Multiphase Oscillator with 40% Spread Spectrum Frequency Modulation and 250Khz Output Frequency

This is a precision, low power, and easy-to-use oscillator that provides multiphase outputs. The output frequency of the project is set to 250Khz, with 40% spread spectrum. The oscillator frequency can be set by resistor R1, and the spread spectrum can be changed using R3. The master oscillator is controlled by the R_SET resistor R1 and has a range of 100khz to 20Mhz. In order to accommodate a wider output frequency range, a programable divider, (divide by 1, 10, 100) is included and can be set using jumper J1.  The integrated programmable multiphase circuit provides either 2, 3, or 4 phase waveforms. The LTC6902’s SSFM capability modulates the oscillator’s frequency by a pseudorandom noise (PRN) signal to spread the oscillator’s energy over a wide frequency band. This spreading decreases the peak electromagnetic radiation level and improves electromagnetic compatibility (EMC) performance. The amount of frequency spreading is programmable by a single additional external resistor R3 (R_MOD). Refer to 4 Phase output waveform diagram.

Features

• Power Input 5V DC
• Default Output Frequency 250Khz
• Default Spread Spectrum 40%
• Jumper for Frequency Division
• Jumper for Multiphase Mode
• 4 Phase Outputs
• PCB Dimensions 20.16 x 15.88 mm

LTC6902 Internal Diagram

Example: 4 Phase, 250Khz Clock with 40% Spreading

• Connect Ph Pin Jumper J2 to VC= Selects 4 Phase Mode, M=4
• Leave Jumper J1 Open N=10 Division
• RSET(R1) =20KOhms Sets F-Out=Fmax=250Khz
• Rmod(R3) =10Kohms Sets the Spreading to 40%

Multiphase Mode, Setting M

• 2-Phase: Connect Jumper2 to GND   M=1
• 3-Phase: Leave Jumper J2 Open        M=3
• 4-Phase: Connect Jumper J2 to VCC M=4

Choosing Programmable Divider Setting (Frequency Divider Setting Jumper J1)

• Jumper J1 Connect to GND = 2.5Mhz Output Frequency
• Jumper J1 Leave Open = 250khz Output Frequency
• Jumper J1 Connect to VCC = 25Khz Output Frequency

Calculating the RSET Resistor Value

• Maximum Frequency excursion (Fmax) is equal to F-Out.
• RSET(R1) =20Kohms(10Mhz/N*M*F-Out)
• N=100,10,1 (Jumper J1)
• M=4,3,1 (Jumper J2)

Calculating the RMOD Resistor Value

• Rmod=20xRSET/Spreading Percentage
• Where the spreading Percentage is defined by the following:
• Spread Percentage =100xFmax-Fmin/Fmax
• Where Fmax is the highest frequency Excursion (Set by the RSET Value) and Fmin is the lowest frequency excursion

Applications

• Switching Power Supply Clock Reference
• Portable and Battery-Powered Equipment
• PDAs, Cell Phones
• Clocking Switched-Capacitor Filters

Schematic

Parts List

Connections

Output waveforms

Gerber View

Photos

Video

LTC6902 Datasheet

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