4 - Component Selection
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, 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. The process is similar for Mouser and other sites.
Using 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 general part category and finding the variant or version you want.
There is a menu on the left that allows you to narrow down your search. Selecting one of these options will open up a sub-menu that will try to narrow down your search query.
For example, selecting 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 are applied. Filters are incredibly important to find the one component you need out of the thousands of others in the same category.
Use them to sort out the components by parameters, such as Value, Manufacturer, Package, etc. There are also a set of checkboxes which are also helpful for filtering components, especially for stock and environmental compliance.
The bottom left Column is where all the actual components are. There is usually a manufacturer-provided picture, along with a short description.
One row will contain all the various specifications for a single component, including dynamic figures like stock and price. These columns can be sorted in ascending or descending order by clicking the up or down arrow 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 under a filter (using the Ctrl or Shift key), and a resistance filter was set with a 200 Ohm to 400 Ohm 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. Some parameters to watch for in an LDO are the input voltage range, output current limit, and the dropout voltage, which is the minimum voltage difference between your input and output voltage for the LDO to function.
In addition, 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. It is also 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
More on these preferred numbers, called “E-series”: https://en.wikipedia.org/wiki/E_series_of_preferred_numbers
Typically, 0603 (0.06” length by 0.03” width) is a good size for a passive component. 0402 is quite smaller and difficult to solder, but can be used to prioritize space-efficiency. 0805 and larger is usually necessary for components with bigger values, and due to their larger physical footprint 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 affects the power rail’s voltage.
It is encouraged at this point to do some research on bulk capacitors and decoupling capacitors and what their differences are.
Selecting capacitors: https://resources.altium.com/p/which-type-capacitor-should-you-use
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 (Vf) 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 maximize and burn it out.
An example datasheet is shown above, where some LED characteristics are shown. Your current limiting resistor value will depend on the voltage of the power rail that the LED is connected to, but typically it is no more than 2kΩ. You may need to do some simple math (Ohm’s Law) to find the right resistor and get the desired current.
Selecting Connectors
Selecting a connector for your PCB is quite open-ended, but some recommend for power connectors to be through-hole type (and not surface mount), such as screw terminals. Molex is a common choice for these rudimentary connectors, but manufacturers themselves 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 (these usually can do 3A or more). Samtec is one popular manufacturer of header pins (but mostly known for their high-density RF connectors).
If you want to try something different, consider implementing a USB connector. The Micro-B standard is quite easy to implement, and USB 2.0 allows for up to 5V 500mA to be arbitrated and pulled by a peripheral. USB 3.0 formally requires 5V 900mA as a minimum capability for all host devices, but most modern devices are all capable of providing 5V 3A.
USB-C is obviously superior than Micro-B when it comes to UX and industry relevance, but its implementation is slightly more complex. It will also do 5V 3A without an advanced PD (“Power Delivery”) handshake. With USB-PD, it can reach up to 240W (48V 5A, USB 3.2 optional spec.)
More about USB connectors and standards:https://en.wikipedia.org/wiki/USB_hardware
Once you have finished selecting all your components for your board, you’ll be ready to move on to your schematic!