Electronic Design

Drive White LEDs Using A Boost Converter In Buck Mode

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Low-voltage halogen lighting for decorative and architectural use continues to gain in popularity. Due to the comparatively short lifetime and low reliability of halogen bulbs, however, high-power LEDs are emerging as the preferred solution. The circuit in the figure offers a novel approach to high-power white LED driving—using a standard boost-converter IC in a voltage-reducing or "buck" mode. This solution offers 96% efficiency and numerous practical advantages over standard topologies, which typically only achieve efficiencies on the order of 85%.

When MOSFET (Q1) is turned on, current flows from the input through the LEDs, parallel smoothing capacitor (C2), inductor (L1), Q1, and sense resistor (R1). The current flows until it reaches its peak, defined by the sense-resistor value and the sense-voltage threshold of the ZXSC310—typically 19 mV.

Upon reaching the associated peak current setting, the MOSFET is switched off for a fixed period of 1.7 ms. During this time, the energy stored in the inductor is transferred to the LEDs via the Schottky diode (D1), thereby maintaining the high-efficiency LED illumination.

The input voltage and the number of serial-connected LEDs are unlimited. For higher input voltages, C1, R2, D1, C2, and Q1 must be scaled appropriately to withstand the voltage range. For a greater number of LEDs, the minimum input voltage must be greater than the combined LEDs' forward-voltage drop.

By employing a voltage-reducing boost-converter topology, a low-side n-channel MOSFET can be used instead of the high-side p-channel MOSFET normally found in a typical buck converter. The n-channel device inherently offers three times lower on-state losses than a p-channel device of equivalent chip size.

Of course, designers can use an n-channel MOSFET in a typical buck-converter topology, but driving it requires additional bootstrapping circuitry. Low-side switching also enables the peak sense current to be referenced to ground. This improves accuracy and reduces noise compared to high-side current sensing.

By using a boost-mode topology in a discontinuous operating mode, the control loop works in current mode, giving the converter cycle-by-cycle control. This makes it inherently stable, which simplifies design when compared to a voltage-mode buck converter.

One added feature of the topology is that as current flows through the LEDs while the inductor is charging, there's a reduction in the peak-to-average current ratio in the LEDs. Thus, a lower peak current can be set for equivalent LED brightness, thereby improving efficiency, reliability, and input-noise performance.

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