Cheapest way to do 2 cells (alkaline) and a 5V USB jack both to 3.3V output - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: April 13, 2019, 01:32:02 am
content: The idea would be, if plugged into USB, the power comes from USB, but then the 5V has to be regulated down to 3.3V.Also need to support 2 alkaline cells (1.6V to 3.2V) that boosts to 3.3V.I've been using some Shottky diodes to combine 5V USB and 3 alkaline cells (2.4V to 4.8V) so the output natually follows the highest voltage (USB if plugged in), but I want to do a dual power source USB and 2 cells that both go to 3.3V.Should I put the 2 cells through a switcher step up to 3.3V and the USB to a linear 3.3V step down and then try to combine them with ?I saw some step up / step down ones, but they were pricy if I was looking at something with leads on it like a soic-8...

author: alank2
author_rank: Super Contributor
timestamp: April 13, 2019, 01:59:03 am
content: Current needs 200mA max.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 13, 2019, 02:11:48 am
content: Linear Tech have some unique buck/boost ICs which include the power Mosfet.I would start from there.And feed it from either USB or battery via Oring diodes.

author: alank2
author_rank: Super Contributor
timestamp: April 13, 2019, 02:17:48 pm
content: I saw one of them, but as the voltage goes lower, so does its current capability. Do you have a part number I should look at? Any other suggestions even if it isn't a single part solution?

author: mariush
author_rank: Super Contributor
timestamp: April 13, 2019, 02:46:09 pm
content: See SEPIC regulators.For example LM2621 :http://www.ti.com/lit/ds/symlink/lm2621.pdfPage 14 has example circuit for 1.25v to 14v -> 3.3v 500mA max and you have explanations on how to pick values if you only need 200mA (for increased efficiency and other things)

author: alank2
author_rank: Super Contributor
timestamp: April 13, 2019, 04:09:14 pm
content: So even if the input is too high, it will still have a 3.3V regulated output? I was reading through the datasheet, but it talks mostly about step up and I don't see it saying much about stepping down.

author: Rolo
author_rank: Regular Contributor
timestamp: April 13, 2019, 05:31:52 pm
content: Take a look at this module, I think it does what you're asking for:https://www.pololu.com/product/2563

author: mariush
author_rank: Super Contributor
timestamp: April 13, 2019, 05:58:28 pm
content: Yes, that's what SEPIC means... seehttps://en.wikipedia.org/wiki/Single-ended_primary-inductor_converterSEPIC converters are noticeable by the presence of two separate inductors, or by an inductor with two windings.Something else you should consider... if you don't care about efficiency when it comes to usb power or heat, you could just use a cheap LDO to get the voltage down to something below 3.3v but above what you'd typically have with batteries, and then simply use a cheaper step-up regulator.For example, 2.8v , 3.0v , 3.3v are standard voltages and there's tens of thousand LDOs out there with these preset outputs ... if you use two rechargeable AA batteries you'd have up to 2.7v (1.35v x 2) .. alkaline batteries are up to around 1.65v, so 3.2v maxFor example, MIC5504-3.3 is 10-11 cents each and can do up to 300mA :https://www.digikey.com/product-detail/en/microchip-technology/MIC5504-3.3YM5-TR/576-4764-1-ND/4864028Same for MIC5501-3 which outputs 3v at up to 300mA :https://www.digikey.com/product-detail/en/microchip-technology/MIC5501-3.0YM5-TR/576-4905-1-ND/5277876Here's a whole lot of them :https://www.digikey.com/short/pp82czSo once you know you have 3.3v or less no matter the source, you can focus on using a step-up / boost regulator only ... and your parameters are less than 3.3v input, probably minimum 1.8v or thereabouts, 3.3v output , around 200mA of current ...Here's a starting point ... all switching regulators that can start with less than 2v and output at least 250mA of current, sorted by price @ 25pcs :https://www.digikey.com/short/pp82twSo you have for example MCP1640 at 50 cents @ 25pcshttps://www.digikey.com/product-detail/en/microchip-technology/MCP1640T-I-CHY/MCP1640T-I-CHYCT-ND/2258621which can do 0.8v ..5.5v step-up to 5.5v 350mA , sync rectifier so no need for diodes...• Up to 96% Typical Efficiency• 800 mA Typical Peak Input Current Limit:-IOUT > 100 mA @ 1.2V VIN, 3.3V VOUT-IOUT > 350 mA @ 2.4V VIN, 3.3V VOUT-IOUT > 350 mA @ 3.3V VIN, 5.0V VOUTOr TPS61322 , 75 cents @ 10pcs, 58 cents @ 100pcs :https://www.digikey.com/product-detail/en/texas-instruments/TPS613221ADBVR/296-50502-1-ND/96856410.9v to 5.5v in, it will do 200mA from 1.5v and upSo basically the overall cost would be less than 1$ but the downside is that the LDO will dissipate some heat taking 5v from USB down to 3.3v or less... at around 250mA, we're talking (5v-3.3v) x0.25a= 0.425 A .. it's not a big amount.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2019, 06:10:37 pm
content: A buck-boost, flyback, or SEPIC regulator is the way to go. SEPIC and flyback regulators can use any controller intended for step-up operation which is convenient because the output switch can be an NPN bipolar or n-channel MOSFET.The major complication is getting full enhancement of a power MOSFET if used from a low input voltage source. There are various ways this is handled.Something like the buck-boost LTC3536 or SEPIC LT8494 or LT1308 would be suitable.

author: alank2
author_rank: Super Contributor
timestamp: April 13, 2019, 06:18:19 pm
content: Thank you guys for all the directions to look at; it will have a microSD card too so there might be some inrush from that being inserted too.

author: bson
author_rank: Unknown
timestamp: April 14, 2019, 08:19:29 pm
content: Quote from: alank2 on April 13, 2019, 06:18:19 pmMake sure to give the SD card a filter cap on Vcc. I like to use 22µF, just something a little bigger than merely decoupling.