LMX2471

The LMX2471 is a low power, high performance delta-sigmafractional-N PLL with an auxiliary integer-N PLL. The deviceis fabricated using National Semiconductor's advanced BiC-MOS process.With delta-sigma architecture, fractional spur compensationis achieved with noise shaping capability of the delta-sigmamodulator and the inherent low pass filtering of the PLL loopfilter. Fractional spurs at lower frequencies are pushed tohigher frequencies outside the loop bandwidth. Unlike ana-log compensation, the digital feedback techniques used inthe LMX2471 are highly resistant to changes in temperatureand variations in wafer processing. With delta-sigma archi-tecture, the ability to push close in spur and phase noiseenergy to higher frequencies is a direct function of the modu-lator order. The higher the order, the more this energy can bespread to higher frequencies. The LMX2471 has a program-mable modulator up to order four, which allows the designerto select the optimum modulator order to fit the phase noise,spur, and lock time requirements of the system.

Programming is fast and simple. Serial data is transferredinto the LMX2471 via a three line MICROWIRE interface(Data, Clock, Load Enable). Nominal supply voltage is 2.5 V.

The LMX2471 features a typical current consumption of 5.6mA at 2.5 V. The LMX2471 is available in a 24 lead 3.5 X 4.5X 0.6 mm package.