Abstract:
Systems, methods and apparatus relating to electronic circuits and signal processing are provided. In one aspect, a circuit is provided that includes a charge-pump operable to supply an output voltage, and a current mirror in communication with the charge-pump. The current mirror is responsive to the output voltage of the charge pump, and is operable to output a relatively constant current and suppress noise from the output voltage.

Description:
BACKGROUND 
   The following disclosure relates to electrical circuits and signal processing. 
   A charge-pump is a useful circuit for a variety of applications. A charge-pump receives a reference voltage input and provides an output voltage to a load circuit of that is a multiple (integer or non-integer) of the reference voltage. A charge-pump can be useful in many types of circuits, for example, a charge-pump can be used as a source for a voltage regulator loop. The voltage regulator loop can operate to provide a consistent desired output voltage over changes in process, voltage, and temperature (“PVT”). 
     FIG. 1  illustrates a conventional charge-pump  100 . Charge-pump  100  includes two identical mirror circuits  102  and  104  connected to output Vcp by two switches S 1  and S 2 . The mirror circuits  102  and  104  provide a step-up voltage to the output Vcp in alternate cycles provided by clocks CLK. Typically, the mirror circuits  102  and  104  are designed to provide a step-up of substantially twice a reference voltage, Vref. Specifically, mirror circuit  102  includes clock controlled digital logic inverters D 1  and D 2  supplied by Vref, capacitors C 1  and C 2 , and transistors M 1  and M 2  defining nodes N 1 , N 2 , and N 3 . 
   When the clock CLK provides a logical 1 signal at node N 1 , node N 2  falls to a logical 0 (due to inverter D 1 ) while node  3  rises to a digital  1  (due to inverter D 2  inverting the logical zero output from inverter D 1 ). The inverters D 1 , D 2  are supplied by Vref so that when either output is a logical 1, the output voltage from the respective inverter is Vref. In the example above (CLK providing a logical 1 at node N 1 ), node  3  rises to a voltage value of Vref. Since the value of node  2  is zero, the voltage at point V 2  decreases causing transistor M 2  to cut off. The resulting voltage at V 1  consequently increases to substantially 2Vref. When the clock cycles so that node N 1  transitions to a logical 0, V 2  continues to bias transistor m 1 , resulting in an output at node V 1  equal to Vref. Switch, S 1 , can be disconnected from output Vcp when V 2  does not equal 2Vref. The mirror circuits  102  and  104  are designed with complementary clock cycles that alternate output voltages of substantially 2Vref in order to provide dual cycle pumping at a near constant supply of 2Vref to output Vcp. 
   Typically, the input voltage to charge-pump  100  is a constant Direct Current (“DC”) source Vref. The output voltage, Vcp, is typically substantially double Vref. However, in a conventional charge-pump the voltage characteristic of Vcp over time is not smooth. Small deviations in the output Vcp can occur as a result of the alternating clock signals. Voltage ripples, or noise, typically occur at a regular period related to the frequency of the clock cycles of the charge-pump. This voltage noise can be significant enough to interfere with circuit functions. 
     FIG. 2  shows a conventional application of a charge-pump  100  as a source for a voltage regulator loop  200 . Voltage regulator loop  200  includes a feedback loop that works to maintain a constant output. Voltage regulator loop  200  includes amplifier  202 , transistors  204  and  206 , and resistors  208 ,  210  and  213 . Charge-pump  100  provides a voltage to gate  212  of transistor  206 , which activates transistor  206 . The gate voltage is designed to bias transistor  206  such that transistor  206  provides a desired output, Vout. If, for example, the output Vout decreases, the feedback loop operates to increase the gate voltage in order to restore the desired output Vout. The noise inherent in the charge-pump output can interfere with the functioning of transistor  206  and result in instability of the regulator loop  200 . For example, large fluctuations in the voltage characteristic can cause transistor  206  to deactivate, shutting down the voltage regulator loop  200 . Additionally, Vref and charge-pump  100  can be susceptible to fluctuations over PVT, causing further variability at gate  212  of transistor  206 . 
   One method of reducing the effect of the voltage noise is to insert a filter into the regulator loop  200 . One typically used filter is a bypass capacitor  214 . The bypass capacitor  214  works to establish an AC ground in a given circuit. The bypass capacitor  214  suppresses the AC component of the output signal. The larger the bypass capacitor  214  the greater the ripple that can be suppressed. However, circuit limitations can prevent the use of large bypass capacitors  214 . For example, since the bypass capacitor  214  is located within the voltage regulator loop  200 , the bypass capacitor  214  changes the loop dynamic. Large capacitance values of bypass capacitor  214  can interfere with the operation of the voltage loop  200  leading to circuit instability. 
     FIGS. 3   a  and  3   b  illustrate the voltage noise at the gate of transistor  206  and the output voltage, Vout, for the voltage regulator loop  200 , respectively. Bypass capacitor  214  reduces the magnitude of the noise, however significant voltage ripple can remain at gate  112  which can cause stability problems with other circuits connected to the voltage regulator loop  200  as well as impact the feedback loop of the voltage regulator. 
   SUMMARY 
   Systems and techniques relating to electronic circuits and signal processing. In general, in one aspect, a circuit is provided that includes a charge-pump operable to supply an output voltage, and a current mirror responsive to the output voltage, and operable to output a relatively constant current and suppress noise from the output voltage. 
   In general, in another aspect, a circuit is provided that includes means for supplying an output voltage and suppression means for suppressing noise from the supplied output voltage including converting the supplied output voltage into a relatively constant current. 
   In general, in another aspect, a method is provided for suppressing noise. The method includes providing an output voltage having an associated noise component, and suppressing the noise component in the output voltage including supplying a relatively constant current in response to the output voltage. 
   In general, in another aspect, an Ethernet transceiver is provided that includes a transmitter, a receiver, a charge-pump, a current mirror, and a voltage regulator. The charge-pump is operable to supply a reference voltage to the current mirror. The current mirror is arranged between the charge-pump and the voltage regulator. The current mirror is operable to provide a relatively constant current to the voltage regulator, and suppress noise from the charge-pump. The voltage regulator is further in communication with at least one of the transmitter and the receiver. The voltage regulator is operable to convert the relatively constant current source into a constant reference voltage, and supply the constant reference voltage to at least one of the transmitter and the receiver. 
   Implementations may include one or more of the following features. The circuit can further include a filter arranged between the charge-pump and the current mirror. The filter can be operable to further suppress noise from the output voltage of the charge-pump. The current mirror can isolate the filter from the load circuit. The filter can include a bypass capacitance. The bypass capacitance can be a bypass capacitor. The load circuit can include a regulator loop operable to generate a consistent output voltage. The load circuit can include a voltage reference generator operable to generate a reference voltage. The load circuit can include a voltage controlled oscillator operable to generate an output signal having a pre-determined oscillation frequency. The current mirror can be operable to reject variations in the output voltage of the charge-pump. 
   The circuit can further include a plurality of charge-pumps each in communication with the current mirror. The current mirror can be operable to suppress noise from an output voltage of the plurality of charge-pumps. The current mirror can be operable to reject variations in the output voltage of the plurality of charge-pumps. The circuit can further comprise one or more filters arranged between the plurality of charge-pumps and the current mirror. The one or more filters can be operable to suppress noise from an output voltage of the plurality of charge-pumps. At least one of the one or more filters can include a bypass capacitor. 
   The circuit can further include a plurality of current mirrors in communication with a plurality of load circuits. Each current mirror can be operable to provide a constant current to a corresponding load circuit and suppress noise from an output voltage of a corresponding charge-pump. At least one of the plurality of load circuits can include a regulator loop. At least one of the plurality of load circuits can include a voltage reference generator. At least one of the plurality of load circuits can include a voltage controlled oscillator. The circuit can further include a filtering means for further suppressing noise from the supplied output voltage. The Ethernet transceiver can be compliant with IEEE 1000BaseT. 
   Implementations may provide one or more of the following advantages. A noise suppression circuit is provided that can suppress noise from a voltage supply, for example a charge-pump. The noise suppression circuit can provide noise suppression for any load circuit that requires a constant current input, such as a voltage regulator loop, voltage reference generator, or voltage controlled oscillator. The current mirror is designed to maintain a relatively constant current output over PVT. Circuit stability is further increased by maintaining a constant current over PVT. By maintaining a constant current output, the current mirror further reduces noise across the current mirror. Circuit noise is reduced by the current mirror because the current mirror provides rejections to voltage variations in order to maintain the constant current. The current mirror also functions to isolate a filter, such as a bypass capacitor from the load circuit, effectively decoupling the filter from the load circuit. In one example, a noise suppression circuit is provided that includes a current mirror that isolates a filter from a voltage regulator loop, which enhances loop stability. Removing the bypass capacitor from the voltage regulator loop allows for the use of a larger capacitance value without impacting the voltage regulator loop function. 
   The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages may be apparent from the description and drawings, and from the claims. 

   
     DRAWING DESCRIPTIONS 
     These and other aspects will now be described in detail with reference to the following drawings. 
       FIG. 1  is a schematic diagram of a conventional charge-pump. 
       FIG. 2  is a schematic diagram of a conventional voltage regulator loop with a charge-pump. 
       FIGS. 3   a–b  are voltage graphs of a conventional voltage regulator loop supplied by a charge-pump. 
       FIG. 4  is a block diagram of a noise suppression circuit positioned between a charge-pump and a voltage regulator loop. 
       FIG. 5  is a schematic diagram of a circuit including a charge-pump, noise suppression circuit, and a voltage regulator loop. 
       FIGS. 6   a–d  are voltage graphs of a voltage regulator loop connected to a noise suppression circuit. 
       FIG. 7  is a schematic diagram of charge-pump, noise suppression circuit, and multiple voltage regulator loops circuit. 
       FIG. 8  is a schematic diagram of a multiple charge-pump, noise suppression circuit, and multiple voltage regulator loops circuit. 
       FIG. 9  is a block diagram of an Ethernet transceiver. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 4  shows a block diagram of a charge-pump current mirror system  400  operable to reduce voltage ripple in a load circuit. Charge-pump  410  is operable to provide a source voltage to a load circuit  430 . The charge-pump  410  can be, for example, the charge-pump  100  as described with respect to  FIG. 1  above. In one implementation, charge-pump  100  receives an input voltage Vref and produces an output voltage Vcp of substantially twice the input voltage Vref. A noise suppression circuit  420  positioned between the charge-pump  410  and the load circuit  420  is operable to suppress voltage ripple from the charge-pump  410  without interfering with the stability and function of the load circuit  430 . In one implementation, noise suppression circuit  420  includes a current mirror  422  and a filter  424  such as a bypass capacitor. The current mirror  422  operates to isolate the filter  424  from load circuit  430  allowing, for example, the use of a large bypass capacitor without impairing the functionality of load circuit  430 . In one implementation, noise suppression circuit  420  is operable to provide a relatively constant current Io to allow proper functioning of load circuit  430 . One example of load circuit  430  is a voltage regulator loop providing a constant output Vout. 
     FIG. 5  illustrates a noise suppression circuit  504  in a circuit  500  including a charge-pump  502  and a voltage regulator loop  506 . Noise suppression circuit  504  includes current source  508  and transistors  510  and  512 , which are operable to provide a current mirror  516 . Noise suppression circuit  504  also includes a filter such as bypass capacitor  514 . Current mirror  516  is operable to provide a constant current, Io, to voltage regulator loop  506  and to reduce the voltage noise from the output of charge-pump  502 . Current mirror  516  provides a constant current output, Io, which works to reject noise from charge-pump  502 . The size of the current source depends on the current requirements of the load circuit. 
   In one implementation, the total mirror current, Io, provided to the load circuit is equal to or less than the current provided by the charge-pump  502 . Bypass capacitor  514  operates, as described above, to suppress the voltage noise inherent in charge-pump  502 . The size of bypass capacitor  514  can be selected based on the magnitude of noise rejection provided by the current mirror. Because bypass capacitor  514  is located outside of the voltage regulator loop  506 , the capacitance value of the bypass capacitor  514  can be increased without significantly impacting the operation of voltage regulator loop  506 . 
   The input reference voltage, Vref, to the charge-pump  502  is sensitive to variations in PVT, which can result in output variations. For example, if Vref increases, the output from charge-pump  502  also increases. However, this type of fluctuation will not affect voltage regulator loop  506  because of the effect of the current mirror  516 . Essentially, current mirror  516  functions to convert the variable voltage input, Vref, into a constant current output, Io, thereby enhancing circuit stability. 
   Voltage regulator loop  506  includes transistors  518  and  526 , resistors  520  and  522 , and amplifier  524 . In one implementation, amplifier  524  is a 2-input amplifier, having as inputs a reference voltage from a voltage source (not shown) and a feedback voltage, which is discussed in greater detail below. Voltage regulator loop  506  is operable to adjust a gate voltage  530  of transistor  518  to provide a constant voltage output  528 . Voltage regulator loop  506  includes a feedback loop that is operable to increase or decrease the gate voltage  530  of transistor  518  as a result of changes to the voltage output  528  (so as to adjust the voltage output  528 ). Voltage regulator loop  506  provided in  FIG. 5  is one example of a regulator loop structure. Other regulator loop structures can be provided. 
   In operation, charge-pump  502  receives a voltage input, Vref, and provides a voltage output of substantially twice Vref. The DC output of charge-pump  502  typically has voltage noise. Current mirror  516  provides a majority of the noise rejection in the noise suppression circuit  504  while a filter, such as bypass capacitor  514 , provides additional noise rejection. When active, current mirror  516  provides a constant current output, Io, from the current mirror that matches a reference current, Iref, at the current source  508 . Current mirror  516  provides a relatively constant current, Io, over PVT variations. 
     FIGS. 6   a–d  illustrate the voltage output graphs for the gate of transistor  518  and for the output voltage  528  as compared with a circuit as shown in  FIG. 2  designed to provide the same gate and output voltages. As shown in  FIG. 6   a , the voltage graph at the gate of transistor  518  illustrates a DC voltage of 2.2V having a voltage ripple of about 1 mV. A comparable circuit without a current mirror (e.g., circuit  200  of  FIG. 2 ) and having a bypass capacitor included within the voltage regulator loop can have a gate voltage ripple of as much as 40 mV ( FIG. 6   b ). As shown in  FIG. 6   c , the voltage graph for the output voltage  528  illustrates a constant DC voltage of about 1.35 V including a voltage ripple of less than 1 mV. Again, a comparable circuit without a current mirror (e.g., circuit  200  of  FIG. 2 ) can have an output voltage ripple of as much as 4 mV ( FIG. 6   d ). Consequently, the use of the noise suppression circuit  504  can provide increased voltage ripple suppression over the prior art without impacting the stability of other circuit elements such as voltage regulator loop  506 . 
     FIG. 7  illustrates an implementation of a noise suppression circuit  702  in a circuit  700  including multiple voltage regulator loops. Noise suppression circuit  702  includes a bypass capacitor  712  and a multiple output current mirror  714 . The multiple output current mirror  714  includes a current source  716  and transistors  718 ,  720 ,  722 , and  724 . Transistors  720 ,  722 , and  724  provide a constant current mirror output, I, that mirrors the reference current, Iref, from current source  716 . 
   In operation, a charge-pump  704  receives a voltage input, Vref, and provides a voltage output of substantially twice Vref. The DC output of charge-pump  704  typically has substantial voltage ripple, which is initially suppressed by the bypass capacitor  712  of the noise suppression circuit  702 . When active, current mirror  714  provides a constant current output, I, from each output transistor (i.e., transistors  720 ,  722 , and  724 ) of the current mirror  714  that matches the reference current, Iref, of the current source  716 . The current mirror  714  can be designed to provide relatively constant current output, I, over PVT. Circuit stability is also increased by maintaining a constant current over PVT. The output transistors  720 ,  722 , and  724  provide a constant current to voltage regulator loops  706 ,  708 , and  710  respectively. As with the single output noise suppression circuit described above, each output of the multiple output current mirror  714  further suppresses the voltage ripple at each voltage regulator loop (e.g., voltage regulator loops  706 ,  708 , and  710 ). Three voltage regulator loops are shown in  FIG. 7 , however, any number of voltage regulator loops can be included. The number of regulator branches is limited only by the size of the charge-pump. Multiple charge-pumps can be used to increase the supply size as described below with respect to  FIG. 8 . 
     FIG. 8  shows another implementation of a noise suppression circuit  802  in a circuit  800  including multiple voltage regulator loops and multiple charge-pumps. A circuit including multiple voltage regulator loops may require multiple charge-pumps in order to supply the required voltage to the voltage regulator loops. Multiple charge-pumps can be coupled in parallel to provide the necessary supply to the voltage regulator loops. Noise suppression circuit  802  includes a bypass capacitor  812  and a multiple output current mirror  814 . The multiple output current mirror  814  includes a current source  816  and transistors  818 ,  820 ,  822 , and  824 . Transistors  820 ,  822 , and  824  provide a constant current mirror output, I, that mirrors the current, Iref, from current source  816 . 
   In operation, charge-pumps  826  and  828  provide the supply voltage to the voltage regulator loops  830 ,  832 , and  834  through noise suppression circuit  802 . The DC output of each charge-pump  826  and  828  typically has substantial voltage ripple, which is initially suppressed by the bypass capacitor  812  of the noise suppression circuit  802 . When active, current mirror  814  maintains a constant (e.g., over PVT variations) current output, I, from each output transistor (i.e., transistors  820 ,  822 , and  824 ) of the current mirror  814  that matches the reference current, Iref, of the current source  816 . The output transistors  820 ,  822 , and  824  provide a constant current to voltage regulator loops  830 ,  832 , and  834  respectively. As with the single output noise suppression circuit  504  described above with respect to  FIG. 5 , each output of the multiple output current mirror  814  further suppresses the noise at each voltage regulator loop (e.g., voltage regulator loops  830 ,  832 , and  834 ). Two charge-pumps are shown in  FIG. 8 , however, any number of charge-pumps can be included to provide the required supply voltage to the voltage regulator loops. 
   Charge-pumps  410 ,  502 ,  704 ,  826 ,  828  and noise suppression circuits  420 ,  504 ,  702 ,  802  can be used in a wide range of applications, for example, in an Ethernet transceiver  900  (hereafter referred to as transceiver  900 ) as shown in  FIG. 9 . A charge-pump (e.g., charge-pump  410 ,  502 ,  704 ,  826 ,  828 ) supplies a reference voltage to a noise suppression circuit (e.g., noise suppression circuit  420 ,  504 ,  702 ,  802 ). The noise suppression circuit, in response to the supplied reference voltage, can supply a relatively constant current source to a voltage regulator  922 . Voltage regulator  922  converts the current source into a reference voltage that can be supplied to each of a receiver  902  and a transmitter  904 . 
   Transceiver  900  can be compliant with IEEE 1000BaseT. 
   Although only a few implementations have been described in detail above, other modifications are possible. The load circuit is illustrated as a voltage regulator loop, however, other circuit topologies can benefit from reduced noise. For example, other circuits requiring a constant current input can be supported such as a voltage reference generator and a voltage controlled oscillator. The current mirror illustrated as part of the noise suppression circuit is one exemplary current mirror. Other current mirror topologies, which provide a constant current output can be implemented to provide similar effects as disclosed above. The filter provided by the noise suppression circuit is shown as a bypass capacitor. Other filters can be implemented to suppress circuit noise. Since the filter is located outside of the load, for example the voltage regulator loop, the type and size of the filter can vary without impacting load circuit function. 
   Other implementations may be within the scope of the following claims.