Regulating voltage for battery driven devices

[asmunderik]

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!

[tszaboo]

"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.[2] 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.

[asmunderik]

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!

[tszaboo]

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 REG102 http://www.ti.com/product/reg102. That even comes in SOT223 that I like.

[asmunderik]

Okay, i will check those! You provided me with some hugely helpfull information! Thanks alot!

[SaintGimp]

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!