220.127.116.11 Voltage References
18.104.22.168.1 Voltage Reference Fundamentals
A wide variety of voltage references are available today. The most common ones are based on the action of either a zener diode or a bandgap cell with additional circuitry included to obtain good temperature stability. Although discrete zener diodes are available in voltage ratings as low as 1.8 V to as high as 200 V, with power-handling capabilities in excess of 100 W, their tolerance and temperature characteristics are unsuitable for many
applications. Therefore discrete zener diode based references have additional circuitry to improve performance. One commonly used version is the temperature-compensated zener diode, particularly for voltages above 5 V.
The operation of a bandgap reference is based on specific characteristics of diodes operating at the same current but at different current densities. Bandgap references are available with output voltage ratings of about 1.2 V to 10 V. The principal advantage of these devices is their ability to provide stable low voltages such as 1.2, 2.5, or 5 V. However, bandgap references of 5 V and higher tend to have more noise than equivalent zener-based references. This is due to the fact that in bandgap references, higher volt-ages are obtained by amplification of the 1.2 V bandgap voltage by an internal amplifier. Their temperature stability is also below that of zener-based references.
In the commercial domain of semiconductors, there are several options today for voltage references such as (a) zener diodes, (b) buried zener diodes, (c) bandgap-based devices, and (d) XFET™ and FGA™. A comprehensive account of these technologies is available in .
22.214.171.124.2 Reverse-Biased Diode-Based Voltage References
The most common and simple way to achieve a reference source is to use a reverse-biased diode, or a zener diode as it is commonly called, where it enters into a voltage breakdown region. A zener diode has two distinctly different breakdown mechanisms: zener breakdown and avalanche breakdown. The zener breakdown voltage decreases as the temperature increases creating a negative temperature coefficient (TC). The ava-lanche breakdown voltage increases with temperature (positive TC). This is illustrated in Figure 1.8. The zener effect dominates usually below 5 V, and the avalanche effect dominates above 6 V. By the use of additional diode (in forward-biased mode) in series with an avalanche-type diode, it is possible to achieve a better temperature stability in a reference circuit .
126.96.36.199.3 Bandgap References
This is one of the popular solutions to achieve a very stable reference source in a regulator circuit. The concept behind this circuit is to have two base emitter junctions operating at
different current densities  where temperature compensation can be easily maintained. A circuit diagram of a bandgap reference is shown in Figure 1.9. This circuit, developed by Paul Brokaw, is called Brokaw bandgap circuit. Transistors Q1 and Q2 are operating at the same current but at different current densities. This is achieved by fabricating Q2 with a larger emitter area than Q1. Therefore the base-emitter voltages of the two transistors are different. This difference is dropped across R2 . Extrapolated to absolute zero, VBE is equal to 1.205 V, the bandgap voltage of silicon, and has a predictable, negative temperature coefficient of –2 mV/°C. By adding a voltage to VBE that has a positive temperature coefficient, a bandgap reference can, at least theoretically, generate a constant voltage at any temperature.The base-emitter voltage difference is given by
where J1 and J2 are the current densities of transistors Q1 and Q2 respectively. Since the sum of the two transistor currents flows through R1, the voltage across R1 can be expressed as
Using Equations (1.28) and (1.29),
Therefore V2 is the sum of VBE and the scaled ∆VBE. It is shown that if ratio of the emitter areas of the two transistors is eight, the temperature coefficients of VBE and ∆VBE cancel each other. The op-amp raises the bandgap voltage V2 to a higher voltage at the output of the reference. There are many variations of this basic circuit in commercial bandgap references by Analog Devices Inc., USA, and readers can refer to their applica-tion notes for details.
Bandgap references typically provide voltages ranging from 1.2 V to 10 V. The advan-tage of bandgap references is their ability to provide voltages below 5 V. The greatest appeal of bandgap devices is the ability to function with operating currents from mil-liamps down to microamps. Commercial IC bandgap references have additional features such as multiple calibrated voltages. Because most bandgap references are constructed in monolithic form, they are relatively inexpensive. However, their temperature coef-ficient could be sometimes inferior to that of temperature -compensated zener-based references. This is due to second-order dependencies of ∆VBE on temperature.
188.8.131.52.4 Buried-Zener References
The above two types of common reference sources have their own advantages and dis-advantages. Another development to compete with disadvantages of these types was the buried-zener reference where some process improvements were used to get a lower noise and improved stability. Figures 1.10(a) and 1.10(b) depict some comparison of the device structure in relation to a regular zener diode. The device comes with a heating element to stabilize the temperature as shown in the commercial example of LM199 from National Semiconductor. Due to the difference in construction, it has achieved far superior performance, which can be summarized by,
- Very low initial error, between 0.01% and 0.05%
- Ultralow temperature coefficient, from 0.05 to 10 ppm/°C
- Ultralow noise level of less than 10 µV peak to peak, in the frequency band of 0.1 to 10 Hz
- Long-term stability of typically less than 25 ppm/1000 hours
More details can be found in , with historical developments occurring in Silicon Valley, USA. This will also provide more details on other state-of-the-art devices such as the XFET™ and Intersil/Xicor FGA™ types. Reference  provides some comparison of zener device families and bandgap families commonly available. Appendix A provides an overview of XFET™.
184.108.40.206.5 Quality Measures of Voltage References
An ideal voltage reference would have the exact specified voltage, and it would not vary with time, temperature, input voltage or load conditions. However, as it is not possible to fabricate such ideal references, manufacturers provide specifications informing the user of the device’s important quality parameters.
220.127.116.11.5.1 Output Voltage Error This is the initial untrimmed accuracy of the refer-ence at 25°C at a specified input voltage. This is specified in millivolts or a percentage. Some references provide pin connections for trimming their initial accuracy with an external potentiometer.
18.104.22.168.5.2 Temperature Coefficient The temperature coefficient of a reference is its average change in output voltage as a function of temperature compared with its value at 25°C. This is specified in ppm/°C or mV/°C.
22.214.171.124.5.3 Line Regulation This is the change in output voltage for a specified change in input voltage. Usually specified in %/V or µV/V of input change, line regulation is a measure of the reference’s ability to handle variations in supply voltage.
126.96.36.199.5.4 Load Regulation This is the change in output voltage for a specified change in load current. Specified in µV/mA, %/mA, or ohms of DC output resistance, load regulation includes any self-heating effects due to changes in power dissipation with load current.
188.8.131.52.5.5 Long-Term Stability This is the change in the output voltage of a reference as a function of time. Specified in ppm/1000 hrs at a specific temperature, long-term stability is difficult to quantify. As a result, manufacturers usually provide only typical specifications based on device data collected during the characterization process.
184.108.40.206.5.6 Noise Although the above are the most important quality parameters of a voltage reference, noise is particularly of importance in certain applications such as A/D or D/A converters. In such applications, the noise from the reference should be less than 10% of the LSB value of the converter. Therefore the higher the resolution of the con-verter, the lower should be the noise generated from the reference. Noise depends on the operating current of the reference, and is generally specified over a particular bandwidth and for a particular current. The specified bandwidths are 0.1–10 Hz (low-frequency noise) and 10 Hz–10 kHz (high-frequency noise).