1. Field of the Invention
The present invention relates to a bandgap voltage and current reference generator circuit wherein two inter-dependent feedback loops around a current-output comparator are used to simultaneously generate voltage and current references, thereby supporting operation down to very low supply voltages.
2. Discussion of the Prior Art
Bandgap references are well known for obtaining a reference voltage that is relatively constant over a substantial temperature range. The basic concept is to combine two potentials, one having a positive temperature coefficient and one having a negative temperature coefficient. The sum of these two potentials is made equal to the semiconductor bandgap potential extrapolated to absolute zero temperature. For silicon, this value is close to 1.2 volts.
Typically, the negative temperature coefficient potential is obtained from a forward-biased PN junction, i.e., the emitter-base junction in a conducting transistor operated at a current that will produce a voltage drop of about 600 mV at 300.degree. K. This voltage has a negative temperature coefficient of about 2 mV/.degree.C. The positive temperature coefficient is obtained from a .DELTA.V.sub.BE -producing circuit that develops a 600 mV potential at about 300.degree. K. This voltage has a positive temperature coefficient of about 2 mV/.degree.C. Thus, when these two voltages are combined at 300.degree. K., a 1.2 V potential is produced with close to zero temperature coefficient.
The .DELTA.V.sub.BE potential is typically produced by operating a pair of transistors at substantially different current densities. This can be done by ratioing the transistor areas and passing equal currents, or by using matched area devices and ratioing the currents. If desired, a combination of transistor size and current ratioing can be employed. The low-current-density transistor includes a series resistor. The two devices are equivalently connected in parallel so that the differential voltage drop (.DELTA.V.sub.BE) appears across the resistor. Typically, at 300.degree. K. and a current-density ratio of 10, the .DELTA.V.sub.BE will be about 60 mV. This value, when multiplied by 10, produces a voltage of about 600 mV having a positive temperature coefficient.