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LT3754 16-Channel x 50mA LED Driver FEATURES
n n n n n n n n n n n n n n
DESCRIPTION
The LT(R)3754 is a 16-channel LED driver with a step-up DC/DC controller capable of driving up to 45V of LEDs. Each channel contains an accurate current sink with 2.8% current matching. Channels follow a master programmable current to allow between 10mA to 50mA of LED current per string. Channels can be paralleled for higher LED current. Output voltage adapts to variations in LED VF for optimum efficiency and open LED faults do not affect the operation of connected LED strings. The LT3754 allows a PWM dimming range up to 3000:1 and an analog dimming range up to 25:1. Operating frequency can be programmed from 100kHz up to 1MHz using a single resistor or synchronized to an external clock. Additional features include: programmable maximum VOUT for open LED protection, a fault flag for open LED, programmable LED current derating vs temperature, micropower shutdown and internal soft-start. The LT3754 is available in a thermally enhanced 5mm x 5mm 32-pin QFN Package.
Up to 45V of LEDs x 50mA, 16-Channel LED Driver Wide Input Range : 6V to 40V 2.8% LED Current Matching at 20mA (Typ 0.7%) Up to 3000:1 True Color PWMTM Dimming Range Single Resistor Sets LED Current (10mA to 50mA) LED Current Regulated Even for PVIN > VOUT Output Adapts to LED VF for Optimum Efficiency Fault Flag + Protection for Open LED Strings Protection for LED Pin to VOUT Short Parallel Channels for Higher LED Current Programmable LED Current Derating vs Temperature Accurate Undervoltage Lockout Threshold with Programmable Hysteresis Programmable Frequency (100kHz to 1MHz) Synchronizable to an External Clock
APPLICATIONS
n
Automotive, Notebook and TV Monitor Backlighting
L, LT, LTC and LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 7199560, 7321203.
TYPICAL APPLICATION
92% Efficient, 36W Backlight LED Driver
PVIN 24V VIN 10V 4.7F 10H 5 2.2F **** 0.015 VOUT LT3754 LED1 LED2 * * * * LED15 LED16 FAULT OVPSET GND RT 20k 39.2k 5.76k 10k 2.2nF ISET VC SYNC * 16 CHANNELS * * * VIN
3754 TA01
Worst-Case Channels LED Current Matching (Normalized to 16-channel Average)
0.8
UP TO 45V OF LEDs PER STRING LED CURRENT MATCHING (%)
4.7F INTVCC 499k 4.7F SENSE SHDN/UVLO 40.2k CTRL PWM REF 20k TSET 30.9k 11k VIN GATE
0.4
0.0
* * *
* * *
* *
* *
-0.4
RISET = 14.7k (I(LED) = 20mA) -0.8 0 25 50 75 100 -50 -25 JUNCTION TEMERATURE (C)
125
3754 TA01
100k
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LT3754 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW OVPSET PWM CTRL TSET ISET REF VC RT 24 LED16 23 LED15 22 LED14 33 21 LED13 20 LED12 19 LED11 18 LED10 17 LED9 9 10 11 12 13 14 15 16 GATE INTVCC VIN SYNC SENSE SHDN/UVLO FAULT VOUT
VOUT, LED1-16 .........................................................60V VIN, SHDN/UVLO, FAULT ...........................................40V INTVCC ......................................................................13V INTVCC above VIN ...................................................+0.3V PWM, CTRL, SYNC .....................................................6V VC ...............................................................................3V VREF , RT, ISET, TSET, OVPSET .......................................2V SENSE......................................................................0.4V Operating Junction Temperature Range (Notes 2,3) ...........................................-40 C to 125 C Storage Temperature Range.................-65 C to 150 C
32 31 30 29 28 27 26 25 LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 1 2 3 4 5 6 7 8
UH PACKAGE 32-LEAD (5mm 5mm) PLASTIC QFN TJMAX = 125C, JA = 34C/W, JC = 3C/W EXPOSED PAD (PIN 33) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3754EUH#PBF LT3754IUH#PBF TAPE AND REEL LT3754EUH#TRPBF LT3754IUH#TRPBF PART MARKING* 3754 3754 PACKAGE DESCRIPTION 32-Lead (5mm x 5mm) Plastic QFN 32-Lead (5mm x 5mm) Plastic QFN TEMPERATURE RANGE -40C to 125C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LT3754 ELECTRICAL CHARACTERISTICS
PARAMETER INPUT BIAS, REFERENCE Minimum Operational VIN (To Allow GATE Switching) VC = 1.5V VIN = INTVCC (Shorted) VIN INTVCC VIN = INTVCC (Shorted) VIN INTVCC CTRL = 0.1V, PWM = 0V CTRL = 0.1V, PWM = 1.5V, (Not Switching) LED1-16 = 1.2V SHDN/UVLO = 0V, VIN =6V SHDN/UVLO = 0V, VIN = 40V SHDN/UVLO = 0V, VIN = INTVCC = 4.5V SHDN/UVLO = 0V, VIN = INTVCC = 13V IVIN < 20A
l l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VOUT = 6V, RISET = 14.7k unless otherwise noted.
CONDITIONS MIN TYP MAX UNITS
4.2 5.5 4.5 6 4.2 9.5 0.1 10 20 0.3 1.414 1.6 1.450 0.7 1.476 2.4 0 1.485 0.01 2
4.5 6.0 14 40 5.7 12 2 10 20 40 1.538 3.2 1.524 0.05
V V V V mA mA A A A A V V A A V %/V mV
Operational VIN VIN Quiescent Current
VIN Shutdown Current (VIN INTVCC) (Not Shorted) VIN Shutdown Current (VIN = INTVCC (Shorted)) SHDN/UVLO Threshold (Micropower) (Falling) (VSD) SHDN/UVLO Threshold (UVLO) (Falling) (Stop Switching) (VUV) SHDN/UVLO Pin Current VREF Voltage VREF Line Regulation VREF Load Regulation OSCILLATOR Frequency: fOSC (100kHz) Frequency: fOSC (1MHz) fOSC (1MHz) Line Regulation RT Pin Voltage Minimum Off-Time Minimum On-Time SYNC Input High Threshold SYNC Input Low Threshold SYNC Input Current SYNC Frequency Range LINEAR REGULATOR (INTVCC) INTVCC Regulation Voltage Dropout (VIN - INTVCC) INTVCC UVLO (+) INTVCC UVLO (-) INTVCC Current Limit OVP/ LED ERROR AMPLIFIERS Transconductance (OVP) Voltage Gain (OVP) Transconductance (LED)
SHDN/UVLO = VUV - 50mV SHDN/UVLO = VUV + 50mV IVREF = 0A IVREF = 0A, 6V < VIN < 40V 0 < IVREF < 150A (Max) RT = 523k RT = 39.2k RT = 39.2k, 6V < VIN < 40V RT = 39.2k (Note 5) (Note 5)
l l
l l
92 0.90
101 1 0.1 1.6 170 190
112 1.10 0.2 250 250 2.2
kHz MHz %/V V nS nS V V A A
0.6 SYNC = 0V SYNC = 5V RT = 523k RT = 39.2k VIN = 12V IINTVCC = 10mA (Start Switching) (Stop Switching)
l
0 25 0.12 1.2 6.65 7 250 3.8 3.4 44 57 4 5 33 1.5 1.5 7.35
MHz MHz V mV V V mA mhos V/V mhos
IVC = 2.5A IVC = 2.5A
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LT3754 ELECTRICAL CHARACTERISTICS
PARAMETER Voltage Gain (LED) VC Source Current (Out of Pin) VC Sink Current (OVP) VC Sink Current (LED) VC Output High (clamp) (VCOH) VC Output Low (clamp) (VCOL) VC Switching Threshold (VCSW) SENSE AMP SENSE Input Current (Out of Pin) SENSE Current Limit Threshold Current Mode Gain SENSE Over Current Limit Threshold LED CURRENT / CONTROL ISET Pin Voltage LEDx Current (20mA) (RISET = 14.7k) LEDx Current Matching (20mA) (RISET = 14.7k) LEDx Current (50mA) (RISET = 5.76k) LED Pin Regulation Voltage TSET Threshold ANALOG DIMMING CTRL Input Current (Out of Pin) LEDx Current (Dimming 25:1) PWM DIMMING PWM Input Low Threshold PWM Input High Threshold PWM Input Current VOUT Pin Current in PWM Mode V(VOUT) = 60V LEDx Leakage Current (PWM = 0V) FAULT DIAGNOSTICS FAULT Output Sink Current LEDx Short Threshold (VSH) (VOUT - VLEDx) LED Open Detection Threshold LED1 = Open, VFAULT = 0.3V VOUT = 12V VOUT = 60V VOUT = 12V 0.3 0.6 6 6 0.5 mA V V PWM = 1.5V PWM = 6V PWM = 1.5V, VLEDx = 1V PWM = 0V, VLEDx = 1V VLEDx = 1V, VOUT = 12V VLEDx = 50V, VOUT = 60V 0.7 1 1.1 6 24 370 20 0.1 0.1 1 2 1.4 V V A A A A A A CTRL = 1V CTRL = 0.04V VLEDx = 1V, CTRL = 0.04V 40 50 0.8 200 200 nA nA mA CTRL = 1.5V VLEDx = 1V, CTRL = 1.5V VLEDx = 1V, CTRL = 1.5V VLEDx = 1V, CTRL = 0.04V
l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VOUT = 6V, RISET = 14.7k unless otherwise noted.
CONDITIONS VC = 1.5V, VLEDx = 0.8V, OVPSET = 1.5V VC = 1.5V, VLEDx = 0.8V, OVPSET = 0V VC = 1.5V, VLEDx = 1.2V, OVPSET = 1.5V MIN TYP 45 10 15 9 2.3 0.8 1.1 65 46 90 52 6
l
MAX
UNITS V/V A A S V V V A
60 110
mV V/V mV V
V(VC)/V(SENSE)
100 1.00
19.29 47.85
20.2 0.7 50.1 1.1 630
21.11 2.8 52.35
mA % mA V mV
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LT3754 ELECTRICAL CHARACTERISTICS
PARAMETER GATE DRIVER GATE Driver Output Rise Time GATE Driver Output Fall Time GATE Output Low GATE Output High OUTPUT VOLTAGE VOUT Over Voltage Protection (OVP) Regulation Voltage OVPSET Input Current (Out of Pin) OVPSET = 0.22V OVPSET = 1V OVPSET = 0.22V, VOUT =12V 12.5 57 40 200 V V nA VIN = 12V, CL = 3300pF (Note 4) VIN = 12V, CL = 3300pF (Note 4) IGATE= 0A INTVCC = VIN = 7V IGATE = 0A 6.95 30 30 0.1 nS nS V V
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VOUT = 6V, RISET =14.7k unless otherwise noted.
CONDITIONS MIN TYP MAX UNITS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3754E is guaranteed to meet performance specifications from 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
LT3754I is guaranteed to meet performance specifications from -40C to 125C junction temperature. Note 3: For Maximum Operating Ambient Temperature, see Thermal Calculations in the Applications Information section. Note 4: GATE rise and fall times are measured between 10% and 90% of INTVCC voltage. Note 5: See Duty Cycle Considerations in the Applications Information.
TYPICAL PERFORMANCE CHARACTERISTICS
Worst-Case Channels LED Current Matching (Normalized to 16-channel Average)
0.8 21.00 RISET = 14.7k LED CURRENT MATCHING (%)
TA = 25C, unless otherwise noted. LED Current vs CTRL Pin Voltage
55 50 45 40 35 30 25 20 15 10 5 RISET = 5.76k 7.32k 9.76k 14.7k 29.4k
LED Current vs Junction Temperature
0.4
20.50 LED CURRENT (mA) LED CURRENT (mA) 25 75 100 -25 0 50 JUNCTION TEMERATURE (C) 125
0.0
20.00
-0.4
19.50
RISET = 14.7k (I(LED) = 20mA) -0.8 0 25 100 -50 -25 50 75 JUNCTION TEMERATURE (C)
125
19.00 -50
0 0.00
0.25
0.50
0.75 1.00 CTRL (V)
1.25
1.50
3754 G03
3754 G01
3754 G02
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LT3754 TYPICAL PERFORMANCE CHARACTERISTICS
LED Current Waveforms 3000:1 PWM Dimming (100Hz)
1.525 I(LEDx) 20mA/DIV 1.505 VREF VOLTAGE (V)
TA = 25C, unless otherwise noted. SHDN/UVLO Threshold vs Junction Temperature
1.525
VREF vs Junction Temperature
I(L) 1A/DIV PWM 10V/DIV 5s/DIV
3754 G04
1.485
1.465
1.445 -50
SHDN/UVLO PIN VOLTAGE (V)
1.505
1.485
1.465
-25 0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
1.445 -50
-25 0 25 50 75 100 JUNCTION TEMERATURE (C)
125
3754 G05
3754 G06
SHDN/UVLO Pin (Hysteresis) Current vs Junction Temperature
2.80 SHDN/UVLO PIN CURRENT (A) 2.70 2.60 2.50 2.40 2.30 2.20 -50 VIN CURRENT (A) 5
VIN Shutdown Current vs Junction Temperature
VIN = 6V, SHDN/UVLO = 0V 12 10 VIN CURRENT (mA) 8 6 4 2 0
VIN Quiescent Current vs VIN
RISET = 14.7k PWM = 1.5V, NO SWITCHING, V(LED1-16) = 1.2V, CTRL = 0.1V
4
3
2
PWM = 0V, CTRL = 0.1V
1
-25 0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
0 -50
0 25 50 75 100 -25 JUNCTION TEMPERATURE (C)
125
0
5
10
15
20 25 VIN (V)
30
35
40
3754 G07
3754 G08
3754 G09
VIN Quiescent Current vs Junction Temperature
15 VIN = 6V, RISET = 14.7k, CTRL = 0.1V SWITCHING FREQUENCY (kHz) 1050 1100
Switching Frequency vs Junction Temperature
2.5
VC High Clamp, Active and Low Clamp Levels vs Junction Temperature
VC HIGH CLAMP 2.0 VC PIN VOLTAGE (V)
VIN CURRENT (mA)
10 PWM = 1.5V, NO SWITCHING, V(LED1-16) = 1.2V, CTRL = 0.1V 5 PWM = 0V, CTRL = 0.1V
1.5
1000 RT = 39.2k
VC ACTIVE (SWITCHING)
1.0 VC LOW CLAMP 0.5
950
0 -50 -25
0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
900 -50 -25
0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
0.0 -50 -25
0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
3754 G10
3754 G11
3754 G12
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LT3754 TYPICAL PERFORMANCE CHARACTERISTICS
INTVCC vs Current, Junction Temperature
7.0 ILOAD = 10mA, 20mA, 30mA 6.0
TA = 25C, unless otherwise noted. INTVCC, UVLO(+), UVLO(-) vs Junction Temperature
8 7 VIN = 12V INTVCC (REGULATED)
INTVCC vs Current, Junction Temperature
VIN = 6V, PWM = 0V
6.9 5.5 INTVCC (V) INTVCC (V) INTVCC (V) 6.8 ILOAD = 40mA 6.7 6 5 4 ILOAD = 10mA ILOAD = 20mA ILOAD = 30mA ILOAD = 40mA 125 4.5 -50 -25 0 25 50 75 100 JUNCTION TEMPERATURE (C) 125 3 2 -50 -25 INTVCC UVLO(+) INTVCC UVLO(-)
5.00
VIN = 8V, PWM = 0V 6.6 -50 -25 0 25 50 75 100 JUNCTION TEMPERATURE (C)
0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
3754 G13
3754 G14
3754 G15
INTVCC Current Limit vs Junction Temperature
60 VIN = 6V, INTVCC = 0V 57.5 INTVCC CURRENT (mA) 55 SENSE PIN VOLTAGE (mV) 55.0 52.5 50.0 47.5 45.0 42.5 40 -50 -25 0 25 50 75 100 JUNCTION TEMPERATURE (C) 125 60.0
SENSE Threshold vs Junction Temperature
70 60 50 OVP (V) 40 30 20
Overvoltage Protection (OVP) Level vs Junction Temperature
OVPSET = 1.0V
50
INDUCTOR PEAK CURRENT THRESHOLD (CYCLE-BY-CYCLE)
45
OVPSET = 0.22V 10 0 -50
40.0 -50
-25 0 25 50 75 100 JUNCTION TEMPERATURE (C)
125
0 25 50 75 100 -25 JUNCTION TEMPERATURE (C)
125
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3754 G17
3754 G18
VOUT - V(LEDx) Short Threshold vs Junction Temperature
7.00 6.75 SHORT THRESHOLD (V) 6.50 TIME (ns) 6.25 6.00 5.75 5.50 5.25 5.00 -50 0 25 50 75 100 -25 JUNCTION TEMPERATURE (C) 125 125 VOUT = 12V VOUT = 60V 200 175 250
Minimum ON and OFF Times vs Junction Temperature
120 CGATE = 3300pF 225 MINIMUM ON-TIME TIME (ns) 100 80
GATE Rise/Fall Times vs GATE Capacitance
VIN = 8V, INTVCC = 7V, RT = 523k
FALL TIME 60 40 20 0 RISE TIME
MINIMUM OFF-TIME 150
100 -50
0 25 50 75 100 -25 JUNCTION TEMPERATURE (C)
125
0
5
10 CL (nF)
15
20
3754 G21
3754 G18
3754 G20
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LT3754 PIN FUNCTIONS
LEDx (Pin 1-8,17-24): 16 LED Driver Outputs. Each output contains an open collector constant current sink. LED currents are programmable from 10mA to 50mA using a single resistor at the ISET pin. Channel matching is 2% with an absolute current accuracy of 3%. Connect the cathode of each LED string to an LED pin. Connect the anode of each LED string to VOUT. Channels can be paralleled for greater LED current or individually disabled (connect LED to VOUT). SENSE (Pin 9): The Current Sense Input for the Control Loop. Connect this pin to the sense resistor in the source of the external power MOSFET. GATE (Pin 10): Drives the gate of an N-channel MOSFET from 0V to INTVCC. INTVCC (Pin 11): A 7V LDO supply generated from VIN and used to power the GATE driver and some control circuitry. Must be bypassed with a 4.7F capacitor to GND. VIN (Pin 12): Input Supply Pin. Must be locally bypassed with a 1F capacitor to ground. SHDN/UVLO (Pin 13): The SHDN/UVLO pin has an accurate 1.476V threshold and can be used to program an under voltage lockout (UVLO) threshold for system input supply using a resistor divider from supply to ground. A 2.4A pin current hysteresis allows programming of UVLO hysteresis. SHDN/UVLO above 1.476V turns the part on and removes a 2.4A sink current from the pin. SHDN/UVLO < 0.7V reduces VIN current < 20A. If the shutdown function is not required, it should be forced above 1.476V or connected directly to VIN. FAULT (Pin 14): Active low if any or all LED strings have an open fault or if any/all LED pins have been shorted to VOUT. If fault(s) removed, FAULT flag returns high. Fault status is only updated during PWM high state and latched during PWM low. SYNC (Pin 15): Allows synchronization of boost converter switching frequency to an external clock. RT resistor should be programmed for fOSC 20% below SYNC frequency. If unused, connect to GND. VOUT (Pin16): Boosted Output Voltage of the Converter. Connect a capacitor from this pin to ground. Connect the anode of each LED (string) to VOUT.
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RT (Pin 25): A resistor to ground programs switching frequency fOSC between 0.1MHz and 1MHz. VC (Pin 26): Output of Both Transconductance Error Amplifiers for the Converter Regulation Loop. The most commonly used gm error amplifier (LED) regulates VOUT to ensure no LED pin falls below 1V. The other gm error amplifier (OVP) is activated if all LEDs fail open and a regulated maximum VOUT is required. Connect a resistor and capacitor in series from the VC pin to ground. PWM (Pin 27): Input Pin for PWM Dimming Control. Above 1.4V allows converter switching and below 0.7V disables switching. The PWM signal can be driven from 0V to 6V. If unused, connect to VREF. OVPSET (Pin 28): Programs maximum allowed VOUT regulation level if all LEDs are open circuit. CTRL (Pin 29): CTRL pin voltage below 1V controls maximum LED current. CTRL voltage can be set by a resistor divider from VIN, VREF or an external voltage source. LED current derating versus temperature is achievable if the voltage programmed at the CTRL pin has a negative temperature coefficient using an external resistor divider from VREF pin with temperature dependent resistance. TSET (Pin 30): Programs LT3754 junction temperature breakpoint past which LED current will begin to derate. VREF (Pin 31): 1.485V Reference Output Pin. This pin can supply up to 150A. Can be used to program CTRL, TSET and OVPSET pin voltages using resistor dividers to ground. ISET (Pin 32): Resistor to Ground Programs LED pin current. See Table 6 in the Applications Information Section. Exposed Pad (Pin 33): GND. The ground for the IC and the converter. The package has an exposed pad (Pin 33) underneath the IC which is the best path for heat out of the package. Pin 33 should be soldered to a continuous copper ground plane under the device to reduce die temperature and increase the power capability of the LT3754.
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LT3754 BLOCK DIAGRAM
13 SHDN/UVLO 600k 1.476V 12 11 VIN INTVCC 7V(REGULATED) UVLO( ) = 3.8V, UVLO( ) = 3.4V R S VC Q EN SLOPE OSC 52mV PEAK CURRENT GATE SYNC RT 25 REF 1.485V
+ -
10 15
-+ + -
INTVCC_UV VIN_UV SHDN_UV 27 31 PWM VREF EN 1.485V 4.2V( ) 3.7V( )
-+
EN
HICCUP__MODE 6V SOFT START LED LOGIC
+ -
EN
29
CTRL
1V
+ + + -
SS LED CURRENT CONTROL
PWM 56R
CHANNEL X
1.1V
-+
VPTAT 30 TSET 32 ISET 33 EXPOSED PAD (GND) 26 VC
Figure 1. LT3754 Block Diagram
OPERATION
The operation of the LT3754 is best understood by referring to the typical application circuit on the front page and the Block Diagram in Figure 1. The LT3754 drives 16 strings of LEDs by using a constant switching frequency, current mode boost controller to generate a single output voltage VOUT for the top (anode) of all LED strings. LED string current is generated and controlled by connection of the bottom LED in each string (cathode) to a current source contained in each corresponding LED pin. Each LED pin contains an accurate current sink to ground, programmable between 10mA to 50mA using a single resistor at the ISET pin. LED channels can be paralleled to achieve higher LED currents. For applications requiring less than 16 strings of LEDs, channels can be paralleled or disabled (connect LED pin to VOUT before startup). For optimum efficiency, VOUT regulates to the lowest possible voltage allowable to maintain regulated current in each LED string. Any OPEN LED fault is indicated by the FAULT pin driven low without effecting the operation of the connected LED strings. The Block Diagram in Figure 1 illustrates the key functions of the LT3754. It can be seen that two external supplies, VREF and INTVCC, are generated by the LT3754. The VREF pin provides a precision 1.485V output for use with external resistors to program the CTRL, OVPSET and TSET input pins. The INTVCC pin provides a regulated 7V output to supply the gate driver for the boost controller GATE pin. An accurate 1.476V threshold on the SHDN/UVLO pin combined with a SHDN/UVLO pin current hysteresis allows a programmable resistor divider from VIN to SHDN/UVLO
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+ -
LED AMP
OVERVOLTAGE AMP
28
OVPSET
3754 BD
+ -
+ - + -
100mV OVER CURRENT
SENSE VOUT
9 16
LEDx 1-8, 17-24 FAULT
14
R
9
LT3754 OPERATION
to define the turn on/off voltages for VIN. SHDN/UVLO pin current switches from 2.4A to 0A when SHDN/UVLO pin voltage exceeds 1.476V. The LT3754 constant switching frequency is programmable from 100kHz up to 1MHz using a single resistor at the RT pin to ground. A SYNC pin is also provided to allow an external clock to define the converter switching frequency. The GATE output provides a 0.8A peak gate drive for an external N-channel power MOSFET to generate a boosted output voltage VOUT using a single inductor, Schottky diode and output capacitor. With LED strings connected from VOUT to every LED pin, the lowest voltage on each LED pin is monitored and compared to an internal 1V reference. VOUT is regulated to ensure the lowest LED pin voltage of any connected LED string is maintained at 1V. If any of the LED strings are open, the LT3754 will ignore the open LED pin. If all of the LED strings are open VOUT charges up until a user programmable OVP (overvoltage protection) level is reached. This programmable OVP level allows the user to protect against LED damage when the LED strings are opened and then reconnected. Since the LT3754 boost controller uses a current mode topology, the VC pin voltage determines the peak current in the inductor of the converter and hence the duty cycle of the GATE switching waveform. The basic loop uses a pulse from an internal oscillator to set an RS flip-flop and turn on the external power MOSFET. Current increases in the MOSFET and inductor until the VC commanded peak switch current is exceeded and the MOSFET is then turned off. Inductor current is sensed during the GATE on period by a sense resistor RS in the source of the external N-channel power MOSFET. As with all current mode converters, slope compensation is added to the control path to ensure stability for duty cycles above 50%. Any over current fault condition in the MOSFET turns off the MOSFET and triggers soft start internally. In this fault mode the LT3754 only allows MOSFET turn-on approximately every 2ms. This hiccup mode significantly reduces the power rating required for the MOSFET. LED current programming and dimming can be achieved using the ISET, CTRL and PWM pins. A single resistor at the ISET pin programs LED current. Analog dimming of LED brightness is achieved using the CTRL pin below 1V. PWM dimming of LED brightness is achieved by controlling the duty cycle of the PWM pin. For robust operation the LT3754 monitors system conditions and performs soft start for startup after any of the following faults: VIN, SHDN or INTVCC voltages too low or MOSFET current too high. The LT3754, when entering these faults, discharges an internal soft start node and prevents switching at the GATE pin. When exiting these faults the LT3754 ramps up an internal soft start node to control VC pin voltage rise and hence control MOSFET peak switch current rise. In addition the soft start period gradually ramps up switching frequency from approximately 33% to 100% of full scale. The LT3754 monitors each LED pin voltage. If the LED string has an open fault (V(LEDX)<0.5V) the FAULT flag is pulled low. For LED protection, the LT3754 CTRL pin allows an LED current derating curve to be programmed versus the ambient temperature of the LED strings. An NTC resistor placed close to the LEDs decreases CTRL pin voltage and hence decreases LED current as LED ambient temperature increases. The LT3754 also allows it's own junction temperature to be monitored and regulated by derating LED currents when a junction temperature programmed by the TSET pin is exceeded.
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LT3754 APPLICATIONS INFORMATION
INTVCC Regulator Bypassing and Operation The INTVCC pin is the output of an internal linear regulator driven from VIN and is the supply for the LT3754 gate driver. The INTVCC pin should be bypassed with a 10V rated 4.7F low ESR, X7R or X5R ceramic capacitor to ensure stability and to provide enough charge for the gate driver. For high enough VIN levels the INTVCC pin provides a regulated 7V supply. Make sure INTVCC voltage does not exceed the VGS rating of the external MOSFET driven by the GATE pin. For low VIN levels the INTVCC level will depend on VIN and the voltage drop of the regulator. The INTVCC regulator has an undervoltage lockout which prevents gate driver switching until INTVCC reaches 3.8V and maintains switching until INTVCC falls below 3.4V. This feature prevents excessive power dissipation in the external MOSFET by ensuring a minimum gate drive level to keep RDS(ON) low. The INTVCC regulator has a current limit of 44mA to limit power dissipation inside the I.C. This current limit should be considered when choosing the N-channel power MOSFET and the switching frequency. The average current load on the INTVCC pin due to the LT3754 gate driver can be calculated as: IINTVCC = Qg * fOSC where Qg is the gate charge (at VGS = INTVCC) specified for the MOSFET and fosc is the switching frequency of the LT3754 boost converter. It is possible to drive the INTVCC pin from a variety of external sources in order to remove power dissipation from the LT3754 and/or to remove the INTVCC current limitation of 44mA. An external supply for INTVCC should never exceed the VIN pin voltage or the maximum INTVCC pin rating of 13V. If INTVCC is shorted to the VIN pin, VIN operational range is 4.5V to 13V. Inductor A list of inductor manufacturers is given in Table 1. However, there are many other manufacturers and inductors that can be used. Consult each manufacturer for more detailed information and their entire range of parts. Ferrite cores should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating. Also ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. Values between 2.2H and 33H will suffice for most applications. The typical inductor value required for a given application (assuming 50% inductor ripple current for example) can be calculated as: 1 1 * *V VOUT fOSC IN VIN L= V 0.5 * OUT * ILEDx * 16 VIN 1- where: VOUT = (N * VF) + 1V (N = number of LEDs per string), VF = LED forward voltage drop, ILEDx = LED current per string Example: For a 12W LED driver application requiring 16 strings of 10 LEDs each driven with 20mA, and choosing VIN = 12V, VOUT = (3.75V * 10) + 1V = 38.5V, ILEDx = 20mA and fOSC = 1MHz the value for L is calculated as 1 1 )* * 12V 3.2 106 L= = 16.5H 0.5 * 3.2 * 20mA * 16 (1-
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Table 1. Inductor Manufacturers
MANUFACTURER Sumida Wurth Elektronik Vishay Coilcraft Coiltronics PHONE NUMBER 408-321-9660 605-886-4385 402-563-6866 847-639-6400 561-998-4100 WEB www.sumida.com www.we-online.com www.vishay.com www.coilcraft.com www.cooperet.com
Schottky Rectifier The external diode for the LT3754 boost converter must be a Schottky diode, with low forward voltage drop and fast switching speed. Table 3 lists several Schottky manufacturers. The diodes average current rating must exceed the application's average output current. The diode's maximum reverse voltage must exceed the maximum output voltage of the application. For PWM dimming applications be aware of the reverse leakage of the Schottky diode. Lower leakage current will drain the output capacitor less during PWM low periods, allowing for higher PWM dimming ratios. The companies below offer Schottky diodes with high voltage and current ratings.
Table 3. Schottky Rectifier Manufacturers
MANUFACTURER Diodes, Inc. On Semiconductor Zetex Vishay Siliconix PHONE NUMBER 805-446-4800 888-743-7826 631-360-2222 402-563-6866 WEB www.microsemi.com www.onsemi.com www.zetex.com www.vishay.com
Input Capacitor The input capacitor of the LT3754 boost converter will supply the transient input current of the power inductor. Values between 2.2F and 10F will work well for the LT3754. Use only X5R or X7R ceramic capacitors to minimize variation over voltage and temperature. If inductor input voltage is required to operate near the minimum allowed operational VIN for the I.C., a larger capacitor value may be required. This is to prevent excessive input voltage ripple causing dips below the minimum operating input voltage. Output Capacitor Low ESR ceramic capacitors should be used at the LT3754 converter output to minimize output ripple voltage. Use only X5R or X7R dielectrics as these materials retain their capacitance over wider voltage and temperature ranges than other dielectrics. The output capacitance requirements for several LED driver application circuits are shown in the Applications Information section for various ILED, VIN, VOUT, L and fOSC values. Some suggested capacitor manufacturers are listed in Table 2.
Table 2. Ceramic Capacitor Manufacturers
MANUFACTURER TDK Kemet Murata Taiyo Yuden AVX PHONE NUMBER 516-535-2600 408-986-0424 814-237-1431 408-573-4150 843-448-9411 WEB www.tdk.com www.kemet.com www.murata.com t-yuden.com www.avxcorp.com
Power MOSFET Selection Several MOSFET vendors are listed in Table 4. Consult the factory applications department for other recommended MOSFETs. The power MOSFET selected should have a VDS rating which exceeds the maximum Overvoltage Protection (OVP) level programmed for the application. (See "Programming OVP level" in the Applications Information section). The MOSFET should also have a low enough total gate charge Qg (at 7V VGS) and a low enough switching frequency (fOSC) to not exceed the INTVCC regulator current limit, where loading on INTVCC pin due to gate switching should obey, IGATE = Qg * fOSC 44mA
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In addition, the current drive required for GATE switching should also be kept low in the case of high VIN voltages (see "Thermal Considerations" in the Applications Information section). The RDS(ON) of the MOSFET will determine d.c. power losses but will usually be less significant compared to switching losses. Be aware of the power dissipation within the MOSFET by calculating d.c. and switching losses and deciding if the thermal resistance of the MOSFET package causes the junction temperature to exceed maximum ratings.
Table 4. MOSFET Manufacturers
MANUFACTURER Vishay Siliconix International Rectifier Fairchild PHONE NUMBER 402-563-6866 310-252-7105 972-910-8000 WEB www.vishay.com www.irf.com www.fairchildsemi.com
RS where
52mV * 0.7 IL(PEAK )
1 0.5 IL(PEAK ) = * 16 * ILEDx * 1+ 2 1- D V OUT(MAX ) , D = MOSFET duty cycle = S VIN(MIN) VOUT(MAX) = N * VF(MAX ) + 1V N = number of LEDs in each string, VF(MAX ) = max imum LED forward voltage drop, VIN(MIN) = min imum input voltage to the inductor,
(
)
Power MOSFET: Current Sense Resistor The LT3754 current mode boost converter controls peak current in the inductor by controlling peak MOSFET current in each switching cycle. The LT3754 monitors current in the external N-channel power MOSFET by sensing the voltage across a sense resistor (RS) connected between the source of the FET and the power ground in the application. The length of these tracks should be minimized and a Kelvin sense should be taken from the top of RS to the sense pin. A 52mV sense pin threshold combined with the value of RS sets the maximum cycle-by-cycle peak MOSFET current. The low 52mV threshold improves efficiency and determines the value for RS given by:
and the 0.5 term represents an inductor peak-to-peak ripple current of 50% of average inductor current. The scale factor of * 0.7 ensures the boost converter can meet the peak inductor requirements of the loop by accounting for the combined errors of the 50mV sense threshold, ILEDx, RS and circuit efficiency. Example: For a 12W LED driver application requiring 16 strings of 10 LEDs each driven with 20mA, and choosing VIN(MIN) = 8V, VOUT(MAX) = (4V * 10)+1V = 41V and ILEDx = 20mA, the value for RS is chosen as: RS 52mV * 0.7 52mV * 0.7 41 IL(PEAK ) * 16 * 0.02 * (1+ 0.25) 8 52mV * 0.7 17.7 m 2.05
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The power rating of RS should be selected to exceed the I2R losses in the resistor. The peak inductor current should be recalculated for the chosen RS value to ensure the chosen inductor will not saturate. Power MOSFET: Overcurrent and Hiccup Mode For severe external faults which may cause the external MOSFET to reach currents greater than the peak current defined by RS and the 52mV sense pin threshold described above, the LT3754 has an overcurrent comparator which triggers soft start and turns off the MOSFET driver for currents exceeding, ID(OVERCURRENT ) = 100mV RS Soft Start To limit inductor inrush current and output voltage during startup or recovery from a fault condition, the LT3754 provides a soft start function. The LT3754 when entering these faults will discharge an internal soft start node and prevent switching at the GATE pin for any of the following faults: VIN, SHDN/UVLO or INTV CC voltages too low or MOSFET current too high (see the timing diagram in Figure 2). When exiting these faults the LT3754 ramps up an internal soft start node at approximately 0.5V/ms to control VC pin voltage rise and hence control MOSFET switch current rise. In addition the soft start period gradually ramps up switching frequency from approximately 33% to 100% of full scale. The conditions required to exit all faults and allow a soft start ramp of the VC pin are listed in Figure 2. An added feature of the LT3754 is that it waits for the first PWM pin active high (minimum 200ns pulse width) before it allows
In this fault mode the LT3754 only allows MOSFET turn on for approximately 100ns every 2ms. This hiccup mode significantly reduces the power rating required for the MOSFET.
GATE VC VC MIN CLAMP 0.4V 0.1V 0.5V/ms VBE (VC SWITCHING THRESHOLD) VBE
SS (INTERNAL) 0.5V/ms 0.4V 0.1V
ANY OF THE FOLLOWING FAULTS TRIGGERS SOFT START LATCH WITH GATE TURNED OFF IMMEDIATELY: VIN < 3.7V, SHDN < 1.476V, INTVCC < 3.4V IDSS (EXTERNAL MOSFET) > 100mV/RS
SOFT-START LATCH SET:
SOFT-START LATCH RESET REQUIRES ALL CONDITIONS SATISFIED: SS (INTERNAL) < 0.2V, VIN 4.2V, SHDN > 1.476V, INTVCC > 3.8V, IDSS (EXTERNAL MOSFET) < 100mV/RS, PWM > 1.4V (FOR AT LEAST 200ns)
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Figure 2. LT3754 Fault Detection and Soft Start Timing for VC Pin and Internal SS Node
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the soft start of VC pin to begin. This feature ensures that during startup of the LT3754 the soft start ramp has not timed out before PWM is asserted high. Without this `wait for PWM high' feature, systems which apply PWM after VIN and SHDN/UVLO are valid, can potentially turn on without soft start and experience high inductor currents during wake up of the converter's output voltage. It is important to note that when PWM subsequently goes low, the soft start ramp is not held at its present voltage but continues to ramp upwards. If the soft start ramp voltage was held every time PWM goes low, this would cause very slow startup of LED displays for applications using very high PWM Dimming ratios. Shutdown and Programming Undervoltage Lockout The LT3754 has an accurate 1.476V shutdown threshold at the SHDN/UVLO pin. This threshold can be used in conjunction with a resistor divider from the system input supply to define an accurate undervoltage lockout (UVLO) threshold for the system (Figure 3). An internal hysteresis current at the SHDN/UVLO pin allows programming of hysteresis voltage for this UVLO threshold. Just before part turn on, an internal 2.4A flows from the SHDN/UVLO
VSUPPLY R1
pin. After part turn on, 0A flows from the SHDN/UVLO pin. Calculation of the turn on/off thresholds for a system input supply using the LT3754 SHDN/UVLO pin can be made as follows : R1 VSUPPLY OFF = 1.476 1+ R2 VSUPPLY ON = VSUPPLY OFF + (2.4A * R1) An open drain transistor can be added to the resistor divider network at the SHDN/UVLO pin to independently control the turn off of the LT3754. Programming Switching Frequency The switching frequency of the LT3754 boost converter can be programmed between 100kHz and 1MHz using a single resistor (RT) connected from the RT pin to ground (Figure 4). Connect the RT resistor as close as possible to the RT pin to minimize noise pick up and stray capacitance (see "Circuit Layout Considerations" in the Applications Information section). Table 5 shows the typical RT values required for a range of frequencies.
1000 900 SWITCHING FREQUENCY (kHz) 800 700 600 500 400 300 200 100 0 100 200 300 400 RT (k) 500 600
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13 SHDN/UVLO 600k
- +
R2 OFF ON
1.476V
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Figure 3. Programming Undervoltage Lockout (UVLO) with Hysteresis
Figure 4. Switching Frequency vs RT
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Selecting the optimum frequency depends on several factors. Higher frequency allows reduction of inductor size but efficiency drops due to higher switching losses. Lower frequency allows higher operational duty cycles to drive a larger number of LEDs per string from a low input supply but require larger magnetics. In each application the switching frequency can be tailored to provide the optimum solution.
Table 5. Switching Frequency vs. RT (1% resistors)
SWITCHING FREQUENCY (kHz) 100 200 300 400 500 600 700 800 900 1000 RT (k) 523 249 158 115 90.9 73.2 60.4 51.1 44.2 39.2
Table 6. LED Current vs. RISET (1% resistors) LED Current per CHANNEL (mA)
10 20 30 40 50 RISET (k) 29.4 14.7 9.76 7.32 5.76
An extra 50ns should be added to these tested timings to account for errors in the rise/fall times of the GATE and DRAIN of the external MOSFET and the d.c. resistance of the external MOSFET and inductor. Synchronizing to an external clock The SYNC pin allows the LT3754 oscillator to be synchronized to an external clock. The SYNC pin can be driven from a logic level output, requiring less than 0.6V for a logic level low and greater than 2.2V for a logic level high. SYNC pin high or low periods should exists for at least 100ns. If unused, the SYNC pin should be tied to ground. To avoid loss of slope compensation during synchronization, the free running oscillator frequency (fOSC) of the LT3754 should be programmed to 80% of the external clock frequency. Programming LED Current The current source to ground at each LED pin is programmed using a single resistor RISET connected from the ISET pin to ground according to the following equation: I (LEDX ) 295 ( ) A (CTRL > 1.1V ) RISET
Duty Cycle Considerations When designing the LT3754 LED driver for a given application, the duty cycle requirements should be considered and compared to the minimum/maximum achievable duty cycles for the LT3754 GATE pin. If required, the LT3754 switching frequency can be programmed to a lower value to meet the duty cycle requirements. In general, the minimum/maximum GATE duty cycles required for a particular application are given by: MIN Duty Cycle = GATE Minimum On-Time * Switching Frequency fOSC MAX Duty Cycle = 1 - (GATE Minimum Off-Time * Switching Frequency fOSC) The typical values for LT3754 GATE pin minimum on and off times versus temperature are shown in the Typical Performance Characteristics. The range of GATE pin minimum on time and off times are given in the electrical specifications.
See Table 6 for resistor values and corresponding programmed LED.
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Analog Dimming The LT3754 allows for LED dimming (brightness reduction) by analog dimming or by PWM dimming. Analog dimming uses the CTRL pin voltage below 1V to reduce LED brightness by reducing LED current. For CTRL pin voltage below 1V, the current in each LED pin is given by: I (LEDX ) CTRL * 295 (0.04 < CTRL < 1V ) RISET
PWM TPWM TON(PWM) (= 1/fPWM)
INDUCTOR CURRENT
LED CURRENT
MAX ILED
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For CTRL pin voltages below 40mV (greater than 25:1 dimming) the LED current will approach zero current. The CTRL pin voltage can be derived from a resistor divider from VREF pin to ground or generated from an external source. If analog dimming is not required, the pin can be directly connected to the VREF pin. The only drawback of analog dimming is that reducing LED current to reduce the brightness of the LED also changes the perceived color of the LED. PWM Dimming Many applications require an accurate control of the brightness of the LED(s). In addition, being able to maintain a constant color over the entire dimming range can be just as critical. For constant color LED dimming the LT3754 provides a PWM pin and special internal circuitry to achieve up to a 3000:1 wide PWM dimming range. This is achieved by operating the LED at it's programmed current and then controlling the on time of that LED current. The duty cycle of the PWM pin controls the on time of each LED pin current source (Figure 5). For maximum PWM dimming ratios (low PWM duty cycles) it is important to be able to turn LED currents on/off as quickly as possible. For PWM low, the LT3754 turns off the boost converter, turns off all LED channel currents and disconnects the VC pin and internal VOUT resistor divider connected to the OVP error amplifier. This allows the part to quickly return to the last state of operation when the PWM pin is returned high.
Figure 5. PWM Dimming Waveforms
Some general guidelines for LED current dimming using the PWM pin (see Figure 5): (1) PWM Dimming Ratio (PDR) = 1/(PWM Duty Cycle) = 1/TON(PWM) * fPWM (2) Lower PWM frequency (fPWM) allows higher PWM dimming ratios (Typically choose 100Hz to maximize PDR and to avoid visible flicker which can occur for display systems with refresh rates at frequencies below 80Hz) (3) Higher fOSC value improves PDR (allows lower TON(PWM)) but will reduce efficiency and increase internal heating. In general, minimum operational TON(PWM) = 3 * (1/fOSC) (4) Lower inductor value improves PDR (5) Higher output capacitor value improves PDR (6) Choose the Schottky diode for the LT3754 boost converter for minimum reverse leakage current. See "LED Current vs PWM Duty Cycle" in the Typical Performance Characteristics section.
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LT3754 APPLICATIONS INFORMATION
Programming LED Current Derating (Breakpoint and Slope) versus LED Ambient Temperature (CTRL Pin) LED datasheets provide curves of maximum allowed LED current versus ambient temperature to warn against damaging of the LED (Figure 6). The LT3754 LED driver improves the utilization and reliability of the LED(s) by allowing the programming of an LED current derating curve versus the ambient temperature of the LED(s). Without the ability to back off LED currents as temperature increases, many LED drivers are limited to driving the LED(s) at 50% or less of their maximum rated currents. The LT3754 allows the temperature breakpoint and the slope of LED current versus ambient temperature to be programmed using a simple resistor network shown in Figure 7. Without the ability to back off LED current as the ambient temperature of the LED(s) increases, many LED drivers are limited to driving the LED(s) at only 50% or less of their maximum rated currents. This limitation requires more LEDs to obtain the intended brightness for the application. The LT3754 allows the LED(s) to be programmed for maximum allowable current while still protecting the LED(s) from excessive currents at high temperature. This is achieved by programming a voltage at the CTRL pin with a negative temperature coefficient using a resistor
250 225 200 175 150 125 100 -50 RNTC -25 0 25 50 75 TA-TEMPERATURE (C) 100 125 A B C D
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divider with temperature dependent resistance (Figures 7 and 8). A variety of resistor networks and NTC resistors with different temperature coefficients can be used to achieve the desired CTRL pin voltage behaviour versus temperature. The current derating curve in Figure 6 uses the resistor network shown in option C of Figure 7. Table 7 shows a list of NTC resistor manufacturers/ distributors. There are several other manufacturers available and the chosen supplier should be contacted for more detailed information. To use an NTC resistor to monitor the ambient temperature of the LED(s) it should be placed as close as possible to the LED(s). Since the temperature dependency of an NTC resistor can be nonlinear over a wide range of temperatures it is important to obtain a resistor's exact values over temperature from the manufacturer. Hand calculations of CTRL voltage can then be performed at each given temperature and the resulting CTRL voltage plotted versus temperature.
Table 7. NTC Resistor Manufacturers
MANUFACTURER Murata Electronics North America TDK Corporation Digi-key WEB www.murata.com www.tdk.com www.digikey.com
31 RESISTOR OPTION A R1 29
REF LT3754 CTRL
IF - FORWARD CURRENT (mA)
LT3754 PROGRAMMED LED CURRENT DERATING CURVE
R2
OPTION A TO D
RY
RY
RNTC
RX RNTC
RNTC
RX
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Figure 6. LED Current Derating vs LED Ambient Temperature
Figure 7. Programming LED Current Derating Curve vs Ambient Temperature (RNTC Located on LED PCB)
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1.50
Using the TSET Pin for Thermal Protection The LT3754 contains a special programmable thermal regulation loop that limits the internal junction temperature of the part. Since the LT3754 topology consists of a single boost controller with sixteen linear current sources, any LED string voltage mismatch will cause additional power to be dissipated in the package. This topology provides excellent current matching between LED strings and allows a single power stage to drive a large number of LEDs, but at the price of additional power dissipation inside the part (which means a higher junction temperature). Being able to limit the maximum junction temperature allows the benefits of this topology to be fully realized. This thermal regulation feature provides important protection at high ambient temperatures, and allows a given application to be optimized for typical, not worst-case, ambient temperatures with the assurance that the LT3754 will automatically protect itself and the LED strings under worst-case conditions. The operation of the thermal loop is simple. As the ambient temperature increases, so does the internal junction temperature of the part. Once the programmed maximum junction temperature is reached, the LT3754 begins to linearly reduce the LED current, as needed, to try and maintain this temperature. This can only be achieved when the ambient temperature stays below the desired maximum junction temperature. If the ambient temperature continues to rise past the programmed maximum junction temperature, the LEDs current will be reduced to approximately 5% of the full LED current. While this feature is intended to directly protect the LT3754, it can also be used to derate the LED current at high temperatures. Since there is a direct relationship between the LED temperature and LT3754 junction temperature, the TSET function also provides some LED current derating at high temperatures.
1.25 CTRL VOLTAGE (V)
1.00 RESISTOR OPTION A 0.75
0.50
0.25
0
10 20 30 40 50 60 70 TA - AMBIENT TEMPERATURE (C)
80
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Figure 8. Programmed CTRL Voltage vs Temperature
If calculation of CTRL voltage at various temperatures gives a downward slope that is too strong, alternative resistor networks can be chosen (B,C,D in Figure 7) which use temperature independent resistance to reduce the effects of the NTC resistor over temperature. Murata Electronics provides a selection of NTC resistors with complete data over a wide range of temperatures. In addition, a software tool is available which allows the user to select from different resistor networks and NTC resistor values and then simulate the exact output voltage curve (CTRL pin behavior) over temperature. Referred to on the website as the `Murata Chip NTC Thermistor Output Voltage Simulator', users can log onto www.murata.com/designlib and download the software followed by instructions for creating an output voltage `VOUT' (LT3754 CTRL pin voltage) from a specified VCC supply (LT3754 VREF pin voltage). At any time during selection of circuit parameters the user can access data on the chosen NTC resistor by clicking on the link to the Murata catalog. For a detailed example of hand calculations using an NTC type resistor divider to program CTRL pin voltage, read the LT3478 LED driver data sheet section Programming LED Current Derating vs Temperature under Applications Information.
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Two external resistors program the maximum IC junction temperature using a resistor divider from the VREF pin, as shown in Figure 9. Choose the ratio of R1 and R2 for the desired junction temperature. Figure 10 shows the relationship of TSET voltage to junction temperature, and Table 8 shows commonly used values for R1 and R2.
31 R2 30 R1
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Programming Overvoltage Protection (OVP) level The LT3754 LED driver provides optimum protection to the LEDs and the external MOSFET by providing a programmable maximum regulated output voltage limit using the OVPSET pin. The Overvoltage Protection (OVP) level is programmed as: OVP (MAXIMUM REGULATED VOUT) = 57 * OVPSET If every LED string fails open or the multiple string LED display becomes disconnected the LT3754 LED driver loop regulates to the programmed OVP level. The OVP level should be programmed to a level high enough to regulate the LED strings but low enough to prevent damage to the power switch and to minimize the voltage across the LED pins upon reconnection of the LED strings. Recommended OVP level is given by: OVP(RECOMMENDED) = 1.2 * ((N * VF) + 1V) where: N = number of LEDs in each string, VF = maximum LED forward voltage drop and the scaling factor of 1.2 accounts for variation in the generation of OVP from OVPSET pin voltage and startup logic requirements.
VREF LT3754 TSET
Figure 9. Programming the TSET Pin
950 900 VTSET THRESHOLD (mV) 850 800 750 700 650 600 550 500 0 25 50 75 100 125 JUNCTION TEMPERATURE (C) 150
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VPTAT
Figure 10. Programing the TSET Pin Threshold
Example: For a converter operating with 10 LEDs per string at a maximum forward voltage of 4V per LED, the OVP level should be programmed to: OVP(RECOMMENDED) = 1.2 * ((10 * 4) + 1V ) = 49.2V For OVP = 49.2V, OVPSET = V 49.2 = 0.863V 57
Table 7. Resistor Values to Program Maximum IC Junction Temperature (VREF (Typical) = 1.485V)
TJ (C) 100 115 130 R1 (k) 24.9 28.0 30.9 R2 (k) 20 20 20 TSET (V) 0.824 0.866 0.902
The OVPSET pin voltage can be generated using a resistor divider from the REF pin.
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LED Open Circuit and PWM Dimming Ratios The LT3754 monitors each LED pin voltage to determine if the LED string has an open fault (LED pin voltage < 0.5V). If an open LED fault occurs, the FAULT flag is pulled low. To avoid false detection of faults during the initial converter startup when VOUT is low, the LT3754 ignores low LED pin voltages until VOUT reaches 90% of its maximum allowed OVP level. Once this condition is met, the LT3754 monitors all LED pins for open LED faults. To avoid false detection of faults during PWM dimming edges (where LED pins can possibly ring and trip fault detection levels) the LT3754 only monitors/updates fault conditions during PWM high (and only after a blank duration of 2s following each PWM rising edge). LED Short Circuit A short circuit fault between the positive terminal of an LED string (VOUT) and the negative terminal of the LED string (LEDx pin) causes the channel to be disabled in order to protect the internal current source. A resistive short is allowed as long as (VOUT -VLEDx) < 6V. Loop Compensation Be sure to check the stability of the loop with the LEDs connected (LED regulation loop) and disconnected (Overvoltage Protection (OVP) regulation loop). Various application circuits are shown in the datasheet which cover a range of VIN, VOUT, fOSC, output power and inductor current ripple values. For application requirements which deviate from the circuits shown in the datasheet be sure to check the stability of the final application over the full VIN range, LED current range (if analog dimming) and temperature range. Be aware that if the VC pin components represent the dominant pole for the converter loop and they have been adjusted to achieve stability, the VC pin might move more slowly during load transient conditions such as an all-LEDs-open fault. A slower moving VC pin will add to VOUT overshoot during an all-LEDs-open fault. An alternative compensation approach is to place the dominant pole of the converter loop at the output. This requires an increased output capacitor value but will allow a much reduced Vc capacitor. The combination will allow VC to move more quickly and VOUT to move more slowly resulting in less overshoot during an all-LEDs-open fault. Thermal Considerations The internal power dissipation of the LT3754 comes from 3 main sources: VIN quiescent current (IQ total), VIN current for GATE switching (IGATE) and the LT3754 LED current sources. Since the maximum operational VIN voltage is 40V, care should be taken when selecting the switching frequency and type of external power MOSFET since the current required from VIN for GATE switching is given by, IGATE = fOSC * Qg where Qg is the gate charge (at VGS = INTVCC) specified for the MOSFET and fOSC is the programmed switching frequency for the LT3754. A low Qg MOSFET should always be used when operating the LT3754 from high VIN voltages. The internal junction temperature of the LT3754 can be estimated as: TJ = TA + [VIN * (IQTOTAL + (fOSC * Qg)) + (16 * I(LEDX) * 1.1V)] * JA where, TA is the ambient temperature for the LT3754 IQTOTAL represents the VIN quiescent current for the LT3754 (not switching, PWM = 1.5V and CTRL = 0.1V) - illustrated in the Typical Characteristics Graphs - plus the base currents of active channels (typically 16 * I(LED)/75). JA is the thermal resistance of the package (34C/W for the 5mm x 5mm QFN package).
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Example : For a 12W LED driver application requiring 16 strings of 10 LEDs each driven with 20mA, VIN = 24V, fOSC = 1MHz, Qg (at 7V VGS) = 15nC, I(LEDX) = 20mA, and an 85C ambient temperature for the LT3754 IC, the LT3754 junction temperature can be approximated as: TJ = 85C + [24 * (9.5mA + (16 * 20mA/75) + (1MHz * 15nC)) + (16 *20mA * 1.1V)] * 34 = 85C + [(24 * 28.8mA) + (320mA * 1.1V)] * 34 = 85C + (0.691W + 0.35W) * 34 = 85C + 35C TJ = 120C The exposed pad on the bottom of the package must be soldered to the ground plane. The ground plane should be connected to an internal copper ground plane with vias placed directly under the package to spread out the heat generated by the LT3754. Circuit Layout Considerations As with all switching regulators, careful attention must be given to PCB layout and component placement to achieve optimal thermal, electrical and noise performance. The exposed pad of the LT3754 is the only ground connection for the IC. The exposed pad should be soldered to a continuous copper ground plane underneath the device to reduce die temperature and maximize the power capability of the IC. An analog ground is down bonded to the exposed pad near the RT and VC pins. ISET, RT and VC components should be connected to an area of ground copper near these pins. The OVPSET track should be kept away from fast moving signals and not loaded with an external capacitor. GATE pin turn off currents escape through a downbond to the exposed pad near the GATE pin. This area of copper should be the power ground (PGND) connection for the inductor input capacitor, INTVCC capacitor and output capacitor. A separate bypass capacitor for the VIN pin of the IC may be required close the VIN pin and connected to the copper area associated with analog ground. To minimize MOSFET peak current sensing errors the sense resistor (RS) should have Kelvin connections to the SENSE pin and the power ground copper area near the pin. The MOSFET drain rise and fall times are designed to be as short as possible for maximum efficiency. To reduce the effects of both radiated and conducted noise, the area of the copper trace for the MOSFET drain should be kept as small as possible. Use a ground plane under the switching regulator to minimize interplane coupling. The Schottky diode and output capacitor should be placed as close as possible to the drain node to minimize this high switching frequency path.
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22
92% Efficient, 36W LED Driver, 1MHz Boost, 16 Strings, 50mA Per String
PVIN 24V L1 10H D1 UP TO 45V OF LEDs PER STRING
4.7F 50V
VIN 10V VIN GATE SENSE 0.015 LT3754 VOUT * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * M1 5 2.2F 100V
4.7F 25V
4.7F 10V
INTVCC
VIN
TYPICAL APPLICATIONS
499k
100k
FAULT
SHDN/UVLO
40.2k
GND
SYNC
PWM DIMMING
PWM
ANALOG DIMMING
CTRL
REF
20k
TSET
30.9k ISET 5.76k 2.2nF 10k VC
11k
LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 LED16
3754 TA03
OVPSET RT
20k
39.2k
LED Current Waveforms 3000:1 PWM Dimming (100Hz)
I(LEDx) 20mA/DIV
L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS360
I(L) 1A/DIV PWM 10V/DIV 5s/DIV
3754 G04
LT3754
23
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LT3754
TYPICAL APPLICATIONS
24
17W LED Driver, 1MHz Boost, 16 Strings, 1MHz Per String
L1 10H D1 UP TO 36V OF LEDs PER STRING VIN GATE SENSE 0.02 LT3754 VOUT * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * M1 4 2.2F 50V LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 LED16
3754 TA04
VIN 10V TO 14V
4.7F 25V
4.7F 10V
INTVCC
1M
1M
100k
FAULT
SHDN/UVLO
232k
CTRL
110k
GND
SYNC
PWM
REF
20k
TSET
19.1k ISET 9.76k 2.2nF 10k VC
OVPSET RT
30.9k
20k
39.2k
L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7308DN D1: DIODES, INC. DFLS160
3754f
LT3754 TYPICAL APPLICATIONS
31W LED Driver, 400kHz Boost, 3 Strings, 250mA Per String
L1 10H 4.7F 50V VIN 4.7F 10V INTVCC GATE SENSE 0.007 100k 1M FAULT SHDN/UVLO 232k GND LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 ISET 5.76k VC 5.1k 4.7nF
3754 TA06
D1
VIN 8V TO 36V
UP TO 42V OF LEDs PER STRING 10 2.2F 100V
M1
LT3754 VOUT LED16
* * *
* * *
* * *
SYNC PWM CTRL REF 20k TSET 15k OVPSET RT 30.9k 23.2k 115k
PWM DIMMING ANALOG DIMMING
L1: COOPER BUSSMANN HC9-100-R M1: VISHAY SILICONIX Si7850DP D1: DIODES, INC. PDS560
3754f
25
LT3754 TYPICAL APPLICATIONS
14W LED Driver, 700kHz Boost, 4 Strings, 80mA Per String
12VIN 10V TO 14V L1 15H 4.7F 25V VIN 4.7F 10V VIN 1M 100k FAULT SHDN/UVLO LT3754 VOUT INTVCC GATE SENSE 0.02 * * * * * * * * * M1 5 2.2F 100V D1 UP TO 45V OF LEDs PER STRING
* * *
GND SYNC PWM CTRL REF 20k TSET 11k OVPSET RT 30.9k 20k 60.4k ISET 14.7k VC
PWM DIMMING ANALOG DIMMING
LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 LED11 LED12 LED13 LED14 LED15 LED16
3754 TA06
7.5k 4.7nF
L1: SUMIDA CDRH8D38 M1: VISHAY SILICONIX Si7308DN D1: DIODES, INC. DFLS160
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26
LT3754 PACKAGE DESCRIPTION
DH Package 32-Lead Plastic QFN (5mm x 5mm)
(Reference LTC DWG # 05-08-1693)
0.70 0.05
5.50 0.05 4.10 0.05 3.50 REF (4 SIDES) 3.45 0.05 3.45 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 0.75 0.05 R = 0.05 TYP 0.00 - 0.05 BOTTOM VIEW--EXPOSED PAD R = 0.115 TYP 31 32 0.40 1 2 3.45 0.10 0.10 PIN 1 NOTCH R = 0.30 TYP OR 0.35 45 CHAMFER
3.50 REF (4-SIDES)
3.45
0.10
(UH32) QFN 0406 REV D
0.200 REF NOTE: 1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE M0-220 VARIATION WHHD-(X) (TO BE APPROVED) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
0.25
0.05
0.50 BSC
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
27
LT3754 RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS VIN(MIN) = 4.5V, VIN(MAX) = 40V, VOUT(MAX) = 60V, 3,000:1 True Color PWM Dimming, ISD = <1A, 3mm x 3mm QFN-16 MSOP-16E LT3755/LT3755-1 High Side 40V, 1MHz LED Controller with True Color 3,000:1 PWM Dimming
LT3756/LT3756-1 High Side 100V, 1MHz LED Controller with VIN(MIN) = 6.0V, VIN(MAX) = 100V, VOUT(MAX) = 100V, 3,000:1 True Color PWM Dimming, ISD = <1A, 3mm x 3mm QFN-16 MSOP-16E True Color 3,000:1 PWM Dimming LT3598 LT3599 LT3595 LTC3783 LT3517 LT3518 LT3486 44V, 1.5A, 2.5MHz Boost 6-Channel 20mA VIN(MIN) = 3V, VIN(MAX) = 30V(40VMAX), VOUT(MAX) = 44V, 1,000:1 True Color PWM Dimming, ISD = <1A, 4mm x 4mm QFN-24 LED Driver 44V, 2A, 2.5MHz Boost 4-Channel 100mA LED Driver 45V, 2.5MHz 16-Channel Full Featured LED Driver High Side 36V, 1MHz LED Controller with True Color 3,000:1 PWM Dimming 1.3A, 2.5MHz High Current LED Driver with 3,000:1 Dimming 2.3A, 2.5MHz High Current LED Driver with 3,000:1 Dimming VIN(MIN) = 3V, VIN(MAX) = 30V(40VMAX), VOUT(MAX) = 44V, 1,000:1 True Color PWM Dimming, ISD = <1A, 4mm x 4mm QFN-24 VIN(MIN) = 4.5V, VIN(MAX) = 45V, VOUT(MAX) = 45V, 5,000:1 True Color PWM Dimming, ISD = <1A, 5mm x 9mm QFN-56 VIN(MIN) = 3.0V, VIN(MAX) = 36V, VOUT(MAX) = 40V, 3,000:1 True Color PWM Dimming, ISD = <20A, 4mm x 5mm DFN-16 TSSOP-16E VIN(MIN) = 3.0V, VIN(MAX) = 30V, VOUT(MAX) = 45, 3,000:1 True Color PWM Dimming, ISD = <1A, 4mm x 4mm QFN-16 VIN(MIN) = 3.0V, VIN(MAX) = 30V, VOUT(MAX) = 45, 3,000:1 True Color PWM Dimming, ISD = <1A, 4mm x 4mm QFN-16
Dual 1.3A, 2MHz High Current LED Driver VIN(MIN) = 2.5V, VIN(MAX) = 24V, VOUT(MAX) = 36V, 1,000:1 True Color PWM Dimming, ISD = <1A, 5mm x 3mm DFN, TSSOP-16E VIN(MIN) = 2.8V, VIN(MAX) = 36V, VOUT(MAX) = 40V, 1,000:1 True Color PWM Dimming, ISD = <10A, 5mm x 7mm QFN-10
LT3478/LT3478-1 4.5A, 2MHz High Current LED Driver with 3,000:1 Dimming LT3496
Triple Output 750mA, 2.1 MHz High VIN(MIN) = 3.0V, VIN(MAX) = 30V, VOUT(MAX) = 40V, 3,000:1 True Color PWM Dimming, Current LED Driver with 3,000:1 Dimming ISD = <1A, 4mm x 5mm QFN-28 VIN(MIN) = 4.0V, VIN(MAX) = 36V, VOUT(MAX) = 13.5V, 400:1 True Color PWM Dimming, ISD = <1A, TSSOP16E VIN(MIN) = 4.0V, VIN(MAX) = 36V, VOUT(MAX) = 13.5V, 3,000:1 True Color PWM Dimming, ISD = <1A, TSSOP20E VIN(MIN) = 2.8V, VIN(MAX) = 16V, VOUT(MAX) = 36V, 1,000:1 True Color PWM Dimming, ISD = <10A, 5mm x 7mm QFN-10
LT3474/LT3474-1 36V, 1A (ILED), 2MHz, Step-Down LED Driver LT3475/LT3475-1 Dual 1.5A(ILED), 36V, 2MHz, Step-Down LED Driver LT3476 Quad Output 1.5A, 2MHz High Current LED Driver with 1,000:1 Dimming
3754f
28 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0809 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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