4 - Component Selection
- Michael Botros
- Jerry Tian
1 - Introduction
Component selection is an important skill to have as an EE. It can involve much research, calculation, datasheet reading, and intuition to source components effectively. In order to make the circuit required in the design challenge, you’ll need to select its components. In simpler terms, shopping is an important skill!
2 - Component Suppliers
Components for PCB projects can be sourced from supplier marketplaces where manufacturers and distributors sell their components
In order to get the components you need for your LDO, there exists large online marketplaces where large amounts of Manufacturers and Distributors put up their electrical related products up for sale usually in massive bulk quantities along with detailed information on each of their products.
Some common examples of these marketplaces are sites like Digikey and Mouser although you may also look at direct manufacturer websites for parts as well.
Either of these sites can be used for this bootcamp but this section will cover how to use Digikey to find components, although usually the process is the same for Mouser and other sites.
How to use Digikey
When you open Digikey, you are greeted with a page like this.
The top search bar can be used to search for any component you are looking. Every part has a manufacturer part number that can allow you to find the part immediately as opposed to looking up a part in general and finding the variant or version you want.
There is also a menu on the left that allows you to narrow down your search, selecting one of these options will also open up a sub-menu that will try to narrow down your search.
For example if I select Integrated Circuits->PMIC->DC DC Converter, I am finally greeted with a different looking page.
This scary looking page is where you can find the exact components you need.
The top section is where all the filters all applied. Filters are incredibly important to find the 1 component you need out of something like 10000 of the same category of component.
Filters sort out the components by things like Value, Manufacturer, Types etc…
There are also a set of checkboxes which are also helpful for filtering components, especially for stock and compliance to certain standards.
The bottom left Column is where all the actual components are. There is usually a picture of sorts along with a short description.
The row of that component contains all the various specifications for the component, usually that you set with your filters. Things like stock and price are also shown directly to the right of the component. These columns can be sorted in ascending or descending order by clicking the up or down arrows on the top of the column.
An example of some filters being applied to a resistor selection can be seen below. The “In Stock“ checkbox was selected, multiple manufacturers were selected using the Ctrl or Shift key, a resistance range was put on the top by specifying and maximum and minimum resistance range.
Selecting a component from the components list will bring you to the following page.
On this page, all the specifications are shown along with price and stock. Note the datasheet link on the page. This link will yield a pdf with all the detailed information about the part. The datasheet is essential to understanding how to use a component.
Components Needed
In order to make your LDO circuit, there are certain external passive components that are needed. Typically, an LDO circuit looks like the one in the schematic below. It requires a main LDO IC which stores the power FET and error amplifier required to regulate the voltage output of the LDO.
In addition to this IC, you will likely need to source an input capacitor, feedback resistors and an output capacitor.
Keep in mind, the exact components you will need will entirely depend on the LDO IC that you select so think about the order in which you need to source your components!
Selecting an LDO IC
An LDO datasheet will likely give example schematics with values which are a great starting point to selecting your passive components. A simple LDO will have the schematic shown in the photo above.
An example electrical characteristics table is shown below. Things to watch out for in an LDO is the input voltage range, output current limit and also the dropout voltage which is the minimum voltage difference between your input and output voltage.
Also ensure the IC you are selecting has enough stock, this value should be at least in the thousands to be able to reliably source it.
Selecting Resistors
When selecting resistors from Digikey, make sure to use YAGEO as a manufacturer.
Also, it is important to select standard resistor values instead of finding the closest resistance value on Digikey based on a calculation.
See standard resistor case sizes here: Resistor Sizes and Packages
See standard resistance values here: Standard Composition Resistor Values
Typically 0603 (0.06” length by 0.03” width) is a good size for a component. 0402 is quite small and difficult to solder but can be used to be space efficient while 0805 is usually necessary for components with higher values but they are larger and as a result can usually be assembled easier.
Selecting Input and Output Capacitors
Input and output capacitors require specific capacitance values to properly filter noise on the voltage line and also supply energy in case a large transient or current rush drops the voltage line.
It is encourage at this point to do some research on bulk capacitors and decoupling capacitors and what their differences are.
When selecting a capacitor from Digikey, we typically like to select from Murata for ceramic capacitors.
We use the Murata SimSurf tool to figure out the effective capacitance of our capacitors since the effective capacitance of a capacitor actually changes with the voltage applied across it. This is called the DC Bias phenomenon.
Selecting LEDs
Selecting LEDs are pretty straightforward on Digikey. Most indicator LEDs on boards are chip surface mount components that look like this.
Notice the green marker that helps differentiate the anode vs the cathode. Different manufacturers have different labelling methods so check with the datasheet to make sure which side is the anode and cathode as this is very relevant for your schematic and PCB footprint.
Another thing to be careful regarding LEDs is their forward voltage drop. The forward voltage drop is the approximate drop in voltage across the LED when it is on, so your driving voltage needs to be higher than this forward voltage value. LEDs also need a current limiting resistor in series with the LED so that the current through the LED doesn’t burn it out.
An example datasheet is shown above where the LED characteristics are shown. Your current limiting resistor depends on the voltage of the input or output line that the LED is connected to but typically it is no more than 2kΩ. You may need to do some simple math to find the right resistor to get the target limited current you want.
Selecting Connectors
Selecting your connector is quite open, but some recommendations for connectors especially for power is to pick through hole connectors (not surface mount) and also screw terminals. Molex is usually a standard choice but manufacturers are not super important for this board. Male header pins are also another option for connectors, although they are not meant for high current applications.
Mating pitch is critical for male pin headers so stick with 2.54mm as a standard and positions and rows options vary depending on requirements.
Once you have finished selecting all your components for your board, you are ready to move on to making your schematics for your board.