High Power Controller Drives High Power LEDs, Regulates Solar Cells, and Charges Batteries, Steps Down 60V Inputs Luke Milner The best LED drivers accurately regulate LED current for consistent color reproduction and modulate it rapidly for high contrast dimming. They also recognize and survive short and open circuits, monitor and report current levels, guard against overheating, and protect weak power supplies from excessive load currents. A standard switching converter would require a number of additional expensive amplifiers, references and passive components to fulfill these responsibilities. In contrast, the LT®3763 LED driver-controller has these functions built in—reducing BOM costs, saving board space and improving reliability. The LT3763 is more than just a high performance LED driver. Its rich feature set simplifies the design of other demanding applications, such as safe charging of a sealed lead-acid batteries, or maximum power point regulation for a solar panel, or a combination of both. The LT3763 performs these tasks with high efficiency, even at input voltages reaching 60V. prevent overshoot and pulls down the FAULT pin to mark the occasion. The LT3763 is designed to provide flickerfree LED dimming as shown in Figure 2. This is achieved by pulling PWMOUT low whenever PWM is low and thereby disconnecting the LED, by similarly disconnecting the compensation network at VC , and resynchronizing internal switching clocks to the PWM pulse. These maneuvers ensure that subsequent pulses are identical, that DRIVING LEDs Figure 1 shows the LT3763 configured as a high power LED driver. A potentiometer at the CTRL1 pin permits manual adjustment of the regulated LED current from 0 to 20A. For thermal regulation of the LED current, a resistor with a negative temperature coefficient is mounted near the LED and connected from the CTRL2 pin to GND. The resistor network at the EN/UVLO pin programs the LT3763 to shut down if the input voltage falls to less than 10V. The resistor network at the FB pin defines an open-circuit condition as when the output reaches 6V, and should that ever happen, the LT3763 automatically reduces the inductor current to 8 | April 2013 : LT Journal of Analog Innovation the inductor current rises as fast as possible to satisfy the programmed LED current level, and that the LED light never flickers. The LT3763 can be configured as in Figure 3 to deliver 350W with 98% efficiency from a 48V input. An internal regulator supplies the drivers of the TG and BG pins with enough power for each to drive two of the external NMOS power switches. Higher power applications can be built by connecting LT3763s in parallel, Figure 1. A single high power LED (20A) driver with analog and PWM dimming RSENSE_IN 2.5mΩ VIN 10V TO 30V REN1 84.5k RFILTA 1k REN2 15.4k CFILT 1µF RFILTB 1k RHOT 45.3k IVINN VIN TG CBOOST 220nF VREF LT3763 CTRL2 RNTC 470k 50k LTspice IV circuits.linear.com/620 IVINP EN/UVLO CREF 2.2µF CIN2 100µF CIN1 4.7µF BOOST M1 L1 1.5µH SW INTVCC BG CTRL1 RS 2.5mΩ VOUT 6V, 20A MAXIMUM COUT 220µF ×2 RFAULT 47.5kΩ CVCC 22µF D1 M2 RSA 10Ω RSB 10Ω GND 50Ω 1nF 50Ω 1nF FBIN SENSE+ IVINMON SENSE– PWMOUT ISMON M3 FAULT PWM SYNC RT RT 82.5k CS 33nF FB SS CSS 10nF VC RC 59k CC 4.7nF L1: COILCRAFT XAL1010-152 M1: RENESAS RJK0365 M2: RENESAS RJK0453 M3: IR IRFH6200 RS: VISHAY WSL25122L500FEA RFB1 47.5k RFB2 12.1k design features The output voltage can be as high as 1.5V less than input voltage, making it possible to charge three sealed lead-acid batteries in series (up to 45V) from a 48V supply with the simplicity of a standard buck converter. small. Once the trickle charge phase is complete, the charger should allow the batteries’ voltages to decay to a relaxed level before finally settling at and holding that final voltage indefinitely. PWM 10V/DIV VSW 50V/DIV IL 5A/DIV 5µs/DIV Figure 2. PWM dimming performance of the circuit in Figure 1 so that current is shared equally between the two controllers. This configuration also illustrates how the SYNC pin can be used to synchronize the parallel connected LT3763s to an external clock. The high output voltage rating of the LT3763 enables 35V at the output with the simplicity of a standard buck converter. The output voltage can be as high as 1.5V less than input voltage, and the configuration in Figure 4 makes use of this feature to charge three sealed lead-acid batteries in series (up to 45V) from a 48V supply. The combined current and voltage regulation loops on the LT3763, and its LED fault handling circuitry, nearly make it a complete battery charger. Only a single additional transistor is required to form a complete battery charging system. The resistor divider at the FB pin has been designed to program the charging voltage to 45V. As in the case of an open-circuit, when the voltage reaches 45V, the LT3763 automatically reduces the current to prevent overshoot as shown in Figure 5. When their combined voltage decays to the newly programmed value, the LT3763 begins switching again and provides a sustaining current necessary to maintain the output voltage indefinitely. As an added Figure 3. 350W white LED driver VIN 48V REN1 374k REN2 124k IVINP IVINN VIN EN/UVLO TG VREF CREF 2.2µF LT3763 CTRL2 SENSE+ SENSE– IVINMON 3V 0V 400kHz RT 200k RFAULT 100k CVCC 22µF M2 ×2 FB CSS 10nF VC RC 5k CC 5nF VOUT 37V, 10A MAXIMUM LED1 RFB1 931k FAULT SS RS 5mΩ CS 1nF PWMOUT ISMON PWM SYNC RT L1 6µH COUT 10µF ×6 GND FBIN M1 ×2 SW BG CTRL1 CBOOST 220nF BOOST INTVCC INTVCC CIN2 100µF CIN1 4.7µF CHARGING BATTERIES The battery charger shown in Figure 4, like all chargers, must be able to precisely regulate the batteries’ rated charging current (constant current mode) until the battery voltages reach the limit set by their chemistry. The charger must maintain that voltage (constant voltage mode) without overshoot until the current drawn by the trickle-charging batteries becomes very Subsequently, during trickle charging, the battery draws less current over time. When the charging current reduces to ten percent of the regulated current (C/10 battery specification), the LT3763’s opencircuit fault condition is triggered. The resulting high-to-low transition at the FAULT pin is used to turn off the gate of the added transistor M3 and remove the resistor RFB3 from the feedback network. The programmed output voltage is thereby lowered, and the LT3763 stops switching to allow the batteries to relax on their own. LED1: LUMINUSPT-121 L1: COILTRONICS HC2-6R0 M1, M2: RENESAS RJK0851 RS: VISHAY WSL25125L000 RFB2 30.9k April 2013 : LT Journal of Analog Innovation | 9 The LT3763 is a versatile step-down buck converter that integrates many complex features essential for LED drivers, solar harvesters and battery chargers. A PWM driver and current monitors are included with fault detection, current limiting, input and output voltage regulation. RSENSE_IN 15mΩ VIN 48V RFILTA 1k CFILT 1µF IVINP ENABLE CREF 2.2µF RFILTB 1k IVINN CIN1 1µF VIN EN/UVLO TG VREF LT3763 CTRL2 CBOOST 220nF BOOST BG RFAULT 47.5kΩ CVCC 22µF M2 GND FBIN SYNC RT FB SS VC RC 8.06k CC 4.7nF RSB 10Ω 12V + 12V + RFB1 402k FAULT CSS 10nF RSA 10Ω + CS 33nF PWMOUT ISMON PWM VOUT 45V, 3.3A MAXIMUM RS 15mΩ 12V SENSE– IVINMON RT 82.5k L1 12µH COUT 20µF SENSE+ INTVCC M1 SW INTVCC CTRL1 CIN2 47µF L1: WÜRTH 74471112 M1, M2: INFINEON BSC100N06LS3 M3: VISHAY VN2222LL RS: VISHAY WSL2512R0150 RFB2 12.1k RFB3 178k M3 Figure 4. 3.3A, six-cell (36V) SLA battery charger benefit, the FAULT pin transition serves as a signal that the trickle charging has begun. A well-designed solar panel power supply requires an intelligent combination of current and voltage regulation. In an optimum design, a converter must sense the voltage on the panel and adjust the current it draws to maintain the input voltage at the panel’s maximum power point. If it draws too much current, the voltage of the high impedance panel will collapse. If it draws too little current, available light energy is essentially wasted. to sense the input voltage and adjust the voltage on the current control pin. The LT3763 includes this function at the FBIN pin. Simply tie CTRL1 high, to the 2V reference available at VREF, and add a voltage divider from VIN to FBIN. When the voltage at FBIN falls to nearly 1.205V, the internal amplifier automatically overrides the CTRL1 voltage and reduces the load current. This regulates the input voltage (the voltage of the solar panel) at the maximum power point for the panel. The resistor divider on the FBIN pin is shown in Figure 6 and can be customized to fit the requirements of any solar panel. In many common solutions, a solar panel controller designer would use an amplifier In the configuration shown in Figure 6, the converter can generate whatever inductor REGULATING SOLAR PANELS 10 | April 2013 : LT Journal of Analog Innovation current, up to 5A, is required to hold the panel voltage at 37V. Input voltage feedback is via the voltage divider at the FBIN pin, which in turn regulates the inductor current to what is actually necessary to hold the panel at peak power in any given light condition. As shown in Figure 7, the process of charging a battery with a solar panel looks very similar to charging with a low impedance supply as before. The difference is that the regulated inductor current (charge current) is not preset by the designer, but is instead adjusted on the fly via the feedback loop regulating input voltage. This effectively minimizes charge time, since input power is maximized at all times, regardless of panel illumination. Since the LT3763 has the capability of regulating input voltage and current, as well as output voltage and current, and provides a fault flag with C/10, it can easily be used with a wide variety of solar panels to charge many different types of batteries. Figure 5. 36V SLA battery charging cycle FAULT 10V/DIV IL 2A/DIV VOUT 50mV/DIV AC-COUPLED 50s/DIV design features Figure 6. 70W solar energy harvester with maximum power point regulation PANEL VOLTAGE UP TO 60V 37V VIN REG POINT RSENSE_IN 10mΩ D1 RFILTA 1k D2 CFILT 1µF IVINP ENABLE Dn CREF 2.2µF RFILTB 1k IVINN TG VREF LT3763 CTRL2 In each of the applications presented here, the LT3763 provides an additional service by monitoring the input and output current levels. Voltages across the IVINP and IVINN pins ranging from 0 to 50mV are amplified with a gain of 20, and the resulting voltage appears at the IVINMON pin. The voltage at the ISMON pin is an identical amplification of the voltage across the SENSE+ and SENSE– pins, as shown in Figure 8. These signals are helpful in systems that must verify the current provided to LEDs or measure the efficiency of voltage conversion. They can also help to estimate the power provided by a solar panel or to monitor the current trickling into a charging battery as it decays to zero. Due to the discontinuous input current of a step-down buck converter, a lowpass filter is typically necessary at the IVINP and IVINN pins as shown in Figure 1 and Figure 4. A much smaller filter at the SENSE+ and SENSE– pins may also be useful Figure 7. Solar powered SLA battery charging VREF CTRL1 FBIN SENSE– IVINMON INTVCC + RFAULT 47.5kΩ CVCC 22µF M2 RSA 10Ω 12V RSB 10Ω SYNC RT CS 33nF PWMOUT ISMON PWM RT 82.5k VOUT RS 10mΩ 14V MAXIMUM GND SENSE+ RFBIN2 12.1k L1 12µH SW BG RFBIN1 348k circuits.linear.com/621 M1 CBOOST 100nF BOOST INTVCC RNTC 470k LTspice IV CIN2 100µF VIN EN/UVLO RHOT 45.3k MONITORING CURRENT LEVELS CIN1 4.7µF RFB1 121k FAULT FB SS CSS 10nF VC RC 26.1k CC 4.7nF in filtering high frequency noise, but it is not necessary. Even with these filters, the monitors are fast enough to track reasonably short PWM pulses as shown in Figure 8. Nevertheless, if a designer is more concerned with average current levels than instantaneous current levels, then additional lowpass filters can be easily added to the ISMON and IVINMON pins. SUMMARY The LT3763 is a versatile step-down buck converter that integrates many complex features essential for not only LED drivers, L1: COILCRAFT MSS1278-123 M1, M2: INFINEON BSC100N06LS3 M3: VISHAY VN2222LL RS: VISHAY WSL2512R0100FEA RFB2 12.1k RFB3 182k M3 but solar harvesters and battery chargers as well. A PWM driver and current monitors are included with fault detection, current limiting, input and output voltage regulation. Due to its high voltage rating, all of these features can be utilized to illuminate long strings of LEDs or charge stacks of batteries. Available in a 28-lead TSSOP package, the LT3763 is a compact, complete, and efficient power system. Visit www.linear.com/LT3763 for data sheets, demo boards and other applications information. n Figure 8. Current monitor outputs in an LED driver application with PWM dimming FAULT 10V/DIV VIN 50mV/DIV AC-COUPLED IL 5A/DIV IL 2A/DIV ISMON 500mV/DIV VOUT 50mV/DIV IVINMON 200mV/DIV 50s/DIV 100µs/DIV April 2013 : LT Journal of Analog Innovation | 11