Decoupling capacitors

[lilimike]

This is probably a stupid question and it was probably asked before but I searched and couldn't find any occurrence.
If decoupling capacitors need to be as close as possible to the IC they are protecting, why are they not integrated to ICs?
I am sure there is a good reason and looking forward to find out.

Mike

 

[crutschow]

Two good reasons. Size and cost. Integrating a capacitor would make the IC package much larger and more expensive since practical cap sizes used to decouple ICs are much larger than the IC chip itself.

 

[Mosaic]

Well, for higher speed apps etc, the capacitor required may need to be accurately calculated. Also PCB requirements may drive the need for compound bypassing.

Have a look through here.

Capacitor Design Data, and Decoupling Placement, How-To

 

[lilimike]

Fair enough, I was under the impression that within an IC there were already some caps but I guess not.
While I am at it, is it acceptable to place a surface mount cap under a DIL IC on the same layer that is, as the IC pins are long enough for the chip to sit on the cap?

Mike

 

[lilimike]

Well, for higher speed apps etc, the capacitor required may need to be accurately calculated. Also PCB requirements may drive the need for compound bypassing.

Have a look through here.

Capacitor Design Data, and Decoupling Placement, How-To

Thanks for the link.

Mike

 

[BrownOut]

Fair enough, I was under the impression that within an IC there were already some caps but I guess not.
While I am at it, is it acceptable to place a surface mount cap under a DIL IC on the same layer that is, as the IC pins are long enough for the chip to sit on the cap?

Mike

We usually call it a DIP, not DIL. From an electrical standpoint, there would be no reason to not do it. From a maintence standpoint, I think it would not be wise. Very often, the caps are installed on the opposite side of the board, under the IC.

 

[shortbus=]

There are IC sockets available with a cap built in, but more expensive than a standard socket and cap.

 

[audioguru]

Opamps have a tiny 30pf frequency compensation capacitor built-in.
30pF is 3334 times smaller than a 0.1uF supply decoupling capacitor.

 

[lilimike]

We usually call it a DIP, not DIL. From an electrical standpoint, there would be no reason to not do it. From a maintence standpoint, I think it would not be wise. Very often, the caps are installed on the opposite side of the board, under the IC.

Actually I always call it a DIP but I guess I had DIL in my head as I was just adding some devices to a schematic in Eagle and they call it DIL.

My question originated from a circuit I did with the goal to have the cap on the other layer under the chip but I forgot to switch its layer and it is when came the time to solder that I had realized my mistake, that's when I figured it still had a nice fit under a DIP chip.

About maintenance issue does a 0.1µ cap ever go bad?

MIke

 

[SABorn]

I call it DIP to, but undersand DIL is the correct term. ( Direct In Line )

Pete.

 

[BrownOut]

Actually
About maintenance issue does a 0.1µ cap ever go bad?

MIke

Not really, but it makes inspection impossible.

 

[lilimike]

Very good point!

Mike

 

[audioguru]

I call it DIP to, but undersand DIL is the correct term. ( Direct In Line )

Pete.

No.
DIP means Dual In line Plastic.
DIL means Dual In line.

 

[lilimike]

I would think DIP means Dual Inline Package!

Edit: Wikipedia

Mike