LINEAR Application Note 95 Data Handbook

Application Note 95

March 2004

Simple Circuitry for Cellular Telephone/Camera Flash Illumination A Practical Guide for Successfully Implementing Flashlamps Jim Williams and Albert Wu

INTRODUCTION Next generation cellular telephones will include high qual- Their line source light output is hundreds of times greater ity photographic capability. Improved image sensors and than point source LEDs, resulting in dense, easily diffused optics are readily utilized, but high quality “Flash” illumi- light over a wide area. Additionally, the flashlamp color nation requires special attention. Flash lighting is crucial temperature of 5500°K to 6000°K, quite close to natural for obtaining good photographic performance and must light, eliminates the color correction necessitated by a be quite carefully considered. white LED’s blue peaked output.

FLASH ILLUMINATION ALTERNATIVES FLASHLAMP BASICS Two practical choices exist for flash illumination—LEDs Figure 2 shows a conceptual flashlamp. The cylindrical (Light Emitting Diode) and flashlamps. Figure 1 compares glass envelope is filled with Xenon gas. Anode and cathode various performance categories for LED and flashlamp electrodes directly contact the gas; the trigger electrode, approaches. LEDs feature continuous operating capability distributed along the lamp’s outer surface, does not. Gas and low density support circuitry among other advan- breakdown potential is in the multikilovolt range; once tages. Flashlamps, however, have some particularly breakdown occurs, lamp impedance drops to ≤1Ω. High

important characteristics for high quality photography. , LTC and LT are registered trademarks of Linear Technology Corporation.

PERFORMANCE CATEGORY FLASHLAMP LED Light Output High—Typically 10 to 400× Higher Than LEDs. Line Source Low. Point Source Output Makes Even Light Distribution

Output Makes Even Light Distribution Relatively Simple Somewhat Difficult

Illumination vs Time Pulsed—Good for Sharp, Still Picture Continuous—Good for Video Color Temperature 5500°K to 6000°K—Very Close to Natural Light. No Color 8500°K—Blue Light Requires Color Correction

Correction Necessary

Solution Size Typically 3.5mm × 8mm × 4mm for Optical Assembly. Typically 7mm × 7mm × 2.4mm for Optical Assembly,

27mm × 6mm × 5mm for Circuitry—Dominated by Flash 7mm × 7mm × 5mm for Circuitry Capacitor (≈6.6mm Diameter; May be Remotely Mounted)

Support Circuit Complexity Moderate Low Charge Time 1 to 5 Seconds, Dependant Upon Flash Energy None—Light Always Available Operating Voltage Kilovolts to Trigger, 300V to Flash. ISUPPLY to Charge ≈ 100m A Typically 3.4V to 4.2V at 30m A per LED Continuous— and Currents to 300m A, Dependant Upon Flash Energy. Essentially Zero 100m A Peak. Essentially Zero Standby Current

Standby Current

Battery Power Consumption 200 to 800 Flashes per Battery Recharge, Dependent ≈120m W per LED (Continuous Light)

Upon Flash Energy ≈400m W per LED (Pulsed Light)

Partial Source: Perkin Elmer Optoelectronics

Figure 1. Performance Characteristics for LED and Flashlamp-Based Illumination. LEDs Feature Small Size, No Charge Time and Continuous Operating Capability; Flashlamps are Much Brighter with Better Color Temperature

an95f

AN95-1