Voltage Regulator - Page 1
Forum section: electronics / beginners

author: staticshock
author_rank: Contributor
timestamp: June 06, 2019, 05:17:06 pm
content: I have op-amps in my circuit that need +/- 16.5V or roughly (16V-18V). However, the DC Voltage available is 28V and will need to step it down. Using LP2951 and simulating in PSpice I can get +16.5V, but either I'm clueless or if my brain is fried at the moment I'm not sure of how to get -16.5V. Any help?

author: bob91343
author_rank: Super Contributor
timestamp: June 06, 2019, 05:58:57 pm
content: You didn't say the current drain. There are IC regulators both pos and neg for a wide variety of voltages. If you need more current you can add a power transistor.

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 06, 2019, 06:28:29 pm
content: If the supply voltage of the op-amp is bigger than u need, then it will still work fine, as long it's it not over the rated limit. But if the same supply is used in biasing/etc, then some values of parts would be changed.LM317 and LM337 chips are very common and cheap adjustable +/- regulators, like LM7812 and LM7912 +12/-12 regulators.

author: mariush
author_rank: Super Contributor
timestamp: June 06, 2019, 06:33:55 pm
content: There are voltage doubler/inverter ICs which can take some voltage and output the negative.For example40mA max : TC7662ACPAhttps://www.digikey.com/product-detail/en/microchip-technology/TC7662ACPA/TC7662ACPA-ND/11528780mA max : TC962CPAhttps://www.digikey.com/product-detail/en/microchip-technology/TC962CPA/TC962CPA-ND/115289So you could have any ldo to produce 16.5v and then use one of the chips above to produce -16.5vIf you want something cheaper, you could go with one of the 34063 or 33063 switching regulators and configure them in inverting mode.There's calculator on Sourceforcehttps://sourceforge.net/projects/mc34063uc/and there's online calculators as well :http://www.nomad.ee/micros/mc34063a/

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 06, 2019, 07:01:57 pm
content: Voltage doubler circuits:https://en.wikipedia.org/wiki/Voltage_doubler

author: Dave
author_rank: Super Contributor
timestamp: June 07, 2019, 06:25:07 am
content: The voltage requirement between the two rails (33V) exceeds your supply voltage, so you will need some kind of switching topology to reach that voltage. Linear regulators alone (like your LP2951) are not going to work.If your circuit demands a low current, you may be able to get away with just a capacitive charge pump, but for higher currents you're going to need inductors. If you don't feel like designing your own from the ground up (might be a bit overwhelming for a beginner), I suggest you tryTI's Webench.Observe their recommended layout if you're going to be designing your own board, because layout isveryimportant in SMPS design. Put your design on the forum and have others critique it before sending it in for fabrication.

author: staticshock
author_rank: Contributor
timestamp: June 07, 2019, 02:03:24 pm
content: Thanks for the replies. I will look into them asap. Further research has led me to TPS7A3901 with dual positive and negative outputs. Following the datasheet I've tried replicating it to give my desired outputs but from my simulation it doesn't appear to be correct because it's giving me millivolts. Can someone verify my connections? In addition, I'm not sure about EN and PAD pins...

author: bob91343
author_rank: Super Contributor
timestamp: June 07, 2019, 04:07:25 pm
content: I don't know this part but from the diagram and nomenclature I suspect it's a positive and a negative regulator in the same package. As such, it requires two sources, one positive and one negative.The PAD terminal is, by my guess, just the substrate and should be grounded.

author: staticshock
author_rank: Contributor
timestamp: June 07, 2019, 06:18:46 pm
content: Apparently I didn't simulate it with enough run time. After increasing, it is functioning as expected. And as I guessed, the EN is tied to INP and PAD is grounded.

author: staticshock
author_rank: Contributor
timestamp: July 03, 2019, 05:00:28 pm
content: TPS7A3901 is only available as a surface mount component. Are there adapters I can use so that I can use it on a breadboard?

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2019, 05:12:16 pm
content: Adapters are readily available. I bought some from China on ebay and they are cheap and work perfectly.I didn't look at the number of pins you need so maybe they don't make one of sufficient complexity but it pays to look.