The figure below describes a simple power supply for a minimalistic expansion board design using an external 12-VDC, 3-A power supply connected directly to the DC input path.
Any DC input must have adequate protection, for example a reverse-current blocking diode, a fuse, and an ESD transient suppression diode. This DC input path must also have sufficient decoupling capacitors before and after the +VSYS buck regulator.
If the expansion board is designed with +VDC_IN connected to a primary power source, as shown below, then the module begins to boot when power is applied. For designs not intended to boot when power is connected (remote or secondary enabling), VDCIN_SENSE must be raised to, or above, +VSYS for the module boot sequence to begin.
Refer to Power Delivery, Signaling, and Reset for the current VDCIN_SENSE and +VSYS voltage ranges.
A buck regulator on the expansion board is necessary to generate module +VSYS power at a controlled 5 V and 5 A regulation. This buck regulator circuit must have a safe input voltage range that is higher than the DC-input supply path provides under any condition.
The +VDD1 and +VDD3 from the module ramp and stabilize before the module code execution begins. These two rails can be used to enable additional voltage regulators and load switches on the expansion board, and those regulator and switches in turn provide power to platform devices.
Simple Power Supply Diagram
Power-on Timing Sequence
See Power On Timing Sequence (cold boot) in the Expansion Board Hardware Guide for more information.