External duty cycle control of a computer mainboard Buck Converter - Page 1
Forum section: electronics / beginners

author: astaire
author_rank: Newbie
timestamp: July 14, 2014, 03:12:34 pm
content: Hi all!I would like to build a 1-5V, 300ADC power supply, with accurate (0.1%) output voltage adjustment. The idea is to use several multi-phase buck converters from server motherboards, and somehow control the duty cycle with analog voltage or current.The board I am experimenting with now has four 5-phase ADP3189 buck controllershttp://media.digikey.com/pdf/Data%20Sheets/Analog%20Devices%20PDFs/ADP3189.pdf. Looking at ADP3189 functional diagram, I thought that putting the voltage control on COMP pin would do the job. But all I get is 0.87V output independent of control voltage, and acoustic noise when control voltage is above 1.7V. Could you comment on this? I might have wrong idea of COMP pin, or there are security features that I fail to by-pass.In a datasheet for another buck controller, ISL 63xx (http://www.intersil.com/content/dam/Intersil/documents/isl6/isl6306.pdf), there is a paragraph on positive/negative output voltage offset. I might try the boards with these controllers, if ADP3189 is not suitable.I would appreciate comments on what pins I need to disconnect, ground etc. to achieve analog control on either ADP3189 or ISL6327 and by-pass possible error conditions.

author: rob77
author_rank: Super Contributor
timestamp: July 14, 2014, 03:21:58 pm
content: please post your actual schematic...

author: astaire
author_rank: Newbie
timestamp: July 14, 2014, 03:46:04 pm
content: The actual schematics should be pretty close to typical application schematics, which I attach here. Just all 5 phases instead of 4.I de-soldered a resistor at COMP and connected 1k voltage divider with 3.5V battery.

author: rob77
author_rank: Super Contributor
timestamp: July 14, 2014, 03:55:54 pm
content: i had a look at the datasheet... that ADP3189 is very specific - it's designed for CPU core voltage regulators. you'll not get any higher voltage than the max 1.6V.Quoteeven if you have a look at the datasheet - you'll see that with increasing values on the 8bit VID input, the voltage decreases. - no way to program the chip for higher than 1.6V.if the 0.5V - 1.6V range is enough for you, then you can control that converter via the VID digital input (e.g. with a micro) , otherwise you have to look after a different polyphase controller.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2014, 03:58:25 pm
content: COMP is a voltage output, don't attempt to overdrive it. Use COMP (out) and FB (-in) as an inverting amplifier: 100k from COMP to FB, 100k from FB to 'input'.Tie all the 'input' lines together and drive them from a common voltage regulating error amplifier.You'll also want to tie off all the LLSET junk, so all the controllers act like dumb current sources. You could probably still use one controller as the master. Probably something in the appnote about wiring units together.Tim

author: rob77
author_rank: Super Contributor
timestamp: July 14, 2014, 04:26:22 pm
content: actually it might work that way, but i have serious concerns regarding stability of regulation. excluding the load line part will decrease the stability (load transient) , dividing the feedback will decrease the stability even further. and last but not least - the inductors were calculated for a specific output voltage and duty-cycle range... cheating the feedback will push the whole thing out of SOA.only option would be to build a new regulator from scratch reusing the chip and the mosfets from that MB and calculate the correct values for the inductors...feedback loop... etc..but definitely i would really look after a different suitable controller for building a 0-5V @ 300A - those controllers are not cheap, but not even super expensive.