# Current Context and Research Trends in Linear DC-DC Converters
## 7 Current Context of Portable Power Supply Designs  
The power supply design is considered an integral part of the overall system design of a portable device due to important design constraints determined by the user requirements, such as longer battery life and reduced power consumption. The most common storage technology that is used in today's power supplies is the Li-ion battery [55]. Driven by the volume of the portable device market, the Li-ion batteries provide the best combination of power density and cost. The typical operating range of Li-ion batteries is 3.0 V to 4.2 V, which is a very convenient voltage because fewer cells are required in many applications [56]. Li-ion batteries have a high energy density and also the self-discharge rate of Li-ion batteries is very low; typically 5% in the first 4 h and dropping down to around 1 to 2% per month [57]. In addition, Li-ion batteries have a higher number of lifetime charge-discharge cycles and they do not require regular maintenance, unlike other available batteries that require regular discharge to overcome the memory effect [58]. There is no requirement for priming Li-ion batteries as there is for nickel-based rechargeable batteries. Apart from these advantages, there are two major drawbacks of Li-ion batteries, which are poor robustness and flammability [59]. As a result, a high degree of protection is required against overcharge and excessive discharge. Portable devices normally have a combination of subsystems that require different voltage levels. As shown in Figure 9[60], modern portable devices such as mobile phones, tablets, and portable electronic medical devices consist of many subsystems: batteries, memory, display, peripherals, etc. [60]. Those components operate at various voltage levels based on their requirements. When designing powering techniques for these subsystems, it is very important to verify that it produces the desired voltage level regardless of the following:  
1. Variation of the current consumed by the subsystem. 2. Fluctuation of the battery voltage in the portable device. 3. Variations due to operating conditions of the device, such as temperature. Power supply requirements for a given application depend on many factors, and we can divide these into two different areas, namely, (I) primary specifications and (II) secondary specifications. Primary specifications include:  
1. Output rail voltage. 2. Maximum load current. Figure 9: Subsystems of a portable device [60]. * Output impedance of the power supply. * Noise and ripple allowed. * Input voltage range. * Efficiency. * Weight/volume of the power supply. * Secondary specifications include:
* Transient response capability and transient response. * Radio frequency interference/electromagnetic interference (RFI/EMI). * Reliability and the age of the power supply. Usually, for very large power requirements for a given output voltage and maximum current requirement, end-to end-efficiency (ETEE) is a critical factor, and this value depends on the design of the power supply. SMPSs are theoretically 100% efficient, based on the assumption that switches, inductors, and capacitors used are ideal. However, six secondary losses associated with a DC-DC converter reduce the ETEE to a lower value, and these six main losses are summarised in Table 4.  
In a linear regulator, Items 2 and 3 are nonexistent, and Item 1 is mainly dependent on the input to output voltage difference. LDO achieves higher efficiency by maintaining the input to output voltage difference at a very small value, usually in the range of a few tens of millivolts to fractional volts, giving a high ETEE of approximately of \(V_{out}/V_{in}\). When a small volume or weight is a primary target for a DC-DC converter, the switch-mode approach pushes the designer to move into higher frequencies, and this, in turn, increases the contribution by Items 2, 3, and 4. However, in an LDO, Items 2 and 3 are completely nonexistent, while capacitor losses are only related to the ripple of the power supply.