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1、 Semiconductor Components Industries, LLC, 2003 October, 2003 Rev. 01Publication Order Number: AND8135/DAND8135/DEfficient High Power Flash LightPrepared by: Michael Bairanzade ON SemiconductorINTRODUCTIONAlthough the xenon lamps dominate the standard camera market, they are not applicable in the po
2、rtable phone. As a matter of fact, the large reservoir capacitor and high voltage associated with the xenon flash make such a concept not suitable for the cellular phone equipments. The semiconductor based White LED devices provide the right choice when limited flash light becomes necessary to illum
3、inate a photographic scene. This paper depicts the basics of the xenon concept and details a typical White LED flash application.XENON LAMP CONCEPTA low pressure of a rare gas mixture fills a glass envelope with two ends electrodes on both sides. In steady state, the voltage across the electrodes is
4、 set to a value well below the trigger voltage as depicted Figure 1. At this point, no current flows and the system is stable until a trigger voltage is applied to the third electrode. This high voltage pulse, in the 1 kV range, comes from a transformer built with a small magnetic tore triggered by
5、an abrupt discharge of the capacitor C2 (see Figure 2).Figure 1. Xenon Flash Breakdown VoltageVIVtrigVCCThe gas is ignited and the plasma generates a bright flash, the typical duration being 2 ms for consumer applications.Depending upon the type of flash involved, the amount of energy stored into ca
6、pacitor C1 can be a low 10 Joule (small camera) to thousand of Joule for professional applications.Figure 2. Basic Xenon FlashTRIGGERGNDT1R1 470 kC2 100 nF/ 300 VX1XENON TUBE+260 V+123C2 4.7 ?F/ 300 VS1The net advantage of such a concept is the very short pulse, making easy snap shot photos to captu
7、re moving stuffs. The drawbacks are the large physical size of the reservoir capacitor and the recycle time needed to recharge the capacitor between two shots (in the 5 sec range for consumer applications). Clearly, these drawbacks make the xenon based flash not suitable for hand held cellular phone
8、, with limited size and energy supply.WHITE LED FLASHTo overcome the physical size limitation, the flash concept is to make profit of the high efficiency, in term of light, coming from the modern white LED. With a 4 V forward drop voltage, such diodes do not need extra high voltage trigger pulse, th
9、ey are extremely fast to turn ON/OFF and all the associated electronic circuit can be housed inside a standard portable phone. Since the white LED have electrical characteristics similar to the standard LED (see Figure 3), one must provide a constant forward current to control the device.APPLICATION
10、 NOTEhttp:/AND8135/Dhttp:/ 23.5 V4.0 V100 mA1 ?AFigure 3. Typical White LED CharacteristicsVRRIFIR5 VVF10 mAConsequently, a standard voltage source cannot be used straightforward and an extra ballast is necessary to set up the current. On the other hand, the flash must be capable to operate over the
11、 typical battery voltage spread (2.8 V to 5.2 V) and a more suitable structure than a simple linear voltage regulator is mandatory. To achieve such constraints, ON Semiconductor has developed a full family of white LED drivers, among which the NCP5007 can fulfill the flash application demands.TYPICA
12、L FLASH APPLICATIONSince the battery voltage ranges from a low 2.8 V to a high 5.2 V, the simplest and economic way to handle this span is to arrange the white LED in series as depicted Figure 4. Such a layout avoid the leakage current during the stand by mode operation (most of the time, the flash
13、is not activated!).The circuit, built around the NCP5007, is designed to support both the low beam current and the high flash pulse as requested during the capture of a photo. The DC/DC boost converter, associated to the sense resistor R1, provides a constant current to the load to properly bias the
14、 white LEDs. With an internal 200 mV voltage reference (Vref), the chip minimizes the drops along the battery supply path.Low Power Beam Operating ModeGenerally speaking, this mode of operation is used to prelight the scene to be capture in order to minimize the redeye effect. The NMOS transistor Q1
15、 is biased OFF and R1 provides the feedback voltage to regulate the load current. The value of R1 is derived from the Ohms law:R1 ?Vref Iout With a typical 4 mA operating bias of the LED during the illumination of the scene, the sense resistor is 51 . The current can be dynamically modulated, if nec
16、essary, by using the EN signal pin 3 as a digital control: such a mode of operation is depicted in the NCP5007 data sheet. The same pin can be used to control the DC/DC by a bit from the external CPU. Of course, a more powerful light can be provided by setting the sense resistor accordingly. From a practical stand point, capacitor C2 is mandatory to avoid large spikes during the energy transfers from the inductor L1 and the white LEDs. Moreover, such a capacitor
