# Current Context and Research Trends in Linear DC-DC Converters
## 3 Linear/Low-Dropout Regulators Operating Principle  
The basic concept of a linear regulator is to use a series pass element to act as a variable resistor against the input voltage fluctuations and load current changes to supply a regulated output voltage [10]. Transistors or a combination of transistors are used as the series pass element and using a feedback control loop, as shown in Figure 4a. The voltage drop across the series pass element is controlled to regulate the output voltage via the reference voltage (\(V_{\textit{ref}}\)).  
Figure 3: Efficiency vs. switching frequency variation in switching devices.  
As shown in Figure 4b, a voltage comparator is used to generate the feedback signal, and a simple voltage divider circuit is used as the feedback. With the assumption of ideal op-amps, output voltage \(V_{out}\) can be formulated as in Equation (3) [11].  
\[V_{out}=V_{ref}\left[1+\frac{R_{x}}{R_{y}}\right] \tag{3}\]  
The efficiency of the linear regulator (\(\eta\)) approximately can be calculated by Equation (4) [11] as follows:  
\[\eta=\frac{V_{out}I_{out}}{V_{in}I_{in}}\approx\frac{V_{out}I_{out}}{V_{in}(I_ {out}+I_{q})}\approx\frac{V_{out}}{V_{in}\left[1+\frac{I_{q}}{I_{out}}\right]} \tag{4}\]  
where \(I_{q}\) is the current used by the control circuit, which is usually referred to as ground pin current. The efficiency of the linear regulator can therefore be improved by designing a control circuit with minimum ground pin current. In addition, it is important to note that efficiency is \(V_{out}/V_{in}\) when the ground pin current is negligibly small.  
Compared with the SMPS, there are specific advantages of using linear regulators such as low cost, low noise, and design simplicity, and the major advantage of using linear regulators is the excellent transient response to load current fluctuations [3]. However, linear regulators can only be used in applications where the input and the output voltage difference is very small. Otherwise, the efficiency is compromised. Low-dropout regulators (LDOs) are introduced to overcome the efficiency issues associated with linear regulators.  
The name LDO is denoted because of the small voltage difference in dropout (\(V_{d}\)) between the input (\(V_{in}\)) and the output (\(V_{out}\)) voltages (see Equation (5) [11]).  
\[V_{d}=V_{in}-V_{out} \tag{5}\]  
Power consumption across the LDO (\(P_{D}\)) can be formulated as in Equation (6).  
\[P_{D}=[V_{in}\ -V_{out}\ ]I_{out}+V_{in}I_{q} \tag{6}\]  
where \(I_{q}\) is the quiescent current. When the dropout voltage \(V_{d}\) is high, the power dissipated through the LDO is high. The first term of Equation (6) represents the power dissipated through the series pass element. Therefore, LDOs are typically used in applications where the dropout voltage is very small. In addition, the lower the dropout voltage, the higher the efficiency is. The second part of Equation (6) represents the power consumed by the controlling circuits. Most commercially available LDOs have a very small ground pin current. As a result, the power consumed by the control circuit is very low, and this improves the efficiency of the LDO.  
An LDO uses a single transistor as the series pass element. PMOS and the NMOS are the two widely used CMOS devices and, of the two, NMOS have the lowest ON resistance. However, when using an NMOS device as the pass element, the charge pump is required  
Figure 4: (**a**) Linear regulator functional diagram; (**b**) basic linear regulator configuration.  
to drive the NMOS, which makes the design complex and requires more PCB area. Due to the aforementioned disadvantages of NMOS, generally, PMOS is used as the pass element of the LDOs. Further details are given by [PERSON] [12].