Test method of LCD control driving circuit based on digital test system
- May 03, 2018 -

                 

As liquid crystal displays have become more and more widely used in daily life, their core components, the types of LCD drive circuits, and the demand for them have also increased. Under normal circumstances, the test of the LCD control driving circuit is completed on the LCD circuit dedicated test system, but due to its high price, the test cost is also significantly increased, becoming a bottleneck restricting the mass production of the LCD control driving circuit. In view of the above reasons, the article proposes a test method for LCD control driving circuit based on digital test system, so as to realize low cost and high quality testing of LCD control driving circuit. At the same time, according to the characteristics of LCD control driving circuit, combined with practical experience, some test techniques for LCD control driving circuits are introduced.


1 Introduction


LCD display devices have been widely used in many occasions because of their outstanding advantages such as low voltage driving and low power consumption. Especially for portable electronic products, the application of liquid crystal displays such as STN and TFT has made rapid progress. The analog output of the LCD Driver IC (LCD Driver IC) directly drives various LCD display panels and controls the operation of pixel arrays of various LCD monitors. It is the core device on LCD displays, and the quality of the LCD control driver circuit is directly determined. The effect of liquid crystal display, so the design of this type of circuit test program is also particularly important. This article mainly introduces the simple test method of the LCD control driving circuit based on the digital test system and some tips that the author summarizes in practice.


2 Difficulty of testing LCD drive circuit


2.1 Many pins


The number of driving pins of the LCD control driving circuit is as few as tens, and as many as thousands, the corresponding test equipment must configure a large number of test channels, generally reaching 256 to 512 channels or even 1024 channels.


2.2 Fine Pin Drive Voltage


For a 4096-color ordinary color display, RGB three colors have 16 levels of gray for each color, corresponding to 16 levels of drive voltage, that is 16 (R) & TImes; 16 (G) & TImes; 16 (B) = 4096, If it is a true color display, each color is 256 levels of gray, corresponding to 256 drive voltages. Therefore, the test equipment must be able to quickly and accurately measure the stepped analog signal output by the LCD driver device with a resolution of millivolts. This is particularly important because the driving voltage is stable and uniform and has a decisive effect on the LCD display.


2.3 Wide range of output drive voltage


The output driving voltage of the LCD control driving circuit is much higher than the 5V voltage of the ordinary CMOS device, even reaching more than 30V, and because of the particularity of the LCD display screen, the polarity of the driving voltage needs to be constantly reversed. Therefore, for the test equipment, the measurement range must reach 30V or more, and it can cope with the polarity change of the driving voltage.


2.4 Others


For some display driver circuits, the test equipment needs powerful signal analysis software to perform arithmetic processing on the analog voltage data sampled in the test channel to obtain the specific color information of each pixel and determine the state of the device.


3 LCD Control Drive Circuit Test Method


It can be seen from the typical problems when the LCD control driving circuit is simply listed above that the testing of such circuits places high requirements on the testing capabilities of the test equipment, so that the best test equipment is for the LCD control driving circuit. Non-LCD circuit dedicated test system, such as the world's largest test equipment company Advantest's T6371, T6373, ND1, ND2, etc., in 2008 Teradyne's D750Ex, and the current LCD driver The Yokoga wa TS670 and TS6700 are mainly used in IC packaging and mass production. TS670 and TS6700 support only one single LCD driver IC at most, and the output pin count is up to 736 pins. However, due to the promotion of multi-channel technology products (LCD TVs, etc.), the current output pin count is from 300 feet to 400 feet. The foot is sharply raised to 800 feet to more than 1 000 feet. Yokogawa's ST6730, Advantech's ND1 and ND2, Teradyne's D750Ex, etc. can support this (ND2 can support pin counts up to 1 500 Above the feet, D750Ex can support up to 2 400 feet).


However, taking into account the corresponding increase in the cost of the test, for some LCD control drive circuit can also use a digital test system for simple testing. The following will introduce the testing method of LCD control driving circuit based on the digital test system platform.


The LCD control driving circuit, like other ordinary circuits, requires testing of some conventional test items and also has some special test methods because of its own characteristics.


3.1 Functional Test


Like the general logic circuit, the function test of the LCD control driving circuit needs to verify each function module of the circuit. However, the output level of the LCD driving signal output terminal of the LCD control driving circuit is not a logic level of “0” or “1” of a general logic device, but a stepped analog signal. When a digital test system is used for testing, the same segment can be tested. The code selects two threshold levels for two tests to achieve a basic test of the LCD drive output.


3.1.1 Programming Tips


Some LCD drive circuits have internal RAM storage areas, which need to be written at least in checkerboard mode to write 0101, 1010 data to test their read and write functions, so that adjacent address units are in different logic level states, and sometimes even need to write. Enter all 0 and all 1 data to fully cover such functional tests.


3.1.2 Programming Tips II


Functional test codes sometimes need to be written by themselves, rather than by the designer through logic simulation. At this time, combining the factors of shortening the test time and reducing the test cost requires careful consideration of the functional test methods so as to fully cover all functions of the circuit. , And can effectively reduce the test time. This depends on its own understanding of the circuit's function and practical experience.


For example, an LCD control driver circuit needs to read and write bidirectional data ports through the circuit to complete the transfer of commands and data during functional test, and then cooperate with other logic units to display the written data on the LCD output port. The test of its internal RAM unit can be verified completely through the bi-directional port, no longer need to be sent to the LCD driver output, and the bi-directional port read/write speed can be much faster than the display output, so when the RAM is tested in this way, it can be appropriate Accelerate the reading and writing clock frequency to reduce test time.


3.2 parameter test


The other parameter tests of the LCD drive circuit are basically the same as the general digital circuit. Here are some special parameters that need attention.


3.2.1 LCD Output Drive Test


As mentioned before, in all parameters of the LCD driver circuit, the LCD output driver (or LCD output voltage deviation, LCD output on-resistance) is the key parameter. It has a decisive influence on the display effect of LCD display devices, especially for display devices with larger specifications (more pixel points), the number of drive output pins of the LCD control driving circuit is more if the pins are under the same load. If the output voltage deviation is too large, the display color of each pixel on the LCD display will be inconsistent. Therefore, the output voltage deviations of all drive output pins of the LCD control driving circuit under the same load must be tested one by one. To ensure that they are all within the allowable range.


Usually, the test time of the DC parameter test unit of the test system is several to several tens of milliseconds. Therefore, the greater the number of drive output pins of the circuit, the longer the test time of this item will be, and the test production cost of the circuit will also increase. . The better test method is:


(1) For LCD-specific test systems, there are multiple digital samplers (DigiTIzer) that can be used to continuously sample voltages so that the circuit can complete this test in a relatively short period of time. For example, Yokogawa's ST6730 test system is configured with a digital sampler using each LCD output pin, while Advantech's test system is equipped with a digital sampler for every 8 LCD output pins.


The schematic diagram of the digital sampler test method is shown in Figure 1.


The




Figure 1 Schematic diagram of digital sampler test method


(2) Some test systems have a load pin (AcTIve load). If the operating voltage of each segment of the LCD drive device under test is within the allowable conditions of the system hardware, and there are enough test channels, each can also be used. The method for carrying out the function test of the LCD driver output pin with load is convenient and time-saving to complete the test of this parameter at the same time as the function test. The schematic diagram of this method is shown in Figure 2.


3.2.2 dynamic partial pressure leakage test


This parameter is not the main parameter in the LCD control driving circuit specification, but when using a digital system to test this type of circuit, adding this parameter to the test can effectively improve the fault coverage rate of the circuit. The specific test method is:


Write data, so that the circuit LCD driver output terminal can display normally in checkerboard mode, and then conduct dynamic leakage current test at each voltage level input end of the circuit.




Fig. 2 Schematic diagram of the functional test method for driving the output pin with load


4 Conclusion


With the development of science and technology, the variety of LCD driver circuits is also changing with each passing day. For this series of circuits, the test methods are also different for different circuit performances. This article only introduces the test method of the LCD control driving circuit based on the digital test system, and shares some test tips that the author summarizes in practice. It is suitable for low-cost, high-quality testing of LCD control driving circuits.