LTC1871 SMPS controller with novel error amplifier connection - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: October 01, 2017, 10:29:33 am
content: Hello,The attached schematic (and LTspice simulation) is a current mode Flyback using the LTC1871 controller.However, I have connected the internal transconductance error amplifier up in the manner of a “voltage error amplifier”.Do you know why the datasheet does not mention the possibility of doing this?It certainly eases the feedback loop compensation.LTC1871 datasheet:http://cds.linear.com/docs/en/datasheet/1871fe.pdfIncidentally, the below is a discussion on the same topic….http://www.edaboard.com/thread304352.html

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2017, 12:08:15 pm
content: Quote from: treez on October 01, 2017, 10:29:33 amI suspect Linear Technology assumes that the designer either understands how transconductance amplifiers may be configured or is going to rely on their tested application examples. Application note 149 discusses this in connection with Linear Technology designs and shows that your configuration suffers from increased switching noise without a decoupling capacitor from the transconductance output to ground.QuoteYou did not demonstrate this in thewww.edaboard.comdiscussion and your implementation looks more complicated.Words of wizdom from Linear Technology application note 19:Transformer Wire Backwards - Those dots indicate polarity, not smashed flies.Electrolytic Capacitors Installed Backwards - This is no problem until you bend over to see what is wrong - then "bang," a personal demonstration of explosive venting.Didn't Read the Data Sheet - Then you shall have no pie.

author: ocset
author_rank: Super Contributor
timestamp: October 01, 2017, 02:50:18 pm
content: QuoteThanks, i appreciate that, but the voltage error amplifier version of that external post was satisfactory enough.The fact is you cant get a proper type 3 error amplifier with the "transconductance error amplifier" rendition of the LTC1871.

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2017, 04:23:16 pm
content: Quote from: treez on October 01, 2017, 02:50:18 pmI would not say proper but instead not as flexible and the comment in the discussion you linked said the same thing. So does this TIapplication note.Type III Compensator Using OTA:An OTA does not offer the same design flexibility as an Op-Amp. However, IC designers like thestructure because of the small semiconductor die size it requires. The lack of virtual groundmakes the lower side divider resistor enters the equations and requires attention when derivingpoles and zeros placement. ...If you want to convert the OTA into a operational amplifier, you can do that by adding a buffer to its output before the feedback network.

author: ocset
author_rank: Super Contributor
timestamp: October 01, 2017, 07:33:20 pm
content: QuoteThanks, the thing is, if you notice, the simulation i posted in post 1 seems to work fine. I dont think i would want to add anything else, though i appreciate that in theory the buffer is needed.Nobody ive ever come across seems to be able to say that what i have done in the top post schem is "wrongful".

author: David Hess
author_rank: Super Contributor
timestamp: October 02, 2017, 07:06:12 pm
content: Quote from: treez on October 01, 2017, 07:33:20 pmIf it works, then it works. I suspect it would work better with an output buffer to drive the feedback network and the OTA (operational transconductance amplifier) output compensated to make an operational amplifier but that the improvement would be insignificant in these applications for extra complexity, a poor payoff.It is rare but sometimes OTAs are configured with feedback from the high impedance output. You might take a look at how the different forms of compensation for 301 and 308 (1) type operational amplifiers work. The compensation can be around the transconductance gain stage inside the operational amplifier like a standard operational amplifier, or it can be around the transconductance gain stage and output stage (2), or it can be from the transconductance output to ground like an OTA, or in inverting applications, feedforward compensation may be used.I am not an expert on this but I think I see where that noise comment in the TI application note comes from. The high impedance current output of an OTA when used with external feedback allows high frequency noise from the inverting input to go directly to the output with the feedback network providing a low frequency offset. When the output network goes to ground instead, the output impedance drops with frequency and this noise is filtered out or simply not present since there is no path around it.(1) Be careful of the Linear Technology schematic of the 308. It has a confusing drafting error involving compensation pin 8. I have an interest in operational amplifiers which provide access to the transconductance node because it can be used for clamping in fast response error amplifiers and nobody makes a precision OTA.(2) 2 pole compensation as shown in the 301 datasheet.