What the accuracy numbers mean for a thermal management loop
The TMP421AQDCNTQ1 reports local die temperature and a remote diode-junction temperature over a single I²C/SMBus interface, with 12-bit resolution giving 0.0625°C per LSB. The remote channel is tighter at ±1°C in the 15°C–85°C window, but degrades to ±5°C at the extremes. For a closed-loop fan controller or overtemperature shutdown, the remote accuracy in the normal operating band is the figure that governs the trip threshold margin — the ±5°C at -40°C or 125°C is a survival spec, not a control spec. An engine ECU or transmission controller that sees 105°C on a hot day still has margin before the silicon's absolute maximum junction temperature.
Supply tolerance and power-up sequencing
The part powers up in shutdown mode by default, drawing minimal current until the host sends a one-shot conversion command or a programmable limit is set — this avoids a current spike on a cold-start rail that is still ramping. The SOT-23-8 package (2.9 mm × 1.6 mm body, 0.65 mm pitch) fits a standard four-layer board with one via per signal. The exposed die attach pad on the bottom is not a thermal pad — it is the ground connection, so the PCB copper under the part should be a solid ground plane pour, not a split island.
Active production and supply posture
ROHS3 compliant per the manufacturer.