MAX1978's maximum output is ±4.3V/±3A so it cannot support 12V@5A output. This is due to the internal design, which has two buck regulators. The output of MAX1978 is the output voltage difference of the two buck regulators. MAX1978 regulates the output voltage by regulating the output of the two buck regulators. Due to the fact that the output of a buck converter is less than the input, it is not possible for the output voltage to be greater than the input voltage, which is between 3 and 5.5V.