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 ().
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