LT3476 High Current Quad Output LED Driver Features Description True Color PWMTM Dimming Delivers Up to 5000:1 Dimming Ratio (In Boost Configuration) n LED Current Regulation with High-Side Sense n VADJ Pin Accurately Sets LED Current Sense Threshold Over Range 10mV to 120mV n Four Independent Driver Channels with 1.5A, 36V Internal NPN Switches n Frequency Adjust Pin: 200kHz to 2MHz n High Efficiency Conversion = Up to 96% n Open LED Protection n Low Shutdown Current < 10µA n Wide V Range: 2.8V to 16V IN n Thermally Enhanced, 38-Lead, 5mm × 7mm QFN Package The LT®3476 is a quad output DC/DC converter designed to operate as a constant-current source for driving high current LEDs. A fixed frequency, current mode architecture results in stable operation over a wide range of supply and output voltages. A frequency adjust pin allows the user to program switching frequency between 200kHz and 2MHz to optimize efficiency and external component size. n The LT3476 senses output current at the high side of the LED. High side current sensing is the most flexible scheme for driving LEDs, allowing buck, boost or buckboost configurations. Each current monitor threshold is trimmed to within 2.5% at the full scale of 105mV. With an external sense resistor, the user programs the output current range of each channel. Each of the four regulators is independently operated by that channel’s PWM signal. This PWM feature allows precise adjustment of the color mixing or dimming ratio of the LED source. Dimming ratios up to 1000:1 can be achieved. Applications n n n n RGGB Lighting Automotive and Avionic Lighting TFT LCD Backlighting Constant-Current Sources L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application 100W Quad 1A × 8 LED Driver PVIN 33V 100mΩ 0.22µF 100mΩ LED3 1A 1A CAP4 100mΩ LED2 LED1 UP TO 8 LEDS CAP3 CAP2 CAP1 100mΩ 1000:1 PWM Dimming at 100Hz 1A 0.22µF PWM 5V/DIV LED4 2.2µF ×4 1A 0.22µF ILED 500mA/DIV 0.22µF 5µs/DIV 10µH VIN 2.8V TO 16V PWM1-4 SHDN 2.2µF 10µH SW1 CAP1-4 LED1-4 VIN PWM1-4 SHDN SW2 10µH SW3 LT3476 GND 3476 TA02 10µH 1.05V SW4 REF VADJ1-4 4.99k 100k VC1-4 RT 3476 TA01 21k 1nF 3476fb 1 LT3476 Absolute Maximum Ratings Pin Configuration (Note 1) VIN.............................................................................16V PWM1-4, SHDN.........................................................16V SW1-4, LED1-4, CAP1-4............................................36V REF, RT, VADJ1-4, VC1-4...............................................2V Operating Junction Temperature Range (Notes 2 and 3)....................................... –40°C to 125°C Maximum Junction Temperature........................... 125°C Storage Temperature Range................... –65°C to 125°C Lead Temperature (Soldering, 10 sec).................... 300°C NC VIN PWM2 PWM1 VADJ2 VADJ1 VC2 TOP VIEW 38 37 36 35 34 33 32 VC1 1 31 NC LED1 2 30 NC CAP1 3 29 SW1 CAP2 4 28 SW1 LED2 5 27 SW2 RT 6 26 SW2 39 GND REF 7 25 SW3 LED3 8 24 SW3 CAP3 9 23 SW4 CAP4 10 22 SW4 LED4 11 21 NC 20 NC VC4 12 NC SHDN PWM3 PWM4 VADJ3 VC3 VADJ4 13 14 15 16 17 18 19 UHF PACKAGE 38-LEAD (5mm × 7mm) PLASTIC QFN TJMAX = 125°C, θJA = 34°C/W, θJC = 2°C/W EXPOSED PAD (PIN 39) IS GND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3476EUHF#PBF LT3476EUHF#TRPBF 3476 38-Lead (5mm × 7mm) Plastic QFN –40°C to 85°C LT3476IUHF#PBF LT3476IUHF#TRPBF 3476 38-Lead (5mm × 7mm) Plastic QFN –40°C to 125°C LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3476EUHF LT3476EUHF#TR 3476 38-Lead (5mm × 7mm) Plastic QFN –40°C to 85°C LT3476IUHF LT3476IUHF#TR 3476 38-Lead (5mm × 7mm) 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. 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, (Note 3) otherwise specifications are at TA = 25°C. SW1-4 = 5V, VIN = 3.3V, SHDN = 3.3V, RT = 21k to GND, PWM1-4 = 3.3V, VADJ1-4 = REF, CAP1-4 = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN VIN Operating Range Full-Scale LED Current Monitor Threshold Over CAP1-4/LED1-4 Operating Range l One-Tenth Scale LED Current Monitor Threshold VADJ1-4 = 100mV 10µA ≥ IREF ≥ –200µA l MAX UNITS 16 V 102 100 105 107 108 mV mV 8 12 16 mV 2.2 CAP1-4/LED1-4 Operating Range REF Output Voltage TYP 2.8 1.032 33.5 1.050 1.063 V V 3476fb 2 LT3476 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, (Note 3) otherwise specifications are at TA = 25°C. SW1-4 = 5V, VIN = 3.3V, SHDN = 3.3V, RT = 21k to GND, PWM1-4 = 3.3V, VADJ1-4 = REF, CAP1-4 = 5V, unless otherwise noted. PARAMETER CONDITIONS REF Line Regulation 2.8V ≤ VIN ≤ 16V MIN TYP MAX 0.003 UNITS %/V Quiescent Current in Shutdown SHDN = 0V 0.1 Quiescent Current Idle PWM1-4 = 0V 5.5 Quiescent Current Active (Not Switching) VC1-4 = 0V 22 30 mA Switching Frequency RT = 8.25k RT = 21k RT = 140k 2000 1000 200 2300 1150 240 kHz kHz kHz 1700 850 160 Nominal RT Pin Voltage 10 µA mA 1.26 V 84 76 90 98 % % % Current Out of Pin –10 20 100 nA PWM1-4 = 0V –20 0 20 nA Maximum Duty Cycle RT = 8.25k (2MHz) RT = 21k (1MHz) RT = 140k (200kHz) VADJ1-4 Input Bias Current VC1-4 Idle Input Bias Current EAMP GM (∆IVC/∆VCAP-LED) VC Output Impedance 1.5 210 µS 3 MΩ SW1-4 Current Limit Static Test SW1-4 VCESAT ISW = 1.3A to GND 350 SW1-4 Leakage Current SHDN = 0V 0.1 CAP1-4 Overvoltage Protect Threshold 33.5 CAP1-4/LED1-4 Idle Input Bias Current PWM1-4 < 0.4V, CAP = LED = 5V CAP1-4/LED1-4 Input Bias Current CAP = LED = 5V l SHDN Input High Voltage l 5 µA 100 nA 35 V l PWM1-4 Input High Voltage l µA 1.5 V V 16 PWM1-4 Input Low Voltage A mV 0.4 SHDN Pin Current 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: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. 2.5 70 SHDN Input Low Voltage PWM1-4 Pin Current 2 30 µA 0.4 V 1.5 V 50 100 µA Note 3: The LT3476E is guaranteed to meet specifications from 0°C to 85°C junction temperature. Specifications over the –40°C to 85°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3476I is guaranteed to meet performance specifications over the –40°C to 125°C operating junction temperature range. 3476fb 3 LT3476 Typical Performance Characteristics V(CAP-LED) Threshold vs VADJ 150 TA = 25°C, unless otherwise noted. Current Limit vs Duty Cycle 10000 2.5 TA = 25°C Oscillator Frequency vs RT 2 90 60 30 0 MINIMUM 1.5 FOSC (kHz) CURRENT LIMIT (A) V(CAP-LED) THRESHOLD (mV) TYPICAL 120 1 0.5 0 0.3 0.6 0.9 VADJ (V) 0 1.5 1.2 20 0 60 40 DUTY CYCLE (%) 80 3476 G01 1150 CURRENT LIMIT (A) 2 104 1.5 1 0.5 103 102 –45 –20 55 30 80 5 TEMPERATURE (°C) 105 0 –45 130 –20 55 30 80 5 TEMPERATURE (°C) Reference Voltage 1.045 –20 55 30 80 5 TEMPERATURE (°C) 1050 1000 950 900 850 –45 130 105 130 3476 G07 55 30 80 5 TEMPERATURE (°C) –20 130 Quiescent Current 25 VADJ = 1.05V 107 PWM 1-4 = 3.6V 20 106 105 104 VC = GND, NOT SWITCHING TA = 25°C 15 10 PWM 1-4 = 0V 5 103 102 105 3476 G06 INPUT CURRENT (mA) V(CAP-LED) THRESHOLD (mV) 1.050 1.040 –45 105 RT = 21k 1100 V(CAP-LED) Threshold vs V(CAP) 108 1.065 1.055 1000 3476 G05 3476 G04 1.060 100 Oscillator Frequency vs Temperature OSCILLATOR FREQUENCY (kHz) 107 105 10 3476 G03 2.5 106 1 RT (kΩ) Switch Current Limit vs Temperature 108 V(CAP-LED) THRESHOLD (mV) 100 100 3476 G02 V(CAP-LED) Threshold vs Temperature, VADJ = VREF VREF (V) 1000 0 5 10 20 15 VCAP (V) 25 30 35 0 0 4 8 12 16 VIN (V) 3476 G08 3476 G09 3476fb 4 LT3476 Typical Performance Characteristics SHDN and PWM Pins Current vs Voltage 1.4 PIN THRESHOLD (V) PWM 1-4 80 60 40 36.0 CAP1-4 OVERVOLTAGE THRESHOLD (V) 1.6 100 CURRENT (µA) CAP Pins Overvoltage Threshold vs Temperature SHDN and PWM Pins Threshold vs Temperature 120 SHDN 1.2 1.0 0.8 0.6 20 0 TA = 25°C, unless otherwise noted. 0 4 8 12 16 PIN VOLTAGE (V) 0.4 –45 –20 55 30 80 5 TEMPERATURE (°C) 3476 G10 105 130 3476 G11 35.5 35.0 34.5 34.0 33.5 33.0 –45 –20 55 30 80 5 TEMPERATURE (°C) 105 130 3476 G12 Pin Functions VC1, VC4, VC3, VC2, (Pins 1, 12, 13, 38): Error Amplifier Compensation Pin. When PWM is low, VC pin floats external compensation capacitor to save state for next cycle. LED1, LED2, LED3, LED4, (Pins 2, 5, 8, 11): NonInverting Input of Current Sense Error Amplifier. Connect directly to LED current sense resistor terminal. Switcher will regulate this node to a voltage of 0.1 • VADJ below the CAP node. Also connected to CAP node through external sense resistor and to anode of LED string. Do not allow this pin to float independently of corresponding CAP input pin. In applications where the LED current is low and the PVIN changes widely, connect the output filter capacitor to LEDn. CAP1, CAP2, CAP3, CAP4, (Pins 3, 4, 9, 10): Inverting input of current sense error amplifier. Connect directly to other terminal of LED current sense resistor. Also connected to output filter capacitor and cathode of external Schottky rectifier. CAP greater than the overvoltage protect threshold will inhibit switching. RT (Pin 6): Oscillator Programming Pin. Place resistor connected to GND to program oscillator frequency. REF: (Pin 7): Reference Output Pin. Connect to VADJ pin to get full-scale LED current. Connect to resistor dividers to program VADJ pins to values lower than 1.05V. Bypass to local GND with 0.1µF capacitor. VADJ4, VADJ3, VADJ2, VADJ1, (Pins 14, 15, 36, 37): LED Current Adjustment Pin. Sets voltage across external sense resistor between CAPn and LEDn. Connect directly to REF for full-scale threshold of 105mV, or use signal vales between GND and REF to modulate LED current. VADJ pin input range is 1.25V maximum. PWM4, PWM3, PWM2, PWM1, (Pins 16, 17, 34, 35): Signal low turns off the channel—disables the main switch, reduces quiescent supply current to the channel, and causes the VC pin for the channel to become high impedance. SHDN (Pin 18): Shutdown Pin. Higher than 1.5V turns the device on. NC (Pins 19, 20, 21, 30, 31, 32): Not Used. Connect to GND (Pin 39) for better heat dissipation. SW4, SW3, SW2, SW1, (Pins 22, 23, 24, 25, 26, 27, 28, 29): Switch Pin. Connect to external inductor and anode of external Schottky rectifier. Minimize area of SW trace and use a GND plane to reduce EMI. Adjacent pins of same name are internally connected. VIN (Pin 33): Input Supply Pin. Must be locally bypassed. GND (Pin 39): Signal and Power GND. Solder exposed pad directly to ground plane. The exposed pad metal of the package provides both electrical contact to ground and good thermal contact to the printed circuit board. It must be soldered to the circuit board for proper operation. 3476fb 5 LT3476 Block Diagram PVIN 33V CBYP 2.2µF EXTERNAL COMPONENTS BUCK MODE RSNS (EXT) 0.1Ω CAP 3, 4, 9, 10 LED 2, 5, 8, 11 VADJ 14, 15, 36, 37 1.25V PWM 16, 17, 34, 35 + + – Q3 A4 ERROR AMPLIFIER THERMAL LIMIT 145°C VC 1, 12, 13, 38 VIN 3V 25k PWM – Q2 + DRIVER R A2 PWM COMPARATOR Q1 MAIN SWITCH Q S + ∑ RSW 0.02Ω A3 – VIN 33 ISRC 300µA REF 7 V1 + RAMP GENERATOR CURRENT SENSE AMPLIFIER 200kHz to 2MHz OSCILLATOR – 1.05V OVERVOLTAGE DETECT RSET1 20kΩ IDLE MODE SW 22-29 35V A1 – 10µH LED ARRAY + RSET 2kΩ CFILT 0.1µF Q4 6 RT LT3476 CHANNEL NC 19, 20, 21 30, 31, 32 SHUTDOWN 18 SHDN 3476 BD Operation The LT3476 is a constant-frequency, current mode regulator with an internal power switch. Operation can be best understood by referring to the Block Diagram. At the start of each oscillator cycle, the SR latch is set, which turns on the Q1 power switch. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset, turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the voltage across the internal resistor RSET and the voltage across the external current sense resistor RSNS. In this manner, the error amplifier sets the correct peak switch current level to regulate the current through RSNS. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. 6 The current regulated in RSNS can be adjusted by changing the voltage across RSET using the VADJ input pin. The amplifier A4 regulates current in Q3 to produce a voltage across RSET equal to VADJ. This current flowing through transistor Q3 also produces a voltage across RSET onetenth the magnitude of the VADJ input and level shifted to the CAP input. The voltage across RSET is limited to 125mV (typ) by the separate 1.25V input on A4. The average current regulated in RSNS can also be adjusted for dimming 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 charge state of the VC pin will be saved on the external compensation capacitor. This feature reduces transient recovery time because when the PWM input again transitions high, the demand current for the switch returns to the value just before PWM last transitioned low. 3476fb LT3476 Applications Information Layout Hints The high speed operation of the LT3476 demands careful attention to board layout. Several items are worthy of note. The exposed pad of the package is the only GND terminal of the IC and is also important to thermal management for the IC, so it is crucial to achieve a good electrical and thermal contact between the exposed pad and the ground plane of the board. Also, the Schottky rectifier and the capacitor between GND at the cathode of the Schottky are in the high frequency switching path where current flow is discontinuous. These elements should be placed so as to minimize the path between SW and the GND of the IC. To reduce EMI, it is important to minimize the area of the SW trace. Use a GND plane under SW to minimize interplane coupling to sensitive signals. To obtain good current regulation accuracy and eliminate sources of channel-to-channel coupling, the CAP and LED inputs of each channel of the LT3476 should be run as separate lines back to the terminals of the appropriate sense resistor. Since there is a small DC input bias current (~50µA) to the LED and CAP inputs, resistance in series with these inputs should be minimized, otherwise there will be an offset. Finally, the bypass capacitor on the VIN supply to the LT3476 should be placed as close as possible to the VIN terminal of the device. Open-Circuit Protection/Overvoltage Lockout The LT3476 has independent internal overvoltage/opencircuit protection (OVP) for all four converters, sensed through their respective CAP inputs. The purpose of the OVP feature is to protect the main switch of the device from damage. In the boost configuration, if the LEDs are disconnected from the circuit or fail open, the converter output voltage at CAP is clamped at the OVP voltage of 35V (typ). Figure 1 shows the transient response of the step-up converter application with LED1 disconnected. With LED1 disconnected, the converter switches at current limit as the output ramps up to OVP. Upon reaching the OVP clamp voltage, the converter will switch with a reduced current limit to regulate the converter output voltage at the OVP clamp. In the buck mode application shown in the Block Diagram, should the external supply for CAP exceed the OVP clamp, then switching will be inhibited for the converter. In order for the overvoltage protection feature to adequately protect the switch, it is important that the CAP input sample a voltage at or near the highest voltage reached by the SW node. As a result, this OVP function will not provide adequate protection from open load events in isolated power configurations such as the 1:1 flyback, since input and output voltage magnitudes must be summed to obtain the voltage seen by the switch. 35V V(CAP) 20V LED DISCONNECT HERE I(SW) 1A/DIV 0A 20µs/DIV 3476 F01 Figure 1. LED Disconnect Transient Setting the Switching Frequency The switching frequency of the LT3476 is set by an external resistor connected between the RT pin and GND. Do not leave this pin open. Also, do not load this pin with a capacitor. A resistor must always be connected for proper operation. See Table 1 below or see the Oscillator Frequency vs RT graph in the Typical Performance Characteristics for resistor values and corresponding switching frequencies. Table 1. Switching Frequency vs RT SWITCHING FREQUENCY (kHz) RT (kΩ) 200 140 400 61.9 1000 21 1200 16.2 2000 8.25 In general, a lower switching frequency should be used where either very high or very low switch duty cycle operation is required, or higher efficiency is desired. Selection of a higher switching frequency will allow use of smaller value external components and yield a smaller solution size and profile. Also for high frequency PWM dimming, a higher switching frequency (shorter switching period) will give better dimming control since for turning on the 3476fb 7 LT3476 Applications Information switch, the state of the PWM pin is sampled only during a narrow time slot at the beginning of each switch period. Inductor Selection The inductors used with the LT3476 should have a saturation current rating of 2.5A or greater. For best loop stability results, the inductor value selected should provide a ripple current of 350mA or more. For buck (step-down) or boost (step-up) configurations, and using a 21kΩ resistor on RT (TSW ~ 1µs), inductor values from 4.7µH to 10µH are recommended for most applications. In the buck mode, the inductor value can be estimated using the formula: DBOOST VLED VCAP DBOOST • TSW (µS) • VIN , ∆I − VIN V = CAP VCAP VIN is the input voltage and VCAP is the voltage across the LED string. Table 2 below provides some suggested components and vendors. Table 2. Inductors VALUE (µH) IRMS (A) DCR (Ω) HEIGHT (mm) CDRH6D38-100 10 2.0 0.028 4.0 CDRH5D28-5R3 5.3 1.90 0.028 3.0 CDRH73-100 10 1.68 0.072 3.4 D63CB 10 1.49 0.042 3.5 D63CB 4.7 2.08 0.026 3.5 4.7 1.80 0.047 2.5 PART NUMBER Sumida Toko Cooper-ET SD25-4R7 In the buck configuration, the capacitor at the input to the power converter has large pulsed currents due to the current returned through the Schottky diode when the switch is off. For best reliability, this capacitor should have low ESR and ESL and meet the ripple current requirement, IRMS = ISW • ((1− D) • D) where D is the switch duty cycle. A 2.2µF ceramic type capacitor placed close to the Schottky and the ground plane is usually sufficient for each channel. VLED is the voltage across the LED string and VCAP is the input voltage to the converter. In the boost mode, the inductor value can be estimated using the formula: L(µH) = For proper operation, it is necessary to place a bypass capacitor to GND close to the VIN pin of the LT3476. A 1µF, or greater, capacitor with low ESR should be used. A ceramic capacitor is usually the best choice. D • T (µS) • (VCAP − VLED ) L(µH) = BUCK SW , ∆I DBUCK = Input Capacitor Selection Output Capacitor Selection The selection of output filter capacitor depends on the load and the converter configuration, i.e., step-up or step-down. For LED applications, the equivalent resistance of the LED is typically low, and the output filter capacitor should be sized to attenuate the current ripple from the inductor to 35mA or less. The following equation is useful to estimate the required capacitor value: CFILT = 2 • TSW RLED A typical filter capacitor value for RLED = 5Ω and TSW = 1µs is 0.47µF. For loop stability, consider the output pole is at the frequency where closed loop gain should be unity, so the dominant pole for loop compensation will be established by the capacitor at the VC input. For the LED boost applications, to achieve the same LED ripple current the required filter capacitor value is about five times larger than the value calculated above due to the pulsed nature of the source current. A 2.2µF ceramic type capacitor placed close to the Schottky and the ground plane of the IC is usually sufficient for each channel. As the output capacitor is subject to high ripple current, ceramic capacitors are recommended due to their low ESR and ESL at high frequency. 3476fb 8 LT3476 Applications Information Ceramic type capacitors using X7R dielectric are best for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor or larger case size to get the required capacitance at the operating voltage. Always check that the voltage rating of the capacitor is sufficient. Table 3 shows some recommended capacitor vendors. Table 3. Low-ESR Surface Mount Capacitors VENDOR TYPE SERIES Taiyo-Yuden Ceramic X5R, X7R AVX Ceramic X5R, X7R Murata Ceramic X5R, X7R Compensation Design The LT3476 uses an internal transconductance error amplifier whose VC output compensates the control loop. The external inductor, output capacitor, and 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. The component values shown in the typical applications circuits yield stable operation over the given range of input-to-output voltages and load currents. For most buck applications, a small filter capacitor (1µF or less) across the load is desirable. In this case, a 10nF compensation capacitor at VC is usually quite adequate. A compensation resistor of 5kΩ placed between the VC output and the compensation capacitor minimizes channelto-channel interaction by reducing transient recovery time. The boost configuration will have a larger output capacitor, 2.2µF to 10µF. The following circuit techniques involving the compensation pin may be helpful where there is a large variation in programmed LED current, or a large input supply range is expected. At low duty cycles (TON less than 350ns) and low average inductor current (less than 500mA), the LT3476 may start to skip switching pulses to maintain output regulation. Pulse-skipping mode is usually less desirable because it leads to increased ripple current in the LED. To improve the onset of pulse-skipping behavior, place a capacitor between the SW node and the compensation capacitor that is 1:1000 the value of the compensation capacitor. In the buck configuration, an additional technique is available. The filter capacitor between the CAP node and the LED bottom (see the Typical Application on the first page) can be moved to between the LED top and the LED bottom. This circuit change places the inductor ripple current through the sense resistor, which improves pulse-skipping behavior. There is usually less than 1% impact to the current regulation point. Diode Selection The Schottky rectifier conducts current during the interval when the switch is turned off. Select a diode with VR rated for the maximum SW voltage. For boost circuits that may use the output disconnect feature, the diode should be rated for at least 40V. It is not necessary that the forward current rating of the diode equal the switch current limit. The average current IF through the diode is a function of the switch duty cycle, so select a diode with forward current rating of IF = 1.5A • (1-D). If using the PWM feature for dimming, it may also be important to consider diode leakage from the output (especially at hot) during the PWM low interval. Table 4 has some recommended component vendors. Table 4. Schottky Diodes VR (V) IAVE (A) VF AT 1A (mV) 40 1 550 DFLS140L 40 1 550 B140 HB 40 1 530 40 1 540 PART NUMBER On Semiconductor MBRM140 Diodes Inc. NXP Semiconductor PMEG4010EJ Programming the LED Current The LED Current is programmed using an external sense resistor in series with the load. This method allows flexibility in driving the load (i.e., sensing one of several parallel strings) while maintaining good accuracy. The VADJ input sets the voltage regulation threshold across the external sense resistor between 10mV and 120mV. A 1.05V reference output (REF) is provided to drive the VADJ pins either 3476fb 9 LT3476 Applications Information through a resistor divider, or connected directly to REF to give the full-scale threshold of 105mV. A DAC may also be used to drive the VADJ pins. The VADJ pins should not be left open. If the VADJ input is connected to a voltage higher than 1.25V, the default regulation threshold across CAP and LED is 125mV (typ). The VADJ pin can also be used in conjunction with a PTC thermistor to provide overtemperature protection for the LED load as shown in Figure 2. Dimming Control There are two methods to control the current source for dimming using the LT3476. The first method, popular with LED applications, uses the PWM pin to modulate the 1.05V VREF 20k 25k VADJ1-4 470 PTC 3476 F01 current source between zero and full current to achieve a precisely programmed average current. To make this method of current control more accurate, during the quiescent phase the switch demand current is stored on the VC node. This feature minimizes recovery time when the PWM signal goes high. The minimum PWM on- or off-time will depend on the choice of operating frequency through the RT input pin. For best current accuracy, the minimum PWM low or high time should be at least ten switching cycles. This guideline has two reasons: first to allow the output to reach steady state before shutting off, and second because the oscillator is not synchronized to the PWM signal and there may be as much as one switching cycle delay from PWM going high to the start of switching. This delay, however, does not apply to the negative transition of the PWM signal. The minimum PWM low/high time can be reduced to five switching cycles if a disconnect switch is used in the LED current path. The second method of dimming control uses the VADJ pin to linearly adjust the current sense threshold during the PWM high state. The LED current programming feature augments the PWM dimming control, possibly increasing total dimming range by a factor of ten. Figure 2. Overtemperature Protect Circuit 3476fb 10 LT3476 Typical Applications Buck Mode 100W Quad 1A × 8 LED Driver CAP2 CAP1 100mΩ 100mΩ 0.22µF L1 10µH D1 L2 10µH SW1 CAP1-4 LED1-4 VIN PWM1-4 SHDN PWM1-4 SHDN 2.2µF L3 10µH SW2 2.2µF ×4 1A 0.22µF D2 VIN 2.8V TO 16V LED4 1A 0.22µF 100 100mΩ LED3 1A 1A Efficiency vs LED Current for Buck Mode CAP4 100mΩ LED2 LED1 UP TO 8 LEDS CAP3 0.22µF D3 L4 10µH D4 LT3476 92 5 LEDS 88 80 4.99k VIN = 3.3V PVIN = 33V WHITE LEDS VF = 3.6V AT 1A 0 0.2 0.6 0.4 LED CURRENT (A) 0.8 1.0 3476 TA05b 100k VC1-4 RT GND 7 LEDS 84 1.05V SW4 REF VADJ1-4 SW3 96 EFFICIENCY (%) PVIN 33V 21k 3476 TA05 1nF L1 TO L4: TOKO A916CY-100M D1 TO D4: DIODES, INC. DFLS140 5V to 25V Buck-Boost Mode Driver for 2 Series 350mA LEDs Buck-Boost Mode Efficiency vs LED Current PVIN 5V TO 25V 90 LED1 L1 10µH LED2 300mΩ 300mΩ D1 D2 L2 10µH L3 10µH LED3 2.2µF LED4 300mΩ D3 D4 CAP3 2.2µF 85 PVIN = 10V L4 10µH 300mΩ CAP2 CAP1 350mA EFFICIENCY (%) 350mA 350mA 350mA 80 70 CAP4 2.2µF 2.2µF VIN 3.3V PWM1-4 SHDN 2.2µF SW2 LT3476 GND SW3 SW4 REF VADJ1-4 VC1-4 RT 3476 TA03 VIN = 3.3V 2 WHITE LEDS VF = 3.6V AT 1A 65 60 SW1 CAP1-4 LED1-4 VIN PWM1-4 SHDN PVIN = 5V 75 50 100 200 250 150 LED CURRENT (mA) 300 350 3476 TA03b 1.05V 4.99k 44.2k 600kHz 2.2nF L1 TO L4: COOPER COILTRONICS MPI4040R3-100R D1 TO D4: NXP PMEG4010 3476fb 11 LT3476 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) 0.70 ± 0.05 5.50 ± 0.05 5.15 ± 0.05 4.10 ± 0.05 3.00 REF 3.15 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 5.5 REF 6.10 ± 0.05 7.50 ± 0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 ± 0.10 0.75 ± 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 × 45° CHAMFER 3.00 REF 37 0.00 – 0.05 38 0.40 ±0.10 PIN 1 TOP MARK (SEE NOTE 6) 1 2 5.15 ± 0.10 5.50 REF 7.00 ± 0.10 3.15 ± 0.10 (UH) QFN REF C 1107 0.200 REF 0.25 ± 0.05 0.50 BSC R = 0.125 TYP R = 0.10 TYP BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE M0-220 VARIATION WHKD 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 3476fb 12 LT3476 Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 11/11 Updated Features, Absolute Maximum Ratings, Pin Configuration, Order Information, Electrical Characteristics notes, Typical Performance Characteristics, and Pin Functions sections. Revised Table 4, moved drawings to Typical Applications section, and updated Related Parts list. Changed RT pin to RT pin and VC pin to VC pin throughout data sheet. 1 to 5 9, 10, 11, 14 1 to 14 3476fb 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. 13 LT3476 Typical Application Quad Boost 200mA × 8LED Driver PVIN 14V TO 18V L1 47µH L2 47µH D1 D2 2.2µF L3 47µH L4 47µH D3 D4 Boost Efficiency vs LED Current 2.2µF UP TO 8 LEDS CAP3 CAP2 0.25Ω LED1 LED2 200mA PWM1-4 SHDN 2.2µF 110k 100k 0.25Ω LED3 200mA SW1 VIN 3.3V 2.2µF 2.2µF CAP4 0.25Ω LED4 200mA SW2 200mA SW3 SW1 SW2 VIN PWM1-4 SHDN REF VADJ1-4 LT3476 SW4 CAP1-4 LED1-4 8 LEDS 92 88 80 10pF VC1-4 6 LEDS VIN = 3.3V PVIN = 14V WHITE LEDS VF = 3.6V AT 1A 84 SW4 SW3 GND 96 2.2µF EFFICIENCY (%) CAP1 0.25Ω 100 SW1-4 4.99k RT 0 100 200 300 LED CURRENT (mA) 400 500 3476 TA04b 2.2nF 44.2k 600kHz 3476 TA04 L1 TO L4: COILCRAFT MSS1038-473 D1 TO D4: NXP PMEG4010EJ Related Parts PART NUMBER DESCRIPTION COMMENTS LT3496 40V, Triple Output 750mA, 2.1MHz High Current LED Driver with 3000:1 Dimming with PMOS Disconnect FET Drivers VIN: 3V to 30V, VOUT(MAX) = 40V, 3000:1 True Color PWM Dimming, ISD < 1μA, 4mm × 5mm QFN-28 Package LT3492 60V, Triple Output 750mA, 1MHz High Current LED Driver with 3000:1 Dimming with PMOS Disconnect FET Drivers VIN: 3V to 30V, VOUT(MAX) = 60V, 3000:1 True Color PWM Dimming, ISD < 1μA, TSSOP-28 and 4mm × 5mm QFN-28 Packages LT3754 60V, 1MHz Boost 16-Channel 40mA LED Driver with True Color 3000:1 PWM Dimming and 2% Current Matching VIN: 4.5V to 40V, VOUT(MAX) = 60V, True Color PWM Dimming = 3000:1, ISD < 1μA, 5mm × 5mm QFN-32 Package LT3755/LT3755-1/ High Side 40V, 1MHz LED Controller with True Color 3000:1 LT3755-2 PWM Dimming VIN: 4.5V to 40V, VOUT(MAX) = 75V, 3000:1 True Color PWM Dimming ISD < 1μA, 3mm × 3mm QFN-16 and MSOP-16E Packages LT3598 44V, 1.5A, 2.5MHz Boost 6-Channel 20mA LED Driver VIN: 3V to 30V (40VMAX), VOUT(MAX) = 44V, 1000:1 True Color PWM Dimming, ISD < 1μA, 4mm × 4mm QFN-24 Package LT3599 44V, 2A, 2.5MHz Boost 4-Channel 100mA LED Driver VIN: 3V to 30V (40VMAX), VOUT(MAX) = 44V, 1000:1 True Color PWM Dimming, ISD < 1μA, 4mm × 4mm QFN-24 Package LT3518 2.3A, 2.5MHz High Current LED Driver with 3000:1 Dimming VIN: 3V to 30V, VOUT(MAX) = 45V, 3000:1 True Color PWM Dimming, ISD < 1μA, 4mm × 4mm QFN-16 and TSSOP-16E Packages with PMOS Disconnect FET Driver LT3486 Dual 1.3A, 2MHz High Current LED Driver VIN: 2.5V to 24V, VOUT(MAX) = 36V, 1000:1 True Color PWM Dimming, ISD < 1μA, 4mm × 4mm QFN-16 and TSSOP-16E Packages LT3478/LT3478-1 4.5A, 2MHz High Current LED Driver with 3000:1 Dimming VIN: 2.8V to 36V, VOUT(MAX) = 40V, 3000:1 True Color PWM Dimming, ISD < 1μA, TSSOP-16E Package LT3956 High Side 80V, 3.5A, 1MHz LED Driver with True Color 3,000:1 PWM Dimming VIN: 6V to 80V, VOUT(MAX) = 80V, True Color PWM Dimming = 3000:1, ISD < 1μA, 5mm × 6mm QFN-36 Package 3476fb 14 Linear Technology Corporation LT 1111 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2006