LT3598 6-String 30mA LED Driver with 1.5% Current Matching DESCRIPTION FEATURES n n n n n n n n n n n n n n True Color PWM™ Dimming Delivers Up to 3000:1 Dimming Ratio Drives Six Strings of LEDs at Up to 30mA 1.5% Accurate LED Current Matching Wide Input Voltage Range: 3.2V to 30V Output Voltage Up to 44V Regulates Current Even When VIN > VOUT Disconnects LEDs in Shutdown Programmable Open LED Protection (Regulated) OPENLED Alert Pin Programmable LED Current Derating Adjustable Frequency: 200kHz to 2.5MHz Synchronizable to an External Clock Parallel Channels for Higher Current per LED String Thermally Enhanced 4mm × 4mm QFN Package APPLICATIONS n n n Notebook Computer Display Medium Size Displays Automotive LCD Display The LT®3598 is a fixed frequency step-up DC/DC converter designed to drive up to six strings of LEDs at an output voltage up to 44V. LED dimming can be achieved with analog dimming on the CTRL pin, and with pulse width modulation dimming on the PWM pin. The LT3598 accurately regulates LED current even when the input voltage is higher than the LED output voltage. The switching frequency is programmable from 200kHz to 2.5MHz through an external resistor. Additional features include programmable overvoltage protection, switching frequency synchronization to an external clock, LED current derating based on junction temperature and/or LED temperature, LED string disable control, OPENLED alert pin and output voltage limiting when all LED strings are disconnected. The LT3598 is available in a thermally enhanced 24-pin (4mm × 4mm) QFN package. 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. Protected by U.S. Patents, including 7199560, 7321203. TYPICAL APPLICATION LED Current Matching 90% Efficient LED Driver for 60 White LEDs PVIN 8V TO 40V 10μH 1.5 2.2μF SW VOUT 1.00M 100k OPENLED SHDN SHDN PWM PWM SYNC SYNC ALL SIX LED STRINGS 1.0 VO_SW VIN MATCHING (%) VIN 5V 2.2μF 4.7μF FB 30.9k LT3598 0.5 0 –0.5 RT 51.1k –1.0 VREF 10k CTRL 100k TSET ISET SS 60.4k 47pF 100k 14.7k –1.5 –50 LED1 LED2 LED3 LED4 LED5 LED6 GND VC 10nF –25 0 25 50 75 100 125 TEMPERATURE (°C) 20mA 3598 TA01b 2.61k 15nF 3598 TA01a 3598f 1 LT3598 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) GND SHDN VIN VOUT VO_SW SW TOP VIEW SHDN ................................................................VIN + 3V VIN, OPENLED ...........................................................30V SW Voltage ...............................................................45V VOUT, VO_SW Voltage .................................................44V LED1 to LED6............................................................44V PWM, SYNC, CTRL, RT, SS, VC ...................................6V VREF, FB Voltage..........................................................6V ISET, TSET.....................................................................6V Operating Junction Temperature Range (Note 2).................................................. –40°C to 125°C Maximum Junction Temperature........................... 125°C Storage Temperature Range................... –65°C to 150°C 24 23 22 21 20 19 LED1 1 18 VREF LED2 2 17 SS LED3 3 16 RT 25 LED4 4 15 PWM 13 NC TSET 9 10 11 12 FB 8 VC OPENLED 7 ISET 14 SYNC LED6 6 CTRL LED5 5 UF PACKAGE 24-LEAD (4mm × 4mm) PLASTIC QFN TJMAX = 125°C, θJA = 37°C/W EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3598EUF#PBF LT3598EUF#TRPBF 3598 24-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C LT3598IUF#PBF LT3598IUF#TRPBF 3598 24-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VSHDN = VIN unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN Minimum Operating Voltage TYP MAX 3 3.2 V 30 V 1.230 1.260 1.260 V V 0.01 0.03 %/V Maximum Operating Voltage Reference Voltage l 1.216 1.210 UNITS Reference Voltage Line Regulation 3 < VIN < 30V, VC = 0.3V Maximum VREF Pin Current Out of Pin FB Pin Bias Current VFB = 1.230V (Note 3) 100 FB Error Amp Transconductance ΔI = 5μA 300 μmhos 600 V/V 200 FB Error Amp Voltage Gain FB Pin Voltage Current Loop Amp Transconductance 1.22 μA 1.24 21 250 1.26 nA V μmhos 3598f 2 LT3598 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VSHDN = VIN unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS Current Loop Amp Voltage Gain 80 V/V VC Sink Current 10 μA Quiescent Current VSHDN = 5V, PWM = 0V, Not Switching Quiescent Current in Shutdown VSHDN = 0V ISET Voltage VCTRL = 1.5V, VTSET = 1.5V, RISET = 14.7kΩ LED Current RISET = 14.7kΩ LED String Current Matching 20mA LED Current 0.985 19.5 l LED Open Detection Threshold OPENLED Sink Current 3.5 5 mA 0 1 μA 1.000 1.015 V 20 20.7 mA ±0.5 ±1.5 % 0.2 0.25 V 2 Minimum LED Regulation Voltage mA 0.8 V LED1-6 Leakage Current VLED1-6 = 1V, VOUT = 5V, PWM = 0V VLED1-6 = 42V, VOUT = 44V, PWM = 0V 0.1 0.2 1 2 μA μA CTRL Pin Bias Current VCTRL = 0.8V (Note 4) 50 125 nA Switching Frequency RT = 309kΩ RT = 51.1kΩ RT = 14.7kΩ 190 1 2.5 209 1.1 2.75 kHz MHz MHz 171 0.9 2.25 TSET Voltage Maximum Switch Duty Cycle RT = 309kΩ RT = 51.1kΩ RT = 14.7kΩ l 90 87 80 mV 95 90 86 % % % Switch Current Limit (Note 5) Switch VCESAT ISW = 0.5A 0.12 Switch Leakage Current VSW = 40V 0.2 5 μA SHDN Pin Current VSHDN = 0V, VIN = 0V VSHDN = 5V, VIN = 5V 0.1 30 1 60 μA μA SHDN Voltage High 1.5 602 2 VSS = 0.1V PWM Input High Voltage 5 V 10 PWM = 3.3V SYNC Input High Voltage V 15 μA V 0.1 0.4 V 1 μA 1.5 V SYNC Input Low Voltage SYNC Pin Bias Current 0.4 1 PWM Input Low Voltage PWM Pin Bias Current SYNC = 0V SYNC = 3.3V VO_SW Switch Resistance 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 LT3598E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, A V 1.6 SHDN Voltage Low Soft-Start Charging Current 2.5 25 0.1 0.4 V 50 1 μA μA 1000 Ω characterization and correlation with statistical process controls. The LT3598I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note 3: Current flows out of FB pin. Note 4: Current flows out of CTRL pin. Note 5: Current limit guaranteed by design and/or correlation to static test. Current limit is independent of duty cycle and is guaranteed by design. 3598f 3 LT3598 TYPICAL PERFORMANCE CHARACTERISTICS SHDN Pin Turn-On Threshold SHDN Pin Current 1.5 Quiescent Current 50 6 125°C 1.4 1.3 –50°C 40 25°C QUIESCENT CURRENT (mA) SHDN PIN CURRENT (μA) SHDN THRESHOLD (V) 45 35 30 25 20 15 10 5 4 3 5 1.2 –50 –25 0 25 50 75 100 0 125 0 5 10 TEMPERATURE (°C) 20 15 VSHDN (V) 25 2 –50 35 Reference Voltage Oscillator Frequency VIN = 5V 1.225 1.220 50 75 100 125 Current Limit 2.8 2.5MHz 2.5 2.4 2.0 CURRENT (A) VIN = 30V OSCILLATOR FREQUENCY (MHz) VIN = 40V 25 3598 G03 3.0 1.230 0 3598 G02 1.240 1.235 –25 TEMPERATURE (°C) 3598 G01 REFERENCE VOLTAGE (V) 30 1.5 1MHz 1.0 2.0 1.6 1.2 0.5 0.8 200kHz 1.215 –50 –25 0 25 50 75 100 0 –50 125 –25 0 25 50 75 100 0.4 –50 125 –25 TEMPERATURE (°C) TEMPERATURE (°C) 0 25 50 75 3598 G05 3598 G04 125 3598 G06 Soft-Start Pin Current VCESAT 0.40 100 TEMPERATURE (°C) Feedback Pin Voltage 15.0 1.24 12.5 1.23 0.35 0.25 ISS (μA) VCESAT (V) FEEDBACK VOLTAGE (V) 25°C 0.30 125°C 0.20 –50°C 0.15 0.10 10.0 7.5 VC = 1.5V 1.22 VC = 1V 1.21 0.05 0 0 0.25 0.50 0.75 1.00 1.25 1.5O SWITCH CURRENT (A) 5.0 –50 –25 0 25 50 75 100 125 –25 0 25 50 75 100 125 TEMPERATURE (°C) TEMPERATURE (°C) 3598 G07 1.20 –50 3598 G08 3598 G09 3598f 4 LT3598 TYPICAL PERFORMANCE CHARACTERISTICS LED Current vs PWM Duty Cycle LED Current vs Temperature 100 1 0.1 0.01 20 LED CURRENT (mA) LED CURRENT (mA) LED CURRENT (mA) 25 20.2 10 20.1 20.0 19.9 0.1 1 10 PWM DUTY CYCLE (%) 19.7 –50 100 15 10 5 19.8 0.001 0.01 –25 0 25 50 75 100 125 0 TEMPERATURE (°C) 3.0 1.0 2.5 OPENLED CURRENT (mA) 1.5 0.0 –0.5 –1.5 –50 –25 0 25 50 75 100 125 1.0 1.2 PWM 5V/DIV SW 20V/DIV 2.0 1.5 IL 1A/DIV ILED1 50mA/DIV 1.0 20μs/DIV 0 –50 –25 0 25 50 75 100 3598 G15 125 TEMPERATURE (°C) TEMPERATURE (°C) 3598 G14 3598 G13 LED Current Waveforms (0.1% PWM) LED Current Waveforms (90% PWM) LED Current Waveforms (90% PWM) PWM 5V/DIV PWM 5V/DIV PWM 5V/DIV SW 20V/DIV SW 20V/DIV SW 20V/DIV IL 1A/DIV ILED1 50mA/DIV IL 1A/DIV ILED1 50mA/DIV IL 1A/DIV ILED1 50mA/DIV 2μs/DIV 0.4 0.8 0.6 CTRL VOLTAGE (V) LED Current Waveforms (0.1% PWM) 0.5 –1.0 0.2 3598 G12 OPENLED Sink Current LED Current vs Temperature 0.5 0 3598 G11 3598 G10 MATCHING (%) LED Current vs CTRL Voltage 20.3 3598 G16 100μs/DIV 3598 G17 5μs/DIV 3598 G18 3598f 5 LT3598 PIN FUNCTIONS LED1-6 (Pins 1, 2, 3, 4, 5, 6): LED String Output. Connect the bottom cathode of each LED string to these pins. Tie pins to VOUT if the string is not used. driving the PWM pin provides accurate dimming control. The PWM signal can be driven from 0V to 5V. If unused, the pin should be connected to VREF. OPENLED (Pin 7): Open LED Flag When Any LED String Opens. The output is open-collector. Tie a resistor to other supply for open LED flag function. RT (Pin 16): A resistor to ground programs switching frequency between 200kHz and 2.5MHz. For SYNC function, choose the resistor to program a frequency 20% slower than the SYNC pulse frequency. Do not leave this pin open. ISET (Pin 8): Programs LED Current for Each String. Connect a 14.7k resistor between ISET and GND to program each LED string current to 20mA. A 47pF capacitor on the ISET pin reduces current ripple in each LED string. CTRL (Pin 9): LED Current Control. If the CTRL pin is not used, tie this pin to VREF through a 10k to 20k resistor. VC (Pin 10): Error Amplifier Output Pin. Tie the external compensation network to this pin. FB (Pin 11): Feedback Pin for Overvoltage Protection. Reference voltage is 1.230V. Connect the resistive divider tap here. Minimize trace area at FB. Set VOUT according to VOUT = 1.230(1 + R2/R1) when overvoltage protection occurs (see Figure 2). TSET (Pin 12): An external resistor divider from VREF programs a decrease in LED current versus internal junction temperature (setting temperature breakpoint and slope). If the TSET pin is not used, tie this pin to VREF. NC (Pin 13): No Connection. SYNC (Pin 14): Frequency Synchronization Pin. This input allows for synchronizing the operating frequency to an external clock. The RT resistor should be chosen to program a switching frequency 20% slower than SYNC pulse frequency. This pin should be grounded if this feature is not used. PWM (Pin 15): Input Pin for PWM Dimming Control. Above 1V allows converter switching and below 1V disables switching with VC pin level maintained. A PWM signal SS (Pin 17): Soft-Start Pin. Place a soft-start capacitor here. Upon start-up, a 10μA current charges the capacitor. Use a larger capacitor for slower start-up. Leave open if not used. VREF (Pin 18): Bandgap Voltage Reference. Internally set to 1.230V. This pin can supply up to 100μA. Can be used to program the CTRL pin voltage using resistor dividers to ground. GND (Pin 19): Ground. Tie directly to local ground plane. SHDN (Pin 20): Shutdown Pin. Tie to 1.6V or more to enable the device. Tie below 0.4V or less to disable device. Do not float this pin. VIN (Pin 21): Input Supply Pin. Must be locally bypassed with a capacitor to ground. SW (Pin 22): Switch Pin. This is the collector of the internal NPN power switch. Minimize the metal trace area connected to this pin to minimize EMI. VOUT (Pin 23): Output Pin. This pin provides power to all LEDs. VO_SW (Pin 24): Drain of an Internal PMOS. The internal PMOS disconnects the feedback resistors from the VOUT pin during shutdown and the PWM transitioned to low. Exposed Pad (Pin 25): Ground. The Exposed Pad must be soldered to the PCB. 3598f 6 LT3598 BLOCK DIAGRAM 20 SHDN 21 VIN 16 RT 14 SYNC 22 SW OSCILLATOR 1.230VREF SLOPE 3 VC + Q1 Q – 10 S A2 R + SS 17 A3 GND – GND VOUT PWM 15 PWM DIMMING LOGIC VO_SW + 19 25 23 24 OVP gm VREF FB – 18 + 11 0.8V LED gm – CTRL 9 OPENLED OPENLED DETECTION TSET 12 VPTAT – + VOUT A1 LED1 LED2 LED DRIVE CIRCUITRY 1 2 LED3 LED 3 DISABLE LED4 DETECTION 4 LED5 ISET 8 7 LED6 5 6 3598 F01 Figure 1. Block Diagram 3598f 7 LT3598 OPERATION The LT3598 uses a constant-frequency, peak current mode control scheme to provide excellent line and load regulation. Each string can drive up to 30mA with 1.5% matching accuracy between strings. Operation can be best understood by referring to the Block Diagram in Figure 1. LT3598 has a built-in boost converter which converts the input voltage to a higher output voltage to drive LEDs. The LED strings are connected to current sources where the current level is set with an external resistor on the ISET pin. The LED1 to LED6 voltages are monitored for output voltage regulation. During normal operation, when all LEDs are used, the lowest LED pin voltage (LED1 to LED6) is used to regulate the output voltage to ensure all LED strings have enough voltage to run the programmed current. For any unused LED strings, tie their LED pins to VOUT. An unused LED string is no longer in the regulation loop, nor does it affect open LED detection. Never allow unused LED strings to be left open. The basic loop uses a pulse from an internal oscillator to set the SR latch and turn on the internal power NPN switch Q1. The signal at the noninverting input of the PWM comparator (A2 slope) is proportional to the sum of the switch current and oscillator ramp. When slope exceeds VC (the output of the gm amplifier), the PWM comparator resets the latch. The switch is then turned off, causing the inductor current to lift the SW pin and turn on an external Schottky diode connected to the output. Inductor current flows via the Schottky diode charging the output capacitor. The switch is turned on again at the next reset cycle of the internal oscillator. During normal operation, the VC voltage controls the peak switch current limit and, hence, the inductor current available to the output LEDs. Dimming of the LEDs is accomplished by pulsing the LED current using the PWM pin. When the PWM pin is low, switching is disabled and the error amplifier is turned off so that it does not drive the VC pin. Also, all internal loads on the VC pin are disabled so that the state of the VC pin is maintained on the external compensation capacitor. This feature reduces transient recovery time. When the PWM input again transitions high, the peak switch current returns to the correct value. The LT3598 uses the FB pin to provide overvoltage protection when all LED strings are open. There is an internal PMOS switch between VOUT and VO_SW that is controlled by the PWM signal. During the PWM off-period, this PMOS is turned off, allowing for higher dimming range and lower current during shutdown. A resistor divider is connected between the VO_SW pin and ground, which sets the overvoltage protection voltage. If the LED1-6 pin voltage is below 0.2V (for a certain delay after 80% of the programmed output voltage is reached), the string is treated as an open LED string. As a result, OPENLED lag is set. If a LED string is open in the middle of the operation, the regulation will continue. OPENLED detection is disabled during the start-up phase to avoid erratic flag generation. An LED string that is disabled by connecting its LED pin to VOUT is not an open LED condition. During normal operation, if an LED string is open and has the lowest LED pin voltage, the output voltage will regulate itself to find another LED string that has the lowest LED pin voltage at about 0.8V. If the open LED string has an LED voltage above 0.8V, the output voltage will remain the same. When the LED string is open, it is no longer in the regulation loop. The OPENLED detection is active only when the PWM signal is enabled. To avoid spurious OPENLED detection and high PWM dimming ratio, more output capacitance is recommended to allow less voltage drop on VOUT. During start-up, 10μA of current charges the external soft-start capacitor. The SS pin directly limits the rate of voltage rise on the VC pin, which in turn, limits the peak switch current. Soft-start also enables the switching frequency foldback to provide a clean start-up for the LT3598. Current limit protects the power switch and external components. 3598f 8 LT3598 APPLICATIONS INFORMATION Inductor Selection Table 1 lists several inductors that work well with the LT3598, however, there are many other manufacturers and devices that can be used. Consult each manufacturer for detailed information on their entire range of parts. Ferrite core inductors should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating. Also, ensure that the inductor has a low DCR (copper wire resistance) to minimize I2R power losses. Values between 4.7μH and 22μH will suffice for most applications. Inductor manufacturers specify the maximum current rating as the current where inductance falls by a given percentage of its nominal value. An inductor can pass a current greater than its rated value without damaging it. Consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct. Table 1. Recommended Inductors L (μH) MAX DCR (Ω) CURRENT RATING (A) B1015AS-100M #817FY-4R7M 1123AS-4R7M 10 4.7 4.7 0.07 0.06 0.12 2.2 2.26 1.90 TOKO www.toko.com 74454068 74454010 7447745100 6.8 10 10 0.055 0.065 0.12 2.2 2 1.7 Würth Electronics www.we-online.com CDH74NP-120L CDH74NP-150L CDRH6D38-100 12 15 10 0.065 0.083 0.038 2.45 2.10 2.00 Sumida www.sumida.com IHLP-2525BD-01 10 0.129 2.5 Vishay www.vishay.com SD25-4R7-R 4.7 0.056 1.83 Cooper www.cooperet.com LPS4018-472ML 4.7 0.200 1.8 Coilcraft www.coilcraft.com PART VENDOR Capacitor Selection Low ESR (equivalent series resistance) ceramic capacitors should be used at the output to minimize the output ripple voltage. Use only X5R or X7R dielectrics, as these materials retain their capacitance over wider voltage and temperature ranges than other dielectrics. A 4.7μF to 10μF output capacitor is sufficient for most high output current designs. Table 2 lists some suggested manufacturers. Consult the manufacturers for detailed information on their entire selection of ceramic parts. Table 2. Recommended Ceramic Capacitor Manufacturers Taiyo Yuden 408-573-4150 www.t-yuden.com AVX 843-448-9411 www.avxcorp.com Murata 770-436-1300 www.murata.com Kemet 408-986-0424 www.kemet.com United Chemi-Con 847-696-2000 www.chemi-con.com Diode Selection Schottky diodes, with their low forward voltage drop and fast switching speed, must be used for all LT3598 applications. Do not use P-N diodes. Table 3 lists several Schottky diodes that work well. The diode’s average current rating must exceed the application’s average output current. The diode’s maximum reverse voltage must exceed the application’s output voltage. A 2A diode is sufficient for most designs. For PWM dimming applications, be aware of the reverse leakage current of the diode. Lower leakage current will drain the output capacitor less, allowing for higher dimming range. The companies below offer Schottky diodes with high voltage and current ratings. Table 3. Suggested Diodes PART MAX MAX REVERSE CURRENT VOLTAGE (A) (V) MANUFACTURER B250A B240A SBR140S3 SBM340, PDS340 2 2 1 3 50 40 40 40 Diodes, Inc. www.diodes.com HSM150G HSM150J 1 1 50 50 Microsemi www.microsemi.com SS3H9 3 90 Vishay www.vishay.com 3598f 9 LT3598 APPLICATIONS INFORMATION Overvoltage Protection The output voltage should be set higher than the normal LED string operating voltage. Under normal operation, LED1 to LED6 pin voltages are monitored and provide feedback information to the converter for output voltage regulation given the programmed LED current. The maximum output regulation loop is activated only when all LEDs are open. The LT3598 uses the FB pin to provide regulated overvoltage protection when all LED strings are open. A resistor divider is connected between the VO_SW pin and ground (Figure 2). There is an internal PMOS switch between VOUT and VO_SW, which is controlled by the PWM signal. The PMOS switch addition prevents the feedback resistor divider from draining the output capacitor during PWM off-period, allowing for a higher dimming range without falsely tripping the OPENLED flag. It also reduces the system current in shutdown. This PMOS has about 1k resistance, so select FB resistors taking this resistance into account. Programming Maximum LED Current Maximum LED current is programmed by placing a resistor between the ISET pin and ground (RISET). The ISET pin resistor can be selected from 10k to 100k. The LED current is programmed according to the following equation: To set the maximum output voltage, select the values of R1 and R2 (see Figure 2) according to the following equation: ILED ≈ ⎛ R2 ⎞ VOUT(MAX ) = 1 . 230 V ⎜ 1 + ⎟ ⎝ R1⎠ LT3598 294V RISET See Table 4 and Figure 3 for resistor values and corresponding programmed LED current. LED current can also be adjusted by programming the CTRL pin voltage. VOUT VO_SW Table 4. RISET Value Selection for LED Current R2 FB LED CURRENT (mA) RESISTOR ON ISET PIN (k) 3mA 97.6 10mA 29.4 20mA 14.7 30mA 9.76 R1 3598 F02 Figure 2. Overvoltage Protection Voltage Feedback Connections 30 LED CURRENT (mA) 25 20 15 10 5 0 0 20 40 60 80 100 RISET (k) 3598 F03 Figure 3. RISET Value Selection for LED Current 10 3598f LT3598 APPLICATIONS INFORMATION LED Current Dimming Two different types of dimming control can be used with the LT3598. The LED current can be set by modulating the CTRL pin or the PWM pin. For some applications, a variable DC voltage that adjusts the LED current is the preferred method of brightness control. The CTRL pin voltage can be modulated to set the dimming of the LED string (see Figures 4 and 5). As the voltage on the CTRL pin increases from 0V to 1.0V, the LED current increases from 0 to the programmed LED current level. As the CTRL pin voltage increases beyond 1V, it has no effect on the LED current. 35 For wide PWM dimming range, higher switching frequency and lower PWM frequency configuration are needed. Special considerations are required for component selection and compensation network. Please contact factory for optimized components selection if very high dimming ratio is desired. RISET = 9.76k True Color PWM is a registered trademark of Linear Technology Corporation. 30 LED CURRENT (mA) For True Color PWMTM dimming, the LT3598 provides up to a 3000:1 PWM dimming range. This is achieved by allowing the duty cycle of the PWM pin (connected to the IC and an internal switch in series with the LED(s)), to be reduced from 100% to as low as 0.1% for a PWM frequency of 100Hz (Figure 6). PWM duty cycle dimming allows for constant LED color to be maintained over the entire dimming range. 25 20 VREF 15 R2 10 LT3598 CTRL 5 R1 0 3598 F05 0 0.2 0.4 0.6 0.8 1 1.2 CTRL (V) 1.4 1.6 3598 F04 Figure 4. LED Current vs CTRL Voltage Figure 5. LED Current vs CTRL TPWM TONPWM (= 1/fPWM) PWM INDUCTOR CURRENT LED CURRENT MAX ILED 3598 F06 Figure 6. LED Current Using PWM Dimming 3598f 11 LT3598 APPLICATIONS INFORMATION LED Current Derating Using the CTRL Pin A useful feature of the LT3598 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 LT3598 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 CTRL pin with a negative temperature coefficient using a resistor divider with temperature dependent resistance (Figure 7). As ambient temperature increases, the CTRL voltage will fall below the internal 1V voltage reference, causing LED currents to be controlled by the CTRL pin voltage. The LED current curve breakpoint and slope versus temperature is defined by the choice of resistor ratios and use of temperature-dependent resistance in the divider for the CTRL pin. Table 5 shows a list of manufacturers/distributors of NTC resistors. There are several other manufacturers available and the chosen supplier should be contacted for more detailed information. If an NTC resistor is used to indicate LED temperature, it is effective only if the resistor is connected 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 CTRL voltage can then be performed at each given temperature, resulting in the CTRL versus temperature plotted curve. Several iterations of resistor value calculations may be required to achieve the desired breakpoint and slope of the LED current derating curve. Table 5. NTC Resistor Manufacturers/Distributors Murata Electronics North America 770-436-1300 www.murata.com TDK Corporation 516-535-2600 www.tdk.com Digi-Key 800-344-4539 www.digikey.com If calculating the 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 overtemperature. 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 behavior) overtemperature. 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 (CTRL) from a specified VCC supply (VREF). RY VREF R2 RY LT3598 CTRL RNTC RNTC RX RNTC RNTC RX R1 (OPTION A TO D) A B C D 3598 F07 Figure 7 . LED Current Derating vs Temperature Using NTC Resistor 3598f 12 LT3598 APPLICATIONS INFORMATION Using the TSET Pin for Thermal Protection The LT3598 contains a special programmable thermal regulation loop that limits the internal junction temperature of the part. Since the LT3598 topology consists of a single boost converter with six 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 LT3598 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 LT3598 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 LT3598, it can also be used to derate the LED current at high temperatures. Since there is a direct relationship between the LED temperature and LT3598 junction temperature, the TSET function also provides some LED current derating at high temperatures. Two external resistors program the maximum IC junction temperature using a resistor divider from the VREF pin, as shown in Figure 8. Choose the ratio of R1 and R2 for the desired junction temperature. Figure 9 shows the relationship of TSET voltage to junction temperature, and Table 6 shows commonly used values for R1 and R2. Table 6. TSET Junction Temperature TJ (°C) R1 R2 90 100k 67.7k 100 100k 63.3k 110 100k 59k 120 100k 54.9k 900 VREF 850 LT3598 TSET R1 3598 F08 Figure 8. Programming the TSET Pin VTSET THRESHOLD (mV) R2 800 750 700 650 600 550 500 0 25 50 75 100 125 JUNCTION TEMPERATURE (°C) 150 3598 F09 Figure 9. TSET Pin Threshold 3598f 13 LT3598 APPLICATIONS INFORMATION Programming Switching Frequency Switching Frequency Synchronization The switching frequency of the LT3598 should be programmed between 200kHz and 2.5MHz by an external resistor connected between the RT pin and ground. Do not leave this pin open. See Table 7 and Figure 10 for resistor values and corresponding frequencies. The nominal operating frequency of the LT3598 is programmed using a resistor from the RT pin to ground and can be controlled over a 200kHz to 2.5MHz 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 LT3598 must have a frequency between 240kHz and 3MHz, a duty cycle between 20% and 80%, a low state below 0.4V and a high state above 1.5V. Synchronization signals outside of these parameters will cause erratic switching behavior. For proper operation, an RT resistor should be 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. 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. In addition, some applications require very high duty cycles to drive a large number of LEDs from a low supply. Low switching frequency allows a greater operational duty cycle and, hence, a greater number of LEDs to be driven. In each case, the switching frequency can be tailored to provide the optimum solution. When programming the switching frequency, the total power losses within the IC should be considered. Table 7. Switching Frequency SWITCHING FREQUENCY (MHz) RT (k) 2.5 14.7 2 20.5 1.5 29.4 1 51.1 0.5 105 0.2 301 The SYNC pin should be grounded if the clock synchronization feature is not used. When the SYNC pin is grounded, the internal oscillator generates switching frequency to the converter. Soft-Start and Switching Frequency Foldback For many applications, it is necessary to minimize the inrush current at start-up. The LT3598’s soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. Before the SS pin voltage reaches 1V, the switching frequency will also fold back proportional to the SS pin voltage. A typical value for the soft-start capacitor is 10nF. SWITCHING FREQUENCY (MHz) 2.5 2.0 1.5 1.0 0.5 0 10 100 RT (k) 1000 3598 F10 Figure 10. Switching Frequency 3598f 14 LT3598 APPLICATIONS INFORMATION OPENLED FLAG The OPENLED pin is an open-collector output and needs an external resistor tied to a supply (see Figure 11). If any LED string is open during normal operation, the OPENLED pin will be pulled down. The OPENLED flag is only activated after the first PWM edge. The open LED detection is enabled only when the PWM signal is enabled. There is a delay for OPENLED flag generation when the PWM signal is enabled to avoid generating a spurious flag signal. The maximum current the OPENLED can sink is typically 2mA. During start-up (see the Operation section), the open LED detection is disabled. If an LED string is not used and tied to VOUT, the string will not be in any fault detection. LT3598 R1 OPENLED 3598 F11 Figure 11. OPENLED Connection Loop Compensation The LT3598 has an internal transconductance error amplifier for LED current regulation whose VC output compensates the control loop. During an open LED event where all LED strings are open, the VC node also compensates the control loop. The external inductor, output capacitor, and the compensation resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop stability. For typical LED applications, a 15nF compensation capacitor in series with a 3k resistor at VC is adequate. Thermal Considerations The LT3598 provides six 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 0.8V. The higher the programmed LED current, the more power dissipation in the LT3598. For 30mA LED programming current with a 100% PWM dimming ratio, at least 144mW is dissipated within the IC due to current sources. If the forward voltages of the six LED strings are very dissimilar, there can be significant power dissipation. Thermal calculations shall include the power dissipation on current sources in addition to conventional switch DC loss, switch AC loss and input quiescent loss. For best efficiency, it is recommended that all channels have the same number of LEDs, and each string has a similar voltage drop across the LEDs. Board Layout Considerations 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. 3598f 15 LT3598 TYPICAL APPLICATIONS LED Driver for 40 White LEDs with Two Channels Unused L1 10μH PVIN 6V TO 40V C1 2.2μF VIN 5V C3 2.2μF R6 100k D1 VIN SW C2 4.7μF VOUT VO_SW R4 1.00M OPENLED SHDN SHDN PWM PWM SYNC SYNC FB R5 30.9k LT3598 RT R1 51.1k VREF RHOT 10k LED1 LED2 LED3 LED4 LED5 LED6 GND VC CTRL RNTC 100k R8 60.4k TSET ISET R7 100k C5 47pF SS R3 14.7k C4 0.1μF 20mA 3598 TA02a RC 2.61k CC 15nF C1: TAIYO YUDEN GMK325BJ225ML C2: MURATA GRM32ER71H475K C3: TAIYO YUDEN LMK212BJ225MG D1: DIODES, INC. B240A L1: WÜRTH ELEKTRONIK 744777410 RNTC : MURATA NCP18WF104J03RB Efficiency (PWM Dimming) 95 PVIN = 25V 90 EFFICIENCY (%) 85 PVIN = 12V 80 75 70 65 60 55 50 0 10 20 30 40 50 60 70 TOTAL LED CURRENT (mA) 80 90 3598 TA02b 3598f 16 LT3598 TYPICAL APPLICATIONS LED Driver for 30 White LEDs with 60mA Each String L1 10μH PVIN 6V TO 40V C1 2.2μF VIN 5V C3 2.2μF R6 100k SHDN CTRL PWM SYNC VIN D1 SW C2 4.7μF VOUT VO_SW OPENLED SHDN CTRL PWM SYNC RT R4 1.00M FB R5 30.9k LT3598 R1 51.1k VREF RHOT 10k CTRL RNTC 100k R8 60.4k TSET ISET R7 100k C5 47pF SS LED1 LED2 LED3 LED4 LED5 LED6 GND VC C4 0.1μF R3 9.76k 60mA RC 2.61k CC 15nF 3598 TA03a C1: TAIYO YUDEN GMK325BJ225ML C2: MURATA GRM32ER71H475KA88L C3: TAIYO YUDEN LMK212BJ225MG D1: VISHAY SS3H9 L1: WÜRTH ELEKTRONIK 744777410 RNTC : MURATA NCP18WF104J03RB Efficiency (PWM Dimming) Dimming Range (1000:1 PWM) at 125°C Junction Temperature, 10ms Period EFFICIENCY (%) 100 95 PVIN = 25V 90 PVIN = 12V PWM 5V/DIV 85 SW 20V/DIV 80 75 ILED1 100mA/DIV 70 2μs/DIV 65 3598 60 0 20 40 60 80 100 120 140 160 180 TOTAL LED CURRENT (mA) 3598 TA03b 3598f 17 LT3598 TYPICAL APPLICATIONS Auto Battery Powered Driver for 20 LEDs with 90mA Each String L1 4.7μH PVIN 6V TO 40V C1 2.2μF VIN 5V C3 2.2μF VIN D1 SW C2 4.7μF VOUT VO_SW 100k R4 1.00M OPENLED SHDN CTRL PWM SHDN CTRL PWM SYNC RT FB R5 30.9k LT3598 R1 51.1k VREF R10 20k CTRL R8 60.4k TSET ISET R7 100k C5 56pF SS LED1 LED2 LED3 LED4 LED5 LED6 GND VC C4 0.1μF R3 9.76k 90mA RC 5.11k CC 6.8nF 3598 TA04a C1: NIPPON CHEMI-CON KTS500B225M32NOTOO C2: MURATA GRM32ER71H475KA88L C3: TAIYO YUDEN LMK212BJ225MG D1: VISHAY SS3H9 L1: WÜRTH ELEKTRONIK 7447785004 Dimming Range 1000:1 PWM, 10ms Period (125°C Junction Temperature) Efficiency 95 EFFICIENCY (%) 90 PWM 5V/DIV 85 80 ILED1 100mA/DIV 75 70 65 5μs/DIV 3598 TA04c 60 0 20 40 60 80 100 120 140 160 180 TOTAL LED CURRENT (mA) 3598 TA04b 3598f 18 LT3598 PACKAGE DESCRIPTION UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 0.70 ±0.05 4.50 ± 0.05 2.45 ± 0.05 3.10 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD 0.75 ± 0.05 R = 0.115 TYP PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER 23 24 0.40 ± 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 2.45 ± 0.10 (4-SIDES) (UF24) QFN 0105 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.50 BSC NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-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 0.15mm ON ANY SIDE, IF PRESENT 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 3598f 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. 19 LT3598 TYPICAL APPLICATION LED Current Matching vs Temperature 90% Efficient LED Driver for 60 White LEDs PVIN 8V TO 40V 100 VIN SW C2 4.7μF VOUT SHDN PWM PWM SYNC SYNC 90 R4 1.00M OPENLED SHDN FB R5 30.9k LT3598 RT 95 VIN = 25V VO_SW 100k EFFICIENCY (%) C1 2.2μF D1 L1, 10μH VIN 5V C3 2.2μF R1 51.1k VIN = 16V 85 80 75 70 65 VREF RHOT 10k CTRL RNTC 100k R8 60.4k TSET ISET R7 100k C5 47pF R3 14.7k SS 60 LED1 LED2 LED3 LED4 LED5 LED6 GND VC C4 10nF 0 20 40 60 80 100 TOTAL LED CURRENT (mA) 3598 TA05b 20mA RC 2.61k CC 15nF 120 3598 TA04a RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3474/ LT3474-1 36V, 1A (ILED), 2MHz, Step-Down LED Driver VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 400:1, ISD < 1μA, TSSOP-16E Package LT3475/ LT3475-1 Dual 1.5A (ILED), 36V, 2MHz, Step-Down LED Driver VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 3000:1, ISD < 1μA, TSSOP-20E Package LT3476 Quad Output 1.5A, 2MHz High Current LED Driver with 1000:1 Dimming VIN: 2.8V to 16V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1, ISD < 10μA, 5mm × 7mm QFN-10 Package LT3477 3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver VIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1μA, QFN and TSSOP20E Packages LT3478/LT3478-1 4.5A, 42V, 2.5MHz High Current LED Driver with 3000:1 VIN: 2.8V to 36V, VOUT(MAX) = 42V, True Color PWM Dimming = 3000:1, Dimming ISD < 3μA, TSSOP16E Package LT3486 Dual 1.3A, 2MHz High Current LED Driver VIN: 2.5V to 24V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1, ISD < 1μA, 5mm × 3mm DFN and TSSOP-16E Packages LT3496 45V, 2.1MHz 3-Channel (ILED = 1A) Full Featured LED Driver VIN: 3V to 30V (40VMAX), VOUT(MAX) = 45V, True Color PWM Dimming = 3000:1, ISD < 1μA, 4mm × 3mm QFN-28 Package LT3497 Dual 2.3MHz, Full Function LED Driver with Integrated Schottkys and 250:1 True Color PWM Dimming VIN: 2.5V to 10V, VOUT(MAX) = 32V, IQ = 6mA, ISD < 12μA, 2mm × 3mm DFN-10 Package LT3498 2.3MHz, 20mA LED Driver and OLED Driver with Integrated Schottky VIN: 2.5V to 12V, VOUT(MAX) = 32V, IQ = 1.65mA, ISD < 9μA, 2mm × 3mm DFN-10 Package LT3518/LT3517 2.3A/1.3A 45V, 2.5MHz Full Featured LED Driver with True Color PWM Dimming VIN: 3V to 30V (40VMAX), VOUT(MAX) = 42V, True Color PWM Dimming = 3000:1, ISD < 5μA, 4mm × 4mm QFN-16 Package LT3590 48V, 850kHz 50mA Buck Mode LED Driver VIN: 4.5V to 55V, Dimming = 200:1 True Color PWM, ISD < 15μA, 2mm × 2mm DFN-6 and SC70 Packages LT3592 36V, 2.2MHz, 500mA Buck Mode LED Driver VIN: 3.6V to 36V, True Color PWM Dimming = 10:1, ISD < 1μA, 2mm × 3mm DFN-10 and MSOP-10E Packages LT3595 45V, 2.5MHz 16-Channel Full Featured LED Driver VIN: 4.5V to 55V, VOUT(MAX) = 45V, True Color PWM Dimming = 5000:1, ISD < 1μA, 5mm × 9mm QFN-56 Package 3598f 20 Linear Technology Corporation LT 0908 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008