Powering battery operated device from USB  - Page 1
Forum section: electronics / projects

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 14, 2016, 12:38:34 am
content: Hello!In the past i have build a few items that can be powered from a battery or USB, but these devices all used a LiIo cell. That makes it fairly easy to power the whole thing of the battery or a USB port when charging.But this time around i am gooing with two AA cells that go through a step up regulator and power a 3.3V device.This thing (just a µC with a display, some bits and bobs and a nifty case) is tweaked for efficiency when running of battery.It may have a USB port and i am playing with the idea to have this device draw its power from the USB port, when connected to a computer, to save battery runtime.I have come up with three ideas that either involve a enough MosFETs that the part of the board can be called a MosFET cemetary and the efficiency goes down the drain.Fairly expensive and stupidly easy smd reed relais or a fairly stupid but equally easy solution were i feed the 5V from the USB port through a linear regulator to drop it below 3V and then feed that into a stepup regulator to get back to 3.3V again.Has someone encountered this problem before? How dit you solve it?Greetings,Peter

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2016, 12:49:20 am
content: LM3940 will supply up to 1 A @ 3.3V.

author: Skimask
author_rank: Super Contributor
timestamp: May 14, 2016, 12:49:46 am
content: I use a TPS63001 Buck/Boost converter.Input is above 3.3v? Buck it down to 3.3v.Input is below 3.3v? Boost it up to 3.3v

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 14, 2016, 01:39:53 am
content: Hello and thank you both for the reply!Even though i have a stick sitting next to me right now, i had not thought of a Buck-Boost.That would solve one problem.But, i have one other, wich is the important one. I have to have both supplys meet at some point.If it would not be a Battery operated thing, i would just wack two diodes in there and i still can use a diode for the USB supply.But when i put a diode on the battery, i waste a bit of voltage.I need to prevent the USB supply feeding into the battery, every solution i can come up with does at some point have problems with a depleted battery.That is were the LDO or the grave of FETs or diodes comes into play.Mhm, i can not explain it properly, my apologies.I will try to make some schematics tomorrow.

author: Skimask
author_rank: Super Contributor
timestamp: May 14, 2016, 01:46:17 am
content: USB -> TP4056 PCB -> Li-Ion Battery|-> CircuitUSB power feeds the TP4056 PCB (search it on ebay and youtube, Julian Ilett (?) has a good video on them).TP4056 charges the Li-Ion battery as needed thru one set of outputs.Another set of outputs on the TP4056 PCB feeds the circuit.No USB? Runs off battery.USB? Runs off battery while USB charges battery.

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 14, 2016, 01:54:14 am
content: Quote from: Skimask on May 14, 2016, 01:46:17 amThank you, yes, that is really easy. Circuits like that i have build many times.But, i do not want to use a Lithium cell this time.The device should be powered of two aa cells in series. That means i can have annything from 0 to 3V and i can not have current flowing into the battery.I know, that is a rather odd thing to do, i have chosen AA cells this time because they need to be replaceable if the thing runs flat. Even when you do not have spares around, you can just run down to the shop and buy some.And beeing high efficiency/low current i can not use annything that takes up a lot of power while running of battery.Greetings,Peter

author: Skimask
author_rank: Super Contributor
timestamp: May 14, 2016, 02:10:54 am
content: Then use a P-channel MOSFET instead of a diode. Google it. Loses the Vdrop.

author: ez24
author_rank: Super Contributor
timestamp: May 14, 2016, 04:58:27 am
content: Quote from: PeterFW on May 14, 2016, 01:54:14 amI suggest that you change the title of this post and include "AA" in it. Use all the space to ask your question.