buckdc/dc converterldopower supply
I am currently designing a circuit where we have a lipo battery powering two rails:
Option 1: use 2 buck boost converter to go up from [3V-4.2V] to 5V one side and to 3.3V on the other side
Option 2: use 1 buck boost converter to go up to 5V creating rail 1, then use rail 1 + a LDO or DC-DC to go from 5V to 3.3V to create rail 2
What is it better to do and why?
You can sortof do it, but it's probably not worth the trouble. You would have to add a diode in front of the 5V output cap, and a pass element in front of the 3.3V output cap. The switcher regulates the 5V output, and a comparator turns on the pass element whenever the 3.3V output is low. In other words, the comparator via the pass element steals current that would otherwise go to the 5V supply, but the feedback system makes sure the 5V supply gets enough current to keep it at the right voltage. Like I said, probably not worth it.
To do this in one switcher somehow will require a inductor that can deliver 3A average. It won't be much worse to split that into two inductors that each only have to deliver 1.5V.
Another possibility is to have the switcher make 5V, then linearly regulate that to 3.3V. This wastes 2.6W in heat.
You could buck to make the 5V, then use a open loop fixed duty cycle buck converter with 3.3V/5.5V = 66% duty cycle to make the 3.3V from there.
Looking at the application circuit, it seems that they have made on channel an LDO and another a Buck to save cost and component count.
Why not 2x bucks? Because a buck requires more components (inductor and better capacitor) than LDO. In this case, the combination of buck and LDO is a compromise of efficiency vs. cost and size.
It's not uncommon to use an LDO to power RF communication, analog front ends, because these can be susceptible to the EMI. At the same time, the digital section can be powered from a buck.
Best Answer
Certainly the second option is the more cost effective. An LDO does not require as much external component as a buck-boost converter. This soultion saves some money by reducing the PCB size and the componenet costs.
On the other hand an LDO won't be as efficient as a buck-boost converter, but it all depends on the current demand of the bluetooth chip. I suppose we are talking about max 30 - 40 mA peak consumption here. That will be \$ \text{1.7 V} \times \text{40 mA} = \text{68 mW} \$ wasted energy. And it is the peak, most of the time the bluetooth chip will draw much less. So I would definietly go with the second option.
In general the switching converters efficiency is worse at light loads than at high (though you could find one with light-load-efficiency feature) so an LDO could be even better here.