Improving the LM3x7 regulator circuit - Page 5 - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 24th March 2004, 10:32 PM   #41
diyAudio Retiree
 
Join Date: Oct 2002
Location: Spain or the pueblo of Los Angeles
Question I think you should go and read the data sheet first of all

I don't know where you pulled those numbers out the air from but I think you have a misconception of what actually constitutes bandwidth for a regulator.

The bandwidth of a regulator at high frequencies as seen by the load is determined by the output capacitor and the bandwidth of the error amp is not the determining factor after a certain point. The gain bandwidth of the error amp will control the impedance as a function of frequency until the impedance rises to the point where it is higher than the impedance of the capacitor. That's why designers put capacitors on the output of regulators. Believe it or not three terminal regulators are used to power digital circuits with the right decoupling caps! It is quite possible to improve the noise and PSSR ratio over the bog standard implementations usually applied to 317/1086/337 type regulators. I think the idea was to milk the best performance out of a three terminal regulator with a simple circuit using good parts. There are applications where this circuit would be used as a preregulator for a more exotic regulators as well. I would think there is some interest in optimizing the circuit since even Mr. Carr weighed in on it. There are other chips with higher bandwidths that can be used for a regulator of about the same complexity (a couple more parts) and power dissipation as a 317........ but you would not be interested in that either I imagine.
  Reply With Quote
Old 24th March 2004, 11:27 PM   #42
jcarr is offline jcarr  United States
diyAudio Member
 
Join Date: Jul 2002
Location: Tokyo, Japan
Dear Dimitri:

How are you doing?

When an experienced designer uses a 3-terminal regulator together with fast digital or video-bandwidth circuitry, I doubt if he expects that the regulator by itself will be able to accomplish the entire task. Rather than expecting that the regulator itself be fast enough to keep the load circuitry happy, the designer primarily wants the regulator will keep the local bypass caps (placed at the load to reduce the effects of trace inductance) charged up, and it is these caps that will be called upon to supply the power to the load circuit as demanded.

OTOH, when using a discrete high-speed regulator, the designer may in fact want the regulator to do most of the work, and in this case local bypass capacitors may be used very sparingly.

In other words, the thinking differs for the above two situations, and in audio some well-known designers express a preference for the regulator-as-cap-charger philosophy, while other successful audio designers have stated that they want the regulator itself to have enough speed for the load. I use both concepts on occasion, depending on my mood and budget .

Besides, it is fun to try to make a Yugo perform like a Porsche (at least a 924 ).

Frank, if you connect C2 from the base of Q2 to the collector (ground), you _will_ have a low-pass filter, and the value of R2 will be added to the time constant to lower the corner frequency even more. So you get a lower drive impedance into Q2 because the value of R2 will no longer be added on top of the impedance curve of C2, plus you reduce the noise where it really matters - at the base of Q2.

I am not quite sure what you mean by R4/R8, as R9 corresponds to R4 in the negative schematic, while R8 in the negative corresponds to R3 in the positive.

hth, jonathan carr
__________________
http://www.lyraconnoisseur.com/, http://www.lyraaudio.com
  Reply With Quote
Old 25th March 2004, 01:16 AM   #43
diyAudio Retiree
 
Join Date: Oct 2002
Location: Spain or the pueblo of Los Angeles
Red face Vicodin and posting don't mix......

"R8 in the negative corresponds to R3 in the positive."

Thanks, the base resistor is a small value resistor of 100 ohms or less
R8 or R3 are about a couple of orders of magnitude larger and determime the filter corner frequency. You also might not want to discharge the cap through the base of the transistor during fault conditions. Don't forget that Q1 and Q2 are high Hfe and Ft devices. you may not want to chuck the base resistors just yet........
  Reply With Quote
Old 25th March 2004, 01:43 AM   #44
jcarr is offline jcarr  United States
diyAudio Member
 
Join Date: Jul 2002
Location: Tokyo, Japan
Fred:

>R8 or R3 are about a couple of orders of magnitude larger and determime the filter corner frequency.<

I experimented with a wide range of values for R2, from 33 ohms up to 4.7kohms. In the interests of further circuit simplification I also tried deleting it entirely.

If you use SMD/MELF resistors and can physically get the junction of R3/R4 really close to the base of Q2, you may find that you don't need any additional base stopper resistors at all, in which case R2 can be eliminated. Ditto on the negative side.

But if you can't make a physically tight layout, and significant pcb or lead trace remains between the junction of R3/R4 and the base of Q2, I would leave R2 intact (and get the trace/lead length between R2 and the base of Q2 as short as possible), Otherwise Q2 may exhibit signs of instability.

Besides, Fred's point is valid. Fault protection should always be considered.

Look at the schematic, consider what you can accomplish with the board layout, and weigh the possibilities of some kind of fault condition occuring (including human error).

regards, jonathan carr
__________________
http://www.lyraconnoisseur.com/, http://www.lyraaudio.com
  Reply With Quote
Old 25th March 2004, 03:32 AM   #45
diyAudio Retiree
 
Join Date: Oct 2002
Location: Spain or the pueblo of Los Angeles
Default reading between the lines

"If you use SMD/MELF resistors and can physically get the junction of R3/R4 really close to the base of Q2, you may find that you don't need any additional base stopper resistors at all, in which case R2 can be eliminated. Ditto on the negative side."

I'm going to go out on a limb and assume that when your refer to the junction of R3/R4 you mean the connection to C2. I think that R3/R4 are large enough values to pretty well damp the trace inductance to their junction. The trace inductance and capacitor's internal and lead inductance of C2 is my real concern. Keeping C2 as close as possible to Q2's collector and base leads would be my first priority. I have a 5uF 200 V cap on my protoboard and a little damping seemed a very good idea. Shorting the input or output of the regulator will likely discharge the cap through the reverse biased base emitter junction of the transistor. The data sheet says not to exceed -5 volts from base to emmitter. If this supply gets shorted you will easily damage a transistor without current limiting provided by a base resistor. Too many years working on high reliability telecom circuits makes me worry about stuff like this. Also, I really wanted to avoid clamp diodes due to the RF paths provided by their junction capacitance. 100 ohms divided by a beta of 400 is 0.25 ohms and is going to add very little to the output impedance of the transistor follower. It is surprising how much there is to consider for a one transistor circuit isn't it?
  Reply With Quote
Old 25th March 2004, 04:07 AM   #46
Account Disabled
 
Join Date: Apr 2003
Location: US
Default Re: apples and oranges

Quote:
Originally posted by Fred Dieckmann
What I ended up building is a one transistor addition to the three terminal regulator.

but how do we know for sure that the circuit is correctly captured?
  Reply With Quote
Old 25th March 2004, 08:49 AM   #47
dimitri is offline dimitri  United States
diyAudio Member
 
dimitri's Avatar
 
Join Date: Oct 2002
Location: retired
Send a message via ICQ to dimitri
Fred Dieckmann wrote: I don't know where you pulled those numbers out the air from but I think you have a misconception of what actually constitutes bandwidth for a regulator.

Dear Fred, I hope that you had measured the gain bandwidth of the error amp before you made attempt to improve the the LM3x7 regulator circuit. Otherwise you can read publications from AENG, which were cited here many rimes.

OK, you stick to regulator-as-cap-charger philosophy and would like to investigate the audibility of the output cap (and other components). Jonathan has already reported this.

You use the emitter follower in the feedback path, so you make the dc loop gain Vout/1.25V times higher than in datasheet circuit. You will get 10 time lower output impedance (about 2mOhm) in the frequency band from dc to several hundreds of Hertz. The ac loop gain remains unchanged. Another thing is that you eliminate the influence of the adjustment current as you drive the adjustment pin from low impedance node. But you add the temperature drift of BE junction -2,2mV/C to the initial reference 1,25V. The reference have the temperature stability -0.1mV/C in the range 25-75C, so you have worsened it significantly.

Is your “enhancement” valuable? From the components (and posts ) number point of view – yes. From the other points – doubtful…


Thank you, Jonathan, I will wait, I’m not in a hurry
  Reply With Quote
Old 25th March 2004, 11:21 AM   #48
Account Disabled
 
Join Date: Apr 2003
Location: US
Quote:
Originally posted by dimitri
Is your “enhancement” valuable? From the components (and posts ) number point of view – yes. From the other points – doubtful…

Hi, that's what we amature designers do: adding useless things like those "enhancements" so we can feel better,

I agree with you, dimitri. It seems enhancing things we don't know (and cannot capture correctly) is pretty funny.
  Reply With Quote
Old 25th March 2004, 11:34 AM   #49
diyAudio Retiree
 
Join Date: Oct 2002
Location: Spain or the pueblo of Los Angeles
Default only about an order of magnitude after all

'You will get 10 time lower output impedance (about 2mOhm) in the frequency band from dc to several hundreds of Hertz."

Yes, and similar improvements in PSRR. Changes approaching an order of magnitude are usually considered significant for most design work.

"But you add the temperature drift of BE junction -2,2mV/C to the initial reference 1,25V. The reference have the temperature stability -0.1mV/C in the range 25-75C, so you have worsened it significantly."

22mV over a 10 degree change times around (15/1.85 + 1) for 15 volts out. That's a little bit over 0.2 % I think I can live with that for a power supply voltage. I think you will find the temperature changes inside your audio equipment to be well under 10 degrees after it is warmed up. Even leaving the circuit on the bench and with the ceiling fan on I saw less than 10 mV drift after the circuit came up to temperature. I just don't see anything that bothers me about the temperature stability.

"You use the emitter follower in the feedback path, so you make the DC loop gain Vout/1.25V times higher than in datasheet circuit. You will get 10 time lower output impedance (about 2mOhm) in the frequency band from dc to several hundreds of Hertz. The ac loop gain remains unchanged. "

No the AC loop gain changes........ check the data sheet figures below for the LM317 below to see how much bypassing the ADJ terminal improves PSRR out and Zout over a large frequency range!

'OK, you stick to regulator-as-cap-charger philosophy and would like to investigate the audibility of the output cap (and other components). Jonathan has already reported this.'

Yes...... and I have discussed using good film caps also. The buffering of R3 and R4 with the follower allows much large resistor values and smaller cap values for a given corner frequency. This making the use of a quality film cap in the 1uF to 5uF region easily usable. Doesn't that fall under the capacitor quality issue?

There are better circuits for audio power supplies, not every one wants to go that far but would like to tweak the 317/337 up a few notches in performance. I have seen some sour grapes rational but nothing concerning the actual circuit, that makes it appear to not to deliver improvements in performance with a reasonable, simple and straight forward circuit, as desired by the original poster. I never imagined a one transistor circuit addition could generate any controversy. This one almost falls under the no brainer category and is real easy to go build and test. Try it.
Attached Images
File Type: gif 317data.gif (9.5 KB, 3972 views)
  Reply With Quote
Old 25th March 2004, 01:04 PM   #50
dimitri is offline dimitri  United States
diyAudio Member
 
dimitri's Avatar
 
Join Date: Oct 2002
Location: retired
Send a message via ICQ to dimitri
Dear Fred,
Your improvements in ripple rejection ratio and in output impedance are shown by blue lines.

Fred Dieckmann wrote “Yes, and similar improvements in PSRR”

As you can see from the data the circuit with Cadj=10uF _already_ has nearly the same PSRR as your one (red circle). I always thought that the ripple has the frequency 100-120Hz, why we need the improvement below?

Fred Dieckmann wrote “No the AC loop gain changes........ check the data sheet figures below for the LM317 below to see how much bypassing the ADJ terminal improves PSRR out and Zout over a large frequency range!”

Check yourself ... I have never wrote that we don’t need bypassing the ADJ terminal.
Attached Images
File Type: gif 317datafd.gif (9.6 KB, 3783 views)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Improving a turntable (motor controller circuit) ;) sek Analogue Source 26 15th September 2008 09:44 AM
Improving on LCaudio LowNoise regulator ABO Power Supplies 43 14th January 2005 12:53 PM
Fet regulator circuit HarryHaller Solid State 47 8th January 2003 04:26 PM


New To Site? Need Help?

All times are GMT. The time now is 03:01 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2