Design Notes: 60V, Synchronous Step-Down High Current LED Driver
The meaning of the term “high power LED” is rapidly evolving. Although a 350mA LED could easily earn the stamp of “high power” a few years ago, it could not hold a candle to the 20A LEDs or the 40A laser diodes of today.
High power LEDs are now used in DLP projectors, surgical equipment, stage lighting, automotive lighting, and other applications traditionally served by high intensity bulbs. To meet the light output requirements of these applications, high power LEDs are often used in series. The problem is that several series-connected LEDs require a high voltage LED driver circuit. LED driver design is further complicated by applications that require fast LED current response to PWM dimming signals.
The LT3763 is a 60V synchronous, step-down DC/DC controller designed to accurately regulate LED current at up to 20A with fast PWM dimming. It is a higher voltage version of its predecessor, the LT3743. It can be applied in a number of other applications thanks to its three additional regulation loops:
1) An output voltage regulation loop enables constant output voltage operation. This can be used to provide open-LED protection or charging 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 for maximum power tracking (MPPT) in solar-powered applications.
48V Input to 35V Output, 10A LED Driver Optimized for Efficiency
Figure 1 shows a design that delivers 350W output power to drive up to seven LEDs in series from a 48V source. At this high power level,
dissipated power is a major concern, so high efficiency is critical. Each 1% of efficiency improvement reduces the loss by 3.5W—significant if the total power loss budget is less than 7W. This circuit is optimized to operate with 98.2% efficiency at full load—Figure 2 shows the efficiency reaching 98% when LED current is above 3A and peaking at 98.4% at ~6A.
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