Power controller drives high power LEDs, regulates solar cells and charges batteries: Page 2 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.
a 48V input. An internal regulator supplies the drivers of the TG and BG pins with enough power for each to drive two of the external NMOS power switches. Higher power applications can be built by connecting LT3763s in parallel, so that current is shared equally between the two controllers. This configuration also illustrates how the SYNC pin can be used to synchronize the parallel connected LT3763s to an external clock.

Figure 3. 350W white LED driver

The high output voltage rating of the LT3763 enables 35V at the output with the simplicity of a standard buck converter. The output voltage can be as high as 1.5V less than input voltage, and the configuration in Figure 4 makes use of this feature to charge three sealed lead-acid batteries in series (up to 45V) from a 48V supply.

Charging Batteries

The battery charger shown in Figure 4, 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.

Figure 4. 3.3A, six-cell (36V) SLA battery charger

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 required to form a complete battery charging system.

The resistor divider at the FB pin has been designed to program the charging voltage to 45V. As in the case of an open-circuit, when the voltage reaches 45V, the LT3763 automatically reduces the current to prevent overshoot as shown in Figure 5.

Figure 5. 36V SLA battery charging

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