LM3875 abuse vulnerability - diyAudio
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 13th December 2008, 06:34 AM   #1
Puffin is offline Puffin  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: U.K
Default LM3875 abuse vulnerability

The LM3875 has output protection from a short to ground, and the SPiKe protection is said to protect a short to the supplies.

I managed to blow one up when checking the rail voltages by attaching the DMM to V- before securing the other probe to grd.

Should this have happened?
  Reply With Quote
Old 13th December 2008, 06:49 AM   #2
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
Two possibilities that occur to me:
There was no cap at CI, or
Electrostatic discharge.

Its good that the chip simply quit--because the usual malfunction is full rail voltage into the speaker.
  Reply With Quote
Old 13th December 2008, 07:11 AM   #3
Puffin is offline Puffin  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: U.K
Ci is an optional feeback cap to ensure unity gain at DC?
  Reply With Quote
Old 13th December 2008, 07:16 AM   #4
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
Quote:
Originally posted by Puffin
Ci is an optional feeback cap to ensure unity gain at DC?
The cap at CI may unlevel the frequency response or cause other aberrations common to signal going through capacitors. This is why it is often omitted. That's less durable, but its easier.

To your question: Yes. The amplifier is more durable and dynamic with that cap in place. Although its difficult to select a capacitor for seemly results at CI, it IS possible and you do get the benefit of larger dynamics, a variety of input impedances available (see page 6), as well as more powerful low bass (See overture design spreadsheet) along with less clipping; and, you get a more durable amplifier.
  Reply With Quote
Old 13th December 2008, 10:00 AM   #5
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
Well man, really, I think the reason it broke is that a power cap suddenly discharged, probably with the help of the voltmeter probe and some static electricity as a bonus (ESD).

What I was trying to say, is that without CI in place, the LM3875 would have tried to amplify the zap by about the gain factor, and blew itself out. That's my guess on it.
  Reply With Quote
Old 13th December 2008, 12:36 PM   #6
Puffin is offline Puffin  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: U.K
anyone else got a take on this? The PS rail cap (1,000uf) survived.
  Reply With Quote
Old 13th December 2008, 11:19 PM   #7
diyAudio Member
 
Join Date: May 2008
Default Re: LM3875 abuse vulnerability

Quote:
Originally posted by Puffin
The LM3875 has output protection from a short to ground, and the SPiKe protection is said to protect a short to the supplies.

I managed to blow one up when checking the rail voltages by attaching the DMM to V- before securing the other probe to grd.

Should this have happened?
Here is a quote from National's Application Note 898 (John deCelles, 1993)
Quote:
It should be noted that
SPiKe protection was enabled after 200 us of current limiting
in Figure 23 and Figure 24, but is in general dependent upon
the case temperature, the transistor operating current and
voltage, and its power dissipation versus time.
The protection circuit has a reaction time. The short could have blown the transistor(s) before SPiKe could react.

Here is another quote from the same document
Quote:
One note to make about this protection scheme is that the
current limitation is not sustained indefinitely. In essence, the
output shorts to either supply rail should not be sustained for
any period of time greater than a few seconds. Frequent
temporary shorts from the output to either supply rail will be
protected, however, continued testing of the circuitry in this
manner is not guaranteed and is likely to cause degradation
to the functionality and long-term reliability of the device.
This means that the short must not be too long either (no pun intended), and that SPiKe is neither immortal nor infallible.
__________________
If you've always done it like that, then it's probably wrong. (Henry Ford)
  Reply With Quote
Old 14th December 2008, 04:26 AM   #8
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
We were just discussing fuses on another thread, so howabout some fast-blo at the speaker output + side? The no-load operating tolerances are so much higher.

OMG! This could help me too! I managed to somehow purchase a boxfull of counterfeit LM1875 and while usable as TDA2050's, they are prone to mysterious failures. Now counting in a fuse holder, the wonderful sale price was not so economical after all. Oh well. Its probably more-seemly to blow a fuse instead. It might give me some clues as to the cause of failure, because of such immediate notice that a fuse can give.

Thanks for the ideas!!
  Reply With Quote
Old 14th December 2008, 12:31 PM   #9
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
a fuse is too slow to protect the semiconductors.

Where a fuse can be good is for long term overload when a current limiting circuit prevents immediate damage to the semiconductor, but continued power dissipation increases the device temperature.
__________________
regards Andrew T.
  Reply With Quote
Old 14th December 2008, 12:46 PM   #10
Puffin is offline Puffin  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: U.K
This was a split second flash as the probe connected!
  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
JFET's ready for abuse Magura Pass Labs 39 24th March 2009 11:01 PM
looking for methods to abuse amps east electronics Solid State 34 31st July 2008 05:39 PM
Reporting an abuse caused Thread Close? jkeny Everything Else 6 20th April 2008 06:50 AM
My wacky budget n00bie 3-way thread. Abuse wanted! Spasticteapot Multi-Way 1 5th January 2007 12:17 AM
sample policy abuse mirlo Parts 1 19th December 2006 05:41 AM


New To Site? Need Help?

All times are GMT. The time now is 05:39 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