# Ignition Output Module

## Purpose

This module drives the existing dual-ended, two-pin dumb ignition coil. The
coil primary is battery-fed and switched on its low side by an STMicroelectronics
VBG08H-E smart ignition IGBT (`VBG08HTR-E` for tape-and-reel assembly).

The VBG08H-E integrates the ignition IGBT, high-voltage clamp, coil-current
limit, thermal protection, soft shutdown, and a coil-current threshold flag.
Its 8 A maximum operative coil-current rating is compatible with V1 only after
the final coil's primary characteristics and required dwell current are
confirmed.

## Power path

![Ignition power](../diagrams/ignition-power.svg)

- `HVC` connects to one coil-primary terminal; the other terminal connects to
  `VBAT_PROT`.
- `VS` is supplied from `VBAT_PROT`. Place a 100 nF ceramic and 10 µF local
  capacitor at `VS`, following the VBG08H-E reference circuit.
- `VBAT_PROT` protection must maintain the VBG08H-E within its 5.4-28 V
  operating supply range during normal operation and expected local transients.
- Do not place a conventional flyback diode across the coil primary. The
  VBG08H-E's internal high-voltage clamp provides the rapid primary-current
  collapse required to generate a spark.

## Command and inhibit path

`INP` is active high: a rising edge begins coil dwell and a falling edge
causes the normal spark event.

![Ignition command](../diagrams/ignition-command.svg)

The VBG08H-E input-high requirement is 3 V. A 5 V buffer is used rather than
direct STM32 drive to preserve high-level margin across MCU output tolerance,
temperature, and ignition noise.

`EN` is active low:

```text
SAFE_IGNITION_INHIBIT = NOT(ENGINE_PERMIT)
```

When `ENGINE_PERMIT` is removed by the single-input deadman interlock or the
MCU, `SAFE_IGNITION_INHIBIT` becomes high. If a dwell is active, this invokes
the VBG08H-E soft shutdown so coil current is removed without intentionally
generating a spark. The 1 kOhm resistors on `INP` and `EN` follow the
manufacturer's reference circuit and limit transient current into the device's
control pins.

The command traces shall be short, have a continuous quiet reference, and be
routed away from the `HVC` switching node and coil connector. The VBG08H-E
already includes input filtering; no external RC filter shall be added until
its timing effect has been analysed.

Fit a low-capacitance Zener clamp directly at each `INP` and `EN` power-IC pin,
on the VBG side of its 1 kOhm series resistor. Each clamp returns to the VBG
local Kelvin-ground reference at the `IGN_PGND` joining point, not to remote
`DGND`. The clamp voltage shall tolerate the valid 5 V logic high while
limiting positive transients before the VBG08H-E input-overvoltage threshold.
On `EN`, the protection network must preserve the fail-safe high/inhibit state
and must not create a path that can pull `EN` low during a fault.

## Current-flag diagnostic

`C.F.` is an open-drain output. It is low while coil current is below its
internal threshold and releases when the current reaches the nominal 6.5 A
threshold. The threshold is not externally programmable.

![Ignition flag](../diagrams/ignition-flag.svg)

The pull-up voltage must not exceed 5.5 V; `+3V3_MAIN` is suitable. The signal
is a diagnostic confirmation of primary-current buildup and is not part of the
hardware ignition shutoff path. Firmware shall use it to detect and report an
under-current/late-current condition while retaining a separately defined
maximum dwell time.

## Ground and layout

Define `IGN_PGND` as the ignition module's high-current return domain.

![Ignition ground](../diagrams/ignition-ground.svg)

- `PGND1` and `PGND2` must use equal-length, equal-width, and similarly
  shaped copper paths to the `IGN_PGND` joining point. The VBG08H-E uses these
  paths in its internal current-sensing scheme; imbalance shifts its current
  limit and current-flag threshold.
- The VBG08H-E `GND` pin connects locally to that same joining point through a
  short, separate Kelvin/reference trace. It does not carry coil current.
- Do not separately connect the VBG08H-E `GND` pin to the remote `DGND` plane.
  `IGN_PGND` and `DGND` meet only at the deliberate power-entry ground star,
  preventing parallel ground-return paths.
- Keep the coil-current loop compact and isolate its routing from MCU, ADC,
  trigger-input, and communication traces.

## Remaining validation before schematic freeze

1. Identify the coil and measure/obtain its primary resistance, inductance,
   operating current, and permitted dwell characteristics.
2. Confirm that worst-case stored energy and current remain within the
   VBG08H-E repetitive operating limits at the 16.8 V battery maximum.
3. Verify thermal performance with the selected PCB copper area and enclosure.
4. Validate spark noise, `C.F.` behavior, input margins, and the effective
   engine-permit shutdown behavior on hardware.