# ADOM custom electronic load: R0 architecture

Design in progress; no PCB or fabrication release yet.

## Proposed envelope
25 W continuous, 24 V maximum and 3 A maximum, simultaneously constrained. Initial DUT is the TPS54202 5 V / 1 A buck molecule. Ambient target 0–40 C is provisional. Cooling must establish the actual derating curve. Current setting target is 1 mA increments; calibrated accuracy target ±(0.2% reading + 1 mA), subject to full error budget and bench verification. These are design targets, not achieved specifications.

## Power and control
DUT+ -> fuse / reverse-protection path -> linear MOSFET -> four-terminal low-side shunt -> DUT-. An analog op-amp loop controls shunt voltage from a precision DAC. MCU supervises setpoints and measures voltage/current; the analog loop controls current continuously. USB CDC is primary, a 3.3 V UART is provisioned on machine contacts for future automatic wiring. Do not route USB D+/D- as arbitrary pincer wires.

Auxiliary 5 V from PS2 supplies logic and fan independently of DUT. A small boost stage supplies the MOSFET gate amplifier. USB power-domain and backfeed protection must be settled before schematic freeze. No galvanic isolation is claimed; DUT return, interface return and fixture grounds must be reviewed.

## Candidate parts
- IXTP64N10L2: TO-220 linear-rated MOSFET, 100 V, DC SOA graph at case 75 C, thetaJC max 0.35 C/W. Source: Littelfuse datasheet revision XZ 10/09/2023, pages 2 and 6. Mouser 300 on hand at $23.98 single / $14.07 at 100, retrieved 2026-10-07.
- DAC60501ZDGSR: 12-bit DAC, zero-scale reset, internal reference, same package family as the more costly DAC80501. Candidate $3.49 single. Resolution is not accuracy.
- OPA189IDBVR: precision zero-drift current servo candidate, max 3 uV offset at 25 C. Supersedes OPA197 candidate to improve low-current error.
- INA226: shunt/bus telemetry, ±81.92 mV shunt range. Exact Mouser stocked tape variant is INA226AIDGST; do not silently substitute packaging in purchasing.
- STM32G0B1KCT6: native USB, UART, I2C, timers and watchdog; industrial temperature variant. Pin multiplexing still to be checked against the manufacturer drawing.
- TPS61040: low-power boost candidate for gate-drive supply.
- Sunon MF40200V1-1000U-G99: candidate 40x40x20 mm 5 V three-wire tachometer fan. Heatsink/duct performance is not inferred from free-air CFM.

## Independent protection requirements
Gate defaults OFF without MCU power or during reset. A hardware watchdog and over-current/over-temperature shutdown must clamp the gate independently of software. Fan tach timeout, remote command lease expiry and implausible sensors latch OFF in firmware; explicit re-arm required. Reverse polarity, shorted pass transistor, connector current capability, auxiliary brownout and inductive lead interruption require analysis. A gate clamp cannot disconnect a failed-short MOSFET.

## Manufacturing
AIFlow loaded adom-3rdpartyfab-2L: 1.6 mm board, 1 oz copper. Apply conservative general 0.20 mm isolation, 0.25 mm signals and 0.60/0.30 mm via pad/drill unless a justified footprint escape needs otherwise. Power conductors sized separately. The pack comments and executable rules disagree on minimum via annulus (0.25/0.15 diameter implies 0.05 mm); the proposed via provides 0.15 mm annulus. Pack edge/clearance rules cover only tracks, so project-wide pad/zone checks must supplement them.

## Evidence boundaries
Research and a public datasheet-review clip are captured. Native KiCad 10 and shared service are available. Fusion app-state query timed out; no existing document was changed. AIFlow pre-board timeline is open, but its ordered KiCad steps require component/library/model proofs before the test-plan step. Design work is tracked as an explicit design stage; no gate has been bypassed.