The Art of Reading a Datasheet

Make no mistake about it, the first page of a data sheet was written by marketing.

This is the first lesson a young engineer needs to learn and is also a lesson typically learned the hard way. Every single specification on the first page (or pages) of a datasheet needs to be met with the question “under what conditions?”.

This really applies to any datasheet, but I am specifically referring to semiconductor data sheets in this article. Everything from max current to ON-state resistance to min/max trip levels are subject to conditions. It could be voltage, current, temperature, or duty cycle just to name a few, but something will affect the “spec” you see on the first page and is not to be taken at face value.

Of course, temperature dependence is one of the best examples that applies to all semiconductors. The specification for the “Thermal Resistance” of the package changes everything when it comes to other parameters typically listed on the first page. This is generally given in °C/Watt and is given from junction to case (Ѳjc) or junction to ambient (Ѳja). This is very often overlooked even by experienced engineers, especially if they tend to only do digital design and have not gotten into power or analog design.

The maximum junction temperature is almost always between 125 and 150°C (but again, always check your data sheet). By using the maximum power dissipated by the device and the Ѳja value one can calculate the maximum ambient temperature for the device.

Of course, things like heat sinking and air cooling can have dramatic effects on these numbers, but let’s stick to the most basic calculation first. If you have a part with a maximum junction temperature of 150°C and a Ѳja = 100°C/Watt and your device is dissipating 1W then here is the calculation: Temperature rise of junction = 100°C/Watt * 1W = 100°C. So, the junction temperature will be 100°C above the ambient temperature. This means that 50°C is the absolute maximum ambient temperature for this device in this design.

It is also very important to note that ambient temperature is not necessarily room temperature. I have seen many products incorrectly labeled as working to 70°C or 85°C simply based on the general version of the semiconductors (being commercial or industrial). The ambient temperature means the ‘ambient temperature SEEN by the part’. So, if your PCB is inside of an enclosure and it gets 20°C hotter than room temperature, then you must subtract another 20°C. So, in the example above, the max room temperature is now 30°C.

These calculations supersede anything else you read in data sheet such as commercial or industrial grade parts and the calculations need to be made for almost every part on the PCB.

So, if you are just starting out and one of your first assignments is to find an alternate source for a part on a current product, make sure you ask “under what conditions?” and look at every graph of every specification throughout the data sheet.

Love to hear feedback or questions