Single ground is recommended: SGND. A multilayer such as 4 layers board is recommended so that one solid SGND is dedicated for return current path, referred to the layout example.
Apply multiple different capacitors for different frequency range on decoupling circuits. Each capacitor has different ESL, Capacitance and ESR, and they have different frequency response.
Avoid long traces close to radiation components, and place them into an internal layer, and it is preferred to have grounding shield, and in the end, add a termination circuit;
Analog circuit such as ADC sensing lines needs a return current path into the analog circuitry; digital circuit such as GPIO, PMBus and PWM has a return current path into the digital circuitry; although with a single plane, still try to avoid to mix analog current and digital current.
Don’t use a ferrite bead or larger than 3 Ω resistor to connect between V33A and V33D.
Both 3.3VD and 3.3VA should have local 4.7 µF decoupling capacitors close to the device power pins, add visas to connect decoupling caps directly to SGND.
Avoid negative current/negative voltage on all pins, so Schottky diodes may need to clamp the voltage; avoid the voltage spike on all pins more than 3.8 V or less than –0.3 V, add Schottky diodes on the pins which could have voltage spikes during surge test; be aware that a Schottky has relatively higher leakage current, which can affect the voltage sensing at high temperature.
If V33 slew rate is less than 2.5 V/ms the RESET pin should have a 2.21-kΩ resistor between the reset pin and V33D and a 2.2-µF capacitor from RESET to ground. For more details please refer to the UCD3138 Family - Practical Design Guideline This capacitor must be located close to the device RESET pin.
Configure unused GPIO pins to be inputs or connect them to the ground (DGND or SGND).
Select the cap ratio as shown below. Use 2.2 µF between V33D and BP18, and 1 µF between BP18 and DGND or SGND.
9.2 Layout Example
Figure 9-1 UCD3138 40-Pin Layout Example
Figure 9-2 UCD3138 40-Pin Layout Example (Internal Layer)