AN3692| Application Note
Maxim/Dallas > App Notes > CLOCK GENERATION AND DISTRIBUTION OSCILLATORS/DELAY LINES/TIMERS/COUNTERS POWER-SUPPLY CIRCUITS PROTOTYPING AND PC BOARD LAYOUT SIGNAL GENERATION CIRCUITS Keywords: EconOscillators, DS1090, oscillator
Nov 18, 2005
APPLICATION NOTE 3692
Frequency Calculator for the DS1090
This application note presents an easy-to-use frequency calculator for determining the proper ordering option, discrete bias resistor selection, and input settings for the DS1090 low-frequency, resistor-programmable, spread-spectrum EconOscillatorTM. The DS1090 is capable of generating a spread-spectrum, square wave output between 125kHz and 8MHz. This spread-spectrum system function is important when EMI is a concern for the end application, such as a switched-mode power supply.
Need for the Calculator
The data sheet for the DS1090 shows: 1) how to select the proper ordering part number, 2) how to manually calculate the external bias resistor value necessary to achieve the desired output frequency, and 3) how to set the external pin connections to achieve the desired output modulation. The Frequency Calculator, however, is an intuitive, fast method for the user to experiment and reach an optimal solution with minimum effort.
The Calculator
The DS1090 Frequency Calculator, which can be downloaded from our web site, was developed using Excel 97 and should function on any platform with Excel 97 (or any newer version). A screen shot of the calculator is shown in Figure 1.
User Inputs
The user must input, at the very minimum, the Item 1 value. 1. The first input is the Desired Output Frequency (fOUT), which is entered in kHz. Allowable frequencies range from 125.0kHz to 8000.0kHz. An error message will appear if an incorrect value is entered. The Frequency Calculator will then present the DS1090 ordering information (specifying the correct divisor), Master Oscillator Frequency in kHz, and bias resistor (RSET) value necessary to achieve the desired output center frequency. The Master Oscillator Frequency (fMOSC) is displayed solely to assist the user in calculating the dithering frequency (Item 3), and is not part of the ordering information. Bias to 91k . resistors will range in value from approximately 45k 2. The second input is the Desired Dither Amount. If output dithering is not required, the default value of '0' corresponds to 0% (no dither). Note that the Dithering Frequency (fMOD) value shows "Disabled". Otherwise, the user should enter '2', '4', or '8' in the Item 2 box, represent
ing the 2%, 4%, or 8% output frequency dithering range available. Resulting circuit connections for pins J0 and J1 are displayed in the INPUT PIN SETTINGS section of the calculator. If the Desired Dither Amount is '0', skip Item 3 and connect pins J0, J1, JC0, and JC1 to ground as indicated. Do not float any of the four dither control input pins.
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