The Power Management Integrated Circuit (PMIC) reference design layouts use four-layer and six-layer PCBs. The use of at least four layers is recommended.
The key benefit of using a PCB design with four or more layers is the proper incorporation of ground planes very close to the power and signal routings. Ground planes improve power and signal integrity for the circuit by improving return current paths, reducing crosstalk between signals, and reducing unwanted Electromagnetic Interference (EMI).
It is beneficial to have a large ground plane without discontinuities, since lower frequency return currents spread across the plane. For higher harmonic return currents, for instance in switching regulators, a ground plane ensures the lowest impedance for the current.
Other benefits of ground planes are controlled impedance in transmission lines, such as RF signals, and heat sinking for self-heating components, which improves the overall longevity of components.
It is possible to route the Quad Flat No-lead Package (QFN) on a two-layer board, but in that case, care must be taken to ensure a solid ground plane under the high current paths, such as DC/DC input, output, and switch traces.
If the Wafer Level Chip Scale Package (WLCSP) is used on a board with via in small pads, it is recommended to use via capping to ensure good solderability.
Other components on the board might have requirements for the stack-up, like 50 Ω impedance on RF lines and Universal Serial Bus (USB) routing differential impedance. Using microvias and a four-layer PCB might also set restrictions on the stack-up, as an aspect ratio of 1:1 is common for laser cut vias.
For more information, see nPM1300 PMIC reference design or nPM1304 PMIC reference design.