Laptop VGA signal switching reference design

Overview

Since IBM developed the PC in 1987, analog VGA signals have been a part of the PC. Today, most business laptops require a docking station and projector. Almost all projectors have a VGA port, which is the most common way to connect to a laptop. Although digital connectivity methods such as DVI™ and HDMI™ continue to emerge, most projectors still only support VGA.

The need for VGA support will continue for many years before the digital standard can completely replace the blue VGA connector commonly used in laptops. The MAX4885E low-capacitance VGA switch introduced by Maxim is designed for this switching function.

The MAX4885E has nearly zero current consumption and is integrated in a 4mm × 4mm package. The device contains most of the switches and active components required for a discrete solution. With ±15kV HBM (Human Body Model) ESD protection on all outputs, designers can eliminate a large number of ESD protection components, thereby reducing costs and saving board space.

MAX4885E optimized for VGA switching

RGB switch

RGB switches require a wider frequency band. The MAX4885E contains 3 SPDT switches, with a bandwidth of greater than 900MHz under a 50Ω load; and a bandwidth of greater than 600MHz under a 75Ω load commonly used in video systems. The QSXGA format (2560 × 2048) requires a bandwidth of approximately 500MHz to ensure that the third harmonic is passed and the waveform quality is maintained. Some designers use traditional "bus switches" with 12pF capacitance, while the MAX4885E's capacitance is only 6pF. In addition, traditional bus switches also require ESD protection diodes, which further reduces bandwidth and increases system cost.

DDC switch

The MAX4885E also uses DDC switches, using a pair of SPDT n-channel FETs to switch the SDA and SCL signals. By controlling the switching output, the system is connected only to the monitor used. Since only one device is connected at a time, switching outputs further reduces the capacitance of the DDC circuit. In addition, all outputs have ±15kV (HBM) ESD protection, eliminating the need for additional ESD protection diodes. The FET gate switches to the V _L level, which is the same voltage as the GPU's I/O voltage (2.5V to 3.3V). The DDC signal is actually an I²C signal and requires pull-up resistors on both sides of the switch. The signal connected to the monitor may be as high as 5.5V, requiring protection and level shifting of the GPU. By biasing the FET gate to the same voltage as the GPU, the FET provides effective protection to the GPU when the signal exceeds the V _L level¹. Using two SPDT n-channel FETs, the GPU has only one capacitive load to avoid damage caused by high voltage and ESD strikes.

Horizontal and vertical sync level shifting and buffering

The horizontal synchronization and vertical synchronization electrical signals of the GPU need to be converted into TTL level signals. The monitor's pull-up pulls these signals to +5.5V. The MAX4885E has a pair of level-shifting buffers that receive signals from 0.8V to 2V and then convert them to TTL outputs; the device is capable of providing ±8mA current drive to meet VESA specifications. The output is referenced to 5V, so there is no voltage compatibility issue. The row and field outputs are ±15kV (HBM) ESD protected, requiring no additional ESD protection diodes.

Integrated LC filter for harmonic stabilization

The MAX4885E integrates switches, FETs, and buffers in a tiny 4mm × 4mm TQFN package for switching VGA signals. However, many systems require bandwidth-limited filters to filter out harmonics and reduce emissions. The MAX4885E does not include such a filter. Passive components are too bulky, and active filters consume considerable current. If three operational amplifiers/filters are integrated inside the MAX4885E, the current consumed will reach 100mA - it is difficult for a notebook computer to accept such high losses. Therefore, an LC filter is used inside the device, which can achieve the same function without consuming current. The MAX4885E consumes less than 5µA in idle mode, and the horizontal and vertical synchronization signal buffers consume several mA when driving a monitor.

High integration reduces component count

As can be seen from Table 1 , MAX4885E can replace up to 14 standard devices. It is worth emphasizing that the package size of MAX4885E is only 16mm².