In some of the embedded applications, we might have to switch between two USB ports. For this purpose a DPDT switch is used. Let us list down important criteria to select a DPDT switch to be used as USB switch.
- Band Width
- Bandwidth is the important criteria. For example, if you are using the switch for a USB for High Speed USB, the Band width must be 240 MHz
- Capacitance
- USB is a high speed interface. Signal Integrity is an important criteria for this interface. While selecting the switch it is important to note that switch must have low capacitance so that there are no signal integrity issues.
- ON Resistance
- Wherever we use a switch, the ON resistance is an important criteria. Lower the ON resistance lesser the voltage drop of the signal, lesser the power dissipation and designer need not bother about the thermal issues
- Protection
- USB switches shall have several protection mechanisms like the ESD protection, over voltage protection. While the ESD protection is understood, Voltage protection is important when high voltage is seen on the D+, D- pins due to unintentional short to any other higher voltage section
- Power consumption
- Power consumption is a problem to be dealt with in many applications. When selecting/using the USB switch, it is important to see that the power drawn by switch under normal conditions and leakage current are low. The pins of USB switch must be in high impedance when the power is OFF to help for low leakage current
- Supply Voltage
- While most of the USB switches operate within the given voltage range, the higher voltage rating is sometimes important depending on the power tree on your board.
- Propagation Delay
- The time taken for the output from the input is provided must be as small as possible. A Propagation delay in the ps range is required.
- Low crosstalk
- Check the specifications of the switch if there is crosstalk rejection. When the signal passes through switch there must be not be any additional noise added by the switch
Other important specifications include:
- ON Time of the switch
- OFF Time of the switch
- Break-Before-Make Delay of the switch
- Isolation specification
USB switches are generally used in:
- Tablets
- Mobiles
- Smart Watches
- Cameras
- USB hub
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