Design considerations for low-current SMPS application

Question

I've got a project I'm working on that is powered by a single Lithium-Ion battery (let's say design range is 2.5V to 4.2V), and a nRF52832-based MCU. I've decided to design a power topology that goes from raw battery to a "system" rail of 2.5V, and then feed that to all devices + the nRF52's internal DC/DC regulator.

My design target for "sleeping" power consumption here (i.e., idling until event occurs) is around 300uA or so. I don't expect peak currents on the rail to ever go past 20mA or so as well -- going by the nRF datasheet numbers (taken with grain of marketing salt), they claim ~15mA or so of current consumption.

So, I've obviously got a concern about the quiescent current (Iq) of my chosen regulator, which I'm basically stuck with based on the manufacturer. But, I also have a concern about a buck controller going into pulse-skipping or similar mode and giving me a great EMI/EMC problem to deal with.

The largest possible voltage delta (4.2V - 2.5V) gives me some pause using a LDO/linear part (~55% efficiency). I've looked at the AP3401, primarily because it is incredibly cheap. The datasheet is very light on the details, but it mentions pulse-skipping once. I imagine running at very low currents relative to the 1A number is going to put me in that range.

This TI TPS62740 naturally has a better datasheet, and seems like a "better" part for this / intended for these low-currents, but it also appears to quite literally be 8x the cost of the AP3401.

Summing up:

1. Recommendations / literature on very low-current switching regulator applications?
2. Experience with / stories about dealing with EMI-related issues with a regulator that is basically always pulse-skipping?
3. Suck it up and go with a linear regulator? Some kind of scheme that switches based on the system state?
4. Recommended solutions from previous experience? Target volume I believe is around 10K units/yr.

Answer 1

Forget about EMI. Unless you do a terrible job on the layout, you will not have radiated emissions problems from your DC-DC converter. The way I see it, the tradeoff is cost vs battery life. If the battery life is OK, I would stick with the LDO for greater simplicity, interchangeability and lower cost. Also note that the efficiency calculation should not be done at 4.2V, but at 3.7V (the average). So it is 2.5/3.7 = 67%. Quiescent current will also probably be lower with the LDO. Quiescent current may dominate your battery life, if the product will spend most of its time in standby. A simple SOT-23 LDO should handle the required current easily. These are available from many vendors with similar/same pinouts, so you won't be single-sourced (make sure to choose an easily crossable part).

If you do go to a switcher, try to find one that is pin compatible across multiple vendors. These are slightly less interchangeable than LDO's, but you should be able to easily produce a layout that accommodates multiple DC-DC converters. I believe quite a few small buck converters will automatically go into LDO mode at low load (rather than skip-pulse mode). So you might not need to worry about skip pulse anyway.

Do not use proprietary parts that are not compatible with parts from other vendors. You will regret it later when lead time goes to 26 weeks, and you are single-sourced for that part.