Bridging 2 LM4780 chips

akunec · 15th April 2005, 12:20 AM

I want to run a bridged LM4780 off a +- 30v power supply. The datasheet said 25v power supply is a maximum for an 8 ohm load.

I was thinking, could I use 2 seperate 4780 chips, each running only 1 channel to increase the heat transfer area to the heatsink? Would this allow 30v rails?

Also, http://home.mira.net/~gnb/audio/lm4780.html shows a pin diagram of the 4780. It shows V- B attached to the Die. If I only powered channel A, would it be possible to mount the chip directly to a heatsink, without an isolating pad? It doesnt show any connection between V- A and the Die.

Leolabs · 15th April 2005, 01:43 AM

Running only on one channel,what a waste?

Why not parrallel them to improve driving capability???

jackinnj · 15th April 2005, 02:32 AM

best advised to plug the values into the design tool:
http://www.national.com/appinfo/audi...gn_Guide15.xls

you need a big heat sink if you are going to run bridged mode at 30V -- you may want to try using two LM3886's

akunec · 15th April 2005, 03:04 AM

Shouldn't 2 LM4780 chips have better heat transfer capabilities than 2 LM3886s. I will only be using 1 channel of each 4780 chip.

akunec · 15th April 2005, 03:20 AM

According to the spreadsheet, it should be capable of outputting 180watts.

Each LM4780 chip must be capable of dissipating 50w of heat. Is it capable of doing this?

SpittinLLama · 15th April 2005, 05:29 AM

You will get a little better thermals running only one channel of each LM4780 * 2 verses running 2 x 3886. That big package on the LM4780 has some very good thermals. But you are wasting 2 perfectly good channels in the process. Running in bridge mode and an 8 ohm load with +/-30V rails is going to be pretty demanding. The LM4780 datasheet spec saying only +/-25V with 4 ohms loads is running both channels in phase (worse case, generally how National does it). So if you run only one channel on each it will be better but still pretty hot (btw, from what I understand, National does thermal testing with 20Hz sine waves, much worse than any music source).

If you want to keep heat down and get plenty of power then run each LM4780 in parallel and as 1/2 a bridge circuit. This would be what is call bridge/parallel or BPA. Check out AN-1192 App Note from National. Basically, each channel of the LM4780 is going to see 8 ohms in this set up so well within thermal specs. Doing this should get you in the 170W range unless you power supply is not strong enough. Good luck.

-SL

Edit: Plug in the datasheet numbers for supply and load impedance. Then read the total Pd/IC and this will tell you what they think is OK for power dissipation.

akunec · 15th April 2005, 06:13 AM

On www.linkwitzlab.com SL uses a 30v supplies with 2 3886 chips. The biggest reason I don't want to use BPA mode is that I don't have the soldering skills to make that many connections on that package. I tried making a parelleled 4780 before and it gets really hot as my feedback resistors weren't perfectly matched. If my feedback resistors aren't perfectly matched on a bridged amp, it really doesn't matter.

SpittinLLama · 15th April 2005, 06:55 PM

Then he is living on the edge of thermal stability unless he has some thermal design AND is using the TA package. Testing with sine waves will cause the LM3886's SPiKe protection to kick in for +/-30V, bridge mode and driving 8 ohms with most heat sinks.

-SL

(I didn't read much on the link.)

akunec · 16th April 2005, 06:06 PM

Here's a response for National:

Hello, Alex

In response to your request:

This integrated circuit must be connected to both Vs+ and Vs- without regard to
the fact that channel B is not used!

We recommend you use the bridged mono ckt shown in figure 2 on page 5 of the
datasheet. This configuration uses both
amplifiers, and Bridge-Tied-Load is the best way to dump maximum power into the
load.

The datasheet gives a relatively good treatment of how to caluclate maximum
allowable power.

The maximum differential differential |V+| - |V-| is 84V, so you can operate up
to ±42V safely.

Regards,

Design Support Group
http://www.national.com

Please provide a detailed explanation of your technical question?
I am considering using only 1 channel of the LM4780. Since it seems that only V- on channel B is connected to
the die, would it be possible to use only ch A and attach the die directly to a heatsink, without the use of an isolator
pad?

Would it maybe be possible to use higher than the max supply voltage +-25volts, provided that you only used 1
channel, and mounted it directly to a large heatsink, thus improving the thermal conductivity?

SpittinLLama · 17th April 2005, 07:25 AM

The package is always connected to -V through the die. To avoid issues it is best to do as they recommend and connected all the -V and +V pins. Just leave the Mute resistor off of the channel you don't want to use. That way the other channel will be in Mute all the time. If you don't use a isolation washer then the heat sink will be at -V too, as you already know.

-SL

15th April 2005, 12:20 AM	#1
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	Bridging 2 LM4780 chips I want to run a bridged LM4780 off a +- 30v power supply. The datasheet said 25v power supply is a maximum for an 8 ohm load. I was thinking, could I use 2 seperate 4780 chips, each running only 1 channel to increase the heat transfer area to the heatsink? Would this allow 30v rails? Also, http://home.mira.net/~gnb/audio/lm4780.html shows a pin diagram of the 4780. It shows V- B attached to the Die. If I only powered channel A, would it be possible to mount the chip directly to a heatsink, without an isolating pad? It doesnt show any connection between V- A and the Die.

15th April 2005, 01:43 AM	#2
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Running only on one channel,what a waste?Why not parrallel them to improve driving capability???

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15th April 2005, 02:32 AM	#3
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	best advised to plug the values into the design tool: http://www.national.com/appinfo/audi...gn_Guide15.xls you need a big heat sink if you are going to run bridged mode at 30V -- you may want to try using two LM3886's

15th April 2005, 03:04 AM	#4
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	Shouldn't 2 LM4780 chips have better heat transfer capabilities than 2 LM3886s. I will only be using 1 channel of each 4780 chip.

15th April 2005, 03:20 AM	#5
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	According to the spreadsheet, it should be capable of outputting 180watts. Each LM4780 chip must be capable of dissipating 50w of heat. Is it capable of doing this?

15th April 2005, 05:29 AM	#6
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	You will get a little better thermals running only one channel of each LM4780 * 2 verses running 2 x 3886. That big package on the LM4780 has some very good thermals. But you are wasting 2 perfectly good channels in the process. Running in bridge mode and an 8 ohm load with +/-30V rails is going to be pretty demanding. The LM4780 datasheet spec saying only +/-25V with 4 ohms loads is running both channels in phase (worse case, generally how National does it). So if you run only one channel on each it will be better but still pretty hot (btw, from what I understand, National does thermal testing with 20Hz sine waves, much worse than any music source). If you want to keep heat down and get plenty of power then run each LM4780 in parallel and as 1/2 a bridge circuit. This would be what is call bridge/parallel or BPA. Check out AN-1192 App Note from National. Basically, each channel of the LM4780 is going to see 8 ohms in this set up so well within thermal specs. Doing this should get you in the 170W range unless you power supply is not strong enough. Good luck. -SL Edit: Plug in the datasheet numbers for supply and load impedance. Then read the total Pd/IC and this will tell you what they think is OK for power dissipation.

15th April 2005, 06:13 AM	#7
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	On www.linkwitzlab.com SL uses a 30v supplies with 2 3886 chips. The biggest reason I don't want to use BPA mode is that I don't have the soldering skills to make that many connections on that package. I tried making a parelleled 4780 before and it gets really hot as my feedback resistors weren't perfectly matched. If my feedback resistors aren't perfectly matched on a bridged amp, it really doesn't matter.

15th April 2005, 06:55 PM	#8
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	Then he is living on the edge of thermal stability unless he has some thermal design AND is using the TA package. Testing with sine waves will cause the LM3886's SPiKe protection to kick in for +/-30V, bridge mode and driving 8 ohms with most heat sinks. -SL (I didn't read much on the link.)

16th April 2005, 06:06 PM	#9
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	Here's a response for National: Hello, Alex In response to your request: This integrated circuit must be connected to both Vs+ and Vs- without regard to the fact that channel B is not used! We recommend you use the bridged mono ckt shown in figure 2 on page 5 of the datasheet. This configuration uses both amplifiers, and Bridge-Tied-Load is the best way to dump maximum power into the load. The datasheet gives a relatively good treatment of how to caluclate maximum allowable power. The maximum differential differential \|V+\| - \|V-\| is 84V, so you can operate up to ±42V safely. Regards, Design Support Group http://www.national.com Please provide a detailed explanation of your technical question? I am considering using only 1 channel of the LM4780. Since it seems that only V- on channel B is connected to the die, would it be possible to use only ch A and attach the die directly to a heatsink, without the use of an isolator pad? Would it maybe be possible to use higher than the max supply voltage +-25volts, provided that you only used 1 channel, and mounted it directly to a large heatsink, thus improving the thermal conductivity?

17th April 2005, 07:25 AM	#10
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	The package is always connected to -V through the die. To avoid issues it is best to do as they recommend and connected all the -V and +V pins. Just leave the Mute resistor off of the channel you don't want to use. That way the other channel will be in Mute all the time. If you don't use a isolation washer then the heat sink will be at -V too, as you already know. -SL