This page will show you how to mux power supplies, and more specifically how to switch over an alternative power source (typically a battery) when the main voltage rail is shut down. Such a system must provide an uninterrupted source of power to avoid any data loss. Typical applications include backup of memory chips or real time clocks.
This battery backup switchover / failover is also often referred to as “ORing” power supplies.
Loss of main power can be problematic in many different situations. For instance, when power is abruptly lost, you might want to save some state on a micro-controller. Old game consoles were often storing save games into a RAM, which loses all data once power is disconnected. Such use cases need some form of automatic switch-over to a secondary power source: a battery.
We therefore needs a circuitry capable of switching from one to the other, automatically and without power loss to the powered system.
As illustrated above, given two power sources, how to control which one is delivering power and switch seamlessly from one to the other?
The simple solution: diodes
The simplest solution to this problem is simply to add a diode on each voltage source.
This only works if the main power source voltage is higher than the battery voltage. For a 3.3V or 5V application with a 3V coin-cell battery as backup, this works well. However, the main issue with this naive approach is that the voltage drop (forward voltage) across the diode might be too high for the system.
Nowadays, this can be mitigated by using extremely low forward voltage Schottky diodes. You can readily find 250mV@1A diodes on the market, and they don’t carry an especially high premium.
That being said, this simple circuit wastes a lot of energy; and it does not regulate the switchover very well. It may very well be that the temporary voltage dip followed by the current surge during the switch prove themselves inadequate for your design. In that case, a more active design should be preferred.
There are lots of way to achieve better performances, so do continue reading if you are not impressed by simple diodes.
The simple solution 2: throw an IC at it
Whenever there is an electronics engineering problem to solve, you can be almost certain that there is an integrated circuit designed just for that. And it applies to this issue as well in the form of the Intersil (now Renesas) ICL7673.
As stated by the manufacturer, this simple IC does everything that is expected of an automatic battery backup switchover:
- Automatically Connects Output to the Greater of Either Input Supply Voltage
- If Main Power to External Equipment is Lost, Circuit Will Automatically Connect Battery Backup
- Reconnects Main Power When Restored
There are two downsides to using this specific IC worth considering. First of all it is expensive ($2.5 a pop on Digikey at the time of writing this) despite the manufacturer claim of being low cost. Secondly, it is a very niche IC that came in 1999. Over 20 years later it is still produced but you run the risk of integrating a component that might be discontinued.
This IC is perfect for a low volume/hobby project where you do not want to spend time engineering a conventional solution. As stated by the datasheet itself, “The Intersil ICL7673 offers unique performance advantages over conventional means of switching to a backup supply”.
But what is this “conventional” mean? The answer to that is “ideal diodes”.
The recommended “state of the art” solution: Ideal Diodes
What if we could find an “ideal diode”? A diode that would be a perfect conductor when forward biased, and a perfect insulator when reverse biased? That would instantly solve this battery failover circuitry problem.
These perfect components do not exist, but the semiconductor industry has come so close to it that using ideal diodes to “ORing” or “MUXing” power supplies is the defacto recommended standard. More specifically, the switchover is achieved using ideal diodes and “ideal diodes controllers”, which are often combined as a two-in-one integrated circuit.
An ideal diode controller drives an external N channel MOSFET. When forward voltage is applied, the controller turns the MOSFET ON, simulating an almost perfect diode with near 0 voltage drop. When voltage is reversed, the integrated body diode of the MOSFET blocks reverse current. It is that simple.
With this in mind it is very easy to design dual power supploes with two controllers and two MOSFETs:
However, this adds four discrete components to the BOM so of course the semiconductor industry has came up with dozens of ICs to further integrate the whole solution. For instance, many ICs integrate the MOSFET with the controller.
When it comes to power delivery and management, the usual suspects are always lurking close, Texas Instruments and Linear Technology (now Analog Devices) to name them. These two companies have a lot to offer in this regard; but I’ll just name a few worth looking at below.
In Practice: Texas Instruments’ LM74700 & LM66100
The LM74700 is just a diode controller, so it requires an external MOSFET to act as battery backup circuitry. It is a typical use case for the IC.
In the case of the LM66100, the MOSFET is integrated to the IC. It is highly recommended for an automatic battery switchover circuit. As an added bonus, the chip is also really affordable (US$0.4 per unit).
In Practice: Analog Devices’ LTC4411 & LTC4412
Analog Devices also provides interesting ICs for the job in the form of LTC4411 and LTC4412. They are strictly identical, but the LTC4411 integrates the N channel MOSFET while the LTC4412 needs an external fet of your choice, similarly to Texas Instruments’ offering.
The recommended application by the manufacturer slightly differs though, as Analog Devices simply recommends a typical Schottky diode for the main power source, while the battery backup is controlled by the ideal diode. This makes sense as typically power loss is more of an issue with the battery backup as compared to the main voltage rail where the loss can be afforded.
Sadly, all Analog Devices solutions carry a big premium this IC is not an exception. From known distributors, one of these puppies costs a whopping US$4! Despite the elegant simplicity of the design, it makes it really hard to recommend over Texas Instruments.
You can now add a state of the art battery backup circuitry to your designs by adding about a single US dollar to your bill of materials.
If you’re not that bothered with the implementation, an IC or the good old diodes might be up the task. At the end of the day, the engineering solution will be determined by the project constraints, but all three solutions are valid.
If you would like to dig further, power supplies muxing goes beyond this use case and can become quite involved once you throw into the mix more than two power supplies and “prioritizers”. That being said, they will always involve ideal diodes has the mechanism behind.
- Primer on PowerPath Controllers, Ideal Diodes and Prioritizers [mirror] by Analog Devices
- Ideal Diodes and Prioritizers [mirror] by Analog Devices
- Basics of Ideal Diodes [mirror] by Texas Instruments
- Seamless Switchover for Backup Power Reference Design [mirror] by Texas Instruments