Current mode charge pump: topology, modeling and control

Current-mode control, the standard method to control inductive switching power supplies can also be used in switched capacitor converters. Here, regulation is achieved by operating one transistor in the charge pump output stage as a controlled current source. Compared with voltage mode operation where the transistor on-resistances is regulated, current mode keeps the small signal output resistance of the converter stage high even with high load currents. This always keeps the output pole, which is the dominant pole in current mode charge pumps at low frequencies even at heavy loads and makes the system easy to stabilize. Provided that the output pole is kept three to five times smaller than any parasitic internal poles, than a first order system is possible. When the controlled current source is placed in the discharge path then the converter always runs with a fully charged flying capacitor, which improves transient response. The universal second order current mode charge pump small signal model presented in this paper consists of a gm-stage error amplifier and a gm-output stage as regulated section. The model also includes the output resistance of the gm-stages, parasitic capacitances, the internal R-C compensation and the external components like the feedback divider, output resistor and load capacitor. The flying capacitor is modeled with a voltage source. By giving this voltage source different values the model can be used with any charge pump topology (doubler, tripler, inverter....). Experimental results show that by comparing the transient response the small signal model well matches with the measured performance. Since model can be used for AC and transient analysis an early prediction of stability and dynamic behavior of the converter is possible. Like inductive current mode converters the maximum output current is limited in the regulated section, which makes the converter short circuit protected.