Generating Stable Voltage For Multiple Applications

Voltage regulators and voltage references are essential to most electronic circuits, whether analog, digital, or a combination of the two. Although these circuits can appear deceivingly simple, much experience in worst-case analysis has shown them to also be the source of a large percentage of problems or issues as part of larger circuit designs. Often, these issues are due in part to a lack of understanding of the actual complexities of these seemingly simple circuits. Another major contributor is a general lack of data from integrated-circuit (IC) manufacturers—data that is critical to completing a successful and robust design.

First, there is some confusion about the types and descriptions of different devices commonly used to provide a stable voltage source. A reference diode is a two-port element that does not provide a voltage by itself. The reference diode is either a Zener diode or bandgap shunt regulator, requiring an external current source. There is no significant difference between a reference diode and a Zener diode, other than the fact that a reference diode is generally a temperaturecompensated device and is often available with tighter initial tolerances than Zener diodes. A Zener diode allows current flow not only in the normal forward direction but also in the reverse direction when the applied voltage is larger than the breakdown voltage (or Zener knee voltage). A conventional diode will not allow significant current flow if it is reverse biased below its reverse breakdown voltage.

A voltage reference is a three-port device that provides a precision output voltage when an appropriate input voltage is connected. The reference is typically capable of either sourcing or sinking current, often to approximately 10 to 20 mA. Adding to the confusion, there are devices that offer operation as either a two-port reference diode or as a threeport reference device.

A linear voltage regulator is a threeport device that provides a regulated output voltage when an appropriate input voltage is connected. The regulator is generally capable of sourcing but not sinking current, and is usually designed for much higher output current than a reference. Typical current capabilities are from 500 mA to several amperes, though some regulators have been developed with capability exceeding 100 A.

The key performance metrics for a regulated voltage source are:

• ripple and noise,
• power supply rejection ratio (PSRR),
• absolute voltage accuracy,
• temperature coefficient,
• output current capability,
• input voltage range,
• control loop stability,
• output impedance, and
• sink current and source current.

It may be obvious upon inspection that a subset of this list may be of interest to any one discipline, but no single discipline will be concerned with the entire list. For example, a designer of battery-powered equipment will be concerned about operating current but is not likely to be concerned with ripple rejection. An analogto- digital-converter (ADC) designer is concerned with noise, absolute accuracy and, depending on the application, PSRR. An RF circuit designer is not generally concerned with absolute accuracy, but is very concerned about output noise and ripple rejection. Many of these designers may be concerned with output impedance or the manifestations resulting from the output impedance variations. For example, a logic designer needs to be concerned with the impact of large dynamic currents. ADCs often present dynamic current loads on the reference, although much smaller than in highly integrated logic systems or computers. The performance requirements for each of these regulated sources are dependent on the discipline to which they are applied. There is not a “one size fits all” solution.

A Zener diode or reference diode must be fed from a current source to properly regulate the voltage across the device. The current for which the device is designed to provide the specified voltage is different for each diode, but is always specified by the parameter I_zt. At this operating current, the device provides a specified temperature coefficient and specified impedance. At other operating currents, these parameters will be different than when working at I_zt. In the case of precision reference diodes, the device is generally temperature compensated and the accuracy of the compensation is often provided for different operating currents.

Since the diode has a specified impedance, the determination of PSRR is easily calculated as a voltage divider, created by the source resistor and the diode impedance. In 1970, Kent Walters, then at Motorola and more recently at Microsemi, received a patent for a circuit using a current-limiting diode (CLD) in conjunction with a precision, temperature-compensated Zener diode to provide an accurate voltage reference over a wide range of input voltage and operating temperature conditions. The benefit of the CLD is that the impedance is much higher than the equivalent resistor, resulting in greatly improved PSRR, as well as greatly reduced input power when used in a wide voltage range application.

There are several things to note about the capabilities of the simple reference circuit shown in Fig. 1:

• The PSRR can be very good, limited primarily by the Zener diode impedance and the capacitance of the CLD.
• Since the circuit does not include a feedback loop, it is unconditionally stable, allowing any value of capacitor to be placed on the output.
• The circuit can sink or source current, limited by the value of the CLD and the power capability of the Zener diode; however, the circuit will only provide precision temperature compensation at a single current, which results in the Zener diode current being equal to Izt.
• The voltage of the device cannot be adjusted, other than by selection of the Zener diode.

To overcome some of these short- comings, the Zener diode is buffered, which is most commonly accomplished by the addition of a transistor, as shown in Fig. 2, or an operational amplifier, as shown in Fig. 3. The addition of the buffer allows the Zener reference diode to operate at a fixed current, while the operational amplifier allows the reference to either sink or source an increased output current. The use of a transistor buffer increases the output source current, but cannot sink current. The maximum output current is typically in the range of +10 to +20 mA. Although the addition of the buffer does allow a much greater range of output current while maintaining the Zener reference diode at a precise current (I_zt), several new comments apply:

• The addition of an opamp to the circuit introduces a feedback loop and so it is simple to cause the reference to have poor stability or even to oscillate in response to the addition of output capacitors.
• The PSRR is severely degraded, since the operational amplifier or transistor has a finite and generally poor PSRR compared with the CLD and Zener reference diode combination. This is especially true for frequencies above several kilohertz.
• It is common for the output impedance of the buffered circuit to be much greater than that of the CLD and Zener reference diode combination, especially at frequencies above several kilohertz.
• The operational amplifier adds some additional tolerances, such as VOS and additional noise terms.
• The output voltage is easily adjusted or trimmed, by resistors, during manufacture.

Voltage regulators are three terminal devices that include a voltage reference, which can be a reference diode type device or a band-gap voltage reference, an error amplifier, and a power stage. Voltage regulators often include additional functions such as current limit, soft-start, and over-temperature protection, to name a few. The typical characteristics of a voltage regulator are summarized as follows:

• The tolerance of the voltage reference is generally not as good as that of a reference.
• The output noise is generally much higher than that of a reference.
• The voltage regulator can generally source current, but not sink current.
• The voltage regulator is very sensitive to load capacitance and can be easily destabilized or made to oscillate due to selection of output filter components. The stability criteria are often neglected from datasheets and many devices do not have external compensation capability.
• The stability is dependent on operating conditions, such as input voltage and load current.
• The voltage regulator is capable of much greater output currents than a reference. Reference diodes, voltage references, and voltage regulators are not functionally interchangeable. Each device has benefits and drawbacks that must be evaluated on a case-by-case basis. Different disciplines require different performance characteristics, while component manufacturers tend to provide a “one-sizefits- all” solution. The end user often adds elaborate filtering and external circuits in order to obtain “improved” performance. Owing to insufficient data and a less than complete understanding of the intricacies of the devices, intended improvements often create performance issues that are difficult, and sometimes impossible, to correct.

Reference
1. Cecil Kent Walters, United States Patent 3,549,988, “Temperature Compensated Reference Voltage Circuitry Employing Current Limiters and Reference Voltage Diodes,” filed Jan. 2, 1968, granted Dec. 22, 1970.