Display LVDS interface technology principle and detailed introduction

    After the development of Shenzhen Hongjia Technology Research and Development Department, our company has mastered mature LVDS LCD screen technology. Currently, there are 2.6-inch LVDS screens with a resolution of 800*480 and 7-inch LVDS screens with a resolution of 1024*600 in mass production. And 8-inch LVDS and 10.1-inch LVDS. Mainly used in industrial control and industry customization customer groups.
LVDS technical principle and detailed introduction
     With the increasing popularity of the Internet, all kinds of communication devices are becoming more and more popular among consumers, which leads to a sharp increase in the demand for data transmission. In addition, digital TV, high-definition TV, and color images all require higher bandwidth. Therefore, system design engineers must rely on analog technology to design circuit systems and support data transmission. Low-voltage differential signaling (LVDS for short) is one such analog technology that engineers can use to design mixed-signal systems. LVDS uses high-speed analog circuit technology to ensure that copper wires can support data transmission above gigabits.
   1 Introduction to LVDS
LVDS (Low Voltage Differential Signaling) is a low-swing differential signal technology that enables signals to be transmitted at a rate of several hundred Mbps on differential PCB pairs or balanced cables. Its low-voltage amplitude and low-current drive output achieve low noise and low power consumption.
For decades, the use of a 5V supply has simplified the interface between logic circuits of different technologies and vendors. However, with the development of integrated circuits and the requirement for higher data rates, low-voltage power supply has become an urgent need. Reducing the power supply voltage not only reduces the power consumption of high-density integrated circuits, but also reduces the heat dissipation inside the chip, which helps to improve the integration level.
LVDS receivers can tolerate at least ±1V variations in ground voltage between the driver and receiver. Since the typical bias voltage of the LVDS driver is +1.2V, the sum of the voltage variation of the ground, the driver bias voltage, and the noise coupled in lightly, is a common-mode voltage at the receiver input with respect to the receiver ground. This common mode range is: +0.2V～+2.2V. The suggested input voltage range of the receiver is: 0V～+2.4V.
   2 Design of LVDS system
The design of LVDS system requires that the designer should have experience in ultra-high-speed single-board design and understand the theory of differential signaling. It is not very difficult to design a high-speed differential board. The following will briefly introduce the points of attention.
2.1 PCB board
(A) Use at least 4 layers of PCB (from top to bottom): LVDS signal layer, ground layer, power layer, TTL signal layer;
(B) Isolate the TTL signal and the LVDS signal from each other, otherwise TTL may be coupled to the LVDS line, it is best to put the TTL and LVDS signals on different layers separated by power/ground;
(C) Locate the LVDS driver and receiver as close as possible to the LVDS end of the connector;
(D) Use distributed multiple capacitors to bypass LVDS devices, with surface mount capacitors placed close to the power/ground pins;
(E) The power layer and the ground layer should use thick lines, do not use 50Ω wiring rules;
(F) Keep the PCB ground plane return path wide and short;
(G) The ground planes of the two systems should be connected by cables utilizing ground return copper wires (gu9ound return wire);
(H) Use multiple vias (at least two) to connect to the power plane (line) and ground plane (line), and surface mount capacitors can be soldered directly to the via pads to reduce wire stubs.
2.2 Wires on board
(A) Both microstrip and stripline have good performance;
(B) Advantages of microwave transmission lines: generally have higher differential impedance and do not require additional vias;
(C) Stripline provides better shielding between signals.
2.3 Differential lines
(A) Use controlled impedance lines that match the differential impedance and termination resistance of the transmission medium, and make the differential line pairs as close as possible to each other (less than 10mm) immediately after leaving the integrated chip, which can reduce reflections and ensure coupling The noise received is common mode noise;
(B) Match the lengths of the differential line pairs to reduce signal distortion and prevent electromagnetic radiation from causing phase differences between signals;
(C) Do not rely solely on the autorouting function, but carefully modify it to achieve differential impedance matching and achieve isolation of differential lines;
(D) Minimize vias and other factors that cause line discontinuity;
(E) Avoid 90° traces that will cause resistance discontinuity, and use arcs or 45° folded lines instead;
(F) Within a differential pair, the distance between the two wires should be as short as possible to preserve the common-mode rejection of the receiver. On the printed board, the distance between the two differential lines should be as consistent as possible to avoid discontinuity in the differential impedance.
2.4 Terminal
(A) Use terminal resistors to achieve the maximum match to the differential transmission line. The resistance value is generally between 90 and 130Ω, and the system is also
This termination resistor is required to generate a differential voltage for proper operation;
(B) It is best to use a surface mount resistor with an accuracy of 1 to 2% to connect the differential line. If necessary, you can also use two resistance values of
50Ω resistor with a capacitor in between to ground to filter out common mode noise.
2.5 Unused pins
All unused LVDS receiver input pins are floating, all unused LVDS and TTL output pins are floating, and unused TTL transmit/driver input and control/enable pins are connected to power or ground.
2.6 Media (cable and connector) selection
(A) Using a controlled impedance medium, the differential impedance is about 100Ω, and no large impedance discontinuity will be introduced;
(B) Balanced cables (such as twisted pairs) are generally better than unbalanced cables simply for reducing noise and improving signal quality;
(C) When the cable length is less than 0.5m, most cables can work effectively. When the distance is between 0.5m and 10m, CAT
3 (Categiory 3) Twisted-pair cables are effective, cheap and easy to buy. When the distance is greater than 10m and high speed is required, it is recommended to use CAT 5 twisted-pair cables.