Specifying Driver Requirements

When designing an EL drive scheme, it is essential to model the characteristics of the EL device, taking into consideration the drive scheme. The EL designer needs to pay close attention to the current levels required to drive his display and make sure that the drivers can sink and/or source the required levels. The drive current requirements for the refresh drive scheme can be determined by examining worst-case voltage excursions and estimated display capacitance. As reported in Proceedings of the SID, vol 23/2. 1982, page 87, the equivalent circuit shown in Figure 8-89 can be used in the analysis of the drive requirements. For purposes of discussion, some simplifying assumptions are made. First, each display element (pixel) is purely capacitive with C = Ce,Ce being the same for all pixels. Next, all points that are at the same potential are treated as if they were connected together.

Figure 8-89. Panel Equivalent Circuit

The equivalent circuit shown in Figure 8-89 is for a panel having N rows and M columns. This circuit ignores the negligible capacitance between adjacent rows and between adjacent columns. The panel is driven one row at a time by selecting one of the N rows and m of the M elements in that row. If one row and m columns are selected, the m elements are turned on and the m columns are capacitively coupled to the N minus 1 floating rows. Each of these N minus 1 floating rows is coupled to M minus m off columns which are again coupled to the one selected row. Consider the following example:

During the refresh, the row driver would be required to source current through the clamp diode. Since all the N rows are pulled up, the equivalent circuit is as shown in Figure 8-90. The worst case is when all the rows are at zero volts and all are switched to Vpos. Suppose Vpos is 200V and a 10µS rise time is required, then the clamp diode must be able to deliver at least:

The current requirements for the column drivers can also be calculated as above.

Figure 8-90. Refresh Equivalent Circuit