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Unless you are working with a purely AC system, your PCB will need to receive stable DC voltage to operate properly. A voltage regulator circuit provides the necessary DC voltage with fixed magnitude, even if the input voltage (line) or the output current (load) changes. Some circuits are more sensitive to voltage fluctuations than others, and some power lines are noisier than others. Any designer should understand how to choose the right voltage regulator for their board. Let’s take a look at the different types of DC regulators and go through some factors to consider when selecting a voltage regulator.
Types of Voltage Regulators
Voltage regulators are normally placed on the output from a full-wave rectifier circuit in order to remove the remaining ripple waveform. There is more than one way to classify a voltage regulator, but for the moment we will stick to linear and switching regulators. These regulators can be added to a layout relatively easily and provide reasonably stable output voltage for most applications.
Linear Regulators
Linear regulators use a BJT or FET to stabilize the supply voltage and are controlled with an amplifier. The amplifier compares the output voltage from the regulator to a precision reference and changes the transistor to maintain a constant output voltage. Linear regulators always step-down the output voltage (i.e., input voltage > output voltage). Linear regulators with low drop-out voltage are called low dropout regulators (LDO). Linear regulators provide the following advantages:
- Low Noise: As there is no switching involved, these regulators generate low noise and are best used for powering sensitive circuits. In contrast, switching regulators are inherently noisy as they switch frequently in order to maintain the output voltage.
- Low power: When properly designed, linear regulators can operate with quite low quiescent current. Switching regulators use complex feedback systems and end up using higher quiescent power. When operating as LDOs, these regulators can have very high efficiency (~90%).
- Low cost: Linear regulators are less costly, and they are easy to add to a layout. They do not require too many components and filters. Usually, a capacitor is placed across the output to help regulate the output voltage.
Example linear regulator circuit
Switching Regulators
A switching regulator converts an input DC voltage to a more stable DC output with a power MOSFET or BJT switch. The output from a switching regulator is usually filtered and used to reduce switching noise on the output voltage. There are three types of switching regulators: buck (step-down), boost (step-up), and buck-boost (can provide step-up or step-down). Switching regulators provide the following advantages:
- High efficiency: Since switching regulators work in either off or on mode, they tend to be more efficient. They can provide 90% or greater efficiency, which is very difficult in most linear regulators unless they are designed to as LDOs.
- Step-up configuration: Linear regulators can only step down the input voltage, but a boost switching regulator can step-up the voltage. This is particularly useful in the case where a large voltage is required for a short period of time (e.g., the backlight in a camera).
- Thermal performance: Linear regulators are less efficient, which means they tend to dissipate more heat and need a heat sink. Most switching regulators do not need heat sinks.
The buck and boost characteristics can be controlled with a PWM waveform, which makes this regulator ideal for use in applications where specific voltage levels are required. For example, you can use the PWM output from a microcontroller to power another circuit with a specific voltage level. There are many different ways to build a switching regulator IC, and we can’t possibly show every possible circuit. If you are interested in learning more about specific circuit layouts for switching regulators, take a look at the datasheets in your parts library.
Important parameters for choosing regulators
One question I often see on forums is how to choose a voltage regulator for different applications. There is no single answer to this question. You should consider the following characteristics when looking for a voltage regulator; note that these aspects apply to both linear and switching regulators:
Output voltage
If you need a step-up regulator, then you will have to use a switching regulator in boost configuration. If you are using multiple supplies in a system, typically one regulator is used for each supply. Regulators usually supply a fixed output voltage, although there are some regulators available on the market which you can use with some adjustable settings.
Efficiency vs. Noise
Switching regulators provide higher efficiency than linear regulators, but they generate more noise. Unnecessary electronic noise can interfere with other circuits when the regulator outputs high current. If you want your product to hit the market, it can be more difficult to receive EMC certification. If other circuits on your board are sensitive (e.g., purely analog components), then it’s better to choose a linear regulator.
Selecting a regulator with high power losses can make it almost impossible to meet efficiency goals. Linear regulators are less noisy, but they are very inefficient (except LDO), meaning some power gets converted to heat. If your regulator will run at high current, you should include heat sinks on the board. If you don’t have room on the board for a heat sink or if power dissipation is a concern, then a switching regulator might be a better option.
These 7805 linear voltage regulators from Fairchild include a heat sink on the back of the package.
Transient response
When output current changes quickly, there is a small spike at the output. The voltage regulator takes some time before it switches back to the same voltage. This is called a transient response. The transient response is usually a function of the output capacitance and load current. A fast transient response ensures that the regulator can deliver the required power. Check your component datasheets and look for the recommended bypass capacitor you should include on the regulator’s output.
Voltage Regulator Layout Considerations
Once you have chosen the right voltage regulator for your design, you’ll need to place it in the proper location on the board. In general, you will need one capacitor between output and ground, and one between the input and ground as close to the pins as possible. You should also carefully design traces so that they can carry the required current without overheating.
If you take a look at some of the open-source projects in Upverter’s™ project library, you’ll find some good examples of regulator layouts that you can use as a reference for your next project.
Boost converter control board from James Fotherby, created in Upverter.
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