# 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 ```text 4S LiPo | +-- input fuse, reverse-polarity protection, TVS/transient protection | +-- VBAT_PROT ---------------------------------> ignition branch | injector branch | protected low-side outputs | protected VBAT sense | +-- synchronous buck --------------------------> +5V_MAIN | | internal 5 V circuitry | | | +-- eFuse / load switch ------------------> +5V_AUX | | external logic-level outputs | | | +-- eFuse / load switch, ferrite/filter --> +5V_SENS | protected sensor supply | +-- synchronous buck --------------------------> +3V3_MAIN | | MCU digital supplies | | 3.3 V logic / communications | | | +-- ferrite bead / filtering -------------> +5V_AUX | MCU analogue domain | analogue front ends | ADC reference | +-- regulated buck-boost ----------------------> +12V_SENS 12 V sensor supply crank/cam Hall sensors ``` ## 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. ```text +5V_SENS ----> sensor excitation | +--> sensor output -- matched divider --> ADC sensor channel | +--> +5V_SENS sense - matched divider --> ADC 5-V-sense channel ``` Firmware shall calculate the sensor signal as a ratio of the two ADC readings: ```text 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. ```text +5V_SENS -- precision pull-up --+-- thermistor -- sensor ground | +-- matched divider/filter --> ADC thermistor channel +5V_SENS ------ matched divider/filter -----------------> ADC 5-V-sense channel ``` 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.