# A New Control Scheme for the Buck Converter
###### Abstract  
In this paper, a new control scheme for buck converters was proposed. The buck converter utilizes the dual control loop to improve transient response and has the constant switching frequency. The control scheme is mainly as follows: (a) The switch-ON time is regulated by the constant frequency mechanism. (b) The switch-OFF time is regulated by the output voltage. The spec/features of the proposed converter are listed as: (1) The buck converter has an output of 1.0-2.5 V for the input of 3.0-3.6 V. The load current ranges from 100 mA to 500 mA. (2) The actual current sensor is not required. (3) The simulation results show that the recovery time is less than 1.6 us during load changes. (4) The variation in switching frequency is smaller than 1.05% over the output range of 1.0-2.5 V. (5) This circuit can be fabricated in future by UMC 0.18 um 1P6M CMOS processes. This paper depicts the control scheme, theoretical analysis, and implementation. switched-capacitor (SC) converters; switched-inductor (SL) converters; current mode control (CMC); voltage mode control (VMC); adaptive ON time (AOT); constant ON time (COT); pulse width modulation (PWM); peak current mode (PCM); average current mode (ACM) +
Footnote †: journal: Applied  
general, common control methods include: pulse width modulation (PWM) and peak current mode (PCM). For PWM, the feature is to fix switching frequency, and then change the pulse width to control the switches (S\({}_{1}\), S\({}_{2}\) in Figure 1). However, PCM usually has a variable switching frequency, and the switches ON/OFF will depend on the inductor current. In addition, whether for PWM or PCM, if the system is in steady state, when the switch (S\({}_{1}\)) starts to turn ON (Figure 1), the inductor current is zero. We refer to this as discontinuous-conduction mode. On the contrary, when the switch (S\({}_{1}\)) starts to turn ON, and the inductor current is not zero, we refer to this as continuous-conduction mode. In addition, the feedback stability is also an important topic that will be discussed in later sections. As an overview of the SL converters, the control modes can be classified into voltage mode control (VMC) [10] and current mode control (CMC) [11; 12; 13; 14]. In general, since CMC has more feedback paths than VMC, CMC should perform better than VMC in regard to the dynamic response and the voltage regulation. This is the reason why most current control schemes are based on CMC. In the conventional CMC, the control scheme requires a current sensor to get information about the inductor current. Therefore, how to sense inductor current is an important issue and is discussed in much of the literature. The related research has been summarized and analyzed in [15]. Several control modes based on CMC have been revealed to regulate the output voltage, such as: constant on time (COT), average current mode (ACM), constant off time (CFT), peak current mode, and adaptive on time (AOT) [16; 17; 18; 19]. The author of [15] uses a virtual inductor current sensor to obtain the inductor current traces instead of the real current sensor (Figure 1). The proposed scheme in [15] can greatly reduce hardware effort. However, it still needs the virtual inductor current sensor. The authors of [20; 21] propose a control scheme that does not require the current sensor but only senses the output voltage. In [20], the switch-ON time is regulated by the voltage difference between the input and output voltages (Figure 2). In [21], the switch-ON time (T\({}_{\text{ON}}\)) of S\({}_{1}\) is adaptive and regulated by the output voltage (Figure 3). Different from [20] and [21], in this paper, the switch-ON time is regulated by the constant switching frequency mechanism. In contrast to [22], the constant switching frequency mechanism consists of only a phase frequency detector, a charge pump, and a low pass filter. Figure 1: Buck topology with a virtual inductor current sensor. In this paper, a new SL buck converter with constant switching frequency will be proposed.