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.
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