AND8423/D Former Catalyst Document Number DN9 CAT32 White LED Driver Efficiency and Inductor Value Tradeoffs http://onsemi.com Prepared by: Fabien Franc, Applications Manager ON Semiconductor APPLICATION NOTE The inductors used are 3.3 mH and 6.8 mH from Panasonic, ELJEA (type 1210). The setup uses the ON Semiconductor CAT32EVAL1 evaluation board. The four white LEDs are from Nichia, NSCW100. This application brief shows the effect of the inductor value on efficiency when the CAT32 is used as a White LED driver. In addition, it shows the output voltage VOUT and the switch SW pin waveforms and describes how efficiency is defined and measured. Panasonic ELJEA6R8 Zetex ZHCS400 L1 6.8mH VIN SW VIN 3.7V VOUT White LEDs CAT32 + C1 4.7mF SHDN LED RSET GND C2 1mF 15mA R1 Figure 1. Test Setup with CAT32 driving 4 LEDs − for L = 6.8 mH, the efficiency is 82% − for L = 3.3 mH, the efficiency is 78% A 4% improvement in efficiency is gained with the larger inductor value. This is true for a given setup and for this specific type of inductor. Efficiency The CAT32 efficiency is defined as the ratio of the power dissipated through the LEDs (not including the loss in the Schottky diode) to the power from the input supply: Efficiency + P LEDńP IN + ǒV OUT * V LEDǓ I LEDńǒV IN I INǓ The efficiency is measured for an input supply of 3.7 V. The efficiency for LED current between 15 mA and 20 mA is as follows: © Semiconductor Components Industries, LLC, 2009 August, 2009 − Rev. 0 1 Publication Order Number: AND8423/D AND8423/D Signal Waveforms (VOUT). The measurement is done for a 3.7 V supply and for a 15 mA LED current flowing through the 4 LEDs. The signal waveforms below show the switch pin voltage (Vsw), the inductor current (IL), and the output voltage Vsw, IL, VOUT with L = 6.8 mH Vsw, IL, VOUT with L = 3.3 mH For the smaller inductor value of 3.3 mH: • The switching noise on VOUT is greater • The charging and discharging time are shorter • The peak inductor current is greater • The ringing frequency on SW is higher (when there is no current in the inductor) In summary, reducing the inductor value has some impact on the overall power efficiency, the inductor peak current, the switching noise and the ringing. Information on Nichia and their LEDs can be found at: www.nichia.com. ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner. PUBLICATION ORDERING INFORMATION LITERATURE FULFILLMENT: Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada Email: [email protected] N. American Technical Support: 800−282−9855 Toll Free USA/Canada Europe, Middle East and Africa Technical Support: Phone: 421 33 790 2910 Japan Customer Focus Center Phone: 81−3−5773−3850 http://onsemi.com 2 ON Semiconductor Website: www.onsemi.com Order Literature: http://www.onsemi.com/orderlit For additional information, please contact your local Sales Representative AND8423/D