Vmx-Pi Power Management - Kauai Labs VMX-pi Reference Manual

VMX-pi Hardware Reference Manual (version 1.10) 14

VMX-pi Power Management

VMX-pi Power Management ensures reasonable system behavior in exceptional events including Input
Power under-voltage, and also external device power over-current and short-circuit conditions.
External Device Over-current and short-circuit management
When the VMX-pi 5V and 3.3V regulators that provide power to External Devices detect a current draw
from those external devices exceeding the maximum current level (.5A), current is either limited to the
maximum, or alternatively the current is completely removed, depending upon software configuration.
This design ensures that sufficient power is reserved for proper operation of the VMX-pi microcontroller
and the Raspberry Pi processor and its USB peripherals - even when External Device short-circuit
conditions occur.
Input Power under-voltage management
In Input Power under-voltage situations, VMX-pi is designed to preserve power to the VMX-pi
microcontroller and the Raspberry Pi processor, at the expense of any peripherals which may be
attached. Specifically, VMX-pi's power management scheme prioritizes the Raspberry PI power supply
first, then the VMX-pi microcontroller second, and lowest priority is given to any External Devices. This
design ensures that critical components are able to maintain state (e.g., software algorithms, IMU
calibration coefficients, or buffers of recently received CAN packets).
Figure 5 VMX-pi power supply voltage regulator dropout curve
As shown in the VMX-pi voltage regulator dropout curve above, the VMX-pi voltage regulator can
guarantee the full 3A (15W) power output as long the Input Power voltage is above 5.75V.
The Raspberry Pi 3 USB power supply – which is designed to power external USB devices - will brown out
when the VMX-pi 5V voltage regulator output drops below 4.75V. At the maximum 3A load, this
corresponds to a VMX-pi input voltage of 5.75V. For a more typical lightly-loaded Raspberry Pi
consuming 1A, this corresponds to a minimum input voltage of 5V. Note also that the Raspberry Pi
processor will continue to function until the VMX-pi input voltage reaches 4.5V, and the VMX-pi