The ideal voltage regulator maintains constant output voltage despite varying input voltage, load current, and temperature conditions. Realistically, these influences affect the regulator’s output voltage. In addition, the regulator’s own internal inaccuracies affect the overall circuit performance. This section discusses the major error contributors, their effects, and suggests some possible
solutions to the problems they create.
REGULATOR REFERENCE TECHNIQUES
There are several reference techniques employed in integrated circuit voltage regulators. Each provides its particular level of performance and problems. The optimum reference depends on the regulator’s requirements.
Zener Diode Reference
The zener diode reference, as illustrated in Figure 5-6, is the simplest technique. The zener voltage itself, Vz, forms the reference voltage, VREF.
This technique is satisfactory for relatively stable supply voltage and load-current applications. The changing zener current results in a change in the zener diode’s reference voltage, Vz. This zener reference model is illustrated in Figure 5-7.
Constant-Current Zener Reference
The zener reference can be refined by the addition of a constant-current source as its supply. Driving the zener diode with a constant current minimizes the effect of zener impedance on the overall stability of the zener reference. An example of this technique is illustrated in Figure 5-8. The reference voltage of this configuration is relatively independent of changes in supply voltage and load current.
In addition to superior supply voltage independence, the circuit illustrated in Figure 5-8 yields improved temperature stability. The reference voltage, VREF, is the sum of the zener voltage (Vz) and the base-emitter voltage of Q1[VBE(Q1)]. A low temperature coefficient can be achieved by balancing the positive temperature coefficient of the zener with the negative temperature coefficient of the base-emitter junction of Q1.
Another popular reference is the band-gap reference, which developed from the highly predictable emitter-base voltage of integrated transistors. Basically, the reference voltage is derived from the energy-band-gap voltage of the semiconductor material [Vgo(silicon) = 1.204 V]. The basic band-gap configuration is illustrated in Figure 5-9. The reference voltage, VREF, in this case is:
The resistor values of R1 and R2 are selected in such a way that the current through transistors Q1 and Q2 are significantly different (I1 = 10I2). The difference in current through transistors Q1 and Q2 also results in a difference in their respective base-emitter voltages. This voltage differential [VBE(Q1) – VBE(Q2)] will appear across R3. Application of transistors with sufficiently high gain results in current I2 passing through R3. In this instance, I2 is equal to:
By analyzing the effect of temperature on VREF it can be shown that the difference between two similar transistors’ emitter-base voltages, when operated at different currents is:
Band-gap reference voltage is particularly advantageous for low-voltage applications (VREF = 1.2 V) and it yields a reference level that is stable even with variations in supply and temperature.