In battery operated devices which have removable batteries, you usually need to prevent the batteries from being connected the wrong way round to prevent damage to the electronics. If that is not possible by physical means you need to include some electronic protection. Physical protection can simply mean a polarized connector or a battery with offset connections (as with most smartphone lithium batteries). For AAA or AA style batteries there are holders which are designed so if the battery is placed the wrong way round, one end will not make contact. There are still circumstances where physical means aren’t possible such as with most button cells or if the user can connect power by wires to screw terminals. This could apply to non-battery operated equipment as well and is likely to apply to automotive electronics.
A simple series diode is one solution but also wastes power. The chances are that with a battery operated device you don’t want to waste power, particularly if your supply voltage is already quite low and so the loss of 0.3V or 0.4V from a Schottky diode will be significant and unacceptable. For many automotive applications, a small voltage drop may not matter.
Similar to power source switching there are “ideal diodes” available from companies such as Linear Technology (Power path Controllers). They use a MOSFET switch and additional circuitry to make a low loss switch without the voltage drop associated with a simple diode. If you want to use one of these ideal diodes for reverse polarity protection, check the specifications carefully to make sure they can provide that function ñ most don’t. The LTC4412, for example, does provide reverse protection, the LTC4411 doesn’t. The LTC4412 requires an external MOSFET though, whereas the LTC4411 has it built in.
In general with Linear Technology, if the description includes the word “controller” it needs an external MOSFET. The LTC4359 is a specific device aimed at automotive reverse battery protection and will tolerate 40V of reverse power.
Simple power path controllers for reverse polarity protection are not easy to find. Power switching functions are often part of a more complex IC such as a battery charger IC and won’t necessarily include reverse polarity protection either. A simple P-channel MOSFET is one possibility to replace a simple diode, depending on your voltage and current requirements. An example is shown below.
The actual choice of components and values will depend on the application, but this is a good starting point for discussion. R2 represents the load. The zener diode will protect against exceeding the recommended gate-source voltage and may not be required, depending on your input voltage range and the MOSFET used. The capacitor is to ensure the circuit works when there is a rapid change in input voltage polarity. Without the capacitor, this is what will happen if the input voltage rapidly goes from -5V to +5V and back:
While it protects against a static reverse polarity (the first 10µs), the circuit doesn’t initially provide protection when the polarity suddenly reverses and any connected circuitry could be damaged. Adding the capacitor changes the performance to this:
Now you have a clean transition from OFF to ON and back to OFF again with minimal reverse voltage getting through. You can reduce the 80mV of reverse voltage that does get through by increasing the capacitor size. The small current glitches you can see are the current needed to charge and discharge the capacitor. A different MOSFET will require different value components to work correctly. For example, a larger MOSFET will probably require a larger capacitor to prevent the transient reverse polarity voltage.
Note that you need to be careful to connect the MOSFET as shown otherwise the body diode will conduct when the input voltage is reversed and you will have failed to achieve your objective. The same care needs to be taken when using the Linear Technology ideal diodes with external MOSFETs. When connected as shown, the body diode actually conducts when the input voltage is positive until there is enough input voltage to turn on the MOSFET. If the MOSFET threshold is low enough, the body diode and the MOSFET could start to conduct at around the same input voltage.
Using a proprietary chip such as the LTC4349 saves some of the design work so you will have a working solution with less effort – but at a higher component cost compared to a discrete solution. And, if you are designing for an automotive application, you need to ensure that your design meets the requirements of appropriate standards such as ISO7637-2.