Technical Information Paper White LEDs Driver Circuits (LCD Back Light & Camera Flash) ! No. 0018 (Ver.001) Dec. 2003 GENERAL DESCRIPTION [Contents] White LEDs are widely used to backlight LCDs on cellular phones, PDAs, and digital cameras. Torex has control solutions for such LEDs, incorporating a DC/DC converter or charge pump system. Introduced below are essential brightness control systems for such backlighting. An application example of a camera flash using white LEDs is also described below. CIRCUIT SERIES 1 XC9106 CIRCUITRY / BRIGHTNESS CONTROL METHODS Step-up DC/DC Converter / CE pin, Voltage control 2 Step-up DC/DC Converter XC9103 / CE pin, PWM control 3 Step-up Constant Voltage Controlled Charge Pump XC9801 / CE pin, PWM control 4 Step-up DC/DC Converter Application Circuit XC9103 / Light & flash for mobile phones & digital cameras ! CIRCUIT 1. XC9106 : STEP UP DC/DC CONVERTER [ White LED Forward Current ] Using the XC9106 series, step-up DC/DC converter, this system controls brightness by varying the current (IF) passed through white LEDs in proportion to the reference voltage (Vref) input from an external source. The FB pin voltage is determined by the voltage applied to the CE pin. IF = VCE VCE : CE pin voltage (0.8V ~ 2.5V) RFB2 Note : The CE pin has chip enabling capability. By connecting it to the ground, the operation of the brightness control system stops. [ IF-VCE ] MOSFET XP151A13A0MR Forward Current IF (mA) 30 L 10uH SD XB0ASB03A1BR CFB 0.047uF VDD EXT CL 4.7uF IF XC9106D003MR VIN=2.7V ~4.2V CIN 2.2uF CE GND FB Enable/ DC VOLTAGE CONTROL RFB2 100 ohm 25 20 15 10 5 0.5 1 1.5 2 2.5 CE Voltage VCE (V) ! CIRCUIT 2. XC9103 : STEP UP DC/DC CONVERTER This system, incorporating the XC9103 series, a step-up DC/DC converter, controls brightness by varying the illumination time of white LEDs, making use of PWM control at the CE pin. White LEDs brighten or fade by increasing or decreasing the duty ratio of the PWM signal. The PWM signal input to the CE pin is controllable between 60 and 100 Hz. [ CE 100Hz Input, IF & DCE Wave form @ Duty : 20% ] IF 2 VCE 1 L 10uH MOSFET XP151A13A0MR 1ch 2V SD XB0ASB03A1BR CFB 0.047uF VDD EXT CL 4.7uF C IN 2.2uF CE GND Enable/PWM BRIGHTNESS CONTROL 20uS/div IF [ CE 100Hz Input, IF & DCE Wave form @ Duty : 80% ] XC9103D093MR VIN=2.7V ~4.2V 2ch 10mA FB RFB2 51ohm IF 2 VCE 1 1ch 2V 2ch 10mA 20uS/div TOREX SEMICONDUCTOR LTD. TIP No. 18 1 http://www.torex.co.jp 3 Technical Information Paper White LEDs Driver Circuits (LCD Back Light & Camera Flash) ! CIRCUIT 3. XC9801 : STEP UP CONSTANT VOLTAGE CONTROLLED CHARGE PUMP This system incorporates the XC9801 series, which is a stepup constant-voltage control charge pump, and performs PWM control at the CE pin for brightness control, varying the illumination time of white LEDs. The parallel connection of LEDs allows the capacitor breakdown voltage to be low, thus facilitating reduction of mounting area. The PWM signal input to the CE pin is controllable between 1 and 10 kHz. [ CE 1kHz Input, IF & DCE Wave form @ Duty : 10% ] IF 2 VCE 1 VOUT=4.5V 1ch 1V SENSE VIN Enable/PWM BRIGHTNESS CONTROL CIN 1mF C1+ XC9801B453KR CE VIN= 3.6V RCE 470kohm 400uS/div 2ch 10mA VOUT C1 0.47uF C1- GND C2 1uF R1 51ohm PGND [ CE 1kHz Input, IF & DCE Wave form @ Duty : 50% ] IF R1 51ohm R1 51ohm R1 51ohm IF 2 VCE 1 1ch 1V ! 400uS/div 2ch 10mA CIRCUIT 4. APPLICATION EXAMPLE : LIGHT & FLASH FOR MOBILE PHONES & DIGITAL CAMERAS This application example makes use of Tr 1 and a resistor added to Circuit 2 to instantaneously increase the current (IF) passed through white LEDs. The circuit is best suited to lights or flashes on cellular phones and digital cameras. The brightness of white LEDs increases when Tr 1 is turned on, as the current (IF) passed through white LEDs increases instantaneously. The white LEDs return to their original brightness when Tr 1 is turned off. When Tr 1 is off, the brightness is determined by the resistance value of RFB 2. [ White LED Forward Current] Tr1 : OFF IF = Vref Vref : 0.9 + 2.0% RFB2 Tr1 : ON IF = Vref R0 * R0 = RFB2 x R RFB2 + R Note : Refer to the manufacturer's technical data sheet L 10uH MOSFET XP151A13A0MR SD XB0ASB03A1BR for the maximum value of pulse forward current. CDD 1uF CFB 0.047uF VDD EXT CL 10uF IF [ Tr1-G 10Hz Input, IF & Tr1 Wave form @ Duty : 5% ] XC9103D093MR VIN=2.7V ~4.2V CIN 10uF CE GND FB RFB2 1k ohm PULSE CONTROL R 12 ohm IF 2 G Tr1 Tr1 1 1ch 2V 2ch 50mA 40mS/div TOREX SEMICONDUCTOR LTD. TIP No. 18 2 http://www.torex.co.jp