This aim of this project is to build a small, adjustable DC/DC converter with an input from 1v -> 5v and an output of 5v->12v at up to 1.2A. With a 3v to 9v conversion at 1A it is 85% efficient. Photos taken during the design process are here.
After much searching I found and successfully tested the LM2623 (datasheet here). This great little converter chip does just what I need. The only problem is that it’s tiny! So this project is the one where I finally conquer my SMT fears!
I prototyped the board first with through hole and then with SMT components. The schematic and board are available here.
I used pretty much default component values from the datasheet. The only thing I did different was to add a trimmer for adjusting the output voltage.
All components are specified to 2A. The last numbers in the list below are all farnell part numbers. It works out at £4 each plus vat for 10 kits.
- C1 22uF 1759435
- C2 100uF 9695672
- C3 4.7pF 1650903
- RF1 1M trimmer 1174187
- RF2 100k 1576183
- R3 150k 9335978
- D1 schottky diode 1680071
- L1 4.7uH 1864113
- IC1 LM2623 8207445
SMT is a different technique to through hole, and requires some different tools. The most important things are a magnifying lens of some kind, liquid flux (I use a pen with good results) and solder braid to remove excess solder. Fine tweezers and a fine tipped soldering iron are also very handy.
Before soldering a component, apply flux onto the pads. Then place the component and check alignment. Apply a bit of solder to the tip of your iron and touch it to the pad while holding the component still with the tweezers. The flux helps the solder flow into the gap between the pad and the component’s pin. Now the component is attached, check the alignment and then move on to the other leg(s). This time apply the solder to the join like soldering through hole. Finally, go back and touch up the first pin you soldered. Check everything thoroughly with the lens after soldering. Having to go back to fix things can mean removing other components first!
For the tiny chip, solder it in place as above, and carefully check alignment! Then apply solder to all the pins and don’t worry about bridging. Use some of the fine braid to remove the excess and you should see a beautiful joint after!
Before starting it is worth scraping the white silkscreen off L1’s pads – this is a mistake I made when making a custom device in Eagle.
Place the components in this order: C3, R3, RF2, C1, IC1 (then test), L1, D1 (stripe on the right), RF1, C2.
After placing IC1, check these connections with a multimeter:
- pins 1 & 5 -> ground
- 2, 6 & 7 -> + out
- 3 -> bottom pad of C3
- 4 -> left hand pad of RF1
- 8 -> left side of D1
Check the input resistance is large (above 1M). Then set your PSU to 3v and if you have current limiting set this low, 100mA is fine. Apply the current and you should see about 8.5v on the output. Use a screwdriver to adjust the trimmer and set your desired voltage.
If you get 0v or the supply voltage out then you probably have a bad connection somewhere. I’ve found bad joints on the big cap (C1), and the diode. I’ve also missed a pin a few times on the IC, but if you check the connections as I’ve listed above then this shouldn’t be an issue.
If you get a large negative voltage it will be because C1 isn’t connected properly.
At very low start up voltages (<1.5v) the board will look like a short circuit because the DC/DC controller shorts the input to charge the inductor. It needs about 2A to start at these low voltages, and then the current will drop to normal.
A beginner SMT artist will take about 30 minutes to build and test – as long as there are no mistakes!
- Bigger pads for trimmer and C2
- Remove silkscreen over L1 pads (actually tdocu layer)
- Indication of positive side of D1