# Design of a single cell low voltage reverse battery protection circuit

I have a design that will run off of a single cell 1.5V AA battery and I wanted to provide some reverse battery protection to it while also maximizing my run time. The classic way to do this is just with a FET.

But then I was reading a TI app note where they note that when a battery is drained to 0.8V the voltage is not sufficient to fully turn on the FET and it will operate in the linear region. Therefor the RDSon is much higher and the voltage drop will not allow the regulator (which is placed after the protection) to turn on.

Their solution in the app note is a regulator they have with a Vaux output that charges up to 2.5V before turning on the main output. This Vaux pin is used to pull up the gate of the FET thus turning it on. The part they use is a little too small for me and I was trying to figure out a way to get just this feature discretely or with some additional IC.

Has anyone run into an application like this before or have any advice?

I looked for some charge pumps but they stopped around 0.9V input where I want to get down to 0.7V (their circuit can do 0.5). I looked for some other regulators and I've considered using their regulator just for it's vaux capability :) Finally one AA battery is a hard requirement, I don't have a choice here.

Here's the TI circuit for reference:

• What do you mean when you say the part is too small for you? Not enough current, or too small a foot print for you to solder? (You can buy little boards that will adapt SMD's to DIP's and such.) – George Herold Feb 3 '17 at 17:01
• Sorry by too small I meant insufficient output current – confused Feb 3 '17 at 17:11

The mosfet that you have shown is valid ,but the gate volts available is of course only the battery volts .Finding a mosfet that will give low on resistance at say 1 Volt will be hard,if not impossible. Remember that the gate source voltage must be significantly above the threshold volts .There are devices that I have not used that have a gate source threshold of 600mV .They may not give low enough on resistance for you on a single alkaline cell.There are osc circuits that could be rectified to make a realistic gate voltage .I have used a modified dynatron that starts on 700mV and can run down to 300mV on a TEG application.There are other osc circuits like joule thief and the blocking osc that I have not tried but could work .At start up the mosfet has no gate volts so the body diode could waste say 600mV .Placing a resistor across the DS of the revpol mosfet will assure low voltage start up .The resistor value is low enough to provide good starting and high enough to keep revpol prospective currents safe .

Seems like a lot of hassle for just a reverse battery protection.

An alternative solution I can think of:

simulate this circuit – Schematic created using CircuitLab

If the battery is reversed, a lot of current flows through the diode. Many circuits (chips) can handle such a small reverse voltage but do check that. The large current blows the fuse. For an evel lower diode voltage drop, use a Schottky diode but beware that these leak reverse current at higher temperatures.

You could make the fuse a standard replaceable glass fuse or a polyfuse (these recover after some time).

Yes another solution is a mechanical solution. I once worked on a product running on a single AAA cell. It had a non-electronic reverse battery protection. The positive battery contact was recessed slightly into the plastic case. Therefore the flat negative terminal of a battery cannot make contact with it. But the positive battery pole protrudes a little bit, just enough to make contact. Have a look in some devices (like a remote control) which work on AA or AAA cells and you'll see what I mean.

• Thanks I considered a fuse but it's not very user friendly so I'm still hunting for an electrical solution. I need more of a whoops I put it in backwards turn it around type of thing. – confused Feb 3 '17 at 17:48
• As I suggested, have a look at the polyfuse: en.wikipedia.org/wiki/Resettable_fuse It "resets" after it has cooled down. No need to replace a polyfuse. – Bimpelrekkie Feb 3 '17 at 18:29
• I like the polyfuse suggestion. These devices are very user-friendly. – Paul Elliott Feb 4 '17 at 6:21

You could mimic the functionality using analog /discrete components but my guess is that it is not going to be worth it. First off, you may end up with lower efficiency overall. If this is not a production design you could consider using a third party board like this one from SF.