Powering 3.3V Microcontroller and 5V Sensor from 3.7V Li-Po Battery—Best Method?

Hello!, 

I have a 3.7 Volt Li-Po Battery and I would like to supply two different loads, a 3.3 Volt microcontroller and 5 Volt sensor. Is there any way to do this...possible just using buck converters or a LM317 Voltage regulator.

Thank you for any help you might have! 

You will need a buck-boost or SEPIC converter. Fortunately you are trying to solve a very common problem so there are multiple parts specifically made for this application. If you are looking for optimum battery life then you want both the 3.3 and 5V outputs to be generated from a switching converter.

To generate the 3.3V output from a single LiPo cell, you need a buck-boost because the battery voltage will range between 4.2V (charge cutoff) and about 2.5V (discharge cutoff). This means you may need to increase or decrease the voltage to generate your 3.3V rail.

You can also use a 5V boost converter with a 3.3V LDO on the 5V output if you are not interested in efficiency.

If you post detailed requirements (current for each rail and how much noise you can tolerate on each rail), then someone can recommend a switching controller.

{I see that Dean posted while I was typing and submitting my reply, his reply is probably more digestible!}
The battery range is from 4.2 down to around 3 volts, I have assumed that you would want to make full use of that span in making the suggestions below.
How you go about a final solution therefore depends on what sort of efficiency and simplicity you require.
1) If efficiency is not an issue, then boost the battery to 5V, use the raw SMPS output for the 5 volts, and a linear regulator to get down to 3.3volts (you need a regulator with a drop out voltage less than 1.7 volts at the load current you need, and good power supply rejection at the SMPS switching frequency or LC filters if you need low noise). This has the advantage of using the simplest topology for the SMPS, they can be bought off the shelf.
2) If the 5 volt supply requires only a low current (tens of mA at most), then you could use a buck-boost converter for your 3.3 volt supply, and convert the 3.3 volts to 5 volts using a switched capacitor converter, with the possibility of a linear regulator on the output. Disadvantage is more complex SMPS topologies, neither are likely to be available off the shelf, you will probably have to do your own PCBs.

There are a few other possibilities, depending on what requirements have to be met for the overall system.
1) If you need low noise on the 5 volt supply, switch to something over 5 volts, and use a high power supply rejection LDO linear regulator or additional LC filtering for the 5 and the same regulator used in the first scenario for the 3.3 volt supply.
2) If you require very high efficiency, use a buck-boost SMPS (SEPIC or ZETA are other more esoteric options, the ZETA has low ripple output, but a high side switch) since the input volts are both above and below the 3.3 volts you require. A boost converter will do the 5 volts.

At switching frequencies above 500Khz, which are most common these days, to keep the inductor sizes small, if you need low switching noise, you are likely to have to resort to LC filters on the SMPS outputs, since there a not many linear regulators that have sufficient power supply rejection above those frequencies (TI and ADI make some). You have to pay close attention to grounding and wiring layout when using SMPS.
What you need to do is work out from the data sheets what the current demands and voltage tolerances are for the two supplies, what noise they will tolerate, what sort of efficiency you require, and work out the best option from there.
Cheers,
Richard