With the increase in the number of medium power (1W to 4W) LED string applications, the demand for compact, high-efficiency, high-performance LED drivers has also increased. The LT3519 LED drivers meet the needs of a wide range of applications, including LCD displays, automotive and avionics applications, architectural and industrial lighting, portable projections and scanners. Its 16-pin MSOP package includes precise LED current regulation, PWM and analog dimming for brightness control, and open circuit protection with fault detection. The device also features a small size and high efficiency.
    The 400kHz LT3519 LED driver includes an integrated 750mA/45V peak power switch, compensation assembly, and low-leakage Schottky diode for a simple, compact design. Despite the high level of integration, the device can be used in a variety of topologies, including boost, SEPIC, buck mode, or buck-boost mode. To maximize versatility, the ANODE of the Schottky diode and the emitter (SW) pin of the internal power switch are drawn out separately so that the SEPIC coupling capacitor can be inserted between the two. Select the internal compensation assembly to match the output capacitance of 2.2μF to 4.7μF in the above topology. The integrated compensation network combined with current mode control produces a fast and stable transient response.
    The device also includes OPENLED detection and fault reporting. Use the resistor divider to set the overvoltage protection output voltage to prevent the LED string from opening. In the event of a failure, a small pull-up resistor can make the open collector OPENLED output pin effective.

4W boosted LED driver
    The simple step-up LED driver in Figure 1 drives an LED up to 38V with 100mA current over the automotive input voltage range. For automotive, avionics and industrial solutions, 400kHz switching frequencies are common. The driver combines high efficiency, small size inductors and capacitors, and high PWM dimming capability while avoiding frequencies in the AM broadcast band. A small inductor with a rated saturation current of about 750mA, a few ceramic capacitors and a few delicate resistors are all the components needed to complete the design. As shown in Figure 2, using an integrated LT3519 PWM dimming architecture and an extremely low leakage integrated Schottky diode, slender PWM dimming MOSFETs can be used to provide more than 1000:1 PWM dimming at 120Hz.
    At 120Hz, the 1000:1 dimming ratio is extremely high for a 400kHz switching regulator. Since a higher switching frequency usually corresponds to a higher PWM dimming ratio, the dimming ratio can be further improved by selecting a driver with a higher frequency. In this case, avoiding the AM band means jumping to 2MHz, which reduces the maximum duty cycle and efficiency. The LT3519's 400kHz switching frequency does what a 2MHz converter cannot: The LT3519 delivers a high duty cycle at a voltage as low as 6VIN at 38VLED; Delivers up to 89% efficiency at 12VIN. If PWM dimming is not required, the MOSFET M1 can be removed and the analog dimming (CTRL) pin can be used to adjust the steady LED current below 100mA for simple brightness control.

2.4W SEPIC LED driver
    When the voltage of the LED string is within the voltage range of the input rail, the SEPIC topology is required. SEPIC produces a high PWM dimming ratio and also provides short circuit protection. The SEPIC shown in Figure 3 drives a 16V LED with a current of 150mA in the input range of 4V to 24V. Because the ANODE of the integrated clamping diode is drawn through a pin independent of the NPN power switch emitter (SW), the coupling capacitor is very easy to insert between the two. The maximum voltage of the SW pin is slightly higher than the sum of the input voltage and the output voltage, so the 45V/750mA integrated power switch meets these specifications well.

Figure 1. The 4W boost LED driver has 89% efficiency at 12VIN

The integrated PWM dimming achieves 1000:1 dimming at 20Hz

Figure 3. SEPIC LED driver with short circuit protection