Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 15th April 2005, 12:20 AM   #1
akunec is offline akunec  Canada
diyAudio Member
 
Join Date: Aug 2004
Location: Alberta
Default 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.
  Reply With Quote
Old 15th April 2005, 01:43 AM   #2
Leolabs is offline Leolabs  Malaysia
diyAudio Member
 
Leolabs's Avatar
 
Join Date: Dec 2004
Location: Bukit Mertajam
Send a message via MSN to Leolabs
Running only on one channel,what a waste?Why not parrallel them to improve driving capability???
  Reply With Quote
Old 15th April 2005, 02:32 AM   #3
diyAudio Member
 
jackinnj's Avatar
 
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
  Reply With Quote
Old 15th April 2005, 03:04 AM   #4
akunec is offline akunec  Canada
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.
  Reply With Quote
Old 15th April 2005, 03:20 AM   #5
akunec is offline akunec  Canada
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?
  Reply With Quote
Old 15th April 2005, 05:29 AM   #6
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.
  Reply With Quote
Old 15th April 2005, 06:13 AM   #7
akunec is offline akunec  Canada
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.
  Reply With Quote
Old 15th April 2005, 06:55 PM   #8
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.)
  Reply With Quote
Old 16th April 2005, 06:06 PM   #9
akunec is offline akunec  Canada
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?
  Reply With Quote
Old 17th April 2005, 07:25 AM   #10
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Bridging 4 lm4780 boards? juan_caliente Chip Amps 1 17th April 2007 11:51 PM
bridging 2x paralleled LM4780 per channel - few questions bikehorn Chip Amps 6 28th April 2006 06:13 PM
Paralleling LM4780 chips? Dominick22 Chip Amps 29 26th February 2006 10:23 PM
bridging lm4780 kit joecool85 Chip Amps 1 23rd January 2006 06:26 AM
Brian GT LM4780 Bridging juan_caliente Chip Amps 15 4th September 2005 10:58 AM


New To Site? Need Help?

All times are GMT. The time now is 04:48 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2