3.5v Boost converter using an MCP1640 - Page 1
Forum section: electronics / projects

author: eddieate
author_rank: Newbie
timestamp: June 09, 2018, 05:15:46 pm
content: Hi GuysSo I wanted to power some white LEDs from two AA cells (NiMH or Alkaline).I couldn't find any converter PCBs that would fit in the space available (12x38mm) so I decided to have a go at designing my own.Circuit;https://ibb.co/fNKpv8Top Copper;https://ibb.co/h23choBottom Copper;https://ibb.co/eEOtNoAll layers;https://ibb.co/bJSL2oThis is not the first iteration of the board, I realized the 0805 inductor I specified probably wasn't going to cut the mustard and I added top and bottom ground planes this time.I'd really appreciate any advice as I've not never had a PCB made before and can't shake the feeling I'm missing something!Thanks, Ed.

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2018, 08:08:09 pm
content: This isn't the right way to do it. An LED need to be driven from a regulated constant current source, not a voltage source and it's better to connect multiple LEDs in series, rather than parallel.The MCP1640 is unsuitable for this. Use a specialised LED driver IC, rather than a generic boost regulator.

author: eddieate
author_rank: Newbie
timestamp: June 09, 2018, 08:41:14 pm
content: Hi Hero,thanks for the reply, I had looked at some LED driver ICs like the AP3012KTR-G1https://uk.rs-online.com/web/p/dc-dc-converters/9210499/But they all seem to have rather high input voltage requirements, 2.6v in the case of the AP3012KTR-G1.So I did a bit of googling and found the ZXSC380 would that be more appropriate?https://docs-emea.rs-online.com/webdocs/154d/0900766b8154d593.pdfThanks, Ed.

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2018, 09:38:02 am
content: Quote from: eddieate on June 09, 2018, 08:41:14 pmThe ZXSC380 looks good to me and the schematic is very simple too. Much easier, than even the classic Joule thief circuit.What are your requirements? How much power do you need, into how many LEDs?

author: eddieate
author_rank: Newbie
timestamp: June 10, 2018, 09:50:51 am
content: So I redid the PCB using an ZXSC380, It barley seems worth having one with just two components on it, the chips and inductor are only £6 now so I'll probably just get them and make it up on some vero board.To be clear I'm just driving 2 LEDs. I believe about 20mA per LED.Ed.

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2018, 10:32:20 pm
content: Quote from: eddieate on June 10, 2018, 09:50:51 amThen the ZXSC380 should be fine.By the way, did you consider the traditional Joule thief circuit? The IC is probably better, but this is a cost sensitive application and regulating the brightness is unimportant, it'll be fine.

author: eddieate
author_rank: Newbie
timestamp: June 11, 2018, 11:22:48 am
content: Size is the main consideration, most of the joule thief's I see are either using a big toroid or have a much higher component count than the ZXSC380 circuit.You've already saved me £25 from the MCP1640 design before I even ordered the PCB.The only thing now is, I'm not sure what inductor is a good choice, I was going off the datasheet when I selected the one for the MCP1640.Could I get away with something like this?https://uk.rs-online.com/web/p/leaded-inductors/1911147/Or would this be more appropriate?https://uk.rs-online.com/web/p/wire-wound-surface-mount-inductors/8987908/Thankk, Ed.

author: mikerj
author_rank: Super Contributor
timestamp: June 11, 2018, 11:33:09 am
content: Quote from: Hero999 on June 10, 2018, 10:32:20 pmLooking at the datasheet for the ZXSC380, the only way to control LED current for a given input voltage is by the inductor value, and with the 3v input the OP wants to use, the output current is quite high with a small adjustment range (60-70mA) which would enforce parallel connection for typical 5mm LEDs. If he drops down to single cell operation the currents look more reasonable.

author: eddieate
author_rank: Newbie
timestamp: June 11, 2018, 11:50:27 am
content: Quote from: mikerj on June 11, 2018, 11:33:09 amHi Mike,I could always parallel the cells up rather than series them, I need to replace the battery contacts anyway.Ed.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 11, 2018, 02:08:27 pm
content: As mentioned, using a proper LED driver would be better.I suggest picking one with an under-voltage lock-out feature (UVLO). The CAT4238 for instance would be a decent fit for a two-AA cells power supply, with its 1.9V threshold.I strongly recommend against using any kind of boost converter directly powered by batteries without a proper under-voltage cut-off adapted to said batteries (either built-in such as in the above IC, or that you would implement adding a voltage supervisor). As the batteries' output voltage falls, input current draw will raise and may eventually lead to catastrophic behavior when approaching complete discharge, depending on the batteries chemistry. It may lead to anything from not much to severe heating, to fire, or even to explosion. I have tested this a few years back, and most NiMH AA batteries would heat up to above 70°C when reaching complete discharge when directly loaded by a step-up converter without cut-off. I think alkaline batteries tended to be more resilient to this. Some other chemistries can be worse. Zinc-air batteries for instance when treated this way can swell a lot and even explode (also tested!)

author: eddieate
author_rank: Newbie
timestamp: June 11, 2018, 06:15:38 pm
content: Quote from: SiliconWizard on June 11, 2018, 02:08:27 pmThanks for the advice and recommendation there, I would like to avoid the batteries getting to 70*c or exploding if at all possibleI'm doing some baking tonight but after I'll whip up a CAT4238 circuit.One question, as I'm only driving two LEDs, can I get away with a smaller inductor than the one in the example circuit? They specced this one;https://www.mouser.co.uk/ProductDetail/Sumida/CDC5D23BNP-470KC?qs=ttApTud31Jbu9cEUdVut%252bQ==Thanks, Ed.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 12, 2018, 03:30:13 pm
content: Quote from: eddieate on June 11, 2018, 06:15:38 pmWell, at 5.8mmx5.8mm, it seems small enough to me!You may get away with a lower value inductor but at the expense of increased ripple.The Sumida CDRH3D28/LDNP-470NC is just 4x4x3 and is also a good fit for this. You can certainly make the whole circuit fit in something like 10mmx10mm.