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Body Electronics

Driving high-power LEDs and charging batteries with 98,5% efficiency

June 12, 2012 | Keith Szolusha, Linear Technology | 222902301
Driving high-power LEDs and charging batteries with 98,5% efficiency The LT3791 is a 4-switch synchronous buck-boost DC/DC converter that regulates both constant-current and constant-voltage at up to 98.5% efficiency with a single inductor. It can deliver hundreds of watts and features a 60V input and output rating, making it ideal for driving high power LED strings and charging high voltage batteries when both step-up and step-down conversion is needed. It can also be used as a constant voltage buck-boost regulator with current limiting and monitoring for both input and output.
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Buck-Boost Controller Drives 100W LED String for Airplane and Truck Lights

Airplanes and big trucks with 24V batteries need powerful, efficient and robust headlights and spotlights. Figure 1 shows a 33.3V, 3A (nine Luminus SSR-90 LEDs) buck-boost LED driver that runs from 15V to 58V input with up to 98.5% efficiency.

The 4-switch synchronous topology drives high power LEDs with minimal switch power loss (and minimal temperature rise). Unlike other topologies, the LT3791 buck-boost can be shorted from LED+ to both LED and GND and, as a feature, can be programmed to latch off or keep trying to turn back on if the short is removed. Diagnostic output flags report both short-circuit and open-LED conditions.

The solution in Figure 1 features up to 100:1 PWM dimming at 100Hz for accurate brightness adjustment without color shift and analog LED dimming when a PWM oscillator is not present.


Figure 1. 1558VIN to 33.3V 3A LED Driver with up to 98.5% Efficiency. For full resolution click here.


Figure 2. Efficiency of Figure 1

36V, 2.5A SLA Battery Charger

The buck-boost converter shown in Figure 3 charges a 36V 12Ah SLA battery at 44V with 2.5A DC from a 9V to 58V input.

Specially integrated C/10 current sensing and battery voltage detection drops the battery voltage from its charging voltage (44V) to its float voltage (41V) when the battery is near full charge. The OPENLED flag is used to change the state of the charger from charge to float. When the battery voltage drops far enough, voltage feedback returns the charger to its charge state.

The LT3791 can be tailored to charge a range of battery chemistries and capacities from a variety of input sources regardless of the voltage relationship between them. An external microcontroller can be programmed and used to create a maximum power point tracking device to charge the battery from a solar panel. The output diagnostics and dimming input pins make this a simple interface for high power solar panel applications design.

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