Voltage References and Voltage Regulators

Dileeka Dia

1.1 Introduction

Almost all electronic systems utilize a regulated power supply as an essential requirement. Most systems need a precision voltage reference as well. In the past, the task of voltage regulation was tediously accomplished with discrete devices. Today, with integrated circuit voltage references and regulators, this task has been significantly simplified. Not only can an extremely high precision be obtained, but also an extremely high degree of temperature stability.

The performance of today’s electronic devices such as microprocessors, test and measuring instruments, and sophisticated portable and handheld equipment is directly related to the quality of the supply voltage. This results in the need for tight regulation, low noise, and excellent transient response. The designer now has a wide choice of fixed, adjustable, and tracking voltage regulators, with many also incorporating built-in protection features.

One of the fastest growing markets in the world of power regulation is for switching regulators. These offer designers several important advantages over linear regulators, the most significant being size and efficiency. In addition, the ability to perform step-up, step-down, or voltage inverting functions is an attractive feature.

The old linear regulator is not totally out of business. The proliferation of battery-powered equipment in recent years has accelerated the development and usage of low-dropout (LDO) voltage regulators. Compared to a standard linear regulator, the LDO regulator using PNP transistors can maintain its output in regulation with a much lower voltage across it. While the NPN transistor requires about 2 V of headroom voltage to regulate, the LDO typically will work with less than 500 mV of input-to-output voltage differential. This reduced input voltage requirement is advantageous in battery-powered systems, since it translates directly into fewer battery cells (Simpson, 1996). In low-dropout applications, the efficiency advantage of switching regulators no longer is as great. A linear regulator design on the other hand offers several desirable features, such as low output noise and wide bandwidth, resulting in excellent transient response.

This chapter describes the basics of voltage references, linear and switching regulators, and continues to discuss the state-of-the-art components available, the advantages and disadvantages of different types of devices, their application environments as well as the basics of regulator design using these components.

1.2 Voltage References

1.2.1 Voltage Reference Fundamentals

A wide variety of voltage references are available today. However, all base their performance on the action of either a zener diode or a bandgap cell. Additional circuitry is 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. The most popular reference is probably 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 different current densities. Bandgap references are available with output voltage ratings of about 1.2 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 because, in bandgap references, higher voltages are obtained by amplification of the 1.2 V bandgap voltage by an internal amplifier. Their temperature stability also is below that of zener-based references.

1.2.2 Types of Voltage References

1.2.2.1 Zener-Based Voltage References

Zener diodes are semiconductor PN junction diodes with controlled reverse-bias properties, which make them extremely useful as voltage references. The V-I characteristics of an ideal zener diode is shown in Figure 1–1 (a) and a simple regulator circuit based on it in Figure 1-1 (b).

The reverse characteristics show that, at the breakdown point, the knee voltage is independent of the diode current. This knee voltage or the zener voltage is controlled by the amount of doping applied in the manufacturing process. In the simple regulator circuit shown in Figure 1–1 (b), as long as the zener diode is in its regulating range, the load voltage VL remains constant and equal to the nominal zener voltage, even when the input voltage and the load resistance varies over a wide range. If the input voltage increases, the diode maintains a constant voltage across the load by absorbing the extra current and keeping the load current constant. If the load resistance decreases, the extra current required to keep the load voltage constant is facilitated by a decrease in the current drawn by the zener diode.

In the preceding simplified analysis, the temperature dependence of the zener voltage was not taken into account. The stability of the output with temperature is a prime requirement of a voltage reference. Not only does the zener voltage vary with temperature, its variation also depends on the type of breakdown that occurs.

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 avalanche breakdown voltage increases with temperature (positive TC) (Pryce, 1990). This is illustrated in Figure 1–2. The zener effect and the avalanche effect dominate at low and high currents, respectively.

Although, theoretically, it is possible to select the operating point of a zener diode so that the two temperature coefficients will cancel out each other, in practical IC zener-based voltage references, a conventional forward-biased diode is used in series with a zener operating in the avalanche mode. A forward-biased diode has a negative TC, and this cancels the positive TC of the zener diode.

A simple zener-based voltage reference IC is shown in Figure 1–3. In this circuit, R4 provides the startup current for the diodes, thus setting the positive input of the op amp at V2. R3 sets the desired bias current for the diodes. Manufacturers set the output voltage to a value different from that of V2 through the ratio R1 to R2. By trimming this resistor ratio, the output voltage can be set to the desired accuracy. Also, by trimming R3, the bias current can be optimized to a point where a minimum TC is obtained.

TC specifications as low as 1 ppm/°C are possible with zener-based voltage reference ICs (Pryce, 1990).

1.2.2.2 Bandgap References

Similar to zener-based references, bandgap references also produce the sum of two voltages having opposite temperature coefficients. One voltage is the forward voltage of a conventional diode (the base-emitter junction of a transistor), which has a negative temperature coefficient. The other is the difference between the forward voltages of two diodes with the same current but operating at two current densities. A circuit diagram of a bandgap reference is shown in Figure 1–4.

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 Ql. Therefore, the base-emitter voltages of the two transistors are different. This difference is dropped across R2.

Extrapolated to absolute 0, 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, which has a positive temperature coefficient, a bandgap reference, at least theoretically, can generate a constant voltage at any temperature.

The base-emitter voltage difference is given by

VBE=kTqlnJ1J2

Where J1 and J2 are the current densities of transistor Q1 and Q2, respectively. Since the sum of the two transistor currents flow through R1, the voltage across R1 can be expressed as

1=2R1R2ΔVBE

Also,

2=VBE+V1

Usinig 1.2 and 1.3,

2=VBE+2R1R2ΔVBE

Therefore, V2 is the sum of VBE and the scaled ∆VBE Knapp (1998) shows that, if the emitter areas of the two transistors is eight, the temperature coefficients of VBE and ∆VBE...