A step-down regulator followed by a step-up regulator usually called a buck-boost regulator. The figure show you with the National LM5118 basic buck-boost operation circuit for the automotive power management solution.
to the LM5118 datasheet, Q1 and Q2 are active when it is in the
buck-boost mode for the same time interval of each cycle. “D” is the
duty cycle of Q1 and Q2 and the initial order of buck-boost transfer
function is VOUT/VIN = D/(1-D). It is noted that the regulator can be
designed to operate with VIN less than 4 volts, but VIN must be at least
5 volts during start-up.
In the LM5118 datasheet you can find its features such as Ultra-wide input voltage range from 3V to 75V, Smooth transition between buck and boost modes, Oscillator synchronization capability, Enable input wide bandwidth error amplifier, and etc.
Find complete read of National LM5118 basic buck-boost regulator circuit for automotive power management solution in the pdf filetype (source: national.com).