Voltage inverter using dc-dc converter - Page 1
Forum section: electronics / beginners

author: newbie666
author_rank: Regular Contributor
timestamp: September 13, 2023, 01:44:37 pm
content: Hi everyone,I need to generate a negative voltage rail in a circuit powered by 9-12V dc input. The negative rail voltage should be inverted positive rail voltage, so if the circuit is powered by 9V then the circuit should generate -9V, if powered by 12V then negative rail should be -12V etc.The max current draw on both rails is 400mA.Now there are some inverting charge pumps that do just that (like LM2776) but their voltage range is usually too low and they can supply less than 100mA so I think I will have to use a dc-dc converter to achieve what I want.My problem is that all of the dc-dc converter ICs regulate their output to a fixed voltage that is set with feedback resistors. How do I design an inverting dc-dc converter that just flips input voltage to the identical, negative value?

author: jwet
author_rank: Frequent Contributor
timestamp: September 14, 2023, 02:08:19 am
content: You're in a no man's land of charge pump power supplies. There are a few that will do the volts (7662) but not the current by a long shot. You're going to have to use a real inverting switcher with an inductor or transformer. The topology is close to that of a buck converter and most bucks will operate in this mode.An oldie but goodie is the LT1074 but most any buck will do it. Look in the app circuits towards the end of the 1074 DS linked below. They also have an old app note (AN44) that goes though this in better detail. Almost any buck converter that can handle the volts and amps can do the same but the LT1074 was famously used in a lot of -9V .5amp power supplies on LAN cards back in the day and this application is well documented. You might want to use this topology/technique on a more modern buck converter. You can easily do this with a modern 1+ MHz switcher in a SOT package in 1/8 the board area and for half the cost- progress. Look a the LT1616 for a candidate.Making it dual tracking will take a bit of trickery and there are a few ways to do it. The easiest would be to have the feedback for the negative inverter developed from a divider that goes from the positive input (9-12) to the negative rail with the FB pin being at the correct voltage when Vout = -Vin. For the LT1074, this is 2.21v, most bucks are 1.25 or so, it doesn't really matter. This will be an asymmetric voltage divider. Use 1% or better resistors or pots with a limited range to dial in the voltage- there is a lot of tolerance stack-up in this rig and you want to minimize what you can. What you'll really be regulating is the total voltage output of the supply but if your input is 9-12 the negative will be the opposite.Some possible gotchas or other comments to watch for:Sequencing- some circuits don't like to have pos and neg supplies doing odd things at startup or shutdown. LCD displays are famous for this- standard op-amps and other amplifiers don't care much but you're usually better off to bring positive up first. You can hold off the negative generator with its shut down or by shunting its FB to do sequencing.Dual tracking requirement- a lot of dual polarity circuits don't care much if they have assymetric supplies, you could just create -12v regulated from you 9-12v input. If you look a the PSRR requirement, you'll see numbers approaching 100 dB at DC.There are some clever cascode transistor type feedback tricks that are sometimes used. Try the preceding first, it will likely work.Link to 1074 DS and AN44- good reading.https://www.analog.com/media/en/technical-documentation/data-sheets/1074fds.pdfhttps://www.analog.com/media/en/technical-documentation/application-notes/an44fa.pdfHave fun.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 14, 2023, 02:19:05 am
content: Rather than make the negative voltage track, why not just regulate both the positive and negative supplies to +-9V or something slightly less. Why would it need to vary?

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2023, 05:25:12 am
content: Quote from: jwet on September 14, 2023, 02:08:19 amThat is simpler and should be more accurate than what I was thinking of.If the converter operates in continuous conduction mode, then the output can be tied to the input by using a constant duty cycle, but then the output voltage will vary with the forward voltage drop of the rectifier and switch degrading accuracy.

author: jwet
author_rank: Frequent Contributor
timestamp: September 14, 2023, 04:14:54 pm
content: Also, the LT1076 is the 2 amp version of the family and would be more appropriate, I knew there were two and got the numbers mixed. LT1074 has a 5 amp switch and cost a lot more. I would use a modern equivalent buck anyway.

author: newbie666
author_rank: Regular Contributor
timestamp: September 18, 2023, 09:02:25 am
content: Thank you for all the replies. an44fa is a great resource I wasn't aware of and I gave it a proper read. I've selected TPS5430 for my switcher as it has a wider input range (up to 36V) which means my circuit could operate close to +-18V max.I really like the idea with voltage divider between V+ and V- and this is what I'll try next. The reason for tracking positive voltage is that I want to be able to power the circuit with as low as 9V if necessary at the cost of headroom, but also be able to use higher voltage for better performance essentially.Thanks again, now I'll get to buildingM.