(A) process variation tolerant on-chip CMOS thermometer for auto temperature compensated self-refresh of low-power mobile dram

Abstract

Smaller transistors mean that capacitors are charged less uniformly, which increases the self-refresh current in the DRAMs used in mobile devices. Adaptive self-refresh using an on-chip thermometer can solve this problem. In this thesis, a PVT tolerant on-chip CMOS thermometer specifically designed for controlling the refresh period of a DRAM will be proposed for low power mobile DRAM. Two types of on-chip CMOS thermometer including a novel temperature sensor is proposed, which is implemented in two different DRAM process technologies integrated into mobile LPDDR2 and LPDDR3 products. The on-chip thermometer incorporating in mobile LPDDR2 chip is fabricated in a 44nm DRAM process with a supply of 1.1V. The sensor has a temperature sensitivity of −3.2mV/°C, over a range of 0°C to 110°C. Its resolution is 1.94°C and is only limited by the 6.2mV step of the associated resistor ladder not by its own design. The high linearity of the sensor permits one-point calibration, after which the errors in 61 sample circuits ranged between −1.42°C and +2.66°C. The sensor has an active area of 0.001725mm2 and consumes less than 0.36μW on average with a supply of 1.1V. To improve the overall performance including ultra-low operation voltage, temperature sensitivity, low power consumption, high linearity regardless of process skew variations and high productivity improved by one point calibration, the folded type on-chip thermometer incorporating in mobile LPDDR3 chip which fabricated in a 29nm DRAM process with a supply of 1.1V and 0.8V will be proposed. This folded type sensor exhibits further upgrading properties such as a temperature sensitivity of −3.2mV/°C@1.1V &−3.13mV/°C @0.8V, over wide range of -40°C to 110°C. Its resolution is 1.85°C@1.1V & 1.98°C@0.8V and is only limited by the 6.2mV step. The more linearity of folded type sensor permits one-point calibration, after which the errors in 494 sample circuits ranged between −1.94°C and +1.61°C. The folded type sensor has an active area of 0.001606mm2 and consumes less than 0.19μW@1.1V & 0.14μW@0.8V on average slightly more than unfolded type sensor.