backupbatteriespowerswitch-mode-power-supply
I have a project running with MCU supplied with a 24V power supply regulated via an SMPS to 9V
also, I have Li-ion battery pack 9V
how to connector these two sources to run the MCU with battery pack when the power failure occurs?
or
what is the best way to switch from battery to power supply and power supply to battery?
There are a number of ICs that will do these functions. I would go to the Maxim and Linear Technology sites. The downside is that most of these devices is that they are only in surface mount packages and a lot of them come in packages without leads (QFN, LGA).
A part like the LTC4088 is nice. It has an input for the battery, an input for the USB (or wall adapter) and a switching regulator.
You could also use separate ICs (but you are probably better off with a single). There is a device that Linear Tech calls a PowerPath controller (LTC4412) that performs the switchover function. You could add a MAX1555 Li-ion charger for a low cost charger and add the LTC4412. Also there are devices called "Ideal Diodes" that will perform switchover. These devices are FETs so you get a much lower voltage drop than a Schottky.
The best solution depends on a couple of things you haven't mentioned -- system current, battery capacity and recharge rates.
Question 1: Option 1. This is a more reliable indicator of the actual battery voltage. The drop out of the regulator will be affected by current.
Question 2: Use the regulator's enable input if it has one. This saves you having to add an extra transistor. This will reduce the amount of current drawn by the regulator.
There are two ways you might implement this:
1) Use a potential divider to create the Enable voltage. When the output voltage of the divider goes below the enable threshold voltage, then the regulator switches off. The problem with this is that the voltage threshold of the enable input is pretty flexible:
The threshold is somewhere between 0.4v and 1.2v! This isn't going to be accurate enough for your purposes.
Instead, I'd suggest using a comparator:
Best Answer
Diodes are a very simple option if you've got the voltage headroom.
Another good reference design is the arduino power selection circuit:
The P channel mosfet on the USB VIN prevents back powering of the usb line, and also doesn't introduce a diode voltage drop.
One thing you'll need to check for safety is the leakage from the 24v supply into the battery. Most semi-conductors like diodes leak a little bit in the uA range. If you're not careful the battery could be overcharged by this and potentially burst into flames!
There are old-school options you could also consider. For example, use a spdt relay with a coil connected to your power supply. Connect COM to the MCU, NO to the power supply, and NC to the battery. You'll need a large cap or something to supply power for a few ms while the relay switches.