L DESIGN IDEAS 4W LED Driver Includes Power Switch, Compensation Components and by Keith Szolusha Schottky in 16-Pin MSOP Introduction As the number of applications for medium power (1W–4W) LED strings grows, so does the need for compact, efficient, high performance LED drivers. The LT3519 LED driver satisfies the needs of a wide variety of applications, including LCD displays, automotive and avionic applications, architectural and industrial lighting, portable projection and scanners. It’s 16-pin MSOP package includes accurate LED current regulation, small size, high efficiency, PWM and analog dimming for brightness control and open circuit protection with fault detect. Easy Layout: Integrated Power Switch, Compensation Components and Schottky The 400kHz LT3519 LED driver features an integrated 750mA 45V peak power switch, integrated compensation components and an integrated low leakage Schottky diode, making designs simple and small. Despite this high level of integration, it can be used in a wide variety of topologies, including boost, SEPIC, buck mode or buck-boost mode. For maximum versatility, the Schottky diode anode (ANODE) and internal power switch emitter (SW) pins are separately pinned out, so a SEPIC coupling capacitor can be inserted between these two. The internal compensation components are chosen to match the 2.2µF VIN 6V TO 30V 68µH VOUT 1µF 4.7µF SW ANODE CATHODE VIN ISP 2.49Ω 1M 100k ISN SHDN/UVLO 243k 1M 1M LT3519 VREF FB CTRL OPENLED GND PWM M1 PWM Figure 1. A 4W boost LED driver with 89% efficiency at 12VIN to 4.7µF output capacitors in all of the topologies mentioned above. The integrated compensation network combined with current mode control yields fast and stable transient response. OPENLED detection and fault reporting are included. A simple resistor divider sets the overvoltage protection output voltage in case of an open LED string and a small pull-up resistor is all that is needed to assert the open collector OPENLED output pin during a fault. 4W Boost LED Driver The simple boost LED driver in Figure 1 drives up to 38V of LEDs at 100mA from an automotive input voltage range. The 400kHz switching frequency is common for automotive, avionic and industrial solutions; it combines continued on page 39 VIN 4V TO 24V C3 2.2µF L1A 68µH L1B 68µH t C1 1µF t RSENSE 1.68Ω 1M 100k SHDN/UVLO 1M 432k VREF 1M FB CTRL C1: TDK C3216X7R1H105K C2: TDK C3216X7R1E475K C3: TDK C3216X7R1E225K L1: COILTRONICS DRQ74-680-R (COUPLED INDUCTOR) M1: VISHAY SILICONIX Si2318DS IL 0.3A/DIV VIN = 12V 2µs/DIV Figure 2. Integrated PWM dimming yields 1000:1 dimming at 120Hz 36 C2 4.7µF ISN LT3519 16V LED 150mA 69.8k OPENLED GND ILED 0.1A/DIV VOUT SW ANODE CATHODE VIN ISP 158k PWM 5V/DIV 38V LED 100mA 29.4k 137k PWM M1 PWM 5V NOTE: VIN = 6V RISING TURN ON VIN = 4V FALLING UVLO VIN > 9V FULL LED CURRENT AND FOLDBACK BELOW VOUT 18.5V OVERVOLTAGE PROTECTION Figure 3. A SEPIC LED driver with short-circuit protection Linear Technology Magazine • September 2009 DESIGN IDEAS L VIN 3.3V to 5V VIN1 C2 150µF C1 22µF x2 VOUT1 CLKOUT1 CLKIN2 SVIN1 FB1 RUN1 ITH1 LTM4616 (15mm s 15mm s 2.8mm) PLLLPF1 MODE1 C3 22µF x2 ITHM1 VOUT 1.8V 16A MAX C4 100µF x4 PHMODE1 TRACK1 MGN1 VIN2 VOUT2 SVIN2 FB2 RUN2 ITH2 RSET 2.49k ITHM2 MODE2 PHMODE2 GND TRACK2 MGN2 Figure 3. LTM4616 with 16A parallel operation 100 Whether you require a single 16A high current output or dual 8A outputs with sequencing, the LTM4616 provides a simple and efficient solution. L IOUT1 5VIN 80 EFFICIENCY (%) Conclusion 3.3VIN 90 IOUT2 70 60 50 0A 40 0A OUTPUT CURRENT (2A/DIV) two regulators drive equal output current even during soft-start, as shown in Figure 5. 30 TIME (2ms/DIV) 20 10 0 0 2 4 6 8 10 12 14 16 18 Figure 5. Balanced current sharing for even heat dissipation [5VIN to 1.8VOUT at 16A] LOAD CURRENT (A) Figure 4. Efficiency: single 1.8V output LT3519, continued from page 36 high efficiency, small inductor and capacitor size, and high PWM dimming capability while avoiding frequencies in the AM broadcast band. A small inductor with about 750mA saturation current rating, a few ceramic capacitors and several tiny resistors are all that are needed to complete the design. As shown in Figure 2, the tiny PWM dimming MOSFET can be used to provide over 1000:1 pwm dimming at 120Hz using the integrated LT3519 PWM dimming architecture and an extremely low leakage integrated Schottky diode. A 1000:1 dimming ratio at 120Hz is exceptionally high for a 400kHz switching regulator. It can be tempting to bump up the dimming ratio by choosing a higher frequency driver, since in general, higher switching frequency corresponds to higher PWM dimming ratios. In this case, avoiding the AM band means jumping Linear Technology Magazine • September 2009 to 2MHz, which in the end reduces the maximum duty cycle and the efficiency. The 400kHz switching frequency of the LT3519 does what 2MHz converters cannot do: it provides high duty cycle for operation down to 6VIN with 38VLED and as high as 89% efficiency at 12VIN. If PWM dimming is not needed, the MOSFET M1 can be removed and the analog dimming (CTRL) pin can be used to adjust the regulated LED current below 100mA for simple brightness control. 2.4W SEPIC LED Driver When the LED string voltage is within the input rail voltage range, a SEPIC topology is called for. The SEPIC produces a high PWM dimming ratio and also gives short-circuit protection. The SEPIC in Figure 3 drives 16V LEDs at 150mA from a 4V to 24V input range. Since the anode of the integrated catch diode (ANODE) is made available at a pin independent of the npn power switch emitter (SW), the coupling capacitor is easily inserted between the two. The maximum voltage that the SW pin sees is a little above the input voltage plus the output voltage, so the 45V 750mA integrated power switch is a perfect match for these specifications. Conclusion The 400kHz LT3519 is a 4W LED driver that integrates a number of required components, including a 45V, 750mA power switch, a low leakage Schottky diode and compensation components. It also features PWM dimming, overvoltage protection and OPENLED fault detection, making it a small, simple, and efficient choice for automotive, avionic, industrial and other LED driver applications. L 39