White LEDs have recently made great leaps in performance by squeezing extreme luminous density into ultra-compact packages. The latest white LEDs have numerous advantages over more conventional lighting, including improved spectral characteristics, shorter response time, greater luminescence, greater durability, longer life, decreased size and lower cost.
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 LEDs solution is unsurpassed in versatility and performance when applied to many lighting applications, including automotive running, dome, dash, and emergency lights, portable flash lights, architectural detail lighting, edge-lit signs, airport emergency and taxi guide lights, medical lighting, and decorative lighting.
One challenge in using white LEDs in many applications is powering them with the wide input voltage range that batteries present. A battery-to-LED DC/DC converter must both step up and step down the source voltage to a typical 6.8V LED forward voltage (Lumileds LXHL-LW6C) and keep the LED current constant (for constant luminosity).
Figure 1 shows an 800kHz SEPIC converter that provides 700mA driving current for the Luxeon V series LED from a wide input voltage range (3.3V to 17V). The output of the converter is 6.8V—the forward voltage of the LXHL-LW6C. U1 is a thermally enhanced LT3436EFE with a high-efficiency 3A on-board power switch, features that simplify regulator design and layout while providing 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.
Figure 1: LT3436EFE/LT1783IS5 SEPIC drives Luxeon V white LED with 700 mA constant current and 75% efficiency.
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The LED is turned on by pressing the ON/OFF button, 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 poses 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 3.5V input), the size of L2 can be reduced by using the CDRH4D28 10μH inductor as opposed to the CDRH6D28 22μH inductor. The larger inductor is needed when there is higher overall input current and less ripple current is required to limit peak switch current to under the 3A maximum rating of the IC.