1.  Field of the Invention
The invention relates generally to the field electronics, and more particularly to charge pump circuits that can be used in systems such as phase locked loops. 
2.  Related Art
Charge pumps are used in a variety of electronic circuits.  Charge pumps are commonly found in the phase locked loops (PLL's).  In a PLL, a charge pump typically receives to input signals, one of which drives the charge pump to increase the charge at its output, and the other of which drives the charge pump to decrease the charge at its output.  As the charge at the output of the charge pump increases or decreases, a voltage is developed with respect to ground.  This voltage is applied to the input of a voltage controlled oscillator (VCO) to control the frequency of the signal that is output by the VCO. 
Typically, the design of a charge pump must take into account a trade-off between the range of output voltages that can be generated by the charge pump and the amount of ripple in the charge pump's output current.  This trade-off is controlled, for the most part, by the size of the switching transistors through which the output current flows.  Larger switching transistors allow greater current to flow at the output of the charge pump, enabling a wider range of voltages.  The larger transistors, however, also produce a greater ripple in the current.  Smaller switching transistors reduce the amount of ripple in the output current, but they also limit the range of voltages that can be generated by the charge pump. 
It would therefore be desirable to provide a charge pump that can generate a wider range of voltages at its output without increasing the amount of ripple in the output current.  Alternatively, it would be desirable to provide a charge account that can generate the same range of voltages at its output with a reduced amount of ripple in the output current. 
Another problem that is present in conventional charge pumps is their inability to directly accept low-voltage input signals.  Typically, the analog components of a conventional PLL (including the charge pump) operate at a higher supply voltage than the digital components of the PLL (e.g., the phase and frequency detector that provides the input signals to the charge pump.)  Because the control signals generated by the phase and frequency detector have a lower voltage range than the signals expected by the charge pump, the charge pump's interpretation of the control signals is subject to increased errors resulting from noise in the signals.  Since the charge pump cannot reliably interpret the low-voltage signals directly from the phase detector, it is necessary to implement a converter to convert the low-voltage signals output by the phase detector in to higher-voltage signals that are acceptable as inputs to the charge pump. 
It would therefore be desirable to provide a charge pump that can directly accept the low-voltage signals from the phase detector, thereby eliminating the need for the converter circuitry to increase the voltage of the control signals.