Electronic Design

Wide Input-Voltage SEPIC Drives High-Power White LEDs

Recently, white LEDs have made sizable performance gains by squeezing extreme luminous density into ultra-compact packages. The latest white LEDs hold numerous advantages over more conventional lighting, including improved spectral characteristics, shorter response time, greater luminescence, greater durability, longer life, and decreased size.

The Lumileds Luxeon V, for instance, boasts up to 50 times the flux, and at least 20 times the luminous density, of a standard through-hole LED. When paired with an efficient and simple switching regulator, a white LED solution is unsurpassed in versatility and performance when applied to many lighting applications, including automotive running, dome, dash, and emergency lights; portable flashlights; architectural detail lighting; edge-lit signs; airport emergency and taxi guide lights; medical lighting; and decorative lighting.

One challenge when using white LEDs in many applications involves powering them with the wide input-voltage range presented by batteries. A battery-to-LED dc-dc converter must both step up and step down the source voltage to a typical 6.8-V LED forward voltage (Lumileds LXHL-LW6C) and keep the LED current constant (for constant luminosity).

The figure shows an 800-kHz single-ended primary inductance converter (SEPIC) that supplies 700-mA driving current for the Luxeon V series LED from a wide input-voltage range (3.6 to 17 V). The converter's output is 6.8 V—the forward voltage of the LXHL-LW6C. U1 is a thermally enhanced LT3436EFE with a high-efficiency, 3-A on-board power switch, features that simplify regulator design and layout. It can supply enough current capability for the LED driver without overheating. U2 (a LT1783IS5 op amp in a SOT-23 package) converts the LT3436 from a current-mode, constant-output voltage switching regulator to a current-mode, constant-current source converter with minimal power loss in the sense resistor.

Pressing the ON/OFF button turns on the LED, connecting the shutdown pin to VIN. Releasing the ON/OFF button puts the IC in shutdown, limiting the power consumption of the IC from the battery source. One advantage the SEPIC configuration provides over a more common boost application is the complete output disconnect during IC shutdown.

If the output LED is physically disconnected from the circuit, R7, R6, and Q1 limit the output voltage to prevent any damage to the IC. If low-voltage operation is not expected (below a 3.6-V input), the size of L2 can be reduced by using the CDRH4D28 10-µH inductor as opposed to the CDRH6D28 22-µH inductor.

The larger inductor is needed when there's higher overall input current. Less ripple current is required to limit peak switch current to under the 3-A maximum rating of the IC.

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