Designing a 60V Buck-Boost LED driver with up to 98% efficiency

November 03, 2016 // By Keith Szolusha
Synchronous buck-boost converters with four power switches can deliver very high efficiency while providing both step-up and step-down DC/DC conversion.

Combining the capabilities of two separate converters (buck and boost) has the advantage of reduced solution size and cost. Four-switch converters should be able to operate with just two switches for highest efficiency when only step-up or step-down conversion is needed. However, they must also be able to utilize four-switch operation as VIN and VOUT approach each other, and to transition gracefully between these regions of operation. Combining control loops for two-switch boost, two-switch buck, and four-switch operation, and designing nearly flawlessly transitions between these regions of operation has its challenges. However, the next generation of buck-boost converters masters these challenges and more. 

The LT8391 60V four-switch buck-boost LED driver is designed to drive high power LEDs up to 250W and to flawlessly transition between two-switch boost, four-switch buck-boost, and two-switch buck regions of operation. A patent pending four-switch buck-boost current-sense resistor control scheme provides a simple, yet masterful method for the IC to run in peak current mode control in all regions of operation with a single sense resistor. This new generation buck-boost LED driver features spread spectrum frequency modulation and internally generated PWM dimming, which work together. The LT8391 has flicker-free PWM dimming with both internal and external PWM dimming, even when spread spectrum is turned on (another patent pending technique).

Figure 1. LT8391 4V to 60V Four-Switch Synchronous Buck-Boost LED Driver Powers a 25V 2A (50W) String of LEDs at Up to 98% Efficiency.

Figure 2. (a) The Efficiency of Figure 1 50W LED Driver Has a Peak of 98% and Ranges from 95% to 97% Throughout the Typical 9V to 16V Automotive Input Range. (b) The LT8391 Peak Inductor Current Limit Can Maintain Stability with Reduced Output Power at Low VIN.

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