOL) SS Reset Threshold SS High Level (VOH) Soft-Start (SS) Pin Charge Current Soft-Start (SS) Pin Discharge Current
4.5 4.5
6 6.3 270 1 41 12.3
6.8 7.5
A A mV A V V
1010 655 325 70 101 67 33 7
1050 700 350 100 105 70.5 35.5 10 40 200 1 1 1 0.15 0.25 1.5 12 350
1090 730 375 130 109 74 38 13
mA mA mA mA mV mV mV mV nA nA
Full Scale LED Current VSENSE (LT3478) CTRL1 = VREF, VSENSE = VVOUT - VLED
1.2 50 100
V nA nA V V V A A
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LT3478/LT3478-1 ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3478EFE/LT3478EFE-1 are guaranteed to meet performance specifications from 0C to 125C junction temperature. Specifications over the -40C to 125C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3478IFE/LT3478IFE-1 are guaranteed over the full -40C to 125C operating junction temperature range. Note 3: This IC includes over-temperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when over-temperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 4: For maximum operating ambient temperature, see the "Thermal Calculations" section in the Applications Information section. Note 5: The maximum operational voltage for VIN is limited by thermal and efficiency considerations. Power switch base current is delivered from VIN and should therefore be driven from the lowest available power supply in the system. See "Thermal Calculations" in the Applications Information section. Note 6: For LT3478, parameter scales * (RSENSE/0.1).
TYPICAL PERFOR A CE CHARACTERISTICS
LED Current vs CTRL1
1400 TA = 25C CTRL2 = VREF (FOR LT3478 SCALE BY 0.1/RSENSE) LED CURRENT (mA) 1400
LED CURRENT (mA)
LT3478-1 700
LT3478-1 700
LED CURRENT (mA)
1050
350 VREF 0 0 0.35 0.70 CTRL1 (V) 1.05 1.40
3478 G01
CTRL1 Pin Current vs Temperature
50 CTRL1 PIN CURRENT X (-1) (nA) CTRL1 = 0.1V SWITCH VCE (SAT) (mV) 240
40
CURRENT LIMIT (A)
30 CTRL1 = 0.35V 20 CTRL2 = VREF CTRL1 AND CTRL2 PINS INTERCHANGEABLE 10
CTRL1 = 0.7V
CTRL1 = 0.9V 0 -50 -25 50 75 100 0 25 JUNCTION TEMPERATURE (C)
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LED Current vs Temperature
(FOR LT3478 SCALE BY 0.1/RSENSE) ILED = 1050mA, CTRL1 = CTRL2 = VREF 1050 1000
LED Current vs PWM Duty Cycle Wide PWM Dimming Range (3000:1)
TA = 25C VIN = VS = 12V 6 LEDS AT 500mA PWM FREQ = 100Hz 100 CTRL1 = 0.5V CTRL2 = VREF FOSC = 1.6MHz L = 2.2H 10
350 ILED = 100mA, CTRL1 = 100mV, CTRL2 = VREF 0 -50 -25 50 75 100 0 25 JUNCTION TEMPERATURE (C) 125
1
0 0.01
0.1 1 10 PWM DUTY CYCLE (%)
100
3478 G03
3478 G02
Switch VCE (SAT) vs Switch Current
TA = 25C 7.0
Switch and Inductor Peak Current Limits vs Temperature
210
6.5 SWITCH INDUCTOR 5.5
180
6.0
120
60
5.0
0 125 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 SWITCH CURRENT (A)
3478 G05
4.5 -50 -25 50 75 100 0 25 JUNCTION TEMPERATURE (C)
125
3478 G04
3478 G06
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LT3478/LT3478-1 TYPICAL PERFOR A CE CHARACTERISTICS
VREF vs Temperature
1.28 1.60
1.50 SHDN (V)
SHDN PIN CURRENT (A)
1.26
VREF (V)
1.24
1.22
1.20
1.18 -50 -25 0 25 50 75 100 JUNCTION TEMPERATURE (C)
VIN Shutdown Current vs Temperature
50 SHDN = 0V 14 12 40 VIN CURRENT (mA)
30 VIN = 36V VIN = 20V 10 VIN = 2.8V 0 -50 -25 0 25 50 75 100 JUNCTION TEMPERATURE (C) 125
8 6 4 2 0 0 3 6
VIN CURRENT (mA)
VIN CURRENT (A)
20
VS, L, SW Shutdown Currents vs Temperature
4 SWITCH PEAK CURRENT LIMIT (A) SHDN = 0V VS = L = SW = 36V 7 6 5 4 3 2 1 0 125
PIN CURRENT (A)
2 I(VS PIN) = I(L PIN)
0 -50 -25 0 25 50 75 100 JUNCTION TEMPERATURE (C)
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SHDN Threshold vs Temperature
15
SHDN Pin (Hysteresis) Current vs Temperature
JUST BEFORE PART TURNS ON 10
1.40
5
1.30
125
1.20 -50 -25 50 75 100 0 25 JUNCTION TEMPERATURE (C)
125
AFTER PART TURNS ON 0 -50 -25 50 75 100 0 25 JUNCTION TEMPERATURE (C)
125
3478 G07
3478 G08
3478 G09
VIN Quiescent Current vs VIN
14 12 10 8 6 4 2
VIN Quiescent Current vs Temperature
10
TA= 25C VC = 0V 9 12 15 18 21 24 27 30 33 36 VIN (V)
3478 G11
VIN = 2.8V V = 0V 0C -50 -25 0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
3478 G10
3478 G12
Switch Peak Current Limit vs Duty Cycle
I(SW PIN)
TA= 25C 0 20 40 60 DUTY CYCLE (%) 80 100
3478 G19
3478 G18
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LT3478/LT3478-1 TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency vs RT
10000 TA = 25C SWITCHING FREQUENCY (MHz) 1.20 1.15 SWITCHING FREQUENCY (kHz) 1.10 1.05 1.00 0.95 0.90 0.85 100 1 10 RT (k)
3478 G13
VOUT CLAMP (V)
1000
100
SS Pin Charge Current vs Temperature
14 SS PIN CURRENT (A) (OUT OF PIN) 1.8 1.5 13 1.2 VC (V) 12 0.9 0.6 11 0.3 10 -50 -25 50 75 100 0 25 JUNCTION TEMPERATURE (C)
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Switching Frequency vs Temperature
RT = 31.6k 43.0 42.5 42.0 41.5 41.0 40.5 40.0 39.5 125
Open-Circuit Output Clamp Voltage vs Temperature
OVPSET = 1V
1000
0.80 0 25 -50 -25 50 75 100 JUNCTION TEMPERATURE (C)
39.0 0 25 -50 -25 50 75 100 JUNCTION TEMPERATURE (C)
125
3478 G14
3478 G15
VC Pin Active and Clamp Voltages vs Temperature
VC CLAMP
VC ACTIVE THRESHOLD
125
0 0 25 -50 -25 50 75 100 JUNCTION TEMPERATURE (C)
125
3478 G16
3478 G17
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LT3478/LT3478-1 PI FU CTIO S
SW (Pins 1, 2): Switch Pin. Collector of the internal NPN power switch. Both pins are fused together inside the IC. Connect the inductor and diode here and minimize the metal trace area connected to this pin to minimize EMI. VIN (Pin 3): Input Supply. Must be locally bypassed with a capacitor to ground. VS (Pin 4): Inductor Supply. Must be locally bypassed with a capacitor to ground. Can be shorted to VIN if only one supply is available (see L (Pin 5) function). L (Pin 5): Inductor Pin. An internal resistor between VS and L pins monitors inductor current to protect against inrush current. Exceeding 6A immediately turns off the internal NPN power switch and discharges the soft-start pin. Input current monitoring can be disabled by connecting the inductor power supply directly to the L pin and leaving the VS pin open (requires local bypass capacitor to GND on L pin; not VS pin). VOUT (Pin 6): Output voltage of the converter. Connect a capacitor from this pin to ground. Internal circuitry monitors VOUT for protection against open LED faults. LED (Pin 7): Connect the LED string from this pin to ground. An internal (LT3478-1)/external (LT3478) resistor between the VOUT and LED pins senses LED current for accurate control. OVPSET (Pin 8): Programs VOUT overvoltage protection level (OVP) to protect against open LED faults. OVP = (OVPSET * 41)V. OVPSET range is 0.3V to 1V for an OVP range of typically 12.3V to 41V. VC (Pin 9): Output of the transconductance error amplifier and compensation pin for the converter regulation loop. VREF (Pin 10): Bandgap Voltage Reference. This pin can supply up to 100A. Can be used to program CTRL1, CTRL2, OVPSET pin voltages using resistor dividers to ground. SHDN (Pin 11): The SHDN pin has an accurate 1.4V threshold and can be used to program an undervoltage lockout (UVLO) threshold for system input supply using a resistor divider from supply to ground. A 10A pin current hysteresis allows programming of undervoltage lockout (UVLO) hysteresis. SHDN above 1.4V turns the part on and removes a 10A sink current from the pin. SHDN = 0V reduces VIN current < 3A. SHDN can be directly connected to VIN. If left open circuit the part will be turned off. CTRL1 (Pin 12): CTRL1 pin voltage is used to program maximum LED current (CTRL2 = VREF). CTRL1 voltage can be set by a resistor divider from VREF or an external voltage source. Maximum LED current is given by: (LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps (LT3478) Max LED Current = 0.1 Min(CTRL, 1.05) * Amps RSENSE (linear for 0.1V < CTRL1< 0.95V ; CTRL2 = VREF) For maximum LED current, short CTRL1 and CTRL2 pins to VREF. CTRL2 (Pin 13): The CTRL2 pin is available for programming a decrease in LED current versus temperature (setting temperature breakpoint and slope). This feature allows the output LED(s) to be programmed for maximum allowable current without damage at higher temperatures. This maximizes LED usage and increases reliability. A CTRL2 voltage with negative temperature coefficient is created using an external resistor divider from VREF with temperature dependant resistance. If not used, CTRL2 should be tied to VREF. PWM (Pin 14): Input pin for PWM dimming control. Above 1V allows converter switching and below 1V disables switching with VC pin level maintained. With an external MOSFET placed in series with the ground side of the LED string, a PWM signal driving the PWM pin and MOSFET gate provides accurate dimming control. The PWM signal can be driven from 0V to 15V. If unused, the pin should be connected to VREF. RT (Pin 15): A resistor to ground programs switching frequency between 200kHz and 2.25MHz. SS (Pin 16): Soft-Start Pin. Placing a capacitor here programs soft-start timing to limit inductor inrush current during start-up due to the converter. When inductor current
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LT3478/LT3478-1 PI FU CTIO S
exceeds 6A or VOUT exceeds OVP, an internal soft-start latch is set, the power NPN is immediately turned off and the SS pin is discharged. The soft-start latch is also set if VIN and/or SHDN do not meet their turn on thresholds. The SS pin only recharges when all faults are removed and the pin has been discharged below 0.25V. Exposed Pad (Pin 17): The ground for the IC and the converter. The FE package has an Exposed Pad underneath the IC which is the best path for heat out of the package. Pin 17 should be soldered to a continuous copper ground plane under the device to reduce die temperature and increase the power capability of the LT3478/LT3478-1.
BLOCK DIAGRA
SHDN 11 10A
-
1.4V
+
VIN 3
REF 1.24V
VREF 10
CTRL1 12 CTRL2 13
1.05V
+
-
1000
RS
TO OVERVOLTAGE DETECT CIRCUIT 8 OVPSET 15 RT 17 EXPOSED PAD (GND) 9 VC
3478 F01
Figure 1
8
-
+
+
-
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VS 4 5
L
SS 16 VC OVERVOLTAGE DETECT
SW 1, 2 VOUT 6
9.5m
-
OVPSET
+ -
57mV
UVLO
INRUSH CURRENT PROTECTION
+
SOFT-START
100
RSENSE 0.1 (INTERNAL FOR LT3478-1) LED 7
RSENSE (EXTERNAL FOR LT3478)
PWM DETECT
OSC
S R
Q
Q1 LED LED LED GM LED 1V PWM 14
PWM
-
+
+ + + -
SLOPE COMP Q2
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LT3478/LT3478-1 OPERATIO
The LT3478/LT3478-1 are high powered LED drivers with a 42V, 4.5A internal switch and the ability to drive LEDs with up to 1050mA for LT3478-1 and up to 105mV/RSENSE for LT3478. The LT3478/LT3478-1 work similarly to a conventional current mode boost converter but use LED current (instead of output voltage) as feedback for the control loop. The Block Diagram in Figure 1 shows the major functions of the LT3478/LT3478-1. For the part to turn on, the VIN pin must exceed 2.8V and the SHDN pin must exceed 1.4V. The SHDN pin threshold allows programming of an undervoltage lockout (UVLO) threshold for the system input supply using a simple resistor divider. A 10A current flows into the SHDN pin before part turn on and is removed after part turn on. This current hysteresis allows programming of hysteresis for the UVLO threshold. See "Shutdown Pin and Programming Undervoltage Lockout" in the Applications Information Section. For micropower shutdown the SHDN pin at 0V reduces VIN supply current to approximately 3A. Each LED driver is a current mode step-up switching regulator. A regulation point is achieved when the boosted output voltage VOUT across the output LED(s) is high enough to create current in the LED(s) equal to the programmed LED current. A sense resistor connected in series with the LED(s) provides feedback of LED current to the converter loop. The basic loop uses a pulse from an internal oscillator to set the RS flip-flop and turn on the internal power NPN switch Q1 connected between the switch pin, SW, and ground. Current increases in the external inductor until switch current limit is exceeded or until the oscillator reaches its maximum duty cycle. The switch is then turned off, causing inductor current to lift the SW pin and turn on an external Schottky diode connected to the output. Inductor current flows via the Schottky diode charging the output capacitor. The switch is turned back on at the next reset cycle of the internal oscillator. During normal operation
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the VC voltage controls the peak switch current limit and hence the inductor current available to the output LED(s). As with all current mode converters, slope compensation is added to the control path to ensure stability. The CTRL1 pin is used to program maximum LED current via Q2. The CTRL2 pin can be used to program a decrease in LED current versus temperature for maximum reliability and utilization of the LED(s). A CTRL2 voltage with negative temperature coefficient can be created using an external resistor divider from VREF with temperature dependant resistance. Unused CTRL2 is tied to VREF. For True Color PWM dimming, the LT3478/LT3478-1 provide up to a 3000:1 wide PWM dimming range by allowing the duty cycle of the PWM pin (connected to the IC and an external N-channel MOSFET in series with the LED(s)) to be reduced from 100% to as low as 0.033% for a PWM frequency of 100Hz. Dimming by PWM duty cycle, allows for constant LED color to be maintained over the entire dimming range. For robust operation, the LT3478/LT3478-1 monitor system performance for any of the following faults : VIN or SHDN pin voltages too low and/or inductor current too high and/or boosted output voltage too high. On detection of any of these faults, the LT3478/LT3478-1 stop switching immediately and a soft-start latch is set discharging the SS pin (see Timing Diagram for SS pin in Figure 11). All faults are detected internally and do not require external components. When all faults no longer exist, an internal 12A supply charges the SS pin with a timing programmed using a single external capacitor. A gradual ramp up of SS pin voltage limits switch current during startup. For optimum component sizing, duty cycle range and efficiency the LT3478/LT3478-1 allow for a separate inductor supply VS and for switching frequency to be programmed from 200kHz up to 2.25MHz using a resistor from the RT pin to ground. The advantages of these options are covered in the Applications Informations section.
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LT3478/LT3478-1 APPLICATIO S I FOR ATIO
Inductor Selection Several inductors that work well with the LT3478/LT3478-1 are listed in Table 1. However, there are many other manufacturers and inductors that can be used. Consult each manufacturer for more detailed information and their entire range of parts. Ferrite cores should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating. Also ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. Values between 4.7H and 22H will suffice for most applications. Inductor manufacturers specify the maximum current rating as the current where inductance falls by a given percentage of its nominal value. An inductor can pass a current greater than its rated value without damaging it. Aggressive designs where board space is precious will exceed the maximum current rating of the inductor to save space. Consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct.
Table 1. Suggested Inductors
MANUFACTURER PART NUMBER CDRH104R-100NC CDRH103RNP-4R7NC-B CDRH124R-100MC CDRH104R-5R2NC FDV0630-4R7M UP4B-220 IDC (A) 3.8 4 4.5 5.5 4.2 7.6 INDUCTANCE (H) 10 4.7 10 5.2 4.7 22 MAX DCR (m) 35 30 28 22 49 34 L x W x H (mm) 10.5 x 10.3 x 4.0 10.5 x 10.3 x 3.1 12.3 x 12.3 x 4.5 10.5 x 10.3 x 4.0 7.0 x 7.7 x 3.0 22 x 15 x 7.9 MANUFACTURER Sumida www.sumida.com Toko www.toko.com Cooper www.cooperet.com
Table 2. Ceramic Capacitor Manufacturers
MANUFACTURER Taiyo Yuden AVX Murata PHONE NUMBER (408) 573-4150 (803) 448-9411 (714) 852-2001 WEB www.t-yuden.com www.avxcorp.com www.murata.com
Table 3. Suggested Diodes
MANUFACTURER PART NUMBER UPS340 B520C B530C B340A B540C PDS560 MAX CURRENT (A) 3 5 5 3 5 5 MAX REVERSE VOLTAGE 40 30 30 40 40 60 WEB Microsemi www.microsemi.com Diodes, Inc. www.diodes.com
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Capacitor Selection Low ESR (equivalent series resistance) ceramic capacitors should be used at the output to minimize the output ripple voltage. Use only X5R or X7R dielectrics, as these materials retain their capacitance over wider voltage and temperature ranges than other dielectrics. A 4.7F to 10F output capacitor is sufficient for most high output current designs. Some suggested manufacturers are listed in Table 2. Diode Selection Schottky diodes, with their low forward voltage drop and fast switching speed, are ideal for LT3478/LT3478-1 applications. Table 3 lists several Schottky diodes that work well. The diode's average current rating must exceed the application's average output current. The diode's maximum reverse voltage must exceed the application's output voltage. A 4.5A diode is sufficient for most designs. For PWM dimming applications, be aware of the reverse leakage current of the diode. Lower leakage current will drain the output capacitor less, allowing for higher dimming range. The companies below offer Schottky diodes with high voltage and current ratings.
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LT3478/LT3478-1 APPLICATIO S I FOR ATIO
Shutdown and Programming Undervoltage Lockout The LT3478/LT3478-1 have an accurate 1.4V shutdown threshold at the SHDN pin. This threshold can be used in conjunction with a resistor divider from the system input supply to define an accurate undervoltage lockout (UVLO) threshold for the system (Figure 2). SHDN pin current hysteresis allows programming of hysteresis voltage for this UVLO threshold. Just before part turn on, 10A flows into the SHDN pin. After part turn on, 0A flows from the SHDN pin. Calculation of the on/off thresholds for a system input supply using the LT3478/LT3478-1 SHDN pin can be made as follows: VSUPPLY OFF = 1.4 [1 + R1/R2)] VSUPPLY ON = VSUPPLY OFF + (10A * R1) An open drain transistor can be added to the resistor divider network at the SHDN pin to independently control the turn off of the LT3478/LT3478-1.
VSUPPLY R1 11 SHDN
-
1.4V
R2 OFF ON
+
SWITCHING FREQUENCY (kHz)
10A
Figure 2. Programming Undervoltage Lockout (UVLO) with Hysteresis
With the SHDN pin connected directly to the VIN pin, an internal undervoltage lockout threshold exists for the VIN pin (2.8V max). This prevents the converter from operating in an erratic mode when supply voltage is too low. The LT3478/LT3478-1 provide a soft-start function when recovering from such faults as SHDN <1.4V and/or VIN <2.8V. See details in the Applications Information section "Soft-Start".
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Programming Switching Frequency The switching frequency is programmed using an external resistor (RT) connected between the RT pin and ground. The internal free-running oscillator is programmable between 200kHz and 2.25MHz. Table 4 shows the typical RT values required for a range of switching frequencies. Selecting the optimum switching frequency depends on several factors. Inductor size is reduced with higher frequency but efficiency drops due to higher switching losses. In addition, some applications require very high duty cycles to drive a large number of LEDs from a low supply. Low switching frequency allows a greater operational duty cycle and hence a greater number of LEDs to be driven. In each case the switching frequency can be tailored to provide the optimum solution. When programming the switching frequency the total power losses within the IC should be considered. See "Thermal Calculations" in the Applications Information section.
10000 TA = 25C 1000
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Figure 3. Switching Frequency vs RT Resistor Value Table 4. Switching Frequencies vs RT Values
SWITCHING FREQUENCY (MHz) 2.25 1 0.2 RT (k) 9.09 31.6 200
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Programming Maximum LED current Maximum LED current can be programmed using the CTRL1 pin with CTRL2 tied to the VREF pin (see Figures 4 and 5). The maximum allowed LED current is defined as: (LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps (LT3478) Max LED Current = 0.1 Min(CTRL1, 1.05) * Amps RSENSE LED current vs CTRL1 is linear for approximately 0.1V < CTRL1 < 0.95V For maximum possible LED current, connect CTRL1 and CTRL2 to the VREF pin.
1400 TA = 25C CTRL2 = VREF (FOR LT3478 SCALE BY 0.1/RSENSE) 900 800 If FORWARD CURRENT (mA) 700 600 500 400 300 200 100 0 0 50 75 25 TA AMBIENT TEMPERATURE (C) 100 EXAMPLE LT3478-1 PROGRAMMED LED CURRENT DERATING CURVE LUXEON V EMITTER CURRENT DERATING CURVE
1050 LED CURRENT (mA)
LT3478-1 700
350 VREF 0 0 0.35 0.70 CTRL1 (V) 1.05 1.40
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Figure 4. LED Current vs CTRL1 Voltage
LT3478/LT3478-1 10 R2 13 12 R1 VREF CTRL2 CTRL1 LED
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(LT3478) RSENSE
VOUT
Figure 5. Programming LED Current
Programming LED Current Derating vs Temperature A useful feature of the LT3478/LT3478-1 is the ability to program a derating curve for maximum LED current versus temperature. LED data sheets provide curves of
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maximum allowed LED current versus temperature to warn against exceeding this current limit and damaging the LED (Figure 6).
Luxeon V (Maximum) and LT3478-1 (Programmed) Current Derating Curves vs Temperature
LUXEON V EMITTER (GREEN, CYAN, BLUE, ROYAL BLUE) JA = 20C/W
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Figure 6. LED Current Derating Curve vs Ambient Temperature
Without the ability to back off LED current as temperature increases, many LED drivers are limited to driving the LED(s) at only 50% or less of their maximum rated currents. This limitation requires more LEDs to obtain the intended brightness for the application. The LT3478/LT3478-1 allow the output LED(s) to be programmed for maximum allowable current while still protecting the LED(s) from excessive currents at high temperature. This is achieved by programming a voltage at the CTRL2 pin with a negative temperature coefficient using a resistor divider with temperature dependent resistance (Figures 7 and 8). CTRL2 voltage is programmed higher than CTRL1 voltage. This allows initial LED current to be defined by CTRL1. As temperature increases, CTRL2 voltage will fall below CTRL1 voltage causing LED currents to be controlled by CTRL2 pin voltage. The choice of resistor ratios and use of temperature dependent resistance in the divider for the CTRL2 pin will define the LED current curve breakpoint and slope versus temperature (Figure 8). A variety of resistor networks and NTC resistors with different temperature coefficients can be used for programming
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CTRL2 to achieve the desired CTRL2 curve vs temperature. The current derating curve shown in Figure 6 uses the resistor network shown in option C of Figure 7.
10 R2 R4 13 12 R1 R3 VREF LT3478/LT3478-1 CTRL2 CTRL1
OPTION A TO D
RY
RY
RNTC
RNTC
RX RNTC
RNTC
A
B
C
D
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Figure 7. Programming LED Current Derating Curve vs Temperature (RNTC Located on LEDs PCB)
1100 CTRL1, CTRL2 PIN VOLTAGES (mV) 1000 900 800 700 600 500 400 300 200 LED CURRENT = MINIMUM 100 OF CTRL1, CTRL2 R3 = OPTION C 0 0 25 50 75 TA AMBIENT TEMPERATURE (C) CTRL2 CTRL1
100
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Figure 8. CTRL1, 2 Programmed Voltages vs Temperature
Table 5 shows a list of manufacturers/distributors of NTC resistors. There are several other manufacturers available and the chosen supplier should be contacted for more detailed information. To use an NTC resistor to indicate LED temperature it is only effective if the resistor is connected as close as possible to the LED(s). LED derating curves shown by manufacturers are listed for ambient temperature. The NTC resistor should be submitted to the same ambient temperature as the LED(s). Since the temperature dependency of an NTC resistor can be nonlinear over a wide range of temperatures it is important
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to obtain a resistor's exact values over temperature from the manufacturer. Hand calculations of CTRL2 voltage can then be performed at each given temperature and the resulting CTRL2 curve plotted versus temperature. Several iterations of resistor value calculations may be required to achieve the desired breakpoint and slope of the LED current derating curve.
Table 5. NTC Resistor Manufacturers/Distributors
MANUFACTURER Murata Electronics North America TDK Corporation Digi-key
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www.murata.com www.tdk.com www.digikey.com
If calculation of CTRL2 voltage at various temperatures gives a downward slope that is too strong, alternative resistor networks can be chosen (B, C, D in Figure 7) which use temperature independent resistance to reduce the effects of the NTC resistor over temperature. Murata Electronics provides a selection of NTC resistors with complete data over a wide range of temperatures. In addition, a software tool is available which allows the user to select from different resistor networks and NTC resistor values and then simulate the exact output voltage curve (CTRL2 behavior) over temperature. Referred to as the `Murata Chip NTC Thermistor Output Voltage Simulator', users can log onto www.murata.com/designlib and download the software followed by instructions for creating an output voltage VOUT (CTRL2) from a specified VCC supply (VREF). At any time during selection of circuit parameters the user can access data on the chosen NTC resistor by clicking on a link to the Murata catalog. The following example uses hand calculations to derive the resistor values required for CTRL1 and CTRL2 pin voltages to achieve a given LED current derating curve. The resistor values obtained using the Murata simulation tool are also provided and were used to create the derating curve shown in Figure 6. The simulation tool illustrates the non-linear nature of the NTC resistor temperature coefficient at temperatures exceeding 50C ambient. In addition, the resistor divider technique using an NTC resistor to derive CTRL2 voltage inherently has a flattening characteristic (reduced downward slope) at higher temperatures. To avoid LED current exceeding a maximum
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allowed level at higher temperatures, the CTRL2 voltage curve may require a greater downward slope between 25C and 50C to compensate for that loss of slope at higher temperatures. Example: Calculate the resistor values required for generating CTRL1 and CTRL2 from VREF based on the following requirements: (a) ILED = 700mA at 25C (b) ILED derating curve breakpoint occurs at 25C (c) ILED derating curve has a slope of -200mA/25C between 25C and 50C ambient temperature Step1: Choose CTRL1 = 700mV for ILED = 700mA CTRL1 = VREF/(1 + R2/R1) R2 = R1 * [(VREF/CTRL1) - 1] For VREF = 1.24V and choosing R1 = 22.1k, R2 = 22.1k [(1.24/0.7) - 1] R2 = 17k (choose 16.9k) CTRL1 = 1.24/(1 + (16.9/22.1)) CTRL1 = 703mV (ILED = 703mA) Step 2: Choose resistor network option A (Figure 7) and CTRL2 = CTRL1 for 25C breakpoint start with R4 = R2 = 16.9k, RNTC = 22k (closest value available) CTRL2 = 701mV (ILED = Min(CTRL1, CTRL2) * 1A = 701mA) Step 3: Calculate CTRL2 slope between 25C and 50C CTRL2 (T) = 1.24/(1 + R4/RNTC (T)) at T = TO = 25C, CTRL2 = 701mV at T = 50C, RNTC (T) = RNTC (TO).ex, x = B [(1/(T + 273) - 1/298)] (B = B-constant; linear over the 25C to 50C temperature range) For RNTC B-constant = 3950 and T = 50C x = 3950 [(1/323) - 1/298] = -1.026
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RNTC (50C) = RNTC (25C).e-1.026 RNTC (50C) = 22k * 0.358 RNTC (50C) = 7.9k CTRL2(50C) = 1.24/(1 + 16.9/7.9) = 395mV CTRL2 slope (25C to 50C) = [CTRL2(50C) - CTRL2(25C)]/25C = (395 - 701)/25 = -306mV/25C ILED slope = -306mA/25C The required ILED slope is -200mA/25C. To reduce the slope of CTRL2 versus temperature it is easier to keep the exact same NTC resistor value and B-constant (there are limited choices) and simply adjust R4 and the type of resistor network used for the CTRL2 pin. By changing the resistor network to option C it is possible to place a temperature independent resistor in series with RNTC to reduce the effects of RNTC on the CTRL2 pin voltage over temperature. Step 4: Calculate the resistor value required for RY in resistor network option (c) (Figure 7) to provide an ILED slope of -200mA/25C between 25C and 50C ambient temperature. CTRL2 (25C) = 0.7V = 1.24/(1 + (R4/(RNTC(25C)+ RY)) R4 = 0.77 (RNTC(25C) + RY) (a) for -200mA/25C slope CTRL2(50C) = 0.7 - 0.2 = 0.5 CTRL2(50C) = 0.5V = 1.24/(1 + (R4/(RNTC + RY)) R4 = 1.48 (RNTC(50C) + RY) (b) Equating (a) = (b) and knowing RNTC(25C) = 22k and RNTC(50C) = 7.9k gives, 0.77 (22k + RY) = 1.48 (7.9k + RY) 17k + 0.77 RY = 11.7 k + 1.48 RY RY = (17k - 11.7k)/(1.48 - 0.77) RY = 7.5k
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The value for R4 can now be solved using equation (a) where, R4 = 0.77 (RNTC(25C) + RY) = 0.77 (22k + 7.5k) R4 = 22.7k (choose 22.6k) ILED slope can now be calculated from, ILED slope = [CTRL2(50C) - CTRL2(25C)]/25C where CTRL2 (50C) = 1.24/(1 + 22.6/(7.9 + 7.5)) = 503mV and CTRL2 (25C) = 1.24/(1 + 39.2/(22 + 28.7)) = 699mV giving ILED slope (from 25C to 50C) = 503mV - 699mV/25C = -196mV/25C => ILED slope = -196mA/25C Using the Murata simulation tool for the resistor network and values in the above example shows a CTRL2 voltage curve that flattens out as temperatures approach 100C ambient. The final resistor network chosen for the derating curve in Figure 6 used option C network with R4 = 19.3k, RNTC = 22k (NCP15XW223J0SRC) and RY = 3.01k. Although the CTRL2 downward slope is greater than -200mA/25C initially, the slope is required to avoid exceeding maximum allowed LED currents at high ambient temperatures (see Figure 6). PWM Dimming Many LED applications require an accurate control of the brightness of the LED(s). In addition, being able to maintain a constant color over the entire dimming range can be just as critical. For constant color LED dimming, the LT3478/LT3478-1 provide a PWM pin and special internal circuitry to allow up to a 3000:1 wide PWM dimming range. With an N-channel MOSFET connected between the LED(s) and ground and a PWM signal connected to the gate of the MOSFET and the PWM pin (Figure 9), it is possible to control the brightness of the LED(s) based on PWM signal duty cycle only. This form of dimming is superior to dimming control using an analog input voltage (reducing CTRL1 voltage) because it allows constant color to be maintained during dimming. The maximum current
VIN SHDN VREF CTRL2 CTRL1 OVPSET RT VC PWM LED D1 LT3478/ LT3478-1 (LT3478) RSENSE VS L SW VOUT
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for the output LED(s) is programmed for a given brightness/color and "chopped" over a PWM duty cycle range (Figure 10) from 100% to as low as 0.033%.
D2 COUT PWM DIMMING CONTROL
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Figure 9. PWM Dimming Control Using the LT3478/LT3478-1
TPWM TONPWM PWM
(= 1/fPWM)
INDUCTOR CURRENT
LED CURRENT
MAX ILED
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Figure 10. PWM Dimming Waveforms Using the LT3478/LT3478-1
Some general guidelines for LED Current Dimming using the PWM pin (see Figure 10): (1) PWM Dimming Ratio (PDR) = 1/(PWM duty cycle) = 1/(TONPWM * fPWM) (2) Lower fPWM allows higher PWM Dimming Ratios (use minimum fPWM = 100Hz to avoid visible flicker and to maximize PDR) (3) Higher fOSC value improves PDR (allows lower TONPWM) but will reduce efficiency and increase internal heating. In general, minimum operational TONPWM = 3 * (1/fOSC). (4) Lower inductor value improves PDR
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(5) Higher output capacitor value improves PDR (6) Choose the schottky diode (D2, Figure 9) for minimum reverse leakage See Typical Performance Characteristics graph "LED Current vs PWM Duty Cycle". Soft-Start To limit inrush current and output voltage overshoot during startup/recovery from a fault condition, the LT3478/ LT3478-1 provide a soft-start pin SS. The SS pin is used to program switch current ramp up timing using a capacitor to ground. The LT3478/LT3478-1 monitor system parameters for the following faults: VIN <2.8V, SHDN <1.4, inductor current >6A and boosted output voltage >OVP. On detection of any of these faults, the LT3478/LT3478-1 stop switching immediately and a soft-start latch is set causing the SS pin to be discharged (see Timing Diagram for the SS pin in Figure 11). When all faults no longer exist and the SS pin has been discharged to at least 0.25V, the soft-start latch is reset and an internal 12A supply charges the SS pin. A gradual ramp up of SS pin voltage is equivalent to a ramp up of switch current limit until SS exceeds VC. The ramp rate of the SS pin is given by: VSS/t = 12A/CSS
SW SS FAULTS TRIGGERING SOFT-START LATCH WITH SW TURNED OFF IMMEDIATELY: VIN < 2.8V OR SHDN < 1.4V OR VOUT > OVP OR I(INDUCTOR) > 6A
0.65V (ACTIVE THRESHOLD) 0.25V (RESET THRESHOLD) 0.15V SOFT-START LATCH RESET: SOFT-START LATCH SET: SS < 0.25V AND VIN > 2.8V AND SHDN > 1.4V AND VOUT < OVP AND I(INDUCTOR) < 6A
Figure 11. LT3478 Fault Detection and SS Pin Timing Diagram
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To limit inductor current overshoot to <0.5A when SS charges past the VC level required for loop control, the CSS capacitor should be chosen using the following formula: CSS(MIN) = CC (7.35 - 0.6(ILED * VOUT/VS)) Example: VS = 8V, VOUT = 16V, ILED = 1.05A, CC = 0.1F, CSS(MIN) = 0.1F (7.35 - 0.6(1.05 * 16/8)) = 0.612F (choose 0.68F). High Inductor Current "Inrush" Protection The LT3478/LT3478-1 provide an integrated resistor between the VS and L pins to monitor inductor current (Figure 1). During startup or "hotplugging" of the inductor supply, it is possible for inductor currents to exceed the maximum switch current limit. When inductor current exceeds 6A, the LT3478/LT3478-1 protect the internal power switch by turning it off and triggering a soft-start latch. This protection prevents the switch from repetitively turning on during excessive inductor currents by delaying switching until the fault has been removed. To defeat inductor current sensing the inductor supply should be connected to the L pin and the VS pin left open. See details in the Applications Information section "Soft-Start". LED Open Circuit Protection and Maximum PWM Dimming Ratios The LT3478/LT3478-1 LED drivers provide optimum protection from open LED faults by clamping the converter output to a programmable overvoltage protection level (OVP). In addition, the programmable OVP feature draws zero current from the output during PWM = 0 to allow higher PWM dimming ratios. This provides an advantage over other LED driver applications which connect a resistor divider directly from VOUT. An open LED fault occurs when the connection to the LED(s) becomes broken or the LED(s) fails open. For an LED driver using a step-up switching regulator, an open circuit LED fault can cause the converter output to exceed the voltage capabilities of the regulator's power switch, causing permanent damage. When VOUT exceeds OVP, the
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LT3478/LT3478-1 immediately stop switching, a soft-start latch is set and the SS pin is discharged. The SS latch can only be reset when VOUT falls below OVP and the SS pin has been discharged below 0.25V (Figure 11). If the LED(s) simply go open circuit and are reconnected, however, the OVP used to protect the switch might be too high for the reconnected LED(s). The LT3478/LT3478-1 therefore allow OVP to be programmable to protect both the LED driver switch and the LED(s). (The minimum allowable OVP for normal operation for a given LED string depends on the number of LEDs and their maximum forward voltage ratings.) OVP is programmed using the OVPSET pin (front page), given by, OVP = (OVPSET * 41)V where the programmable range for the OVPSET pin is 0.3V to 1V resulting in an OVP range of 12.3V to 41V. The OVPSET pin can be programmed with a single resistor by tapping off of the resistor divider from VREF used to program CTRL1. If both CTRL1 and CTRL2 are connected directly to VREF (maximum LED current setting) then OVPSET requires a simple 2 resistor divider from VREF. Thermal Calculations To maximize output power capability in an application without exceeding the LT3478/LT3478-1 125C maximum operational junction temperature, it is useful to be able to calculate power dissipation within the IC. The power dissipation within the IC comes from four main sources: switch DC loss, switch AC loss, Inductor and LED current sensing and input quiescent current. These formulas assume a boost converter architecture, continuous mode operation and no PWM dimming. (1) Switch DC loss = PSW(DC) = (RSW * IL(AVE)2 * D) RSW = switch resistance = 0.07 (at TJ = 125C) IL(AVE) = POUT/( * VS) POUT = VOUT * ILED = converter efficiency = POUT/(POUT + PLOSS)
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VS = inductor supply input D = switch duty cycle = (VOUT + VF - VS)/(VOUT + VF - VSAT) VF = forward voltage drop of external Schottky diode VSAT = IL(AVE) * RSW (2) Switch AC loss = PSW(AC) = tEFF(1/2)IL(AVE)(VOUT + VF)(FOSC) tEFF = effective switch current and switch VCE voltage overlap time during turn on and turn off = 2 * (tISW + tVSW) tISW = ISWITCH rise/fall time = IL(AVE) * 2ns tVSW = SW fall/rise time = (VOUT + VF) * 0.7ns fOSC = switching frequency (3) Current sensing loss = PSENSE = PSENSE(IL) + PSENSE(ILED) PSENSE(IL) = IL(AVE)2 * 9.5m PSENSE(ILED) = ILED2 * 0.1 (4) Input quiescent loss = PQ = VIN * IQ where IQ = (6.2mA + (100mA * D)) Example (Using LT3478-1): For VIN = VS = 8V, ILED = 700mA, VOUT = 24.5V (7 LEDs), VF = 0.5V and fOSC = 0.2Mhz, = 0.89 (initial assumption) IL(AVE) = (24.5 * 0.7)/(0.89 * 8) = 2.41A D = (24.5 + 0.5 - 8)/(24.5 + 0.5 - 0.17) = 0.684 TEFF = 2 * ((2.41 * 2)ns + (24.5 + 0.5) * 0.7)ns = 45ns Total Power Dissipation: PIC = PSW(DC) + PSW(AC) + PSENSE + PQ PSW(DC) = 0.07 * (2.41)2 * 0.684 = 0.278W PSW(AC) = 45ns * 0.5 * 2.41 * 25 * 0.2MHz = 0.271W PSENSE = ((2.41)2 * 0.0095) + ((0.7)2 * 0.1) = 0.104W PQ = 8 * (6.2mA + (100mA * 0.684)) = 0.597W PIC = 0.278 + 0.271 + 0.104 + 0.597 = 1.25W
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Local heating from the nearby inductor and Schottky diode will also add to the final junction temperature of the IC. Based on empirical measurements, the effect of diode and inductor heating on the LT3478-1 junction temperature can be approximated as: TJ (LT3478-1) = 5C/W * (PDIODE + PINDUCTOR) PDIODE = (1 - D) * VF * IL(AVE) 1 - D = 0.316 VF = 0.5V IL(AVE) = 2.41 PDIODE = 0.316 * 0.5 * 2.41 = 0.381W PINDUCTOR = IL(AVE)2 * DCR DCR = inductor DC resistance (assume 0.05) PINDUCTOR = (2.41)2 * 0.05 = 0.29W The LT3478/LT3478-1 use a thermally enhanced FE package. With proper soldering to the Exposed Pad on the underside of the package combined with a full copper plane underneath the device, thermal resistance (JA) will be about 35C/W. For an ambient temperature of TA = 70C, the junction temperature of the LT3478-1 for the example application described above, can be calculated as: TJ (LT3478-1) = TA + JA(PTOT) + 5(PDIODE + PINDUCTOR) = 70 + 35(1.25) + 5(0.671) = 70 + 44 + 4 = 118C In the above example, efficiency was initially assumed to be = 0.89. A lower efficiency () for the converter will increase IL(AVE) and hence increase the calculated value for TJ. can be calculated as: = POUT/(POUT + PLOSS) POUT = VOUT * ILED = 17.15W PLOSS (estimated) = PIC + PDIODE + PINDUCTOR = 1.92W = 17.15/(17.15 + 1.92) = 0.9
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If an application is built, the inductor current can be measured and a new value for junction temperature estimated. Ideally a thermal measurement should be made to achieve the greatest accuracy for TJ. Note: The junction temperature of the IC can be reduced if a lower VIN supply is available - separate from the inductor supply VS. In the above example, driving VIN from an available 3V source (instead of VS = 8V) reduces input quiescent losses in item(4) from 0.597W to 0.224W, resulting in a reduction of TJ from 118C to 105C. 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 (Figure 12). The exposed pad of the LT3478/LT3478-1 (Pin 17) is the only GND connection for the IC. The exposed pad should be soldered to a continuous copper ground plane underneath the device to reduce die temperature and maximize the power capability of the IC. The ground path for the RT resistor and VC capacitor should be taken from nearby the analog ground connection to the exposed pad (near Pin 9) separate from the power ground connection to the exposed pad (near Pin 16). The bypass capacitor for VIN should be placed as close as possible to the VIN pin and the analog ground connection. SW pin voltage rise and fall times are designed to be as short as possible for maximum efficiency. To reduce the effects of both radiated and conducted noise, the area of the SW trace should be kept as small as possible. Use a ground plane under the switching regulator to minimize interplane coupling. The schottky diode and output capacitor should be placed as close as possible to the SW node to minimize this high frequency switching path. To minimize LED current sensing errors for the LT3478, the terminals of the external sense resistor RSENSE should be tracked to the VOUT and LED pins separate from any high current paths.
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CVS CVIN VS VOUT VIN SCHOTTKY DIODE OUTPUT CAPACITOR
SW INDUCTOR L RSENSE (LT3478 ONLY)
Figure 12. Recommended Layout for LT3478/LT3478-1 (Boost Configuration)
TYPICAL APPLICATIO S
15W, 6 LEDs at 700mA, Boost LED Driver
L1 10H VIN 8V TO 16V C1 4.7F 25V D1 C2 10F 25V PWM 5V/DIV LT3478-1 LED 700mA INDUCTOR CURRENT 1A/DIV ILED 0.5A/DIV SS VC CC 0.1F RT RT 69.8k 2s/DIV PWM DIMMING RATIO = 1000:1 (SEE EFFICIENCY ON PAGE 1)
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VIN SHDN VREF
VS
L
SW OUT
R1 45.3k R4 54.9k
CTRL2 OVPSET
CTRL1 R2 130k L1: CDRH104R-100NC D1: PDS560 Q1: Si2318DS LEDs: LUXEON III (WHITE) 3.3V 0V 100Hz PWM DIMMING RATIO = 1000:1 R3 10k Q1 PWM CSS 1F
fOSC = 500kHz
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(CONNECT MULTIPLE GROUND PLANES THROUGH VIAS UNDERNEATH THE IC) GND LT3478/LT3478-1 SW SW VIN VS L VOUT LED OVPSET R R 1 2 3 4 5 6 POWER GND SOLDER THE EXPOSED PAD (PIN 17) TO THE ENTIRE COPPER GROUND PLANE UNDERNEATH THE DEVICE 16 SS 15 RT 14 PWM 13 CTRL2 12 CTRL1 11 SHDN CSS RT R R R R EXPOSED PAD 7 10 VREF PIN 17 8 ANALOG GND 9 VC C VIN BYPASS CAP CF RC CC
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LT3478-1 PWM Dimming Waveforms
fPWM = 100Hz
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17W, 15 LEDs at 350mA, Boost LED Driver plus LT3003
VS 8V TO 14V C1 4.7F 16V C3 3.3F 10V
VIN 3.3V
VIN SHDN VREF
VS
LED
CTRL1 OVPSET
EFFICIENCY (%)
R1 24k
CTRL2
R2 100k
PWM CSS 1F
L1: CDRH104R-5R2 D1: PDS560 LEDs: LUXEON I (WHITE)
3.3V 0V VOUT 100Hz PWM DIMMING RATIO = 3000:1 VIN
16W, 12 LEDs at 350mA, Buck-Boost Mode LED Driver plus LT3003
VS 12V TO 16V C1 4.7F 25V C3 3.3F 10V
VIN 5V
VIN SHDN VREF
VS
LED
EFFICIENCY (%)
R1 24k
CTRL2 CTRL1 OVPSET
R2 100k
PWM CSS 1F
SS
L1: CDRH105R-8R2 D1: PDS560 D2: 7.5V ZENER LEDs: LUXEON I (WHITE) 3.3V 0V 100Hz PWM
DIMMING RATIO = 200:1
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SS
L1 5.2H
D1
VOUT C2 3.3F 25V 90
Efficiency vs Input VS
VIN = 3.3V ILED = 350mA fOSC = 1MHz PWM DUTY CYCLE = 100%
L
SW OUT
85 LT3478-1 1.05A
80
VC VC CC 0.1F
RT RT 31.6k
75 15 LEDs (5 SERIES x 3 CHANNELS) LUXEON I (WHITE) 8 LED1 VMAX VIN PWM LED2 LT3003 GND VEE LED3 OT1 OT2 VC 10 VS (V) 12 14
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fOSC = 1MHz
70
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L1 8.2H
D1
VOUT C2 10F 50V 90 85 80
Efficiency vs Input VS
VIN = 5V ILED = 350mA fOSC = 500kHz PWM DUTY CYCLE = 100%
L
SW OUT
LT3478-1
1.05A
75 70 65 60
VC VC CC 0.1F C4 1F
RT RT 69.8k
55 50 12 13
12 LEDs (4 SERIES x 3 CHANNELS) LUXEON I (WHITE) 14 VS (V) 15 16
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fOSC = 500kHz D2 VOUT LED1 VMAX VIN PWM LED2 LT3003 GND VEE LED3 OT1 OT2 VC
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LT3478/LT3478-1 TYPICAL APPLICATIO S
4W, 1 LED at 1A, Buck-Boost Mode LED Driver
VIN 3.8V TO 6.5V NiMH 4x C1 10F 10V VIN ON OFF SHDN VREF R1 100k CTRL2 CTRL1 OVPSET R2 L1: CDRH105R-6R8 34k D1: B320 Q1: Si2302ADS Q2: Si2315BDS LED: LUXEON III (WHITE) 3.3V 0V 1kHz PWM DIMMING RATIO = 200:1 R3 10k PWM CSS 1F SS VC RT RT 69.8k 55 R5 510 50 3 Q1 4 SINGLE LED LUXEON III (WHITE) 5 VIN (V) 6 7
3478 TA06b
VS
LT3478-1
LED
1A R4 510
Q2
EFFICIENCY (%)
U
L1 6.8H
D1 C2 4.7F 16V 80
Efficiency vs VIN
ILED = 1A fOSC = 500kHz 75 PWM DUTY CYCLE = 100% 70 65 60
L
SW OUT
CC 0.1F
fOSC = 500kHz
3478 TA06a
34781f
21
LT3478/LT3478-1 TYPICAL APPLICATIO S
24W, 4 LEDs at 1.5A, Buck Mode LED Driver
PVIN 32V C1 3.3F 50V RSENSE 0.068 1.5A
TYPICAL EFFICIENCY = 90% FOR CONDITIONS/COMPONENTS SHOWN (PWM DUTY CYCLE = 100%, TA =25C) Q2
VIN 3.3V
L1: CDRH105R-100 D1: PDS560 Q1: 2N7002 Q2: Si2319DS LEDs: LXK2 (WHITE)
22
U
4 LEDs R4 365 C3 10F 25V
L1 10H
R5 510
C2 4.7F 10V
D1 VIN SHDN VS L OUT LED SW PWM R3 10k LT3478 PWM DIMMING RATIO = 3000:1 3.3V SS CSS 1F VC RT 0V RT 69.8k 100Hz Q1
VREF R1 24k CTRL2 CTRL1 OVPSET R2 100k
CC 0.1F fOSC = 500kHz
3478 TA07a
34781f
LT3478/LT3478-1 PACKAGE DESCRIPTIO U
FE Package 16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation BC
3.58 (.141) 4.90 - 5.10* (.193 - .201) 3.58 (.141) 16 1514 13 12 1110 6.60 0.10 4.50 0.10
SEE NOTE 4
9
2.94 (.116) 0.45 0.05 1.05 0.10 0.65 BSC
6.40 2.94 (.252) (.116) BSC
RECOMMENDED SOLDER PAD LAYOUT
12345678 1.10 (.0433) MAX
0 - 8
4.30 - 4.50* (.169 - .177)
0.25 REF
0.09 - 0.20 (.0035 - .0079)
0.50 - 0.75 (.020 - .030)
0.65 (.0256) BSC
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE
0.195 - 0.30 (.0077 - .0118) TYP
0.05 - 0.15 (.002 - .006)
FE16 (BC) TSSOP 0204
4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE
34781f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LT3478/LT3478-1 TYPICAL APPLICATIO
VS 8V TO 16V C1 4.7F 25V C3 3.3F 10V
VIN 3.3V
VIN SHDN VREF
VS
LED
250mA
EFFICIENCY (%)
R1 8.25k
CTRL2 CTRL1 OVPSET
R2 10k
PWM CSS 1F
L1: CDRH6D28 D1: ZLLS1000 Q1: Si2318DS LEDs: LUXEON I (WHITE) 3.3V 0V 100Hz PWM
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 LED Ballaster with 3,000:1 True Color PWM Dimming 36V, 1A (ILED), 2MHz,Step-Down LED Driver with 400:1 True Color PWM Dimming Dual 1.5A(ILED), 36V, 2MHz,Step-Down LED Driver 3,000:1 True Color PWM Dimming Quad Output 1.5A, 2MHz High Current LED Driver with 1,000:1 True Color PWM Dimming 42V, 3A, 3.5MHz Boost, Buck-Boost, Buck LED Driver with Analog/ PWM Dimming 3A, 3.5MHz Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection and Analog/PWM Dimming Dual 1.3A , 2MHz High Current LED Driver with 1,000:1 True Color PWM Dimming High Current LED Controller with 3,000:1 True Color PWM Dimming COMMENTS VIN: 5V to 18V, VOUT(MAX) = 36V, ISD <1A, MS10 Package VIN: 3V to 48V, ISD <5A, MSOP10 Package VIN: 4V to 36V, VOUT(MAX) = 13.5V, ISD <1A, TSSOP16E Package VIN: 4V to 36V, VOUT(MAX) = 13.5V, ISD <1A, TSSOP20E Package VIN: 2.8V to 16V, VOUT(MAX) = 36V, ISD <10A, 5mm x 7mm QFN Package VIN: 2.5V to 25V, VOUT(MAX) = 40V, ISD <1A, QFN, TSSOP20E Packages VIN: 2.5V to 24V, VOUT(MAX) = 40V, ISD <1A, 4mm x 3mm DFN, TSSOP16E Packages VIN: 2.5V to 24V, VOUT(MAX) = 36V, ISD <1A, 5mm x 3mm DFN, TSSOP16E Packages VIN: 3V to 36V, VOUT(MAX) = Ext FET, ISD <20A, 5mm x 4mm DFN, TSSOP16E Packages
24 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2007
U
6W, 6 LEDs at 250mA, Boost LED Driver
L1 10H D1 C2 3.3F 25V RSENSE 0.42 LT3478
Efficiency vs Input VS
100 VIN = 3.3V ILED = 250mA 95 fOSC = 2MHz PWM DUTY CYCLE = 100% 90 85 80 75 70
L
SW OUT
SS
VC
RT RT 10k
CC 0.1F
65 60 8 10 6 LEDs = LUXEON I (WHITE) 12 VS (V) 14 16
3478 TA05b
fOSC = 2MHz
Q1 R3 10k
3478 TA05a
34781f LT 0107 * PRINTED IN USA

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