1. Technical Field of the Invention
This invention relates generally to integrated circuits and more particularly to band-gap references.
2. Description of Related Art
Integrated circuits are used in an abundance of electronic devices ranging, for example, from handheld games to computers to communication systems to home appliances and beyond. Integrated circuits can be manufactured using a variety of processes including bipolar, CMOS, gallium arsenide, and silicon germanium. Of these processes, CMOS is the most popular due to its flexibility to support various circuit topologies, its circuit density (i.e., amount of transistors per die area), and its cost. CMOS integrated circuits, however, are not perfect. For instance, the performance of the components fabricated utilizing a CMOS process varies over temperature and also varies from integrated circuit to integrated circuit. Multiple techniques have been developed to compensate for these variations including match component designs, band-gap references, calibration circuits, et cetera.
Band-gap voltage references are used on almost every integrated circuit to provide a fixed reference voltage that does not drift over temperature and may be designed to be process variant independent or process variant dependent. Typically, a band-gap circuit is designed to provide a 1.2 volt reference that does not vary over temperature. This is typically done by taking advantage of the known temperature related properties of CMOS transistors. As is known, a base emitter voltage (VBE) of a CMOS transistor that is emulating a bipolar transistor decreases over temperature. As is further known, the slope of the VBE versus temperature curve varies based on the size of the transistor, where a smaller transistor has a greater slope than a larger transistor. Based on this property, a positive slope difference ratio may be produced over temperature between the two transistors of different sizes. This difference ratio may be scaled to have an equal but opposite slope of the VBE versus temperature curve for the smaller transistor. Utilizing these inversely proportional curves, a temperature independent band-gap voltage reference is achieved.
The band-gap voltage reference can be resistor-independent or resistor-dependent. The resistor-dependent band-gap voltage reference is one that produces a voltage that, from integrated circuit to integrated circuit varies due to process variations inherent in the CMOS integrated circuit fabrication process of producing resistors. Circuits whose operations are resistor-dependent use resistor-dependent band-gap voltage references. For example, an amplifier with resistive loads is a circuit whose operation is resistor-dependent. In particular, the process variations of the resistive load (i.e., the resistor value, for integrated circuit to integrated circuit varies) affect the gain of the amplifier. By utilizing a resistor-dependent band-gap voltage reference for such circuits, the process variations that affect the circuit also affect the band-gap voltage reference in a similar manner such that, from integrated circuit to integrated circuit, the circuit performs in a substantially similar manner.
A resistor-independent band-gap voltage reference is one that, from integrated circuit to integrated circuit, produces a substantially similar voltage reference. Circuits whose performance are not affected by process variations in fabricating resistors, but are dependent on an accurate voltage reference use resistor-independent band-gap voltage references. For example, analog-to-digital converters, digital-to-analog converters and other digital circuits are circuits that use a resistor independent bandgap voltage reference.
Many integrated circuits include circuits whose performance is resistor-dependent and circuits whose performance is resistor-independent. To accommodate both types of circuits, the integrated circuit includes two band-gap references: one that is resistor-dependent and one that is resistor-independent.
A band-gap voltage reference, whether resistor-independent or resistor-dependent, includes at least three stacked transistors per leg, which requires a supply voltage of at least 2.1 volts. Such a restriction presents a significant problem as the CMOS process evolves to allow integrated circuits to be powered from voltage sources of 1.8 volts and below. For these low supply voltage CMOS integrated circuits, the band-gap reference will not operate properly thus will not provide a reliable band-gap voltage reference.
Therefore, a need exists for a low supply voltage band-gap reference that can be extended to supply both a resistor-dependent band-gap reference and a resistor-independent band-gap reference.