DC-DC Converter - Off-the-Shelf or Roll-Your-Own...?

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Quick call for opinions here... :)

In order to reduce the thermal loading on the power supply of my project, I'm considering adding a DC-DC converter that will drop the voltage from 12V to 5V.

I've been looking over the datasheets for some of the switching controllers that Linear offers - the LT1767 and LTC1474 in particular - but I've lately been starting to wonder...

Given that this will likely be a one-of project, is it worth my while to go through the calculations for putting together a custom switchmode converter, or could I just as easily buy a pre-assembled module - such as the Innoline R-78AA-1.0 or the V-Infinity V78XX-500-SMT - to do the same thing?

From what I've been able to piece together so far, the parts cost for building my own is roughly the same as a fully-assembled and tested module... but are there other factors I should be considering?

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Quote:
Given that this will likely be a one-of project,
Then get an off the shelf type. Do you know how much current you need? for 0.5A-1A you can get 7805 drop in units from Recom or Mornsun.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Quote:
From what I've been able to piece together so far, the parts cost for building my own is roughly the same as a fully-assembled and tested module... but are there other factors I should be considering?

The satisfaction that you got the parts, designed it yourself and built it. Oh yeah, the knowledge learned that may be useful elsewhere in the future??
Maybe just for fun?

JIm

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just be careful to still provision for filtering (inductor and some cap pads) the supply a great deal if you have and require your analog to be nice and clean. this has definitely bitten me on some past projects.

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John - those were basically my thoughts as well. :)

If things work the way I expect them to (!) then the maximum current-draw will be in the 350mA range... and 'idle' draw somewhere around 50-60mA.

One of the features I'm looking at in the CUI and Innoline models is the low-voltage lockout capability. Since this is going to be driving 12VDC relays from a battery pack, I want it to stay off when the charge is below 8V - the 3-pin replacements for a 7805 (at least, the ones I've seen...) don't have that.

Jim - yeah... there IS that. And I'll likely draw up BOTH options, just to stretch my mind. :)

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National and several others also have DC-DC converter modules. They take a few external caps, and thats it. Price is a bit more than an IC and inductor, For one-off, its hardly worth rolling your own except for the learning aspect.

Jim

 

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I've done boost and inverter circuits for designs with precise requirements and which were planned to be produced in quantity. However, I second the vote for COTS stuff for one-offs. My regs of choice are the V-Infinity V78XX series. While they may cost a little more (i.e., $9), the cost in time to design my own (unless I have an existing design) is worth more.

Science is not consensus. Science is numbers.

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The one aspect that's giving me the most grief is keeping the AC pathways short, and avoiding jumpers, on a single-sided PCB. I find myself placing at least one trace underneath the chip... which is considered unwise, based on the datasheets from Linear, National, Analog and Micrel that I have been going over.

The math for selecting the components isn't all that difficult - but the board is giving me fits... :shock:

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There are many choice of all-in-one power modules out there. The usage is like a 3 termial regulator but they are dc-dc indeed.
LMZ14201 form NS is a good choice.
If size doesn't that matter, another more cost effective product is VMP3201

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One of the irritating things I've been finding out about the off-the-shelf modules is the low switching frequency. The ones I've seen from CUI, Recom and Texas Instruments have all been in the 300kHz range.

The difficulty arises because the project I have in mind uses a MaxStream XBee-Pro 900MHz RF link... and the users' manual states that if the unit is powered with an SMPS, the switching frequency should be over 500kHz to prevent interference. Now... I AM adding a linear regulator between the switchmode supply and the device, to allow for on/off control via the microcontroller, but I don't really know how much difference that would make.

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Check out the 78SR series stuff from murata, drop in replacement for t220 5v regulator, 500khz switching freq,1.5a max, ~$5 in single qty.

I plan on using the 5v version in one of my current projects.

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Hadn't seen that one. Looks promising - except that I want to put a linear regulator following it, and it seems to be set at 5V.

Since I'm looking at a possible 6.5V output requirement, they are out of the running... :(