Converter buck/boost with n°2 out

Your solution that uses a buck for 5V then a boost for 12V is a viable option.  For the boost stage in such a configuation, you have several choices from Ti including LM3668.  You may also consider a different approach--a single-stage, dual output buck-boost like LM2585/6/7.  I will ask the engineer of our Simple Switcher group to comment and respond.

For your 1st output you might consider a Sepic converter based on LM2585S-ADJ. Attached is an approximate schematic. If it doesn't meet your needs let us know and we can try a different approach.  What factors are most important?  Size Cost Efficiency etc.

0284.LM2585S-ADJ_9_to_25_In_12_Out_Sepic_Schematic Prints.pdf

Alan Martin TI - SVA - Simple Switcher Customer Support Group

Thanks

I power @ 5v an RF module and little brushed motor, instead with 12V an camera.
I use 9-25 Vin ( battery )

According to you, what is the best solution for noise and size ?

use SEPIC for 12V and separated COT for 5V
or COT @ 5V and after Boost it to 12V?

Is possible solution with only one and little inductor ?

Thanks

if can help we can reduce Iout ( 400ma ) @ 12V

If good:

i use LM25085 to buck Vin9-25 in Vout 5V

After i use boost for Vout 12V and 400ma ( I use 5V by LM25085 )

Important is Footprint....max 400mmq

Is good use MC34063 ?

thanks

Or...if is correct buck and after boost....

Is better MC34063 or TPS61085?

Thanks

I think that if you chain converters, for your application it is better to make 12V first. If you made 5V first, it would have to be a 4A converters, 2A for the load and 2A for the boost. If you make 12V first, it can be a 2A PS, 1A for the load and 1A for the following buck for 5V.

In any case, If I were you, I would make two independent power supplies. Just in case, for many reasons. For example, you may discover that you want the 5V to wake up first to power a micro that will enable the 12V later... or the other way round.

yes, no bad your idea....two separated power supply.

But....for 12V, I need little size ( under 400mmq ), only 1 inductor, good ripple out.....what is good IC for this ?

and for 12V I have ....Vin: 9-25V

thanks

Did you try to use Webench?

Input the voltage range you want, the output voltage, the output current for each voltage... I tried that for you in the case of the 12V and got LOTS of solutions, most of them based in SEPIC configuration. Go give it a try for yourself

All the best

Albert

thank you very much.

I try it, but Ive difficult to find good solution for radio equipment

Acconding to you.... what is good IC for free of radio interference.?.....( I know that after design is important...but what is good start )

Internal or external switch ? ( internal is more simply for my design )

SEPIC or FLYBACk ?

Thanks

My intuition says that internal switch is better since connections will be shorter. I don't have an answer regarding SEPIC or FLYBACK. Flyback has the advantage of closing the flux inside the core. But Sepic also has solutions with coupled inductors, and the flux would be also contained. You should maybe ask this question on the RF forum and of course, make a search on the Internet for relevant material.

Moreover, synchronizing the power supplies (and adding a phase shift) can be very good to reduce EMI, as is using spread speactrum techniques for the clock input. Also,  pulse skipping for light loads improves EMI.

Hello Again Silvio,

Lets look at the output power requirements of your revised spec.

12V @ 0.4A is 4.8W.  and

5V @ 2A is 10W.

For maximum efficiency the 5V supply should be synchronously rectified. So take a look at the LM3102 a 2.5A synchronous regulator.  Run it at around 400kHz for best efficiency.

Next you should consider trying a sepic with two individual inductors rather than a two winding 1:1 transformer. Probably the smallest approach is an LM2735YMY.  Feed its VIn input and enable from the 5V rail but run the sepic input inductor from the 9...25V input.  The maximum switch voltage of the LM2735 is only 25V.  The required switch voltage for this sepic is 25 + 12 = 37.  To acheive this you will need a cascoded external switch such as the 60V Siliconix Si7414DN-T1-E1.

Cascading the supplies where you first generate the 5V and from that the 12V would require about 4A of 5V power and push you into a synchronous controller such as LM3150 for the buck converter Overall efficiency will be lower because of the compound efficiency penalty. But If you want to try a small 5V to 12V boost use the LM2735YMY, It will probably be fine if your thermal constrains aren't too high.

It sounds like one of your questions is whether the complete solution can be accomplished with a single small inductor. Sorry but no it can't. You might be able to get all two or three inductors to be the same part number but that will increase system size as they have different requirements for optimization.

You will need an LC filter section in front of the radio 5V input to keep switcher noise isolated from it. A fairly large LC in front of the motor stage input would also be a good idea.

EMI control starts with proper layout techniques.

Alan Martin - TI - SVA

Thanks for reply.

I check your solution tomorrow....
Today I try some solution with Webench and I see this IC:

For 5V.... LM22676...is simple and I see that it have very good ripple.... ( or I see TPS54332...is better than LM22676 ? )
according to you....Is LM22676 good for low NOISE application with radio equipments ?

For 12V...

1 solution )   if I use in direct in Vin ( 9-25 )....I see LM3478 in SEPIC .... but I cant undestand that it works well for low noise equipments

2 solution ) if i use my 5V ( I change with LM22678 or LM22679 5A ).....I I use TPS61175......but I cant undestand that it works well for low noise equipments

what do you think ?

Thanks for your help