EMI and regulator Question

bambino

Here is a shot from Diptrace showing a board I'm putting together. My problem is that the board will be mounted to the back of a 5" LCD screen that, naturally has·a Transformer on it for the Backlight. I have positioned my digital components as far away as possible, and used the area closest to it for my regulator and Heatsink. I am planning on having the area under my heat sink, on both sides of the board, copper poured.

What rail, or no rail at all, should I connect the Thermals to in this case to keep EMI out of my Power supply!

All the components are not on the pic, but the area to the upper right is as populated as it is going to get.

Paul Baker

You have to check the datasheet for the regulator, most have the thermal pad grounded, but a few have the tab connected to Vo, we ran into that problem when we did the first prototype of the protoboard. Had to snip the pins and inject 3.3V to test the rest of the circuit. Then we took another board and instead of soldering it to the pad we placed heat resistant celluloid tape to prevent the tab from making electrical contact to double check that was the only error with the board.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

bambino

That is the way of the lt1085, Tab is tied to output. So it would be easy to tie the thermal to 5v. But isn't more heat carried away by ground? and would the noise from the transformer affect the props circuits with the thermal tied to either!

It may be no big deal with the propeller, but the first prototype of this project had a max1270 ADC chip setting approxamately where that regulator is an it was completly incoherent until I moved it away down and to the left!

bambino

I just check the Heatsink and I don't know what it is made of , but it is non-conductive(Electrically), so the copper poar can be easily made to be in contact or not. So I guess what I'm really asking is if it is ok toe connect the pour to ground and not induce to much noise in the circuit.

Else I can just isolate the copper!

Paul Baker

If it's electrically isolated i suppose you could tie it to whatever you want, but i would still be inclined to tie it to Vo just in case there is an error (such as a secure screw chipping the non-conductive coating)

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

bambino

Thanks !