I know this question has been covered multiple times already; but as far as my reading goes I couldn't get any clear answers yet.
I am building a circuit based on a LiPo input source (4.2V to 3.3V) and I plan to shut-off any power withdraw at 3.3V in order to save the LiPo lifetime. (using voltage comparator on EN pin). The maximum current draw is around 500-600 mA, but for 80-90% of the time, the system will be on lower power mode (around 100mA). The operating voltage is 3.3V for as long as possible; until the LiPo voltage reached 3.3V (bottom discharge limit)
Therefore the dilemma and if I got my reading correct, so far this is what I understood. Under high load it is not favorable to use an LDO because of the increase of drop-out voltage; however it seems that a DCDC step-down would do the job perfectly. But according to my case I would like to have a second opinion; is there any reason to use one or another ?
LDO : http://www.ti.com/lit/ds/symlink/tps737.pdf
DCDC Step-down : http://www.ti.com/lit/ds/symlink/lm3671.pdf
edit : In addition; what do you think about the benefit of using an LDO that would not track bellow 3.3V and therefore get rid of the voltage reference? As opposed to the DCDC that will track voltage well bellow 3.3, hence the need to shutdown system.
Thank you.
Best Answer
Reducing a 4.2V source to 3.3V with the linear regulator represents an efficiency of about 3.3V / 4.2V ≈ 78.57% regardless of current. The efficiency increases as the source voltage decreases, up to 3.3V / 3.4V ≈ 97.06% (assuming a 100mV dropout worst case), with a small drop to 3.2V / 3.3V ≈ 96.97% before the cutoff is reached.
The switching regulator has an efficiency of about... we'll call it 90% at the currents you are considering. The linear regulator will only reach 90% efficiency with a source voltage of 3.3V / 0.9 ≈ 3.67V, which the battery will reach with about 35-45% of its energy left.
It is also possible that the final switching regulator design will have less efficiency than given in the datasheet, in which case the linear regulator is at even more of an advantage. And of course below about 80% it would be silly to use the switching regulator at all.
So the switching regulator will probably last longer on the same amount of battery assuming a high enough efficiency, but requires additional passives compared to a linear regulator and is noisier due to the pass element turning on and off. The linear regulator has a simpler design with less noise, but will likely burn through the battery a bit faster.