High-intensity white LEDs are ideal for backlighting of small, color LCD displays used in handheld devices such as cellular phones and PDAs. When compared to other backlight solutions, white LEDs provide higher reliability and lower power consumption.
But portable devices powered by a single Li-ion cell require a step-up voltage conversion to drive white LEDs over the entire voltage range of the Li-ion cell (2.7 to 4.2 V). This is because the forward voltage drop of a typical white LED is 3.6 to 4 V. Shown here is a constant-current backlight supply using the LTC3200 inductorless regulator to drive four white LEDs.
The LTC3200's inductorless architecture and 2-MHz switching frequency result in a small circuit size—less than 1.1 mm in height and 28 mm2 in area. The constant-current source illustrated in the figure accurately controls the LED current regardless of variations in the forward voltage drop of the LEDs. This results in constant light output from different white LEDs.
The LED current is programmed via the RPROGRAM resistor. Using the feedback voltage of the LTC3200 (VFEEDBACK = 1.4 V), the LED current is regulated according to the formula:
ILED = VFEEDBACK/RPROGRAM
In this circuit, an 86.6-Ω resistor is used to set the LED current at 16 mA. To maintain the same current in each LED, the paralleled LEDs also are ballasted with 86.6-Ω resistors.
The brightness of the LEDs can be adjusted by changing the duty cycle of the PWM signal at the external MOSFET connected to the shutdown pin. A lower duty cycle will result in lower brightness, whereas a higher duty cycle will result in higher brightness. For the best results, a PWM signal in the range of 200 Hz to 1 kHz should be used.