What do I do with the NC-pins on th LM4780?

rho · 9th April 2004, 05:58 PM

Where do you guys connect the NC-pins on th LM4780? Should I leave then un-connected or could I use them to optimise the power-routing on the PCB? It is possible to route wider tracks or add bigger copper-planes when using the NC-pins as "passive-power-pins". Bad idea?

Fred Dieckmann · 9th April 2004, 06:12 PM

NC stands for No Connect. Here in Rio Linda that means not to connect them to anything.............

Sometimes access to test points on an IC are via such " extra "pins. If you want the exrtra real estate for traces cut them off. BTW great waffles man........

pinkmouse · 9th April 2004, 06:14 PM

I agree with Fred, chop 'em off!

Fred Dieckmann · 9th April 2004, 06:17 PM

Man! I thought I had been moderated over waffles for a sec there.....

pinkmouse · 9th April 2004, 06:21 PM

Quote:

Originally posted by Fred Dieckmann
Man! I thought I had been moderated over waffles for a sec there.....

C'mon Fred I'm not always moderating you...

rho · 9th April 2004, 06:44 PM

I get the message.

BrianGT · 9th April 2004, 07:14 PM

I would prefer to not clip the pins off of the chip, and leave them on to offer the chip better support, with respect to the connection from the pcb to the chip.

Then, on the pcb layout, the pins are simply not tied to anything.

This worked out quite well on my LM3875 layout. The chip sits much better with all the pins attached.

Another issue, is that the pins on the LM4780 are so small, and I would be afraid that I would cut off too many, or the wrong ones. I bought a few from Digikey, and am going to try to make an amp with them in the future.

--
Brian

rho · 9th April 2004, 07:44 PM

Maybe I can check for ESD diodes first. In most cases there would be some kind of ESD protection between the supplies and in- or-output-pins. If there is an open circuit that would mean the pin is not a TP.

I too prefer to keep all the pins attached. In some cases the stress from cutting off some pins is too much for the leadframe and can damage bond-wires of neigbouring pins. Atleast that's yhe case with smaller packages. I don't have any experiance with this type of package but I will not risk it.
Also the comment about stability is very true.

...yeah we do have tasty waffles here in Belgium.....ever tried some real Belgiun "frieten"? (The best French fries are not French

)

BrianGT · 10th April 2004, 04:14 AM

Quote:

Originally posted by rho
I too prefer to keep all the pins attached. In some cases the stress from cutting off some pins is too much for the leadframe and can damage bond-wires of neigbouring pins. Atleast that's yhe case with smaller packages. I don't have any experiance with this type of package but I will not risk it.
Also the comment about stability is very true.

...yeah we do have tasty waffles here in Belgium.....ever tried some real Belgiun "frieten"? (The best French fries are not French )

Yeah, keeping all the pins really helps for stability. Man, I was looking at the LM4780, and there are 27 pins, and only 7 of them are NC pins, so I don't think that it is worth the effort to get rid of 7 pins. They all look the same, and with the close spacing, there would be a good chance of cutting off the wrong pin. Also, you made a good point about the bond wires, I didn't consider that.

I was in Belgium a couple of summers ago, and I did have some French Fries and Waffles, and they were quite good.

--
Brian

Bricolo · 10th April 2004, 06:46 AM

I second Fred's advice.
The manufacturers should even call them DNC: Do Not Connect!

A friend of mine built a TDA7294 amp last week, and on the fisrt channel he tied the NC pins to ground.
The chip exploded at the first powerup

PS: and the best fries are french

9th April 2004, 05:58 PM	#1
rho diyAudio Member Join Date: Sep 2003 Location: Peer, Belgium	What do I do with the NC-pins on th LM4780? Where do you guys connect the NC-pins on th LM4780? Should I leave then un-connected or could I use them to optimise the power-routing on the PCB? It is possible to route wider tracks or add bigger copper-planes when using the NC-pins as "passive-power-pins". Bad idea?

9th April 2004, 06:14 PM	#3
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	I agree with Fred, chop 'em off! __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

9th April 2004, 06:17 PM	#4
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	Phewwww Man! I thought I had been moderated over waffles for a sec there.....

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9th April 2004, 06:44 PM	#6
rho diyAudio Member Join Date: Sep 2003 Location: Peer, Belgium	I get the message.

9th April 2004, 07:14 PM	#7
BrianGT diyAudio Moderator Emeritus Join Date: Jan 2002 Location: near Atlanta, GA	I would prefer to not clip the pins off of the chip, and leave them on to offer the chip better support, with respect to the connection from the pcb to the chip. Then, on the pcb layout, the pins are simply not tied to anything. This worked out quite well on my LM3875 layout. The chip sits much better with all the pins attached. Another issue, is that the pins on the LM4780 are so small, and I would be afraid that I would cut off too many, or the wrong ones. I bought a few from Digikey, and am going to try to make an amp with them in the future. -- Brian

9th April 2004, 07:44 PM	#8
rho diyAudio Member Join Date: Sep 2003 Location: Peer, Belgium	Maybe I can check for ESD diodes first. In most cases there would be some kind of ESD protection between the supplies and in- or-output-pins. If there is an open circuit that would mean the pin is not a TP. I too prefer to keep all the pins attached. In some cases the stress from cutting off some pins is too much for the leadframe and can damage bond-wires of neigbouring pins. Atleast that's yhe case with smaller packages. I don't have any experiance with this type of package but I will not risk it. Also the comment about stability is very true. ...yeah we do have tasty waffles here in Belgium.....ever tried some real Belgiun "frieten"? (The best French fries are not French )

10th April 2004, 06:46 AM	#10
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	I second Fred's advice. The manufacturers should even call them DNC: Do Not Connect! A friend of mine built a TDA7294 amp last week, and on the fisrt channel he tied the NC pins to ground. The chip exploded at the first powerup PS: and the best fries are french

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