1. Field of the Invention
The current invention relates to drivers for resistive loads, and in particular to drivers that control the brightness of a liquid crystal display (LCD) backlight system by regulating the electrical current through an LED array.
2. Description of the Related Art
A growing number of electronic devices use LCDs to provide information to the user. LCDs work, generally, by permitting or preventing light from passing through electrically controlled “portholes.” Three categories of LCD displays are reflective, transmissive, and transflective. The light necessary for the LCD display to be visible can be provided by reflected ambient light, as in reflective LCD displays. In many situations, particularly where ambient light is meager, it is advantageous to provide a backlight for the LCD, as in transmissive and transflective LCDs, to make the display easier to read.
Transmissive LCDs, such as LCD computer monitors, are only used with a backlight. Transflective LCDs, such as those used on many popular portable audio devices, can be used either with a backlight (as in a transmissive LCD) or with reflected ambient light (as in a reflective LCD display). One type of backlight for transmissive displays (as used herein, the term includes transflective LCDs in transmissive mode) is based on an array of light emitting diodes (LEDs).
The brightness of a transmissive display that uses an LED-array backlight can be varied by controlling the amount of current flowing through the LEDs. The higher the current, the brighter the transmissive display appears. In order to control the current through the LEDs, a feedback loop is typically used. The feedback loop includes a current-sense resistor, i.e., a resistor of relatively low resistance connected, on one end, to a device of interest, e.g., the LED array, and, on the other end, to a common node (e.g., ground), wherein the voltage across the resistor is indicative of the current through it and thus through the device of interest. This method is considered sufficiently reliable for current values down to the tens of milliamps. However, prior-art backlight drivers do not reliably detect smaller electrical currents, such as those measured in microamps, since smaller voltages are more difficult to accurately detect.