LT3476 High Current Quad Output LED Driver DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ True Color PWMTM Dimming Delivers Up to 5000:1 Dimming Ratio (In Boost Configuration) LED Current Regulation with High-Side Sense VADJ Pin Accurately Sets LED Current Sense Threshold Over Range 10mV to 120mV Four Independent Driver Channels with 1.5A, 36V Internal NPN Switches Frequency Adjust Pin: 200kHz to 2MHz High Efficiency Conversion = Up to 96% Open LED Protection Low Quiescent Current 22mA in Active Mode <10μA in Shutdown Mode Wide VIN Range: 2.8V to 16V 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. 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. U APPLICATIO S ■ ■ ■ ■ , 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. RGGB Lighting Automotive and Avionic Lighting TFT LCD Backlighting Constant-Current Sources U TYPICAL APPLICATIO 100W Quad 1A × 8 LED Driver PVIN 33V CAP1 CAP2 100mΩ 100mΩ LED1 UP TO 8 LEDS 2.2μF ×4 1A 0.22μF 10μH PWM 5V/DIV LED4 1A 0.22μF 10μH 100mΩ LED3 1A 0.22μF CAP4 CAP3 100mΩ LED2 1A 1000:1 PWM Dimming at 100Hz 0.22μF 10μH ILED 500mA/DIV 5μs/DIV 10μH 3476 TA02 1.05V VIN 2.8V TO 16V PWM1-4 SHDN 2.2μF SW1 CAP1-4 LED1-4 VIN PWM1-4 SHDN SW2 SW3 LT3476 GND SW4 REF VADJ1-4 4.99k 100k VC1-4 RT 3476 TA01 21k 1nF 3476fa 1 LT3476 U U U W W W AXI U RATI GS U ABSOLUTE PI CO FIGURATIO (Note 1) NC VIN PWM2 PWM1 VADJ2 VC2 VADJ1 TOP VIEW VIN ............................................................................16V PWM1-4, SHDN ........................................................16V SW1-4, LED1-4, CAP1-4 ...........................................36V REF, RT, VADJ1-4, VC1-4 ................................................2V Operating Temperature Range (Note 2).... –40°C to 85°C Maximum Junction Temperature........................... 125°C Storage Temperature Range................... –65°C to 125°C Lead Temperature ................................................. 300°C 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 EXPOSED PAD (PIN 39) IS GND (MUST BE SOLDERED TO PCB) U W U ORDER I FOR ATIO 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 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 Consult LTC Marketing for parts specified with wider operating temperature ranges. 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 ● 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 2.8 Over CAP1-4/LED1-4 Operating Range ● One-Tenth Scale LED Current Monitor Threshold VADJ1-4 = 100mV 10μA ≥ IREF ≥ –200μA REF Line Regulation 2.8V ≤ VIN ≤ 16V Quiescent Current in Shutdown SHDN = 0V ● MAX UNITS 16 V 102 100 105 107 108 mV mV 8 12 16 mV 36 V 1.063 V 2.2 CAP1-4/LED1-4 Operating Range REF Output Voltage TYP 1.032 1.050 0.003 0.1 %/V 10 μA 3476fa 2 LT3476 ELECTRICAL CHARACTERISTICS The ● 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 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 TYP MAX UNITS mA 1.26 V % % % RT = 8.25k (2MHz) RT = 21k (1MHz) RT = 140k (200kHz) 84 76 90 98 VADJ1-4 Input Bias Current Current Out of Pin –10 20 100 nA VC1-4 Idle Input Bias Current PWM1-4 = 0V –20 0 20 nA Maximum Duty Cycle EAMP GM (ΔIVC/ΔVCAP-LED) 200 VC Output Impedance μS 3 SW1-4 Current Limit Static Test 1.5 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 2 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. 5 μA V 100 nA μA 0.4 V 1.5 V 16 PWM1-4 Input Low Voltage PWM1-4 Pin Current mV 70 SHDN Pin Current PWM1-4 Input High Voltage A 35 SHDN Input Low Voltage SHDN Input High Voltage MΩ 2.5 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. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. 3476fa 3 LT3476 U W TYPICAL PERFOR A CE CHARACTERISTICS V(CAP-LED) Threshold vs VADJ 150 TA = 25°C, unless otherwise noted. Oscillator Frequency vs RT Current Limit vs Duty Cycle 10000 2.5 TA = 25°C 2 90 60 30 MINIMUM 1.5 FOSC (kHz) CURRENT LIMIT (A) V(CAP-LED) THRESHOLD (mV) TYPICAL 120 1 0.5 0 0 0.3 0.6 0.9 VADJ (V) 1.2 0 1.5 100 20 0 60 40 DUTY CYCLE (%) 80 Oscillator Frequency vs Temperature 1150 OSCILLATOR FREQUENCY (kHz) 107 2 1.5 1 0.5 103 102 –45 –20 55 30 5 TEMPERATURE (°C) 80 0 –45 105 –20 55 30 5 TEMPERATURE (°C) Reference Voltage 1.055 1.050 1.045 55 30 5 TEMPERATURE (°C) 1000 950 900 850 –45 80 105 3476 G07 55 30 5 TEMPERATURE (°C) –20 105 80 3476 G06 Quiescent Current 25 VADJ = 1.05V 107 PWM 1-4 = 3.6V 20 INPUT CURRENT (mA) V(CAP-LED) THRESHOLD (mV) 108 –20 1050 V(CAP-LED) Threshold vs V(CAP) 1.065 1.040 –45 1100 105 80 RT = 21k 3476 G05 3476 G04 1.060 1000 3476 G03 2.5 CURRENT LIMIT (A) V(CAP-LED) THRESHOLD (mV) 108 104 100 RT (kΩ) Switch Current Limit vs Temperature 105 10 3476 G02 V(CAP-LED) Threshold vs Temperature, VADJ = VREF 106 1 100 3476 G01 VREF (V) 1000 106 105 104 VC = GND, NOT SWITCHING TA = 25°C 15 10 PWM 1-4 = 0V 5 103 0 102 0 5 10 20 15 VCAP (V) 25 30 35 0 4 8 12 16 VIN (V) 3476 G08 3476 G09 3476fa 4 LT3476 U U U PI FU CTIO S 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. 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 contract to ground and good thermal contact to the printed circuit board. It must be soldered to the circuit board for proper operation. 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. 3476fa 5 LT3476 W BLOCK DIAGRA 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 + + – Q3 PWM 16, 17, 34, 35 THERMAL LIMIT 145°C 25k PWM – Q2 + RSET1 20kΩ IDLE MODE VC 1, 12, 13, 38 VIN 3V OVERVOLTAGE DETECT – A4 SW 22-29 35V A1 ERROR AMPLIFIER 1.25V DRIVER R A2 Q1 MAIN SWITCH Q S PWM COMPARATOR + ∑ RSW 0.02Ω A3 – VIN 33 ISRC 300μA REF 7 V1 + RAMP GENERATOR CURRENT SENSE AMPLIFIER 200kHz to 2MHz OSCILLATOR – 1.05V 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 U OPERATIO 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. 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. 3476fa 6 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) 3476fa 7 LT3476 U U W U APPLICATIO S I FOR ATIO will give better dimming control since for turning on the 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: DBUCK • TSW (μS) • (VCAP − VLED ) , ΔI V = LED VCAP DBOOST DBOOST • TSW (μS) • VIN , ΔI − 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) Output Capacitor Selection 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. 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. L(μH) = DBUCK Input 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: T CFILT = 2 • SW 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. 3476fa 8 LT3476 U W U U APPLICATIO S I FOR ATIO 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 channel-to-channel interaction by reducing transient recovery time. The boost configuration will have a larger output capacitor, 2.2μF to 10μF. 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 PART NUMBER VR (V) IAVE (A) VF AT 1A (mV) 40 1 550 On Semiconductor MBRM140 Diodes Inc. DFLS140L 40 1 550 B140 HB 40 1 530 40 1 540 Philips Semiconductor PMEG4010EJ 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 3476fa 9 LT3476 U W U U APPLICATIO S I FOR ATIO Programming the LED Current Dimming Control 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 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. 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 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. 1.05V VREF 20k 25k VADJ1-4 470 PTC 3476 F01 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 3476fa 10 LT3476 U TYPICAL APPLICATIO 5V to 25V Step-Up/Step-Down Driver for 2 Series 350mA LEDs PVIN 5V TO 25V 350mA 350mA 350mA 10μH 10μH LED1 10μH LED3 300mΩ CAP1 300mΩ CAP3 2.2μF SW1 CAP1-4 LED1-4 VIN PWM1-4 SHDN PWM1-4 SHDN 2.2μF LED4 300mΩ CAP2 2.2μF VIN 2.8V TO 16V 10μH LED2 300mΩ 350mA CAP4 2.2μF SW2 SW3 LT3476 GND 2.2μF SW4 REF VADJ1-4 1.05V VC1-4 RT 3476 TA03 47k 47nF U PACKAGE DESCRIPTIO UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701) 5.00 ± 0.10 (2 SIDES) 3.15 ± 0.10 (2 SIDES) 0.75 ± 0.05 0.00 – 0.05 0.70 ± 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 × 45° CHAMFER 37 38 0.40 ±0.10 PIN 1 TOP MARK (SEE NOTE 6) 5.50 ± 0.05 (2 SIDES) 1 2 4.10 ± 0.05 (2 SIDES) 3.15 ± 0.05 (2 SIDES) 7.00 ± 0.10 (2 SIDES) 5.15 ± 0.10 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 5.15 ± 0.05 (2 SIDES) 6.10 ± 0.05 (2 SIDES) 7.50 ± 0.05 (2 SIDES) 0.40 ±0.10 0.200 REF RECOMMENDED SOLDER PAD LAYOUT 0.200 REF 0.00 – 0.05 0.75 ± 0.05 0.25 ± 0.05 0.50 BSC R = 0.115 TYP BOTTOM VIEW—EXPOSED PAD (UH) QFN 0205 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 3476fa 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. 11 LT3476 U TYPICAL APPLICATIO Quad Boost 100mA × 8LED Driver PVIN 2.8V TO 5V 4.7μH CAP1 2.2μF UP TO 8 LEDS 4.7μH 2.2μF CAP2 CAP3 1Ω 1Ω LED1 4.7μH 4.7μH 2.2μF LED2 100mA 2.2μF CAP4 1Ω 1Ω LED3 2.2μF LED4 100mA 100mA 100mA 1.05V VIN 2.8V TO 5V PWM1-4 SHDN 2.2μF SW1 SW2 CAP1-4 LED1-4 VIN PWM1-4 SHDN SW3 LT3476 GND SW4 REF VADJ1-4 5k 100k VC1-4 RT 3476 TA04 21k 47nF RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 1.5A, Constant-Current, 1.4MHz, Boost Converter VIN(MIN) = 5V, VIN(MAX) = 18V, VOUT(MAX) = 36V, Dimming = Analog/PWM, ISD < 1μA, MS10 Package LTC3454 1A Synchronous Buck-Boost LED Driver VIN(MIN) = 2.7V, VIN(MAX) = 5.5V, VOUT(MAX) = 5.5V, Dimming = 4-Levels of Adj, ISD < 1μA, DFN-10 Package LTC3474 1A (ILED), 36V, 2MHz, Step-Down LED Driver VIN(MIN) = 4V, VIN(MAX) = 36V, VOUT(MAX) = 13.5V, 400:1 True Color PWM, ISD < 1μA, TSSOP-16E Package LT3475 Dual 1.5A (ILED), 36V, 2MHz, Step-Down LED Driver VIN(MIN) = 4.0V, VIN(MAX) = 36V, VOUT(MAX) = 13.5V, Dimming = 3000:1 True Color PWM, ISD < 1μA, TSSOP-20E Package LT3477 3A, 42V, 3.5MHz Boost, Buck-Boost, Buck LED Driver VIN(MIN) = 2.5V, VIN(MAX) = 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1μA, QFN and TSSOP-20E Packages LT3479 3A, Full-Featured DC/DC Converter with Soft-Start and Inrush Current Protection VIN(MIN) = 2.5V, VIN(MAX) = 24V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1μA, DFN and TSSOP Packages 3476fa 12 Linear Technology Corporation LT 0707 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006