Switch VGA signal in laptop
Abstract: This article introduces a solution that uses the MAX4885E low-capacitance VGA switch to implement video switching between a laptop and a docking station. The current consumption of the MAX4885E is almost zero, integrated in a 4mm × 4mm package. The device contains most of the switches and active components required for discrete solutions, thereby saving costs and reducing the number of components required. All outputs of the device have ± 15kV Human Body Model (HBM) ESD protection.
The ideal device that supports switching between the docking station and the VGA connector must not only manage and control all switches, but also provide sufficient ESD protection for all devices connected to the output.
The MAX4885E VGA switch is optimized for signal switching between laptops and docking stations. The device includes three high-frequency (approximately 950MHz) RGB switches, two low-frequency DDC signal clamp switches, and a pair of line and field signal levels Converted buffer.
Optimizing VGA signal switching In order to properly handle analog VGA signals, seven signals should be switched to the computer's connector or docking station port. If no switch is used, then each port may be connected to an unused signal, and the increased capacitance at the output of the DAC needs to be considered, which will cause bandwidth loss. The video signal is usually a standard 75Ω, and the video DAC is usually a current source driving a 75Ω load. The display is also a 75Ω load, which forms a dual-terminal load system, and the maximum VGA driver that the DAC can provide for a 75Ω load is 0.7V.
Many designers have tried to design videos that support 1920 × 1200, 60 Hz, and the pixel rate ≈ 6.0 ns. If the rising and falling edges of the signal are allowed to be 1.7ns, the signal can meet the requirements of VESA (Video Electronics Standards Association), so:
tR = 2.2RC, where R = 37.5Ω
If tR of DAC is 0, the tR of the system can be obtained from the above formula. However, the DAC in a typical application without additional load, tR is generally 1ns. Suppose that the delay of the DAC consists of two parts: the internal delay and the rise time caused by the output capacitor. For the above application, it is assumed that the internal delay of the DAC is 300ps, and the value is 8pf. Adding 8pf of the switch, the total capacitance is ≈16pF. Therefore, for most of these applications, the f3dB bandwidth of the traditional analog switch is 400MHz. Although it is sufficient to support a 140Msps signal, the DAC occupies most of the margin of the entire system.
The MAX4885E VGA switch greatly reduces the number of components. The MAX4885E is a complete 1: 2 VGA switch with an RGB bandwidth greater than 700MHz and a capacitance less than 7pF. In addition, all outputs are equipped with ± 15kV HBM (Human Body Model) ESD protection. No additional ESD protection circuit is required, which not only saves costs, but also eliminates external capacitors used for ESD protection.
The MAX4885E also has two important functions: it can convert low-level line and field signals to 5.0V TTL compatible logic levels according to the VESA standard (Figure 1). The provided line and field signals can drive dual loads. Like RGB signals, the line and field signals are also protected with ± 15kV static electricity, and no additional components are needed to protect their output. By connecting the VL pin to + 3.3V, the line and field signals can be converted from lower levels to TTL compatible levels.
Figure 1. Schematic diagram of horizontal and vertical signal level conversion
The MAX4885E can also provide DDC (display data control) switches, level clamping, and ESD protection. By switching the DDC signal, the capacitive load is separated. These signals are compatible with I²C standards and have a maximum load capacitance of 700pF. If two loads are connected to the same output, the capacitive load may be out of range. The MAX4885E allows only one load to be connected at a time. Similar to other functions, the DDC input also has ± 15kV ESD protection.
The MAX4885E's DDC switch also provides an additional function for this type of application: voltage clamping. The DDC voltage from the display is usually pulled up to + 5V through a 2.2kΩ resistor (Figure 2). The DDC driver does not allow such a high voltage, and generally only allows a maximum voltage of + 3.3V. The DDC signal is compatible with the I²C standard, with pull-up resistors on both sides of the switch. The switch itself consists of a pair of n-channel transistors.
Figure 2. Schematic of n-channel FET transistor clamping
Due to the influence of the body diode, the n-channel FET can only pass the gate voltage within about 0.7V. When the voltage is close to the gate voltage, the channel resistance increases because the bias voltage is not large enough. When both sides of the switch are pulled up to their respective supply voltages, the n-channel is almost completely clamped. It can transmit signals close to ground potential, and the switch can be pulled down easily. If the signal range on the + 5V side is: 0.5V to 4.8V, the signal swing on the 3.3V side is 0.5V to 3.3V, as shown above (left side).
Conclusion As of 2015, analog VGA signals will still be used in mainstream computers, even for a longer time. HD monitors require a signal rise / fall time of less than 1.6ns. In order to achieve such a fast time, the DAC output must maintain as small a capacitance as possible. The MAX4885E VGA switch has a capacitance of less than 7pF and is available in a tiny 4mm × 4mm package. All outputs of the device have ± 15kV ESD protection, and no additional protection circuits are needed for RGB, line, field output terminals and DDC signals. The MAX4885E also adds a low-voltage to TTL logic level converter / buffer to further improve the functional requirements. Line and field signals can be converted from approximately 0.5V to 1.5V to 5V TTL logic level. The output with electrostatic protection can provide ± 8mA drive for the load after buffering, and the rise / fall time is less than 5ns. In short, the MAX4885E can provide low capacitance switching, clamp protection, and ESD protection. The MAX4885E is ideal for docking switches in notebook computers, and provides an evaluation (EV) board to facilitate device testing and program evaluation.
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