DC Power Supply Design

By Raymond Sung, Patrick Chan, Jason Mah and Andrew Sung

Some people might be wondering how to obtain different regulated voltages than what is available by tapping off the power-supply of the Altera UP1 board. (for those groups who want to use another FPGA in their designs, this application note might be of even more use to you) Furthermore, the available power supplies in the EE labs are often damaged somewhat from overuse and use of these supplies directly might be harmful to sensitive circuitry. If you need an external DC power supply in your designs then read on.

While it is possible to obtain different voltages and power supplies using discrete components as you did in other course work, i.e. EE 340 Shunt Regulator using a Zener, this practice is not recommended for actual designs. This is because the performance of external hardware often depends on having a DC supplies with low-ripple in the mV range. That is where the use of integrated circuit power supplies comes in.

Basically, there are three types of DC-DC power supplies available in integrated circuits. The three of them, in increasing order of complexity are: linear regulators, switched capacitor (sometimes called charge pumps), and switching regulators (sometimes called buck/boost regulators). Depending on your application, you might have to choose one of the more complicated supplies to meet your hardware requirements.

Linear voltage regulators are the simplest power supplies to use. Some electronics engineers call them plug-n-play supplies because of this. With a linear voltage supply, you can obtain a regulated, low-ripple, DC output from an unregulated DC input. However, the input voltage must be greater than the desired output voltage by a factor called the dropout voltage. This is an important point because you can never obtain an output voltage greater than . Many manufacturers boast that their linear regulators have low-dropout. This is a major concern only if your input voltage is slightly above the desired output voltage. Otherwise, save your money and opt for a linear regulator that does not boast a low-dropout. A typical +5V linear voltage regulator is the MAX667 by MAXIM.

As one can easily see, the connections for these voltage regulators are simple. Connect a battery or other power source to the input of the IC, apply a ground and put a small output capacitor on the output pin. (many linear regulators require an input filtering capacitor as well, always read the data sheet thoroughly). When reading the datasheet for these types of DC supplies, the parameters of most interest are the output voltage and output current. The linear regulator IC must be able to support your desired output voltage, usually 5V or 3.3V for CMOS logic. Linear regulators also come in varieties which generate negative voltages for negative power rails (op-amps, A/D, D/A converters, LCDs often need them). Most regulators are adjustable to any desired voltage by merely adding a resistor divider network, so this is often not a major concern. However, the rated output current is important to consider. The output current is the maximum current that you can draw from the power supply without damaging it. Before, choosing a power supply, have a good idea of the power dissipation in your circuit. This will make life much easier for you later on. (Just a note for the interested, the rated dropout for a linear regulator is proportional to the current drawn from it, the MAX667 for instance is rated for 350mV dropout at 200mA. As an example, this means that you in order to regulate to 5V while drawing 200mA, that the input voltage must be equal to at least 5+0.35=5.35V). Incidentally, not only are linear regulators easy to use, but they also introduce very little noise to the rest of your circuit. This could not be said about the more complicated power supplies that we will discuss later.

A switched capacitor DC-DC supply is more versatile than a linear regulator. These converters come in different varieties which allow one to double the voltage, invert the voltage (to produce negative voltage rails), double then invert, etc. You can also find models which step-up or step-down voltages in contrast to linear regulators which can only step down voltages. Furthermore, they come in regulated or unregulated versions depending on your needs. If you already have a regulated voltage and want to double it for instance, then you don’t need a charge pump that produces regulated output since these often cost more.

A schematic of the typical operating circuit for a charge pump, the MAXIM MAX682 is as follows:

Up until now, it seems that charge-pumps are superior to linear regulators because you can do so much more with them. However, as in most engineering applications, trade-offs exist for the added versatility. The first problem with switched-capacitor converters is that there are often up to three or four external capacitors needed for proper operation. These capacitors have to be a special type having low effective series resistance (low-ESR) to minimize the output ripple. The IC manufacturer will state the low ESR requirements more specifically and give suitable component manufacturers. Keep in mind however, that low-ESR capacitors are probably not stocked in the EE stores room are often relatively expensive to buy. With recent electronic component distributors that guarantee next day delivery, such as Digikey, obtaining low ESR capacitors is not a problem at least. Furthermore, these DC-DC converters are inherently noisier than linear regulators because of how they operate. Basically, charge pumps transfer charge from the input to the output using a flying-capacitor. The flying capacitor is allowed to charge-up and then this charge is transferred to the output after a certain period of time as determined by an internal timing circuit. The switching action often occurs at relatively high frequencies from several kHz to several MHz. What the switching does is that it often introduces noise to the rest of your circuit. This can be a problem if you intend to use sensitive electronics that implement Radio Frequency, for instance. Also, if you have another timed circuit operating at the same frequency or even a sub-harmonic of the capacitor switching frequency, then that timed circuit may not operate properly due to noise. Manufacturers often allow adjustment of the switching frequency, with performance trade-offs involved, to eliminate the noise problem, although this does take more effort on the part of the designer. Furthermore, the maximum output current is often limited to around 300mA or less for these type of converters. (For the interested, power conversion efficiency; the input voltage that is actually converted to the output voltage and not dissipated as heat; is rather low, around 70-80%. Conversion efficiency is important in battery and power-saving applications) That coupled with the fact that linear regulators are limited to around the same maximum current output leads us to the most complicated of DC-DC converters, the switching regulator.

Switching regulators are often the most versatile of power supplies. These regulators are separated into three types: boost or step-up (for increasing the input voltage), buck or step-down (to decrease the input voltage) and buck/boost (dual-mode functionality for increasing or decreasing the input voltage). These power supplies come in varieties that can source anywhere from hundreds of milliAmperes to several Amperes of current. They can also step-up or step-down voltages from very extreme values and guarantee very high power conversion efficiencies. They are used extensively in home appliances, computers, and other high voltage electrical circuits. A typical DC-DC switching regulator might be the MAXIM MAX1678. This one is a boost/step-up type.

This regulator for instance, can regulate up to 5V from a single AA 1.2V battery. The input and output capacitors, have to be of the low-ESR type as in the case of switched capacitor supplies. Furthermore, some switching regulators require a special type of diode to implement something called a synchronous rectifier. As one could easily notice from the diagram, an inductor is required to properly operate a switching regulator. This inductor has to be of the correct inductance value and current rating as specified by the IC manufacturer. The problem with inductors is that they introduce unwanted EMI into the rest of the circuit. That coupled with the fact that switching regulators use a switching action to produce its desired output voltage, makes this supply the worst for generating circuit noise. It is common practice to separate the switching power supply as far away as possible from other analog or digital circuitry. (As a matter of interest, the output ripple of a switching regulator is proportional to the current drawn from it) Use of a switching power supply, might be necessary in some instances, such as motor applications. However, one generally tries to avoid having to use this supply in projects unless its benefits are essential.

 This application note introduces the reader to integrated power supplies that he or she might need for their EE 552 projects. The advantages and disadvantages of the three commonly available DC-DC converters were discussed. Upon reading this application note, the reader should have a better idea of how to pick a power supply to generate different voltages for their applications.

 Any errors, additions or suggestions please contact Raymond Sung (sung@ee.ualberta.ca)