Electronic Design

Headphone Audio Shares Connector With High-Speed USB Data

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

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