Multiple voltages off a single source

Question

I'm building a controller for project that has hobby servos. These typically need ~5V to run. The controller will be an ATmega 2560, running at 3.3V / 8MHz. Looking at the reference design for the Arduino board that uses the same ATmega chip, they're using a 1 amp LDO to regulate input voltage down to what they need.

The question has two parts:

1. If I need both voltages available to me, I assume I should just use two voltage regulators, set for their respective output voltages. Is there a reason to daisy-chain them (feeding the 3.3V one from the 5.0 one?) or should I have them both pull from the battery (12V)?

2. Given that I'll be running this off a battery source that'll be somewhere between 9 and 12.4 volts (a 3-cell LiPo battery), should I use an LDO, or a buck regulator (I'm a bit out of my depth when it comes to power management). For some reason I'm thinking that an LDO will not be as efficient as a buck regulator at what seems like a substantial voltage difference.

Answer 1

It's much better to use switching regulators than LDOs, especially with a in-out voltage difference that you will have, because of the heat dissipation.

However one of the problems with using switching regulators for the hobbyist is that almost all of them come in surface mount packages -- of the 20,592 switching regulators currently listed on Digi-Key, just 128 come in DIP packages.

If you can get by with a maximum current draw of 1.5A on either the 5v or 3.3v rails, then I recommend the MC34063, which comes in an 8-pin DIP and costs just 62 cents from Digi-Key. (Or use a switcher for the 5.0v rail, and a LDO for the 3.3v.)

One of the drawbacks of using switching regulators is that you do have to surround it with a number of components, some of which set the voltage and current for adjustable regulators like this one. But all of these components can be easily found as through-hole parts. Here is a configuration for a 25v to 5v 1/2A step-down converter. You'll want to get an inductor with twice the current rating as your output.

If you have surface mount capabilities, then I suggest using a dual regulator like the TPS54295 in your case. It has a capacity of 2A for each rail, and comes in a fairly friendly 16-TSSOP package. It is available from Digi-Key for $2.58. The extra components needed are similar to the schematic shown above, times 2.

Answer 2

You would only need to daisy-chain them if the 5V regulator had a minimum load that could be met if followed by the 3.3V regulator. But keep in mind that the earlier regulators will need to supply both the current required for their rail as well as the current required for the later regulators.

Since the voltage drop is so large, using a switching regulator is recommended. The 80% or so (minimum, usually) efficiency from the switcher is much higher than what you can get from the linear. OTOH, if you were only dropping a single Li-ion cell to 3.3V-3.6V then a linear LDO regulator would be the way to go.

Answer 3

First, I think you should have a quick read of some general overview on battery voltage selection for your projects, and in general the power stage selection is important. I have written up an informative power point presentation that I delivered to the members of my university's robotics club. You can see it here link to PDF document about power systems for robots

The basic answer to your Q1. is given the requirement of both power rails, I suggest you use a buck converter with 1-2 amp output at 5V for the servos, and 'daisy chain' a 3.3V linear (LDO) regulator off the 5V rail for your digital systems. Use lots of capacitors to prevent brown-outs and other issues that can occur if your servos pull the 5V rail too low (if they get jammed, or you are abusing their range of movement too much).

For heat dissipation purposes, you could also put an LDO (5V, 1A) on the 12V rail for the servo, and have an independent supply for the 12V -> 3.3V of your digital system on another regulator IC. This method is better if you are stuck with just a few adjustable linear regulators - and safer by having the supplies independent of each other to avoid brown-outs due to the servo stalling. If you were to daisy chain the regulators, you may still have the drop-out issue, but mostly you should avoid pulling any more current than necessary from the 12V -> 5V regulator, for heat reasons. The low current 12->3.3V LDO should be fine by itself, but it's constant draw if daisy chained to the 12->5V LDO may be detrimental.

To answer Q2. I suggest you try to use some pre-made (cheap, from lots of places online) buck regulators to get your 5V and 3.3V supplies each connected to the battery. If you can only get one buck regulator, get it for the 5V rail and use an LDO for the 3.3V supply. Again, big fat capacitors to help under pulsed loads, such as a servo moving back and forth quickly!

I hope that helps. Please check out that PDF document I made, it may help you understand why, and it gives you some example scenarios for what to choose and when.