Buck converter IC packages

[Alex Eisenhut]

Looks like the majority of SO-8 packages with exposed paddle have the paddle go to ground. Makes sense, that's where the copper will be. Why would a manufacturer put the Vin on the exposed paddle?

http://www.digikey.ca/product-detail/en/AOZ1284PI/785-1689-1-ND/4900905

The Vin copper area would be tiny. Granted, the chip has a pretty low RDSon and no synchronous rectifier, so I doubt it would get too hot. But what's the technological reason for it? The converter happens to have a higher max Vin than the parts with grounded exposed paddle.

[T3sl4co1l]

Lower supply inductance (but, higher ground inductance -- yuck!!)
More convenient, better heatsinking (substrate is drain, on modern VDMOS and etc.)
More clearance (though I've seen 100V capable controllers in TSSOP, so that doesn't matter)

Why would the area be tiny?  If you need cooling, you make that pour as big as it needs to be...

Tim

[Alex Eisenhut]

Lower supply inductance (but, higher ground inductance -- yuck!!)
More convenient, better heatsinking (substrate is drain, on modern VDMOS and etc.)
More clearance (though I've seen 100V capable controllers in TSSOP, so that doesn't matter)

Why would the area be tiny?  If you need cooling, you make that pour as big as it needs to be...

Tim

It's for my TO-3 fetish. With the input LC filter, and the rest of the space taken up by the diode and coil, that doesn't leave much room. I guess there's a thermal path through the input coil, fuse and eventually the input pin and the system PCB.

I just wondered why it's hard to find these little guys with input voltage up to 36V and a grounded pad.

[Phoenix]

Why would a manufacturer put the Vin on the exposed paddle?

Maybe because the Vin is a high power input, the gound is a signal level input. Ground inductance would be less important as the high frequency return current would flow external to the IC.