White-LED driver backlights LCD and keypad
Designers widely use white LEDs to backlight color LCDs and keypads in handheld devices, such as cell phones, MP3 players, GPS navigators, and PDAs. Their spectrum and brightness represent near-ideal light sources. One possible configuration for a phone or a phone/PDA combination is to have a group of three LEDs to light the display and six LEDs for the keypad. Figure 1 shows a method for driving all the LEDs with a single driver IC from Catalyst Semiconductor (www.catsemi.com), the CAT32. Power comes from a single lithium-ion battery cell. A FET switch can independently turn off the group of six LEDs. The shutdown input, , on the CAT32 turns off all the LEDs. LED brightness is a direct function of the current running through the LED, and typical values are 10 to 20 mA for white LEDs. The CAT32 regulates a constant current through the LED pin. Resistor R1 connected to the RSET pin adjusts the LED current.
The driver is a step-up converter using an inductor to boost the voltage, such as that from a lithium-ion battery, and bias multiple LEDs in series. The RS resistors in series with the LEDs balance out the current between the groups of LEDs. The LED forward voltage, typically around 3.5V, experiences some variation from one LED to the other for a given bias current, and the series resistors also compensate for that situation. By using series resistors with values of 75Ω, a current of approximately 15 mA flows in all LEDs. The display uses Nichia (www.nichia.com) NSCW335 side-view LEDs, and top-view Nichia NSCW100 LEDs backlight the keypad. You define efficiency as the ratio of the power dissipated in the LEDs, including the power in the series resistors but not including the loss in the Schottky diode, to the total input power. Figure 2 shows the efficiency of the circuit for a load of 15-mA current in the LEDs and for supply voltage ranging from 3 and 4.2V, corresponding to the charge-discharge cycle of lithium-ion batteries. The expression for efficiency is as follows:
where VLED is the voltage across the LED, ILED is the LED current, RS is the series resistor, and IIN is the current from the input supply. The efficiency is approximately 84% for a 3.6V supply. If you now consider only the power the LEDs dissipate, excluding all the other losses, the following formula gives the net efficiency (approximately 75% with a 3.6V supply):