Headphone Audio Shares Connector With High-Speed USB Data

Dec. 15, 2006
Today's trend toward more functionality in portable products affects everything, including I/O ports. The FSA201 and FSA221 multimedia switches from Fairchild Semiconductor let designers run audio through their product's USB connector when it

Today's trend toward more functionality in portable products affects everything, including I/O ports. The FSA201 and FSA221 multimedia switches from Fairchild Semiconductor let designers run audio through their product's USB connector when it isn't being used for data. In that case, a cell phone could use either switch to supply audio to a headset or exchange data with a laptop via the same connector (see the figure).

Both products are MOSFET-based double-pole/double-throw (DPDT) analog switches, yet they differ in audio and data bandwidth. The FSA201 handles USB 1.1 full-speed ((12 Mbits/s) data, and the FSA221 handles high-speed ((480 Mbits/s) USB 2.0 data.

One of the larger challenges in handling USB 2.0 high-speed data is meeting the USB spec's low-jitter requirements. Testing involves a series of measurements on the interface's differential signaling lines over a single 488-bit data packet to demonstrate compliance with specifications for rise/fall times, under/overshoot, and jitter, as well as comparing test results with the USB 2.0 eye mask. The FSA221 achieves full compliance.

Both chips provide automatic VBUS detection when they encounter a live USB connection, but with user-override of the VBUS detection. Audio specs include wide USB 3-dB bandwidth: better than 720 MHz for the FSA221 and more than 250 MHz for the FSA201. The chips also can handle either standard ac-coupled signals and direct-coupled signals that swing both positive and negative.

Another advantage of the negative-swing capability is the elimination of audible pop transients when switching between inputs. Insertion losses are low (RON is 3), and inputs come with 10-kV electrostatic-discharge protection

The USB 1.1 device comes in a 1.6- by 2.1-mm MicroPak and costs $100 in 1000-unit quantities. The USB 2.0 high-speed device comes in a 1.4- by 1.8- by 0.55-mm UMLP and costs $1.25.

Fairchild Semiconductor
www.fairchildsemi.com

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