# Board Design -Need Help: how much PAD area

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#### manju_bhat

##### Junior Member level 3
Board Design -Need Help

Hi

I am planning to use a step down converter for one of my application. The IC is LT3693 , Linear make and the output requirement is 5V @ 2A . At full laod, IC generates a lot of heat. Since Ic is MSOP package, a proper heat sink has to be designed & PCB Pad area has to be given as heat sink. The power across IC is 2W. Could anyone suggest me how much PAD area need to be given exactly for such application. I do not have any handy calculation. Please help.

thanks.

#### VVV

##### Advanced Member level 5
Re: Board Design -Need Help

How did you actually calculate the power dissipation of the chip to be 2W? This chip is supposed to deliver up to 3.5A, so i am wondering how come you have 2W dissipated at 2A only?

If you look at the graph on the first page you can see that at 34V input, with a 2A load the efficiency is about 85%. That means the input power is 5*2/0.85=11.76W. Thus, the power lost is 1.76W. This includes the diode and the inductor. The diode dissipates about: 0.5V*2A*(1-5/34)=0.85W.
Neglecting the power lost in the inductor, the chip should only dissipate 1.76-0.85=0.91W.

Anyway, I would say about 1 square in of Cu will give you about 35C/W. So to keep the junction at 100C you can dissipate 2W with a maximum ambient of about 30C. Is that going to be sufficient?

If you agree the chip dissipates only about 0.9W, as shown above, then with that 1 square in of Cu you can have a maximum ambient of about 70C, which is generally very acceptable, even for inductrial equipment.

#### manju_bhat

##### Junior Member level 3
Re: Board Design -Need Help

Hello VVV ,

Thanks for your reply. You are absolutely right regarding the power calculation & I agree with your logic. But you say that 1 square inch of Cu should be good enough. Do you have any PDF or any document which support that. I agree that , it may be rule of thumb. I would like to have handy calculations or any Application note which describes the same. Thanks for your suggestion.

#### VVV

##### Advanced Member level 5
Re: Board Design -Need Help

Well, try this: http://www.fairchildsemi.com/an/AB/AB-15.pdf#page=1

Although it refers to MOSFETs, the surface area will give you much the same Rth.

There are other appnotes out there, but I don't seem to find them right now. Try the ON Semi webiste and search for power dissipation.

#### manju_bhat

##### Junior Member level 3
Re: Board Design -Need Help

thanks everybody

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