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( File format: PDF, 249037 )
Resistor Set Oscillator with Spread Spectrum Modulation
Linear Technology Inc.

Description
The LTC6908 is an easy-to-use precision oscillator that provides 2-outputs, shifted by either 180 or 90. The oscillator frequency is programmed by a single external resistor (RSET) and spread spectrum frequency modulation (SSFM) can be activated for improved electromagnetic compatibility (EMC) performance. The LTC6908 operates with a single 2.7V to 5.5V supply and provides rail-to-rail, 50% duty cycle square wave outputs. A single resistor from 10k to 2M is used to select an oscillator frequency from 50kHz to 10MHz (5V supply). The oscillator can be easily programmed using the simple formula outlined below: fOUT =10MHz ? 10k/RSET The LTC6908s SSFM capability modulates the output frequency by a pseudorandom noise (PRN) signal to decrease the peak electromagnetic radiation level and improve EMC performance. The amount of frequency spreading is ? xed at 10% of the center frequency. When SSFM is enabled, the rate of modulation is selected by the user. The three possible modulation rates are fOUT/16, fOUT/32 and fOUT/64.
Features
LTC6908-1: Complementary Outputs (0/180) LTC6908-2: Quadrature Outputs (0/90) 50kHz to 10MHz Frequency Range One External Resistor Sets the Frequency Optional Spread Spectrum Frequency Modulation for Improved EMC Performance 10% Frequency Spreading 400?A Supply Current Typical (V+ = 5V, 50kHz) Frequency Error 1.5% Max (TA = 25C, V+ = 3V) 40ppm/C Temperature Stability Fast Start-Up Time: 260?s Typical (1MHz) Outputs Muted Until Stable Operates from a Single 2.7V to 5.5V Supply Available in Low Pro?le (1mm) ThinSOT and DFN (2mm 3mm) Packages




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