AN2243 Application note

Transcription

1 Application note Step up converter for camera flash light Introduction STCF01 is a dedicated IC to drive up to four white LEDs with constant current in camera flash for cellular phones. It provides up to 300mA over an input voltage range of 2.6V to 5.5V. A high switching frequency (1.5MHz) allows the use of tiny surface-mount components, as well as the use of current levels set resistors. Only an inductor, a Schottky diode, two capacitors and two resistors are required. Besides, a low output ripple is guaranteed by the current mode PWM topology and by the use of low E.S.R. SMD ceramic capacitors. The device is also thermal protected and available in QFN 3x3mm 10L package. Figure 1. Assembled Board March 2006 Rev 1 1/

2 Contents Contents 1 Main Characteristics Typical Application Schematic External Components Efficiency Application With 2/3/4 WLEDs PCB Design Rules Conclusion Revision history /13

3 Figures Figures Figure 1. Assembled Board Figure 2. Typical Application Schematic Figure 3. STCF01-100mA Figure 4. STCF01-300mA Figure 5. Board Layout - Top View Figure 6. Board Layout - Bottom View Figure 7. Components Layout /13

4 Tables Tables Table 1. Bill Of Material Table 2. Revision history /13

5 Main Characteristics 1 Main Characteristics The purpose of this application note is to provide a brief summary of the specifications and the functionality of the application board. The STCF01 includes a step-up converter, whose input is connected directly to the battery. The output voltage of the step-up is automatically determined using a duty-cycle control based on current feedback information. Using a dedicated pin, two levels of current can be selected for the LEDs. The amount of each level of current can be set using an external resistor. Compared to the linear current control technique, this control method allows the best efficiency performances according to the selected current requirement to be obtained, avoiding losses on the linear element. The shutdown pin allows power saving when the camera flash is not used, providing a current consumption of less than 0.1uA. If it is held to GND the output current continuously flows through LEDs. The minimum efficiency guaranteed in the typical application is higher than 80%. In some conditions the efficiency can reach values equal to 90% (two WLeds with 100mA current). The device works with a minimum Vin=2.6V, and it has an Over Voltage Protection on the output guaranteed at the minimum value equal to 16.5V. This value ensures proper operation with a maximum of four White LEDs in series. The maximum Iout current is limited by the inductor peak current internally set at Typ. 1.2A. This feature allows saving the battery life from intensive use of the flash. The SEL pin allows to select high and low current values flowing to the White LEDs. The two current values are set through external resistors Rhc and Rlc according to the following formula: I LED (FLASH)= 2100*(V I -V HC )/R HC I LED (TORCH)= 2400*(V I -V LC )/R LC High logic level on SHDN pin puts the device in shutdown mode. If it is set to Low the flash or torch mode is activated and the device provides the requested current in less than 200us. This fast turn-on time makes the device suitable for single shoot and multiple shoot operation modes. 5/13

6 Typical Application Schematic 2 Typical Application Schematic Figure 2. Typical Application Schematic Suggested Values: RLC=12kΩ RHC=4kΩ C1=2.2µF C2=4.7µF L1=10µH 6/13

7 External Components 3 External Components This application requires few external components: two ceramic capacitors (Cin=2.2uF, Cout=4.7uF), one inductor (L=10uF), one Schottky diode and two resistors. For the input and output capacitors, it is recommended to use ceramic capacitors with low ESR. For a good stability of the device supplied by a low input voltage of 2.6V at maximun ratings, it is recommended to use 2.2uF/6.3V as a minimum value of input capacitor and 4.7uF/16V as a minimum value of output capacitor. In the application with 4 White LEDs, becuase of the higher output voltage, we suggest to use a Cout with 25V dielectric instead of a normal 16V dielectric. Regarding the inductor, a 10uH shielded thin inductor with low DC series resistance of wiring is recommended for this application, with maximum peak current of about 1.3A. During normal Ton/Toff transient time, the current in the inductor can continue to flow thanks to the Schottky diode. It is very important to use a diode with a very low threshold, in order to have high efficiency and a good current capability. We have used the STPS1L40M diode in our application and obtained good results. To set the current value through external resistors, thick metal film resistors 0805 package size are suggested (Rlc=12kOhm and Rhc=4kOhm). All LEDs with forward voltage from 2.7 to 5V are feasible for use with STCF01. Through Rhc and Rlc resistors it is possible to set the LEDs current respectively in flash mode and torch mode. Table 1. Bill Of Material Symbol Parameter Type Supplier Typ. Value Unit L Inductor VLF5014A TDK 10 uh Cin Input Cap. Ceramic Low ESR TDK 2.2 uf Cout Output Cap. Ceramic Low ESR TDK 4.7 uf D Schottky STPS1L40M ST 1.5 A Rhc H Current Set Thick film type ROHM 4K Ohm Rlc L Current Set Thick film type ROHM 12K Ohm LEDs SMD White Led LWH1035N LUXPIA 30 ma Even if we suggest a particular brand, the device is able to work properly also with other brands but final results are not guaranteed. It is possible to use the STCF01 camera flash in one LED configuration with a power supply ranging from 2.6V up to the LED forward voltage (3.4V to 4.0V). 7/13

8 Efficiency Application With 2/3/4 WLEDs 4 Efficiency Application With 2/3/4 WLEDs As showed in the Figure 3. and Figure 4., the STCF01 can reach an efficiency equal to 90% in two WLEDs typical application. Figure 3. STCF01-100mA Figure 4. STCF01-300mA 8/13

9 PCB Design Rules 5 PCB Design Rules Figure 5. shows the recommended board layout in order to reach the best efficiency for this application. Due to the high switching frequency and peak current, it is important to optimize the application environment reducing the length of the PCB traces and placing all the external component close to the device. Also switched high-energy loops should be as small as possible to reduce EMI. It is recommended to use a PCB with at least two sides. In order to reduce the power dissipation, it can be also useful to solder the exposed pad to GND. Figure 5. Board Layout - Top View Original size 30mm 9/13

10 PCB Design Rules Figure 6. Board Layout - Bottom View Figure 7. Components Layout 10/13

11 Conclusion 6 Conclusion The STCF01 is a white LED driver optimized for driving two to four LEDs from a Li-Ion battery input. It features 16.5V output voltage, 1.5MHz internal power switch, internal soft start, open load voltage protection and optimized internal compensation. The STCF01 is ideal for camera flash in cellular phones requiring, small circuit size and very high efficiency (up to 90%). 11/13

12 Revision history 7 Revision history Table 2. Revision history Date Revision Changes 22-March Initial release. 12/13

13 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries ( ST ) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein. UNLESS OTHERWISE SET FORTH IN ST S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. UNLESS EXPRESSLY APPROVED IN WRITING BY AN AUTHORIZE REPRESENTATIVE OF ST, ST PRODUCTS ARE NOT DESIGNED, AUTHORIZED OR WARRANTED FOR USE IN MILITARY, AIR CRAFT, SPACE, LIFE SAVING, OR LIFE SUSTAINING APPLICATIONS, NOR IN PRODUCTS OR SYSTEMS, WHERE FAILURE OR MALFUNCTION MAY RESULT IN PERSONAL INJURY, DEATH, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE. Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any liability of ST. ST and the ST logo are trademarks or registered trademarks of ST in various countries. Information in this document supersedes and replaces all information previously supplied. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America 13/13