Kind of a novice question but I wanted to check if I am right about something. I like the convenience of the UDN2981 source driver to provide switched power to for multiple loads. However, I would like to drive loads that requires about 1.5x higher amperage than the specified limit of the chip. Would it be OK to physically stack two of these chips on top of each other to double the current draw capacity that I am able to deliver? I have verified that it seems to work, but I am looking for some sage advice as to whether this is truly OK to do.
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2 Answers
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The output stage of the UDN2981 is bipolar, so you must always fear the negative temperature coefficient – if one of the chips heats up more than the other, it will hog more current and this results in a quick thermal runaway of the stack.
So, two of them tightly stacked might be okay. Three – not, because the one in the middle inevitably gets much warmer than the outer ones.
Putting balancing resistors between Vdd and Vs is possible (or on each output) but still seems a bad idea.
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You would be better off using two or three of the source drivers in one package in parallel to get your increased drive capability. Since they would be on the same device the bipolar output transistors would more similar and at almost same temperature so there would be a little less concern with current hogging.
Then allocate other drive circuits to an additional package put down on the circuit board.
answered Feb 6, 2018 at 10:07
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1\$\begingroup\$ I was about to write the same in my answer, but if you do that, all the simplicity goes away and you should think of another driver chip instead. \$\endgroup\$– JankaCommented Feb 6, 2018 at 20:38