Modeling of Cross-Coupled AC-DC Charge Pump Operating in Subthreshold Region

This paper proposes a circuit model of a cross-coupled CMOS AC-DC charge pump (XC-CP) operating in the subthreshold region. The aim is to improve the efficiency of designing XC-CPs with a variety of specifications, e.g., input and output voltages and AC input frequency. First, it is shown that the output resistance (Ro) of XC-CP is much higher than those of CPs with single diodes (SD-CP) and ultra-low-power diodes (ULPD-CP) as charge transfer switches (CTSs). Second, the reason behind the above feature of XC-CP, identified by a simple model, is that the gate-to-source voltages of CTS MOSFETs are independent of the output voltage of the CP. Third, the high but finite Ro of XC-CP is explainable with a more accurate model that includes the dependence of the saturation current of MOSFETs operating in the subthreshold region on the drain-to-source voltage, which is a function of the output voltage of CP. The model is in good agreement with measured and simulated results of XC-, SD-, and ULPD-CPs fabricated in a 250 nm CMOS.