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78 DESIGN007 MAGAZINE I JULY 2021 Cherie Litson: Look for the watts, or calculate it yourself—remember Ohm's law and Watt's Law? What kind of heat do you need to get rid of ? How much copper do you have on your board? en go to the online calculators (Sat- urn PCB Toolkit comes to mind) and crunch the numbers. If you still have questions, call the component manufacturer and talk with the electrical engineers there. Figure 1 is a great chart for you to keep around. Carl Schattke: e first thing I want to know is how many watts the part is going to need to dis- sipate. en, how am I going to be able to do that? A copper land area is not the only way to get heat away from a part. What's the environ- ment? We can get heat away with convection and conduction, or some combination of that. If you do not have thermal modeling tools avail- able, the lowest risk is going to be following the guidelines on the data sheet. Usually, you need less area than what is called out on a datasheet, but you may have a better method of dissipat- ing the heat than they do. You may have a large ground plane, or some structure that acts as a conductor away from the part, or some active cooling components that draw heat away. Lee Ritchey: Deciding on how much copper area is needed for heat dissipation requires a good thermal modeling tool. Any other approach is simply guessing. Chris Young: I suggest reading the following application report from Texas Instruments: AN-1520A Guide to Board Layout for Best ermal Resistance for Exposed Packages. is report details an approach to making low ther- mal resistance connections to copper regions within a PCB. A datasheet will typically give you a copper area and a copper weight. Rarely will datasheets give details about the environ- ment where the test/characterization measure- ments were conducted. is leaves you in a bind because there is not enough data present to work your way back to any specific and "safe" requirements for your board. I have found that many IC manufacturers will do whatever is pos- sible to show the best possible performance of the part in the datasheet. I have also found that this "whatever is possible" is not a reason- able approach to manufacturable designs. e most reasonable course of action is to build a prototype and take measurements in the environment(s) you expect your design to be operating in. DESIGN007 When a datasheet calls out an area of copper for heat dissipation, how do I interpret that requirement for my board? Figure 1: This handy chart allows users to calculate watts, volts, amps or ohms. Q

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