Power controller drives high power LEDs, regulates solar cells and charges batteries: Page 3 of 5

July 08, 2013 //By Luke Milner, Linear Technology Corporation
Power controller drives high power LEDs, regulates solar cells and charges batteries
The best LED drivers accurately regulate LED current for consistent color reproduction and modulate it rapidly for high contrast dimming. They also recognize 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 fulfill these responsibilities.
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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. As an added benefit, the /FAULT pin transition serves as a signal that the trickle charging has begun.

Regulating Solar Panels

A well-designed solar panel power supply requires an intelligent 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 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 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 6 and can be customized to fit the

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