Selecting Inductors to Drive LEDs Application Design Example LED lighting is an exciting and fast growing application. LEDs can provide low cost, reliable lighting for a wide variety of applications ranging from architectural and automotive to signage and handheld devices. The designer faces the challenge of choosing from a vast array of LED manufacturers and device styles. Equally challenging can be the selection of components for the LED driver circuit. Fortunately powerful tools are available that greatly facilitate the power inductor selection. Since LED applications can use LEDs individually as well as parallel or serial arrays, the driver circuit may need to be a voltage step-up, step-down, or both. The Coilcraft DC-DC Converter Inductor Selector tool can be used to select the inductor for all of these driver circuit configurations. Example 1 – SEPIC Converter for LithiumIon Battery, Step-Up/Step-Down Example 1 demonstrates the selection of coupled inductors for SEPIC converters driving a single white LED. Table 1 lists a sampling of typical white LEDs with forward currents of 350 mA. These devices can be driven from a single Li-ion battery, the typical power source of handheld devices. It is desired to operate from as wide a voltage range as possible in order to maximize the battery life per charge. Fsw = Vin = Vo = Io = 400 kHz 2.7 to 4.2 Vdc 3.42 Vdc 350 mA The challenge with this design is that when fully charged or during charging, the Li-ion cell voltage is greater than the output voltage. Conversely the cell voltage will be less than the desired output during the end of the discharge cycle. Therefore, neither a buck nor boost regulator will be sufficient for this purpose and a topology must be chosen that allows both voltage step-up and step-down. The SEPIC topology, using coupled inductors, is becoming increasingly popular for this purpose. The Coilcraft DC-DC Converter Inductor Selector tool can select the inductors for this application. Table 1. Typical White LEDs Company Part Number Vf Io Color Avago ASMT-MW00 3.6 0.35 Cool white www.avagotech.com Cree XRCWHT-L1-0000-00601 3.5 0.35 Cool white www.cree.com Everlight EHP-AX08B/CT01H-P01 3.5 0.35 Cool white www.everlight.com Lite-on LOPL-E011WA 3.8 0.35 white www.us.liteon.com Osram LW-W5SM 3.2 0.35 White www.osram-os.com Philips Lumileds LXK2-PW12-S00 3.42 0.35 White www.philipslumileds.com Seoul Semiconductor W10190 3.5 0.35 Cool white www.seoulsemicon.com Document 732-1 Revised 11/21/08 The inductor selection process begins with choosing the appropriate circuit topology. The results shown in Step 4 can be sorted for the parameters of most interest to the user, whether the emphasis is power efficiency, physical size, or cost. Example 2 – Buck Converter Example 2 demonstrates the use of the Coilcraft DCDC Converter Inductor Selector tool to select the inductor for a buck regulator to drive a single white LUXEON® K2 LED from Philips Lumileds. Philips Lumileds projects that white LUXEON K2 products will deliver, on average, 70% lumen maintenance at 50,000 hours of operation at a forward current of 1000 mA1, so let’s examine what it takes to drive the LED at 1000 mA. The SEPIC topology input screen requests the input/ output specifications, the switching frequency and asks the user to select the allowed ripple current. Assuming a switching frequency of 250 kHz and an input voltage range of 15 to 30 Vdc, drive circuit parameters are: Fsw = Vin = Vo = Io = 250 kHz 15 to 30 Vdc 3.42 Vdc 1000 mA This is all the information needed to proceed to the Coilcraft DC-DC Converter Inductor Selector tool. The first step is to identify the driver circuit topology. The required inductance and current ratings for each winding of the coupled inductor are then displayed in Step 3. Step 2 requires input of the operating parameters: Vin, Vout, Iout, switching frequency and the selection of the allowed peak-peak ripple current. From this information, a list of suitable inductor part numbers is displayed. LUXEON® is a registered trademark of Philips Lumileds Lighting Company 1 LUXEON K2 Datasheet DS51 (6/08) http://www.philipslumileds.com/ Document 732-2 Revised 11/21/08 Note that the input voltage as well as the output current and voltage are specified as part of the design requirement. The switching frequency may represent some design freedom if a driver IC is not yet selected, but generally the only degree of freedom in selecting the inductor value is the amount of ripple current to be allowed. In this case the design is started assuming a 40% peak-peak current ripple. From these inputs the Coilcraft DC-DC Converter Inductor Selector tool returns in Step 3, the required inductance value, Isat and Irms ratings that the desired inductor must have. If extra margin in the current rating to prevent inductor saturation is preferred, sorting by Isat is recommended. From these results the tool searches and finds all Coilcraft inductors that meet the inductance, Isat and Irms criteria. The Coilcraft DC-DC Converter Inductor Selector tool features the flexibility to help the user optimize the inductor selection based on the criteria most important to that application. With multiple results, the user may optimize the inductor selection based on criteria specific to the application. The tool allows a quick sort of the results by user selected parameters. For example, for a handheld mobile device or backlight display, component height may be the most important criteria. Conclusion LED lighting is a growing and exciting application area and Coilcraft design tools can guide the designer quickly and easily to inductors best suited to a variety of applications. Document 732-3 Revised 11/21/08