LM2679 question
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
 9th November 2013, 07:11 PM #1 akis   diyAudio Member   Join Date: Jul 2008 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.
 9th November 2013, 09:11 PM #2 Mark Johnson   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.
akis
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
 LM2679-SimpleModel.jpg (22.4 KB, 39 views)

 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 Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         Dx Classic Amplifiers         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post head_unit Construction Tips 13 12th September 2010 05:38 PM legendaryfrog Multi-Way 4 29th October 2006 01:39 AM imo Solid State 0 18th January 2006 10:10 PM TunaFish Parts 14 12th January 2004 02:23 AM

 New To Site? Need Help?

All times are GMT. The time now is 12:41 AM.