NeoECU-Hardware/Architecture/POWER_ARCHITECTURE.md

NeoECU V1 Power Architecture

Purpose

This document defines the working power architecture for NeoECU V1. The ECU is powered from a 4S LiPo and controls one injector, one dual-ended dumb ignition coil, engine-position Hall sensors, analogue sensors, CAN, and a starter-enable output.

This is an architecture decision document, not a component-selection or schematic document. Current limits, exact protection components, and regulator part numbers remain to be selected after the loads and packaging are confirmed.

Supply Assumptions

• Supply: 4S LiPo battery.
• Nominal voltage: 14.8 V.
• Fully charged voltage: 16.8 V.
• The ECU shall tolerate supply sag during vehicle operation and switching transients from ignition, injector, starter, and harness inductance.
• Unlike a road-car alternator system, V1 is not initially designed around a conventional automotive load-dump event. Input protection and regulators should nevertheless have adequate voltage headroom for realistic local transients.

Rail Tree

Input and Raw-Battery Domain

VBAT_PROT is the protected raw-battery domain. It is produced after the input fuse, reverse-polarity stage, and input transient clamp.

It supplies the loads that must operate from battery voltage:

• ignition-coil primary supply;
• injector supply;
• a protected, scaled battery-voltage measurement for the MCU.

Ignition, injector, and low-side-output branches require their own protection and fault containment. Their high-current returns must not share the sensor or MCU return path; all return domains join deliberately at the power-entry region.

+5V_MAIN

+5V_MAIN is the ECU's primary regulated 5 V rail. It is generated directly from VBAT_PROT by a synchronous buck converter and powers internal 5 V circuitry, including automotive ICs that require 5 V logic or supply voltage.

It is the upstream rail for +5V_AUX and +5V_SENS. External harness-connected loads shall not be connected directly to +5V_MAIN; they use their separately protected branch.

+5V_SENS

+5V_SENS is derived from +5V_MAIN through an eFuse or protected load switch, followed by a ferrite bead and local filtering. It is not shared directly with external digital-output loads.

It provides a controlled, protected 5 V excitation supply for ratiometric pressure sensors and other sensors that require 5 V. The rail shall include a sensor-facing protection/current-limit stage so a harness short cannot bring down +5V_MAIN or the ECU logic supply. Its voltage should be monitored by the MCU.

The ferrite bead and local filtering isolate sensor excitation from switching and digital noise on +5V_MAIN, while the protected branch keeps a sensor-harness fault contained to the sensor domain.

+5V_AUX

+5V_AUX is derived from +5V_MAIN through an eFuse or protected load switch. It supplies external 5 V digital logic-level output circuits, such as a starter-enable command to a separate external control module.

This rail is distinct from +5V_SENS so external digital loads and harness faults cannot disturb sensor excitation. The protection stage shall isolate a short on +5V_AUX without inhibiting +5V_MAIN or the rest of the ECU. It shall include appropriate output protection and current limiting for the external interface.

+5V_AUX is not an actuator supply. If a starter-enable or other command must drive a relay or another inductive load, it shall use an appropriate protected low-side sink output instead of a 5 V logic-level output.

+3V3_MAIN

+3V3_MAIN is generated directly from VBAT_PROT by a dedicated synchronous buck converter. It supplies the STM32H747 digital supply pins and the 3.3 V digital loads, including logic-side communications circuitry.

The main 3.3 V rail shall not be made by an LDO from either 5 V rail. The H747 and supporting logic can create a substantial 3.3 V load; an LDO would dissipate the difference between 5 V and 3.3 V as heat. Generating 3.3 V from VBAT_PROT also avoids placing the MCU's current demand on +5V_MAIN. +3V3_MAIN must never be derived from +5V_SENS, since that would couple the logic load to the sensor supply.

+3V3_ANA

+3V3_ANA is derived from +3V3_MAIN through a ferrite bead and local filtering. It supplies the MCU analogue supply/reference domain and analogue front-end circuitry.

The initial implementation should reserve an optional low-noise LDO footprint for this rail. An LDO should be fitted only if analogue-noise testing shows it is necessary; it is not the primary 3.3 V regulator. The analogue rail and its decoupling must follow the STM32H747 supply and ADC layout guidance.

Ratiometric 5 V Sensor Measurements

The MCU ADC reference is +3V3_ANA; it is not expected to be precision-trimmed to track +5V_SENS. Therefore 5 V ratiometric sensor readings are made ratiometric in measurement and firmware rather than by requiring a fixed relationship between the rails.

Firmware shall calculate the sensor signal as a ratio of the two ADC readings:

sensor fraction = ADC(sensor output) / ADC(+5V_SENS sense)

Since the two channels use the same ADC reference, the reference-voltage error and most common gain error cancel. Divider ratio and temperature drift are also minimised by using matched, low-drift resistor networks for the sensor-output and 5-V-sense dividers.

Design requirements:

• The divider ratio must keep both channels within the ADC input range with margin for the maximum sensor-supply tolerance. A nominal ratio near 0.55 maps 5.25 V to approximately 2.9 V.
• Use corresponding filter characteristics on the sensor-output and 5-V-sense channels and sample them close together.
• Apply normal input protection and anti-alias filtering without defeating the intended ratiometric measurement.
• A failed or shorted sensor supply must be detectable from the sensed 5 V reading.

Thermistor Measurements

Thermistor dividers are normally excited from +5V_SENS, consistent with a conventional automotive ECU sensor interface. The thermistor itself is passive and would operate with a 3.3 V excitation, but using the protected 5 V sensor rail gives a common, diagnosable excitation supply for external sensors.

Firmware shall use the ratio of the thermistor-channel and 5-V-sense ADC readings to calculate the thermistor resistance. This cancels variation in both +5V_SENS and the +3V3_ANA ADC reference. Pull-up resistance must be chosen to limit thermistor self-heating while providing sufficient measurement resolution over the required temperature range.

+12V_SENS

The crank and cam sensors are currently described as 12 V, active-low, open-collector-style Hall-effect sensors. They are supplied from +12V_SENS.

+12V_SENS is a regulated 12 V buck-boost rail generated from VBAT_PROT. It provides a stable supply when the 4S LiPo is above or below 12 V and may be used by future 12 V sensor circuits in addition to the crank and cam sensors.

The rail shall include a sensor-facing current-limit or protected high-side switch, filtering appropriate to the harness, and local decoupling at the sensor connector. Its voltage accuracy, ripple, current capability, and current-limit threshold must be specified from the selected Hall sensors and the future 12 V sensor budget.

Hall outputs shall not enter the MCU at the sensor supply voltage. Their input conditioning shall provide protection, noise rejection, and a defined logic-level conversion to protected 3.3 V timer inputs. Output pull-up and level-shift details are to be selected with the final sensor wiring and output type.

Grounding and Layout Intent

• Keep ignition, injector, and starter-switch current returns separate from sensor and MCU ground returns.
• Join these return domains deliberately near the input/power-entry region.
• Keep switching-regulator hot loops compact and away from trigger and ADC signal paths.
• Route sensor supply and sensor-return paths as controlled pairs to the connector where practical.
• Place the analogue front end near its ADC connections and isolate it from ignition and injector switching nodes.

Open Items Before Schematic Freeze

1. Confirm the current budget for each rail, including all expansion I/O.
2. Confirm the Hall-sensor part numbers, current, output type, cable lengths, and pull-up requirements; establish the current budget for future +12V_SENS loads.
3. Select the input transient and reverse-polarity ratings after the battery, wiring, and enclosure arrangement are known.
4. Confirm sensor-supply accuracy, current limit, and diagnostic requirements.
5. Validate ADC noise and ratiometric accuracy on hardware before deciding whether the optional +3V3_ANA LDO is needed.