93 lines
4.0 KiB
Markdown
93 lines
4.0 KiB
Markdown
# Injector Output Module
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## Purpose
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This module drives one conventional high-impedance/saturated fuel injector. It
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uses the STMicroelectronics VNL5050S5-E protected low-side switch
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(`VNL5050S5TR-E` for tape-and-reel assembly).
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The driver provides an integrated power MOSFET, inductive-load clamp, current
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and power limiting, thermal protection, off-state open-load detection, and an
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open-drain status output. It is not a peak-and-hold injector driver; a
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low-impedance injector requires a different architecture.
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## Power path
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- The injector's positive terminal connects to `VBAT_PROT`; its negative
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terminal connects to `DRAIN`.
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- `SUPPLY` connects to `+5V_MAIN`, the internal logic rail. Place local
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decoupling from `SUPPLY` to `SOURCE`, including a nearby 100 nF ceramic
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capacitor.
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- The VNL5050S5-E provides a 41-52 V drain-source clamp for inductive turn-off.
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Injector current-decay/closing time must be verified using the actual
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injector inductance and resistance.
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- Do not add a conventional flyback diode across the injector; it would slow
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its current decay and closing response.
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## Command path
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`INPUT` is active high. It receives only the hardware-permitted injector
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command defined by the single-input deadman interlock.
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```text
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SAFE_INJECTOR_SIG = ENGINE_PERMIT AND MCU_INJECTOR_SIG
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```
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`SAFE_INJECTOR_SIG` is 5 V logic and therefore provides substantial margin
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over the VNL5050S5-E's 2.1 V minimum input-high level. The 1 kOhm `Rprot`
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resistor follows the manufacturer's protection approach. The 10 kOhm pull-down
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is placed on the driver side of `Rprot`; it holds `INPUT` low if the permission
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logic is unpowered, disconnected, or during power sequencing.
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Fit the low-capacitance Zener clamp directly at the VNL5050S5-E `INPUT` pin,
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on the driver side of `Rprot`. Its return is the local `INJ_PGND`/`SOURCE`
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reference, not remote `DGND`. Select its clamp voltage to tolerate the valid
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5 V command high while limiting coupled positive transients at the power-IC
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pin.
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When `ENGINE_PERMIT` is removed, `SAFE_INJECTOR_SIG` goes low and turns the
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injector output off. This is the hardware shutoff path; firmware also clears
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pending injection events when the single `DEADMAN_STATUS` input deasserts.
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## Status diagnostic
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`STATUS` is an open-drain diagnostic output. It is read by the MCU through a
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1 kOhm protection resistor and a `+3V3_MAIN` pull-up.
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The 3.3 V pull-up is within the pin rating and provides a safe MCU logic level.
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The diagnostic path supports off-state open-load detection and indicates
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over-temperature shutdown. The VNL5050S5-E protects against short circuit and
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current overload internally, but its single `STATUS` pin does not provide a
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separate, unambiguous MCU-readable short-circuit/current-limit indication.
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## Ground and layout
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Define `INJ_PGND` as the injector module's high-current return domain.
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- `INJ_PGND` runs directly to the power-entry ground star, where it joins the
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battery return, `IGN_PGND`, `DGND`, and sensor return domains.
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- It must not share copper with those other returns before the star point.
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- `SOURCE` is both the MOSFET power return and the driver's logic reference;
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unlike the ignition driver, this part has no separate Kelvin-sense ground
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pin. Keep the `SUPPLY`-to-`SOURCE` decoupling loop compact and the command
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and status traces away from the `DRAIN` switching node.
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- Provide the drain copper area required by the package thermal guidance.
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## Remaining validation before schematic freeze
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1. Measure injector coil resistance and obtain its part number/datasheet.
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2. Confirm that the injector is high impedance and that its peak current,
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stored energy, and turn-off time fit the VNL5050S5-E limits.
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3. Verify output-stage temperature using the actual injector duty cycle,
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board copper area, and enclosure conditions.
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4. Validate injector opening/closing timing, diagnostic behavior, and ignition
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noise immunity on hardware.
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