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Programmable octal CMOS clock generator IC produces (almost) any frequency, replaces more complex PLL approach

Clock signals for your system: you can never have enough of them, or if you do, they aren't quite the right ones or don’t have the glitch or jitter performance you need. The CMOS Si5350 and Si5351 CMOS ICs from Silicon Laboratories aim to change that situation, by providing octal clock generators with on-chip voltage-controlled crystal oscillators (VXCOs) to synthesize a unique frequency on each output.

Unlike conventional designs which require a separate phase lock loop (PLL) to synthesize output frequencies with non-integer-related outputs, these ICs can synthesize a unique frequency on each output. They do this by integrating the clock synthesis in the clock's output-divider stage rather than the PLL, thus eliminating the need for eight PLLS while providing equivalent clock synthesis capability.

The ICs can develop any combination of eight non-related frequencies from 8 kHz to 133 MHz with 0 ppm frequency error. They target consumer applications which require a plethora of clocks: DVRs, HDTV, gaming consoles, set-top boxes, printers, projectors, blade servers, RAID systems, and femtocell/premise telecom systems. By producing so many frequencies within one IC and with no PLL “farm”, the bill of materials, footprint, and cost are significantly reduced, while power dissipation is cut as well. The ICs can simultaneously provide free-running clocks using a crystal input as well as clocks sync'ed to a reference clock or an analog control-voltage input.

With the clock ICs, a proprietary phase-error cancellation circuit minimizes jitter to under 100 ps, and an (optional) spread spectrum function can be invoked for each output to spread EMI and reduce regulatory concerns (-0.1 to -2.5% down; ±0.1 to ±1.5% center). An output can be switched between output frequencies without glitches, critical for use with audio DACs during sampling-rate changes (that “popping” sound), for example.

Programmability of the Si5350 is via pin control, while the Si5351 is programmable via an I2 C interface. The highly linear VXCO function can be pulled from 10 to 90% of VDD (and eliminates the need for special so-called “pullable” crystals), in contrast to varactor-based approaches which usually can be pulled only across 0.5 or 1 V.

For clock customization, Silicon Labs offers an online ClockBuilder application, which automatically creates Si5350 part numbers; the vendor claims users will get custom, factory-programmed samples in two weeks (far faster than mask-programmed customized clocks). They also offer a USB-based development kit with on-board IDD measurement and independent VDD s to support mixed-signal applications using 2.5 and 3.3 V rails. Operating range is -40 to +85 °C

Price and availability : The clock ICs are housed in 4×4 mm 20-lead QFN, 24-lead QSOP, and 10-lead MSOP packages. They are sampling now, with full production and the end of 2010. Prices span $0.66 (3-output version) to $2.38 (eight outputs) in 10K volumes. The evaluation boards are $150.

For more information : Silicon Laboratories, http://www.silabs.com/pr/clocks .

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