Help in selection of LM3886 Heat sink?

[Cluene]

I'm going to make an amplifier but i'm in a dilemma with the proper dimensions of the heatsink of the IC lm3886 ... would a 1.5 x 2 heat sink be enough? the input voltage of the op-amp is around 33-36v

 

[MrAl]

Hi,

You cant give a dimension like 1x2 or 2x2 or 3x4 for a heat sink with fins, which is what you need. You have to go by the required thermal resistance for the power being dissipated. How much power do you have to dissipate in the LM device?

 

[Cluene]

Hello

um around 50 watts ...

 

[audioguru]

The datasheet shows how to select a heatsink when your power supply voltage is higher than yours.
The LM3886 is usually used with a positive and a negative 35VDC supply which totals 70V. Then its output is 50W into 8 ohms.

For only a 36VDC supply, the LM3886 has an output into 8 ohms of about 16W. If your speaker is 4 ohms then you should have said so.
A graph in the datasheet shows the power dissipation with 4 sets of supply voltages and an 8 ohm speaker. With a plus and minus 18V supply (totalling 36V that you have) then the output power of 16W produces 10W of heat.

The spec's show an absolute maximum chip temperature of 150 degrees c ands a thermal resistance from chip to case of 1 degree C per Watt. The text says thermal compound is about 0.2 degrees C per Watt. An insulator is not mentioned. Maybe your ambient temperature is 40 degrees C.

Derate the maximum chip temperature to 140 degrees C and subtract the ambient of 40 degrees C to get 100 degrees C that the heatsink must cool. You are dissipating 10W of heat so the total of the thermal resistances must be 100 degrees/10W= 10 degrees C per Watt. 10-1-0.2= 8.8 degrees C per Watt is the thermal resistance needed by your heatsink. It will be listed in a catalog by a heatsink manufacturer. It will look something like this:

 

[ronsimpson]

The thermal resistance of a heat sink depends greatly on its environment.
AIR FLOW, AIR FLOW, AIR FLOW
If you close up the box there is no (little) AIR FLOW.
If the heat sink is in the middle of the PCB there is little AIR FLOW.
If there are other hot parts neat the heat sink...........

 

[audioguru]

around 50 watts ...

For 50W into 8 ohms the power supply must total 70V (plus AND minus 35V) but you said the power supply is only 33V to 36V.
You will have an output of only 16W with a 36V supply.

That is the amount of output power but you were asked for the dissipation (the amount of heating).

 

[audioguru]

Don't worry about the heating too much because you will not use the amplifier at full blast all the time (acid rock?).

 

[ronsimpson]

Don't worry about the heating too much because you will not use the amplifier at full blast all the time (acid rock?).

Then we don't have to worry about distortion either. (acid rock?)

 

[audioguru]

Acid rock has only a little distortion. It is mostly noise.

 

[ronsimpson]

Acid rock has only a little distortion. It is mostly noise.

I thought you are a big fan of (acid rock).

Back in my broadcast days, I remember an interview where a guitarist explained that he took the output of his guitar amp and plugged it into the input of a second guitar amp and turned up the volume until the second amp's red clipping light was on 100% of the time.

Engineers spend many hours trying to get the distortion down and then some one pegs the VU meter.

 

[Mr RB]

...
Back in my broadcast days, I remember an interview where a guitarist explained that he took the output of his guitar amp and plugged it into the input of a second guitar amp and turned up the volume until the second amp's red clipping light was on 100% of the time.

Engineers spend many hours trying to get the distortion down and then some one pegs the VU meter.

That's why musicians are in charge of making the music, and engineers are in charge of...
Complaining!

Getting back on topic, I heat tested my LM3876 amp running 40W output power into 8 ohms and the amp chip dissipated 34W. That is a pretty big heatsink needed, and either in free air vertically or with adequate fan forced air flow. That is a much more serious heatsink than AudioGuru suggested.

 

[audioguru]

I heat tested my LM3876 amp running 40W output power into 8 ohms and the amp chip dissipated 34W. That is a pretty big heatsink needed, and either in free air vertically or with adequate fan forced air flow. That is a much more serious heatsink than AudioGuru suggested.

The OP has a 36V supply, not a 70V supply. His amplifier clips when its output exceeds 16W into 8 ohms, not 40W.
A graph in the datasheet does not show such low power but is close. It shows a max of 11W of heat dissipation when the output is 16W into 8 ohms with a 40V supply. Then the heat dissipation is 10W with a 36V supply.

The OP said his supply might be as low as 33V so the output power and heating will be less.

 

[Mr RB]

My apologies AG, the LM3886 is normally used with a dual voltage supply and I assumed when he said 33v he was using +/-33v as would be shown in the datasheet and the schematic he used to build the amp etc.

Maybe the OP can clarify?

 

[audioguru]

My apologies AG, the LM3886 is normally used with a dual voltage supply and I assumed when he said 33v he was using +/-33v as would be shown in the datasheet and the schematic he used to build the amp etc.

Maybe the OP can clarify?

The datasheet for the IC shows a dual polarity supply and a single polarity supply.
The OP said in post #1 that his supply is, "around 33-36V".
He said in post #3 that his amplifier IC dissipates, "um around 50 watts ..." which is extremely high for the IC.