New IC simplifies design of large flat-panel displays

Introduction

Flat panel displays are everywhere. Whether it is in the terminal equipment at home, in the office or in the store, or even in the car, you can see flat panel displays. One of the most common applications of flat panel displays is color TV, especially large flat panel color TVs larger than 22 inches (and tending to 40 inches). In this field, the old cathode ray tube (CRT) technology faces the challenge of new technologies. The most noteworthy ones are thin film transistor liquid crystal displays (TFT LCD) and plasma flat panel displays (PDP).

Of course, CRT is still the dominant large-screen display due to its high resolution, wide grayscale range, and ability to display motion without artifacts. However, the performance of non-CRT displays, such as TFT LCD and PDP technology, continues to improve, providing users with great convenience. Their ultra-thin, streamlined appearance and lighter weight allow them to be hung on the wall. However, the production cost of these new displays is still high, so the selling price is also high. However, TFT LCD continues to improve performance in all aspects: readability, response time, and driving characteristics. In addition, many semiconductor companies with large production equipment believe that they can pay off production costs by using existing production facilities, so production costs will be significantly reduced. Currently, 32-inch and 40-inch TFT LCD displays have been put into production, and 54-inch displays have been sampled.

History of TFT LCD

Liquid crystals were discovered by Austrian botanist Fredreich Rheinizer in 1888. In the mid-1960s, scientists confirmed that liquid crystals deflect light passing through the crystals after being stimulated by external electric charges. Early prototypes were too unstable to be mass-produced, but all that changed when a British researcher introduced a stable liquid crystal material, biphenyl.

TFT LCD is a three-layer structure, the liquid crystal layer is filled between two glass layers. One glass layer is TFT glass, which has the same number of TFTs as pixels displayed. The second layer is the color filter glass layer, which has a color filter to produce colors. The liquid crystal layer moves according to the difference in voltage between the color filter glass layer and the TFT glass layer. The amount of light generated by the backlight is determined by the amount of movement of the liquid crystal layer to produce color. For a correct LCD flat panel display to work, the following three voltages are required:

1. AVDD is the high voltage/current output used to drive the TFT.
2. VON is the high voltage output that biases the TFT, i.e. the on-voltage.
3. VOFF is the reverse voltage output that reverse biases the TFT, i.e. the off-voltage.

New quad-output regulator specifically for TFT LCD flat-panel displays

The LT1943 quad-output adjustable switching regulator provides power for large TFT LCD flat panel displays. This device uses a small 28-pin thermally enhanced TSSOP package to generate 3.3V or 5V logic voltages while having the three output supplies required for TFT LCDs. The input voltage operates in the range of 4.5V to 22V, and the buck regulator provides a low voltage output VLOGIC and up to 2A of current. A high-power boost converter and a flyback converter provide the three independent output voltages AVDD, VON and VOFF required for LCD flat panel displays. The high-side PNP provides a delayed turn-on of the VON signal, which can handle up to 30mA of current.

Protection circuits ensure that if any of the four outputs drops below 10% of the nominal voltage, VON will not operate. All switches are synchronized with an internal 1.2MHz clock, allowing the use of small inductors and ceramic capacitors. The current mode architecture provides excellent transient response. And, for maximum flexibility, all outputs are adjustable. Another very important feature of the LT1943 is its input voltage range of 4.5V to 22V, which allows AC adapters greater than 5V to drive larger flat panel displays. This is very important because 5V AC adapters have thick cables to handle the corresponding power levels, but these cables have inherent high voltage drops, which can cause the output regulation to drop below the specified level, thereby causing the display to fail. Higher voltage AC adapters (such as 19V) have much thinner cables and smaller voltage drops, providing better voltage regulation for the display.

LT1943 Operating Principle

The LT1943 is a highly integrated power IC that includes four independent switching regulators. All four switches have frequency foldback and their own oscillators using current mode control, and the block diagram of each switching power supply is shown in Figure 2. Switching regulator 1 includes a buck regulator and a 2.4A current limit. Switching regulator 2 is a boost regulator and has a 2.6A current limit. Switching regulators 3 and 4 are 0.35A boost regulators. Switching regulator 4 has two feedback pins (FB4 and NFB4) that can directly adjust the positive or negative output voltage.

When power is applied to VIN, the RUN-SS pin begins charging and when it reaches 0.8V, switch 1 is enabled. The RUN-SS pin is used for soft start and can limit the rate at which VLOGIC can rise. Using a larger capacitor in the RUN-SS pin will cause VLOGIC to start up more slowly. Switching regulators 2, 3, and 4 are driven by the BIAS pin, which must be tied to VLOGIC. VLOGIC is the first to go in and when it reaches 2.8V, the SS-234 pin will begin charging to enable switches 2, 3, and 4. At this point AVDD and VOFF will rise, with the rate of rise depending on the size of the capacitor connected to the SS-234 pin. When AVDD reaches approximately 90% of its programmable voltage, the PGOOD pin will drop low. When all outputs reach 90% of their programmable voltages, the CT timer will trigger and the 20uA current source will begin charging the CT pin. When the CT pin reaches 1.1V, the output split PNP turns on, connecting VON. When any of the 4 outputs drops below 10% of the rated voltage, the planar protection circuit will push the CT pin to ground, stopping VON. This prevents the TFT display from sustaining VON surge current, which can cause the entire display to be suddenly lit, which may damage the display.

in conclusion

Regardless of which flat panel display technology will become dominant, the new era of flat panel display technology will inevitably permeate all aspects of life. In the next few years, CRT will still dominate the TV market, but their departure is inevitable. It will last for another 3 to 5 years before TFT LCD becomes mainstream. The main problem of TFT LCD is to further reduce its manufacturing cost, but the ICs required for appropriate power supply are already in use. The LT1943 is a recent example, which has a high level of integration and is the most compact and safe power supply solution currently available on the market for TFT LCD designers.