Micro USB battery charger and 3.3V supply - Page 1
Forum section: electronics / projects

author: Kaxsp
author_rank: Contributor
timestamp: May 29, 2014, 12:58:37 am
content: HiI would like to ask you for advise. I am trying to create a schematic for a usb charger for a battery (3.7 V or any other) and convert it to a 3.3V signal.I have been looking for different models such as:http://www.maximintegrated.com/en/app-notes/index.mvp/id/960In this example, it converts the signal to 5V and then to 3.3V, but i dont need the 5V signal.So, do you know any other example about how to do it? Any help or device that could fit these requirements?Thank you!

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 29, 2014, 01:31:14 am
content: For a single cell lithium ion, you'll do pretty well with just a LDO. The efficiency would be very good 90% or so over much of the discharge curve and there's not much power left below 3.3V anyways.BTW, I actually recommend charging to 4.1V/cell instead of 4.2V like what is normally recommended. You'll get quite a bit more use out of the batteries that way.

author: Kaxsp
author_rank: Contributor
timestamp: May 29, 2014, 11:34:33 am
content: Thank you for your reply, i have been looking for some designs and i found this one which could fit for my design.I will try to explain a little more my issue.This is my current design with no batteries. My device is just working with USB port.Using this componentshttp://www.mouser.com/ds/2/308/MC78LC00-D-110708.pdfhttp://www.micrel.com/_PDF/mic5305.pdfI would like to modify this design to allow a battery charge of 3.7V, but i dont know if it could be easy having my initial schematic.I checked some designs likehttp://www.sunrom.com/media/content/207/images/MCP73831-circuit.gifandhttp://imageshack.us/a/img5/9934/cargador.jpgusing this componenthttp://datasheet.octopart.com/MCP73832T-2ACI-OT-Microchip-datasheet-16123459.pdfbut i dont know if those designs will work fine with no supply issues at 3.3V. Any idea or schematic example?Thank you!

author: mariush
author_rank: Super Contributor
timestamp: May 29, 2014, 11:58:15 am
content: You can use the circuit here:http://www.sunrom.com/media/content/207/images/MCP73831-circuit.gifThat will take care of charging battery from usb.Find either a small linear regulator with low voltage drop or a switching regulator that would take let's say 3.5 - 6v and output 3.3v .For example:max voltage , output voltage , maximum current , voltage drop , package6V 3.3V 50mA 85mV SOT-23http://uk.farnell.com/microchip/tc1014-3-3vct713/ic-v-reg-ldo-50ma-smd-sot-23-5/dp/1331502RL5.5V 3.3V 150mA 210mV SOT-23http://uk.farnell.com/micrel-semiconductor/mic5265-3-3yd5-tr/ic-ldo-reg-150ma-3-3v-tsot23-5/dp/16630855.5V 3.3V 200mA 80mV TSOPhttp://uk.farnell.com/nxp/ld6806td-33p/ldo-3-3v-200ma-5sot753/dp/20955106V 3.3V 250mA 178mV TO-92http://uk.farnell.com/microchip/mcp1700-3302e-to/ic-v-reg-ldo-250ma-to-92-3/dp/1296588You can find most of these in other packages as well.If you need more than 250mA, there are a lot of them, but most need just a bit more voltage above the output, maybe 0.4-0.5v (so they'll still work with at least 3.6-3.7v).You can find lots of chips here, use the filters to pick an output current range and then see what chip is most convenient for you and easy to buy :http://uk.farnell.com/jsp/search/browse.jsp?N=225136http://uk.farnell.com/jsp/search/browse.jsp?N=225134I don't think it would make a difference if you connect it directly to usb or only after the batter charger IC.You could use a very low drop diode after the battery to connect both battery voltage wire and usb voltage wire directly and then to the regulator's input. If there's power from usb, the diode blocks current coming from battery and protects it at same time from the higher usb voltage and the regulator is happy. If there's no voltage from usb, the diode lets current flow from battery into the regulator, but you'll have 50-100mV+ drop on the diode.But this also has another downside... in some rare cases, the USB port on a laptop/computer may not give you more than 100mA without negotiation (usually what's connected to usb has to talk to usb controller and say it needs more than 100mA but in real world most implementations simply allow 500mA or even more on usb port).If this is the case and your project uses more than 100mA then the voltage on the usb port is higher than the battery voltage so it blocks the battery, but at same time you don't get more than 100mA so your linear regulator may no longer output 3.3v, it will enter current limit mode or drop the voltage down.It's up to you how you connect it eventually.

author: Kaxsp
author_rank: Contributor
timestamp: May 29, 2014, 02:17:09 pm
content: Thank you very much for your advises.I did this schematicusinghttp://www.micrel.com/_PDF/mic5305.pdfandhttp://datasheet.octopart.com/MCP73832T-2ACI-OT-Microchip-datasheet-16123459.pdfI think it is right, but i dont know if would be good having a switch close to the battery or it is ok right now.Thank you!

author: Kaxsp
author_rank: Contributor
timestamp: June 01, 2014, 10:05:17 am
content: Hi again,I changed my schematic again to this oneThis schematic is divided in 2 parts. From LiPo to right is based in my current schema powered only by USB 5V. And LiPo to left is what i added based on this designhttps://www.sparkfun.com/datasheets/Prototyping/Batteries/LiPo-Charger-Basic-v10.pdfSo, having this all together, do you think is there any error? Should I put any component between these two schematics? I am specially worried about when the battery is full.Thank you

author: kaindub
author_rank: Regular Contributor
timestamp: June 03, 2014, 02:00:46 am
content: Why do you use the post regulator?Circuits that specify 3.3V working voltage are designed for LiPo batteries, which vary between 3 and 4.2V (fully charged.) The ICs will handle that range.Your post regulator is going to have some 0.7V voltage drop limiting the useable battery capacity to a minimum of 3.7V (3.7 V minus 0.7V is 3.0 volts ) That leaves a lot of capacity unused in the battery.Also saves parts and space.Robert

author: rexxar
author_rank: Frequent Contributor
timestamp: June 03, 2014, 03:03:56 pm
content: Quote from: kaindub on June 03, 2014, 02:00:46 amNo, all 3.3V components aren't designed for lithium supplies. Some are, but definitely not all. Though, many (most?) ICs will tolerate 5V or more on Vcc.Kaxsp, why are you using the MIC5305andthe 78LC33?The Micrel part is probably overkill for your purposes, the 78LC33 will do just fine. Additionally, you don't really want to charge your battery to 3.7V. The 3.7V is anominalfigure. If you just charge to that point, you'll only end up charging it halfway. You need to charge it up to at least 4V. I use 4.2, but usually anywhere from 4 to 4.3 is alright.