Power Electronics

Photoflash Chargers Light the Way for Camera Phones

Xenon flash bulbs remain fixtures in photography even with the migration from film-based cameras to digital still cameras. Typically, a circuit compromised of discrete components was used to charge a photoflash capacitor to a high voltage. That high-voltage energy was then used to fire the flash bulb. With digital still cameras (DSCs) shrinking in size, demand is growing for more integrated charging designs. Semiconductor vendors are responding by developing photoflash charger ICs and reference designs.

Although DSCs are the primary target of these chips, advanced camera phones are emerging as an important application. While existing camera phones have relied largely on LEDs as the flash, the move to higher-resolution/higher-quality photography is creating a need for the improved lighting provided by Xenon bulbs. Although the flashlamp circuit can be implemented with discrete components, space in the camera phone application is limited, as it is in smaller DSCs; thus, IC-based designs are needed.

In addition to size, chips being developed for the flashlamp application are addressing requirements for high efficiency, fast capacitor charging and flexibility to accommodate different capacitor values, charging voltages and charging rates. Unlike discrete circuit solutions, these ICs consume a few microamps or less of current when not charging. They interact with the host controller in the camera, which commands the IC to begin charging and awaits a signal that the capacitor is charged before firing the flashlamp.

Linear Technology (LTC) is targeting the advanced camera phone applications with its LT3468 photoflash capacitor charger, which produces a highly integrated design for quickly and efficiently charging Xenon flashlamps. According to LTC, there are several reasons the new camera phones are opting for Xenon flashlamps rather than the LED flashes used in earlier phones.

The light output from Xenon flashes is hundreds of times that of point source LEDs, resulting in intense, easily diffused light over a wide area. At the same time, the flash is much faster than LEDs, stopping motion and preventing blur. Because Xenon flashlamps operate at a color temperature close to that of natural light, they don't require the color correction necessitated by the blue peaked output from white LEDs.

Also, the Xenon strobe flash approach delivers high-quality photos up to several meters from the subject, rather than the 1-ft to 3-ft limitation of LED-based camera phones. Although Xenon lamps have the advantage in still photography, LEDs are better suited to video photography because they maintain their output over longer time periods. Thus, LEDs will be the light source of choice in videophone applications.[1]

The photoflash capacitor charger circuit based on the LT3468 charges the Xenon flashlamp in as little as 1 sec — a five-fold improvement over discrete solutions. Also, this scheme supports “red-eye” reduction, whereby one or more reduced intensity flashes immediately precede the main flash.

A complete flashlamp circuit must perform two functions: charge the external flashlamp capacitor to approximately 300 V and produce a multi-kilovolt trigger pulse to ionize the Xenon gas in the lamp. By ionizing the gas, the trigger pulse reduces the impedance of the gas to less than an ohm. Once this low-impedance state is reached, the energy in the flash storage capacitor can be discharged through the bulb. The high level of current flowing through the bulb — about a 100-A pulse — causes the lamp to illuminate. The lamp's ability to flash repeatedly is limited by its ability to dissipate heat and the speed with which the capacitor can be charged.

Fig. 1 shows a complete flashlamp circuit based on the LT3468. A patented control technique allows for the use of 5.8-mm × 5.8-mm × 3.0-mm transformers with the capacitor charger. Along with the standard LT3468, two variations — the LT3468-1 and LT3468-2 — are offered. These devices feature a primary current limit of 1.4 A for the standard model, 0.7 A for the -1, and 1 A for the -2. These current limits produce controlled input currents of 500 mA, 225 mA and 375 mA, respectively, which impacts the drain on the battery.

If the battery supports higher current drain, then the photoflash capacitor may be charged more quickly. However, drawing too much current may pull down battery voltage and affect system operation. The charger chips support operation with supplies ranging from 2.5 V to 16 V. If the source is a single Li-ion cell at its 3.6-V nominal voltage, the charging time will be 4.6 sec for the LT3468 and 5.5 sec for the LT3468-1.

With the LTC charger, no output voltage divider is required to sense the output voltage. Instead, the chip monitors T1's flyback pulse as reflected back across the transformer's primary. The LT3468, in conjunction with a standard flyback transformer, charges any size photoflash capacitor with efficiencies greater than 80%. Other features include adjustable output voltage and a thin 5-pin SOT-23 package. Available from stock, the LT3468 is priced at $1.95 each in 1000-piece quantities.

Meanwhile, Zetex has developed a photoflash charger IC, the ZXSC440. This chip increases the battery life of cameras and strobe equipment using Xenon flash tubes by achieving a flyback conversion efficiency as high as 75%. According to Zetex, discrete charging circuits achieve no better than 50% efficiency. However, this charger chip's efficiency is influenced by the choice of switching transistor on the drive output. Complementary bipolar transistors available from Zetex can provide the optimum combination of low VCE(sat) and high gain.

Offered in an 8-pin MSOP, the photoflash charger charges an 80-µF capacitor to 300 V in 3.5 sec from a 3-V supply (Fig. 2). Also, the charger chip operates from a 1.8-V to 8-V supply. The 10,000-piece price is $0.54 each.

Texas Instruments (TI) also is developing a photoflash circuit for DSCs. The device is an integrated photoflash charger and IGBT driver. Operating from a 1.8-V to 12-V supply, the chip charges an external capacitor to a user-selectable, high voltage in the 250-V to 350-V range. Like the other photoflash chargers, it alerts the camera's host system once the capacitor is charged. However, it also awaits an enable signal from the host, causing the IC to fire an external IGBT to generate the trigger pulse that ionizes the Xenon gas (Fig. 3).

The TI chip detects a fully charged condition on the capacitor by sensing the output voltage as it is reflected back across the primary, so no external resistor divider is required. Charging time is readily programmed by setting the charging current. This allows the designer to make adjustments for different states of charge on the battery. When the battery is fully charged, a high charging current can be used to charge the capacitor quickly without pulling down the battery voltage. As battery voltage falls, a lower charging current can be used. Scheduled for release in the third quarter, the TI device will be offered in a 16-pin, 3-mm × 3-mm QFN and a 10-pin MSOP. Unit pricing will be $1.90 in lots of 1000.

Reference

1.Williams, Jim and Wu, Albert. “Simple Circuitry for Cellular Telephone/Camera Flash Illumination. A Practical Guide for Successfully Implementing Flashlamps,” Linear Technology's Application Note 95, March 2004.

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