The input of a DC-DC converter is tied to the output of an upstream supply in the power system. This will define the required input voltage necessary for the DC-DC converter. There are a number of different input options depending on the stability of the voltage. For example, a fixed input voltage can be used when the voltage rail connected to the input is very stable. If, on the other hand, the DC-DC converter is connected to a fluctuating supply, like a battery, a much wider input range is required to account for the changing input voltage conditions. 2:1 and 4:1 type inputs are examples of wide input range devices used to address applications with a volatile input voltage. These numbers represent a comparison between the high and low input voltage limits. An example of a 2:1 input voltage device would be a DC-DC converter that has a nominal input of 12 V but can operate anywhere between 9 V and 18 V while still maintaining a stable output voltage.