LINEAR Tiny Efficient High Power LED Camera Flash Solutions for Cell Phone Applications Design Note 1009 Keith Szolusha
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1. LXCL PWF2 Boost Driver To provide significantly more light output the Lumileds LXCL PWF2 Luxeon Flash LED stacks two PWF1s in se ries onasingle die This effectively provides twice the light of the PWF1 atthe same LED current but doubles the for ward voltage The higher forward voltage of the LXCL PWF2 is always above the battery range so a step up boost solution is sufficient The LT3479 can provide up to 700mA current flash to the PWF2 Figure 4 The LT1618 can flash 350mA through the PWF2 when configured as a boost only With the ability to externally reduce the error amplifier s refer ence voltage the LT3479 drives the LED current directly through a sense resistor tied to the feedback pin without compromising efficiency An output disconnect MOSFET is required to turn off the LED Boost regulators inherently provide adirect path from the input to the output even when the low side switch is shutdown Since LEDs emit light even with very small forward current a complete disconnect in the path to ground is required By placing the FET in series with the LED instead of between FB and GND the LED current and feedback voltage are not compromised maintaining excel lent regulation LUMILEDS is a CDRH5D28 3R0 pees LXCL PWF2 m o o0o 2Z 2 ILep 0 SHUTDOWN 0 SHUTDOWN 700mA FLASH 100mA TORCH v h Ta m 0o0 2Z Figure 4 The LT3479 Boost Regulator Delivers 700mA Constant Curre2. IN 2 7V to 4 2V Vin PVin SW1 Vo 0 1uF 7 TT SCENT TORCH EN1 z D1 LUMILEDS LXCL PWF1 EN2 FLASH EN2 LTC3453 EN1 EN2 ILeD 8 25k 0 0 0 SHUTDOWN 1 Iset2 1 0 150mA TORCH 0 1 350mA 1 1 500mA FLASH DN1009 F01 Figure 1 High Performance LED Flash Driver Yields Greater than 90 Efficiency The LTC3453 Buck Boost Delivers 500mA to the LXCL PWF1 LED Flash with Only Six Tiny External Components LEDs an attractive solution for the now popular 1 megapixel and higher cell phone cameras Linear Technology offers many simple and extremely compact solutions for driving the latest high light out put LEDs including Lumileds Luxeon Flash LEDs These LEDs are up to 12 times brighter than conventional LEDs http www lumileds com solutions solution cfm id 9 Constant Current LED Drivers Switcher based DC DC converters are small and efficient so they are prime candidates for LED driver solutions The problem with a traditional switcher is that it is designed to producea regulated voltage for varying load currents LEDs must be driven with constant current to generate predict able light output Using a typical switcher for constant current involves adding an op amp and additional circuitry which are undesirable in space constrained applications Four new devices solve this problem The LTC 3216 charge pump the LTC3453 buck boost and the LT 1618 and LT3479 boost regulators all inco
3. advertisement DESIGN NOTES Tiny Efficient High Power LED Camera Flash Solutions for Cell Phone Applications Design Note 1009 Keith Szolusha Introduction Cell phones and PDAs are approaching the limit for small form factors but their feature sets are growing with new functions suchas built in cameras and even video cameras The initial in phone cameras were basic low resolution de vices with little in the way of usable flash capability How ever cell phone camera resolutions have improved and sophisticated flash technology is expected to keep pace There are two flash technologies used in digital still cam eras The first uses a Xenon flash lamp to provide a very short high lumen flash with excellent spectral character istics LTC offers Xenon photoflash solutions that are simple compact and efficient see Design Note 345 The second flash technology uses LEDs LED solutions hold the upper hand when simplicity and space is ata premium and they are the only choice when torch or video mode lighting is required LED flash units fit into spaces where even the smallest Xenon photoflash cannot Until recently LEDs could not compete with Xenon solutions in light output and spectral performance relegating them to the realm of low resolution sub 1 megapixel cameras How ever recent gt 10 x performance improvements have made 4 7uH D03314 472 3 3mm x 3 3mm x 1 4mm V
4. gure 1 does this very efficiently with its four internal switches This tiny solution only requires one small and low profile inductor 3 3mm x 3 3mm x 1 4mm and provides 90 LED efficiency The even smaller LTC3216 charge pump LED driver Figure 2 takes a different approach to the same problem By operating in 1x 1 5x or 2x mode it provides constant current to the PWF1 LED with either a step up or step down in voltage The external components are only four small ceramic capacitors and two smaller resistors The LTC3453 can provide up to 500mA LED flash current and the LTC3216 can provide 800mA LED flash current to the PWF1 The LT1618 constant current constant voltage converter Figure 3 has a unique current sense amplifier with the ability to sample current from the input or output By LUMILEDS LXCL PWF1 llep S SD25 10H PMEG2010EA 0 102 1 Vin 3 3V TO 4 2V EN1 TORCH duF 2 ILep 0 SHUTDOWN 0 SHUTDOWN 250mA TORCH 500mA FLASH EN2 FLASH Figure 3 The LT1618 Constant Current Constant Voltage Boost Regulator can Be Configured to Give a Low Parts Count Buck Boost Solution for LXCL PWF1 Luxeon Flash at 500mA Data Sheet Download http www linear com returning the LED from the output to the input this boost converter is given buck boost capability with up to500mA LED flash current The LT1618 solution provides effective performance for cost sensitive applications
5. nt to the LXCL PWF2 High Voltage Luxeon Flash with 85 Efficiency and Output Disconnect For applications help call 408 432 1900 Ext 2759 Linear Technology Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 www linear com dn1009 LT 1004 PRINTED IN THE USA LY MUR LINEAR TECHNOLOGY CORPORATION 2004
6. rporate the required cir cuitry to generate a constant current output while main taining high efficiency operation 4J LTC and LT are registered trademarks of Linear Technology Corporation 2 2uF 2 2uF cit cr c2 cz 2 9V TO 4 4V Vin 2 2uF D1 LUMILEDS LXCL PWF1 LTC3216 EN1 TORCH EN1 EN2 FLASH EN2 N2 ILep 0 0 SHUTDOWN 0 200mA TORCH 1 1 Iset1 Iser2 600mA 800mA FLASH Figure 2 This LXCL PWF1 Luxeon Flash Driver is Extremely Small It Requires Only an LTC3216 Low Noise High Efficiency Charge Pump in a Small DFN Package Along with Four 0603 Capacitors and Two 0402 Resistors 10 04 1009 Buck Boost for LXCL PWF1 Although constant current solutions do not directly regu late or monitor the output voltage of the LED driver circuit the level of the output voltage relative to the input voltage is a key determinant of the type of part required A buck or boost regulator does not have the ability when con verted to constant current to properly regulate the LED currentif the forward voltage of the LED is not respectively below or above the input voltage The wide input voltage range of a lithium ion battery requires that a converter be able to both step up and step down the voltage when the forward voltage of the LED is within the range of the battery s discharge profile in order to achieve steady state regulation of the LED current The LTC3453 buck boost LED driver Fi
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