powerZONE Products for the week of May 24, 2004
Linear Technology Says . . .
LT3468: Technology Increases Illumination With Xenon
Flash In Camera Phones
Simple, Practical Solution for Next-Generation Camera Phones
Linear Technology Corporation, a leading supplier of high performance analog integrated circuits, announced a practical way to create high-quality photographs using cellular phones. Camera phones shipping today have limited photo quality since the photo flash provides minimal illumination and requires the camera to be close to the subject. Linear Technology has enabled the use of a high quality Xenon strobe flash in camera phones in place of LEDs (light emitting diodes), which provide only limited illumination. Using this simple, practical approach with Xenon strobe flashes, makers of cell phones can now produce compact and affordable camera phones with high quality photo capability.
This new generation of camera phones with Xenon flash lamps has several key advantages over current 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. The flash is many times faster than LEDs, stopping motion and preventing blur. In addition, since these new Xenon flash lamps operate at a color temperature close to that of natural light, they do not require the color correction necessitated by the blue peaked output from white LEDs. The result of this improved flash technology is a practical, easy to implement solution for high quality photo-enabled phones. Such cameras deliver high quality photos up to several meters from the subject, rather than the current one to three feet limitation of LED-based camera phones.
The camera phone solution pioneered by Linear Technology involves the use of Xenon strobe flash lamps, a well-established light source that has been in production for many years in film and digital still cameras (DSCs). The Linear Technology solution incorporates the company's patented LT3468 high efficiency, fast photoflash capacitor charger. This device charges a high voltage capacitor, meeting the form factor and efficiency needed for cellular phones. The small footprint and low profile of this photoflash solution allows it to be used in small form factor handsets, significantly improving the photo image quality.
"Next generation cellular phones will deliver high quality photographic capability," stated Jim Williams, Staff Scientist for Linear Technology. "By solving the critical flash illumination challenge, we can look forward to camera phones that provide excellent photographic performance in a small form factor and at a reasonable cost."
Don Paulus, Vice President and General Manager of Linear Technology's
Power Business Unit, added, "This innovation leverages our understanding
of power management solutions for handheld consumer products and promises
to further fuel the camera phone market, which is already growing exponentially
on a worldwide basis. This is a vivid example of how Linear Technology works
closely with customers to develop enabling technologies that enhance both
end products and the user experience. We look forward to continued growth
of our power management business, with the explosion of demand in a broad
array of digital consumer electronic products."
analogZONE Says...
The LT3468 was actually announced in November 2003, but many of us didn't fully understand the significance of what it might mean until Linear very pointedly told us here.
The camera phone market is in an incredible growth spurt and this solution from Linear allows the manufacturers to switch from the rather silly LED flash illumination -- which must be effective over at least six inches! -- to a decent Xenon output. The problem with using such devices is the high voltage required and the time needed to charge the capacitor that powers the output. Here the LT3468 provides a switch output of up to 31 V above the input voltage level (a comparator trips at a nominal 31.5 V above the input) to a flyback transformer which on the secondary with a diode provides a nominal, rectified, 320 V for the flash capacitor.
Three versions of the part offer controlled input currents of 500 mA (LT3468), 375 mA (LT3468-2), and 225 mA (LT3468-1) giving typical charge times of 4.6 s (with 3.6 V input) across 100 µF, 5.7 s across 100 µF, and 5.5 s across 50 µF, respectively. The choice between the parts would be a decision based on the physical size allowable for the flyback transformer, the size allowable for the photoflash capacitor, and the current drain desired. The input voltage range of 2.5 V to 16 V allows for use with supplies from a single Li-ion cell upwards.
On-chip circuitry (two resistors and a transistor) set up an input for a comparator against the on-chip 1.25-V reference to measure the output voltage. An on-chip sense resistor (differing in value between the three parts) in series with the on-chip n-channel switch transistor monitors the ramping current through the transformer and resets the switch at the right point (1.4 A for the LT4368, 1 A for the LT4368-2, and 0.7 A for the LT4368-2). The operation of the circuit is controlled from a "Charge" pin that is taken positive to trigger everything. When the circuit has fully charged the capacitor the "Done" pin (can be set high or low) indicates that.
Choice of the turns ratio of the flyback transformer, its primary and leakage inductance, isolation voltage, primary saturation current, and winding resistances of both primary and secondary are critical and the data sheet gives good coverage about how those factors can be taken into account, as well as suggesting some commercial implementations. The data sheet also recommends some commercially-available dual diodes for the output and explains the choice of the clamp diode necessary for the switch to prevent it going too far below ground when it is turned off.
Quiescent current for the parts when not switching is a typical 5 mA, and if the "Charge" pin is grounded the device is turned off.
This very neat design will change the future of phone camera flash operation and the parts will see adoption in other digital cameras and arenas like emergency strobe lighting.
All three versions of the LT3468 are in production in SOT-23-5 priced at $1.95 in 1000-piece lots.