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How to Switch VGA Signals in a Laptop

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How to Switch VGA Signals in a Laptop

Analog VGA signals have been part of the PC world since IBM introduced PCs in 1987. Today most business-oriented laptops need to work with a docking station and with the vast number of existing projectors. Nearly all projectors have a VGA port which is the only common way for a typical user to hook up a laptop. Although digital connections such as DVI and HDMI are appearing, the vast majority of projectors still only support VGA.

DDC Switching
DDC switching is also done on the MAX4885E, which uses a pair or SPDT n-Channel FETs to do the switching for the SDA and SCL. By actually switching the outputs, the system can only hook up to the monitor in use. Switching the outputs further reduces the capacitance that the DDC circuit will see, since only one device is connected at a time. In addition, all outputs are again protected to ±15kV (HBM), so no additional ESD diodes are needed. The gate of the FET is switched to a voltage level, VL. This voltage is normally the same as the GPU I/O (2.5V to 3.3V). The DDC signals are actually I²C signals, with pullup resistors on both sides of the switch. Since the signals going to the monitor can be as high as 5.5V, the GPU needs to be protected and level shifted.

RGB Switching
RGB switching requires high-bandwidth switches. The MAX4885E contains three SPDT switches that exhibit >900MHz bandwidth at 50W, and > 600MHz at the more common 75W used for video. The QSXGA format (2560 x2048) requires ~500MHz of bandwidth so that the third harmonic is passed and the quality of the waveform is preserved. Some designers use the older "bus switches" with 12pF of capacitance which compares to the 6pF for the MAX4885E. Those older bus switches, moreover, need ESD diodes which reduce the bandwidth further and add cost to the system.

 

The circuit in Figure 1 shows the MAX4885E used in a docking station application for a laptop. All the critical components are present. All ESD concerns are managed, and only one control bit is required to select the dock vs. an internal connector. The circuit only draws a few μA at idle and a few mA to supply the horizontal and vertical buffers.

max4885e_schematic
Application circuit for a VGA connection between a laptop and docking station features the MAX4885A
VGA switch

 

Horizontal and Vertical Level Translation and Buffering
Horizontal and vertical synchronization signals are required to interface the GPU to full TTL-level signals. Pullups on the monitor can, in fact, pull these signals to +5.5V. The MAX4885E has a pair of level-translating buffers that take a signal between 0.8V and 2V and translate it to a full TTL output; the device can supply ±8mA, which meets the VESA specification. The output is referenced to 5V, so there is no issue with voltage compatibility. Again the horizontal and vertical outputs have ±15kV (HBM) ESD protection, so no added diodes are needed.

Integrated LC Filter for Harmonic Stability
The MAX4885E integrates all the key switches, FETs, and buffers typically used for VGA switching into a tiny 4mm x 4mm TQFN package. However, many systems require some form of bandwidth limiting filter to prevent harmonics from radiating. The MAX4885E did not include any filtering for that function. Passive component values would be too large, and an active filter would require considerable current. While the MAX4885E could have integrated a triple-amplifier/filer, that would have made the device draw as much as 100mA—too much to be tolerated in a laptop. Instead, the device’s LC filter draws no current and accomplishes the same task. The MAX4885E draws < 5μA at idle and a few mA when driving the monitor for the horizontal and vertical buffers.

Components Eliminated with the MAX4885E
Components Eliminated with the MAX4885E

http://pdfserv.maxim-ic.com/en/an/AN4370.pdf

Read the Italian version: Come cambiare i segnali VGA in un portatile

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