60V Synchronous Step-Down High Current LED Driver

The meaning of the term “high-power LED” is rapidly evolving. While a 350mA LED might have easily been labeled “high power” a few years ago, it pales in comparison to today’s 20A LEDs or 40A laser diodes. Today, high-power LEDs are widely used in DLP projectors, surgical equipment, stage lighting, automotive lighting, and other applications that have traditionally been illuminated by high-brightness bulbs. To meet the light output requirements of such applications, high-power LEDs are often connected in series. The problem is that multiple LEDs connected in series require a high-voltage LED driver circuit. And applications that require fast LED current response to PWM dimming signals further complicate LED driver design.

The LT3763 is a 60V synchronous, step-down DC/DC controller designed to accurately regulate LED currents up to 20A and achieve fast PWM dimming. It is a higher voltage version of its earlier sibling, the LT3743. The device can be used in many other applications thanks to its three additional regulation loops:

1) An output voltage regulation loop achieves constant output voltage operation. This can be used to provide LED open circuit protection or charge termination for a battery charger.

2) A second current regulation loop can be used to set an input current limit.

3) An input voltage regulation loop can be used to implement maximum power point tracking (MPPT) function in solar-powered applications.

48V Input to 35V Output, 10A LED Driver Optimized for Efficiency

Figure 1 shows a design that can deliver 350W output from a 48V supply to drive up to 7 series LEDs. At these high power levels, power dissipation is a significant issue, so high efficiency is critical. Every 1% improvement in efficiency reduces power losses by 3.5W, which is a significant reduction if the total power loss budget is less than 7W. The circuit is optimized to operate at 98.2% efficiency at full load. As shown in Figure 2, efficiency reaches 98% at LED currents above 3A and peaks at 98.4% at about 6A.

At high voltage, the switching losses of the MOSFET and inductor exceed the conduction losses. The switching frequency is set to 200kHz to minimize switching losses and keep the solution size small. When running at full load, the hot spot of the circuit occurs in the upper MOSFET, which has a temperature rise of less than 50°C (a very good range for MOSFETs).

36V Input to 20V Output, 10A LED Driver with Fastest PWM Dimming Capability

PWM LED dimming is the standard dimming method for high power, high performance lighting applications. In image generation applications such as DLP projectors, fast LED current response to PWM signals is important. Figure 3 shows the LT3763 in an application circuit optimized for fast LED PWM dimming.

The LT3763 has several innovative features designed to achieve fast LED current response to a PWM signal. For a given input voltage, smaller inductance results in faster inductor current ramp-up, which translates into faster LED current response. When a PWM dimming signal is turned on, the circuit takes only a few μs to reach full LED current from zero current. Figure 4 shows the performance of this circuit in a PWM dimming application. Efficiency can reach 97% at full load.

Solar powered battery charger

In addition, the LT3763 can regulate input voltage by adjusting its output current. This is useful for applications that must track peak input power, such as solar-powered battery chargers.

Each solar panel has a maximum output power point, which depends on the illumination, voltage and output current of the solar panel. Generally speaking, peak power can be achieved by keeping the solar panel voltage within a small range, and reducing the output current when necessary to prevent the solar panel voltage from exceeding this range. This is called maximum power point tracking (MPPT).

The LT3763's input voltage regulation loop keeps the solar panel voltage within the maximum power point by adjusting the output current. Constant current, constant voltage (CCCV) operation and C/10 functionality make the device suitable for battery charger applications.

in conclusion

The LT3763 is a 60V, synchronous, high current step-down LED driver controller that can be used to drive the latest high power LEDs with fast PWM dimming response when needed. The LT3763 has three additional voltage and current regulation loops and several powerful features, so its application range is not limited to LED drivers.