Phase change memory (PCM) is an emerging segment of semiconductor memory technology. Phase change memory operation requires a variety of DC power supply voltages to support read and write operations, with some voltages being higher than the external power supply. For example, phase change memory operation requires a variety of voltages to be generated on die to support operation with good DC characteristics of up to μS duration, e.g., ˜0.4 V for bitline precharge, ˜1 V for standard logic and ˜2.5 V for wordline, read sense, and write supply. Tight power supply tolerances are required for resistance sensing and writing PCM arrays with differing load currents and duration.
Voltage charge pump circuits are required to raise the voltages higher than the external power supply. The requirements/demands on the high voltage supply are challenging and include the requirement of low ripple while being able to supply current over a wide range, high current supply capability, and good tolerance. However, voltage charge pump systems developed from other types of memory devices do not serve the unique requirements of phase change memory sufficiently. In other words, they do not supply high voltage over a wide range, with high current capability while also having low ripple.
Charge pumps are used in integrated circuits to provide a boosted supply voltage in applications such as eDRAM memory, FLASH memory, bandgap voltage references, etc. Typical boost circuits first charge a capacitor from an external supply and then transfer the stored charge into a capacitor on a boosted supply net. A ripple voltage, though, develops on the boosted supply net that needs to be minimized. Currently, these ripples are minimized by placing large filter capacitors (also called decoupling capacitance) on the boosted supply net and by the use of multiphase pumps.
For a given design, a charge pump supplies current to drive an intended load. The output load current can vary greatly depending upon the different operating modes or other conditions. Further, the pump output current will increase with higher pump frequency and supply voltage. To regulate the output voltage of a charge pump with good precision and a tight voltage tolerance it is necessary to use a regulator circuit to control a charge pump, typically turning it on and off so at to keep the output voltage within a desired range. Typically, a charge pump provides some excess charge before it can be turned off. In addition, the boosted voltage must fall to a lower potential before a regulator will turn the pump back on. The excess charge and fall in potential is a characteristic of regulation and adds to voltage ripple on the boosted supply net.
More specifically, a charge pump system can be sized to provide adequate current at low supply voltage. However, it will also typically have excessive ripple voltage unless the dcap is sized for the higher charge transfer at the maximum supply voltage. But, decoupling capacitance typically consumes close to 50% of the total pump area in the design and has a significant impact on chip size.