Summer-Less Dual Charge Pump Based PLL With Wide Lock Range Using Analog Frequency Detector
Raghavendra, R G
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Phase Locked Loop (PLL) is an integral component of clock generation circuits. A third order Charge Pump PLL (CPPLL) is most widely employed PLL architecture due to its zero steady state phase error. A monolithic implementation of such a CPPLL presents numerous challenges to PLL designers, the number of such challenges vary depending on the process technology employed and the end application. One such challenge that is worth mentioning is the on-chip integration of the second order passive loop filter. The area occupied by the second order passive loop filter is mainly determined by the zero determining capacitance (CZ). A low loop bandwidth CPPLL has a higher CZ value, and hence consumes a larger die area than a large loop bandwidth CPPLL. Literature survey shows that the problem of higher CZ value in low loop bandwidth CPPLL is addressed by using some form of emulation techniques. A relatively simpler emulation technique is the use of dual charge pump based loop filter. Existing dual charge pump based loop filter consume extra elements (such as summer that need opamps to realize the summer function) for achieving low CZ value. These extra elements consume extra area and additional power. We present two types of Summer-Less Dual Charge Pump (SDCP) based loop filter designs that do not need extra elements and still achieves low CZ value and this is achieved by using a second charge pump in an appropriate way. A test chip was implemented in 0.13µm UMC MMRFCMOS process to verify the presented circuits. The presented SDCP based loop filter circuits are particularly useful in designs employing multiple CPPLL’s and design employing low loop bandwidth CPPLL’s. Another challenge worth-mentioning is the frequency ranges over which the PLL can be locked. The Voltage Controlled Oscillator (VCO) of PLL mainly determines the frequency locking range of a PLL. A typical VCO has a frequency locking range of usually 1:2 to 1:3. The VCO frequency tuning range reduces with reduction in supply voltage. This poses a serious problem in low supply voltage applications that demand a wide frequency locking range, sometimes greater than 1:3. We have addressed this problem of wide PLL lock range, by using an Analog Frequency Detector. A wide frequency lock range is achieved, either by dynamically modifying the VCO or the feedback divider of PLL. Both the approaches are equally feasible. The frequency detector is used for dynamically modifying the VCO or the feedback divider of PLL. Two test chips were implemented to verify the presented Analog Frequency Detector scheme. A testchip implemented in 0.25µm CSM analog process achieves wide frequency lock range by dynamically modifying the feedback divider of PLL. Another testchip implemented in 0.13µm UMC MMRFCMOS process achieves wide frequency lock range by dynamically modifying the center frequency of the VCO. Presented analog frequency detection scheme is particularly useful in applications that demand wide PLL lock range from a single die.