# Injector Output Module

## Purpose

This module drives one conventional high-impedance/saturated fuel injector. It
uses the STMicroelectronics VNL5050S5-E protected low-side switch
(`VNL5050S5TR-E` for tape-and-reel assembly).

The driver provides an integrated power MOSFET, inductive-load clamp, current
and power limiting, thermal protection, off-state open-load detection, and an
open-drain status output. It is not a peak-and-hold injector driver; a
low-impedance injector requires a different architecture.

## Power path

```text
VBAT_PROT --> injector positive terminal --> injector coil --> VNL5050 DRAIN
                                                                  |
                                                           internal low-side MOSFET
                                                                  |
                                                                SOURCE --> INJ_PGND --> power-entry ground
```

- The injector's positive terminal connects to `VBAT_PROT`; its negative
  terminal connects to `DRAIN`.
- `SUPPLY` connects to `+5V_MAIN`, the internal logic rail. Place local
  decoupling from `SUPPLY` to `SOURCE`, including a nearby 100 nF ceramic
  capacitor.
- The VNL5050S5-E provides a 41-52 V drain-source clamp for inductive turn-off.
  Injector current-decay/closing time must be verified using the actual
  injector inductance and resistance.
- Do not add a conventional flyback diode across the injector; it would slow
  its current decay and closing response.

## Command path

`INPUT` is active high. It receives only the hardware-permitted injector
command defined by the dead-man interlock.

```text
SAFE_INJECTOR_SIG --> 1 kOhm Rprot --> INPUT
                                         |-- 10 kOhm ---------> INJ_PGND
                                         `-- low-capacitance
                                             Zener clamp ------> INJ_PGND
```

```text
SAFE_INJECTOR_SIG = ENGINE_PERMIT AND MCU_INJECTOR_SIG
```

`SAFE_INJECTOR_SIG` is 5 V logic and therefore provides substantial margin
over the VNL5050S5-E's 2.1 V minimum input-high level. The 1 kOhm `Rprot`
resistor follows the manufacturer's protection approach. The 10 kOhm pull-down
is placed on the driver side of `Rprot`; it holds `INPUT` low if the permission
logic is unpowered, disconnected, or during power sequencing.

Fit the low-capacitance Zener clamp directly at the VNL5050S5-E `INPUT` pin,
on the driver side of `Rprot`. Its return is the local `INJ_PGND`/`SOURCE`
reference, not remote `DGND`. Select its clamp voltage to tolerate the valid
5 V command high while limiting coupled positive transients at the power-IC
pin.

When `ENGINE_PERMIT` is removed, `SAFE_INJECTOR_SIG` goes low and turns the
injector output off. This is the hardware shutoff path; firmware also clears
pending injection events when it observes a dead-man release.

## Status diagnostic

`STATUS` is an open-drain diagnostic output. It is read by the MCU through a
1 kOhm protection resistor and a `+3V3_MAIN` pull-up.

```text
+3V3_MAIN pull-up --> MCU-side status node --> 1 kOhm Rprot --> STATUS
```

The 3.3 V pull-up is within the pin rating and provides a safe MCU logic level.
The diagnostic path supports off-state open-load detection and indicates
over-temperature shutdown. The VNL5050S5-E protects against short circuit and
current overload internally, but its single `STATUS` pin does not provide a
separate, unambiguous MCU-readable short-circuit/current-limit indication.

## Ground and layout

Define `INJ_PGND` as the injector module's high-current return domain.

```text
VNL5050 SOURCE --> short, low-impedance INJ_PGND copper --> power-entry ground star
```

- `INJ_PGND` runs directly to the power-entry ground star, where it joins the
  battery return, `IGN_PGND`, `DGND`, and sensor return domains.
- It must not share copper with those other returns before the star point.
- `SOURCE` is both the MOSFET power return and the driver's logic reference;
  unlike the ignition driver, this part has no separate Kelvin-sense ground
  pin. Keep the `SUPPLY`-to-`SOURCE` decoupling loop compact and the command
  and status traces away from the `DRAIN` switching node.
- Provide the drain copper area required by the package thermal guidance.

## Remaining validation before schematic freeze

1. Measure injector coil resistance and obtain its part number/datasheet.
2. Confirm that the injector is high impedance and that its peak current,
   stored energy, and turn-off time fit the VNL5050S5-E limits.
3. Verify output-stage temperature using the actual injector duty cycle,
   board copper area, and enclosure conditions.
4. Validate injector opening/closing timing, diagnostic behavior, and ignition
   noise immunity on hardware.