I'm struggling to understand the chart of device current versus area of copper for heat dissipation in the datasheet for an LT4423 ideal diode and load switch. My output current will be around 1A at 12V.
How does one read this chart?
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1\$\begingroup\$ What specifically are you confused by on this chart? \$\endgroup\$– HearthCommented Dec 2 at 17:27
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\$\begingroup\$ Thanks for looking. I simply don't understand the first thing about how to use it. I'm hoping to find the area of copper required to support a specific output current. But (a) 1A is off the bottom of the x-axis; (b) for a particular current, at what point does one stop tracking up the y-direction to find an area? \$\endgroup\$– Sputan DeargCommented Dec 2 at 17:30
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Choose one of the ambient temperatures - that's the upper limit you design for. From now on, only consider the plot marked with that temperature and ignore others.
Choose your diode current. Draw a vertical line at that current.
If the line does not cross the curve of your choosing, the operating current is too high to accommodate with \$<2{\rm\ in}^2\$ of copper.
Where the line intersects the curve, draw a horizontal line.
Read out the needed thermal copper area from the vertical position of the horizontal line.
answered Dec 2 at 17:30
Kuba hasn't forgotten MonicaKuba hasn't forgotten Monica
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\$\begingroup\$ OK thanks. So for example, if ambient temperature is 90C and diode current is 1.75A, then I need a minimum of 0.2 inch-squared? I think I was confused by reading upwards rather than downwards. For my likely conditions of 1A at 12V, it looks like there's essentially no minimum area - have I got that right? \$\endgroup\$ Commented Dec 2 at 17:35
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\$\begingroup\$ @user3486482 The minimum would be dictated by other concerns. I imagine they still assume you have at least the area of the device's recommended footprint. \$\endgroup\$– HearthCommented Dec 2 at 17:43
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\$\begingroup\$ @user3486482 For your example, it's more like 0.3sq-in. The y-axis starts at 0.1 as the zero position... As for specifically 1A, I wouldn't rule out virtually no copper area. \$\endgroup\$– ColinCommented Dec 2 at 17:46
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\$\begingroup\$ Ah OK my mistake. Yes 0.3. I think I get it now. To summarise my learnings: - forget anything more than 2A; - for my chosen current, start from the top of the chart and track downwards to reach the blue line of my chosen ambient temperature and read off the minimum area on the y-axis; - or reach the bottom, in which case as Hearth says, minimum area isn't a thermal consideration at all. Thanks all. \$\endgroup\$ Commented Dec 2 at 17:50