Electronics > Beginners

DC/DC Converter

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sımışka:

--- Quote from: mariush on February 13, 2020, 07:43:39 pm ---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/10492238
high 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 components

Another 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/2769843
https://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AOZ1283PI/785-1803-6-ND/9679281


--- End quote ---

Thanks 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?

Vovk_Z:

--- Quote from: sımışka on February 13, 2020, 06:12:35 pm ---
--- Quote from: Vovk_Z on February 13, 2020, 04:40:13 pm ---TS may want SEPIC (Single-ended primary-inductor converter) type dc-dc.

--- End quote ---

for example LT8711 ?

https://www.analog.com/media/en/technical-documentation/data-sheets/lt8711.pdf

--- End quote ---
Yes. 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.

sımışka:

--- Quote from: Vovk_Z on February 14, 2020, 12:45:41 am ---
--- Quote from: sımışka on February 13, 2020, 06:12:35 pm ---
--- Quote from: Vovk_Z on February 13, 2020, 04:40:13 pm ---TS may want SEPIC (Single-ended primary-inductor converter) type dc-dc.

--- End quote ---

for example LT8711 ?

https://www.analog.com/media/en/technical-documentation/data-sheets/lt8711.pdf

--- End quote ---
Yes. 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.

--- End quote ---

My 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

Vovk_Z:

--- Quote from: sımışka on February 14, 2020, 06:35:44 am ---My 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 ?

--- End quote ---
It 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.

sımışka:

--- Quote from: Vovk_Z on February 14, 2020, 09:53:50 am ---
--- Quote from: sımışka on February 14, 2020, 06:35:44 am ---My 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 ?

--- End quote ---
It 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.

--- End quote ---
Thanks your answer.  for 0-10v output then could it be better to supply to opamp with 24vdc to increase load capacity? ( vcc max 26v)

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