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.
See the figure