NX USB switches device highlight
USB switches are typically used in devices to connect, route, and switch USB data signals between a host and peripherals. USB switches play an important role in overall system performance, relying upon data transfer speed and signal quality to ensure reliable operation between the host and the device. On resistance, switch capacitance, propagation delay, bandwidth, power consumption, and supply voltage are some of the key parameters of USB switches used when selecting a switch.
NXP leverages its high performance mixed signal capabilities to offer faster USB switches with lower RON, higher bandwidth, and wider supply voltage. NXP's USB switches are designed into major smart phone and handheld computer manufacturers and offer excellent performance at a competitive price.
NXP's USB Switches
The NX3DV42 and NX3DV221 are high-bandwidth DPDT switches designed for switching high-speed and full-speed USB signals. These switches can multiplex differential outputs from a USB host device to one of two corresponding outputs. Select and enable control pins provide design flexibility. With bandwidth as high as 1GHz, USB 1.1 and USB 2.0 data signals can be switched with minimal distortion and phase noise. USB signal switching calls for as low on resistance and capacitance as possible while keeping the bandwidth high enough to pass the data signals without attenuation. The characteristics of the two switches in the DPDT also need to be matched to ensure the data line symmetry.
Both NXP USB switches are available in extremely small 10-pin, leadless QFN packages to minimize board space and cost. Designed with robust input structures, the NX3DV42 and NX3DV221 provide ±12kV HBM (Human Body Model) ESD (electrostatic discharge) protection.
The primary difference between NXP's NX3DV42 and NX3DV221 is that the NX3DV221 includes a charge pump for a higher bandwidth (1GHz). Additionally, the NX3DV221 offers an over-supply voltage tolerance up to 5.5V. The NX3DV221 targets TI's TS3USB221A, which is designed for desktop/laptop applications. The NX3DV42 targets Fairchild's FSUSB42 which is designed for portable applications such as mobile phones with a much lower ICC(ON). While the pinout differs between the two USB switches, each offers identical pinout compared to its primary competition for easy drop-in replacement.
NXP's USB switches give you faster time-to-market through improved design flexibility from their superior high bandwidth, increased supply voltage, and best-in-class ESD protection. The superior design can compensate for other design deficiencies or allow more robust operation under challenging enrivonmental conditions. Additionally, NXP's USB switches are the choice of major OEMs due to our competitive pricing and superior supply. NXP is the largest volume supplier of logic in the world and can meet the most demanding supply requirements. NXP's USB switches are produced in multiple sites for assured supply, and our outstanding quality makes us a top automotive supplier.
With a typical on resistance of 4Ω, both the NX3DV42 and NX3DV221 result in minimal signal loss in a USB path with differential driver and receiver with terminations. Variation of RON with supply, temperature, and switch voltage is kept at a minimum for both switches so that the change in amplitude at receiver is also minimal. Typical on resistance flatness of 0.7Ω for these switches results in output rise and fall times that are very close to each other, which is critical for data integrity.
Capacitance of a switch in the on state is critical for switching high speed USB signals. The on capacitance of a switch slows down the signals and results in degradation. NXP offers lower typical on state capacitance of 6pF to ensure better signal integrity for USB data signals.
The USB switch should add negligible delay to the data signal passing through it. Since there are no buffers involved in the switching path, the USB switch can be modeled as a series resister, and the only delay should be the time taken by signal to get to the die and out again on the board. With 0.25ns of typical delay, NXP's USB switches are the ideal choice for switching the USB data without adding any additional latency in system.
The combination of on resistance and capacitance of the USB switch determines the signal bandwidth. On resistance is inversely proportional to the size of switch, so, with a larger die, the on resistance is lower, but at the same time, on capacitance increases with the larger die. NXP offers excellent bandwidth of up to 1GHz for USB switching and, at the same time, keeps the size of switch very small by using the innovative design techniques like a charge pump unlike some competitors.
USB 2.0 (Hi-Speed USB) requires a data rate of 480Mbps and that the system passes an eye diagram test to assure compliance. The eye diagram simultaneously displays the D+ and D- traces from a valid source, and resembles a human eye. An eye diagram shows the rise and fall times of data signals along with the amplitude, jitter, skew, overshoot, and undershoot of signals. Figure 1 shows the 480Mbps D+ and D- signals in a typical system without a switch in the path. It can be seen that the signals are well outside the restricted area represented by an eye-like mask, with a typical eye opening of ±400mV.
Figure 1: Typical Eye-Pattern for 480 Mbps USB Signals without Switch
If we assume that the system has a switch on board and a connector at the output, then the signal must be outside the eye as required to pass the signals without timing errors or information loss. On a system board layout, the thickness and length of traces along with the spacing between them determines the impedance seen by differential data signals and affects the rise and fall times, overshoots, and undershoots seen in the eye diagram.
Bandwidth of the system is a key parameter that affects the eye diagram. The sloped lines, left and right, give the permissible rise and fall times for the system. The D+ (non-inverted data) signal has to pass without touching the eye mask in middle. The -3dB bandwidth of NXP's USB switches is near 1GHz and typical output skew is about 0.1ns. This provides generous room to keep the differential data signals out of the switch mask, when the switch is inserted as a 1-to-2 demultiplexer with a single host or as a 2-to-1 multiplexer with two hosts. Figure 2 shows a typical eye pattern for 480 Mbps USB 2.0 signals when passed through the normally closed channel of an NXP USB switch.
Figure 2: Typical Eye-Pattern for 480 Mbps USB Signals through the Normally-Closed Path of the NXP USB Switch
With low cross-talk (-40dB) and low on resistance mismatch (0.1Ω) between the switches, the eye pattern for the normally open channel is shown in Figure 3, which is quite similar to the pattern seen in Figure 2.
Figure 3: Typical Eye-Pattern for 480 Mbps USB Signals through the Normally-Open Path of the NXP USB Switch
NXP's USB switches are ideal for any application that includes differential USB signals and the port. USB port isolation, connector sharing, and signal multiplexing are some of the most common uses for USB switches. They are often used to share or route USB ports and to isolate flash memory, media players, digital cameras, and other removable devices. The switches are designed to be compliant with USB 2.0 Full Speed (FS) and Hi-Speed (HS), with bandwidths in excess of 900MHz.
Figure 4 shows how a phone's USB connection can be multiplexed by using a USB switch in a smart phone. Increasing resolutions of LCD panel displays and cameras in smart phone designs generate a requirement for larger storage devices, such as embedded storage or external, reduced-size memory cards. Most cell phones use standalone hard-drive controllers with a USB interface to communicate with a PC host. When a hi-speed I/O port for the baseband processor is also used for synchronizing address books or other data, sharing a single USB port becomes a challenge. The design is simplified by multiplexing the phone's USB connection.
Figure 4: Switching between Wireless and Storage in a USB Adapter
One of the most common requirements is in switching between different data standards, for example, between a UART and a USB interface. Handset manufacturers like to retain the capability of offering a choice of data-transmission standards to their customers, but they cannot afford the area needed for a separate connector for each interface. The easiest solution is to multiplex a number of pins on a common connector. An example of this is shown in Figure 5:
Figure 5: UART and USB Port Sharing
All NX USB Switches
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Below is a brief overview video of our NX3DV42 USB switch.
NXP makes a small evaluation board featuring the NX3DV221 USB switch. The video below demonstrates this board and the performance of the NX3DV221 USB switch.
To obtain the NX3DV221 evaluation board, please contact your local sales office for further details.
NX3DV42 Competitive Comparison
For the NX3DV42, the major competition is Fairchild's FSUSB42 USB switch. With a higher bandwidth of 950MHz, NXP's NX3DV42 offers better signal integrity and eye test pattern as compared to Fairchild's FSUSB42 which has a lower bandwidth of 720MHz. Also, the wider supply voltage range (1.65V to 4.3V) for our NX3DV42 offers flexibility in system design as compared to the FSUSB42 which has a more-limited supply voltage range of 3.0V to 4.3V. The table below compares key parameters of the NX3DV42 and major competitors, including the FSUSB42. The NX3DV42 is an ideal drop-in replacement for Fairchild's part and is offered by NXP, the largest logic volume supplier in the world, and at competitive pricing.
NX3DV221 Competitive Comparison
The main competitor for NXP's NX3DV221 switch is TI's TS3USB221A. The key advantage of NX3DV221 is the higher bandwidth of up to 1GHz. The table shows the comparison of key electrical parameters along with package types between the NX3DV221 and similar switches from the competition. NXP's NX3DV221 is comparable to or beats the specifications of these switches with excellent pricing and a multi-sourced, high-volume supply advantage.