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New components shall not be sourced if a suitable alternative already exists in our library.
Vendor
Digikey is the preferred vendor since they have the largest variety of parts and they give us a discount on our orders. Other vendors may only be used if a desired component does not exist on Digikey and modifications cannot be made to the circuit to utilize components available on Digikey.
Stock
Components used should be “well in stock”, that is, there are more than just a few units and it is foreseeable that it will remain in stock into the future. It is typically months between first choosing a component and when you finish the board and order it, and we want to avoid OOS re-spins.
Packages
If specific standards are not specified in the component category sections below, standard and widely available packages shall be used. This facilitates component interchangeability and greatly speeds up the component creation process since we already have many footprints in our library. Only if a non-standard package has specific advantages, or when a unique part is only available in that package, can they be used.
Resistors
Manufacturer
Stackpole is preferred for SMD resistors due to their low cost and high availability. Other vendors may be used when parts are only available elsewhere.
Tolerance
All resistors, except for high power resistors required to dissipate a lot of heat, shall have a tolerance of 1% or less.
Package
Resistors shall be 0603 SMD parts unless higher power dissipation or higher voltage tolerance is required. Typical 0603 resistors can handle 50mW and up to 75V, and offer a good balance between component density and ease of assembly, rework, and probing for debug.
Power
Resistors shall generally be specced to handle twice the expected continuous load, to allow for more margin and reduce the effects of resistance’s temperature dependence. The change in resistance with respect to temperature is given by the part’s temperature coefficient.
Note that some resistors claim very high power ratings. These are often under certain conditions where case temperature is kept to 25C through external heat sinking. This is not realistic for us, since keeping a part at room temperature while under full load requires sub ambient cooling. Thus, appropriate derating is required along with external heat sinking for high power applications.
Capacitors
Type
MLCCs are preferred for most applications due to their low ESR and high capacitance per volume. Film or electrolytic capacitors may be used in high voltage or high capacity applications.
Manufacturer
Samsung generally has the lowest prices. For applications that require detailed characteristics, Murata caps will have that information in their simsurfing tool.
Tolerance
A tolerance of 10% is preferred, with 20% being acceptable. Higher precision parts may be used if warranted by the application.
Package
MLCC 0603 SMD unless the application calls for characteristics requiring a larger package size.
Voltage Rating
At least 2x the expected maximum voltage in the application or 40V higher, whichever is less.
A capacitor with the highest voltage rating for a desired capacitance and package should be chosen if the cost is not significantly different.
MLCC Dielectric
X7R: preferred when available
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Note that even X7R capacitors experience a significant drop in capacitance at higher voltages, typically > 50% drop at a 12V bias for 0603s. Graphs of capacitance vs DC bias for Murata caps can be found on their online simsurfing tool.
Inductors
Current
The maximum rated continuous current shall be at least 50% higher than the expected application. This value is generally covered by thermal limitations, the part will heat up due to IR losses.
The saturation current shall be below the expected peak current of the application. Beyond this point, the inductor will rapidly lose inductance, and can let much higher currents through than what is expected. Care should be taken especially in SMPS applications to account for the peak of the expected ripple current in the inductor.
Diodes
Package
The preferred package for low power applications is SOD-323. The preferred package for high power applications is DO-214AC.
Rectifying Diodes
Standard PN junction diodes can be used in rectifying applications where efficiency is not a concern. For cases where higher efficiency is desired, Schottky diodes can be used as they have a lower forward voltage.
TVS and Zener Diodes
TVS and Zener diodes are similar in their behaviour but differ in their construction. Both are used for their reverse breakdown characteristics.
TVS diodes are suitable for high instantaneous current, such as ESD or other forms of transient voltage suppression. Zener diodes have more precise breakdown thresholds, and are suitable for handling continuous reverse current such as for voltage regulation.
Fuses
Voltage
Fuses shall be rated for at least 25% higher than what the application requires.
FETs
Current
The rated current shall be at least double the expected application. The maximum current is typically defined by the thermal limit of how much heat the part can handle due to IR losses. Note that especially for high current FETs, the max current is given for specific conditions, often at a case (surface) temperature (Tc) of 25C. In reality, it is not realistic to keep the surface of the part at 25C under full load, as this would require sub ambient cooling. Thus, appropriate component derating must be performed.
LEDs
Indicator
Low power indicator LEDs are 0603 package. LEDs shall not be blue, as brief flashes on power-up/down looks like arcing.
Lighting
LEDs required in high power applications are of the JE2835 series.