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Check my figures for LM317 ripple
Check my figures for LM317 ripple
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Old 10th November 2017, 01:59 PM   #11
mchambin is offline mchambin  France
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This sounds awfull, may be, there is a mistake in the datasheet.
I've used a LM317 at Vo = 48v ...........!!!!!!! Microphone +48 phantom power.

I do not understand their Vin - Vout = 15V.
Usually we don't want such a high voltage drop.
At 500mA this asks for a heatsink to evacuate 7.5 W

It regulates with a voltage drop as low as 3V; So, even with safety margins this 15V doesn't make sense to me. Am I missing something
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Old 10th November 2017, 04:42 PM   #12
mchambin is offline mchambin  France
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Quote:
Originally Posted by mchambin View Post
I do not understand their Vin - Vout = 15V.
Indeed, 15V doesn't make sense.
That is a typo. It is clear from other graphics nearby.
Read Vin - Vout = 5V.
5V is a usual operating choice.

The OnSemi datasheet gives a much better ripple rejection 80dB typical, instead of 64dB typical on the TI datasheet.

Beware of the noise figure. Both datasheets give 0.003% Vo noise rms.
So a 15V power supply is expected to be 0.45 mV noisy. So the 0.1 mV ripple I calculated do not make much sense. From this I think 10 000uF is overkill, 3000uF would't make any difference.
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Old 10th November 2017, 04:46 PM   #13
kevinkr is offline kevinkr  United States
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Check my figures for LM317 ripple
Quote:
Originally Posted by abraxalito View Post
What's very interesting is that the original (National) DS and TI's current datasheet do not agree on ripple rejection vs frequency. I'm inclined not to believe TI - NS shows its not flat with frequency but rather has a peak about 100Hz and then falls off.
Don't assume that the current TI part is entirely similar to the original NS part, changes in process and design improvements often occur quietly over time and the improvements are reflected in the newer data sheet. While I can't vouch for the accuracy of this particular data sheet we do use a lot of TI parts in our products at work and they generally meet or exceed the published specifications if used with care.
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Old 10th November 2017, 04:51 PM   #14
kevinkr is offline kevinkr  United States
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Check my figures for LM317 ripple
Quote:
Originally Posted by mchambin View Post
Indeed, 15V doesn't make sense.
That is a typo. It is clear from other graphics nearby.
Read Vin - Vout = 5V.
5V is a usual operating choice.

The OnSemi datasheet gives a much better ripple rejection 80dB typical, instead of 64dB typical on the TI datasheet.

Beware of the noise figure. Both datasheets give 0.003% Vo noise rms.
So a 15V power supply is expected to be 0.45 mV noisy. So the 0.1 mV ripple I calculated do not make much sense. From this I think 10 000uF is overkill, 3000uF would't make any difference.
Ripple rejection and output noise are not synonymous, the noise is measured over a specified bandwidth, the ripple is at a specified frequency. 100uV of ripple at 100Hz or 120Hz will not be lost in the total 450uV of noise over say a 20kHz bandwidth.
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Old 10th November 2017, 05:42 PM   #15
mchambin is offline mchambin  France
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Sure ripple and noise are not synonymous.
How do they add ? 0.1 mV and 0.45 mV
Because we have rms values, I think we add this way:
( 0.1^2 + 0.45^2 )^0.5 that gives : 0.46
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Old 10th November 2017, 06:22 PM   #16
kevinkr is offline kevinkr  United States
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Check my figures for LM317 ripple
Nope, they're not referred to the same measurement bandwidths, one is simply uncorrelated broadband noise, and the other is a combination of highly annoying spectrally discrete harmonics of the AC line frequency and in addition the ripple is much more annoying than broadband noise over a 20kHz bandwidth. Take a look at the noise spectrum using an FFT and you should see the discrete tones of the ripple and its harmonics are dominant. Neither tbh are particularly great performance levels, but better the older fixed voltage regulators they were intended to replace.
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Old 10th November 2017, 06:25 PM   #17
kevinkr is offline kevinkr  United States
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Check my figures for LM317 ripple
I thought you might find this interesting if you are looking at ways of reducing noise on the output of inexpensive monolithic voltage regulators. (It's pretty neat stuff)

Finesse Voltage Regulator Noise! |
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Old 10th November 2017, 06:25 PM   #18
Chris888 is offline Chris888  United Kingdom
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Bear in mind that in a typical audio application, the load step on each rail could put far more AC on the rail than either the noise or supply ripple.
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Old 10th November 2017, 08:38 PM   #19
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by mchambin View Post
This sounds awfull, may be, there is a mistake in the datasheet. I do not understand their Vin - Vout = 15V. Usually we don't want such a high voltage drop. At 500mA this asks for a heatsink to evacuate 7.5 W It regulates with a voltage drop as low as 3V; So, even with safety margins this 15V doesn't make sense to me. Am I missing something
Hmmm....maybe. But the datasheet under "Electrical Characteristics" shows the maximum current at Vi-Vo ≤ 15 volts to be 1.5 A min, 2.2A typical. That would be Pd = 22.5 watts min, 33 watts typical. So if it IS an error, it's done in two places. Your application of Phantom Power draws VERY little current (5 mA usually), so it may not be relevant. Another poster correctly noted another error on the datasheet---the reversal of the Cadj =10uF and Cadj = 0 uF on the "Ripple Rejection vs. Frequency" graph; so perhaps the proofreading here is not so great.
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Old 12th November 2017, 10:15 PM   #20
dotneck335 is offline dotneck335  United States
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The On Semiconductor datasheet seems to indicate better performance than the T.I. one. The ripple rejection is ≥80 db w/ 10uF adjustment cap; and that stays constant up to 35 volts output. This would give me about 13.2 uVolts ripple from a OnSemi LM317 with 10,000uf input cap, at 1 ampere of current. Far below what I can hear.
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