Buck converter using LTC1625

Hello,

I'm designing an buck converter that operates from a stable 12V DC source and makes it into 6.3V 8A DC.
I'm trying to prevent using IC's i can't solder(newer packages that require reflow etc, unhumanly small).
So i found LTC1625 from Linear technology which suits my needs.
But it's datasheet isn't as good as TI's for example and i'm running into problems, mainly into picking capacitors for input and output.

Basically i am between two mosfets: SI4410DY and Si4134DY. I'm probably going to order both. SI4410DY is easier to design because i downloaded LTSpice and it had SI4410DY in database but no Si4134DY.

Calculations i have made according to datasheet:
Pt(100)=1.3 [temperature related RDS on changes, using SI4134DY]
RDSon=0.120/8*1.3=0,0115ohm
Ptop=0.60W [Top mosfet power dissipation]
Inductor=7uH
Tjunction=70C+0.6*50=100C (I presume this is the maximum Tjunction, is it really realistic for the IC to have 70C ambient as is in datasheet?)
Resistor divider: R2=35.7kOhm ; R1= 8,3kOhm 1% (Should give 6.3V)

Supply is probably going to drive around 6A but 8A would be good. I don't mind losing on efficency and the components could be quite big.
Now i'm stuck at finding the required input and output capacitors. The datasheet says to use RMS but they don't show good calculations so i'm stuck...
According to Irms=Iomax*Vout/Vin(Vin/Vout-1)^1/2 so if i put my data in Irms=4A, how do i choose capacitor now? I understand that the capacitors combined peak rating must be higher than 4A, what about capacitance and voltage? I presume this is for the Cin capacitor solely.
For Cout i presume i use the formula Vmaxrippleiwant=Ilmax*ESR . So if i want ripple 0.05V then 0.05=3.2*ESR ESR=0,0156ohms? But how would i choose the voltage and capacitance?

Also what would be good software to design this IC, i have eagle cad for student but i think even windows 10 search is better than that... Maybe have to manually insert the package...

Hi,

a bit confusing post.

I recommend to ask only one dedicated question at a time....

So what´s your main question? I´m looking for question marks.

is it really realistic for the IC to have 70C ambient as is in datasheet?)

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In automotive or industrial environment an ambient temperature of 70°C isn´t unusal.
--> But you are free to choose any other ambient temperature that fits to your application.

how do i choose capacitor now?

Click to expand...

There is a chapter in the datsheet. But many parameters depend on your setup or requirements.
Lucky you there are some example circuits. There you may find some "usual" values.
Additionally there is a complete design example. What else information do you need?

what about capacitance and voltage?

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For more detailed capacitor value selection you may use the formula with delta_V_out. The less ripple you want the bigger capacitor you need.
Voltage: As with every application. The capacitor voltage rating needs to be higher than the expected voltage at this node.
How reliable are the voltages? If very reliable then you need less headroom. Like with a regulated input voltage.
But - for example - at a 12V automotive applicaton you won´t use a 12V rated capacitor, because the end_of_charge voltage is 14.4V and you may expect a lot of ripple. I´d not use less than 25V rated capacitors. Tantalum are more prone to fail on short overvoltage peaks...add a higher margin.

ESR=0,0156ohms?

Click to expand...

You will find this information in your capacitor´s datasheet.

Also what would be good software to design this IC, i have eagle cad for student but i think even windows 10 search is better than that...

Click to expand...

Don´t complain about a software as long as you aren´t familiar with it. Dont judge a book by it´s cover. Better learn how to use it.
There are many many users they won´t miss EAGLE anymore.

No need to "design this IC". EAGLE is a schematic an PVB layout sofrware , no IC design software.

Maybe have to manually insert the package...

Click to expand...

LTC1625 comes with standard IC packages. It´s not more effort than copy and paste.
After you have chosen the correct package from any other EAGLE library it´s just two mouse clicks to insert it - (completely with all descriptions...) into your library.

I´m using both. EAGLE and LinearTechnolgy datasheets for decades now.

Klaus