CMOS Clock Generators Deliver Eight Outputs At Any Frequency

Dec. 14, 2010
New clock ICs can generate multiple clocks frequencies in a single chip.

Si5350/51 generators

Many electronic products utilize a clock generator, and many count on several, which can complicate design matters. Communications gear such as wireless gateways and femtocells and consumer products including HDTVs, DVD/DVR/Blu-ray units, and audio/video receivers require multiple clocks with unrelated frequencies. Often, this means multiple clock chips and plenty of design headaches. Silicon Laboratories has a solution via its Si5350/51 any-frequency octal clock generators.

The Si5350/51 generators (see the figure) are intended to replace multiple crystal oscillators, voltage-controlled crystal oscillators (VCXOs), and other clock ICs. Unlike traditional clock generators requiring a separate phase-locked loop (PLL) to synthesize each non-integer-related output frequency, the Si5350/51 synthesize a unique frequency on each of their eight output clocks. By integrating clock synthesis in the output divider stage rather than with the PLL, the Si5350 and Si5351 provide the equivalent clock synthesis capability of eight PLLs while dramatically reducing board space and power consumption. 

The Si5350/51 generate all system clocks (audio, video, interface, and system on chip) and can synthesize any combination of eight unique, non-integer-related frequencies from 8 kHz up to 133 MHz with 0-ppm frequency error. Jitter is less than 100 ps for each output. Three-output versions are also available. The Si5350/51 can simultaneously generate free-running clocks from a 25- to 27-MHz crystal input as well as clocks synchronized to a reference clock or analog control voltage input, allowing multiple board-level timing domains to be clocked from a single device. Spread-spectrum clock outputs are also an option.

The Si5350/51 simplify system-level design by providing glitch-free frequency shifting between two rates on up to six of the device outputs. This frequency-shifting technique simplifies clock synthesis in audio and video applications, which often require a combination of frequencies to support multiple formats and display standards. It also eases processor over-clocking to improve CPU performance in embedded applications and facilitates board-level power savings by enabling a dynamic reduction in output clock frequency.

Custom factory-programmed pin-controlled versions of the Si5350 are available using Silicon’s Web-based ClockBuilder configuration utility, which provides turnkey, application-specific clocks, eliminating the need for field programming hardware and software. The Si5351 versions are in-circuit configurable. Since the Si5350/51 do not require metal mask changes to customize clock frequencies like traditional clock ICs, lead times are less than two weeks for custom clocks. Silicon Labs also offers a field programming kit to enable rapid prototyping of the Si5350 and Si5351.

Samples of the Si5350/51 are available in space-saving 4- by 4-mm 20-QFN, 24-QSOP, and 10-MSOP packages. Production quantities are scheduled to be available at the end of the fourth quarter of 2010. Pricing ranges from $0.66 (three outputs) to $2.38 (eight outputs) in 10,000-unit quantities. The Si535x-20QFN-EVB and Si535x-24QSOP-EVB evaluation boards are now available for $150.

Silicon Laboratories

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