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DS1090U-2 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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DS1090U-2 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 8 page Low-Frequency, Spread-Spectrum EconOscillator _____________________________________________________________________ 7 Detailed Description The DS1090 is a center-dithered, spread-spectrum sili- con oscillator for use as an external clock in reduced- EMI applications. With a combination of factory- programmed prescalers and a user-selected external resistor, output frequencies from 125kHz to 8MHz can be achieved. The output center frequency can be dithered by selecting the desired dither rate and ampli- tude with discrete inputs J0, J1, JC0, and JC1. The DS1090 contains four basic circuit blocks: master oscillator, factory-programmed prescaler, dither gener- ator, and the voltage-bias circuit that provides the feed- back path to the master oscillator for frequency control and dithering functions. Master Oscillator The master oscillator is programmable in the applica- tion by the use of an external resistor (RSET) tied to ground (GND). Resistor values of 45k Ω to 91kΩ vary the square-wave output frequency of the voltage-con- trolled master oscillator (fMOSC) from 8MHz down to 4MHz (see Figure 1). The master oscillator (Hz) frequency can be stated as Factory-Programmed Prescaler The prescaler divides the frequency of the master oscil- lator by 1, 2, 4, 8, 16, or 32 to generate the square- wave output clock (fOSC). This divisor is factory-set and is an ordering option. Dither Generator Spread-spectrum functionality is achieved by a user- configurable divider (determines dither rate), a triangle generator, and a user-configurable dither amplitude cir- cuit (see Block Diagram). The input to the triangle-wave generator is derived from the internal master oscillator and is fed through a user- configurable divider. The settings of control pins JC0 and JC1 determine this dither rate divisor setting (see Table 1), dividing the master clock by 4, 8, 16, or 32. The clock signal is further divided by 128 in the triangle-wave generator, which results in a triangle- wave signal of either 1/512th, 1/1024th, 1/2048th, or 1/4096th of the master oscillator (fMOD), depending upon the user’s divisor setting. The dithering frequency can be also expressed as the result of where Divisor is 4, 8, 16, or 32. f f Divisor MOD MOSC = ×128 f E sistor MOSC ≅ + 3 6461 11 . Re MASTER OSCILLATOR FREQUENCY vs. EXTERNAL RESISTOR SELECTION RSET RESISTANCE (k Ω) 90 80 70 60 50 4 5 6 7 8 9 3 40 Figure 1. Master Oscillator Frequency TIME DITHER AMOUNT (2, 4, OR 8%) IF DITHER AMOUNT = 0% (- 1, 2, or 4% of fMOSC) fMOD 1 (+ 1, 2, or 4% of fMOSC) Programmed fMOSC Figure 2. Center Frequency Dither Diagram JC1 JC0 DITHERING PERCENTAGE (fMOSC/n) DIVISOR SETTING 00 FMOSC / 512 4 01 FMOSC / 1024 8 10 fMOSC / 2048 16 11 fMOSC / 4096 32 Table 1. Dither Rate Divisor Settings |
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