Dual Voltage Power Supply Design.

[Falcon69]

Spent all last night and today working on this. Trying to understand related data sheets to put together this power supply.

How's the design? Is this going to work okay.?

The 5.4V line will maybe see 2.5A while the 5V line may see 1A for max current load. (the reason I have 5.4Volts is because I need to compensate for Voltage drop along 50ft or more of wire to a sensor I'm using)

Included is the schematic and the BoM list for reference.

I'm not sure on some of the size/values/voltages for the Capacitors and Resistors, but I think It's pretty good. All capacitors are Ceramic. The data sheets say Ceramic is fine for those Regulators. X7R or X5R, depending on cost.

One thing I am confused about, and I'm not sure if I even need them, are the C10 and R10 parts. Do I even need the PWRGD pin from the LDO regulator? I couldn't understand what it was for, all I gathered from searching was if I have circuits that require resetting if voltage drops too low. The PWRGD pin puts out a 'Power Good' signal which could activate a transistor or mosfet used to reset all those resets of micro-controllers, etc.. If I need C10 and R10, what do I do with the output (it's just dangling below the 5V & GND output on the schematic)?

If I don't need C10 or R10, what do I do with the PWRGD and Cdelay Pins? Leave them Floating? Connect to Ground?

It was mentioned in another thread to keep all my capacitors at triple the voltage for the input and the output. So I've tried to leave those at 35V for the input, and 16V for the output.  If I can go lower, that would make them smaller chips, thus making the footprint of the overall circuit smaller.

Thanks in advance for any input and help you may offer.  I'm just a beginner, but I'm learning lots over the last couple years browsing this forum.   

[amitchell]

Follow this for buck pcb layout

http://www.diodes.com/_files/evbd/AP65550EV1-EVM%20Manual_R2.pdf

0402 is pretty small by eye, I would consider 0603 as your minimum package.

You are not likely to nail a SMPS first time, and its fun to play and experiment, so saying that...

I like to add jumpers at Vin's at chips so I can disable them or add an RC filter/LC filter or ferrite bead. Sometimes SMPS benefit from LC filters at Vin and/or Vout, it wont cost you much to add the footprints in anyways... a 0R jumper for the first revision. I would populate an additional capacitor foot print or two at the Vin and Vout of the buck. 

[Falcon69]

0402 isn't a problem. I've soldered them before. I made a reflow oven out of a toaster oven and made an air pick up vacuum tool.  It helps alot.

Someday, when I can afford it, I'll get a PnP machine, but only if I win the lottery.

So you are saying I may need 3 Caps in Parallel on the in and out of the Buck?

[amitchell]

What I am getting at is you may need to change or add things around to fit your application 100% and it may be wise on your first spin to be prepared to do that. My first boost converter took 4 revisions before I got it nailed.

Did you say previously that you didn't have a scope? I think they are a needed tool when doing SMPS.

[Falcon69]

I don't have one, nor do I know how to use one...yet.

I don't have the money for one anyway right now.

It's on my list of things to get and play with though.

[Falcon69]

amitchell, is there a place to buy that evaluation board, without the components?  Then I can buy the components I want..and see if they work with that chip...without having to design the PCB right now and having it made?

[Falcon69]

Also, another thing....

If this calculation is correct...

5.402 volts x 3.5A = 18.908 watts
18.908 watts x 100 / 90 (efficiency) = 21.009 watts
21.009 watts / 12 volts = 1.7508A

So, by that logic, the input into VR1 can be a wall plug that is 12V @ 2A?
Also, the Fuse can be a PTC fuse that is 24V x 2A with a trip current of 4A? http://www.mouser.com/ds/2/240/Littelfuse_2920L-39522.pdf

If so, that will simplify the PCB layout (won't have to make fuse clips and solder) and also will lower cost of wall plug (I was planning to buy a 3 or 4A because I thought that's what was needed)

[amitchell]

amitchell, is there a place to buy that evaluation board, without the components?  Then I can buy the components I want..and see if they work with that chip...without having to design the PCB right now and having it made?

I don't think so, but they get you close and you can just change components out, that's what they are meant for. However it looks like a new part and I cannot find any EVM's for purchase. If you can bump your IC BOM budget up to TI prices I would look at them, they are better stocked and EVM's are easy to find.

[Falcon69]

Okay. I couldn't find anywhere to purchase the board either. Ti's components are just too pricy for this application. I need to try and keep the cost down as much as I can.

So, on that PWRGD capacitor C10 and resistor R10...are those needed? And what do I do with the pins for PWRGD and CDelay? Do I just leave them floating?  It didn't say anything about that in the Data sheet, and I don't plan on using the output for anything.

[amitchell]

Okay. I couldn't find anywhere to purchase the board either. Ti's components are just too pricy for this application. I need to try and keep the cost down as much as I can.

So, on that PWRGD capacitor C10 and resistor R10...are those needed? And what do I do with the pins for PWRGD and CDelay? Do I just leave them floating?  It didn't say anything about that in the Data sheet, and I don't plan on using the output for anything.

You can leave both of those floating they are just for indication, like you said to let an MCU know things are correct or to drive an LED light etc. I guess you could run some traces out of them if you wanted to use them in the future.

[Falcon69]

okay, thank you amitchell.  I don't understand what an LED would do. Would it just flash (like on your computer when your harddrive LED flashes) showing that the voltage output of the LDO Regulator has reached it's full 5.0v?

Sounds kinda useless, since the buck converter above it is the main regulator. The LDO may or may not even be used, depending on the application this entire circuit is used for.

I know it's just a couple pennies for the Cap and the Resistor, but then you have to add another 0.02 cents per pad cost for assembly, if I have these mass produced. (Not sure on that yet)

[amitchell]

It would just be on when power is good, likely useless here.