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An open source layout for LM3886?
An open source layout for LM3886?
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Old 28th April 2018, 01:14 PM   #21
diyaudnut is offline diyaudnut  United States
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Originally Posted by Mark Whitney View Post
Remove the PGND plane from under the input and feedback traces/parts. The AGND plane goes under the input and feedback circuits only.

Add the option of a Hum Breaking Resistor between the output GND and AGND. This will
help when you use multiple boards connected to a single transformer.

The LM3886 inputs are high impedance, keeping these traces short will prevent hum.
a quick explanation of the reasoning behind this helps others learn the practical tricks, pitfalls behind layouts. I mean from the perspective of inexperienced designers. There are many books explaining the art, but nothing like practical tit-bits from experienced folks.

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Old 28th April 2018, 06:16 PM   #22
00940 is offline 00940  Belgium
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Trying to address a few suggestions.

- the feedback cap is now clearing the way to the lm3886 screw
- the rf input filter is now ahead of the input cap. It's creating a voltage divider with the input resistor but the signal loss isn't big. The input tracks are significantly shorter. Most of the input tracks are under the signal ground plane only.
- there is now a resistor between signal gnd and power gnd. Easily jumpered if not needed.

I'm yet to be convinced of the need to clear the power gndplane near the lm3886 input pins...
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Old 29th April 2018, 07:04 AM   #23
Mark Whitney is offline Mark Whitney  Netherlands
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Most of the reasoning can be found in the datasheet.


LAYOUT, GROUND LOOPS AND STABILITY
The LM3886 is designed to be stable when operated at a closed-loop gain of 10 or greater, but as with any other high-current amplifier, the LM3886 can be made to oscillate under certain conditions. These usually involve printed circuit board layout or output/input coupling.
When designing a layout, it is important to return the load ground, the output compensation ground, and the low level (feedback and input) grounds to the circuit board common ground point through separate paths. Otherwise, large currents flowing along a ground conductor will generate voltages on the conductor which can effectively act as signals at the input, resulting in high frequency oscillation or excessive distortion. It is advisable to keep the output compensation components and the 0.1 μF supply decoupling capacitors as close as possible to the LM3886 to reduce the effects of PCB trace resistance and inductance. For the same reason, the ground return paths should be as short as possible. In general, with fast, high-current circuitry, all sorts of problems can arise from improper grounding which again can be avoided by returning all grounds separately to a common point. Without isolating the ground signals and returning the grounds to a common point, ground loops may occur. “Ground Loop” is the term used to describe situations occurring in ground systems where a difference in potential exists between two ground points. Ideally a ground is a ground, but unfortunately, in order for this to be true, ground conductors with zero resistance are necessary. Since real world ground leads possess finite resistance, currents running through them will cause finite voltage drops to exist. If two ground return lines tie into the same path at different points there will be a voltage drop between them. The first figure below shows a common ground example where the positive input ground and the load ground are returned to the supply ground point via the same wire. The addition of the finite wire resistance, R2, results in a voltage difference between the two points.



The single-point ground concept should be applied rigorously to all components and all circuits when possible. Violations of single-point grounding are most common among printed circuit board designs, since the circuit is surrounded by large ground areas which invite the temptation to run a device to the closest ground spot. As a final rule, make all ground returns low resistance and low inductance by using large wire and wide traces. Occasionally, current in the output leads (which function as antennas) can be coupled through the air to the amplifier input, resulting in high-frequency oscillation. This normally happens when the source impedance is high or the input leads are long. The problem can be eliminated by placing a small capacitor, CC, (on the order of 50 pF to 500 pF) across the LM3886 input terminals. Refer to External Components Description relating to component interaction with Cf.
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Old 29th April 2018, 07:52 PM   #24
00940 is offline 00940  Belgium
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Mark: all of those points raised by the datasheet are addressed by the current layout.

I'd be more interested to know why you are concerned by the power groundplane under the feedback network.
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Old 29th April 2018, 07:58 PM   #25
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by Mark Whitney View Post
Most of the reasoning can be found in the datasheet.

Cf.
I found the feedback resistor path needs to be as short as possible.
As for minimum gain you can get around it by putting 100pf between + and - inputs. I fell foul of that in a TDA7294 design that had a valve pre amp and I had reduced the gain to around 8. The cap stopped it oscillating.
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Old 29th April 2018, 08:35 PM   #26
Mark Whitney is offline Mark Whitney  Netherlands
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Originally Posted by 00940 View Post
Mark: all of those points raised by the datasheet are addressed by the current layout.

I'd be more interested to know why you are concerned by the power groundplane under the feedback network.
Current in the output leads (which function as antennas) can be coupled through the air to the amplifier input, resulting in high-frequency oscillation.

The power ground plane carries high current. Why would you want to place it under the feedback and/or input circuits?
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Old 29th April 2018, 09:33 PM   #27
DPH is offline DPH  United States
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Quote:
Originally Posted by nigelwright7557 View Post
I found the feedback resistor path needs to be as short as possible.
As for minimum gain you can get around it by putting 100pf between + and - inputs. I fell foul of that in a TDA7294 design that had a valve pre amp and I had reduced the gain to around 8. The cap stopped it oscillating.
Ben already covered the rationale of tapping the Rf between -in and the edge of the Zobel. Likewise, the other compensation networks are in place.
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Old 29th April 2018, 09:43 PM   #28
00940 is offline 00940  Belgium
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@Mark: My antenna theory is rusty but the problem isn't having the power groundplane under the signal tracks. It is more about the couple power tracks-groundplane and how it radiates. Untwisted output leads are way more problematic than having power lines over a continuous groundplane.

Furthermore, at the highish frequencies threatening oscillation, there will be less current flowing in the plane under the feedback network. Current will gather more under the output tracks as frequency goes up.

Anyway, I can check again my smd layout for oscillation. It's even more compact and "worse" under that respect.
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Old 29th April 2018, 11:20 PM   #29
diyaudnut is offline diyaudnut  United States
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Quote:
Originally Posted by Mark Whitney View Post
Most of the reasoning can be found in the datasheet.


LAYOUT, GROUND LOOPS AND STABILITY........

.
Thanks
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Old 30th April 2018, 10:49 AM   #30
levistubby is offline levistubby  United Kingdom
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Remember the old saying "A camel is a horse designed by committee"!
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