This chapter discusses the design of control circuits for low-voltage charge pumps, as illustrated in Fig. 4.1. VS and VPP are the input and output voltages of a charge pump, respectively. The VS can vary due to variations in the open-circuit voltage of the TEG according to fluctuations in the environmental temperature or due to variations in the input current of the CP. On the other hand, the VPP needs to be controlled to the target voltage required by a load, such as a sensor or RF IC, regardless of the variation in VS. Even when the VS is high enough for the CP to operate but not high enough for a bandgap reference or other analog circuits to operate, the power converter is required to work properly. Section 4.1 describes the use of an auxiliary voltage generator to supply a voltage for low-power analog and digital circuits in oscillators, regulators and maximum power point tracking (MPPT) control circuits. A voltage regulator usually only controls the VPP. However, when VS approaches a critical voltage at which the CP flows current from the VPP rather than to the VPP, the CP operation needs to be suspended to hold the charges stored in the decoupling capacitor (which is not shown in Fig. 4.1). Thus, the TEG regulator is designed to detect both VPP and VS and to control the CP properly. The design of the regulator is presented in Sect. 4.2. In Chapter 3 , the optimum design of charge pumps is discussed under the condition that VS is at a certain voltage, particularly at the minimum voltage for design. When VS varies, the optimum number of stages and capacitance per stage should be altered to maximize the output power from the charge pump. In Sect. 4.3, MPPT control is described.