DC to DC Conversion

kinarfi said:

Mike, I'm listening, You'll appreciate this!!...

Click to expand...

You missed a point. Even though the total wasted power is the same, the way you propose configuring the two linear regulators requires that U2 have the same max unregulated input voltage spec as U1. By cascading the two regulators, it reduces the demand on U2, making many more chips available... Besides, the TS has already decided that U1 should be a switcher (because that is where the bulk of the dissipation takes place) , and U2 can be a linear (with a 5V input) because the dissipation there is only (5-3.3)*0.1 = 170mW.

alternatively, OOPS this was supposed to post last night,
I would use a switcher also because of the wasted 2.5 watts and I have not had any problems with them, that I know of, but wanted to see what could be done with linear regulators

First just let me say WOW you guys are too kind it look like you have done loads of work especially Mike and Nigel (even if I don't understand most of it lol)

So, I will look for a inductor that can handle 175ma.

[EDIT] How does this look? [END EDIT]
[EDIT] Even better **broken link removed**? [END EDIT]

Thanks again to all of you, Steve

Hi,

Can anyone just tell me if those inductors are OK? don't want to get PCB's done and have missed
something stupid.

First inductor here, second one **broken link removed**

Steve

Use this:
https://uk.rs-online.com/web/p/non-isolated-dc-dc-converters/7962138/
RS number: 796-2138

This is an all in one dc-dc converter with a wide input range of 7-36V and an output of 5V, rated for 1.5A and can be used in the same way as an LDO. You can then use a 3.3V LDO coming of the 5V

also on the Murata site:
https://power.murata.com/en/product...out-vdc-number/0.6,5.0/vin-range/7-to-36.html

Hi again all,


Well I have spent the last few days trying to sort the PCB tracking out for LT3470 and it's a nightmare!! I can see the recommended PCB layout on the datasheet, but right away I hit a snag because this recommended layout implies a four layer PCB, mine was 2, so I added The extra layers and tried again to copy verbatim the layout, but this is almost impossible (well for me it is using Altium) I made the TSOT-23 footprint and then dropped it onto the PCB, started to use polygon pours to make the areas defined in the datasheet but this just got messy, so then I thought "change the component footprint to have these strange shapes" this again was messy. I think this all comes from not being a expert and not really knowing what I can / cannot do, hence me trying to copy exactly what was recommended, but this is probably not realistic, so here I am again asking "what the hell can I do"


Option are


1. Find someone who has created a layout or the LT3470 using the standard TSOT-23 footprint and just copy it
2. Find a small switch mode regulator PCB like the ones show by Nigel and MisterBenn and copy those as close as possible
3. Give up :-(

Steve

P.S. Forgive me if this does not make sense it's been another long day trying to sort this issue.

Scarr said:

there are physical limits I have that I cannot change. (these are 48mm X 10mm approx)
Steve

Click to expand...

Hy Scarr,

Where are you from. Feel like putting it next to 'Location' on your user page.

What is the third dimension of the space you have available. I would not waste any more time laying out a printed circuit when you can buy a ready made switch mode power supply, as Nigel suggested, but physically smaller to generate the 3.3V at 100mA and a simple three terminal voltage regulator for the 5V at 10mA. Of course, it all depends on the third dimension of your space.

spec

Scarr said:

Hi again all,


Well I have spent the last few days trying to sort the PCB tracking out for LT3470 and it's a nightmare!! I can see the recommended PCB layout on the datasheet, but right away I hit a snag because this recommended layout implies a four layer PCB, mine was 2, so I added The extra layers and tried again to copy verbatim the layout, but this is almost impossible (well for me it is using Altium) I made the TSOT-23 footprint and then dropped it onto the PCB, started to use polygon pours to make the areas defined in the datasheet but this just got messy, so then I thought "change the component footprint to have these strange shapes" this again was messy. I think this all comes from not being a expert and not really knowing what I can / cannot do, hence me trying to copy exactly what was recommended, but this is probably not realistic, so here I am again asking "what the hell can I do"

Click to expand...

Are you looking at a different datasheet than this? Figure 5a on page 13 shows a layout that could easily be done on a two layer PCB. All of the connections are on the top layer with a ground plane on the bottom layer. The feedback connection can be plowed through the ground plane, or routed around the edge of the pattern on the top layer.

Scarr said:

Hi All,

I have a 28vDC supply and want a 5vDC 10mA + 3.3vDC 100mA deriving from it. I have used tried a few LDO regulators just wired up by hand to test the outputs and I have tried two configurations.

First configuration was with the 28vDC feeding both regulators, both regulators can handle 30v in, but for some reason the 3.3v regulator popped and and blew up the HC-O5 I had attached to it, this happened twice after approx 30-40 seconds!! I did it a second time because I thought it might have been a faulty regulator. When I did not have the HC-05 connected the output showed 3.3v without issue.

The second configuration has the 5v regulator output feeding the 3.3v regulator input, but in this configuration the 5v regulator got hot (didn't blow just got hot to the touch) .

So my questions if what is the best way to get 5vDC 10mA + 3.3vDC 100mA from 28vDC supply. both the 5v and 3.3v supply IC's

Thanks

Steve


P.S. I am thinking about using two Zener + resistors but am not sure this is best approach, also if any part of my email does not sense don't flame just ask me to clarify.

Click to expand...

Easy way: use a couple on LM317 (TO-220 package) devices set up for 3.3V and 5V. The 3.3V reg needs a small heatsink like solder down to 1 square inch of copper PCB.