27.8 A 4600μm2 1.5°C (3σ) 0.9kS/s thermal-diffusivity temperature sensor with VCO-based readout

Temperature sensors are widely used in microprocessors to monitor on-chip temperature gradients and hot-spots, which are known to negatively impact reliability [1-4]. Such sensors should be: 1) Small to facilitate floor planning; 2) Fast to track millisecond thermal transients and 3) Easy to trim to reduce the associated costs. Recently, it has been shown that thermal diffusivity (TD) sensors can meet these requirements [5]. TD sensors operate by digitizing the temperature-dependent delay associated with the diffusion of heat pulses through an electro-thermal filter (ETF), which, in standard CMOS, can be readily implemented as a resistive heater surrounded by a thermopile. Unlike BJT-based temperature sensors, their accuracy actually improves with CMOS scaling [6], since it is mainly limited by the accuracy of the heater/thermopile spacing, which in turn is determined by lithography. However, the readout circuitry of prior TD sensors has been based on analog phase-domain ΔΣ ADCs, which are not easily ported to low-voltage technologies, and which occupy much more area than the ETF itself [5].