205 lines
8.7 KiB
Markdown
205 lines
8.7 KiB
Markdown
# NeoECU V1 Power Architecture
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## Purpose
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This document defines the working power architecture for NeoECU V1. The ECU
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is powered from a 4S LiPo and controls one injector, one dual-ended dumb
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ignition coil, engine-position Hall sensors, analogue sensors, CAN, and a
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starter-enable output.
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This is an architecture decision document, not a component-selection or
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schematic document. Current limits, exact protection components, and regulator
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part numbers remain to be selected after the loads and packaging are confirmed.
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## Supply Assumptions
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- Supply: 4S LiPo battery.
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- Nominal voltage: 14.8 V.
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- Fully charged voltage: 16.8 V.
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- The ECU shall tolerate supply sag during vehicle operation and switching
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transients from ignition, injector, starter, and harness inductance.
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- Unlike a road-car alternator system, V1 is not initially designed around a
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conventional automotive load-dump event. Input protection and regulators
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should nevertheless have adequate voltage headroom for realistic local
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transients.
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## Rail Tree
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## Input and Raw-Battery Domain
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`VBAT_PROT` is the protected raw-battery domain. It is produced after the
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input fuse, reverse-polarity stage, and input transient clamp.
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It supplies the loads that must operate from battery voltage:
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- ignition-coil primary supply;
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- injector supply;
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- a protected, scaled battery-voltage measurement for the MCU.
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Ignition, injector, and low-side-output branches require their own protection
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and fault containment. Their high-current returns must not share the sensor or
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MCU return path; all return domains join deliberately at the power-entry
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region.
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## +5V_MAIN
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`+5V_MAIN` is the ECU's primary regulated 5 V rail. It is generated directly
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from `VBAT_PROT` by a synchronous buck converter and powers internal 5 V
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circuitry, including automotive ICs that require 5 V logic or supply voltage.
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It is the upstream rail for `+5V_AUX` and `+5V_SENS`. External
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harness-connected loads shall not be connected directly to `+5V_MAIN`; they
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use their separately protected branch.
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## +5V_SENS
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`+5V_SENS` is derived from `+5V_MAIN` through an eFuse or protected load
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switch, followed by a ferrite bead and local filtering. It is not shared
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directly with external digital-output loads.
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It provides a controlled, protected 5 V excitation supply for ratiometric
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pressure sensors and other sensors that require 5 V. The rail shall include a
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sensor-facing protection/current-limit stage so a harness short cannot bring
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down `+5V_MAIN` or the ECU logic supply. Its voltage should be monitored by
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the MCU.
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The ferrite bead and local filtering isolate sensor excitation from switching
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and digital noise on `+5V_MAIN`, while the protected branch keeps a
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sensor-harness fault contained to the sensor domain.
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## +5V_AUX
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`+5V_AUX` is derived from `+5V_MAIN` through an eFuse or protected load switch.
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It supplies external 5 V digital logic-level output circuits, such as a
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starter-enable command to a separate external control module.
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This rail is distinct from `+5V_SENS` so external digital loads and harness
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faults cannot disturb sensor excitation. The protection stage shall isolate a
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short on `+5V_AUX` without inhibiting `+5V_MAIN` or the rest of the ECU. It
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shall include appropriate output protection and current limiting for the
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external interface.
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`+5V_AUX` is not an actuator supply. If a starter-enable or other command must
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drive a relay or another inductive load, it shall use an appropriate protected
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low-side sink output instead of a 5 V logic-level output.
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## +3V3_MAIN
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`+3V3_MAIN` is generated directly from `VBAT_PROT` by a dedicated synchronous
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buck converter. It supplies the STM32H747 digital supply pins and the 3.3 V
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digital loads, including logic-side communications circuitry.
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The main 3.3 V rail shall not be made by an LDO from either 5 V rail. The H747
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and supporting logic can create a substantial 3.3 V load; an LDO would
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dissipate the difference between 5 V and 3.3 V as heat. Generating 3.3 V from
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`VBAT_PROT` also avoids placing the MCU's current demand on `+5V_MAIN`.
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`+3V3_MAIN` must never be derived from `+5V_SENS`, since that would couple the
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logic load to the sensor supply.
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## +3V3_ANA
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`+3V3_ANA` is derived from `+3V3_MAIN` through a ferrite bead and local
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filtering. It supplies the MCU analogue supply/reference domain and analogue
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front-end circuitry.
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The initial implementation should reserve an optional low-noise LDO footprint
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for this rail. An LDO should be fitted only if analogue-noise testing shows it
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is necessary; it is not the primary 3.3 V regulator. The analogue rail and its
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decoupling must follow the STM32H747 supply and ADC layout guidance.
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## Ratiometric 5 V Sensor Measurements
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The MCU ADC reference is `+3V3_ANA`; it is not expected to be precision-trimmed
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to track `+5V_SENS`. Therefore 5 V ratiometric sensor readings are made
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ratiometric in measurement and firmware rather than by requiring a fixed
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relationship between the rails.
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Firmware shall calculate the sensor signal as a ratio of the two ADC readings:
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```text
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sensor fraction = ADC(sensor output) / ADC(+5V_SENS sense)
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```
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Since the two channels use the same ADC reference, the reference-voltage error
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and most common gain error cancel. Divider ratio and temperature drift are also
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minimised by using matched, low-drift resistor networks for the sensor-output
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and 5-V-sense dividers.
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Design requirements:
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- The divider ratio must keep both channels within the ADC input range with
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margin for the maximum sensor-supply tolerance. A nominal ratio near 0.55
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maps 5.25 V to approximately 2.9 V.
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- Use corresponding filter characteristics on the sensor-output and 5-V-sense
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channels and sample them close together.
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- Apply normal input protection and anti-alias filtering without defeating the
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intended ratiometric measurement.
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- A failed or shorted sensor supply must be detectable from the sensed 5 V
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reading.
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## Thermistor Measurements
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Thermistor dividers are normally excited from `+5V_SENS`, consistent with a
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conventional automotive ECU sensor interface. The thermistor itself is passive
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and would operate with a 3.3 V excitation, but using the protected 5 V sensor
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rail gives a common, diagnosable excitation supply for external sensors.
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Firmware shall use the ratio of the thermistor-channel and 5-V-sense ADC
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readings to calculate the thermistor resistance. This cancels variation in
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both `+5V_SENS` and the `+3V3_ANA` ADC reference. Pull-up resistance must be
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chosen to limit thermistor self-heating while providing sufficient measurement
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resolution over the required temperature range.
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## +12V_SENS
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The crank and cam sensors are currently described as 12 V, active-low,
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open-collector-style Hall-effect sensors. They are supplied from `+12V_SENS`.
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`+12V_SENS` is a regulated 12 V buck-boost rail generated from `VBAT_PROT`.
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It provides a stable supply when the 4S LiPo is above or below 12 V and may be
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used by future 12 V sensor circuits in addition to the crank and cam sensors.
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The rail shall include a sensor-facing current-limit or protected high-side
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switch, filtering appropriate to the harness, and local decoupling at the
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sensor connector. Its voltage accuracy, ripple, current capability, and
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current-limit threshold must be specified from the selected Hall sensors and
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the future 12 V sensor budget.
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Hall outputs shall not enter the MCU at the sensor supply voltage. Their input
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conditioning shall provide protection, noise rejection, and a defined
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logic-level conversion to protected 3.3 V timer inputs. Output pull-up and
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level-shift details are to be selected with the final sensor wiring and output
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type.
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## Grounding and Layout Intent
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- Keep ignition, injector, and starter-switch current returns separate from
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sensor and MCU ground returns.
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- Join these return domains deliberately near the input/power-entry region.
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- Keep switching-regulator hot loops compact and away from trigger and ADC
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signal paths.
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- Route sensor supply and sensor-return paths as controlled pairs to the
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connector where practical.
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- Place the analogue front end near its ADC connections and isolate it from
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ignition and injector switching nodes.
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## Open Items Before Schematic Freeze
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1. Confirm the current budget for each rail, including all expansion I/O.
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2. Confirm the Hall-sensor part numbers, current, output type, cable lengths,
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and pull-up requirements; establish the current budget for future
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`+12V_SENS` loads.
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3. Select the input transient and reverse-polarity ratings after the battery,
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wiring, and enclosure arrangement are known.
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4. Confirm sensor-supply accuracy, current limit, and diagnostic requirements.
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5. Validate ADC noise and ratiometric accuracy on hardware before deciding
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whether the optional `+3V3_ANA` LDO is needed.
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