Design Review: Three Ship-Blocking Faults Caught Before Fab
Description
A client submitted a completed mixed-signal board for an independent review before releasing it to fabrication. The design combined stepper motor drive, switching power conversion, digital isolation, and an audio output stage — four subsystems that each impose different constraints on the same board.
The board was considered finished. The review found three separate faults, each of which would have shipped as a field failure.
Reviewed stack
- DRV8452 stepper motor driver
- TPS54360 wide-input buck converter
- ISO7721 dual-channel digital isolator
- MAX98357 Class-D audio amplifier
- SPH0641 MEMS microphone
Faults Found
1. Switching-frequency resistor miscalculated
The TPS54360 frequency-setting resistor did not produce the intended switching frequency. Left as designed, the converter would have run outside its intended operating point, shifting the noise spectrum into a band the audio stage could not reject and altering thermal behaviour under load.
2. Enable-pin overvoltage
The buck converter enable pin was exposed to a voltage above its absolute maximum rating under a normal input condition — not a fault case. This is the class of error that survives bench testing and fails intermittently in the field, because the damage is cumulative.
3. Digital isolator channel direction reversed
The ISO7721 is a dual-channel isolator with a fixed direction per channel. One channel was wired against its direction of travel, so that signal path would never have functioned. The board would have assembled, powered up, and silently failed on one interface.
Why It Matters
None of these faults are exotic. Each one is a single line in a datasheet that was not cross-checked against the schematic — and each one would have cost a full fabrication and assembly cycle to discover.
A review is measured against the cost of the respin it prevents, not against its own hours.
Deliverables
- Annotated fault list with datasheet references for each finding
- Corrected values and pin assignments
- Severity ranking: ship-blocking vs. improvement
- Review notes formatted for handoff to the original design team
Published without client, project name, or location under NDA. Component selection and findings are unchanged.