how to switch VGA signal on laptop computer

Although the digital video switch has become very popular in 2008, but, as of 2015, the market will still be tens of millions of monitors, projectors analog VGA. Almost all laptops with docking station laptop computer through the bottom of a set of interfaces to switch VGA signals.

    Support the docking station and switch between VGA connector ideal for management control device not only to all the switches, but also for all the devices connected to the output to provide adequate ESD protection.

    MAX4885E VGA switch is optimized for notebook and docking station to switch between the signals, the device includes three-way high-frequency (about 950MHz) RGB switch, two low-frequency DDC signal switches and a pair of clamp for horizontal and vertical signal level conversion buffer.

    Optimization of switching VGA signal

    In order to correctly handle the analog VGA signals, seven signals should switch to the computer connector or a docking station port. If you do not use the switch, then connected to each port may not use the signals need to consider the increase in the DAC output capacitance will cause the bandwidth loss. A standard video signal is usually 75Ω, video DAC is usually a driver 75Ω load current source. Display is 75Ω load, thus forming a double-end load system, and DAC to 75Ω load to the maximum VGA driver is 0.7V.

    Many designers have tried to design to support 1920 × 1200,60 Hz video, pixel rate ≈ 6.0ns. If you allow the signal rise and fall are 1.7ns, the signal to meet the VESA (Video Electronics Standards Association) requirements, therefore:

    tR = 2.2RC, where R = 37.5Ω

    If the DAC's tR 0, can be drawn from the above equation system tR. However, the typical application of the DAC in the absence of additional load conditions, tR typically 1ns. Assuming DAC delay consists of two parts: internal delay and rise time due to the output capacitor. For these applications, assuming the internal DAC delay is 300ps, tolerance is 8pf. Coupled with the switch 8pf, the total capacitance of ≈ 16pF. So, for most such applications, traditional analog switches f3dB bandwidth of 400MHz, although the signal enough to support 140Msps, but the DAC occupies most of the system margin.

    MAX4885E VGA switch significantly reduces the number of components

    MAX4885E is a complete 1:2 VGA switching, RGB has more than 700MHz of bandwidth and less than 7pF capacitors. In addition, all outputs with ± 15kV HBM (Human Body Model) ESD protection, no external ESD protection circuit, not only cost savings, and eliminates the external capacitors used for ESD protection.

    MAX4885E has two important functions: to the low horizontal and vertical signals in accordance with the VESA standards to convert 5.0V TTL compatible logic-level (Figure 1). Provide horizontal and vertical signals drive two loads. As with the RGB signals, horizontal and vertical signals with ± 15kV ESD protection, without adding extra components to protect their output. The VL pin is connected to +3.3 V, horizontal and vertical signals from the lower level into TTL compatible levels.

Figure 1. Horizontal and vertical signal level converter schematic

    MAX4885E can also provide DDC (Display Data Control) switches, level clamping and ESD protection. By switching the DDC signals, separation capacitive load. These signals are compatible with I ² C standard and the maximum load capacitance is 700pF. If both the load connected to the same output, capacitive load may be out of range. MAX4885E only allowed to connect at the same time a load. Similar to other functions, DDC input also has a ± 15kV ESD protection.

    MAX4885E the DDC switch also provides an application for such additional functions: voltage clamp. DDC monitor the voltage from the 2.2kΩ resistor is usually pulled through the +5 V (Figure 2). DDC does not allow such a high drive voltage, typically only +3.3 V maximum voltage. DDC signal compatible with I ² C standard pull switch on both sides. Switch itself consists of a pair of n-channel transistors.

Figure 2. N-channel FET transistor schematic clamp

    Because the impact of the body diode, n-channel FET can only be within the gate voltage of about 0.7V. When the voltage approaches the gate voltage, the bias is not large enough, the channel resistance increases. The switch is pulled on both sides of the respective power supply voltage, n-channel almost completely clamp. Can transmit a signal close to ground potential, the switch is easy to drop down. If the +5 V side of the signal range: 0.5V to 4.8V, the 3.3V to 0.5V signal swing side to 3.3V, as shown above (left).
Conclusion

    As of 2015, will analog VGA signals for the mainstream computer, or even use a longer time. HD monitor required signal rise / fall time of less than 1.6ns. To achieve such a fast time, DAC output capacitance must be kept as small as possible. MAX4885E VGA switch with less than 7pF capacitance, providing a tiny 4mm × 4mm package. All output devices are ± 15kV ESD-protected, RGB, horizontal and vertical output and DDC signal not require additional protection circuitry. MAX4885E also increased the low-voltage logic level to TTL converter / buffer to further improve the functional requirements.  ESD protection has been buffered output can drive load ± 8mA, rise / fall time of less than 5ns. In short, MAX4885E can provide low capacitance switching, protection and ESD protection clamp. MAX4885E is ideal for notebook computer docking station switch, provides assessment (EV) board device easily test and program evaluation.cheap cell phones
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