5.8 A digitally assisted single-point-calibration CMOS bandgap voltage reference with a 3σ inaccuracy of ±0.08% for fuel-gauge applications

Abstract

Accurate voltage references are key building blocks for almost all electronic systems. Specifically, fuel gauge applications benefit from very high precision references to allow for extremely precise measurement of battery voltage and current in order to provide an accurate measurement of the state of charge of the battery.In this work a digitally assisted single-point-trimmed CMOS bandgap voltage reference is presented. Compared to previous art [1][2][3][4], this work achieves a low inaccuracy of ±0.08% (3σ) from -40°C to +120°C. The residual temperature drift is as low as 7 ppm/°C. The key idea is to keep the analog bandgap core simple and only compensate non-PTAT related effects (like offset) by using chopping techniques. The remaining PTAT and chip-to-chip variations can then be cancelled out using a single-point trim. Compared to [1], this work avoids the need for a bulky analog notch filter by minimizing offset using a DAC and a simple digital calibration loop. Finally, the remaining curvature, temperature drift, and stress effects are compensated in the digital domain by means of a temperature sensor, a stress sensor and a lookup table (LUT). In summary, the high precision of the reference voltage is achieved by reducing the analog portion to a minimum and combining this with digital compensation. As a consequence, the analog output voltage of the reference is not fully compensated (but also not needed in our system).Figure 5.8.1 shows a system overview of the voltage reference in the context of a fuel gauge system. The core of the reference is a CMOS bandgap circuit providing an analog reference voltage, V BG . This voltage is used as reference for the high accuracy ΣΔ-ADC [5], which measures the battery cell voltage (or current via a shunt resistor). Since V BG is compensated for non-PTAT errors only, the remaining error also affects the uncorrected ADC result in the digital domain. Here the remaining errors are compensated using a LUT, which is based on correction values from the measured V BG at room temperature (single-point trim). In addition, the LUT is used to provide curvature correction, since this effect is quite stable for devices in a given technology [7]. During calibration, the analog V BG output is passed via a 1 st -order lowpass filter to an on-chip chopped buffer (not shown) to provide a low impedance signal source to the tester. The temperature is measured by an on-chip temperature sensor with ±0.5°C accuracy. Finally, stress effects caused by packaging or aging are compensated for by using a stress sensor [6] and calculating the final corrected ADC result used by the fuel gauge algorithms. Combining all of these techniques, a single-point calibration at wafer level is sufficient to achieve the required accuracy of the overall system. Figure 5.8.2 shows a block diagram of the used bandgap subsystem. PNP transistors Q 1 and Q 2 and resistors R 1 , R 2 , and R 3 form a classical bandgap structure. The common node of the core is driven by M P1 , which is in common source configuration. Wit...

A Low Power Energy-Efficient Precision CMOS Temperature Sensor †

“…Knowing the die temperature of transistors employed in precision sensor systems is often quite important because this information can be used to mitigate the cross sensitivity of a system to temperature [1,2]. In this manner, temperature sensors have been employed to compensate for the temperature dependence of MEMS resonators [1], to compensate for the curvature in a band-gap voltage reference [2], or in temperature measurements and over-temperature protection directly.…”

“…In this manner, temperature sensors have been employed to compensate for the temperature dependence of MEMS resonators [1], to compensate for the curvature in a band-gap voltage reference [2], or in temperature measurements and over-temperature protection directly. In such systems, the inaccuracy of temperature sensors is a significant component of the total error budget, and thus often limits their ultimate performance.…”

A BJT-Based Temperature-to-Digital Converter With ±60 mK ( $3~\sigma$ ) Inaccuracy From −55 °C to +125 °C in 0.16-μm CMOS

“…In precision systems and sensors, knowing die temperature is often quite important, because it can be used to mitigate their cross-sensitivity to temperature [1]- [5]. Temperature-to-Digital Converters (TDCs) have been used to compensate for the temperature dependency of MEMS resonators [1], [2], cancel the self-heating effect in shunt-based current sensors [3], [4], and compensate for curvature in a band-gap voltage reference [5]. In such systems, the TDCs inaccuracy is a significant part of the total error budget, and thus often limits their ultimate performance.…”

Compensation of Mechanical Stress-Induced Drift of Bandgap References With On-Chip Stress Sensor

“…Since then several papers have continuously improved temperature stability e.g. by curvature-correction techniques [2][3][4][5][6][7][8], so that mechanical stress is indeed becoming a relevant limitation of bandgap references. 1 Microelectronic packages generate large mechanical stress on the semiconductor chip [9].…”

“…1 shows a Brokaw-bandgap circuit which we used to provide a regulated internal supply voltage for analog circuitry of the Hall current sensor. The temperature stable bandgap voltage is given by (2) with V T = k b T/q, Boltzmann's constant k b , elementary charge q, and absolute temperature T of the junction.…”