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Download Driver for Flashlight Applications in Mobil Phones Application Note
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Driver for Flashlight Applications in Mobil Phones Application Note Abstract This application note introduces various possibilities for driving a flash light LED along with their most important advantages and disadvantages. Introduction The U-I characteristics of an LED are most often used as a basis when developing driver circuitry. Due to the production technology and the associated process tolerances, not all LEDs of a particular type are the same; rather, they vary within certain limits. grouping current), whereby the costs for the measurement process are reduced. LEDs of a type and within a brightness group have the same forward current; the forward voltage, however, can vary between Uf,min and Uf,max. When designing driver circuitry, it is preferable to wire the LEDs in series so that all LEDs are supplied with the same current and therefore, all have the same brightness. This also has the advantage that the adjustment (series resistance) for all LEDs must only be made once. With parallel circuitry, however, the adjustment must be individually performed for every single LED. Electrical Switching Possibilities Figure 1: U-I-Diagram of LW F65G Figure 1 shows these circumstances for the LW F65G LED. As shown in the figure, the forward voltage (Uf) can vary between lower and upper limits. The diagram also shows the characteristics which are typical for the majority of LW F65G LEDs. Due to this variation, LED brightness measurements and thus the classification according to brightness groups, occurs at a particular, type-specific forward current, (If = January, 2014 The following describes a few possibilities for driving the LW F65G LED in flash and/or torch light modes of operation. When selecting the appropriate driver circuitry, the battery properties should also be taken into account. For the current generation of cell phones, batteries with voltages up to 4 V are typically used. Since the battery discharges during cell phone operation, the voltage also sinks to a certain minimum level (typ. 3.1 - 3.4 V), after which the functionality is impaired or ceases altogether. The voltage changes which arise should be taken into account when designing the driver circuitry. 1. Circuit with series resistance The simplest and most cost effective solution for driving an LED, although most prone to underlying effects, is the use of a series resistance. Page 1 of 6 The resistance allows the current flowing through the LED to be set to a specific level. In operation, LEDs with the same forward voltage illuminate or flash with a particular brightness. Figure 2: Voltage regulator with series resistance Figure 1: Circuit with series resistance For mass production, a typical value for the forward voltage should be specified for the driver circuitry. For LEDs which exhibit a lower forward voltage, more current will flow when a typical series resistance is used, causing the application to be correspondingly brighter. If LEDs with a higher forward voltage are used, the current is reduced, and the application appears accordingly dimmer. In addition, this simple solution does not permit additional modes of operation. These must be implemented with additional switching circuitry. The continual decrease in voltage as the battery discharges leads to an ongoing reduction in brightness. 2. Voltage resistance Regulator with series Compared to a driver circuit with a single series resistance, the use of an additional voltage regulator provides the advantage that the forward voltage is held constant up to a particular discharge level. The influence of the battery is therefore reduced. Variations in brightness due to differing LED forward voltages still arise, however. January, 2014 3. Current Sink An additional, cost effective solution is to drive the LEDs with a current sink. This has the effect that all LEDs are supplied with the same current, independent of their forward voltages. Variations in brightness due to differing forward voltages are thus eliminated. Figure 3: Current sink In this case, the circuitry is also influenced by the available battery voltage, however. A decrease in battery voltage also leads to a general decrease in LED brightness. Moreover, the circuit ceases to function when the operating voltage falls below a certain level. Page 2 of 6 4. Voltage Booster with Current Sink 5. Boosting Current Source For operation of the LEDs at low battery voltages (e.g. 3 V), an additional inductive or capacitive voltage booster is required. Figure 4 shows an example with a current sink. A more complicated and therefore more expensive solution is a circuit with a voltage boosted current source. Figure 5: Boosting Current Source Figure 4: Voltage Booster with Current Sink In spite of additional costs, this circuitry is considerably easier to use and offers additional features such as: The voltage booster only serves to increase the supply voltage to the required LED operating voltage. As with the previous circuits, one is limited to a particular operating mode (flash or flashlight). Various operating modes (flash or flashlight) Greater battery voltage range LED monitoring Thermal Protection etc. In this case, two different designs can be used: either an inductive DC-DC converter or a charge pump. In general, both designs can provide up to 1 A of current for flash mode operation. The main advantage of both designs is that across nearly the entire battery voltage range, a constant current source is available for the LEDs. However, inductive DC-DC converters (Figure 6) use coils and therefore might cause electromagnetic interference, which has to be properly handled. Within their working range, DC-DC converters can convert an input voltage to any output voltage. January, 2014 Page 3 of 6 Figure 6: Example of a synchronous boost converter up to If = 700mA from Texas Instruments Charge pumps (Figure 7) use capacitors instead of coils. For this reason, charge pumps support automatic mode switching of input and output voltage, e.g. 1.5x or 2x mode. This means that the output voltage can only be stepped up in multiples of the input voltage. Regarding a suitable LED driver for a high current flash, please see the links in the appendix or contact OSRAM Opto Semiconductors. Conclusion When one compares the various possibilities for driving flash LEDs, driver circuits which utilize a DC-DC converter or a charge pump are the only recommendable solutions. Both designs minimize variations in LED forward voltage and provide constant current for flash applications. In general, with a low forward voltage (Ufmax= 4.5 V, @ 1000 mA), the LW F65G makes electrical control easier in comparison to other flash LEDs available on the market. Figure 7: Example of a charge pump up to If = 1000mA from Austria Microsystems January, 2014 Page 4 of 6 Appendix Don't forget: LED Light for you is your place to be whenever you are looking for information or worldwide partners for your LED Lighting project. www.ledlightforyou.com Links for LED Flashlight Drivers austriamicrosystems Linear Technologies ON Semiconductor www.austriamicrosystems.com www.linear.com www.onsemi.com www.maxim.com www.monolithicpower.com www.national.com www.st.com www.supertex.com www.ti.com Maxim Monolithic Power National Semiconductor STMicroelectronics Supertex Texas Instruments Author: Andreas Stich ABOUT OSRAM OPTO SEMICONDUCTORS OSRAM, with its headquarters in Munich, is one of the two leading lighting manufacturers in the world. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), offers its customers solutions based on semiconductor technology for lighting, sensor and visualization applications. OSRAM Opto Semiconductors has production sites in Regensburg (Germany) and Penang (Malaysia). Its headquarters for North America is in Sunnyvale (USA). Its headquarters for the Asia region is in Hong Kong. OSRAM Opto Semiconductors also has sales offices throughout the world. For more information go to www.osram-os.com. DISCLAIMER PLEASE CAREFULLY READ THE BELOW TERMS AND CONDITIONS BEFORE USING THE INFORMATION. IF YOU DO NOT AGREE WITH ANY OF THESE TERMS AND CONDITIONS, DO NOT USE THE INFORMATION. The Information shown in this document was produced with due care, but is provided by OSRAM Opto Semiconductors GmbH “as is” and without OSRAM Opto Semiconductors GmbH assuming, express or January, 2014 Page 5 of 6 implied, any warranty or liability whatsoever, including, but not limited to the warranties of correctness, completeness, merchantability, fitness for a particular purpose, title or non-infringement. In no event shall OSRAM Opto Semiconductors GmbH be liable - regardless of the legal theory - for any direct, indirect, special, incidental, exemplary, consequential, or punitive damages related to the use of the Information. This limitation shall apply even if OSRAM Opto Semiconductors GmbH has been advised of possible damages. As some jurisdictions do not allow exclusion of certain warranties or limitations of liability, the above limitations or exclusions may not apply. The liability of OSRAM Opto Semiconductors GmbH would in such case be limited to the greatest extent permitted by law. OSRAM Opto Semiconductors GmbH may change the Information at anytime without notice to user and is not obligated to provide any maintenance or support related to the Information. The Information is based on specific Conditions and, therefore, alterations to the Information cannot be excluded. Any rights not expressly granted herein are reserved. Except for the right to use the Information included in this document, no other rights are granted nor shall any obligation be implied requiring the grant of further rights. Any and all rights or licenses to patents or patent applications are expressly excluded. Reproduction, transfer, distribution or storage of part or all of the contents of this document in any form without the prior written permission of OSRAM Opto Semiconductors GmbH is prohibited except in accordance with applicable mandatory law. January, 2014 Page 6 of 6