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Posted on 23 July 2019

DC-DC Converter Selection for Portable Electronics

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Power-management topology has a direct impact to portables

Portable electronics has forced system designers to focus on the design of their power-management circuitry. Today’s designer must find ways to extend battery life whilst maintaining performance. In addition, many portable applications are becoming even more compact, forcing circuit designs into smaller and smaller form factors.

By Kevin Tretter, Product Marketing Manager, Analogue & Interface Products Division, Microchip Technology Inc.

 

The design of the power-management circuitry in such systems is critical, and an understanding of different DC-DC converter architectures is essential to optimise overall design performance.

DC-DC conversion from a direct current at one voltage potential, to a direct current at a different voltage potential, is a critical function in battery-powered applications, where the voltage across the battery or batteries can change as the cell is depleted. For example, a single AA alkaline battery produces a nominal voltage of 1.5V when fully charged. As the battery cell becomes depleted, this voltage can drop to as low as 0.9V. A DC-DC converter may be required to boost the battery voltage to a different voltage potential, in order to operate various integrated circuits in the system. The choice that the designer must make is which of different DC-DC converter topologies to use: linear regulator, switching regulator or charge pump.

Linear Regulators

A linear regulator uses a voltage-controlled current source to create a given output voltage. There are several types of linear regulator, but for battery-powered applications, the most common is the low dropout regulator (LDO), such as Microchip’s MCP1703. This type of linear regulator uses a P-channel pass transistor as a variable resistor with feedback, in order to regulate the output voltage.

MCP1703 LDO DC-DC linear regulator

As this architecture relies on adjusting a resistive element to create a given output voltage, an LDO, or any linear regulator, can only create an output voltage that is lower than the input voltage. The minimum required differential voltage between the input and the output is called the dropout voltage. Since an LDO uses only a single P-channel transistor, it has a lower dropout voltage than the other linear-regulator topologies.

Another disadvantage of LDOs is that they can be inefficient. The resistive element that is needed to drop the input voltage to the required level dissipates power in the form of heat. This is considered lost energy, as it is being supplied by the input source but is not being delivered to the output. This can also cause thermal issues, as the regulator dissipates more and more energy in the form of heat. A system designer must be aware of the input and output voltage and current requirements. LDOs may not be a viable option for systems that require a large voltage drop and/or a large output current, due to issues caused by self-heating.

Despite these limitations, LDOs do provide some inherent advantages over other types of DC-DC converters. One advantage is that LDOs haven with relatively low noise, as there are no switching elements associated with their use. This can be very important to noisecritical applications that involve sensitive measurements, or to systems that must comply with strict government regulations.

Additional advantages include ease of use and overall size. The most basic LDOs come in 3-pin packages, providing an input, an output and a ground reference. Most LDOs require only a capacitor at the output for stability, which helps to achieve a very small circuit footprint. Finally, LDOs are typically more cost-effective than the other DC-DC regulator architectures.

Switching Regulators

In their most basic form, DC-DC switching regulators use a diode, an inductor and a switch in order to transfer energy from the input to provide a given output. Switching regulators can be configured in a number of topologies, including buck, boost and buck/boost. A buck switching regulator provides a regulated output voltage that is lower than the input voltage, similar to the functionality of a LDO. A boost switching regulator provides an output voltage that is higher than the input. This is a function that cannot be accomplished with a LDO.

Finally, a buck/boost topology provides a regulated output across a range of input voltages that are above and/or below the output.

The inductor inside the switching regulator acts as an energy-storage device, and is much more efficient than the resistive element used in linear regulators. A switching regulator can achieve efficiencies of 85 percent or higher. Except in very specific conditions, an LDO cannot achieve this level of efficiency.

Due to their high levels of efficiency, switching regulators are a better fit for higher-power applications, such as regulating a 24V power bus down to 5V and supplying 500mA to the load. In this example, an LDO would dissipate over 9W of energy in the form of heat, while only delivering 2.5W to the load. For many portable applications, implementing a large heat-sink or a cooling fan to overcome the thermal issues is not an option.

The switching element in a switching regulator is typically a transistor that is turned on and off rapidly. The frequency of this switching varies according to the regulator’s design, and generates harmonics around the switching frequency. Switching noise can be a problem for circuits that involve sensitive measurements, or for products which need to comply with electromagnetic interference (EMI) regulations.

Although IC manufacturers are making switching regulators easier to use, most still switching regulators still require an external inductor. The larger the inductor, the less effect the inductor current ripple will have on the regulated output, therefore, it is not practical to integrate the inductor into the silicon. Depending upon the size of the inductor, this can mean a considerable increase in the overall circuit footprint. The external inductor can also create noise, so additional care must be taken when laying-out the printed circuit board.

Charge Pumps

A charge pump is a DC-DC converter which uses a capacitor as an energy-storage device. Switches connect the plates of the capacitor to the input voltage in such a way that they can double, triple, invert, halve or even create an arbitrary regulated output voltage, depending upon the circuit topology. Because charge pumps charge and discharge capacitors to transfer energy, the amount of output current that this type of converter can provide is relatively low, compared to the other converters mentioned: typically no more than a couple of hundred mA.

In terms of efficiency, noise and footprint, charge pumps fall between linear regulators and switching regulators. The efficiency of charge pumps is better than that of LDOs, although switching regulators can achieve even higher efficiencies. The switches inside a charge pump create the same type of switching noise as a switching regulator, but they do not require an external inductor which radiates switching noise. Charge pumps also require an external capacitor to minimise the voltage ripple applied to the load. This external capacitor needs to be much larger than an internally-switched capacitor and can take valuable circuit-board space.

Summary

The advantages and disadvantages of each of the three DC-DC converter topologies are shown in Table 1.

DC-DC converter topology comparison

Selecting the optimum topology depends on the critical parameters if each application. If extending battery life is a priority, then a high-efficiency switching regulator may be the best option. If noise is a big concern, then a linear regulator could be the best choice. However, regardless of specific system issues such as noise or efficiency, the choice of power-management topology has a direct impact on the overall performance of all portable electronics.

 

 

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