# Current Context and Research Trends in Linear DC-DC Converters
## 8 Future Prospects for LDO Regulators in Power Supplies
### Concept of SCALDO  
When a precharged large capacitor is placed in series with the input path of an LDO, it keeps on accumulating the series drop, reducing the voltage drop across the series pass transistor of the LDO, assuming that the input DC voltage is constant. The voltage change (\(dv\)) of a capacitor with time is given by Equation (7) [72] as follows:  
\[dv=\int_{0}^{t}\frac{1}{C}\,i(t)dt \tag{7}\]  
where \(C\) is the capacitance and \(i(t)\) is the instantaneous current through the capacitor. For a constant load current, if the capacitance is very large, \(dv\) will be very small over time, and it takes a long time to block the DC path. Figure 11a depicts how this simple series connection helps in improving the efficiency of an LDO. If the capacitor \(C_{SC}\) has an initial voltage of \(V_{SC}\), input voltage at the input of LDO (\(V_{in}\)) will be expressed as Equation (8),  
\[V_{in}\,=V_{SC}-V_{p} \tag{8}\]  
As it carries on charging, at a later moment, the LDO will reach its minimum value for the correct operation of the LDO and at that point, the SC will have to be placed in parallel with the input of the LDO, as shown in Figure 11b. During this time, the input voltage  
Figure 11: SCALDO configuration: (**a**) phase 1: supercapacitor charging circuit [72]; (**b**) phase 2: supercapacitor discharging circuit [72]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Range** \\ \hline Rated voltage & V & 1.2–3.8 \\ \hline Capacitance & F & 1–3000 \\ \hline Specific energy density & Wh/kg. & 1–10 \\ \hline Specific power density & W/kg & \textless{}10,000 \\ \hline Cycle life & Cycle number & \textgreater{}50,000 \\ \hline Charge and discharge efficiency & \% & 85–98 \\ \hline Fast charge duration & Seconds & 0.3–30 \\ \hline Fast discharge duration & Seconds & 0.3–30 \\ \hline Shelf life & Years & 20 \\ \hline Operating temperature & \({}^{\circ}\)C & \(-40\) to \(75\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Supercapacitor specifications. source is disconnected, and the SC discharges back to LDO, to maintain the charge balance of the SC. When considering the current flow during phases 1 and 2, the charge balance of the supercapacitor following connection can be formulated as in Equation (9) [72]. \[I_{L}\;\;t_{charging}=I_{L}\;\;t_{discharging} \tag{9}\]  
where \(t_{charging}\), \(t_{discharging}\), and \(I_{L}\) are the charging, discharging times, and current consumed by the load, respectively. By simplifying Equations (9) and (10), it can be formulated as follows. \[t_{charging}=t_{discharging} \tag{10}\]  
With the above connection in Equation (10), the average current drawn from the unregulated power supply (\(I_{AVG}\)) can be formulated as in Equation (11) [72],  
\[I_{AVG}=\frac{I_{L}\;\;t_{charging}}{t_{charging}+t_{discharging}}=\frac{I_{L}}{2} \tag{11}\]  
Assuming there is no current drawn for the control circuit, the efficiency of the SCALDO can be derived as in Equation (12) [72],  
\[\eta_{SCALDO}=\frac{P_{out}}{P_{in}}=\frac{V_{reg}I_{L}}{V_{P}\;\frac{I_{L}}{2 }}=2\frac{V_{reg}}{V_{P}} \tag{12}\]  
From Equation (12), it can be seen that the efficiency of a SCALDO is two times higher than the typical regulator. As shown in Figure 12a [5], an experimental setup of 12 V to 5 V SCALDO was implemented to verify the proposed SCALDO technique. The 12 V to 5 V SCALDO was implemented using MCP1827 LDO from Microchip. As shown in Figure 12c, the maximum theoretical efficiency of a conventional 12-5 V LDO lies between 38-43% [73]. The 12-5 V SCALDO prototype achieved an overall end-to-end efficiency of 70-83.3% [73]. The setup was tested against a load current between 100-800 mA and the switching frequency is between 0.05 to 4 Hz, which is a very low operating frequency compared to SMPS. Instead of using a single supercapacitor depending on the requirements, an array of SCs can be used as a voltage dropper element. This further-extended concept is discussed in [74].