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FEATURES
n n
n n n n n n n n n n n n n
Three 100mA Buck Regulators, Each Drives Up to 10 LEDs with Fast NPN Current Sources Fast Current Sources for <1s Pulse Widths (10,000:1 True Color PWMTM Dimming at 100Hz) LEDs Disconnected in Shutdown Adaptive VOUT for Increased Efficiency 6V to 60V Input Voltage Range 2% LED Current Matching External Resistor Sets LED Current for Each Channel Internal Compensation and Soft-Start Programmable Switching Frequency (200kHz to 1MHz) Synchronizable to External Clock Open LED Detection and Reporting Shorted LED Pin Protection and Reporting Programmable LED Thermal Derating Programmable Temperature Protection 5mm x 8mm Thermally Enhanced QFN Package with a 0.6mm High Voltage Pin Spacing LED Billboards and Signboards Mono, Multi, Full-Color LED Displays Large Screen Display LED Backlighting Automotive, Industrial, and Medical Displays
LT3597 60V Triple Step-Down LED Driver DESCRIPTION
The LT(R)3597 is a 60V triple step-down LED driver capable of achieving 10,000:1 digital PWM dimming at 100Hz with fast NPN current sources driving up to 10 LEDs in each channel. LED dimming can also be achieved via analog control of the CTRL1-3 pin. The step-down switching frequency is programmed between 200kHz and 1MHz. The frequency is also synchronizable to an external clock. The LT3597 provides maximum LED brightness while adhering to manufacturers' specifications for thermal derating. The derate temperature is programmed by placing a negative temperature coefficient (NTC) resistor on the master control pin. The LT3597 adaptively controls VOUT in order to achieve optimal efficiency. Other features include: 2% LED current matching between channels, open LED reporting, shorted LED protection, programmable LED current, and programmable temperature protection.
L, LT, LTC and LTM 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.
APPLICATIONS
n n n n
TYPICAL APPLICATION
Triple Step-Down RGB Single Pixel LED Driver, 100mA Current
VIN 48V 10F 270k 91k BOOST1 VOUT1 97k R 9.1k 100H 3.3F DA1 FB1 VOUT1 LED1 BIAS 10F 3 3 100k FAULT PWM1-3 CTRL1-3 SYNC RT 33.2k (1MHz) ISET1 20k ISET2 20k ISET3 20k GND LT3597 VOUT2 LED2 BOOST3 0.1F 100H SW3 3.3F DA3 FB3 VOUT3 LED3 VREF TSET CTRLM
3597 TA01a
VIN EN/UVLO
BOOST2 0.1F 100H SW2 3.3F DA2 FB2 9.1k G 97k VOUT2
10,000:1 PWM Dimming at 100Hz
0.1F SW1
PWM 2V/DIV
VOUT3 97k 9.1k B 200ns/DIV
3597 TA01b
ILED 50mA/DIV
VCC 5V
10k
82.5k 49.9k
VREF
100k
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LT3597 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW BST2 SW2 46 DA2 44 43 DA3 BST1 2 41 SW3 SW1 4 39 BST3 DA1 6 FB1 7 EN/UVLO 9 TSET 11 VREF 12 CTRLM 13 ISET1 14 ISET2 15 ISET3 16 RT 17 18 19 20 21 22 23 24 25 26 LED1 LED2 LED3 VOUT1 VOUT2 VOUT3 FAULT GND NC 53 GND 37 BIAS 35 FB3 34 FB2 33 CTRL1 32 CTRL2 31 CTRL3 30 PWM1 29 PWM2 28 PWM3 27 SYNC VIN VIN
Input Voltage (VIN), EN/UVLO ...................................60V BOOST1-3 .................................................................85V BOOST Pin Above SW Pin........................................ 25V LED1-3, VOUT1-3 ........................................................42V BIAS, FAULT ..............................................................25V VREF, RT, ISET1-3 , TSET, CTRLM ...................................3V FB1-3, CTRL1-3, PWM1-3, SYNC ...............................6V Operating Junction Temperature Range (Notes 2, 3)........................................-40C to 125C Maximum Junction Temperature........................... 125C Storage Temperature Range...................-65C to 150C
51 50
48
UHG PACKAGE VARIATION: UHG52(39) 52-LEAD (5mm x 8mm) PLASTIC QFN TJMAX = 125C, JA = 32C/W EXPOSED PAD (PIN 53) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3597EUHG#PBF LT3597IUHG#PBF TAPE AND REEL LT3597EUHG#TRPBF LT3597IUHG#TRPBF PART MARKING* 3597 3597 PACKAGE DESCRIPTION 52-Lead (5mm x 8mm) Plastic QFN 52-Lead (5mm x 8mm) 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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LT3597 ELECTRICAL CHARACTERISTICS
PARAMETER VIN Operating Voltage Quiescent Current from VIN Minimum BIAS Voltage Quiescent Current from BIAS EN/UVLO = 0.4V BIAS = 5V, Not Switching BIAS = 0V, Not Switching, Current Out of Pin 0.4 1.47 EN/UVLO = 1.6V EN/UVLO = 1.4V FB = 6V RT = 220k (200kHz) RT = 33.2k (1MHz) ISW1-3 = 100mA 400 225 VSW1-3 = 0V ISW1-3 = 100mA RT = 220k RT = 33.2k 170 900 1.6 RT = 220k RT = 47k VSYNC = 3.3V (Note 4) IVREF = 0A RISET1-3 = 20k, VCTRL = VCTRLM = TSET = 1.5V VTSET = 1.0V RISET1-3 = 20k
l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 24V, BOOST = 29V, BIAS = 5V, EN/UVLO = 5V, PWM1-3 = 3.3V, CTRL1-3 = CTRLM = TSET = 2.0V, VOUT1-3 = 24V, SYNC = 0V, unless otherwise specified. (Note 2)
CONDITIONS
l
MIN 6
TYP
MAX 55 2 4 6 3.1 2 3 150 1.53 6 1.25 200
UNITS V A mA mA V A mA A V V nA A V nA % % mV
EN/UVLO = 0.4V BIAS = 5V, Not Switching BIAS = 0V, Not Switching
2.1 4 3 2 60 0.7 1.51 10 5.1 1.22 95 85 265 510 280 2 200 1000
EN Threshold (Falling) UVLO Threshold (Falling) EN/UVLO Pin Current (Hysteresis) FB1-3 Regulation Voltage FB1-3 Pin Bias Current Maximum Duty Cycle Switch Saturation Voltage Switch Current Limit DA Pin Current to Stop OSC Switch Leakage BST1-3 Pin Current Switching Frequency SYNC Input Low SYNC Input High SYNC Frequency Range SYNC Pin Bias Current Soft-Start Time VREF Voltage Maximum VREF Current ISET1-3 Pin Voltage TSET Voltage for LED Current Derating TSET Pin Leakage Current ILED1-3 LED Current
4 1.15 90 78
700 350 700 230 1100 0.4
mA mA nA mA kHz kHz V V kHz kHz nA ms V A V mV
240
1000 200 2.5
1.96 200
2.0 1.0 540
2.04
200 98 97 100 100 102 103
nA mA mA
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LT3597 ELECTRICAL CHARACTERISTICS
PARAMETER LED String Current Matching LED Pin Voltage LED1-3 Open Detection Threshold LED1-3 Short Protection Threshold (from GND) LED1-3 Short Protection Threshold (from VOUT1-3) LED1-3 Pin Leakage Current PWM1-3 Input Low Voltage PWM1-3 Input High Voltage PWM1-3 Pin Bias Current CTRL1-3 Voltage for Full LED Current CTRL1-3 Pin Bias Current CTRLM Voltage for Full LED Current CTRLM Pin Bias Current FAULT Output Voltage Low FAULT Pin Input leakage Current CTRLM = 3V IFAULT = 200A FAULT = 25V 0.11 200 Note 3: For Maximum Operating Ambient Temperature, see Thermal Considerations in the Applications Information section. Note 4: Guaranteed by design. CTRL1-3 = 6V 1.2 200 1.2 200 1.6 200 PWM1-3 = 3.3V VOUT1-3 = 6V VLED1-3 = 42V 10 1 1.25
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 24V, BOOST = 29V, BIAS = 5V, EN/UVLO = 5V, PWM1-3 = 3.3V, CTRL1-3 = CTRLM = TSET = 2.0V, VOUT1-3 = 24V, SYNC = 0V, unless otherwise specified. (Note 2)
CONDITIONS RISET1-3 = 20k
l
MIN
TYP 0.35 0.35 1.1 0.28
MAX 1.5 2
UNITS % % V V
Adaptive VOUT Loop Enabled
15 2 100 0.4
V V nA V V nA V nA V nA V nA
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 LT3597E 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 LT3597I specifications are guaranteed over the full -40C to 125C operating junction temperature range.
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LT3597 TYPICAL PERFORMANCE CHARACTERISTICS
VIN Quiescent Current
3.0 2.5 VIN CURRENT (mA) 2.0 1.5 1.0 0.5 0 25C VBIAS = 5V 125C VBIAS CURRENT (mA) 3.0 2.5 2.0 1.5 1.0 0.5 0 0 10 20 30 40 VIN VOLTAGE (V) 50 60
3597 G01
TA = 25C, unless otherwise noted.
VBIAS Quiescent Current
VIN = 24V 2.00 125C 25C -40C CURRENT (A)
Shutdown Current
VIN = 55V 1.75 VBIAS = 25V 1.50 1.25 1.00 0.75 0.50 0.25 IVIN IBIAS -25 0 25 50 75 TEMPERATURE (C) 100 125
-40C
-0.5
0
5
15 VBIAS VOLTAGE (V)
10
20
25
3597 G02
0 -50
3597 G03
UVLO Threshold Voltage (Falling)
1.8 1.7 1.6 UVLO VOLTAGE (V) 6 5 4 3 2 1
EN/UVLO Pin Bias Current
2.04
EN/UVLO = 1.4V
VREF Voltage
VREF LOAD = 0A 2.03 2.02 VREF VOLTAGE (V)
CURRENT (A)
1.5 1.4 1.3 1.2 1.1 1.0 -50 -25 0 25 50 75 TEMPERATURE (C) 100 125
2.01 2.00 1.99 1.98
VIN = 24V
VIN = 55V
VIN = 6V
0 -50
EN/UVLO = 1.6V -25 75 0 25 50 TEMPERATURE (C) 100 125
1.97 1.96 -50 -25 0 25 50 75 TEMPERATURE (C) 100 125
3597 G04
3597 G05
3597 G06
Switching Frequency
1.2 1.0 FREQUENCY (MHz) 0.8 0.6 0.4 0.2 0 -50 RT = 220k RT = 33.2k
600 500 400 300 200 100
Current Limit
600
Switch Voltage Drop
VBIAS = 5V 125C 25C -40C 500 400 300 200 100 0
SWITCH
SWITCH VOLTAGE DROP (mV)
CURRENT (mA)
DA
-25
0 25 50 75 TEMPERATURE (C)
100
125
0 -50
-25
0 25 50 75 TEMPERATURE (C)
100
125
0
50
100 150 200 SWITCH CURRENT (mA)
250
3597 G09
3597 G07
3597 G08
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LT3597 TYPICAL PERFORMANCE CHARACTERISTICS
Boost Diode VF
800 700 600 DIODE VF (mV) 500 400 300 200 100 0 0 2 4 6 8 DIODE CURRENT (mA) 10
3597 G10
TA = 25C, unless otherwise noted.
Soft-Start
VEN/UVLO 5V/DIV
60V Switching Waveforms
-40C 25C 125C
VSW 20V/DIV
VSW 20V/DIV
IL 100mA/DIV VOUT 20V/DIV 400s/DIV
3597 G11
IL 100mA/DIV
VOUT 20V/DIV
400ns/DIV
3597 G12
Loop Regulation Voltage
1.4 103 102 LED CURRENT (mA) VFB 101 100 99 98
LED Current
0.50
LED Current Matching
LED CURRENT MATCHING (%)
1.3 VOLTAGE (V)
0.25 CH3 0 CH2 CH1
1.2
CH1
CH2 CH3
1.1
VLED (ADAPTIVE LOOP)
1.0
-0.25
0.9 -50
-25
0 25 50 75 TEMPERATURE (C)
100
125
97 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
-0.50 -50
-25
0 25 50 75 TEMPERATURE (C)
100
125
3597 G13
3597 G14
3597 G15
LED Current vs PWM Duty Cycle
100 10 LED CURRENT (mA) 1 0.1 0.01 0.001 0.0001 0.001
LED CURRENT (mA) 120 100 80 60 40 20 0
LED Current vs CTRL
ILED 100mA/DIV VOUT 10V/DIV VLED 10V/DIV VSW 20V/DIV
Adaptive Loop Operation
0.01
0.1 1 DUTY CYCLE (%)
10
100
3597 G16
0
0.25
0.5 0.75 1 CTRL VOLTAGE (V)
1.25
1.5
400s/DIV
3597 G18
3597 G17
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LT3597 TYPICAL PERFORMANCE CHARACTERISTICS
20,000:1 PWM Dimming (100Hz) TA = 25C, unless otherwise noted.
1,000:1 PWM Dimming (100Hz)
PWM 2V/DIV
PWM 2V/DIV
ILED 50mA/DIV
ILED 50mA/DIV
100ns/DIV
3597 G19
2s/DIV
3597 G20
Open LED Fault
VOUT 5V/DIV VOUT 5V/DIV
Shorted LEDs Fault
VLED 2V/DIV ILED 100mA/DIV VFAULT 5V/DIV
3597 G21
VLED 5V/DIV ILED 100mA/DIV VFAULT 5V/DIV 20ms/DIV
3597 G22
10ms/DIV
FAULT Pin Voltage Low
400 350 FAULT PIN VOLTAGE (mV) 300 250 200 150 100 50 0 0 0.6 0.2 0.4 0.8 FAULT PIN CURRENT (mA) 1
3597 G23
TSET Voltage for Temperature Derating
0.8 120 100 LED CURRENT (mA) 80 60 40 20 0
TSET LED Current Derating
VTSET = 0.675V
125C 25C 40C
0.7 TSET VOLTAGE (V)
0.6
0.5
0.4
0.3 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
25
45
65 85 105 TEMPERATURE (C)
125
3597 G25
3597 G24
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LT3597 PIN FUNCTIONS
BOOST1, BOOST2, BOOST3 (Pins 2, 48, 39): Boost Capacitor Pin. This pin is used to provide a voltage that is higher than the input voltage when the switch is on to supply current to the switch driver. SW1, SW2, SW3 (Pins 4, 46, 41): Switch Pin. Connect the inductor, catch diode and boost capacitor to this pin. DA1, DA2, DA3 (Pins 6, 44, 43): Catch Diode Anode. This pin is used to provide frequency foldback in extreme situations. FB1, FB2, FB3 (Pins 7, 34, 35): Feedback Pin. This pin is regulated to the internal bandgap voltage. The maximum Buck output voltage can be set by connecting this pin to a resistor divider from VOUT1-3. EN/UVLO (Pin 9): Enable and Undervoltage Lockout Pin. Accurate 1.51V threshold. UVLO threshold can be programmed by using a resistor divider from VIN. If function is not required, pin can be tied to the VIN pin. TSET (Pin 11): Thermal Regulation Pin. Programs the LT3597 junction temperature at which LED current begins to derate. VREF (Pin 12): 2.0V Reference Output Pin. This pin can sources up to 200A and can be used to program TSET and CTRLM. CTRLM (Pin 13): Master Control Pin. LED current derating vs temperature is achievable for all channels if the voltage on CTRLM has a negative temperature coefficient using an external NTC resistor in a voltage divider from VREF. ISET1, ISET2, ISET3 (Pins 14, 15, 16): LED Current Programming Pin. A resistor to ground programs full-scale LED current. RT (Pin 17): Switching Frequency Programming Pin. A resistor to ground programs the switching frequency between 200kHz and 1MHz. GND (Pin 18, Exposed Pad Pin 53): Ground Pin. This is the ground for both the IC and the switching converters. Exposed pad must be soldered to PCB ground. VOUT1, VOUT2, VOUT3 (Pins 19, 22, 23): Buck Output. This is the buck regulator output voltage sense into the IC. LED1, LED2, LED3 (Pins 20, 21, 24): Constant Current Sink Pin. These are 3 LED driver outputs, each containing an open collector, constant current sink. All outputs are matched within 2% and are individually programmed up to 100mA using an external resistor at the ISET1-3 pin. Outputs are rated to allow a maximum VOUT1-3 of 42V. Connect the cathode of the LED string to LED1-3. Connect the anode of the LED string to VOUT1-3. FAULT (Pin 25): Fault Detection Pin. Open collector pin used to report open LED faults. FAULT must be externally pulled to a positive supply through a resistor. NC (Pin 26): No Connection Pin. Tie to ground. SYNC (Pin 27): External Clock Synchronization Pin. When an external clock drives this pin, the Buck regulators are synchronized to that frequency. Frequency programmed by the RT pin resistor must be at least 20% less than the SYNC pin clock frequency. PWM1, PWM2, PWM3 (Pins 30, 29, 28): PWM Dimming Control Pin. When driven to a logic high, the LED1-3 current sink is enabled. If PWM dimming is not desired, connect the pin to VREF. Channels can be individually disabled by tying PWM1-3 to ground. CTRL1, CTRL2, CTRL3 (Pins 33, 32, 31): Analog Dimming Control Pin. This pin is used to dim the LED current in an analog fashion. If the pin is tied to a voltage lower than 1.0V, it will linearly reduce the LED current. If feature is unused, connect the pin to VREF . BIAS (Pin 37): Supply Pin. This pin is the supply for an internal voltage regulator to internal analog and digital circuitry. BIAS must be locally bypassed with a 2.2F capacitor. VIN (Pins 50, 51): Input Supply Pin. VIN must be locally bypassed with a 10F capacitor to ground. Pins 50 and 51 are internally fused.
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LT3597 BLOCK DIAGRAM
LT3597 EN/UVLO VREF BIAS START-UP REFERENCE BIAS VIN
+-
BSTn S R Q SWn
SYNC RT OSC
SLOPE COMP
+
SOFT-START AND CLAMP VC
DAn
-
GND
CHANNEL n n = 1-3
+ - + -
1.22V
FBn VOUTn FAULT
540mV PTAT
1.1V
TSET CTRLM CTRLn ISETn PWMn
+ -
CONVERSION AND CONTROL
LED FAULT DETECTION
LOGIC
LEDn LED DRIVE CIRCUITRY
PWM DIMMING LOGIC
GND, EXPOSED PAD
3597 BD
Figure 1. Block Diagram
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LT3597 OPERATION
The LT3597 uses a constant frequency, internally compensated peak current mode control scheme. Operation is best understood by referring to the Block Diagram in Figure 1. Enable and undervoltage lockout (UVLO) are both controlled by a single pin. If the pin falls below 1.51V, an accurate comparator turns off the LED drivers and buck regulators. If the pin continues to fall to less than 0.4V, the part enters shutdown and consumes less than 2A. The LT3597 contains three constant current sink LED drivers. Each of the three LED strings is powered from a dedicated adaptive buck converter in order to achieve maximum efficiency. The frequency of the buck regulators is programmed from 200kHz to 1MHz using an external resistor. The frequency can also be synchronized to an external clock using the SYNC pin. Internal buck compensation and soft-start requires few external components and permits simple board layout. A high-side switch current limit protects the internal switch during its on time, while a low side current limit prevents the switch from turning on in the case of excessive off phase current. Step-Down Adaptive Control Adaptive control of the output voltages achieves superior system efficiency. When a given channel's PWM pin is low, the respective buck regulator output will go to a programmable high output voltage. In this case the buck will enter into a pulse-skipping mode since there is no load connected. This guarantees that the buck output voltage is high enough to immediately supply the LED current once the string is reactivated. As soon as PWM goes high, the output voltage of the buck will drop until there is 1V across the LED current sink. This scheme ensures the best efficiency for each LED channel. Since each LED string is independently driven from a separate buck channel, efficiency is optimized for all three strings even if the number of LEDs is mismatched between the channels. Another benefit of this regulation method is that the LT3597 starts up with 10,000:1 dimming even if the PWM1-3 pulse width is 1s. Since VOUT starts up even if PWM1-3 is low, the part achieves high dimming ratios with narrow pulse widths within a couple of PWM1-3 clock cycles. LED Current Each LED string current can be individually programmed up to a maximum of 100mA with a 2% matching accuracy between the strings. An external resistor on the ISET1-3 pin programs the max current for that string. The CTRL1-3 pin can be used for analog dimming. Digital PWM can be programmed using the PWM1-3 pin. A dimming ratio of 10,000:1 can be achieved at a frequency of 100Hz. Fault Protection and Reporting The LT3597 protects against both open LED and shorted LED conditions. If the LED1-3 pin voltage exceeds 12V while the LED string is sinking current, or if the LED1-3 pin voltage is within 1.25V of VOUT1-3 pin voltage, the channel is disabled until the fault is removed. If the LED string opens, the LT3597 will limit the output to the voltage set by the FB resistor divider. The LT3597 reports a fault on the FAULT pin if any of the LED strings is open or shorted. LED faults are only reported if the respective string PWM signal is high. A fault is also reported if the internal die temperature reaches the TSET programmed derating limit.
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LT3597 APPLICATIONS INFORMATION
Inductor Selection Inductor values between 100H and 470H are recommended for most applications. It is important to choose an inductor that can handle the peak current without saturating. The inductor DCR (copper wire resistance) must also be low in order to minimize I2R power losses. Table 1 lists several recommended inductors.
Table 1. Recommended Inductors
PART LPS6225 MSS1038 MSS1038 MSS1038 CDRH105R CDRW105R CDRH105R CDR6D28MN DS1262C2 DS1262C2 DS1262C2 SLF10145T SLF10145T DR73 DR73 L (H) 100 100 220 470 100 220 470 100 100 220 470 100 220 100 220 DCR () 0.61 0.3 0.76 1.24 0.253 0.50 1.29 0.9 0.17 0.35 1.243 0.26 0.47 0.527 1.05 CURRENT RATING (A) 0.52 1.46 0.99 0.70 1.35 0.94 0.60 0.75 1.5 1.0 0.7 1.0 0.7 0.79 0.53 500 VENDOR Coilcraft www.coilcraft.com 200
Typically 10F capacitors are sufficient for the VIN and BIAS pins. The output capacitor for the buck regulators depends on the number of LEDs and switching frequency. Refer to Table 3 for the proper output capacitor selection.
Table 3. Recommended Output Capacitor Values (VLED = 3.5V)
SWITCHING FREQUENCY (kHz) 1000 # LEDS 1-3 >3 1-3 >3 1-3 >3 COUT (F) 3.3 2.2 4.7 3.3 15 6.8
Sumida www.sumida.com
Diode Selection Schottky diodes, with their low forward voltage drop and fast switching speed, must be used for all LT3597 applications. Do not use P-N junction diodes. The diode's average current rating must exceed the application's average current. The diode's maximum reverse voltage must exceed the application's input voltage. Table 4 lists some recommended Schottky diodes.
Table 4. Recommended Diodes
PART DFLS160 B160 CMMSH1-60 ESIPB MAX CURRENT MAX REVERSE (A) VOLTAGE (V) 1 1 1 1 60 60 60 100 MANUFACTURER Diodes, Inc. www.diodes.com Central www.centralsemi.com Vishay www.vishay.com
Toko www.toko.com TDK www.tdk.com Coiltronics www.cooperet.com
Capacitor Selection Low ESR (equivalent series resistance) capacitors should be used at the outputs 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. Table 2 lists some suggested manufacturers. Consult the manufacturers for detailed information on their entire selection of ceramic surface mount parts.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden AVX Murata Kemet TDK www.t-yuden.com www.avxcorp.com www.murata.com www.kemet.com www.tdk.com
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LT3597 APPLICATIONS INFORMATION
Undervoltage Lockout (UVLO) The EN/UVLO can be used to program the input UVLO threshold by connecting it to a resistor divider from the VIN pin as shown in Figure 2.
LT3597 VIN R2 R1 EN/UVLO
Programming Maximum LED Current Maximum LED current can be programmed by placing a resistor (RISET1-3) between the ISET1-3 pin and ground. RISET1-3 values between 20k and 100k can be chosen to set the maximum LED current between 100mA and 20mA respectively. The LED current is programmed according to the following equation:
+ -
1.51V
ILED1-3 = 2 *
1V RISET1-3
(mA)
3597 F02
Figure 2. EN/UVLO Control
See Table 5 and Figure 3 for resistor values and corresponding programmed LED current.
Table 5. LED Current Programming
RISET1-3 VALUE (k) 20 25 33.3 50 100 LED CURRENT (mA) 100 80 60 40 20
Select R1 and R2 according to the following equation: R2 VIN(UVLO) = 1.51V * 1+ R1 In UVLO an internal 5.1A pull-down current source is connected to the pin for programmable UVLO hysteresis. The hysteresis can be set according to the following equation: VUVLO(HYST) = 5.1A * R2
100
LED CURRENT (mA)
Care must be taken if too much hysteresis is programmed, the pin voltage might drop too far and cause the current source to saturate. Once the EN/UVLO pin falls below 0.4V, the part enters into shutdown.
80
60
40
20
0
0
25
50 RSET1-3 (k )
75
100
3597 F03
Figure 3. RISET1-3 Value for LED Current
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LT3597 APPLICATIONS INFORMATION
LED Current Dimming Two different types of dimming control are available with the LT3597. The LED current can be dimmed using the CTRL1-3 pin or the PWM1-3 pin. For some applications, a variable DC voltage that adjusts the LED current is the preferred method for brightness control. In that case, the CTRL1-3 pin can be modulated to set the LED dimming (see Figure 4). As the CTRL1-3 pin voltage rises from 0V to 1.0V, the LED current increases from 0mA to the maximum programmed LED current in a linear fashion. As the CTRL1-3 pin continues to increase past 1.0V, the maximum programmed LED current is maintained. If this type of dimming control is not desired, the CTRL1-3 pin can be tied to VREF .
120 100 LED CURRENT (mA) 80 60 40 20 0
For True Color PWM dimming, the LT3597 provides up to 10,000:1 PWM dimming range at 100Hz. This is achieved by allowing the duty cycle of the PWM1-3 pin to be reduced from 100% to 0.01% for a PWM frequency of 100Hz (see Figure 5), hence a minimum on-time of 1s and a maximum period of 100ms. PWM duty cycle dimming allows for constant LED color to be maintained over the entire dimming range. Using the TSET Pin for Thermal Protection The LT3597 contains a special programmable thermal regulation loop that limits the internal junction temperature. This thermal regulation feature provides important protection at high ambient temperatures, and allows a given application to be optimized for typical, not worstcase, ambient temperatures with the assurance that the LT3597 will automatically protect itself and the LED strings under worst-case conditions. As the ambient temperature increases, so does the internal junction temperature of the part. Once the programmed maximum junction temperature is reached, the LT3597 linearly reduces the LED current, as needed, to maintain this junction temperature. This can only be achieved when the ambient temperature stays below the maximum programmed junction temperature. If the ambient temperature continues to rise above the programmed maximum junction temperature, the LED current will reduce to less than 20% of the full current. A resistor divider from the VREF pin programs the maximum part junction temperature as shown in Figure 6.
LT3597
0
0.25
0.5
0.75 1 CTRL1-3 (V)
1.25
1.5
3597 F04
Figure 4. LED Current vs CTRL1-3 Voltage
tPWM tON(PWM) PWM1-3 LED1-3 CURRENT MAX ILED
3597 F06
VREF R2 TSET R1
3597 F07
Figure 5. LED Current Using PWM Dimming
Figure 6. Programming the TSET Pin
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LT3597 APPLICATIONS INFORMATION
Table 6 shows commonly used values for R1 and R2. Choose the ratio of R1 and R2 for the desired junction temperature limit as described in Figure 7.
Table 6. TSET Programmed Junction Temperature
TJ (C) 85 100 115 R1 (k) 49.9 49.9 49.9 R2 (k) 97.6 90.9 84.5
LED Current Derating Using the CTRLM Pin Another feature of the LT3597 is its ability to program a derating curve for maximum LED current versus temperature. LED data sheets provide curves of maximum allowable LED current versus temperature to warn against exceeding this current limit and damaging the LED. The LT3597 allows the output LEDs to be programmed for maximum allowable current while still protecting the LEDs from excessive currents at high temperature. This is achieved by programming a voltage at the CTRLM pin with a negative temperature coefficient using a resistor divider with temperature dependent resistance (Figure 8). As ambient temperature increases, the CTRLM voltage will fall below the internal 1V voltage reference, causing LED currents to be controlled by the CTRLM pin voltage. The LED current curve breakpoint and slope versus temperature are defined by the choice of resistor ratios and use of temperature-dependent resistance in the divider for the CTRLM pin.
LT3597 VREF R2 R1 (OPTION A TO D) CTRLM RNTC A B RNTC RX RY RY RNTC C D
3597 F08
The TSET pin must be tied to VREF if the temperature protection feature is not desired.
0.8
0.7 TSET VOLTAGE (V)
0.6
0.5
0.4
0.3 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
RNTC
RX
3597 F07
Figure 7. TSET Voltage for Temperature Derating
Figure 8. Programming the CTRLM Pin
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14
LT3597 APPLICATIONS INFORMATION
Table 7 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. If an NTC resistor is used to indicate LED temperature, it is effective only if the resistor is placed as closely as possible to the LED strings. LED derating curves shown by manufacturers are listed for ambient temperature. The NTC resistor should have the same ambient temperature as the LEDs. 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 value over temperature from the manufacturer. Hand calculations of the CTRLM voltage can then be performed at each given temperature, resulting in the CTRLM versus temperature plotted curve. Iterations of resistor value calculations may be necessary to achieve the desired break point and slope of the LED current derating curve. From the CTRLM voltage, the LED current can be found using the curve shown in Figure 9.
Table 7. NTC Resistor Manufacturers/Distributors
Murata TDK Corporation Digi-Key www.murata.com www.tdk.com www.digikey.com
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 (CTRLM behavior) over temperature. Referred to as the "Murata Chip NTC Thermistor Output Voltage Simulator," users can log onto www.murata.com and download the software followed by instructions for creating an output voltage VOUT (CTRLM) from a specified VCC supply (VREF). The CTRLM pin must be tied to VREF if the temperature derating function is not desired. Programming Switching Frequency The switching frequency of the LT3597 can be programmed between 200kHz and 1MHz by an external resistor connected between the RT pin and ground. Do not leave this pin open. See Table 8 and Figure 10 for resistor values and corresponding frequencies.
Table 8. RT Resistor Selection
SWITCHING FREQUENCY (MHz) 1.0 0.5 0.2 RT VALUE (k) 33.2 80 220
If calculating the CTRLM voltage at various temperatures gives a downward slope that is too strong, use alternative resistor networks (B, C, D in Figure 8). They use temperature independent resistance to reduce the effects off the NTC resistor over temperature.
120
1.2 SWITCHING FREQUENCY (MHz)
0 0.25 0.5 0.75 1 CTRLM (V) 1.25 1.5
3597 F09
100 LED CURRENT (mA) 80 60 40 20 0
1.0 0.8 0.6 0.4 0.2 0
25
50
75
100 125 150 175 200 225 RT (k )
3597 F10
Figure 9. LED Current vs CTRLM Voltage
Figure 10. Programming Switching Frequency
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15
LT3597 APPLICATIONS INFORMATION
Selecting the optimum switching frequency depends on several factors. Inductor size is reduced with higher frequency, but efficiency drops slightly due to higher switching losses. Some applications require very low duty cycles to drive a small number of LEDs from a high supply. Low switching frequency allows a greater range of operational duty cycle and hence a lower number of LEDs can 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. Switching Frequency Synchronization The nominal operating frequency of the LT3597 is programmed using a resistor from the RT pin to ground over a 200kHz to 1MHz range. In addition, the internal oscillator can be synchronized to an external clock applied to the SYNC pin. The synchronizing clock signal input to the LT3597 must have a frequency between 240kHz and 1MHz, a duty cycle between 20% and 80%, a low state below 0.4V and a high state above 1.6V. Synchronization signals outside of these parameters will cause erratic switching behavior. For proper operation, an RT resistor is chosen to program a switching frequency 20% slower than the SYNC pulse frequency. Synchronization occurs at a fixed delay after the rising edge of SYNC. The SYNC pin must be grounded if the clock synchronization feature is not used. When the SYNC pin is grounded, the internal oscillator controls the switching frequency of the converter. Operating Frequency Trade-offs Selection of the operating frequency is a trade-off between efficiency, component size, output voltage and maximum input voltage. The advantage of high frequency operation is smaller component sizes and values. The disadvantages are lower efficiency and lower input voltage range for a desired output voltage. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX) = VD + VOUT tON(MIN) ( VD + VIN - VSW )
where VIN is the typical input voltage, VOUT is the output voltage, VD is the catch diode drop (0.5V) and VSW is the internal switch drop (0.5V at max load). This equation shows that slower switching is necessary to accommodate high VIN /VOUT ratios. The reason the input voltage range depends on the switching frequency is due to the finite minimum switch on and off times. The switch minimum on and off times are 200ns. Adaptive Loop Control The LT3597 uses an adaptive control mechanism to set the buck output voltage. This control scheme ensures maximum efficiency while not compromising minimum PWM pulse widths. When PWM1-3 is low, the output of the respective buck rises to a maximum value set by an external resistor divider to the respective FB pin. Once PWM1-3 goes high, the output voltage is adaptively reduced until the voltage across the LED current sink is 1V. Figure 11 shows how the maximum output voltage can be set by an external resistor divider.
LT3597 VOUT1-3 R2 FB1-3 R1
3597 F11
VOUT1-3
Figure 11. Programming Maximum VOUT1-3
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16
LT3597 APPLICATIONS INFORMATION
The maximum output voltage must be set to exceed the maximum LED drop plus 1V by a margin greater than 10%. However, this margin must not exceed a voltage of 10V. This ensures proper adaptive loop control. The equations below are used to estimate the resistor divider ratio. The sum of the resistors should be less than 100k to avoid noise coupling to the FB pin. R2 VOUT(MAX) = 1.1 VLED(MAX) + 1.1V = 1.2V * 1+ R1 Fault Flag The FAULT pin is an open-collector output and needs an external resistor tied to a supply. If the LED1-3 pin voltage exceeds 12V or if the LED1-3 pin voltage is within 1.25V of VOUT1-3 pins while PWM1-3 is high, the FAULT pin will be pulled low. The FAULT pin will also be pulled low if the internal junction temperature exceeds the TSET programmed temperature limit. There is a 3s delay for FAULT flag generation when the PWM1-3 signal is enabled to avoid generating a spurious flag signal. The maximum current the FAULT can sink is typically 1mA. Thermal Considerations The LT3597 provides three channels for LED strings with internal NPN devices serving as constant current sources. When LED strings are regulated, the lowest LED pin voltage is typically 1V. More power dissipation occurs in the LT3597 at higher programmed LED currents. For 100mA of LED current with a 100% PWM dimming ratio, at least 300mW is dissipated within the IC due to current sources. Thermal calculations must include the power dissipation in the current sources in addition to conventional switch DC loss, switch transient loss and input quiescent loss. In addition, the die temperature of the LT3597 must be lower than the maximum rating of 125C. This is generally not a concern unless the ambient temperature is above 100C. Care should be taken in the board layout to ensure good heat sinking of the LT3597. The maximum load current should be derated as the ambient temperature approaches 125C. The die temperature rise above ambient is calculated by multiplying the LT3597 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3597 is estimated by calculating the total power loss from an efficiency measurement and subtracting the losses of the catch diode and the inductor. Thermal resistance depends on the layout of the circuit board, but 32C/W is typical for the 5mm x 8mm QFN package.
(
)
VOUT(MAX) = VLED(MAX) +1.1V +VMARGIN VMARGIN 10V Minimum Input Voltage The minimum input voltage required to generate an output voltage is limited by the maximum duty cycle and the output voltage (VOUT) set by the FB resistor divider. The duty cycle is: VD + VOUT DC = VIN - VCESAT + VD where VD is the Schottky forward drop and VCESAT is the saturation voltage of the internal switch. The minimum input voltage is: VD + VOUT(MAX) VIN(MIN) = + VCESAT - VD DCMAX where VOUT(MAX) is calculated from the equation in the Adaptive Loop Control section, and DCMAX is the minimum rating of the maximum duty cycle.
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17
LT3597 APPLICATIONS INFORMATION
Board Layout As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To prevent electromagnetic interference (EMI) problems, proper layout of high frequency switching paths is essential. Minimize the length and area of all traces connected to the switching node pin (SW). Always use a ground plane under the switching regulator to minimize interplane coupling. Good grounding is essential in LED fault detection. Proper grounding is also essential for the external resistors and resistor dividers that set critical operation parameters. Both the LT3597 exposed pad and pin 18 are ground. Resistors connected between ground and the CTRL1-3, CTRLM, FB1-3, TSET, ISET1-3, RT and EN/UVLO pins are best tied to pin 18 and not the ground plane.
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18
LT3597 TYPICAL APPLICATIONS
48V 1MHz Triple Step-Down 100mA RGB LED Driver
VIN 48V VIN EN/UVLO 91k BOOST1 VOUT1 97k R 9.1k 100H 3.3F DA1 FB1 VOUT1 LED1 BIAS 10F 3 3 100k FAULT PWM1-3 CTRL1-3 SYNC RT 33.2k (1MHz) ISET1 20k ISET2 20k ISET3 20k GND LT3597 VOUT2 LED2 BOOST3 0.1F 100H SW3 2.2F DA3 FB3 VOUT3 LED3 VREF TSET CTRLM
3597 TA02
10F
270k
BOOST2 0.1F 100H SW2 2.2F DA2 FB2 4.7k B 97k VOUT2
0.1F SW1
VOUT3 97k 3.83k G
VCC 5V
10k
82.5k 49.9k
VREF
100k
Efficiency
90 80 70 EFFICIENCY (%) 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 LED CURRENT PER CHANNEL (mA)
3597 TA02b
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19
LT3597 TYPICAL APPLICATIONS
48V 1MHz Triple Step-Down 10W 100mA White LED Driver (3.6V LEDs)
VIN 48V
10F
270k 91k
VIN EN/UVLO BOOST1
BOOST2 0.1F 100H SW2 2.2F DA2 FB2 VOUT2 LED2 BOOST3 0.1F 100H SW3 2.2F DA3 FB3 3.24k VOUT3 97k 3.24k VOUT2 97k
VOUT1 97k 3.24k
100H 2.2F
0.1F SW1 DA1 FB1 VOUT1 LED1 BIAS LT3597
VCC 5V
10F 3 3
100k FAULT PWM1-3 CTRL1-3 SYNC RT 33.2k ISET1 20k ISET2 20k ISET3 20k
VOUT3 LED3 VREF TSET GND CTRLM
3597 TA03
10k
82.5k 49.9k
VREF
100k
Efficiency
90 80 70 EFFICIENCY (%) 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 LED CURRENT PER CHANNEL (mA)
3597 TA03b
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20
LT3597 TYPICAL APPLICATIONS
24V 200kHz Triple Step-Down 100mA RGB LED Driver
VIN 24V VIN EN/UVLO BOOST1 VOUT1 97k R 9.1k 470H 15F DA1 FB1 VOUT1 LED1 BIAS 10F 3 3 100k FAULT PWM1-3 CTRL1-3 SYNC RT 220k ISET1 20k ISET2 20k ISET3 20k GND LT3597 VOUT2 LED2 BOOST3 SW3 15F DA3 FB3 VOUT3 LED3 VREF TSET CTRLM
3597 TA04
10F
BOOST2 SW2
0.22F
470H 15F 97k
VOUT2
DA2 FB2 9.1k G
0.22F SW1
0.22F
470H 97k
VOUT3
9.1k
B
VREF
Efficiency
90 80 70 EFFICIENCY (%) 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 LED CURRENT PER CHANNEL (mA)
3597 TA04b
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21
LT3597 TYPICAL APPLICATIONS
48V 1MHz Triple Step-Down 20mA RGB LED Driver
VIN 48V VIN EN/UVLO 91k BOOST1 VOUT1 97k R 9.1k 100H 3.3F DA1 FB1 VOUT1 LED1 BIAS 10F 3 3 100k FAULT PWM1-3 CTRL1-3 SYNC RT 33.2k ISET1 100k ISET2 100k ISET3 100k GND LT3597 VOUT2 LED2 BOOST3 0.1F 100H SW3 2.2F DA3 FB3 VOUT3 LED3 VREF TSET CTRLM
3597 TA05
10F
270k
BOOST2 0.1F 100H SW2 2.2F DA2 FB2 4.7k B 97k VOUT2
0.1F SW1
VOUT3 97k 3.83k G
VCC 5V
10k
82.5k 49.9k
VREF
100k
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22
LT3597 PACKAGE DESCRIPTION
(Reference LTC DWG # 05-08-1846 Rev B)
43 41 39 37 6.40 REF 35 34 33 32 31 30 29 28 27 0.70 0.05 44 46 48 2.90 0.05 5.90 0.05 26 25 24 23 22 3.20 REF 21 20 19 18 PACKAGE OUTLINE 2 4 6 0.80 BSC 7 9 11 12 13 14 15 16 0.40 BSC 17 0.20 0.05
UHG Package Variation: UHG52 (39) 52-Lead Plastic QFN (5mm x 8mm)
5.50 0.05 4.10 0.05
50 51
7.10 0.05 8.50 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 51 50 48 46 44 PIN 1 NOTCH R = 0.30 TYP OR 0.35 x 45 CHAMFER 50 51 0.40 0.10 43 43 2 41 4 39 6 7 9 8.00 0.10 11 12 13 14 15 16 17 37 35 34 33 32 31 30 29 28 27 37 35 6.40 REF 34 33 32 31 30 29 28 27 5.90 0.10 2.90 0.10 39 6 7 9 11 12 13 14 15 16 17 0.70 TYP 0.200 REF 0.75 0.05 18 19 20 21 22 23 24 25 26 0.00 - 0.05 NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 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 26 25 24 23 22 21 20 19 3.20 REF BOTTOM VIEW--EXPOSED PAD 18
(UHG39) QFN 0410 REV B
5.00 0.10 PIN 1 TOP MARK (SEE NOTE 6)
0.75 0.05 0.00 - 0.05
R = 0.10 TYP 44
R = 0.10 TYP 46 48
2
41 4 0.20 0.05 0.80 BSC 0.60 TYP
0.40 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.
23
LT3597 TYPICAL APPLICATION
Triple Step-Down RGB Single Pixel LED Driver, 100mA Current
VIN 48V 10F 270k 91k BOOST1 VOUT1 97k R 9.1k 100H 3.3F DA1 FB1 VOUT1 LED1 BIAS 10F 100k FAULT PWM1:3 CTRL1:3 SYNC RT 33.2k ISET1 20k ISET2 20k ISET3 20k GND LT3597 VOUT2 LED2 BOOST3 0.1F 100H SW3 3.3F DA3 FB3 VOUT3 LED3 VREF TSET CTRLM
3597 TA06
10,000:1 Dimming at 100Hz
VOUT2 3.3F 97k 9.1k G PWM 2V/DIV
VIN EN/UVLO
BOOST2 0.1F 100H SW2 DA2 FB2
0.1F SW1
ILED 50mA/DIV VOUT3 97k 9.1k B 200ns/DIV
3597 TA06b
VCC 5V 10F
10k
82.5k 49.9k
VREF
100k
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2A Boost Converter with Internal 4-String 150mA LED VIN: 3V to 30V, VOUT(MAX) = 44V, True Color PWM Dimming = 1000:1, ISD < 1A, 5mm x 5mm QFN-32 and TSSOP-28 Packages Ballaster 16-Channel x 50mA LED Driver with 60V Boost Controller and PWM Dimming 8-Channel x 100mA LED Driver with 60V Boost Controller and PWM Dimming VIN: 6V to 40V, VOUT(MAX) = 60V, True Color PWM Dimming = 3000:1, ISD < 2A, 5mm x 5mm QFN-52 Package VIN: 6V to 40V, VOUT(MAX) = 60V, True Color PWM Dimming = 3000:1, ISD < 2A, TSSOP-28 Package
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24 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0311 * PRINTED IN USA
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2011

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