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A new LED driver-controller that cuts BOM costs, saves board space and improves reliability
Design Engineer, Luke Milner talks CIE through how the new LT3763 from Linear Technology is able to not only drive high power LEDs, but regulate solar cells and charges batteries
he best LED drivers accurately regulate LED current for consistent colour reproduction and modulate it rapidly for high contrast dimming. They also recognise 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 full-fill these responsibilities.
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The LT3763 LED driver-controller, by contrast, has these functions built in, reducing BOM costs, saving board space and improving reliability.
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
Figure 2 to deliver 350W with 98% efficiency from a 48V input. Higher power applications can be built by connecting LT3763s in parallel, so that current is shared equally between the two controllers.
Charging batteries
When used as a battery charger, the LT3763, 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 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.
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
Figure 1. A Single High Power LED (20A) Driver with Analogue & PWM Dimming
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 LT3763 is designed to provide flicker-free LED dimming. 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 re-synchronising internal switching clocks to the PWM pulse. These manoeuvres ensure that subsequent pulses are identical, that 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
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required to form a complete battery charging system. 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 open-circuit 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. 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.
Regulating solar panels A well-designed solar panel power supply requires an intelligent
Figure 2. 350W White LED Driver
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. In many common solutions, a solar panel controller designer would use an amplifier 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
Figure 3. 70W Solar Energy Harvester with Maximum Power Point Regulation
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 3 and can be customized to fit the requirements of any solar panel.
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
complex features essential for not only LED drivers, 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 utilised to illuminate long strings of LEDs or charge stacks of batteries.
Linear Technology |
www.linear.com Components in Electronics February 2014 5
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
solar panels to charge many different types of batteries.
Monitoring current levels In each of these applications, 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.
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. The LT3763 is a versatile step-down buck converter that integrates many
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