When sourcing electronic components, understanding the exact specifications and parameters of the part is critical — and sometimes difficult.
There are a few well-known ways that electronic component manufacturers use to define technical parameters:
- Parameters with minimum and maximum values. This is the best data possible. By specifying the entire range, the manufacturer of the component has performed a great deal of qualification tests in position to guarantee that the component will meet the rated specifications for the entire range of parameters.
- Parameters with typical values only. This is the least useful type of data. These parameters have a statistical character only. The manufacturer doesn't guarantee any range of approved deviations in mass production.
- Parameters that are based on calculations and included in data sheet as reference information. The correct design should take into account the extreme values (worst cases) for all critical parameters. If such parameters are defined in the data sheet with only their typical values, or as reference information, then it is necessary to get more detailed data from the manufacturer.
It is important to underline that manufacturers guarantee operating parameters for the recommended operating conditions only, and not for the absolute maximum ratings where the manufacturer guarantees only that component will not fail. The functional operation of the component at any other conditions beyond those indicated under recommended operating conditions is not guaranteed. Besides, exposure of a component to absolute maximum ratings conditions for extended periods may affect its reliability.
As a rule, all the electrical parameters are given in the specifications for 25°C (room temperature). Inasmuch as most of such parameters depend on ambient temperature, it is important to define them for the entire temperature range. If this information is missed in the specification, it may be achieved by calculations using temperature coefficients such as ppm/°C, mV/°C, etc. Designers have to be particularly sensitive to the current rating for electromechanical components (connectors, switches, relays) and their dependence on temperatures, because manufacturers of such components sometimes forget to mention the specific temperature for a given current rating and its possible derating versus temperature. It is very important to check the status of the data sheet itself before the component is taken for a design of an electronic device. There are five typical statuses of data sheet information:
- Advanced information (for information purposes only). The manufacturer provides the general information about a new component to potential customers. This component doesn't exist yet and it is still in the design stage.
- Preliminary. The design of the component is finished and it is ready for mass production. However, the entire specification is approximate and the manufacturer cannot guarantee all the operating parameters without approval from production lines about the acceptable yield. For statistical data and confirmation of all main parameters in the preliminary specification, it is necessary to manufacture the component in large quantities. This requires a high investment. The component manufacturer is interested in doing this testing at the expense of customers who may purchase these experimental components. The manufacturer does not guarantee the component's parameters in the specification with a preliminary status.
- Active (released for mass production). These components are approved for mass production and sales with guaranteed specifications. After production of components in sufficient quantities for yield confirmation, the manufacturer corrects the component's specification and replaces the preliminary status with final release status. From this moment onwards, the manufacturer guarantees full compliance of the component to its specification. Only components with this active status are recommended for use in electronic designs.
- Not recommended for new design (NRND). These components are not recommended for use in newly designed products. At a certain stage of a component's life cycle, the manufacturer may decide to give the status NRND for existing products because of low market demand or because of the establishment of newer technology. The manufacturer restricts the production of such component and supports only existing customers for a limited period of time. Besides, the status NRND of a component is also a serious warning signal of impending discontinuation of the part.
- Obsolete, end of life, or discontinued. Those are components whose production has already stopped or for which the manufacturer has specified the date of their discontinuation. Under these circumstances the manufacturer is releasing a product change notification (PCN) with two types of dates: last time buy (LTB), the last day permitted for purchase orders, and last time ship (LTS), the last day of shipment for ordered parts.
Needless to say that such components shouldn't be used in designed devices and for existing products. It is necessary to find replacements from other manufacturers (if the original manufacturer doesn't offer a replacement for the discontinued part) or look for an alternative part that will usually require a redesign of the end product.
Designers are often buried in a pile of datasheets. Understanding the real meaning behind the specs, though, saves time and money, and it ensures good design.