Temperature Sensor

A temperature sensor on an integrated circuit is an on-die measurement circuit that monitors the local junction temperature at specific locations on the chip — providing critical data for thermal management, throttling decisions, and reliability protection.

Why On-Die Temperature Sensing?

• Thermal Limits: Every chip has a maximum junction temperature ($T_{j,max}$, typically 105–125°C). Exceeding this causes reliability degradation and eventual failure.
• Hot Spots: Temperature is not uniform across the die — active areas (CPU cores, FPUs) can be 10–30°C hotter than inactive regions. External package sensors miss these hot spots.
• Dynamic Behavior: Temperature changes rapidly during workload transitions — only on-die sensors can track these fast transients.

Temperature Sensor Types

• BJT (Bipolar Junction Transistor) Based: The most accurate and widely used on-die sensor.
• Uses a parasitic PNP or NPN transistor available in CMOS (substrate PNP or vertical NPN).
• The base-emitter voltage $V_{BE}$ is temperature-dependent: $V_{BE} \propto -2$ mV/°C.
• PTAT (Proportional To Absolute Temperature): Difference of $V_{BE}$ at two different current densities: $\Delta V_{BE} = (kT/q) \cdot \ln(N)$ where $N$ is the current density ratio.
• Combined PTAT and CTAT (Complementary TAT) signals yield accurate, linear temperature readings.
• Accuracy: ±1–3°C after calibration.

• Ring Oscillator Based: A ring oscillator whose frequency varies with temperature.
• Simple, all-digital implementation.
• Frequency decreases as temperature increases (at typical operating voltages).
• Less accurate (±5–10°C) but easy to integrate and requires no analog circuits.

• Thermal Diode: A diode-connected transistor whose forward voltage varies with temperature.
• Often used for external readout — the thermal diode is the sensor, and an external IC reads it.
• Standard interface supported by most thermal management ICs.

Temperature Sensor Architecture

• Analog Front-End: The temperature-sensitive element (BJT, diode) produces a voltage proportional to temperature.
• ADC: Digitizes the analog temperature voltage — SAR or sigma-delta ADC, typically 10–12 bits.
• Digital Output: Temperature reading available as a digital value to the power management unit (PMU) or system software.
• Threshold Comparators: Hardware comparators that trigger interrupts when temperature exceeds programmed thresholds — enables immediate thermal throttling without software intervention.

Thermal Management Actions

• Throttling: Reduce clock frequency or inject idle cycles when approaching $T_{j,max}$ — reduces power dissipation.
• DVFS: Lower voltage and frequency to reduce heat generation.
• Fan Control: Adjust cooling fan speed based on die temperature.
• Emergency Shutdown: Hard shutdown if temperature exceeds critical limit — prevents permanent damage.

Temperature sensors are an essential safety and optimization feature of every modern processor — they prevent thermal damage and enable intelligent power management that maximizes performance within thermal constraints.