DC/DC Converter - Page 1
Forum section: electronics / beginners

author: sımışka
author_rank: Contributor
timestamp: February 13, 2020, 01:26:54 pm
content: Hi,The input voltage of the pcb I want to design can be 12vdc or 24vdc. I want to get a constant 12v as the output voltage. So if the input voltage is 12v, the output voltage is 12v, if the input voltage is 24v, I want to get the output voltage again 12v. How can I do it ? Is there such an product?

author: TechCB
author_rank: Newbie
timestamp: February 13, 2020, 01:59:08 pm
content: I don't know the specs of your design (current, power, etc.)… But take a look at the LM7812C datasheet, this voltage regulator may help you with your design.https://www.jameco.com/Jameco/Products/ProdDS/902127.pdf

author: digsys
author_rank: Unknown
timestamp: February 13, 2020, 02:14:23 pm
content: We're going to need more info !! Lots of possibilities, depending on your requirements. #1 What current !!? #2 What output variation ? +/- 5% 10% ? #3 What type of load ? Resistive / Inductive / Capacitive ?

author: sımışka
author_rank: Contributor
timestamp: February 13, 2020, 04:27:19 pm
content: Quote from: TechCB on February 13, 2020, 01:59:08 pmbut for 12v output , Vin must be (14.5 ≤ VIN ≤ 27) in your product.

author: sımışka
author_rank: Contributor
timestamp: February 13, 2020, 04:30:31 pm
content: Quote from: digsys on February 13, 2020, 02:14:23 pm12v or 24v input, 12v at least 1.5A output, maybe additional 5v 1a output. Actually 12v output for sevaral opamps and darlington bjts.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 13, 2020, 04:40:13 pm
content: TS may want SEPIC (Single-ended primary-inductor converter) type dc-dc.

author: sımışka
author_rank: Contributor
timestamp: February 13, 2020, 06:12:35 pm
content: Quote from: Vovk_Z on February 13, 2020, 04:40:13 pmfor example LT8711 ?https://www.analog.com/media/en/technical-documentation/data-sheets/lt8711.pdf

author: frogg
author_rank: Regular Contributor
timestamp: February 13, 2020, 06:32:00 pm
content: google:1. buck converter2. switching regulatorLDO == hot

author: Xiphoid
author_rank: Newbie
timestamp: February 13, 2020, 06:52:04 pm
content: You should check the LM7812C datasheet.

author: mariush
author_rank: Super Contributor
timestamp: February 13, 2020, 07:43:39 pm
content: Ignore everyone recommending linear regulators.You may be able to find one that has a very low voltage drop, something like 0.3-0.4v at 1.5A, but even that is really a lot ... With 12v input you'd be looking at 0.4v x 1.5A = 0.6 watts making it acceptable for 12v input, but at 24v input you're looking at around 15 watts wasted.You'll have to decide if your opamps and whatever else you have can work with around 11v..11.5v, if such voltage would be sufficient.The reason I ask is because if you plan to use a 12v wallwart adapter / power supply you may find that at 1..1.5A there will be some voltage drop over the length of the cable between the wallwart and your product, so you may get only 11.8v or even less.If you choose to use a cheaper buck (step-down) only voltage regulator, such regulator would prefer to have some room between input voltage and output voltage, let's say at least 0.5v or something like that.It would be hard to "optimize" a circuit to work with 24v and output 12v while also get it working with 12v input and 11.9v out or something like that.If you want truly universal, you would have to go with a SEPIC (buck-boost) voltage regulator IC, which can take lower voltages or higher voltages and output a specific voltage - in such scenario I would probably choose to accept 9v...24v and output 12v.There's some other option ... you could have a buck only dc converter circuit and and some small circuit that measures the input voltage and if it's less than let's say 13v, the circuit would turn on a mosfet or relay and take the voltage converter out, passing the 12v directly without conversion.Look at a chip like AP63301 for a buck regulator ..https://www.digikey.com/product-detail/en/diodes-incorporated/AP63301WU-7/31-AP63301WU-7CT-ND/10492238high frequency (500Khz) so small and cheap components, synchronous so no diode required, has an automated "LDO mode" where the input voltage is very close to output voltage , and can do up to 32v and 3A so 1.5A would not be a problem.Datasheet is very detailed and even has example of layout of the componentsAnother easy to use chip would be AOZ1281 (1.8a max, 26v in max) or AOZ1283 (2.5A max, 36v in max) :https://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AOZ1281DI/785-1276-1-ND/2769843https://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AOZ1283PI/785-1803-6-ND/9679281

author: sımışka
author_rank: Contributor
timestamp: February 13, 2020, 08:31:22 pm
content: Quote from: mariush on February 13, 2020, 07:43:39 pmThanks for your reply, i like AP6301 converter and have a question about Vin voltage , for fix 12v Vout voltage , some circuit companents values in the table, okey. but what is the minimum input voltage for 12v output?

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 14, 2020, 12:45:41 am
content: Quote from: sımışka on February 13, 2020, 06:12:35 pmYes. I haven't tried it, but it is right type.But I have to add that there are a lot of usual step down type dc-dc with small enough drop voltage (Vin-Vout). May be they can do a job too. For example XL4016 can give about 11.5 V at output with 12.0 V at the input. May be that is OK too.

author: sımışka
author_rank: Contributor
timestamp: February 14, 2020, 06:35:44 am
content: Quote from: Vovk_Z on February 14, 2020, 12:45:41 amMy main purpose for 12v output supply to VCC of lm2904n opamp in DAC design. Opamp circuit is non inverting and gain is 3, opamp input voltage is 0-3.3v ( 3.3V x 3gain =10v output). for obtain 10v output from opamp, what is the at least opamp VCC ?https://media.digikey.com/pdf/Data%20Sheets/Texas%20Instruments%20PDFs/LM158,258,358,2904.pdf

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 14, 2020, 09:53:50 am
content: Quote from: sımışka on February 14, 2020, 06:35:44 amIt depends on the load.LM2904 with >2kOhm load have about 2 V "drop", so 10 V output needs at least 12 V power. If power is 11.5 V then output will be 9.5 V max.It works better with >10k load - only 1 V drop, so with such a load it could do as you want.

author: sımışka
author_rank: Contributor
timestamp: February 14, 2020, 10:35:15 am
content: Quote from: Vovk_Z on February 14, 2020, 09:53:50 amThanks your answer. for 0-10v output then could it be better to supply to opamp with 24vdc to increase load capacity? ( vcc max 26v)

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 15, 2020, 01:42:10 am
content: Yes, or use another opamp.