JP2008085534A - Voltage controlled oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology capable of compensating variations in a process in an analog circuit of a voltage controlled oscillator (VCO) or the like. <P>SOLUTION: The voltage controlled oscillator has a voltage current conversion circuit 106 for converting input voltage into current, and a ring oscillator 107 in which an oscillation frequency is controlled by the current converted by the voltage current conversion circuit 106, wherein the voltage current conversion circuit 106 has a current mirror circuit 307 for adjusting a frequency range, a lowpass limiter 309 for generating subtraction current, and a current mirror circuit 308 for adjusting variations in a process, and the lowpass limiter 309 has a function for adjusting the subtraction current in accordance with fluctuations of the variations in the process. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a voltage controlled oscillator, and more particularly, to a technique for adjusting fluctuations in frequency control characteristics of a voltage controlled oscillator due to manufacturing process variations.

  For example, in a voltage controlled oscillator (VCO; Voltage Controlled Oscillator, hereinafter referred to as “VCO”), there are techniques described in Patent Documents 1 to 4 and the like.

  Patent Documents 1 and 2 describe a phase locked loop (PLL) circuit having a calibration circuit that optimizes the current flowing through the VCO in accordance with process variations and environmental variations. ing.

  Patent Document 3 describes a PLL circuit equipped with a VCO having a variable resistance circuit that determines an output current in a voltage-current conversion circuit while taking into account variations in process variations.

Patent Document 4 describes an optical disk reproducing apparatus equipped with a data reproducing PLL circuit having a VCO that is not affected by variations in manufacturing processes.
JP 2000-049597 A JP 2003-078410 A JP 2006-033197 A JP 2004-200742 A

  In recent years, miniaturization of processes has progressed, and it has become possible to increase the speed of digital circuits. Even in the design of a DVD (Digital Versatile Disc), a signal processing LSI capable of writing and reading at a high speed is compatible with Blu-ray (trademark) by miniaturizing the use process and increasing the speed of the digital circuit. I am trying to design a (semiconductor integrated circuit).

  However, when an analog circuit is mounted on the same LSI, an increase in leakage current due to process miniaturization and an increase in process variation have become problems. Therefore, it is necessary to compensate for variations in the leakage current.

  For example, in Patent Documents 1 and 2, a process variation is compensated by a calibration circuit. However, in this case, the layout area for the calibration circuit increases. Also, extra time is required to execute calibration when the power is turned on. In addition, there is a concern that noise characteristics may deteriorate due to an increase in circuits. In addition, a screening test of the calibration circuit itself is required.

  Therefore, an object of the present invention is to provide a technique capable of compensating for process variations in an analog circuit such as a VCO.

  The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

  Of the inventions disclosed in the present application, the outline of typical ones will be briefly described as follows.

  That is, the voltage-controlled oscillator according to the present invention includes a voltage-current conversion circuit that converts an input voltage into a current, and a ring oscillator whose oscillation frequency is controlled by the current converted by the voltage-current conversion circuit, and the voltage The current conversion circuit includes a first current mirror circuit that adjusts a frequency range, a low-frequency limiter that generates a subtraction current, and a second current mirror circuit that adjusts process variation, and the low-frequency limiter includes: It has a function of adjusting the subtracting current according to a variation in process variation.

  Of the inventions disclosed in the present application, effects obtained by typical ones will be briefly described as follows.

  In an analog circuit such as a VCO, fluctuations in the oscillation range due to process variations can be suppressed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment, and the repetitive description thereof will be omitted.

  FIG. 1 is a block diagram showing a configuration of a PLL circuit according to an embodiment of the present invention.

  First, an example of the configuration of the PLL circuit according to the present embodiment will be described with reference to FIG. The PLL circuit according to the present embodiment includes, for example, a phase frequency comparator 101, a charge pump 102, a loop filter 103, a VCO 104, a frequency divider circuit 105, and the like.

  The phase frequency comparator 101 detects a phase difference between a reference signal (reference) Fr input to one terminal and a feedback signal Fb input to the other terminal, and outputs a phase comparison signal corresponding to the phase difference. To do. The charge pump 102 drives the loop filter 103 according to the phase comparison signal. The loop filter 103 smoothes the output of the charge pump 102 and outputs the oscillator control voltage of the VCO 104. The VCO 104 oscillates at a frequency corresponding to the oscillator control voltage. A frequency divider circuit 105 is provided between the output Fo of the VCO 104 and the other terminal to which the feedback signal Fb of the phase frequency comparator 101 is input, thereby forming a feedback loop.

  By adopting such a configuration, the PLL circuit is controlled so that the phase and frequency of the reference signal Fr and the feedback signal Fb are matched. Here, an arbitrary positive integer can be selected as the frequency division number N of the frequency divider circuit 105, and the frequency of the signal (Fo) output from the VCO 104 when the PLL circuit converges is N times the reference signal Fr. Become.

  When the above PLL circuit is mounted on an LSI for clock generation, one LSI can operate at various frequencies depending on settings, such as high-frequency operation while the LSI is processing and low-frequency operation during standby. It is desirable to be. Therefore, it is desirable for the PLL circuit to be able to operate in a wide frequency range.

  Similarly, when the PLL circuit is applied to adjust the phase of the clock inside and outside the LSI, the operating frequency of the external bus varies widely according to the target performance of the user, so the PLL circuit has a wide frequency range. It is required to be able to operate.

  FIG. 2 is a schematic diagram showing the electrical characteristics of the VCO 104.

  The VCO 104 includes a voltage-current conversion circuit 106, a ring oscillator 107, and the like. The oscillator control voltage smoothed by the loop filter 103 is converted into a control current by the voltage-current conversion circuit 106, and the oscillation frequency of the ring oscillator 107 is controlled by the control current. As a result, the oscillation frequency of the output (Fo) of VC0104 changes according to the value of the oscillator control voltage. At this time, a leak current is generated in the voltage-current conversion circuit 106.

  FIG. 3 is a schematic diagram illustrating a schematic configuration of the charge pump 102, the loop filter 103, the voltage / current conversion circuit 106, and the ring oscillator 107.

  The charge pump 102 includes current mirror circuits 301 and 302, transistors 303 and 304, and the like. In response to the phase comparison signal from the phase frequency comparator 101, charging is performed via the transistor 303, and discharging is performed via the transistor 304.

  The loop filter 103 includes a resistor 305 and a capacitor 306. The oscillator control voltage is smoothed by the loop filter 103 and input to the voltage / current conversion circuit 106.

  The voltage-current conversion circuit 106 includes current mirror circuits 307 and 308, a low-frequency limiter 309, a transistor 310, and the like. An oscillator control voltage is input to the gate of the transistor 310, the drain current of the transistor 310 changes according to the gate voltage, and the current change is reflected in the control current by the current mirror circuits 307 and 308 and the low-frequency limiter 309. The current mirror circuit 307 has, for example, a configuration of 4 bits / 16 gradations and can adjust the frequency range from the outside. Similarly, the current mirror circuit 308 also has a configuration of 4 bits / 16 gradations, and process variations can be adjusted from the outside. For example, each of the 4-bit currents of the current mirror circuits 307 and 308 has a ratio of 1: 2: 4: 8. The low-frequency limiter 309 generates a subtraction current.

  The ring oscillator 107 includes a current mirror circuit 311 and a plurality of differential amplifiers 312. Delay occurs by the plurality of differential amplifiers 312 and oscillation occurs by inverting the output of the final stage and inputting it to the differential amplifier of the first stage. At that time, since the delay varies depending on the control current, the oscillation frequency is controlled by the control current.

  4 and 5 are explanatory diagrams showing a method of adjusting the subtracting current in the low-frequency limiter 309. FIG. 4 and 5, the horizontal axis represents the oscillator control voltage at the input of the VCO 104, and the vertical axis represents the oscillation frequency of the output of the VCO 104. Further, the controllable range of the charge pump 102 is set to 0.2V to 0.5V. FIG. 4A shows characteristics in a normal state.

  Since a leak current is generated in the voltage-current conversion circuit 106, the oscillation frequency increases as shown in FIG. 4B, and the lower limit specification of the VCO is not satisfied. Therefore, as shown in FIG. 4C, the current is subtracted by the low-frequency limiter 309 to compensate.

  However, the leak current is not constant, and the leak current varies due to process variations such as the threshold voltage Vth of the transistor 310. Therefore, if the current is excessively drawn by the low-frequency limiter 309 as shown in FIG. 5A, the oscillation frequency is too low to satisfy the upper limit specification of the VCO. On the other hand, as shown in FIG. 5B, if the current is not sufficiently drawn by the low-frequency limiter 309, the oscillation frequency will increase too much to satisfy the lower limit specification of the VCO.

  The fluctuation of the leak current is proportional to the speed change of the ring oscillator 107 due to the process fluctuation. The speed change of the ring oscillator 107 due to process fluctuation is adjusted in the LSI selection process by the current mirror circuit 308, and the adjustment result is laser-cut and stored as data in each LSI. By referring to this result, the value of the leakage current is estimated and the subtraction current of the low-frequency limiter 309 is determined. This makes it possible to compensate for process variations in leakage current. Specifically, for example, the low-frequency limiter 309 has a circuit configuration as shown in FIG.

  FIG. 6 is a circuit diagram illustrating a configuration example of the low-frequency limiter 309.

  The low-frequency limiter 309 includes transistors 601, 602, 603, 604, and 605, switches 606, 607, 608, and 609, inverters 610, 611, 612, and 613, for example. The transistors 601, 602, 603, 604, and 605 constitute a current mirror circuit, and a current proportional to a constant current flowing through the transistor 601 flows in each of the transistors 602, 603, 604, and 605, and the sum of them becomes a subtraction current. . The size of each of the transistors 602, 603, 604, and 605 is set to a ratio of 1: 2: 4: 8, and the subtraction current can be adjusted in 4 bit / 16 gradation. At the inputs of the switches 606, 607, 608, and 609, a 4-bit / 16-gradation signal for process variation adjustment is inverted by the inverters 610, 611, 612, and 613. Although not shown, a register is connected to the input side of the inverters 610, 611, 612, and 613, and the subtraction current can be adjusted by rewriting the value of the register. The register is initialized when the power is turned on by the set value by the fuse. The set value by the fuse is linked with the process variation adjustment, and is set by cutting the fuse at the time of LSI selection. Then, the subtracting current operates so as to cancel the leakage current fluctuation with respect to the process variation.

  At the time of LSI selection, the VCO frequency is measured by a tester, the setting is adjusted so that the process variation can be adjusted, the fuse is blown by the laser cutter, and the setting value (4 bit / 16 gradation) is written. In conjunction with this, if the limiter current of the low-frequency limiter 309 is set, the limit current can be determined including process variations, so that the problem due to manufacturing variations in the VCO frequency range can be solved.

  FIG. 7 is an explanatory diagram showing a method for adjusting process variations by the current mirror circuit 308 for process variation adjustment. FIG. 7A shows the VCO characteristics in the normal state.

  When Vth becomes small due to process variations, the ring oscillator 107 becomes slow, and the slope of the frequency / control current decreases as shown in FIG. 7B. Therefore, the current amount is increased by the current mirror circuit 308 for process variation adjustment, and the slope of the control current / oscillator control voltage is increased (current is increased). As a result, the frequency / oscillator control voltage characteristics have the same slope as in the normal state, and process variations are adjusted.

  Further, when Vth increases due to process variations, the ring oscillator 107 becomes faster, and the slope of the frequency / control current increases as shown in FIG. 7C. Therefore, the current amount is reduced by the current mirror circuit 308 for process variation adjustment, and the slope of the control current / oscillator control voltage is reduced (current is reduced). As a result, the frequency / oscillator control voltage characteristics have the same slope as in the normal state, and process variations are adjusted.

  Therefore, according to the voltage controlled oscillator (VCO) of the present embodiment, fluctuations in the VCO oscillation range due to process variations can be suppressed. The reason is as follows.

  That is, the variation of MOS elements (transistors) can be seen by the variation of Vth (threshold voltage). Vth varies from wafer to wafer within the manufacturing range. The current-frequency relationship of the ring oscillator is related to the variation in Vth. When Vth increases, the frequency decreases (slows), and when Vth decreases, the frequency increases (fastens). Therefore, in the LSI selection process, the oscillation frequency at a constant current is observed, adjustments are made to individual LSIs in consideration of process variations, and the results are stored in fuses.

  The point of interest of the present invention is that the leakage current also changes depending on Vth. As Vth increases, the leakage current decreases, and as Vth decreases, the leakage current increases. Therefore, referring to the same result, the current value of the subtractor circuit of the low-frequency limiter is changed. When Vth is increased, the subtraction current is decreased, and when Vth is decreased, the subtraction current is increased. As a result, the current fluctuation can be almost canceled.

  Although the invention made by the present inventor has been specifically described based on the embodiment, the invention is not limited to the embodiment and can be variously modified without departing from the scope of the invention. Needless to say.

  The present invention is applicable to a VCO that requires a wide variable range. This is particularly effective when a VCO is designed by a miniaturization process with a large leakage current or when it is necessary to oscillate at a low speed by a ring oscillator with a small current consumption.

It is a block diagram which shows the structure of the PLL circuit by one embodiment of this invention. It is a schematic diagram which shows the electrical property of VCO. 1 is a schematic diagram illustrating a schematic configuration of a charge pump, a loop filter, a voltage / current conversion circuit, and a ring oscillator in a PLL circuit according to an embodiment of the present invention. (A), (b), (c) is explanatory drawing which shows the adjustment method of the subtraction current in a low-frequency limiter. (A), (b) is explanatory drawing which shows the adjustment method of the subtraction current in a low-frequency limiter. FIG. 5 is a circuit diagram showing a configuration example of a low-frequency limiter in the VCO according to the embodiment of the present invention. (A), (b), (c) is explanatory drawing which shows the method of adjusting the process dispersion | variation by the current mirror circuit of process dispersion | variation adjustment in VCO by one Embodiment of this invention.

Explanation of symbols

101 phase frequency comparator 102 charge pump 103 loop filter 104 VCO (voltage controlled oscillator)
105 Frequency Divider 106 Voltage / Current Converter 107 Ring Oscillators 301, 307, 308, 311 Current Mirror Circuits 303, 304, 310, 601 to 605 Transistor 305 Resistor 306 Capacitor 309 Low Frequency Limiter 312 Differential Amplifiers 606 to 609 Switch 610 613 Inverter

Claims (5)

A voltage-current conversion circuit for converting an input voltage into a current;
A ring oscillator whose oscillation frequency is controlled by the current converted by the voltage-current conversion circuit,
The voltage-current conversion circuit includes a first current mirror circuit that adjusts a frequency range, a low-frequency limiter that generates a subtraction current, and a second current mirror circuit that adjusts process variation,
The low-frequency limiter has a function of adjusting the subtraction current in accordance with a variation in process variation. The voltage controlled oscillator according to claim 1, wherein
The subtracting current of the low-frequency limiter is adjusted in the selection step of the voltage controlled oscillator. The voltage controlled oscillator according to claim 1, wherein
The subtracting current of the low-frequency limiter is adjusted by cutting a fuse. The voltage controlled oscillator according to claim 1, wherein
The subtracting current of the low-frequency limiter is adjusted in conjunction with the second current mirror circuit. The voltage controlled oscillator according to claim 1, wherein
The low-frequency limiter comprises a third current mirror circuit,
The third current mirror circuit includes a plurality of transistors and a plurality of switches for switching the plurality of transistors.

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