Regulating voltage for battery driven devices  - Page 1
Forum section: electronics / beginners

author: asmunderik
author_rank: Newbie
timestamp: December 13, 2016, 10:54:03 am
content: Hello!I'm having trouble wrapping my head around something and thought you guys might help me!This might be very obvius as something always is to someone! hehe.But I tought id ask here to get started getting used to the forum and getting something for my general knowledge bank!Main objective:I am going to power a device with a li-ion 3.7v (charging voltage of 4.2).The source of my circuit is an atmega256rfr2 which ive experiences is a bit reactive to voltage changes.Theres also a few other components, all should work fine at 3.3V.This is where i get a bit mixed up when choosing a power management circuit for this. I thought i'd go with a LDO regulator to get the 3.3V.But wont the regulated voltage exceed to exist before the battery enters it's final stages of discharge at approximately 2.7-2.9V?I was also thinking about an Step up DC-DC converter, this will hold the voltage stable at 3.3 voltage until the battery is at the discharg i want it to be if i implement an power monitor to my mcu.I was hoping i could get a little information about how this is usually done and what kind of power management it is possible to use.I cant imagine this is a case only I have stumbled upon...I'm sorry if its not a direct question and maybe doesnt make much sense or i havent supplied enough information about all that is needed for an answer! And if there is already many posts about it, i tried searching for it but i lack the propper search words it seems!Thanks in advance!

author: tszaboo
author_rank: Super Contributor
timestamp: December 13, 2016, 10:57:09 am
content: "The Li?ion can be discharged to 3V and lower; however, with a discharge to 3.3V (at room temperature), about 92–98% of the capacity is used. Importantly, particularly in the case of lithium ion batteries which are used in the vast majority of portable electronics today, a voltage cut-off below 3.2V can lead to chemical instability in the cell, with the result being a reduced battery lifetime. For this reason, electronics manufacturers tend to use higher cut-off voltages, removing the need for consumers to buy battery replacements before other failure mechanisms in a device take effect."Set up a cutoff of 3.4V, use an LDO, or a stepdown with Vin ~ Vout.

author: asmunderik
author_rank: Newbie
timestamp: December 14, 2016, 07:01:49 am
content: Ahh i see! Great info, thanks!Any tips in what to look for in an LDO? Theres alot of them out there.I guess it depends alot on what you need it for ofcourse...Again, thanks for replaying!

author: tszaboo
author_rank: Super Contributor
timestamp: December 14, 2016, 10:47:20 am
content: I would use an LDO, which has low quiescent current, so it does not drain the battery. Also the input-output voltage drop should be low, some 150-200mV max. I like LDOs which are working with ceramic capacitors. I dont know, how much current you need, but if it is only the Atmaga, I would use something like a REG102http://www.ti.com/product/reg102. That even comes in SOT223 that I like.

author: asmunderik
author_rank: Newbie
timestamp: December 14, 2016, 11:59:33 am
content: Okay, i will check those! You provided me with some hugely helpfull information! Thanks alot!

author: SaintGimp
author_rank: Contributor
timestamp: December 15, 2016, 07:05:06 am
content: I like the MCP1700 (http://ww1.microchip.com/downloads/en/DeviceDoc/20001826C.pdf) for low-power battery-powered applications. It has 1.6 uA quiescent current and a 178 mV dropout voltage at the maximum 250 mA current draw. But if your application is low-power, the dropout voltage decreases linearly with load current. At 20 mA of load current the dropout voltage is only ~20mV, for example. Be aware that the maximum input voltage for this regulator is only 6V, though, so it's not a general purpose regulator. I forgot that once. Poof!