DC feedback at pin 7 of the LM 3886? - diyAudio
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Old 16th August 2011, 10:33 PM   #1
Marc Vi is offline Marc Vi  Netherlands
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Default DC feedback at pin 7 of the LM 3886?

Has somebody thought of using pin 7 of a LM3886 for DC feedback to eleminate offset output voltage by means of a servo circuit?

See equivalent schematic of a LM3886 in the application note: "LM3886 Overture™ Audio Power Amplifier Series High-Performance 68W Audio Power Amplifier w/Mute"

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Old 18th August 2011, 06:25 PM   #2
Marc Vi is offline Marc Vi  Netherlands
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Default Use pin 7 as DC feedback by means of a servo unit

Hello,

Maybe my question is not clear. Normally pin 7 of a LM3886 is connected to ground. If you see the equivalent schematic, pin 7 is the pin which begins inside with a resistor af 10k.

In the case of a single supply voltage circuit, pin 7 is used to bring the output voltage on half the supply voltage (see application note).

So my idea is to use pin 7 for adjusting the output offset to zero. This can be done by means of a servo unit.

I hope somebody will reflect on this idea.

Marc Vi.
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Old 18th August 2011, 07:26 PM   #3
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Sorry, this is not a good idea. The pin must be tied to the ground. If you are interested in a DC servo, please check application note AN-1192.
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Old 18th August 2011, 08:42 PM   #4
jawih is offline jawih  Netherlands
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I think it must be possible, if you have a look at figure 2 from the single-supply application, you can use it as an inputsignal.
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Old 18th August 2011, 10:48 PM   #5
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No, it's not being used as a signal input, it's tied to an artificial ground built around Q1. You're certainly free to experiment, but I wouldn't expect it to work the way you think it should.

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Old 19th August 2011, 01:56 AM   #6
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Is there any advantage to do so (if works) instead of traditional methods? Will it affect internal operating conditions if this pin is not the mid-point of the rails?
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Old 27th November 2011, 01:05 PM   #7
Marc Vi is offline Marc Vi  Netherlands
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As you see in LM4780 / 3886 / 3875 / 1875 Comparison (LM3886 Annotated Equivalent Schematic) pin 7 sets the ground voltage. If you use an input capacitor (which I always advise) to block an input DC voltage but not a feedback capacitor, because that will be an electrolytic one, then a output offset voltage will occurre. By putting a low voltage with a low impedance on pin 7, you can cancell out the output offset voltage. So this pin sets the output on the midpoint of the rails. Pin 7 is apart from the signal ground, so to my opinion it doesn't affect the quality, on the contrary.
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Old 27th November 2011, 01:13 PM   #8
AndrewT is offline AndrewT  Scotland
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Is pin 7 part of the protection systems?
Does pin 7 allow turn off in event of fault/abuse detection?
Does pin 7 allow delay if the supply rails are not up to minimum specified operating voltage?

Can the simplified schematic be relied upon to use features that are not in the datasheet?

There's only one way to find out!
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Old 28th November 2011, 12:35 PM   #9
Marc Vi is offline Marc Vi  Netherlands
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Andrew,

Please see the application note, figure 2 which single supply voltage, where pin 7 is used to set the ground voltage between.
Please see LM4780 / 3886 / 3875 / 1875 Comparison , the annotated equivalent schematic of the LM3886.

Of course I will try out within several months, because I am to busy now. I was just curious to see the opinion of others.

Regards, Marc.
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Old 28th November 2011, 12:43 PM   #10
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Interesting thoughts Marc, but you may be looking to solve a problem that isn't really there.

I've never measured more than about 12mV offset on any properly built chipamp I've made.
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