Patent Abstract:
A differential charge pump with common mode and active regulators is presented. Either type of regulator may be used to improve the performance characteristics of the differential charge pump. The active regulator increases the output range of the differential amplifier. The common mode regulator establishes the common mode voltage of the differential charge pump. The common mode voltage is established independently from external circuitry and does not use a feedback path. The common mode regulator may also be used to establish a mid-rail voltage, which may be used to further improve the output range of the differential amplifier.

Full Description:
GOVERNMENT RIGHTS 
     The United States Government has acquired certain rights in this invention pursuant to Contract No. DTRA01-03-D-0018 and Delivery No. DTRA01-03-D-0018-0001 awarded by the Defense Threat Reduction Agency. 
    
    
     FIELD 
     The present invention relates generally to the field of integrated circuit charge pumps and more particularly to a differential charge pump with active and common mode regulators. 
     BACKGROUND 
     As Integrated Circuits (ICs) continue to become more advanced, the transistors that are used to construct them continue to decrease in size. Decreases in transistor size create changes in operating specifications associated with smaller transistors. Such changes include decreased operating voltages and tighter common mode voltage tolerances. 
     One type of device that is affected by decreased operating voltage and tighter common mode voltage tolerances is a charge pump. Charge pumps are fundamental components of many types of devices. For example, a charge pump may be used to adjust the amount of voltage applied to a low pass filter in a phase locked loop. One such charge pump (a differential charge pump) is illustrated in  FIG. 1 . Charge pump  10  includes a current steerer  12  which is coupled to receive a source current from a current source  14  and a sink current from a current sink  16 . The current steerer distributes the source and sink current to output terminals NEG  18  and POS  20 . Signals applied to a differential control (differential input terminals  22 A-D) may be used to determine a duty cycle associated with the amount of time the source and sink currents are steered to a respective output terminal. 
     The output terminals  18  and  20  may each be coupled to a capacitor that uses the source and sink currents to charge and discharge. By changing the duty cycle, via the differential inputs  22 A-D, the amount of voltage stored on a particular capacitor may be adjusted. For example, if the capacitors are used in a loop filter (in a phase locked loop), the voltage level on each capacitor may be used to differentially control the output frequency of a voltage controlled oscillator. 
     Because the current steerer  12  is comprised of Field Effect Transistors (FETs)  24 - 27 , decreasing operating voltages (which may be associated with decreasing transistor sizes) have a direct impact on the output current range of current steerer  12 . This becomes apparent by examining nodes  28  and  30 . The maximum voltage at node  28  and the maximum voltage at node  30  limit the maximum range of output voltage available for input voltages at FETs  24 - 27 . For example, as operating voltages decrease, a larger percentage of an operating voltage intended for FETs  24 - 27  may be distributed across current source  14  and current sink  16 . When this happens less voltage is available for nodes  28  and  30 , and as a result, the output current range of current steerer  12  is reduced. 
     Another shortcoming with current charge pumps is common mode voltage drift. Common mode voltage drift occur when small asymmetries in the charge pump  12  (or asymmetries in the circuit referencing the charge pump) may cause the positive or negative DC voltage across output terminals  18  and  20  to drift to an undesirable voltage level. For example, due to statistical variations that occur in manufacturing (i.e., semiconductor processing), FET  24  may have a smaller channel length (ΔL), channel width (ΔW), and/or threshold voltage (ΔV t ) than FET  25 . Over time, small differences in current resulting from an asymmetry may cause a capacitor referencing output terminal  20  to store a small increment of charge at each clock cycle. Another capacitor referencing output terminal  18  may not store this increment of charge. A DC voltage, therefore, is established between output terminals and it may grow, or drift, with each clock cycle. 
     Current charge pumps employ a feedback mechanism that monitors the DC, or common mode, voltage. By monitoring the common mode voltage through a feedback path, and adjusting the charge pump based on the feedback, a feedback mechanism may compensate for the asymmetry. This may be done by adjusting the duty cycle applied to the current steerer  12 , for example. Unfortunately, the feedback mechanism increases the complexity of the charge pump and produce additional overhead. The problems associated with asymmetry may also be further exacerbated with decreased transistor sizes. 
     Therefore there is a need for a charge pump that has an output range that is not restricted by device scaling and processing asymmetries. 
     SUMMARY 
     A differential charge pump with open loop common mode is presented. The differential charge pump includes a current steerer that uses a differential control to steer source and sink currents to differential output terminals. In one example, the output terminals are coupled with a common mode regulator which drives a common mode voltage of the differential charge pump. The common mode regulator operates independent from external circuitry and does not require feedback (open loop). 
     In a further example, the common mode regulator includes a voltage driver coupled with a pair of resistances. The voltage driver may be an inverter having an input coupled to an output in order to establish a voltage that is about half of a power supply voltage. The output of the inverter is coupled with the resistances. A resistance value associated with the resistances may be tailored to prevent a substantial portion of the source and sink currents from entering the common mode regulator. 
     In another example, the current steerer is coupled with an active regulator. The active regulator reduces overhead voltage associated with a current source which is used to generate the source current. The operating range of the current steerer is thereby increased. In a further example, a second active regulator may be used to reduce overhead associated with sink current generation. 
     In yet another example, the differential charge pump may employ both an active regulator and a common mode regulator in order to increase operating range and create a desired common mode voltage. These as well as other aspects and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it is understood that this summary is merely an example and is not intended to limit the scope of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain examples are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein: 
         FIG. 1  is a circuit diagram of a differential charge pump; 
         FIG. 2A  is a circuit diagram of a common mode regulator; 
         FIG. 2B  is a circuit diagram of another common mode regulator; 
         FIG. 3A  is a circuit diagram of an active regulator; 
         FIG. 3B  is a circuit diagram of another active regulator; and 
         FIG. 4  is a circuit diagram of a differential charge pump including a common mode regulator and two active regulators. 
     
    
    
     DETAILED DESCRIPTION 
     A differential charge pump including a common mode regulator and/or an active regulator is presented. The common mode regulator and the active regulator are coupled with a current steerer. The common mode regulator establishes, without a feedback path, the common mode voltage level of the charge pump. In doing so, an erroneous voltage build up which may be associated with asymmetries inherent to the charge pump may be mitigated. The active regulator, on the other hand, increases the amount of input voltage that is distributed to the charge pump. As a result, the output range of the charge pump is also increased. 
     Turning now to  FIG. 2A , an example common mode regulator  32  is illustrated. The common mode regulator includes a voltage driver  34  and resistances  36  and  38 . Common mode regulator  32  includes a FET  40 , which may be used to turn the common mode regulator on and off. FET  40  may be excluded from the implementation of other common mode regulators, particularly in common mode regulators that are always on. 
     The output terminals  42  and  44  are respectively coupled to output terminals  18  and  20  of current steerer  12 . The resistances  36  and  38  should each have a resistance value that is high enough to prevent a substantial current from traveling through resistance  36  or  38 . Depending on the design of the common mode regulator  32 , resistances  36  and  38  may have a value such that only a minimal portion of output current (e.g. less than 1%) travels through these resistances. Resistances  36  and  38  may each be a resistor, such as a doped silicon or polysilicon resistor formed in a Complimentary Metal Oxide Semiconductor (CMOS) process, for example. 
     In order to establish a common mode voltage in a differential charge pump, voltage driver  34  is coupled to node  46  which joins resistance  36  and  38 . Voltage driver  34  determines the common mode voltage that is output at output terminals  18  and  20  of the current steerer  12 . Voltage driver  34  may be set to a variety of voltages. For example, the common mode voltage may be determined by the technology node (i.e., 5V, 3V, or 1.6V) or an application that a particular differential charge pump is directed to. 
     Common mode regulator  32  prevents deviation in common mode voltage, and in particular common mode voltage drift, in current steerer  12  by driving the voltages at the output terminals  18  and  20  to the voltage level of the voltage driver  34 . Without common mode regulator  32  (and voltage driver  34 ), deviations in common mode voltage may cause the dynamic range of current steerer  12  to decrease. In addition, other deleterious effects may occur. One effect may be any of the FETS  24 - 27  becoming pinned at a supply or common voltage, thereby further reducing or eliminating the output range. 
     Common mode regulator  32 , however, prevents unwanted charge build up at terminal  18  or  20 , or a voltage from developing across these terminals, by sinking extraneous charge. The common mode regulator  32  may include a ground terminal or common terminal for this purpose. Additionally, because the voltage driver  34  operates independently from the current steerer  12 , extraneous charge will not cause its voltage level to drift over time and, as a result, the common mode regulator  32  is coupled with the current steerer  12  in an open loop. 
     The voltage driver  34  may be designed in a variety of ways. One such voltage driver  48  is illustrated in  FIG. 2   b . Voltage driver  48  includes an inverter  50  having its output coupled in negative feedback to its input. A resistance  52  is also used to couple the input of the inverter to its output. Capacitances  54  and  56  are also included in the voltage driver  48 . 
     The negative feedback configuration of inverter  50  sets the voltage at node  58  to the switching threshold of the inverter  50 . For example, if the switching threshold is at 1.5V the voltage at node  58  will be 1.5V and therefore the common mode voltage of the current steerer will also be set to 1.5V. The switching threshold is determined by the design of inverter  50  and, depending on the application, may be adjustable. 
     Resistance  52  and capacitances  54  and  56  may be used to reduce noise, or glitching, in the switching of FETs  22 A-D located in current steerer  12 . Resistance  52  and capacitances  54  and  56  may be tailored to a specific current steerer or excluded. Other types of tailoring, such as selecting a mid-rail voltage, may be used to maximize the range of output terminals  18  and  20  of current steerer  12 . 
     Another way to maximize the range of output terminals  18  and  20  is to implement an active regulator. The active regulator maximizes the voltage that is applied to nodes  28  and/or  30 . As mentioned above, if a voltage applied to either one of these nodes is distributed across other circuit components subsequent to it being applied to node  28  and/or node  30 , the output range of output terminals  18  and  20  will be reduced. 
       FIG. 3A  illustrates an example active regulator  60 . Active regulator  60  includes a current mirror (formed by FETs  62  and  64 ) and an amplifier  66 . The current mirror mirrors current from current source  14  to output terminal  68 . Amplifier  66  has its inputs coupled to the drains of FETs  62  and  64 . An output of amplifier  66  is coupled to the gates of FETs  62  and  64 . Amplifier  66  may be an operational amplifier, for example. 
     The output of amplifier  66  supplies a voltage that allows both FETs  62  and  64  to turn “on”. The supply voltage, V P , is pulled to the output terminal  68  and to both input terminals of amplifier  66 . The voltages between the drains of FETs  62  and  64  (amplifier  66 &#39;s input terminals) cannot deviate significantly from each other without increasing the current through FETs  62  and  64 . Therefore, the drains of FETs  62  and  64  will both maintain a voltage that is about equal to the supply voltage. In addition, the current mirror will mirror the source current to output terminal  68 . As a result, the active regulator  66  allows the source current to be supplied to output terminal  68  without reducing the voltage level at output terminal  68 . Output terminal  68  may be coupled to node  28  of the current steerer  12  to provide the source current and supply voltage. 
     A second supply, or common supply, voltage can also be coupled with a second active regulator that is coupled to the current steerer  12 . Example active regulator  70  is illustrated in  FIG. 3B . Active regulator  70  also includes a current mirror (FETs  72  and  74 ) coupled to an amplifier  76 . Output terminal  78  is coupled to one input terminal of amplifier  76 . The other input terminal of amplifier  76  is coupled to current sink  16 . The output terminal  78  provides both the sink current and the common supply voltage V N  to node  30  of the current steerer  12 . In the same manner as active regulator  60 , the common supply voltage supplied to output terminal  78  is optimized as it is directly distributed to current steerer  12  and does not have to be “dropped” across current sink  16  prior to being communicated to node  30 . 
     All of the above examples may be used in combination to create a differential charge pump. For example,  FIG. 4  is a circuit diagram of differential charge pump  80  including common mode regulator  32 , active regulator  60 , and active regulator  70  all coupled to current steerer  12 . Differential charge pump  80  offers an improved operating voltage range and a common mode voltage that is determined without feedback. 
     Overall, the above examples describe a differential charge pump that offers an improved output range of lower operating voltages. As described above, these lower operating voltages may be associated with decreasing transistor sizes. The differential charge pump may include active and/or common mode voltage regulators. Although several example circuit structures have been shown, the present application should not be viewed as limited to these examples. A variety of structures and implementations may be realized that would be analogous and apparent to one skilled in the art. Additionally, the output of the differential charge pump may be a voltage or a current. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all examples that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.

Technology Classification (CPC): 7