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Old 5th April 2012, 03:13 PM   #1
epilot is offline epilot  United States
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Default LM3886 overshoot

Hello guys,

I have an LM3886 driving a capacitive load piezo at say 40KHz by a bandwith of say 10KHz.
the pirzo load has a capacitance of almost 0.2uF. I have the problem of ringigng/overshoot at the output of the LM3886 when connected to the said load (the load has an impedance of say 5ohms).Can you guys tell me how to remove/reduce the overshoot problem plz?
the datasheet of the chip suggest using an LR circuit in series with the load (0.7uH & 10 ohms). I did s but the result was not promising. ANy idea plz? did I used the wrong values of lr for my chip at the said freq?
By the way what about the Q of the L used here? How to choice it? I do not know if it is better to have a highe or the lower q for the L here?

Thanks a lot

P.S As my load is capacitive do I need to use any zobbel netwoork too?
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Old 5th April 2012, 04:25 PM   #2
KSTR is offline KSTR  Germany
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Increase noise gain to 100x, 40dB (you don't need lowest THD, do you?).
Use a snubber as recommended (3R, 100nF or bigger) right on the chip from out to V-.
Isolate load with 1R or so (do you need max power transfer?).

Check application notes from Apex (now a sub-company of Cirrus), those are the best to found on most anything about power opamps. AN25, AN1, AN19.
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Old 5th April 2012, 04:37 PM   #3
jcx is offline jcx  United States
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any narrow band transducer will ring, often peaking is deliberately designed for, it is used to increase efficiency

you need to understand the acoustic output requirements and the inherent limitations of narrow band transducers before you can optimally shape the driver impedance with external RLC, possilbly apply EQ

the min gain stability requirement of the decompensated chip amp is also possibly in play as memtioned
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Old 7th April 2012, 03:11 PM   #4
epilot is offline epilot  United States
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Finall I could solve the problem of overshoot by putting a 70uH inductor in series with the piezo load.
the piezo as I said has a capacitance of 0.2uF and an impedance of 5ohms. But is it just right putting such a high inductor in series with the piezo load? The SPL of the piezo load seems to be higher by putting the said inductor. but I am afraid of damaging the piezo load by the inductor, Am I right? furthermore what will happen to the bandwidth of the load by putting such an inductor in series with the piezo load? I guess that the bandwidth would be reduced but I think I read somewhere that the bandwidth of the piezo load would be incrased by adding an inductor to the piezo loads. Any idea please?

Thanks
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