AN-1235| Application Note

Using Smart Charge sharing to Reduce Power and Boost Column Driver Performance

Using Smart Charge Sharing to Reduce Power and Boost Column Driver Performance
Introduction As flat panel displays extend to higher resolutions and refresh rates, the performance requirements of shorter line times are in direct conflict with the system designer's power budget requirements. National Semiconductor's proprietary Smart Charge Sharing technology is able to reduce power consumption while improving the column driver output performance. When used properly, Smart Charge Sharing can reduce the column driver power by up to 40% and improve the settling time of the outputs. This application note will describe how Smart Charge Sharing works, how it differs from other common power saving techniques in column drivers, and how to control Smart Charge Sharing on National's FPD33584 and FPD33620 column drivers. How Smart Charge Sharing Works Smart Charge Sharing works by redistributing the energy stored in the columns of the TFT liquid crystal display. The columns can be driven halfway to their final value without consuming any power. This is possible because in dot or n-line inversion s
chemes, half of the columns are driven to voltages above VCOM and half of the columns are driven to voltages below VCOM. In Figure 1, Figure 2, and Figure 3 below, the basic operation of Smart Charge Sharing is described. In this example, the flat panel display column is approximated as a lumped RC load on the output amplifier of the column driver. For quantitative analysis, the columns should be modeled as a distributed load, but for the purpose of illustrating how charge sharing works, a lumped model is adequate. Figure 1 shows the situation immediately before the start of charge sharing. Alternating columns are at voltages above and below VCOM, respectively. There are a series of switches within the column driver that are capable of shorting all the columns together. Before charge sharing begins, these switches are all open.

National Semiconductor Application Note 1235 Craig Zajac May 2002

AN200464-1

FIGURE 1. Column Voltages Immediately Before Charge Sharing Figure 2 shows the situation during charge sharing. The output amplifiers are disabled (in a hi-Z mode) and the switches are now closed. Current flows from the columns above VCOM to the columns below VCOM, as shown by the arrows. During charge sharing, no power is consumed by the output amplifiers.

AN-1235

2002 National Semiconductor Corporation

AN200464

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