LME49810 - a new cousin for LM4702

[jackinnj]

[Russ White]

More important than the distortion of the test circuit is the distortion of the final amp. And current drive and slew rate will have a lot to do with that. Also you won't have as much XTALK with a properly designed LME49810 circuit(or crcuits, dual mono).

LM4702
XTALK Channel Separation (Note 11) f = 1kHz at Av = 30dB 85dB
SR Slew Rate VIN = 1.2VP-P, f = 10kHz square Wave, Outputs shorted 15 V/μs

LME49810
SR Slew Rate VIN = 1VP-P, f = 10kHz square Wave 50 V/μs(min)

Sure the distortion of the LM4702 looks great in the test configuration (no load, especially not a capacitive one). Driving a realistic power stage I would think the LME49810 would have a clear advantage, especially with MOSFETs.

So while thats an interesting graph, I would say its not very representative of the results you will get with a real power amplifier, even using National's excellent guidelines.

I would say the LME49810 is in part the result of lessons learned on the LM4702.

Cheers!
Russ

[Russ White]

Also notice this graph from the LME49810 DS,

The standard (there is only one part) LM49810 easily surpasses the 4902C and equals (or betters under a real load) the 4902B.

When reading these data sheets always keep in mind, we are talking no load here. Add a load and the story will change dictated by the capability of the driver to adequately meet the demands of the load.

[jackinnj]

the load on the source and sink is different on the two test boards --

with respect to slew rate -- (among other things) this is a function of the compensation capacitor value -- there is a forthcoming application note on compensating all of these driver chips. i have played around with this quite a bit and found that a couple tweaks were necessary to max the bandwidth without having the amplifier self destruct -- found that 30 pF was about right. i also found that the feedback network had to be compensated a bit -- i have run the amp thusly at 140 kHz without catastrophe!

in a new application note, National shows BD139 and BD140 transistors to drive a variety of MOSFETs (Renesas, Magnatec and Vishay) -- this is a good idea.

here's a comparison of the two driver chips -- they probably used 30kHz bw on their distortion analyzer.

[Russ White]

Originally posted by jackinnj
with respect to slew rate -- (among other things) this is a function of the compensation capacitor value -- there is a forthcoming application note on compensating all of these driver chips.

Yes that will be a good app note, but I doubt it will change the maximum slew rate of the LM4702 which is stated in the data sheet. and I have read the FET app note, but you would not need any additional driver to use the LME49810 with many FETs.

[Russ White]

Originally posted by jackinnj

You should note that that is the expensive LM4702B and not the more common LM4702C.

[jackinnj]

Originally posted by Russ White


You should note that that is the expensive LM4702B and not the more common LM4702C.

I was never able to get my Lateral MOSFET amplifier to attain the THD+N% levels of the reference design -- but it was (is) still very, very good -- the "B" has just become available in quantity and is a little more than twice the price of the "C".

I have an LM4702 amp sitting on the bench as we speak -- oscillating at 8.123 MHz.

[Russ White]

Yes I bet the FET LM4702 amp sounds great. But even in the app note I think in most cases your are running the LM4702 very close to its margins with respect to the bias voltage. I guess what I am saying is I would doubt very much that with a real amp the LM4702 performs any better than a LME49810. You can look at the LME49810 as having a built in pre-driver for FETs etc.

Lets say you started with an LM4702 plus BD139 and BD140 transistors to drive FETs. Naturally your THD+N would be much worse than the LME49810 or LM4702 alone.

Also, I just don't see being able to compensate the LM4702 so it could match the LME49810's slew rate. Though I will anxiously await the app note to see what National says.

[glennb]

Originally posted by Russ White
...According to the nice gentleman I spoke with there are more new devices on the way as well... Could it be a new dual part coming based on the 49810? Hmmmmm that would be sweet.

Such as the new LME49860 - 44V Dual High Performance, High Fidelity Audio Operational Amplifier http://www.national.com/pf/LM/LME49860.html ?

The specs look the same as LME49720 http://www.national.com/pf/LM/LME49720.html (aka LM4562 http://www.national.com/pf/LM/LM4562.html) except the power supply limit has been raised +/- 5V.

jackinnj hinted at the 49860 back in March Bob Pease on the New LM4562

[ozonek]

Will NS continue their development on LM4702A (the top grade) while they now have a better design like LME49810? (dual version is coming soon?)