# Patent

## Title
High-speed, high PSRR, wide operating range voltage controlled oscillator

## Abstract

A VCO for a PLL may include a ring oscillator having a power supply controlled in response to the VCO's control voltage input and an inverter having an input coupled to the ring oscillator's output and also supplied with a power supply controlled by the control voltage input. Together, the output of the ring oscillator and the output of the inverter may closely approximate a differential signal. The VCO may include an amplifier for amplifying a differential input to an output in the voltage domain of the system including the PLL. The output of the ring oscillator may be used as an input to the amplifier, and the output of the inverter may be used as the other input. The power supply terminals of the ring oscillator and the inverter may be coupled to outputs of a current mirror. In one implementation, the current mirror may not be cascoded.

## Background
This a continuation of U.S. patent application Ser. No. 09/833,297, filed Apr. 11, 2001, now U.S. Pat. No. 6,566,970 which claims benefit of priority to U.S. Provisional Application Ser. No. 60/266,077, filed Feb. 2, 2001.


BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to the field of voltage controlled oscillators (VCOs) and particularly their use in phase locked loops (PLLs).
2. Description of the Related Art
PLLs are used in a wide variety of applications. In integrated circuits, especially modern integrated circuits operating at high clock frequencies, PLLs are often used to generate the internal clocks for the integrated circuit and to lock the internal clocks to the phase of the clock supplied externally. Phase locking the internal clock to the external clock may aid in providing reasonable setup and hold times for communications on the pins of the integrated circuit. PLLs are also used for clock recovery, data transmission/reception, etc.
Integrated circuits are often designed to be scaleable to different clock frequencies, allowing the integrated circuits to be marketed at different frequency levels/price points and enhancing the usefulness of the integrated circuit in a variety of systems. Accordingly, it is desirable for PLLs to operate properly over a wide range of frequencies while achieving a high degree of noise rejection and fast lock times (the amount of time needed to lock the phase of the internal clock to the external clock).
One factor affecting the frequency of operation of the PLL is a voltage controlled oscillator (VCO) within the PLL. Generally, the PLL measures the difference in phase and/or frequency between the internal clock and the external clock and controls the VCO to change the frequency of the internal clock until phase/frequency lock is achieved. Generally, it is desirable for the VCO to operate over a wide control voltage range (allowing the largest variation in the output frequency of the VCO) while still offering high noise rejection (to reduce errors induced by the noise). Additionally, the VCO must interface properly to the rest of the system, which may be operating over a different voltage range than the VCO.
SUMMARY OF THE INVENTION
A VCO for a PLL may include a ring oscillator having a power supply controlled in response to the VCO's control voltage input and an inverter having an input coupled to the ring oscillator's output and also supplied with a power supply controlled by the control voltage input. Together, the output of the ring oscillator and the output of the inverter may closely approximate a differential signal. The VCO may include an amplifier for amplifying a differential input to an output in the voltage domain of the system including the PLL. The output of the ring oscillator may be used as an input to the amplifier, and the output of the inverter may be used as the other input. Thus, an interface between the low voltages in the VCO and the system may be formed.
The power supply terminals of the ring oscillator and the inverter may be coupled to outputs of a current mirror. In one implementation, the current mirror may not be cascoded. Accordingly, the voltage range available for the ring oscillator (and thus the range of output frequencies available) may be larger than if a cascoded current mirror is used. The VCO may, in one embodiment, be supplied with a regulated voltage supply to improve the noise rejection of the VCO.
Broadly speaking, a circuit for use with a ring oscillator is contemplated. The circuit comprises a current mirror and an inverter. The current mirror has a first input for receiving an input current, a first output for providing a first output current proportional to the input current, and a second output for providing a second output current proportional to the input current. The first output is coupled to a first power supply terminal of the ring oscillator. The inverter is coupled to receive an output of the ring oscillator as an input and having a second power supply terminal coupled to the second output of the current mirror.
Additionally, a VCO is contemplated, comprising: a current mirror, a ring oscillator, and an inverter. The current mirror has a first input for receiving an input current, a first output for providing a first output current proportional to the input current and a second output for providing a second output current proportional to the input current. The input current is responsive to a control voltage input to the VCO. The ring oscillator has an output and a first power supply terminal. The first power supply terminal is coupled to the first output of the current mirror. The inverter has an input coupled to the output of the ring oscillator and has a second power supply terminal coupled to the second output of the current mirror.
Moreover, a method is contemplated. A voltage is provided on a power supply terminal of a ring oscillator responsive to a control voltage. The voltage is also provided on a power supply terminal of an inverter coupled to an output of the ring oscillator, whereby a signal swing on an output of the inverter is similar to a signal swing on the output of the ring oscillator.



BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description makes reference to the accompanying drawings, which are now briefly described.
FIG. 1 is a block diagram of one embodiment of a phase locked loop (PLL).
FIG. 2 is a circuit diagram of one embodiment of a loop filter shown in FIG. 1.
FIG. 3 is a circuit diagram of one embodiment of a charge pump shown in FIG. 1.
FIG. 4 is a timing diagram illustrating operation of portions of one embodiment of the charge pump shown in FIG. 2.
FIG. 5 is a circuit diagram of one embodiment of a voltage controlled oscillator shown in FIG. 1.
FIG. 6 is a circuit diagram of one embodiment of a voltage regulator shown in FIG. 1.
FIG. 7 is a circuit diagram of one embodiment of a power-up control circuit shown in FIG. 6.
FIG. 8 is a circuit diagram of a gain boosted operation amplifier circuit.
FIG. 9 is a block diagram of a carrier medium.



While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to FIG. 1, a block diagram of one embodiment of a phase locked loop (PLL) 10 is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 1, the PLL 10 includes a voltage regulator 12, a phase/frequency detector (PFD) 14, a level shifter 16, a charge pump 18, a voltage controlled oscillator (VCO) 20, a pair of divide by two circuits 22 and 24, a divide by N circuit 26, a loop filter 28, a current generator 30, a configuration register 32, and a pair of output buffers 34 and 36. In the embodiment of FIG. 1, the voltage regulator 12 is coupled to a first power supply which supplies a first voltage (V1). The voltage regulator 12 provides regulated output voltages Vreg1 and Vreg2. The level shifter 16 is powered by Vreg2, while the charge pump 18 and the VCO 20 are powered by Vreg1. The output buffers 34 and 26, the divide by 2 circuits 22 and 24, the divide by N circuit 26, and the PFD 14 are powered by a Vdd supply voltage. The PFD 14 is coupled to receive the output of the divide by N circuit 26 and to receive a reference clock CLKref. The PFD 14 provides Up and Down signals to the level shifter 16, which level shifts the Up and Down signals from the Vdd voltage domain to the Vreg voltage domain corresponding to Vreg1 and Vreg2. The charge pump 18 is coupled to receive the level-shifted Up and Down signals from the level shifter 16 as well as a reference current Iref from the current generator 30. The charge pump 18 provides an output voltage VCtrl to the VCO 20, and the loop filter 28 is coupled to the node between the charge pump 18 and the VCO 20 as well. Both the loop filter 28 and the charge pump 18 are programmable via values in the configuration register 32 to which the loop filter 28 and the current generator 30 are coupled. Particularly, the charge pump 18 is programmable by changing the reference current Iref, as described in more detail below. The VCO 20 provides an output signal to the series connection of the divide by two circuits 22 and 24 (the output of each being buffered by output buffers 34 and 36, respectively). The output of the divide by 2 circuit 24 is fed back to the divide by N circuit 26.
Generally, the Vdd voltage may be the voltage supplied to the circuitry within an integrated circuit also employing the PLL 10. Accordingly, the Vdd voltage may be subject to a large amount of noise (variations in the voltage due to, e.g., digital switching in the integrated circuit, etc.). The V1 voltage may also be subject to noise. In one implementation, the V1 voltage may be the voltage supplied to input/output circuits of the integrated circuit. The Vdd and V1 voltages may be any suitable voltages, and may supply any other suitable circuitry, as desired. In one exemplary implementation, the Vdd voltage may be nominally 1.2 volts and the V1 voltage may be nominally 3.3 volts. However, the voltages may be varied in other implementations. The Vdd voltage may depend, at least in part, on the process technology used to form the transistors within PLL 10. The V1 voltage may depend on the process technology as well as external interface demands.
Since the Vdd and V1 voltages are noisy, they may be unsuitable for powering certain blocks of the PLL 10. Specifically, the charge pump 18 and the VCO 20 may be sensitive to power supply noise, and may function less accurately in the presence of power supply noise. Accordingly, these blocks are supplied with a regulated voltage from the voltage regulator 12 (specifically, Vreg1). In one embodiment described below, the voltage regulator 12 provides regulated voltages with a power supply rejection ratio (PSRR) greater than 40 decibels (dB), while prior voltage regulators in such contexts may have been limited to a PSRR of about 25 dB. The PSRR achieved by various embodiments of the voltage regulator 12 may vary depending upon which features of the voltage regulator as illustrated in FIG. 6 below are implemented, tradeoffs in the voltage regulator design even if the features are implemented, etc. The voltage regulator 12 may provide two regulated voltages (Vreg2 and Vreg1, which may be equal in the illustrated embodiment). The second regulated voltage (Vreg2) supplies the level shifter 16.
The level shifter 16 may serve as a noise reduction device in addition to shifting the Up and Down signals from Vdd voltage swings to Vreg voltage swings. The Up and Down signals provided by the PFD 14 may be noisy, as they are generated in the noisy Vdd voltage domain. The level shifter 16 passes these signals through various active devices (e.g. transistors) and thus the noise level is attenuated in the active devices. Accordingly, the Up and Down signals presented to the charge pump 18 may be less noisy and thus may enhance the accuracy of the charge pump 18 and the VCO 20.
Both the loop filter 28 and the charge pump 18 may be programmable through configuration register 32. The charge pump 18 is programmed by changing the reference current Iref supplied to the charge pump 18 (e.g. by the current generator 30 in the illustrated embodiment). In one embodiment, the current generator 30 may be supplied by the Vreg1 voltage as well (or the Vreg2 voltage, if desired). The loop filter 28 may be programmed by changing the resistor and capacitor values therein. An exemplary loop filter is illustrated in FIG. 2 below.
By adjusting the loop filter 28 and the charge pump 18, the damping, loop bandwidth and lock time of the PLL 10 may be adjusted for a given operating range. Thus, the PLL 10 may be optimized for a selected operating range in a given device, and thus the flexibility of the PLL 10 over a large operating range may be enhanced. As an example, increasing the reference current Iref may increase the rate at which the charge pump 18 may vary the VCtrl voltage, and thus may decrease the lock time of the PLL 10 while decreasing the damping of the loop. Varying the loop filter parameters may change which low and high frequencies of the VCtrl voltage are filtered out. These modifications may affect the lock time, damping, loop bandwidth, and jitter as well.
It is noted that, while the configuration register 32 is illustrated in proximity to the other elements of the PLL 10, the configuration register 32 may be located elsewhere. For example, in one embodiment, the PLL 10 may be included in a system on a chip design which integrates the PLL 10 with one or more processors, memory, bus bridges, etc. The configuration register 32 may be located in one of the processors, and the values stored therein may be transmitted to the PLL 10. Furthermore, other methods for programming the charge pump 18 and the loop filter 28 may be used. For example, pins on the integrated circuit including the PLL 10 may be set to appropriate voltages to provide the selected values. A combination of pins and configuration register values may also be used.
The PLL 10 may be a low power design. For example, by operating the VCO 20 with a Vreg1 supply voltage instead of the higher V1 voltage, the voltage swings of the oscillator within the VCO 20 may be limited to a maximum of approximately Vreg1, thus reducing the power consumed in the VCO 20 as compared to a VCO supplied by the V1 voltage. Additionally, in one implementation, current mirrors within the charge pump 18 and the VCO 20 may be implemented without cascoding (which may increases the device count and thus may lead to increased the power dissipation). Additionally, for VCO 20, cascoding the current mirrors reduces the maximum voltage that may be applied to the ring oscillator and thus would reduce the maximum output frequency of the VCO 20. Cascoded current mirrors are typically used to improve the operation of the current mirrors as current sources by attenuating supply noise. The non-cascoded current mirrors may provide acceptable current source operation since the voltage regulator 12 and the level shifter 16 operate to minimize the noise experienced by the charge pump 18 and the VCO 20. Still further, non-cascoded structures may more readily allow for transistors to remain in saturation, which may improve the performance of the transistors. Additionally, self-biasing may be used to ensure that voltages referenced to the Vdd and/or V1 voltages track changes in those voltages properly.
The voltage regulator 12 provides the PLL 10 with supply decoupling from the V1 power supply. Additionally, in one implementation, the PLL 10 may employ relatively large decoupling capacitance on critical nodes. For example, large decoupling capacitances may be provided on the Vreg2 and Vreg1 nodes. These capacitances may cause a corresponding variation on the Vreg2 and Vreg1 nodes in response to ground variations (“ground bounce”), thus preserving the Vreg2/Vreg1 to ground potential. Furthermore, these decoupling capacitances may substantially reduce the effect of supply variations (“supply bounce”) on the corresponding nodes.
The PFD 14 generally compares the frequency and phase of the clock output by the divide by N circuit 26 to the reference clock CLKref. If the phase of the clock output of the divide by N circuit 26 lags the phase of the reference clock, the PFD 14 activates the Up signal during the lagging period. If the phase of the clock output of the divide by N circuit 26 leads the phase of the reference clock, the PFD 14 activates the Down signal during the lagging period. Any suitable design may be used for the PFD 14, although in one embodiment a dual D flip flop design is used.
As mentioned above, the level shifter level shifts the Up and Down outputs of the PFD 14 from the Vdd voltage to the Vreg voltage. These level-shifted Up and Down signals (UpVreg and DownVreg, respectively) are applied to the charge pump 18. In response to an active Up signal, the charge pump 18 increases the charge on the output node, thus increasing VCtrl. In response to an active Down signal, the charge pump 18 decreases the charge on the output node, thus decreasing VCtrl. Increasing VCtrl causes the VCO 20 to increase its output frequency, while decreasing VCtrl causes the VCO 20 to decrease its output frequency.
It is noted that the Up and Down signals may be activated, or considered to be active, at either a high voltage or a low voltage. In the embodiment of the charge pump 18 illustrated in FIG. 3 below, the Up and Down signals are activated at a high (Vreg1) voltage (and deactivated at a low, or Gnd, voltage). Additionally, in some embodiments such as the one illustrated in FIG. 3 below, the charge pump 18 may use both the true and complement of the Up and Down signals. The complement may be generated by the charge pump 18, the level shifter 16, or the PFD 14, as desired. In this context, a first signal is a “complement” of a second signal if the first signal carries a voltage representing an opposite binary state to the voltage carried by the second signal.
The VCO 20 includes an output circuit which provides a full Vdd swing on the clock output from the VCO 20. This output clock is then divided by 2 (in frequency) by the divide by 2 circuit 22 (thus producing PLL output clock Out1). The second divide by 2 circuit 24 divides the frequency by two again (thus producing PLL output clock Out2, which is also fed back to the divide by N circuit 26). Accordingly, Out1 is 2N times the frequency of CLKref and Out1 is N times the frequency of CLKref (once the PLL 10 has locked to CLKref). Other embodiments may produce only one output clock, or more output clocks, and each output clock may be any multiple of the frequency of the clock reference CLKref using appropriate divide circuits in place of circuits 22, 24, and 26. As used herein, the term “signal swing” refers to a range of voltages which a signal may take on.
It is noted that, while specific embodiments of the loop filter 28, the charge pump 18, the VCO 20, and the voltage regulator 12 are illustrated in FIGS. 2, 3, 5, and 6 below (respectively), other embodiments may use any circuit for each of these elements of PLL 10, in other embodiments. Furthermore, any combination of the embodiments shown in FIGS. 2, 3, 5 and 6 along with conventional embodiments of other elements may be used.
As used herein, the term “power supply” refers to a node which is supplied, during use, with a relatively stable voltage. Thus, in many contexts, the ground node is a power supply node according to this definition, as well as nodes labeled, e.g., V1, Vreg1, Vreg2, Vdd, etc. Thus, coupling to a power supply refers to coupling to the node supplied with the corresponding voltage, during use. Furthermore, references to coupling to a voltage (including ground, or Gnd) or to a voltage supply herein should be interpreted as coupling to the corresponding power supply.
As used herein, a “current mirror” is a circuit having an input for receiving a current and at least one output. The current mirror provides a current on the output which is proportional to the input current. The output current may approximately equal the input current if the transistors forming the current mirror are matched.
Turning next to FIG. 2, a block diagram of one embodiment of the loop filter 28 is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 2, the loop filter 28 includes a programmable resistor R1 in series with a programmable capacitor C1, the series connection coupled between the output node of the charge pump 18 (the VCtrl voltage node) and ground, and a programmable capacitor C2 in parallel with the series connection of R1 and C1. In the illustrated embodiment, the loop filter 28 is programmable in response to configuration register values from the configuration register 32.
In the embodiment of FIG. 2, each of the filter components are programmable, providing a high degree of flexibility. Other embodiments may employ fewer programmable components (e.g. only R1 and C2 may be programmable), as desired.
The programmable components may be realized in any suitable fashion. For example, R1 may comprise two or more resistor elements in any series and/or parallel connection. Each resistor element may be shorted (series connection) or disconnected (parallel connection) via a switch which may be controlled by the configuration register value from the configuration register 32. Similarly, each capacitor may comprise one or more capacitor elements in any series and/or parallel connection, and each capacitor element may be shorted (series connection) or disconnected (parallel connection) via a switch which may be controlled by the configuration register value from the configuration register 32.
In one implementation, the register R1 may be programmable in the range of 0 to 33.2 kilo-ohms (kohms), in steps of 8.3 kohms (under worst case process conditions, or 25.0 kohms in steps of 6.25 kohms nominally); the capacitor C1 may be implemented as a parallel combination of 4 capacitors (each included or excluded programmably) having capacitances of approximately 20 picoFarads, 20 picoFarads, 10 picoFarads, and 5 picoFarads, respectively; and the capacitor C2 may be implemented as a parallel combination of a capacitor of approximately 1.25 picoFarads and a capacitor programmably included or excluded and of approximately 0.5 picoFarads. In one specific implementation, the resistor may be implemented of P+ polysilicon material using a unit cell approach, where each unit cell is approximately 1.184 kohms. The array of unit cells may be surrounded by dummy cells. The dimensions of the polysilicon may be material and/or process dependent, as is known to those of skill in the art. Alternatively, transistors may be used to construct the resistors. The capacitors may, in one implementation, be constructed from transistors with the gate terminal coupled as one capacitor input and the source and drain terminals and the bulk terminal shorted to the other capacitor terminal (e.g. ground).
Turning next to FIG. 3, a circuit diagram of one embodiment of the charge pump 18 is shown. Other embodiments are possible and contemplated. In FIG. 3 and other circuit diagrams herein, PMOS transistors are illustrated with an arrow pointing into the transistor on the source terminal (e.g. transistor Md1 is a PMOS transistor) and NMOS transistors are illustrated with an arrow pointing out of the transistor on the source terminal (e.g. transistor Md4 is an NMOS transistor). Each of the NMOS and PMOS transistors include a gate terminal, a source terminal, and a drain terminal.
In the embodiment of FIG. 3, the charge pump 18 includes current mirroring transistors Mm1, Mm2, and Mm3 which serve to provide a current proportional to the reference current Iref through transistors Mc1 and Mc2 as up and down currents Iup and Idown, respectively. The current mirror is non-cascoded. Additionally, the charge pump 18 includes a series connection of transistors Md1, Ms1, Md2, and Mc2 between the VCtrl output node and the Vreg1 power supply. The source and drain terminals of the Md1 and Md2 transistors are shorted. Coupled between the drain terminal of the Ms1 transistor (and the source terminal of the Mc1 transistor) and the Ground (Gnd) is the transistor Mx. The gate terminals of the Md1 and Md2 transistors are coupled to the Up signal, and the gate terminals of the Ms1 and Mx transistors are coupled to the complement of the Up signal (referred to as “Up bar” herein and shown as the word “Up” with a bar over it in FIG. 3). Shown in dotted form between the source terminal of the Mc1 transistor and the Vreg1 power supply is a parasitic capacitance Cp1. The parasitic capacitance Cp1 represents the total capacitance present at the source of transistor Mc1. The parasitic capacitance Cp1 may include the source to supply parasitic capacitance of transistors Mc1, Ms1, and Md2, as well as the drain to supply parasitic capacitance of transistors Md2 and Mx. Additionally, to the capacitance may be affected by the gate to drain parasitic capacitance of Ms1 and the gate to source parasitic capacitance of Mc1. The gate terminal of the transistor Mc1 is coupled to the gate terminal of transistor Mm3.
Furthermore, the charge pump 18 includes a series connection of transistors Mc2, Md3, Ms2, and Md4 between the VCtrl output node and Gnd. The source and drain terminals of the Md3 and Md4 transistors are shorted. Coupled between the drain terminal of the Ms2 transistor (and the source terminal of the Mc2 transistor) and the Vreg1 power supply is the transistor My. The gate terminals of the Md3 and Md4 transistors are coupled to the complement of the Down signal (referred to as “Down bar” herein and shown as the word “Down” with a bar over it in FIG. 3), and the gate terminals of the Ms2 and My transistors are coupled to the Down signal. Shown in dotted form between the source terminal of the Mc2 transistor and Gnd is the parasitic capacitance Cp2. The parasitic capacitance Cp2 represents the total capacitance present at the source of transistor Mc2. The parasitic capacitance Cp1 may include the source to supply parasitic capacitance of transistors Mc2, Ms2, and Md3, as well as the drain to supply parasitic capacitance of transistors Md3 and My. Additionally, the capacitance may be affected by the gate to drain parasitic capacitance of Ms2 and the gate to source parasitic capacitance of Mc2. The gate terminal of the transistor Mc2 is coupled to the gate terminal of transistor Mm2. Additionally shown in dotted form are the parasitic drain to gate capacitance of transistor Ms2 (Cp3) and the parasitic drain to gate and source to gate capacitances (Cp3/2) of transistor Md3. It is noted that transistors Ms1 and Md2 have similar parasitic capacitances as transistors Ms2 and Md3, respectively, not shown in FIG. 3.
Finally, the charge pump 18 includes: (i) a set of transistors similar in connection to Md1, Ms1, Md2, Mc2, and Mx except the gate terminals are connected to Down or Down bar (as shown in FIG. 3) and the drain terminal of the transistor similar to Mc2 is connected to a Vdummy node (reference numeral 40); and (ii) a set of transistors similar in connection to Mc2, Md3, Ms2, Md4, and My except the gate terminals are connected to Up or Up bar (as shown in FIG. 3) and the drain terminal of the transistor similar to Mc1 is connected to the Vdummy node (reference numeral 42).
Generally, transistors Mc1 and Mc2 are the current devices in the charge pump 18, providing the Iup and Idown currents (respectively) to the output node VCtrl. The transistors Ms1 and Ms2 are the switching transistors used to switch the currents on and off responsive to the Up and Down signals, respectively. The switching of transistors frequently generates noise which may disturb the output of the charge pump 18, and thus reduce its accuracy and effectiveness. For example, when a transistor is switched from on (conducting) to off (not conducting), charge injection may occur. Generally speaking, charge injection occurs if charge in the channel of the switching transistor does not recombine in the channel when the switching transistor is switched to the off state. This charge travels out of the channel onto the source and/or drain nodes of the switching transistor, changing the voltage on these nodes. Since this voltage change is unintentional, the change represents error (noise) and may affect the output voltage VCtrl. Additionally, since the Up and Down signals may transition rapidly to active and inactive states, clock feedthrough (charge transmitted from the gate to the drain or source of the transistor via the parasitic capacitances between the gate and the drain or source) may occur, which also represents error.
The switching transistors Ms1 and Ms2 are placed on the source side of the current transistors Mc1 and Mc2 (i.e. the side opposite the output node VCtrl). Accordingly, any switching error which may occur in switching transistors Ms1 and Ms2 is attenuated through Mc1 and Mc2. As described in more detail below, the charge pump 18 includes several mechanisms for reducing switching error from transistors Ms1 and Ms2. Any switching error not removed via these mechanisms may be attenuated, further reducing its effect on the output node VCtrl.
The mechanisms used in the charge pump 18 for reduction of charge injection errors and clock feedthrough errors will be described next with respect to transistors Ms2, Md3, and Md4. Similar operation occurs with respect to transistors Ms1, Md1, and Md2 (although the voltages that cause switching on and off differ since transistors Ms1, Md1, and Md2 are PMOS transistors while transistors Ms2, Md3, and Md4 are NMOS transistors).
Transistors Md3 and Md4 have gate terminals coupled to the complement of the signal on the gate terminal of transistor Ms2. In the illustrated embodiment, for example, the Down signal is received at the gate terminal of transistor Ms2, while the Down bar signal is received at the gate of transistors Md3 and Md4. Accordingly, during times that transistor Ms2 is being switched on (Down transitioning to a Vreg1 voltage), transistors Md3 and Md4 are being switched off (Down bar transitioning to a Gnd voltage) and vice versa.
The aforementioned structure may reduce charge injection error. When Ms2 is being switched off, and injecting charge onto its drain and/or source, Md3 and Md4 are being switched on. Thus, the channels of Md3 and Md4 acquire charge during approximately the same time period that transistor Ms2 is injecting charge. The injected charge (are a large portion thereof) may be drawn into the channels of Md3 and Md4 as these channels acquire charge, thus reducing the amount of charge actually injected onto the drain and source nodes of transistor Ms2.
The transistors Md3 and Md4 may be carefully sized with respect to transistor Ms2 in order to reduce the charge injection error appropriately. If the transistors are not sized appropriately, the charge injection reduction may not be minimized or may be overcompensated (making the effective charge injection negative). The assumption may be made that approximately half of the charge injected by transistor Ms2 is injected onto the source and the remaining charge injected by transistor Ms2 onto the drain. According, transistors Md3 and Md4 may be sized approximately half of the size of transistor Ms2, thus being capable of drawing the 1⁄2 charge injection from the corresponding source or drain terminal of transistor Ms2. The mobility of the devices affects the amount of charge injection (since the mobility may affect the recombination of charge in the channel), and the mobility may be factored into the sizing of the transistors Md3 and Md4. For example, the transistors Md3 and Md4 may be sized plus or minus 10% of 1⁄2 the size of transistor Ms2.
It is noted that the “size” of a transistor may be the ratio of the channel width to the channel length. Thus, a first transistor may be 1⁄2 the size of a second transistor if the ratio of the channel width and channel length of the first transistor is 1⁄2 the ratio of the channel width and channel length of the second transistor. In general, the channel length of the transistors may nominally be the same for most of the transistors in FIG. 3. Particularly, it may be desirable for the channel length of transistors Ms1, Ms2, Md1, Md2, Md3, and Md4 to be nominally the same since charge injection is related to the transit time across the channel.
The structure of transistors Ms2, Md3, and Md4 may also reduce clock feedthrough error. Illustrated in FIG. 3 is the gate to drain parasitic capacitance of transistor Ms2 (Cp3) as well as the gate to drain and gate to source parasitic capacitance of transistor Md3 (Cp3/2 each). The parasitic capacitances of transistor Md3 are approximately 1⁄2 the parasitic capacitance of transistor Ms2. Additionally, since the source and drain terminals of transistor Md3 are shorted, the parasitic capacitances are in parallel. Accordingly, the sum of the parasitic capacitances of transistor Md3 is approximately Cp3.
Since the gate terminal of transistor Md3 receives the complement of the signal on the gate terminal of transistor Ms2, the clock feedthrough from transistor Md3 to the drain of transistor Ms2 (through a total parasitic capacitance of Cp3) may be approximately the same as the clock feedthrough from transistor Ms2 to the drain of transistor Ms2, but of the opposite polarity. Summing these two clock feedthrough errors may produce a total clock feedthrough error on the drain of transistor Ms2 of approximately zero, thereby reducing the clock feedthrough error. A similar discussion applies to the gate to source and drain parasitic capacitances of transistor Md4 and the gate to source parasitic capacitance of transistor Ms2 (and to the transistors Md1, Md2, and Ms2).
It is desirable to match the currents Iup and Idown, so that equal width pulses of the Up and Down signals produce equal charge transferred to or drawn from the VCtrl output node (respectively). The transistors Mc1 and Mc2 may be sized similarly to nominally provide the same current. Some effects (such as channel length modulation, drain induced barrier loading (DIBL), etc.) can temporarily affect this balance and thus contribute to error on the VCtrl output node. Many of these effects can be reduced by increasing the channel length of the Mc1 and Mc2 transistors (i.e. these transistors may not have the minimum channel length in a given process technology). For example, in one specific implementation, a ratio of the channel length of transistors Mc1 and Mc2 to the other transistors (which may have the minimum channel length for the process) may be in the range of approximately 3-7, and preferably about 5.6, although other ratios may be used. However, increasing the channel length (and the diffusion area width) of these devices leads to larger parasitic capacitances (e.g. the parasitic capacitance Cp2 between the source of transistor Mc2 and ground and the parasitic capacitance Cp1 between the source of transistor Mc1 and Vreg1). The parasitic capacitances Cp1 and Cp2 may act to cause additional current to flow through transistors Mc1 and Mc2, respectively, after the corresponding switch Ms1 and Ms2 has switched off the current flow. This additional current flow is a source of error in the output voltage VCtrl. The larger parasitic capacitances may also require larger switch transistors Ms1 and Ms2 to switch the parasitic capacitances, which may exacerbate clock feedthrough and charge injection errors.
The charge pump 18 employs an active shutoff of the current transistors Mc1 and Mc2 using transistors Mx and My, respectively. The active turnoff of Mc2 by transistor My will be described, and the active turnoff of Mc1 by transistor Mx may be similar, with an appropriate change in polarities of voltages to account for the different transistor types.
Transistor My is a PMOS transistor coupled between the Vreg1 power supply and the source of transistor Mc2. When transistor Ms2 switches off in response to the down signal switching to a ground voltage, transistor My activates and actively charges the source of Mc2 to the Vreg1 voltage. This quick charging of the source of transistor Mc2 ensures that, regardless of the gate voltage of transistor Mc2 (which is less than or equal to Vreg1), the current flow through Mc2 stops quickly.
Another source of mismatch in the Iup and Idown currents for a given Up or Down signal pulse width may be the uneven loading of the Up and Down signals. By providing the sets of transistors illustrated at reference numerals 40 and 42, the load on the Up and Down signals may be balanced. For example, the Up signal is coupled to the gates of transistors Md1, Md2, a transistor similar to Ms2 and a transistor similar to My. The Down signal is coupled to the gates of a transistor similar to Md1, a transistor similar to Md2, transistor Ms2, and transistor My. Accordingly, over process variation and process mismatch, the Up and Down signals should be approximately evenly loaded (and thus edge rates on these signals may be approximately equal). Transistors are said to be similar in this context if the transistors are nominally matched in characteristics (e.g. the same diffusion area width, channel length, etc.).
As mentioned above, transistors Mm1, Mm2, and Mm3 comprise current mirroring for mirroring the reference current Iref to the Iup and Idown currents of Mc1 and Mc2. A non-cascoded current mirror is used in this embodiment, which may reduce power consumption, increase headroom, and ease the task of keeping transistors in saturation. The noise suppression may be provided via the level shifter and voltage regulator, as described above. Other embodiments may employ cascoding of the current mirrors (e.g. for use in embodiments not having the voltage regulator of FIG. 1 or having a different voltage regulator).
FIG. 4 is a timing diagram further illustrating the operation of transistors Ms2, Md3, Md4, and My for an exemplary set of transitions of the Down and Down bar signals. The Down and Down bar signals are represented by waveforms 50 and 52, respectively.
When Down transitions from high to low, charge injection may occur from transistor Ms2 (arrow 54). However, at approximately the same time the down bar signal transitions from low to high, activating transistors Md3 and Md4. Thus, these transistors may draw charge (including the injected charge), offsetting the injected charge (arrow 56).
The clock feedthrough from Down and Down bar is also shown in FIG. 4 (wave forms 58 and 60. Each of wave forms 58 and 60 is shown having approximately the same amplitude (A), since the Down and Down bar signals have the same swing and the parasitic capacitance through which they travel is approximately the same. Since the wave forms 58 and 60 have opposite polarities, the voltages sum to approximately zero at any given point (e.g. arrows 62 and 64).
Finally, a wave form 66 is illustrated in FIG. 4 illustrating an exemplary voltage on the node between the transistor Ms2 and the transistor Mc2. The voltage may have some value between Vreg1 and ground while the transistor Ms2 is on, but than rapidly transitions to Vreg1 when the transistor Ms2 is turned off. The rapid transition occurs in response to the transistor My actively charging the node.
Turning next to FIG. 5, a circuit diagram of one embodiment of the VCO 20 is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 5, the VCO 20 includes an input transistor Mi1, a current mirror including transistors Mm4, Mm5, and Mm6, a ring oscillator 70, an output amplifier 72, an inverter 74, and a self-biasing circuit 76. The input transistor Mi1 has a gate terminal coupled to receive the VCtrl voltage and is coupled between the current mirror and ground. The current mirror transistor Mm5 is coupled to the Vreg1 power supply and to the power supply terminal for the inverters in the ring oscillator 70. The current mirror transistor Mm6 is coupled between the Vreg1 voltage and to the power supply terminal of the inverter 74. A first input to the amplifier 72 is coupled to the output of the ring oscillator 70. The output of the ring oscillator 70 is also coupled to the input of the inverter 74, which is coupled to the second input of the amplifier 72. The self-biasing circuit 76 provides a bias voltage to the amplifier 76, generated from the Vdd power supply. The output of the amplifier 72 is coupled to a pair of series-coupled inverters 78.
Generally, the input transistor Mi1 develops a current in response to the VCtrl voltage input to the gate terminal. The current mirror transistor Mm4 mirrors the current to the current mirror transistors Mm5 and Mm6. The transistor Mm5 provides charge to the power supply terminals of the inverters in the ring oscillator 70. Thus, as the current in the transistor Mm5 increases (reflecting an increase in the control voltage VCtrl), the power supply to the ring oscillator 70 increases and thus the oscillation frequency of the VCO 20 increases. Similarly, as the current in the transistor Mm5 decreases, the oscillation frequency of the VCO 20 decreases. However, the signal swing of the ring oscillator 70 varies with the supply voltage as well, and may be small.
The output amplifier 72 is a differential amplifier providing an amplified, non-differential output to the output buffers 78. The output buffers 78 complete the transition of the output signal to a full Vdd swing. Since the output swing of the ring oscillator 70 is between ground and a voltage which may be small (the power supply voltage provided by the transistor Mm5), the amplifier employs PMOS input transistors (which react to low voltages well). The amplifier requires a differential input, however, and the combination of the inverter 74 supplied with a power supply through transistor Mm6 in combination with the output of the ring oscillator 70 may closely approximate a differential input. If an inverter powered by the Vreg1 power supply were used to provide the second input, the swing of the output signal of that inverter would not match the output of the ring oscillator 70. On the other hand, the power supply voltage for the inverter 74 may be close to the power supply voltage of the ring oscillator 70. Accordingly, although the output of the inverter 74 is slightly delayed with respect to the output of the ring oscillator 70, the output of the inverter 74 may more closely be a complement of output of the ring oscillator 70. Accordingly, the common mode noise rejection of the VCO 20 may be improved using the structure shown in FIG. 5.
It is noted that, in one implementation, the size of transistor Mm6 may be approximately 1⁄3 the size of the transistor Mm5 to provide approximately the same power supply voltage for the inverter 74 and the ring oscillator 70. Other embodiments may change to size ratio to match the number of inverters in the ring oscillator 70. For example, if an embodiment of the ring oscillator 70 includes 5 inverters, the transistor Mm6 may be approximately 1⁄5 the size of the transistor Mm5.
The self-biasing circuit 76 operates to provide a bias voltage for amplifier 72 which is a relatively constant ratio of Vdd over the range of values that Vdd may take on (especially due to switching noise, etc.) and over the range of process variation, operating temperature, etc. The ratio of the bias voltage to Vdd is set by the ratio of the sizes of each transistor in self-biasing circuit 76. The self-biasing may help ensure a gain in the amplifier 72 over the range of Vdd values which may be experienced during operation.
Additionally, the VCO 20 includes a non-cascoded current mirror including transistors Mm4, Mm5, and Mm6. Use of the non-cascoded current mirror (similar to the charge pump 18) allows for power consumption reduction and saturation of devices, as mentioned before. Furthermore, the non-cascoded current mirror results in a lower voltage drop between Vreg1 and the power supply to the ring oscillator 70, allowing a wider range of operation for the ring oscillator 70 than in cascoded current mirroring were used.
It is noted that, while the ring oscillator 70 illustrated in FIG. 5 is shown as three series coupled inverters (with the output of the last inverter connected to the input of the first inverter), any odd number of inverters may be used. Furthermore, if fully differential oscillator stages are used, an even number of stages may be used. As used herein, the term “power supply terminal” refers to a terminal of a circuit designed to be connected to a power supply. In the ring oscillator 70 and inverter 74, the power supply terminal is coupled to another device to allow the voltage supplied, during use, to be varied.
Turing next to FIG. 6, a circuit diagram of one embodiment of the voltage regulator 12 is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 6, the voltage regulator 12 includes transistors Msf1, Msf2, and Msf3 coupled in a source follower configuration. Each of the transistors Msf1, Msf2, and Msf3 have a gate terminal coupled to a node between a resistor coupled to the V1 power supply and capacitor coupled to ground (e.g. the gate terminal of the transistor Msf1 is coupled to the node between the resistor R3 and the capacitor C4). A bandgap generator 80 is used to generate a reference voltage, and an output transistor 82 is coupled to receive the reference voltage and provide it to an operational amplifier (op-amp) 84. The output transistor 82 is coupled to the transistor Msf3 as well, and a decoupling capacitor CDec is coupled between transistor 82 and ground. The op-amp 84 amplifies the reference voltage from the bandgap generator 80 to produce the first regulated voltage Vreg1 to be output by the voltage regulator 12. The output of the op-amp 84 passes through the common source transistor Ccs1 to the output Vreg1 and to an input of a second op-amp 86. The output voltage Vreg1 is passed though a feedback network to the second input of the op-amp 84, where the ratio of the resistors in the feedback network, in conjunction with the gain of op-amp 84, divides the desired output voltage Vreg1 back down to the reference voltage output from the bandgap generator 80. For example, the reference voltage may be 1.2 volts and the output voltage Vreg1 may be 2.0 volts. Additionally, a transistor Mx1 is coupled between the Vreg1 output node and ground, and has its gate terminal coupled to the output node of the op-amp 84 (the gate terminal of the transistor Mcs1). A capacitor Cf1 is coupled between the Vreg1 output node and the output of the op-amp 84. Finally, a decoupling capacitor CDec is coupled between the Vreg1 output node and ground. The second op-amp 86 is connected in a unity gain configuration (the feedback network is a wire) and employs an output transistor Mcs2 coupled between the transistor Msf2 and the Vreg2 output node of the voltage regulator 12. Alternatively, the second op-amp 86 may be coupled in the same fashion as op-amp 84 and in parallel with op-amp 84 to produce the Vreg2 voltage on the Vreg2 output node. Similar to the Vreg1 output node, a decoupling capacitor CDec is coupled between the Vreg2 output node and ground, a transistor Mx2 is coupled between the Vreg2 output node and ground with a gate terminal coupled to the output of the op-amp 86, and a capacitor Cf2 is coupled between the Vreg2 output node and the output of the op-amp 86. The gate terminals of transistors Msf1 and Msf2 are further coupled to a power-up control circuit 88.
The source follower transistors Msf1, Msf2, and Msf3 are used to provide current to the output transistors Mcs1, Mcs2, and 82, respectively. Generally, the source follower connection provides a low output impedance (the output being the terminal of the transistor Msf1, Msf2, and Msf3 coupled to the output transistor Mcs1, Mcs2, and 82, respectively), and thus the source follower connection may be an good voltage source. The transistors Msf1, Msf2, and Msf3 may be sensitive to noise on the V1 power supply, particularly noise transmitted to the gate terminals of these transistors. The resistor/capacitor series connection with the gate terminal of these transistors coupled to the node connecting the resistor and the capacitor forms a low pass filter which may limit the amount of noise from the V1 power supply which is transmitted to the gate terminal of the corresponding transistor. The low pass filter effectively decouples the gate terminal from the V1 power supply and couples the gate terminal to ground through the capacitor within the low pass filter. In one implementation, for example, using a resistance of 5 Mega-ohms and a capacitance of 100 picoFarads, at least 15 dB of noise rejection may be experienced. In this manner, the transistors Msf1, Msf2, and Msf3 may act as better current sources even in the presence of noise. However, other embodiments of the voltage regulator 12 may not employ the resistor/capacitor connections to the gate of the source to follower transistors, as desired, opting for a more conventional source follower connection or diode configuration.
While a low pass filter is used in the illustrated embodiment, any filter could be used depending on the frequencies of noise desired to be filtered. Furthermore, a combination of filters may be used.
Having the source follower transistors Msf1, Msf2, and Msf3 in series with another device may also limit the amount of voltage available below the series connection. Connecting a source follower to the V1 power supply may induce body effect in the source follower transistors, which increases the threshold voltage of the transistors and thus the voltage drop across the transistors. If the voltage drop across the source follower transistors increases, there is less remaining voltage (between the voltage at the output of the source follower and ground) available for use in the remaining circuits (and, in the case of the transistors Msf1 and Msf2, less voltage available for output as Vreg1 and Vreg2). In one example, V1 is 3.3 volts and Vreg1/Vreg2 is 2.0 volts, leaving only 1.3 volts for drops across Msf1 and Msf2 and the corresponding series devices Mcs1 and Mcs2.
The body effect in the transistors Msf1 and Msf2 may be offset (at least partially), by using transistors with lower threshold voltages for transistors Msf1 and Msf2. However, such transistors may also limit the voltages which may be tolerated between any two terminals. For example, in one implementation, the lower threshold transistors may be limited to 1.5 volts across any two terminals. During operation, the lower amount of voltage may not present a problem since most of the voltage between V1 and ground is used for Vreg1/Vreg2 (e.g. in the above example, 1.3 volts separate V1 and Vreg1/Vreg2). During power up of the voltage regulator circuit 12, these transistors may be protected from over-voltage conditions using power-up control circuit 88. Particularly, the gate terminals of the transistors Msf1 and Msf2, during power up, may initially be zero volts (in the absence of power-up control circuit 88). As the power supply V1 is powered to its operating level, the gate terminals of transistors Msf1 and Msf2 charge more slowly to the V1 voltage level (at a rate determined by the RC time constant of the resistor and capacitor coupled to the gate terminals). Thus, during power up, the full V1 voltage (or substantially the voltage, as the RC network charges) may be present between the gate and drain terminals of Msf1 and Msf2. Power-up control circuit 88 provides a voltage (Vg1 and Vg2 in FIG. 6) during power up to ensure that the over-voltage condition does not occur, protecting the Msf1 and Msf2 transistors. For example, in one embodiment, the power-up control circuit 88 may provide a V1 voltage level at Vg1 and Vg2. An example of the power-up control circuit 88 is shown in FIG. 7 below. For embodiments in which the transistors Msf1 and Msf2 are not implemented with lower threshold voltages and/or voltage tolerances, the power-up control circuit 88 may be omitted.
As used herein, the term “power up” refers to the application of a power supply voltage or voltages to a circuit after a period of time in which the circuit has been unpowered by the voltages.
The transistors Mx1 and Mx2 are provided to maintain a relatively constant current load at the Vreg1 and Vreg2 output nodes, respectively (in other words, relatively constant current through transistors Mcs1 and Mcs2, respectively) even though the current demands of the PLL 10 generally vary during operation. By keeping the current through Mcs1 and Mcs2 relatively stable, the regulation of the Vreg1 and Vreg2 voltages may be improved as well as stability, phase margin, and frequency response. It may generally be easier to regulate a voltage for a relatively constant current load, or a current load that varies within a small range. In the context of the PLL 10, stabilizing the current load may have the most effect for output frequencies of the PLL 10 at the lower end of the operating range of PLL 10.
The operation of Mx1 will be described, and the operation of Mx2 may be similar in the context of Vreg2 and Mcs2. If the current flowing out of the voltage regulator 12 through the Vreg1 terminal decreases, the voltage Vreg1 tends to rise (since the current though Mcs2 changes in proportion to its gate voltage, which hasn't yet changed). The increased voltage at Vreg1 leads to an increased voltage at the positive input of the op-amp 84, which leads to an increase in the output voltage of the op-amp 84. The gate terminal of Mx1 is coupled to the output of the op-amp 84, and is an NMOS transistor. Thus, as the output voltage of the op-amp 84 rises, the current in transistor Mx1 increases. Accordingly, the current though transistor Mx1 increases as the current traveling out of the Vreg1 output node (to the PLL 10) decreases. Similarly, as the current traveling out of the Vreg1 output node increases, the current through transistor Mx1 decreases.
Viewed in another way, transistor Mx1 is a current source which provides current inversely proportional to the current exiting the voltage regulator 12 through the Vreg1 output node. Any current source providing such a current may be used, in other embodiments. Furthermore, other embodiments of the voltage regulator 12 may not employ the current sources such as the transistors Mx1 and Mx2, as desired.
It is noted that, while the common source connection of the output transistors Mcs1 and Mcs2 may exhibit a smaller voltage drop from the source of the transistors Msf1 and Msf2 to the output nodes Vreg1 and Vreg2 than a source follower connection, the output impedance may be higher. Decoupling capacitances CDec are included on each node, mitigating the effect of the higher output impedance. Each capacitance CDec may be of any suitable size, but one embodiment may employ approximately 0.6 nanoFarads (nF) of decoupling capacitance.
It is noted that the stability of the op-amp/common source output transistor circuit may be affected by the existence of a dominant pole in the frequency response at both the output node of the op-amp and the output of the common source stage. Compensation capacitors Cf1 and Cf2 may be used to improve the stability be separating the poles. A resistor may also be placed in series with each of the capacitors Cf1 and Cf2 to remove a right half-plane zero in the response.
It is further noted that the use of a low pass filter on the gate terminal of a source follower circuit for noise reduction may be used in other contexts than a voltage regulator. The circuit may be used in any circuitry in which noise rejection is desirable. Similarly, the power-up control circuit 88 may be used to establish a voltage on any node during power up in order to protect a circuit from damage.
It is still further noted that other embodiments of the voltage regulator 12 may employ any combination of the above highlighted features, as desired.
Turning next to FIG. 7, a block diagram of one embodiment of the power-up control circuit 88 is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 7, the power-up control circuit 88 includes a resistor 90 coupled in series with a capacitor 92 between the V1 power supply and ground. The resistor may be implemented with transistors, in one embodiment, or with P+ poly, as desired. The gate terminal of a transistor 94 is coupled to the node between the resistor and the capacitor, and has a source coupled to the V1 power supply and a drain coupled to the Vg1 output. A similar circuit including a resistor 96, a capacitor 98 and a transistor 100 is also included, with the transistor 100 having a drain coupled to the Vg2 output.
The circuit including the resistor 90, the capacitor 92, and transistor 94 will be described. The circuit including the resistor 96, the capacitor 98, and the transistor 100 operates similarly. During power up, the voltage on the V1 power supply rapidly charges to the V1 voltage. However, the gate terminal of the transistor 94 charges from ground at a rate determined by the time constant of the resistor 90 and the capacitor 92. Accordingly, the transistor 94 is turned on and charges the Vg1 output node to V1 rapidly. In this manner, the gate terminal of the transistor Msf1 is held at V1 during power up, reducing the gate to drain voltage on the transistor Msf1 to a tolerable level. As the gate terminal of the transistor 94 charges to V1, the transistor 94 turns off and thus the node Vg1 is not actively charged by the power-up control circuit 88 during normal operation. Thus, the power-up control circuit 88 is active during a time period commencing at power up and ending at a later time determined by the time constant of resistor 90 and capacitor 92. The time period ends when the difference between the voltage of the gate terminal of the transistor 94 and the V1 voltage is less than a threshold voltage of the transistor 94.
While the embodiment of FIG. 7 includes two circuits producing two output voltages, other embodiments may include more circuits. Furthermore, an embodiment in which one circuit is used to supply gate voltages to both Msf1 and Msf2 from a single output node is contemplated.
Turning now to FIG. 8, a block diagram of a gain boosted op-amp 110 is shown. Other embodiments are possible and contemplated. The gain boosted op-amp 110 may be used as the op-amp 84 and/or 86 in FIG. 6, as desired. Furthermore, gain boosting may not be implemented in other embodiments.
Generally, the transistors 112 and 114 are intended as constant current devices. However, if the voltage at nodes 116 and 118 changes during operation (due to the positive and negative differential input voltages In_p and In_n changing), then the current in the transistors 112 and 114 may change. The voltage at nodes 116 and 118 is fed back through circuit 120 to the gate terminals of transistors 122 and 124, respectively. The current through the transistors 122 and 124 may thereby be altered to keep the current through the transistors 112 and 114 approximately constant. The gain boosting may ensure some gain from op-amp 110 over a large range of input voltage, power supply voltage, process, temperature, etc.
Turning next to FIG. 9, a block diagram of a carrier medium 300 including a database representative of PLL 10 is shown. Generally speaking, a carrier medium may include storage media such as magnetic or optical media, e.g., disk or CD-ROM, volatile or non-volatile memory media such as RAM (e.g. SDRAM, RDRAM, SRAM, etc.), ROM, etc., as well as transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link.
Generally, the database of PLL 10 carried on carrier medium 300 may be a database which can be read by a program and used, directly or indirectly, to fabricate the hardware comprising PLL 10. For example, the database may be a behavioral-level description or register-transfer level (RTL) description of the hardware functionality in a high level design language (HDL) such as Verilog or VHDL. The description may be read by a synthesis tool which may synthesize the description to produce a netlist comprising a list of gates from a synthesis library. The netlist comprises a set of gates which also represent the functionality of the hardware comprising PLL 10. The netlist may then be placed and routed to produce a data set-describing geometric shapes to be applied to masks. The masks may then be used in various semiconductor fabrication steps to produce a semiconductor circuit or circuits corresponding to PLL 10. Alternatively, the database on carrier medium 300 may be the netlist (with or without the synthesis library) or the data set, as desired.
While carrier medium 300 carries a representation of PLL 10, other embodiments may carry a representation of any portion of PLL 10, as desired, including any combination of a voltage regulator 12, a charge pump 18, a VCO 20, a loop filter 28, a configuration register 32, a level shifter 16, a PFD 14, a current generator 30, and divide circuits 22, 24, and 26, etc. or portions thereof.
Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

## Claims

1. A voltage controlled oscillator (VCO) comprising:
a current mirror having a first receiving an input current, a first output for providing a first output current proportional to the input current, and a second output for providing a second output current proportional to the input current; 
a ring oscillator having a third output and a first power supply terminal, the first power supply terminal coupled to the first output of the current mirror, 
an inverter having an input coupled to the third output of the ring oscillator and a fourth output, the inverter further having a second power supply terminal coupled to the second output or the current mirror; and 
wherein the third output and the fourth output approximate a differential pair during use.

2. The VCO as recited in claim 1 wherein the current mirror is configured to control a relative magnitude of the first input current and the second input current to supply a first voltage on the first power supply terminal having approximately a same magnitude as a second voltage on the second power supply terminal.

3. The VCO as recited in claim 1 wherein further comprising an amplifier configured to receive differential inputs and to provide a non-differential output, wherein a first of the differential inputs is coupled to the third output of the ring oscillator and the second of the differential inputs is coupled to the fourth output of the inverter.

4. The VCO as recited in claim 3 wherein the amplifier is further configured to receive a bias voltage, and wherein the bias voltage is provided by a self-biasing circuit.

5. The VCO as recited in claim 4 wherein the self-biasing circuit comprises series connection, between a pair of power supplies, of at least two transistors having their gate terminals coupled to their drains, and wherein a node between two of the transistors supplies the bias voltage.

6. The VCO as recited in claim 1 wherein the current mirror comprises; (i) a first transistor having a first terminal, a second terminal, and a first gate terminal; (ii) a second transistor having a third terminal, a fourth terminal, and a second gate terminal; and (iii) a third transistor having a fifth terminal, a sixth terminal, and a third gate terminal; and wherein the first terminal is coupled to the first input and is further coupled to the first sate terminal, and wherein the second terminal is coupled to a power supply, and wherein the second gate terminal and the shied gate terminal are coupled go the first gate terminal, and wherein the third terminal and the fifth terminal are coupled to the power supply, and wherein the fourth terminal is coupled to the first output, and wherein the sixth terminal is coupled to the second output.

7. The VCO as recited in claim 6 wherein the ring oscillator comprises a plurality of inverters coupled in a ring, wherein a number of the plurality of inverters is odd, and wherein a ratio of a size of the second transistor to a size of the third transistor approximately the number of the plurality of inverters.

8. The VCO as recited in claim 1 further comprising a control voltage input, wherein the input current to the current mirror is a function of the control voltage input.

9. A carrier medium comprising a database which is operated upon by a program executable on a computer system, the program operating on the database to perform a portion of a process to fabricate an integrated circuit including circuitry described by the database, the circuitry described in the database including a voltage controlled oscillator (VCO) comprising:
a current mirror having a first input for receiving an input current, a first output for providing a first output proportional to the input current, and a second output for providing a second output current proportional to the input current; 
a ring oscillator having a third output and a first power supply terminal, the first power supply terminal coupled to the first output of the current mirror; 
an inverter having an input coupled to the third output of the ring oscillator and a fourth output, the inverter further having a second power supply terminal coupled to the second output of the current mirror; and 
wherein the third output and the fourth output approximate a differential pair during use.

10. The carrier medium as recited in claim 9 wherein the current mirror is configured to control a relative magnitude of the first input current the second input current to supply a first voltage on the first power supply terminal having approximately a same magnitude as a second voltage on the second power supply terminal.

11. The carrier medium as recited in claim 9 wherein the VCO further comprises an amplifier configured to receive differential inputs and to provide a non-differential output, wherein a first or the differential inputs is coupled to the third output of the ring oscillator and the second of the differential inputs is coupled to the fourth output of the inverter.

12. The carrier medium as recited in claim 11 wherein the amplifier is further configured to receive a bias voltage, and wherein the bias voltage is provided by a self-biasing circuit.

13. The carrier medium as recited in claim 12 wherein the self-biasing circuit comprises a series connection, between a pair of power supplies, of at least two translators having their gate terminals coupled to their drains, and wherein a node between two of the transistors supplies the bias voltage.

14. The carrier medium as recited in claim 9 wherein the current mirror comprises: (i) a first transistor having a first terminal, a second terminal, and a first gate terminal; (ii) a second transistor having a third terminal, a fourth terminal, and a second gate terminal; and (iii) a third transistor having a fifth terminal, a sixth terminal, and a third gate, terminal; and wherein the first terminal is coupled to the first input and is further coupled to the first gate terminal, and wherein the second terminal is coupled the power supply, and wherein the second gate terminal and the third gate terminal is coupled to the firm gate terminal, and wherein the third terminal and the fifth terminal are coupled to the power supply, and wherein the fourth terminal is coupled to the first output, and wherein the sixth terminal is coupled to the second output.

15. The carrier medium as recited in claim 14 wherein the ring oscillator comprises a plurality of inverters coupled in a ring, wherein a number of the plurality of inverters is odd, and wherein a ratio of a size of the second transistor to a size of the third transistor is approximately the number of the plurality of inverters.

16. The carrier medium as recited in claim 9 wherein the VCO further comprises a control voltage input, wherein the input current to the current mirror is a function of the control voltage input.

17. A method comprising:
supplying a first power supply voltage on a first power supply input to a ring oscillator, a first magnitude of the first power supply voltage responsive to a control voltage input; 
supplying a second power supply voltage on a second power supply input to an inverter, the second power supply voltage having the first magnitude the inverter having an input coupled to a first output of the ring oscillator and having a second output, and the first output and the second output approximating a differential pair.

18. The method as recited in claim 17 further comprising amplifying the differential pair to provide a non-differential output.

19. The method as recited in claim 17 wherein the supplying a first power supply voltage and the supplying a second power supply voltage comprise a mirror controlling magnitudes of currents to the first power supply input and the second power supply input.