Technological advancements in drivers for plasma-display panels have enabled their manufacturers to compete with liquid-crystal displays on price while maintaining superior image quality.
by Eric Cirot and Jean-Raphael Bezal
PLASMA-DISPLAY SYSTEMS have gained attention in both the consumer and business markets because of their attractive form factors, screen performance, and declining prices. Along with the increasing size of plasma panels from 42 and 50 in. to 65 in. and beyond, the resolution of plasma-display panels (PDPs) is on the rise as well. Today's consumers demand high-definition (1024 x 768) and full-high-definition (1920 x 1080) panels to future-proof their sets to be compatible with future digital-content formats.
To stay competitive in the intense battle for dominance in the large-flat-panel-TV market between liquid-crystal displays (LCDs) and PDPs, makers are looking at ways to reduce costs while enhancing the consumer experience by tackling technical challenges brought on by increases in panel size and resolution. The improvement in drivers, which are being made smaller and more power-efficient, is one area that has been key to the success of PDPs. Today's drivers must combine affordable high-voltage technology in the output stage and smart features that reduce electromagnetic interference (EMI) with low-voltage high-frequency technology in the input stage.
The latest technological improvements in plasma drivers have allowed higher addressing speeds, enabling PDP makers to move to single-scan technology in large panels. Compared to incumbent dual-scan systems, where drivers on the top and bottom meet in the middle and only need to support current for onehalf of the display height, the number of drivers in single-scan systems is reduced by one-half. However, column (data) drivers in a single-scan system require higher current and higher input-signal frequency to drive the full height of the display. Higher addressing currents generate over-voltages and over-currents on data drivers, raising their operating temperatures and EMI susceptibility.
Figure 1 describes the large current loops created by the electrode and the power electronics (a horizontal loop for the scanning board and a vertical loop for the data driver).
The large currents that flow from data-driver outputs (256 per driver in today's panels) generate electromagnetic interference and high-frequency "overshoots," which affects the driver's reliability. Many data-driver outputs rise at the same time, while just a few may remain at Vpp (see Fig. 2). Very large currentsas high as 20 A flow to the line boards through the panel and electrodes, generating through the parasitic impedances a positive peaking voltage on the line driver and the row electrodes.
Fig. 1: The large current loop created by the electrode and the power electronics (a horizontal loop for the sustaining boards and a vertical loop for the data driver).
When a large number of data lines rise, a large displacement current flows from the data drivers through the plasma-panel capacitance. This displacement current flows through the parasitic inductances of the system, causing overshoots on the data electrodes in the high state. Some of this current flows through resistor R, which represents either a designed-in external resistor or a parasitic resistance. The resistor current causes the power-supply rail and the output of the high-state data driver to overshoot above Vpp, which can be a driver-reliability concern.
A second problem is an unwanted variability in the data-driver rise time. Displacement current induced through the plasma-panel capacitance by the rise of many data drivers causes an overshoot on the scan-driver side. Consequently, the voltage of the data-driver output rises faster than what is normally allowed by the high-side switch-current capability when only a few outputs switch at the same time.
Constant rising slope and spread-spectrum Jitter – features implemented in the latest STMicroelectronics PDP drivers (Fig. 3) – drastically decrease the EMI levels for a wide range of frequencies and reduce over-voltage vis-à-vis the operating voltage, leaving higher guard-band design margins, typically up to 25 V below the maximum operating voltage.
Constant rising slope [Fig 4(a)] controls the rising speed of the outputs, producing a constant rising edge of ~120 nsec even when the output load varies substantially due to differing numbers of data drivers rising. The principle behind the constant rising slope is to generate a ramp signal (for instance, by means of a current source charging a capacitor). The image of this ramp is produced on the output due to a source-follower transistor stage that can handle the variable load currents and still maintain the ramp.
Spread-spectrum jitter [Fig. 4(b)] in STMicroelectronics' new generation of plasma-panel data drivers reduces high peak currents flowing through the electrodes and the line electronics by spreading the output-switching transition for both rising and falling edges. The switching of outputs is de-synchronized to smooth the overall peak current in the power voltage bus and ground and, therefore, avoiding high-frequency radiations. The jitter is ~50 nsec. The pulse width is maintained by ensuring that the pulse-rise jitter has the same magnitude and phase as the pulse-fall jitter. The ~50-nsec jitter is sufficiently small, compared to the ~1000-nsec pulse width, that it does not noticeably alter the addressing characteristics of the display.
Benchmark measurements have shown that delays in output switching, achieved by the spread-spectrum jitter, can cut the peak currents in half, from 6 A for synchronized rising and falling edges in standard data drivers to 3–4 A. Because EMI is linked to the level and the transition speed of the current waveform, STMicroelectronics' data drivers can reduce the EMI level in the range of 5–8 dB compared to that of standard solutions. The implementation of constant rising slope and spread-spectrum jitter helps PDP makers reduce the cost of shielding, while remaining compliant with the industry standards for EMI.
STMicroelectronics has developed a propri-etary technology to address the size and power issues in large-flat-panel-display drivers on the die level. By using a combination of bipolar/CMOS/DMOS (BCD) and silicon-on-insulator (SOI), STMicroelectronics' manufacturing process facilitates size reduction of the high-voltage output stages in display drivers, as well as optimizes power consumption by integrating energy-recovery control.
Advanced energy-recovery solutions play an important role in curbing high temperatures caused by increased power dissipation in single-scan data drivers. To assure reliability and uncompromised performance, PDP makers are moving from direct current (dc) to energy-recovery methods, such as the use of an external energy recovery circuit (ERC) or "charge sharing."
Fig. 3: The latest PDP driver from STMicroelectronics includes patented features such as constant rising slope and spread-spectrum jitter that help decrease EMI levels and reduce over-voltage.
STMicroelectronics engineers have currently filed a patent for an embedded smart-power-saving (SPS) technology that increases energy efficiency in plasma-display drivers by as much as 45% compared with standard dc power-supply solutions. This embedded SPS system requires no additional components (capacitive or inductive) and no dedicated input control. It saves power by transferring the charge from the falling to the rising outputs during the charging or discharging of the load capacitance. This allows the data output switching to occur in two voltage steps, which reduces the 1/2CV2 losses. One of the challenges of this chip design is to efficiently perform the needed two-step switching and still maintain the attributes of constant rising slope and spread-spectrum jitter.
For the largest screen sizes (50–65 in.), highest resolutions (HD/full HD), and harshest temperature constraints, an external power-saving (Weber) circuit remains the most efficient solution, as shown in Fig. 5, though it does add an additional cost. This external circuit embeds an inductive recovery mechanism that enables better recovery efficiency in the resonant mode.
Apart from higher power demands, single-scan driving also requires faster addressing speeds. At the same time, the number of connections and the risk of associated time delays between drivers and the panel controller increase significantly in large-sized high-resolution plasma displays. To enhance the data rate – the trend is to increase addressing speeds above 300 Mbits/sec – new transmission protocols are needed to accelerate and streamline the communication throughout the panel. Different protocols are in competition based on RSDS differential signaling.
STMicroelectronics has recently developed a high-speed single-data-link transmission protocol that will appear in the next generation of STMicroelectronics' PDP data drivers, slated to arrive on the market by the end of this year. This new protocol, which embeds a clock within the data, will allow faster transmissions and a reduction in the number of connections between the PDP controller and data-driver board. This reduction will help save on the size of the data-driver board and on the pin number of the PDP controller, hence reducing overall application cost.
The authors are very thankful to Larry Weber for his valuable comments. •