Many applications require a minimum number of conductors in the link between two areas of a device—for example, a flip-style cell phone. While a Mobile Pixel Link (MPL) device offers a convenient way to transmit high-speed RGB video data across three wires, the LCD display may still need a synchronous serial interface for configuration and control. In addition, the backlight can consist of multiple LEDs that must be driven with a higher voltage, and the voltage must be variable (or its duty-cycle controlled) to adjust the display's intensity.
An I/O expander such as the LP3944, normally used as an LED driver, can supply these functions by using an I2C interface to control both. The I2C interface also can control other peripherals without further increasing the number of wires required for the link.
The LP3944 has eight open-drain outputs. The display's synchronous serial interface is "bit-banged," since its transfer rate can be low and is insensitive to clock variations. The LP3944 is particularly convenient in this application because it contains separate registers to control the upper and lower nibbles of its output lines.
The variable backlight voltage is realized by a simple four-bit resistor network connected to an adjustable switching regulator. This allows 16 brightness levels, which is more than enough for most applications. The values shown are designed for a range of 10.0 to 15.0 V. The Schottky diode protects the I/O lines from excessive voltage if a fault occurs.