FIG. 1 is a schematic diagram of a conventional pixel 100 of an active matrix light-emitting diode (AMLED) display. Pixel 100 includes a light-emitting diode (LED) 105 (e.g., an organic LED or other type of LED), a column driver 108, row drivers 110 and 115 (each coupled to ground), voltage sources 120 and 125 (each coupled to ground), a capacitor 130, and switches 142, 144, 146, and 148 (e.g., semiconductor switches).
The cathode of LED 105 is coupled to the negative terminal of voltage source 120 (the positive terminal being coupled to ground), or directly to ground, while the anode of LED 105 is coupled to a pixel drive transistor (e.g., a switch 142). Switch 142 is also coupled to a node 152, and node 152 is also coupled to switches 144 and 148. Switch 142 is turned ON/OFF by column driver 108 (via switch 146 and a node 156) and capacitor 130 via a node 154.
Switch 148 is coupled to node 156, and is turned ON/OFF by row driver 115 (via a node 158). Node 156 is also coupled to switch 146, and switch 146 is turned ON/OFF by row driver 115 (via node 158).
Pixel 100 also includes a node 160 coupled to switch 144, capacitor 130, and the positive terminal of voltage source 125 (the negative terminal being coupled to ground). Switch 144 is coupled to and turned ON/OFF by row driver 110.
During operation, row driver 115 turns ON switches 146 and 148 to program pixel 100. When switch 146 is ON, current from column driver 108 charges capacitor 130 and provides a voltage at the gate of switch 142, which turns ON switch 142. When switches 148 and 142 are each ON (at the same time as switch 146), current from column driver 108 is supplied to LED 105 (via switch 142) and LED 105 is illuminated.
Row driver 115 then turns OFF switches 146 and 148, and row driver 110 turns ON switch 144 (switch 142 remains ON via capacitor 130). When switches 142 and 144 are both ON, current from voltage source 125 is supplied to LED 105. This is referred to as the “Hold” portion of the cycle. LED 105 remains illuminated until row driver 110 turns OFF switch 144.
The brightness of LED 105 is determined not only by the magnitude of the current supplied, but also by the amount of time current is supplied to LED 105. That is, the longer the period of time LED 105 receives current during the cycle time, the brighter LED 105 appears. Similarly, the shorter the period of time LED 105 receives current, the dimmer LED 105 appears.
A conventional display (not shown) using an array of pixels 100 illuminates the array one row of pixels at a time (via a pair of row drivers 110 and 115 for each respective row) during a cycle time. Furthermore, once illuminated, each row remains illuminated until it is reprogrammed during the next cycle. That is, for each cycle row 1 is illuminated first via a first pair of row drivers, row 2 is then illuminated via a second pair of row drivers, and then row 3 is illuminated via a third pair of row drivers. This process continues until each row is illuminated via a respective pair of row drivers, and each row remains illuminated throughout its cycle.
FIG. 2 illustrates a timing diagram 200 of a conventional array of pixels 100 arranged in a plurality of rows. Timing diagram 200 shows one cycle time, which is typically about 16.6 milliseconds (ms). As illustrated, row 1 is illuminated at time T0 and held ON for the remainder of the cycle time. After row 1 is illuminated, row 2 is illuminated at a time TR (e.g. 0.5 ms) after T0 and held on until its next programming time. As discussed above, this process is repeated for each row until all of the rows in the array are illuminated.
Dimming of the display's luminance while retaining displayed information (e.g. gray shades) may be accomplished by modulating the amplitude of voltage supplies 120 and/or 125, or by turning either supply 125 or 105 OFF at an interval shorter than the cycle time. This is referred to as pulse width modulation of the LED 105 current.
Since each pair of row drivers illuminates the pixels 100 in their respective rows one row at a time, each row may be illuminated for a different amount of time if the PWM is not properly synchronized with each row's programming and hold periods. Furthermore, transients caused by the turning ON or OFF of switch 144 cause a change in the amount of charge on capacitor 130, and a corresponding change in the programmed current through switch 142 resulting in an undesired change in luminance of LED 105, thus causing luminance non-uniformity in the LED 105 array. Moreover, the ability to control the brightness of each LED is limited to the ability to precisely control the amount of current provided to the LED by the current source.
Accordingly, it is desirable to employ apparatus, systems, and methods for dimming the brightness of an array of pixels uniformly without the problems associated with the prior art methods. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.