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Old 14th February 2007, 12:20 PM   #1
Dxvideo is offline Dxvideo  Turkey
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Default LM4780 is burning!


I finished my last LM4780 GC today.. Everything looks ok;
- Total 100mA quiescent current on both rails while muted or input shorted,
- (-)10mV (right) and (-)9mV (left) offsets on outputs and 5mV on inputs
and i havent connected any load or source until now. Because it gets hot as crazy. It increases up to 70C in one minutes! For just first trying; i connected it to a little CPU heatsink without fan.
I checked every connections pin by pin but couldnt find any mistake.
Does anybody have any experience with LM4780s with little heatsink? And is this normal while inputs shorted?
PS: I know that , i asked a lot last days but i wouldnt ask if i dont need!
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Old 14th February 2007, 12:48 PM   #2
rabbitz is offline rabbitz  Australia
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From my experiences, this is one hot running chip and not cool like the LM3875.

I had it attached to a 1.2C/W heatsink and with 35V rails was too hot. I ended up reducing the rails to 25V and all is cool.

From the Overture Design Guide (xls spreadsheet):
35V rails - 8R load 1.1C/W, 4R load 0.18C/W (chip can't use it)
30V rails - 8R load 1.76C/W, 4R load 0.53C/W (chip can't use it)
25V rails - 8R load 2.85C/W, 4R load 1.1C/W

I don't think the chip can utilise a heatsink greater than 1C/W and correct me if I'm wrong, but these have 2 LM3886's inside, so are going to be quite hot in use.

I'm sure the computer heatsink is way too small.

Hope this helps.
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Old 14th February 2007, 12:51 PM   #3
Dxvideo is offline Dxvideo  Turkey
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The PSU is 2x24V.. and NO LOAD connected! But there is a Zobel on outputs! Does zobel sink some current? Do you think its normal?
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Old 14th February 2007, 01:29 PM   #4
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Quote:
Originally posted by rabbitz

I don't think the chip can utilise a heatsink greater than 1C/W and correct me if I'm wrong, but these have 2 LM3886's inside, so are going to be quite hot in use.

I'm sure the computer heatsink is way too small.

You can download the inter-active design tool from National to determine the heat sink value --

http://www.national.com/appinfo/audi...gn_Guide15.xls For +/- 24 VDC with an 8 ohm load the thermal impedance should be 3.1 degrees per watt --

Here's how to determine the heat sink value -- get a 10 to 50 ohm power resistor -- the kind that you can screw onto a heat sink -- get a decent kitchen thermometer -- the metal kind from Williams Sonoma or Bed n Bath, or an electronic outdoor thermometer from RS -- you probably have one anyway -- I attach the heat sink to a string and suspend it from my adjacent work table so that it isn't in contact with any surface -- attach the thermometer to the heatsink -- measure the initial temperature , run an amp through the resistor and take the temperature of the heat sink in one hour -- this will give you the rise in temperature for the number of watts that the resistor has been baking at.
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Old 14th February 2007, 01:43 PM   #5
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Guess you replied to Rabbitz.. I dont worry about my heatsink. I will use a 0,4C/W huge one. My question is;
When the inputz shorted, is it normal to warmimg up to 70C?
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Old 14th February 2007, 01:52 PM   #6
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it depends upon the rail voltage and the efficiency of the connection between the device and heat sink -- if this is a bridged amplifier the effective load is half !

it's normal for these chips to run hot.
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Old 14th February 2007, 01:53 PM   #7
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Hmm ok! For trying i connect it a bit loose i guess.
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Old 14th February 2007, 02:49 PM   #8
rabbitz is offline rabbitz  Australia
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Quote:
Originally posted by Dxvideo
I dont worry about my heatsink. I will use a 0,4C/W huge one.
The chip can't utilise the 0.4C/W heatsink as the LM4780 package can't transfer the heat quick enough..... about 1C/W heatsink max.

It's not an easy chip to mount securely as it has open holes on the ends so I used a clamp made from alumium. This also kept the mounting screws away from the body as when they touched, the chip was grounded to the heatsink.... not good..... see pic.
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Old 14th February 2007, 03:16 PM   #9
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Quote:
Originally posted by rabbitz


The chip can't utilise the 0.4C/W heatsink as the LM4780 package can't transfer the heat quick enough..... about 1C/W heatsink max.

It's not an easy chip to mount securely as it has open holes on the ends so I used a clamp made from alumium. This also kept the mounting screws away from the body as when they touched, the chip was grounded to the heatsink.... not good..... see pic.
That's incorrect.
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Old 14th February 2007, 03:34 PM   #10
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Interesting!
Bigger heatsink always better i guess. Its like, if you use a 2400btu air conditioner where 9000 needed! It gets colder; but if you consider ICs never get flu, its not a problem.
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