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Old 14th July 2013, 09:49 AM   #1
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Default LM338 tracking preregulator and current limit

I have a question regarding the current limit of a LM338 tracking preregulator.

With a single LM338, the input-output differential voltage is determining the DC and pulse current limit. The ideal voltage drop seems to be around 7.5V (see attached graph).

Now if you employ a second LM338 in the fashion of the tracking preregulator, as shown in the datasheet, does the second LM338 become a potential bottleneck? In other words, do I have to match the differential voltage to get the desired current capability?
So for ideal current capability of said tracking preregulator, I'd have to drop 7.5V per regulator, and 15V in total?
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Old 15th July 2013, 11:38 AM   #2
AndrewT is offline AndrewT  Scotland
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I think your logic is correct.
Total Vdrop from average supply voltage to average output voltage = 15Vdrop.

But that is a lot of heat. This requirement may over-rule your peak output capability criteria.
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Old 15th July 2013, 09:31 PM   #3
Ketje is offline Ketje  Belgium
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For me tracking shoud be (Vin-Vout)/2 as well over the first as over the second LM338 so they are heated equally.That's not the case here,(Vin-Vout) of the second LM338 is always 5V.
The first one has to dissipate the rest.
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Old 16th July 2013, 09:11 AM   #4
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Thanks for your answers!

Heat dissipation may become an issue, I agree.
And with values as shown in the datasheet the second LM338 only drops about 5V, that's what got me thinking about current capability.

I had considered using this tracking preregulator for the PSU in my next LM3886 GC project. With the 330VA 2x30VAC xformer I have, I could get to 40VDC rectified, drop 15V on the regulators, and end up with 25VDC regulated, only good enough for a mere 30W into 8 Ohm.
With a single LM338 regulating to 33VDC I would theoretically get close to 60W, and paired with the already outstanding PSRR of the chip itself, end up with a better sounding amp (more dynamical, more headroom, less distortion).
So I think I may put the preregulator design aside for this project.
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Old 16th July 2013, 09:51 AM   #5
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I found this link very handy when optimising Tracking pre regs and it might help you too. It applies to LM338's as well as LM317 types - same principle of operation. Note the adding of extra resistance in series with capacitors ESR to ensure stability and the use of LED's to set the tracking voltage in place of R2.

Using 3-pin regulators off-piste: part 4

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Old 16th July 2013, 10:14 AM   #6
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Thanks John, I've read that linked article before and found it quite helpful, yet it left me wondering about the current capabilities of the LM338 again. The LM117 will do fine with only dropping marginally more than 2.5V to perform at its best (see attachment, graph taken from LM117 datasheet), which is achievable with a single red LED.
For the LM338 it would take three red LEDs at least I recon. I'll definitely try that implementation in another project sometime.
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Old 16th July 2013, 10:41 AM   #7
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Quote:
Originally Posted by Rodeodave View Post
the already outstanding PSRR of the chip itself,
This leaves me wondering if we both have the same DS. The PSRR looks great at very low freqs, and not too bad across the band on the +ve supply, but the -ve rail performance looks to me a million miles from 'outstanding'. What am I missing?
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Old 16th July 2013, 11:11 AM   #8
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Okay, you got me. I was being too generous with that adjective. Coming from Class A, in comparison almost any kind of PSRR is outstanding...
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Old 16th July 2013, 11:17 AM   #9
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Yep. But then a class A doesn't poop on its own patch so needs no PSRR to speak of
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Old 17th July 2013, 04:32 AM   #10
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Hmm, depends; class A loading has a way of varying some and the CRC or CLC filters popular with class A can leave a fair bit to be desired when it comes to line regulation. Supply design for class A is maybe a bit easier for class AB but with either I tend to end up at the same point; if you really want supply artifacts not to be present on the output the result ends up being a composite amp with a regulated control loop. The 3886 wants a controlling op amp to manage its small signal THD anyway.

In this case it might preferable buy a lower voltage trafo and operate the 3886 unregulated than to heat sink two to four 338s with hefty drops plus the 3886s. The high mains, light load corner on a 30V trafo is around 50V depending on exactly how much overvoltage tolerance one designs for. This means the 338s must be floating to stay within their 40V Vin - Vout limit and the 338s must drop the rails to prevent the 3886 for exceeding its 84V Vcc - Vee limit. The low corner's again a matter of choice but, allowing for low mains, high ripple, and dropout, it's around 25V. So heatsinking for something like 50W or so of regulator dissipation is in order. This usually starts me thinking about switchmode.
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