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AN44 Datasheet(PDF) 6 Page - Zetex Semiconductors |
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AN44 Datasheet(HTML) 6 Page - Zetex Semiconductors |
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AN44 www.zetex.com 6 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 For international sales offices visit www.zetex.com/offices Zetex products are distributed worldwide. For details, see www.zetex.com/salesnetwork This publication is issued to provide outline information only which (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contact or be regarded as a representation relating to the products or services concerned. The company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. Europe Zetex GmbH Streitfeldstraße 19 D-81673 München Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Americas Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 usa.sales@zetex.com Asia Pacific Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com Corporate Headquarters Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com Useful formulae for calculations The input power from the battery during TON (assuming discontinuous operation mode) is VIN * IPEAK/2. The average input current from the battery is therefore this current multiplied by the ratio of TON to the total cycle time: It can be seen from this how the average battery current will increase at lower VIN as TON becomes larger compared to the fixed 1.7µs TOFF. This is logical, as the fixed (approximately) LED power will require more battery current at lower battery voltage to draw the same power. The energy which is stored in the inductor equals the energy which is transferred from the inductor to the LED (assuming discontinuous operation) is: ½ * L1 * IPEAK 2 [Joules] Therefore, when the input and the output voltage difference are greater, the LED will have more energy which will be transferred from the inductor to the LED rather than be directly obtained from the battery. If the inductor size L1 and peak current IPEAK can be calculated such that the current just reaches zero in 1.7µs, then the power in the LED will not be too dependent on battery volts, since the average current in the LED will always be approximately IPEAK/2. As the battery voltage increases, the TON necessary to reach IPEAK will decrease, but the LED power will be substantially constant and it will just draw a battery current ramping from zero to IPEAK during TON. At higher battery voltages, TON will have a lower proportional of the total cycle time, so that the average battery current at higher battery voltage will be less, such that power (and efficiency) is conserved. The forward voltage which is across the Schottky diode detracts from the efficiency. For example, assuming VF of the LED is 6V and VF of the Schottky is 0.3V, the efficiency loss of energy which is transferred from the inductor is 5%, i.e. the ratio of the Schottky forward drop to the LED forward drop. The Schottky is not in circuit during the TON period and therefore does not cause a loss, so the overall percentage loss will depend on the ratio of the TON and TOFF periods. For low battery voltages where TON is a large proportion of the cycle, the Schottky loss will not be significant. The Schottky loss will also be less significant at higher LED voltages (more LED's in series) as Schottky drop becomes a lower percentage of the total voltage. I PEAK 2 ---------------- T ON T ON T OFF × --------------------------------- × T ON I PEAK L1 × V BATT V LED – () --------------------------------------------- = |
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