Producing a flyback conversion efficiency as high as 75%, the ZXSC440 photoflash charger IC can dramatically increase the battery life of cameras and strobe equipment using Xeon flash tubes. Alternative discrete charging circuits can at best achieve an efficiency of 50%. The dedicated photoflash controller from Zetex can charge an 80F capacitor to 300V in 3.5 seconds from a 3V supply. Its fast charge capability meets the refresh demands of high performance digital cameras, and its simple configurability ” via an external resistor network ” makes it suitable for use with a wide range of photoflash capacitor values. Provided in the low profile MSOP8 package, the ZXSC440 is easily interfaced with a camera's central microcontroller. A 'charge' pin is used to start and stop photoflash charging and a 'ready' pin indicates when the photoflash is fully charged and ready for shooting. The ZXSC440 photoflash charger also uses negligible current when not charging. When the flash is not in use, a current of just 4.5A is consumed by the controller, helping to further extend battery life. Overall charger 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. At input voltages greater than 4V, Zetex MOSFETs give a comparable performance and simplify the drive circuit. A small amount of hysteresis on the voltage feedback pin shuts the device down as long as the capacitor remains fully charged. Zetex PLC, Oldham, Lancs OL9 9TY, UK.
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