×

Spread spectrum control PLL circuit and its start-up method

  • US 7,986,175 B2
  • Filed: 03/18/2008
  • Issued: 07/26/2011
  • Est. Priority Date: 05/30/2007
  • Status: Active Grant
First Claim
Patent Images

1. A spread spectrum control PLL circuit for outputting a spread spectrum clock signal based on an input reference clock signal, comprising:

  • a charge pump circuit configured to switch charging and discharging of a charging current, depending on a phase difference between a feedback clock signal based on an output clock signal of the PLL circuit and the reference clock signal;

    a loop filter circuit having a filter capacitor which is charged and discharged using the charging current output from the charge pump circuit;

    a voltage controlled oscillator configured to oscillate at a frequency depending on an input control voltage; and

    a calibration circuit configured to adjust at least one of one of the charging current of the charge pump circuit and a capacitance value of the filter capacitor and a gain of the voltage controlled oscillator, depending on a frequency of the reference clock signal,wherein the voltage controlled oscillator includes;

    a selector circuit configured to select as the control voltage one from a first voltage, a second voltage, and an output voltage of the loop filter circuit, andthe calibration circuit includesa timer circuit configured to measure a period of time depending on the frequency of the reference clock signal;

    a counter circuit configured to count pulses of the output clock signal of the PLL circuit or a frequency-divided clock signal thereof during a period of time when the timer circuit is operating; and

    a control circuit configured to control the selector circuit to adjust the gain of the voltage controlled oscillator so that a difference in count value of the counter circuit between when the first voltage is input to the voltage controlled oscillator and when the second voltage is input to the voltage controlled oscillator approaches a target value, andwhereinthe calibration circuit includes;

    a first memory circuit and a second memory circuit configured to store the count value of the counter circuit which is obtained when the first voltage is input to the voltage controlled oscillator and when the second voltage is input to the voltage controlled oscillator, respectively;

    a subtractor configured to calculate a difference between the count values stored in the first and second memory circuits; and

    a comparator configured to compare an output of the subtractor with the target value, andthe control circuit adjusts the gain of the voltage controlled oscillator based on an output of the comparator.

View all claims
  • 1 Assignment
Timeline View
Assignment View
    ×
    ×