News and New Products

CMOS-clock chip replaces crystal, PLL circuit

By Margery Conner, Technical Editor -- EDN, 4/22/2008

Stable, accurate reference frequencies are basic to almost all digital electronics. The most common references combine quartz crystals with clock-generator circuits, often employing a PLL (phase-locked loop). However, temperature changes can affect the bulky, multimodule crystal-PLL circuits, which are less accurate at higher frequencies. To address these problems, Mobius Microsystems aims to obsolete the venerable crystal-oscillator frequency reference with the new MM8511 spread-spectrum clock IC, which the company based on its CHO (CMOS-harmonic-oscillator) technology. The MM8511 CHO replaces the quartz crystal and PLL with a monolithic CMOS die, which generates spread-spectrum clock frequencies without the need for an external resonator.

The device uses an LC-harmonic-oscillator circuit that relies on a high-precision inductor and multiple high-precision capacitors. The chip employs a real-time analog algorithm to monitor the temperature, voltage, and process variables. As any of the parameters vary, the algorithm can switch in a capacitive value to compensate for circuit variables in real time, not only allowing for highly accurate frequencies, but also minimizing phase shift and jitter.

The MM8511 operates from a 3.3V supply and can generate frequencies as high as several hundred megahertz; Mobius has factory-programmed the initial products at common interface frequencies of 10 to 100 MHz and spread-spectrum modulation of 0 to 6%. The MM8511 comes in proprietary eight-pin, 3×3-mm DFN packages and nine-pin, 1.6×1.6-mm CSPs, as well as an eight-pin, 3×6.4-mm TSSOP drop-in replacements for common spread-spectrum PLL ICs. The evaluation board for MM8511 will be available this month, and production samples of the product will be available in July. Price for the chip is $1.35 (1000).