Solution: average the pulse of output current in the same time PCB should be more than 4 layers, and the shorter the wiring part, the better
VLED and VCC are separated into different power supplies add a large voltage stabilizing capacitor to the ground terminal of VLED and VCCNowadays, LED display is more and more widely used. Its footprints can be seen in financial securities, sports, traffic information, advertising transmission, etc. because of the decline of LED cost and the improvement of brightness in recent years, coupled with the advantages of less power consumption, long service life, large viewing angle and fast response speed.
Moreover, the corresponding sizes can be customized according to different locations and needs, and quickly rise into the darling of a new generation of communication media in the market. Its conditions are unmatched by other large-scale display devices. This paper will further explain how to realize high-quality LED display without changing the circuit design and using the rapid response advantage of the driver chip.Overall speed improvement - higher refresh frequency and frame change frequencyThe LED is driven by the current flowing, and the brightness and gray of the LED can be controlled by the pulse width. In short, if the design of the system end is not considered, the refresh frequency is determined by the addressing time (TACC) and the current speed flowing through the LED; The improvement of framerate requires faster addressing time in addition to the support of the system, and there is a strong positive correlation between addressing time and transmission frequency (DCLK) and addressing number.
For example, there is a full-color outdoor display screen whose addressing number is 768. If different frequencies are used, the overall addressing time will be different. The working frequency is 10MHz - > 768x0.1us = 76.8us, and the working frequency is 30MHz - > 768x0.033us = 25.6us. The addressing time difference between the two is three times.The speed of current flowing through the LED determines the refresh frequency of the LED display screen. For example, if an LED display screen has 768 addressing numbers, 30 MHz working frequency, 8 bits for gray scale adjustment, 2 bits for brightness adjustment, and 4 US interval for each subfield; The pulse width displayed by the traditional driver chip is 250ns, while the pulse width of snapdrivetm driver chip is 50ns. There is a significant difference in the refresh frequency between the two
It shows that the gray scale is improved. At present, the OE response time of the common traditional driver chip in the market is about 250ns. If the above example is used, the highest gray scale is 8 bits; That is, R, G and B each have 256 gray scales. Its color is 256x256x256 = 166777216, about 16 million colors. If you want to increase the gray scale to 14 bits, that is, 16384x16384x16384 = 439 billion colors; The refresh rate between the two will also be significantly different The following are the test conditions and results of the driver chip including snapdrivetm Technology launched by Taiwan Xunjie technology. It can be clearly seen from figures 1 and 3 that the output current of the driver chip is still linear under the very small OE pulse width, while the traditional driver chip cannot provide linear output.
Test conditions: VCC = 5V, IOUT = 38.3ma, RL = 47 , CL = 13pf Reduction of distortion rate for drive chips with different output current slopes, we get different results in distortion rate by using simulation software (hspice2007). Simulation conditions: traditional drive chip: ton: 160ns, TOF: 70ns, snapdrivetm drive chip: ton: 15ns, TOF: 15ns, VIN: 5V, IOUT = 20mA, LED equivalent circuit RL: 52 , CL: 10PF, OE pulse width: 250ns To solve the problem of LED heat and increase the service life of LED, as shown in Figure 5, the current output diagram of 50% dutycycle is shown. If the current pulse is dispersed evenly at the same time, it will not affect the output current and LED brightness, but also avoid the phenomenon of LED overheating and early attenuation of service life caused by long-time lighting of LED. Fast response circuit design uses fast response driver chip, although it can improve the gray scale and refresh frequency of LED display screen; However, according to the formula of inductance effect V = l di / dt becomes smaller due to time t; Relatively speaking, the instantaneous voltage increases, so it is easy to produce surge. The author listed several ways to improve the circuit design for readers' reference:
5: Variation of voltage L: parasitic inductance on the circuit Di: differential of current DT: differential of timeThere are several points to pay special attention to in circuit design:
1. PCB should preferably be more than 4 layers, and the power supply and ground should be independent on one layer; The shorter the routing part, the better. 2. Add a large voltage stabilizing capacitor to the ground terminal of VLED and VCC. It is recommended that CP1 and CP2 be 1000 1500uf. 3. VLED and VCC are separated into different power supplies. 4. RC circuit can be added to the frequency input (clock) to reduce its peak value and reduce the impact on electromagnetic interference; It is recommended that RT
Related Description: OE: chip output enable pin. Snapdrivetm: a fast response driver chip developed and produced by Taiwan Xunjie Technology Institute. Its pins are fully compatible with tb62746 and other products.