Designing a 60V Buck-Boost LED driver with up to 98% efficiency: Page 2 of 3

November 03, 2016 //By Keith Szolusha
Designing a 60V Buck-Boost LED driver with up to 98% efficiency
Synchronous buck-boost converters with four power switches can deliver very high efficiency while providing both step-up and step-down DC/DC conversion.

98% Efficient Buck-Boost LED Driver

The LT8391 high power buck-boost LED driver in Figure 1 drives 25V of LEDs at 2A from a wide input voltage range. Efficiency can reach 98% at its highest point. Over the typical automotive battery range of 9V to 16V, the converter operates between 95% and 97% efficiency. With a single high power inductor, the temperature rise is low, even at 50W. At 12VIN, no component rises more than 25°C. At 6VIN, the hot-test component rises less than 50°C with a standard 4-layer PCB and no heat sink or air flow. This leaves room to increase the power output of the converter, making hundreds of watts possible.

The LT8391 operates down to 4VIN, where IIN can get very high. The LT8391 is designed to either handle very high input currents or use its peak switch current limit to operate with stability at low VIN while reducing output power. This allows the converter to run through automotive cold-crank voltages or other drops in VIN without an increase in power component size or cost.

The LT8391 can achieve 1000:1 PWM dimming with no flicker. The high side PWM (TG) MOSFET PWM dims a grounded LED string. It also acts as an overcurrent disconnect during short-circuits.


Internally Generated PWM Dimming

The LT8391 has both standard external PWM dimming and internally generated PWM dimming. LT8391’s unique internal PWM dimming eliminates the need for components such as clocking devices and microcontrollers to generate accurate PWM brightness control at ratios as high as 128:1. The IC’s internally generated PWM frequency, such as 200Hz, is set by a simple resistor on the RP pin. A voltage between 1V and 2V on the PWM pin determines the PWM duty cycle. The duty cycle of the internal dimming is chosen as one of 128 steps and internal hysteresis prevents duty cycle chatter. The <±1% accuracy of internally generated PWM dimming is the same for all regions of operation.

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