The nPM2100 PMIC reference design layouts use four-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 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.
The nPM2100 Quad Flat No-lead Package (QFN) and Wafer Level Chip Scale Package (WLCSP) can be routed on a two-layer board, but in that case, a solid ground plane must be ensured under high current paths, such as boost input, output, and switch nets. For a WLCSP on a two-layer board or a multilayer board without microvias, the GPIO0 and SYSGDEN pins cannot be routed. This limitation should be considered from the perspective of the use case when designing the end product.
Other components on the board might have requirements for the stack-up, like 50 Ω impedance on RF lines and 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. If nPM2100 WLCSP is used on a board with via in small pads, it is recommended to use via capping to ensure good solderability.