LM2679 question - 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 9th November 2013, 07:11 PM   #1
akis is offline akis  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Default LM2679 question

Hello

I have a linear PSU which uses a current limit sensor circuit and a voltage setting circuit. Because it is linear there can be conditions of high power spent on the current limiting transistor (a P-MOSFET actually). I was thinking of adding an LM2679 before the PSU to drop the DC voltage when needed so that we only spend very few volts on the current limiting FET.

So I need a way to control the voltage output of the LM2679. In addition I need to control it in an intelligent way, so that, ideally, it "senses" the final output of the PSU and sets its output voltage to a couple of volts over. That would mean that the PSU is always powered by a DC a couple of volts over what is needed, so there will never be excessive heat generated on the FETs.

Currently I cannot simulate this because it is oscillating. Although I have prototyped the linear PSU on the bench, I have not yet prototyped the LM2679 because it is harder to do.

Could you tell me if the idea is sound or if you can think of another way maybe?

Thanks

Edit: actually we may also spend considerable power on the voltage setting FET.
  Reply With Quote
Old 9th November 2013, 09:11 PM   #2
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
I guess you could use a blazing fast opamp + resistors as a differential amplifier with gain of 1.0, to calculate V(SMPS) - V(AnalogSupplyOut). You want this number to be exactly, what, +3.0 volts? If deltaV > 3V, dont switchmode so hard. If deltaV < 3V, switchmode a little harder.

You talk to the adjustable Vout version of the LM2679 via its FEEDBACK pin. According to the Block Diagram on p.7 of the datasheet, when FEEDBACK is greater than 1.21 volts, the output is deemed to be "too low" and the SMPS pumps harder. When FEEDBACK is less than 1.21 volts, the output is deemed to be "too high" and the SMPS pumps less hard.

So what you really want, is a blazing fast opamp + resistor circuit that computes V(SMPS) - V(AnalogSupplyOut) - 3.00V + 1.21V. In other words,
  • Opamp circuit output = LM2679 FEEDBACK pin input = V(SMPS) - V(AnalogSupplyOut) - 1.79 volts.
That doesn't look so hard. It's just a 40 MHz unity gain stable opamp, a few resistors, and a reference voltage IC. Although the fastest opamps generally run on reduced supply voltages, like plus/minus 6V or even plus/minus 2.5V. So you may need to build and regulate sub-rails.
  Reply With Quote
Old 9th November 2013, 10:32 PM   #3
akis is offline akis  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
I think the LM2679 tries to maintain 1.21V between the feedback and the ground, just like the LM317 tries to maintain 1.25V between out and ref pin. This is a simple model I have built for it.

Why do I need a very fast op-amp? Considering the PSU uses an op-amp to sense the current, one op-amp to drive the current limiting FET and another op-amp to drive the voltage output FET? Edit: I was thinking maybe a delay would be needed to avoid oscillations?
Attached Images
File Type: jpg LM2679-SimpleModel.jpg (22.4 KB, 35 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
Quickie woodworking question! With bonus shellac question... head_unit Construction Tips 13 12th September 2010 05:38 PM
do i REALLY need a crossover?? and an amp question. and an ohm question legendaryfrog Multi-Way 4 29th October 2006 01:39 AM
newbie with input tran. question and wiring question imo Solid State 0 18th January 2006 10:10 PM
Capacitor Question /question on cap TunaFish Tubes / Valves 14 12th January 2004 02:23 AM


New To Site? Need Help?

All times are GMT. The time now is 04:52 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