LT3596IUHG-TRPBF

LT3596 60V Step-Down LED Driver FeaTures n n DescripTion The LT®3596 is a 60V step-down LED Driver. It achieves 10,000:1 digital PWM dimming at 100Hz with fast NPN current sources driving up to 10 LEDs in each channel. 100:1 LED dimming can also be done with analog control of the CTRL1-3 pin. The step-down switching frequency is programmable between 200kHz and 1MHz and is synchronizable to an external clock. The LT3596 also 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 LT3596 adaptively controls VOUT in order to achieve optimal efficiency. Other features include: 1.5% LED current matching between channels, open LED reporting, shorted LED pin protection and reporting, programmable LED current and programmable temperature protection. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation and True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. n n n n n n n n n n n n 300mA Buck Regulator, Drives Up to 10 LEDs per Channel with Fast NPN Current Sources Fast Current Sources for VLED1-3 VLED1-3 = 42V, PWM1-3 = 0V 4.25 1.15 90 80 1.25 750 700 230 1100 0.4 A mA nA mA kHz kHz V V kHz kHz nA ms V µA V mV 240 1000 200 2.2 1.96 200 2.0 1.0 540 2.04 200 98 97 100 100 ±0.35 ±0.35 1.07 0.29 10 1 1.2 15 1.6 200 102 103 ±1.5 ±2 nA mA mA % % V V V V nA 3596f RISET1-3 = 20k l Adaptive VOUT Loop Enabled  LT3596 elecTrical characTerisTics PARAMETER 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 VCTRLM = 3V IFAULT = 200µA VFAULT = 25V 0.10 200 Note 3: For maximum operating ambient temperature, see the High Temperature Considerations section in the Applications Information section. Note 4: Guaranteed by design. VCTRL1-3 = 6V 1.2 200 1.2 200 1.6 200 The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, BOOST = 30V, BIAS = 5V, EN/UVLO = 5V, PWM1-3 = 3.3V, CTRL1-3 = CTRLM = TSET = 2V, VOUT = 24V, SYNC = 0V unless otherwise specified. (Note 2) CONDITIONS MIN TYP MAX 0.4 UNITS 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 LT3596E 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, characterization and correlation with statistical process controls. The LT3596I specifications are guaranteed over the full –40°C to 125°C operating junction temperature range. 3596f  LT3596 Typical perForMance characTerisTics VIN Quiescent Current 2.0 1.8 1.6 BIAS CURRENT (mA) VIN CURRENT (mA) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 10 30 40 20 VIN VOLTAGE (V) T = 125°C T = 25°C T = –40°C 50 60 3596 G01 TA = 25°C, unless otherwise noted VIN, BIAS Shutdown Current 1.4 1.2 1.0 VIN = 55V VBIAS = 25V BIAS Quiescent Current 2.0 VIN = 24V VBIAS = 5V 1.5 CURRENT (µA) T = 125°C T = 25°C T = –40°C 0 5 15 10 BIAS VOLTAGE (V) 20 25 3596 G02 1.0 0.8 0.6 0.4 0.5 0 0.2 0 –50 –25 IVBIAS 50 25 75 0 TEMPERATURE (°C) IVIN 125 –0.5 100 3596 G03 UVLO Threshold 1.8 1.7 EN/UVLO CURRENT (µA) UVLO THRESHOLD (V) 1.6 1.5 1.4 1.3 1.2 1.1 1.0 –50 –25 0 75 50 25 TEMPERATURE (°C) 100 125 6 5 4 3 2 1 EN/UVLO Pin Current VEN/UVLO = 1.4V 2.04 2.03 2.02 VREF VOLTAGE (V) 2.01 2.00 1.99 1.98 1.97 50 25 75 0 TEMPERATURE (°C) 100 125 VREF Voltage 0 –50 –25 1.96 –50 –25 VIN = 55V VIN = 24V VIN = 6V 0 75 50 25 TEMPERATURE (°C) 100 125 3596 G04 3596 G05 3596 G06 Switching Frequency 1.4 SWITCHING FREQUENCY (MHz) 1.2 1.0 0.8 0.6 0.4 0.2 0 –50 –25 RT = 220k RT = 33.2k 1100 1000 CURRENT LIMIT (mA) 900 800 700 600 SW and DA Current Limit 1000 ISW SWITCH VOLTAGE (mV) 900 800 700 600 500 400 300 200 100 50 25 75 0 TEMPERATURE (°C) 100 125 0 Switch Voltage Drop IDA 50 25 75 0 TEMPERATURE (°C) 100 125 500 –50 –25 T = 125°C T = 25°C T = –40°C 0 100 200 300 400 500 600 700 800 SWITCH CURRENT (mA) 3596 G09 3596 G07 3596 G08 3596f  LT3596 Typical perForMance characTerisTics Boost Diode Voltage 1.0 EN/UVLO 5V/DIV SW 50V/DIV IL 500mA/DIV VOUT 20V/DIV 400µs/DIV 3596 G11 TA = 25°C, unless otherwise noted 60V Buck Switching Waveforms Soft-Start BOOST DIODE VOLTAGE (V) 0.8 SW 20V/DIV 0.6 0.4 IL 500mA/DIV VOUT 20V/DIV 400ns/DIV 3596 G12 0.2 0 0 25 50 75 100 125 CURRENT (mA) 150 175 3596 G10 FB and LED Loop Regulation Voltage 1.4 103 102 LED CURRENT (mA) LED Current RISETn = 20k 1.00 0.75 0.50 MATCHING (%) 101 100 99 98 97 –50 –25 LED1 LED2 LED3 50 25 75 0 TEMPERATURE (°C) 100 125 0.25 0 LED Current Matching RISETn = 20k REGULATION VOLTAGE (V) 1.3 VFB 1.2 1.1 VLED –0.25 1.0 –0.50 –0.75 –1.00 –50 –25 0 75 50 25 TEMPERATURE (°C) 100 CH1 CH2 CH3 125 0.9 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 3596 G13 3596 G14 3596 G15 LED Current vs PWM Duty Cycle 100 10 LED CURRENT (mA) 1 0.1 0.01 0.001 0.0001 0.001 0 LED CURRENT (mA) 75 RISETn = 20k 100 LED Current vs CTRLn Voltage RISETn = 20k ILED 100mA/DIV VOUT 10V/DIV VLED 10V/DIV VSW 100V/DIV Adaptive Loop Switching Waveforms (with PWM Dimming) 50 25 200µs/DIV 3596 G18 0.01 0.1 1 10 PWM DUTY CYCLE (%) 100 3596 G16 0 0.25 0.5 0.75 1 CTRLn VOLTAGE (V) 1.25 1.5 3596 G17 3596f  LT3596 Typical perForMance characTerisTics 10,000:1 PWM Dimming at 100Hz PWM 2V/DIV PWM 2V/DIV TA = 25°C, unless otherwise noted PWM Dimming Waveforms (Overlapping PWM Signals) PWM1 5V/DIV ILED1 100mA/DIV PWM2 5V/DIV ILED2 100mA/DIV 1,000:1 PWM Dimming at 100Hz ILED 50mA/DIV ILED 50mA/DIV 200ns/DIV 3596 G19 2µs/DIV 3596 G20 2ms/DIV 3596 G21 PWM Dimming Waveforms (Nonoverlapping PWM Signals) PWM1 5V/DIV ILED1 100mA/DIV PWM2 5V/DIV ILED2 100mA/DIV 2ms/DIV 3596 G22 LED Open-Fault Waveforms VLED 1V/DIV ILED1 100mA/DIV VOUT 20V/DIV VSW 50V/DIV 20µs/DIV 3596 G23 LED Short to VOUT Waveforms VOUT 20V/DIV VLED 20V/DIV ILED 100mA/DIV VFAULT 5V/DIV 10ms/DIV 3596 G24 FAULT Output Voltage 800 0.8 VTSET for LED Current Derating 100 TSET LED Current Derating VTSET = 0.68V FAULT VOLTAGE (mV) 600 VTSET (V) 0.7 80 400 0.5 ILED (mA) 0.6 60 40 20 200 T = 125°C T = 25°C T = –40°C 0 0.25 0.5 0.75 1 1.25 1.5 1.75 IFAULT (mA) 2 0.4 0 0.3 –50 –25 50 0 75 25 TEMPERATURE (°C) 100 125 0 25 45 85 105 65 TEMPERATURE (°C) 125 3596 G27 3596 G25 3596 G26 3596f  LT3596 pin FuncTions FB (Pin 2): Feedback Pin. This pin is regulated to the internal bandgap voltage. The maximum buck output voltage is set by connecting this pin to a resistor divider from VOUT . EN/UVLO (Pin 4): Enable and Undervoltage Lockout Pin. Accurate 1.5V falling threshold. UVLO threshold is programmed by using a resistor divider from VIN. TSET (Pin 6): Thermal Regulation Pin. Programs the LT3596 junction temperature at which LED current begins to derate. VREF (Pin 7): 2V Reference Output Pin. This pin sources up to 200µA and is used to program TSET and CTRLM. GND (Pin 9/Exposed Pad Pin 53): Ground Pin. This is the ground for both the IC and the switching converter. Exposed pad must be soldered to PCB ground. NC (Pins 11, 12, 18, 22, 23, 26, 34, 35, 39, 41, 43): No Connection Pins. Tie to ground if unused. CTRLM (Pin 13): Master Control Pin. LED current derating vs temperature is achievable for all channels if the voltage on CTRLM has a negative coefficient using an external NTC resistor divider from VREF . ISET1, ISET2, ISET3 (Pins 14, 15, 16): LED Current Programming Pin. Resistor to ground programs full-scale LED current. RT (Pin 17): Switching Frequency Programming Pin. A resistor to ground programs switching frequency between 200kHz and 1MHz. For the SYNC function, choose the resistor to program a frequency 20% slower than the SYNC pulse frequency. VOUT (Pin 19): 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 three LED driver outputs, each containing an open collector, constant current sink. All outputs are matched within ±1.5% and are individually programmed up to 100mA using an external resistor at the ISET1-3 pin. Outputs are rated to allow a maximum VOUT of 42V. Connect the cathode of the LED string to LED1-3. Connect the anode of the LED string to VOUT . 3596f FAULT (25): Fault Detection Pin. Open-collector pin used to report open LED faults. FAULT must be externally pulled to a positive supply. SYNC (Pin 27): External Clock Synchronization Pin. When an external clock drives this pin, the buck regulator is synchronized to that frequency. Frequency programmed by the RT pin resistor must be at least 20% slower 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. Channels can be individually disabled by tying PWM1-3 to ground. If PWM dimming is not desired then the pin should be connected to VREF . 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 unused the pin must be connected to VREF . BIAS (37): Supply Pin. This pin is the supply for an internal voltage regulator for analog and digital circuitry. BIAS must be locally bypassed with a 4.7µF capacitor. DA (44): Catch Diode Anode. This pin is used to provide frequency foldback in extreme situations. BOOST (Pin 46): Boost Capacitor Pin. This pin is used to provide a voltage above the input voltage when the switch is on. It supplies current to the switch driver. SW (Pin 48): Switch Pin. Connect the inductor, catch diode and boost capacitor to this pin. VIN (Pins 50, 51): Input Supply Pins. Pins are electrically connected inside the package. VIN must be locally bypassed with a 10µF capacitor to ground.  LT3596 block DiagraM VIN 50-51 4 EN/UVLO BIAS START-UP REFERENCE 7 VREF + – BIAS BOOST 37 46 27 17 SYNC RT OSC S R SLOPE COMP Q SW 48 44 – SOFT-START AND CLAMP VC DA + + – + – 1.21V GND 9 FB 1V 2 540mV PTAT VOUT CHANNELS 1 TO 3 FAULT LED FAULT PROTECTION LOGIC 19 25 6 13 TSET CTRLM CTRLn 33-31 ISETn 14-16 PWMn 30-28 + – CONVERSION AND CONTROL PWM DIMMING LOGIC EXPOSED PAD 53 LED DRIVE CIRCUITRY LEDn 20, 21, 24 3596 F01 Figure 1. Block Diagram 3596f  LT3596 operaTion The LT3596 uses a constant-frequency, peak current mode control scheme to provide excellent line and load regulation. Operation is best understood by referring to the Block Diagram (Figure 1). The bias, start-up, reference, oscillator, TSET amplifier and the buck regulator are shared by the three LED current sources. The conversion and control, PWM dimming logic, LED fault detection, and LED drive circuitry are identical for all three current sources. Enable and undervoltage lockout (UVLO) are both controlled by a single pin. If the EN/UVLO pin falls below 1.51V (typical), an accurate comparator turns off the LED drivers and the buck regulator. If the pin continues to fall to less than 0.4V, the part enters a low quiescent current shutdown mode. The LT3596 contains three constant-current sink LED drivers. These drivers sink up to 100mA with 1.5% matching accuracy between LED strings. The LED strings are powered from the buck converter. The buck converter contains an adaptive loop that adjusts the output voltage based on LED string voltage to ensure maximum efficiency. External compensation and soft-start components are not required, minimizing component count and simplifying board layout. An external resistor programs the buck’s switching frequency between 200kHz and 1MHz. The frequency can also be synchronized to an external clock using the SYNC pin. Step-Down Adaptive Control Adaptive control of the output voltage maximizes system efficiency. This control scheme regulates the output voltage to the minimum that ensures all three LED strings turn on. This accounts for the variation in the forward voltage of the LEDs, and minimizes the power dissipation across the internal current sources. Activation of the adaptive loop is set by the status of the PWMn pins. If any channel’s PWM pin is low, then the buck regulator output ascends to an externally programmed output voltage. This voltage is always set above the maximum voltage drop of the LEDs. This guarantees that the buck output voltage is high enough to immediately supply the LED current once the strings are reactivated. As soon as all of the PWM pins transition high, the output voltage of the buck drops until the adaptive loop regulates the output with about 1V across the LED current sinks. This scheme optimizes the efficiency for the system since the output voltage regulates to the minimum voltage required for all three LED strings. LED Current Each LED string current is individually programmed to a maximum of 100mA with 1.5% matching accuracy between the strings. An external resistor on the ISETn pin programs the maximum current for each string. The CTRLn pin is used for analog dimming. Digital dimming is programmed using the PWMn pin. A dimming ratio of 10,000:1 is achievable at a frequency of 100Hz. Fault Protection and Reporting The LT3596 features diagnostic circuitry that protects the system in the event that a LEDn pin is shorted to an undesirable voltage. The LT3596 detects when the LEDn voltage exceeds 12V or is within 1.2V of VOUT when the LED string is sinking current. If either faulted condition occurs, the channel is disabled until the fault is removed. The fault is reported on FAULT until the fault has cleared. The LT3596 also offers open-LED detection and reporting. If a LED string is opened and no current flows in the string, then a fault is reported on FAULT. A fault is also reported if the internal die temperature reaches the TSET programmed derating limit. LED faults are only reported if the respective PWM signal is high. 3596f 0 LT3596 applicaTions inForMaTion Inductor Selection Inductor values between 100µH and 470µH 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 MSS1038 MSS1038 MSS1246T CDRH10D68 CDRH12D58R DS1262C2 VLF10040 DR124 DR127 DR74 744771220 L (µH) 100 220 470 100 220 470 100 220 100 220 100 220 470 100 220 MAX DCR (Ω) 0.3 0.76 0.935 0.205 0.362 0.67 0.17 0.35 0.22 0.47 0.26 0.56 0.861 0.383 0.40 CURRENT RATING (A) VENDOR 1.46 0.99 1.0 1.5 1.0 1.01 1.5 1.0 1.3 0.9 1.79 1.15 1.6 0.99 1.2 Coilcraft www.coilcraft.com Sumida www.sumida.com Toko www.toko.com TDK www.tdk.com Coiltronics www.cooperet.com Typically 10µF capacitors are sufficient for the VIN and BIAS pins. The output capacitor for the buck regulator 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 (Volts/LED = 3.5V) SWITCHING FREQUENCY (kHz) 1000 500 200 # LEDS 1 to 3 >3 1 to 3 >3 1 to 3 >3 COUT (µF) 6.8 4.7 10 6.8 22 10 Diode Selection Schottky diodes, with their low forward voltage drop and fast switching speed, must be used for all LT3596 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 MAX CURRENT (A) 1 1 1 MAX REVERSE VOLTAGE (V) 60 60 100 Würth Elektronik www.we-online.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 PART DFLS160 CMMSH1-60 ESIPB MANUFACTURER Diodes, Inc. www.diodes.com Central Semiconductor www.centralsemi.com Vishay www.vishay.com Undervoltage Lockout (UVLO) EN/UVLO programs the UVLO threshold by connecting the pin to a resistor divider from VIN as shown in Figure 2. Select R1 and R2 according to the following equation:  R2  VIN(UVLO) = 1.51V •  1+   R1 3596f  LT3596 applicaTions inForMaTion VIN R2 EN/UVLO LT3596 LED Current Dimming The LT3596 has two different types of dimming control. The LED current is dimmed using the CTRLn pin or the PWMn pin. 1.51V R1 Figure 2. EN/UVLO Control In UVLO an internal 5.1µA (typical) pull-down current source is connected to the pin for programmable UVLO hysteresis. The hysteresis is set according to the following equation: VUVLO(HYST) = 5.1µA • R2 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 goes below 0.4V, the part enters shutdown. Programming Maximum LED Current Maximum LED current is programmed by placing a resistor (RISETn) between the ISETn pin and ground. RISETn 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: ILED1-3 = 1V • 2000 RISETn LED CURRENT (mA) LED CURRENT (mA) where RISETn is in kΩ and ILEDn is in mA. See Table 5 and Figure 3 for resistor values and corresponding programmed LED current. Table 5. RISETn Value for LED Current RISETn VALUE (kΩ) 20 24.9 33.2 49.9 100 LED CURRENT (mA) 100 80 60 40 20  + – 3596 F02 For some applications, a variable DC voltage that adjusts the LED current is the preferred method for brightness control. In this case, the CTRLn pin is modulated to set the LED dimming. As the CTRLn pin voltage rises from 0V to 1V, the LED current increases from 0mA to the maximum programmed LED current in a linear fashion (see Figure 4). As the CTRLn pin increases beyond 1V, the maximum programmed LED current is maintained. If this type of dimming control is not desired, the CTRLn pin can be connected to VREF . 100 80 60 40 20 0 0 25 50 RISETn (k ) 75 100 3596 F03 Figure 3. RISETn Value for LED Current 100 RISETn = 20k 75 50 25 0 0 0.25 0.5 0.75 1 CTRLn VOLTAGE (V) 1.25 1.5 3596 F04 Figure 4. LED Current vs CTRLn Voltage 3596f LT3596 applicaTions inForMaTion For True Color PWM dimming, the LT3596 provides up to 10,000:1 PWM dimming range at 100Hz. This is done by reducing the duty cycle of the PWMn pin from 100% to 0.01% for a PWM frequency of 100Hz (see Figure 5). This equates to a minimum on time of 1µs and a maximum period of 10ms. PWM duty cycle dimming allows for constant LED color to be maintained over the entire dimming range. tPWM tON(PWM) VREF R2 LT3596 TSET R1 3596 F06 Figure 6. Programming the TSET Pin Table 6. TSET Programmed Junction Temperature TJ (°C) 85 100 115 R1 (kΩ) 49.9 49.9 49.9 R2 (kΩ) 97.6 90.9 84.5 LED1-3 CURRENT MAX ILED 3596 F05 The TSET pin must be tied to VREF if the temperature protection feature is not desired. 0.8 Figure 5. LED Current Using PWM Dimming 0.7 Using the TSET Pin for Thermal Protection The LT3596 contains a special programmable thermal regulation loop that limits the internal junction temperature. This thermal regulation feature provides important protection at high ambient temperatures. It allows an application to be optimized for typical, not worst-case, ambient temperatures with the assurance that the LT3596 automatically protects 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 LT3596 linearly reduces the LED current, as needed, to maintain this junction temperature. This is only 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 10% of the full current. A resistor divider from the VREF pin programs the maximum IC junction temperature as shown in Figure 6. Table 6 shows commonly used values for R1 and R2. Choose the ratio of R1 and R2 for the desired junction temperature limit as shown in Figure 7. VTSET (V) 0.6 0.5 0.4 0.3 –50 –25 50 0 75 25 TEMPERATURE (°C) 100 125 3596 F07 Figure 7. VTSET for LED Current Derating LED Current Derating Using the CTRLM Pin A useful feature of the LT3596 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 LT3596 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 falls below the internal 1V voltage reference, causing 3596f  LT3596 applicaTions inForMaTion 100 RX VREF R2 LT3596 CTRLM R1 (OPTION A TO D) (8a) (8b) (8c) (8d) RNTC RX RNTC LED CURRENT (mA) RX RNTC 3596 F08 RISETn = 20k RX 75 50 25 Figure 8. Programming the CTRLM Pin 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. Table 7 shows a list of manufacturers/distributors of NTC resistors. There are several other manufacturers available. The chosen supplier should be contacted for more detailed information. Table 7. NTC Resistor Manufacturers/Distributors Murata TDK Corporation Digi-Key www.murata.com www.tdk.com www.digikey.com 0 0 0.25 0.5 0.75 1 CTRLM VOLTAGE (V) 1.25 1.5 3596 F04 Figure 9. LED Current vs CTRLM Voltage 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 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 (CTRLM behavior) over temperature. Referred to as the Murata Chip NTC Thermistor Output Voltage Simulator, users can log on to 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 LT3596 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. Selecting the optimum switching frequency depends on several factors. Inductor size is reduced with higher frequency, but efficiency drops slightly due to higher switching 3596f If an NTC resistor is used to indicate LED temperature, it is effective only if the resistor is placed as close 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 dependence of an NTC resistor is nonlinear as a function of temperature, it is important to obtain its temperature characteristics from the manufacturer. Hand calculations of the CTRLM voltage are then performed at each given temperature using the following equation:  R1  VCTRLM = VREF •   R1+ R2   This produces a plot of VCTRLM versus temperature. From this curve, the LED current is found using Figure 9. Several iterations of resistor value calculations may be necessary to achieve the desired breakpoint and slope of the LED current derating curve.  LT3596 applicaTions inForMaTion 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 so 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. Table 8. RT Resistor Selection RT VALUE (kΩ) 33.2 80.6 220 SWITCHING FREQUENCY (MHz) 1.0 0.5 0.2 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, input voltage and maximum output voltage. The advantage of high frequency operation is smaller component size and value. The disadvantages are lower efficiency and lower maximum output voltage for a fixed input 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 ) 1.2 SWITCHING FREQUENCY (MHz) 1.0 0.8 0.6 0.4 0.2 0 0 55 110 RT (k ) 165 220 3596 F10 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 (~0.4V at max load). This equation shows that slower switching is necessary to accommodate high VIN/VOUT ratios. The input voltage range depends on the switching frequency due to the finite minimum switch on and off times. The switch minimum on and off times are 150ns. Adaptive Loop Control The LT3596 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 any PWMn is low, the output of the buck rises to a maximum value set by an external resistor divider to the FB pin. When all PWMn pins go high, the output voltage is adaptively reduced until the voltage across the LED current sink is about 1V. Figure 11 shows how the maximum output voltage is set by an external resistor divider. VOUT R2 VOUT LT3596 FB R1 3596 F11 Figure 10. Programming Switching Frequency Switching Frequency Synchronization The nominal operating frequency of the LT3596 is programmed using a resistor from the RT pin to ground. The frequency range is 200kHz to 1MHz. In addition, the internal oscillator can be synchronized to an external clock applied to the SYNC pin. The synchronizing clock signal input to the LT3596 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 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. Figure 11. Programming Maximum VOUT 3596f  LT3596 applicaTions inForMaTion The maximum output voltage must be set to exceed the maximum LED drop plus 1.07V by a margin greater than 15%. This ensures proper adaptive loop control. The equation below is used to estimate the resistor divider ratio. The sum of the resistors should be approximately 100k to avoid noise coupling to the FB pin.  R2  VOUT(MAX ) = 1.15 • VLED(MAX ) + 1.07 V = 1.21V •  1+   R1 High Temperature Considerations The LT3596 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. For 100mA of LED current with a 100% PWM dimming ratio, at least 300mW is dissipated within the IC due to current sources. If the forward voltages of the three LED strings are very dissimilar, significant power dissipation will occur. 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. For best efficiency, it is recommended that each LED string have approximately the same voltage drop. In addition, the die temperature of the LT3596 must be lower than the maximum rating of 125°C. This is generally not a concern unless the ambient temperature is above 100°C. Care should be taken in the board layout to ensure good heat sinking of the LT3596. The maximum load current (300mA) must be derated as the ambient temperature approaches 125°C. The die temperature is calculated by multiplying the LT3596 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3596 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 32°C/W is typical for the 5mm × 8mm QFN package. Board Layout As with all switching regulators, careful attention must be paid to the PCB 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 (SW). Always use a ground plane under the switching regulator to minimize interplane coupling. Resistors connected between ground and the CTRL1-3, CTRLM, FB, TSET, ISETn  , RT and EN/UVLO pins are best connected to a quiet ground. ( ) 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. Start-Up At start-up, when VOUT reaches 90% of the FB programmed output voltage, the adaptive loop is enabled. At this point, the LED string with the highest voltage drop is selected. The output voltage reduces until the selected string’s LED pin is about 1V. This regulation method ensures that all three LED strings run their programmed current at a minimum output voltage despite mismatches in LED forward voltage. This minimizes the drop across the internal current sources and maximizes system efficiency. Another benefit of this regulation method is that the LT3596 starts up with 10,000:1 dimming even if the PWMn pulse width is 1µs. Since VOUT starts up even if PWMn is low, the part achieves high dimming ratios with narrow pulse widths within a couple of PWMn clock cycles. 3596f  LT3596 Typical applicaTions 24V 200kHz Step-Down 4W, 100mA LED Driver VIN 24V VIN 270k EN/UVLO BOOST 91k LT3596 SW SYNC 220k PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 SYNC RT DA GND FB VOUT 100nF 470µH 100k 7.32k 10µF BIAS VOUT 100 Efficiency 10µF 80 EFFICIENCY (%) VOUT 60 40 20 0 0 20 60 40 LED CURRENT (mA) 80 100 3596 TA02b BIAS 100k FAULT 90.9k 10k FAULT VREF TSET CTRLM LED1 LED2 LED3 ISET1 ISET2 ISET3 20k 20k 20k 3696 TA02a 49.9k 100k MURATA NCP18WF104 12V 1MHz Step-Down 100mA Single Pixel R-G-B Driver VIN 12V VIN 270k EN/UVLO BOOST 91k LT3596 SW SYNC 33.2k PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 SYNC RT DA GND FB VOUT 100nF 100µH 100k 14.7k 6.8µF BIAS VOUT 100 Efficiency 10µF 80 EFFICIENCY (%) VOUT 60 40 20 0 0 20 60 40 LED CURRENT (mA) 80 100 3596 TA03b BIAS 100k FAULT 90.9k 10k FAULT VREF TSET CTRLM LED1 LED2 LED3 ISET1 ISET2 ISET3 20k 20k 20k 3696 TA03a 49.9k 100k MURATA NCP18WF104 3596f  LT3596 Typical applicaTions 48V 500kHz Step-Down 10W, 100mA LED Driver VIN 48V VIN 270k EN/UVLO BOOST 91k LT3596 SW SYNC 80.6k PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 BIAS 100k FAULT 90.9k 10k TSET CTRLM 49.9k FAULT VREF LED1 LED2 LED3 ISET1 ISET2 ISET3 20k 20k 20k PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 SYNC RT DA GND FB VOUT ~36V PER LED STRING 100k 3.01k 100nF 220µH 6.8µF BIAS 5V BIAS 10µF 4.7µF VOUT 3696 TA04a 100k MURATA NCP18WF104 Efficiency 100 PWM 2V/DIV 10,000:1 PWM Dimming at 100Hz 80 EFFICIENCY (%) 60 ILED 50mA/DIV 40 20 200ns/DIV 0 20 60 40 LED CURRENT (mA) 80 100 3596 TA04b 3596 G19 0 3596f  LT3596 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 44 46 0.05 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 7.10 8.50 9 11 12 0.05 0.05 13 14 15 16 0.40 BSC 17 0.20 0.05 0.05 UHG Package Variation: UHG52 (39) 52-Lead Plastic QFN (5mm × 8mm) 5.50 0.05 4.10 50 51 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 45 CHAMFER 50 51 0.40 0.10 43 43 2 41 4 39 6 7 9 8.00 11 12 13 14 15 16 17 0.10 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 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 3596f 0.75 5.00 0.10 0.05 0.00 – 0.05 R = 0.10 TYP 44 R = 0.10 TYP 46 48 2 PIN 1 TOP MARK (SEE NOTE 6) 41 4 0.20 0.05 0.80 BSC 0.60 TYP 0.40 BSC 26 25 24 23 22 21 20 19 3.20 REF BOTTOM VIEW—EXPOSED PAD 18 (UHG39) QFN 0410 REV B 0.05 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.  LT3596 Typical applicaTion 48V 1MHz Step-Down 8W, 100mA LED Driver (Eight White LEDs per Channel) VIN 48V VIN 270k EN/UVLO BOOST 91k SYNC 33.2k PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 PWM1 PWM2 PWM3 CTRL1 CTRL2 CTRL3 LED1 LED2 LED3 3696 TA01a 10µF BIAS 4.7µF 100nF 100µH 5V BIAS 10,000:1 PWM Dimming at 100Hz 4.7µF VOUT SW LT3596 DA SYNC RT GND FB VOUT 100k 3.65k PWM 2V/DIV ILED 50mA/DIV BIAS 100k FAULT 90.9k 10k FAULT VREF TSET CTRLM 200ns/DIV 3596 TA01b ISET1 ISET2 ISET3 20k 20k 20k 49.9k 100k relaTeD parTs PART NUMBER LT3476 LT3496 LT3590 LT3595 LT3598 LT3599 LT3754 DESCRIPTION Quad Output 1.5A, 2MHz High Current LED Driver with 1000:1 Dimming 45V, 2.1MHz 3-Channel (ILED = 1A) Full-Featured LED Driver 48V, 850KHz 50mA Buck Mode LED Driver 45V, 2MHz 16-Channel Full-Featured LED Driver 44V, 1.5A, 2.5MHz Boost 6-Channel LED Driver 2A Boost Converter with Internal 4-String 150mA LED Ballaster 16-Channel × 50mA LED Driver with 60V Boost Controller and PWM Dimming COMMENTS VIN: 2.8V to 16V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1, ISD < 10µA, 5mm × 7mm QFN-10 Package VIN: 3V to 30V (40VMAX), VOUT(MAX ) = 45V, True Color PWM Dimming = 3000:1, ISD < 1µA, 4mm × 3mm QFN-28 Package VIN: 4.5V to 55V, True Color PWM Dimming = 200:1, ISD < 15µA, 2mm × 2mm DFN-6 and SC70 Packages VIN: 4.5V to 55V, VOUT(MAX) = 45V True Color PWM Dimming = 5000:1, ISD < 1µA, 5mm × 9mm QFN-56 Package VIN: 3V to 30V (40VMAX), VOUT(MAX) = 44V, True Color PWM Dimming = 1000:1, ISD < 1µA, 4mm × 4mm QFN-24 Package VIN: 3V to 30V, VOUT(MAX) = 44V, True Color PWM Dimming = 1000:1, ISD < 1µA, 5mm × 5mm QFN-32 and TSSOP-28 Packages VIN: 6V to 40V, VOUT(MAX) = 45V, True Color PWM Dimming = 3000:1, ISD < 1µA, 5mm × 5mm QFN-32 Package 3596f 0 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0610 • PRINTED IN USA www.linear.com  LINEAR TECHNOLOGY CORPORATION 2010