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15 /Feb / 2023How to Prevent Electromagnetic Interference for TFT Display?
If you have known liquid crystal display, you will know that LCD display TFT is actually a module, namely LCD module, mainly composed by LCD panel, backlight, all light membrane, row drive circuit and sequence control circuit (often called logic board). The called TFT-LCD interface is the interface connecting LCD module and mainboard, namely logic board's signal input interface.
However, for LCD display, TFT-LCD interface has many types including TTL(RGB), LVDS, EDP, RSDS, TMDS, etc. among which the front three kinds are used often. Now let's talk about features of LVDS interface and signal components.
The definition of LVDS interface is LVDS input end interface pin function of LCD side panel, which can be usually known by checking original data (often called screen specifications) of LCD display TFT. Although different LCD interface function has different English indicator, the function can be known according to key letters and numbers sequence, for example, a model of LCD is CLAA170EA02, the English abbreviation of interface table from factory shows two channel data with RXO and RXE, and each channel with 0-3 groups of data signal and 1 group of clock signal (RXOC or RXEC), therefore, this LCD liquid crystal display is a display with 30 needles and 8 locations, and pin function is obvious, namely RXO0- representing first group data 1-, RXO0 + representing first group data 1+; RXE0- representing second group data 1-, RXE0+ representing second group data 1+, and so on.
LVDS is abbreviation of Low Voltage Differential Signaling. LVDS overcomes shortcomings such as big energy consumption of TTL level way broadband transmission high bit rate data, EMI high electromagnetic interference, etc. It is a kind of digital video signal transmission mode. LVDS output interface uses very low voltage swing (about 350mV) to transmit data by difference at two PCB traces or a pair of balanced cable.
LVDS interface can make signals transmit at hundreds of megabits per second at difference PCB traces or balanced cable. By low voltage and undercurrent drive mode, it has advantages of low noise and low energy consumption. LVDS line of LCD TV uses balanced cable, actually twisted pair.
LCD display TFT decodes input signals such as TV, AV, etc. and gets RGB signals, the process is similar to regular TV, and then by RGB-LVDS transition, outputs LVDS signals, and sends to liquid crystal display. Since TFT only identifies TTL (RGB) signals of LCD, LVDS sent to LCD shall be decoded to gain TTL signals.
It thus can be seen that LVDS interface circuit includes two parts: one is main side panel's LVDS output interface circuit (LVDS sending end), the other is LCD side panel's LVDS input interface circuit (LVDS receiver). LVDS sending end turns TTL signals into LVDS signals, and sends signals, through flat cable or flexible cable (often called top screen lines) between drive board and LCD panel, to LVDS decoding IC of LCD side panel, turns serial signals into TTL level parallel signals, and then sends to sequential control and row drive circuits of LCD display TFT.
LVDS signal is mainly composed by data difference and clock difference. Suffix letters are M and P, separately represents normal phase and opposition, namely YX and YXP are a pair of differential line, and CLKOUT M and CLKOUT_P are also a pair of differential line.
Part of LVDS interface signal has set special function control signals, such as LVDS data format selection, display model control, frame rate selection, etc.