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

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 single-input deadman interlock.

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 the single DEADMAN_STATUS input deasserts.

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.

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.

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

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

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