Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1999-05-24
2001-04-10
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C323S313000
Reexamination Certificate
active
06215353
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a temperature-compensated, solid-state voltage reference.
2. Description of the Related Art
Stable voltage references traditionally called “bandgap references” are commonly used in a wide variety of applications, including telecommunications. These references typically combine a small voltage, which is directly proportional to absolute temperature, with a larger voltage, which has a negative temperature coefficient. The two voltages are produced by two different diodes operating at current densities typically in the range of 10:1. The voltage difference between the two is then amplified by a DC amplifier with a gain that is established by the ratio of a polysilicon resistor divider. The goal of this voltage combination is to produce a substantially constant reference voltage over a wide temperature range.
There are, however, several sources of problems one encounters when trying to realize accurate, stable voltages using these conventional temperature-compensating voltage references. For example, the small difference between the voltages is directly proportional to absolute temperature. This voltage difference is typically only about 60 mV, but it varies approximately 0.2 mV per degree Celsius.
Because the voltage that is proportional to absolute temperature is small, the initial amplifier offset voltage can produce large changes in the reference voltage. For example, a 1.2 mV change in amplifier offset voltage can produce a 10 mV change in the initial reference voltage. In addition, small variations in the amplifier offset voltage or stress-induced changes in the polysilicon resistor divider ratio can produce large changes in the reference voltage.
Other sources of error include small variations in temperature between the diode elements themselves. For example, a temperature difference of less than 0.5° C. between diode elements or amplifier input elements can produce a voltage difference of 1 mV at the reference output. Such errors can be significant, particularly during the time after switching from standby to full power in the chip that incorporates the reference.
A typical design goal for a voltage reference is the production of a device with better than one percent accuracy over a wide temperature range. Despite considerable research effort, only a few manufacturers of integrated circuits have been able to obtain such accuracy. Even where this accuracy has been achieved, however, it has typically been necessary to add complex circuits to the basic design discussed above. This, in turn, has required considerable chip area in order to compensate for several undesirable effects in the basic reference circuit. These circuits commonly include, for example, trimming circuits capable of adjusting the reference voltage after wafer probe and packaging.
In addition to increased chip area, these designs normally also require significant testing time to adjust the reference voltage. Unfortunately, reference voltage adjustments made at the wafer probe do not generally hold through the packaging process. Stress induced by the packaging process typically causes an accuracy in the voltage reference of better than one percent to become worse than one percent after packaging.
What is needed is therefore a voltage reference that provides the desired accuracy not only in theory but in practice, even after packaging, that does not require complex additional circuitry or long testing periods, and that can be implemented using easily calibrated and matched components.
SUMMARY OF THE INVENTION
The invention meets this need by providing a stable voltage reference circuit that has a single reference diode junction, which may be implemented, for example, as a single diode junction or as a junction of a diode-coupled transistor. A current generating arrangement alternately generates and applies to the diode junction a first current and a second current. The second current is larger than the first current, and a voltage over the diode junction thereby alternates between a first AC input voltage (V
1
) that has a positive temperature dependence (dV
1
/dT) and a second AC input voltage (V
2
) that has a negative temperature dependence (dV
2
/dT). Combining circuitry is included for adding the first and second input voltages and for thereby generating an output voltage (Vref) substantially constant with absolute temperature.
The current generating arrangement preferably comprises two different current sources—a first current source that generates the first current and a second current source that generates the second current. A first switch then alternately switches the first and second currents into the single reference diode junction.
The combining circuitry preferably includes an amplifier that has, for the first input voltage, a gain substantially equal to the ratio of the negative temperature dependence divided by the positive temperature dependence. The amplifier is preferably part of an amplification arrangement in which the amplifier is an AC amplifier with input and feedback elements that include a monolithic capacitor network.
The preferred embodiment of the voltage reference circuit includes a capacitor network that determines the gain of the amplifier. This capacitor network preferably includes a first capacitor with a first capacitance (C
1
) in a first signal path for the first input voltage and a second capacitor with a second capacitance (C
2
) in a second signal path for the second input voltage, in which both signal paths lead to a summing junction of the amplification means and the ratio of C
2
to C
1
is equal to the gain of the amplification means.
A second switch is preferably included in the invention and is connected, via the second capacitor, to the summing junction. The second switch alternately connects the summing junction, via the second capacitor, to the first input voltage when the first input voltage is equal to a maximum input voltage and otherwise to a system ground.
In the preferred embodiment of the invention, the voltage reference circuit further has a feedback path from the output voltage (Vref) to a summing junction of the amplification amplifier. A third switch is then preferably included in the feedback path to alternately connect the summing junction, via a third capacitor, to either the output voltage Vref or to circuit ground.
REFERENCES:
patent: 5059820 (1991-10-01), Westwick
patent: 5352972 (1994-10-01), Pernici et al.
patent: 5563504 (1996-10-01), Gilbert et al.
patent: 5867012 (1999-02-01), Tuthill
patent: 5945871 (1999-08-01), Kausel et al.
Cunningham Terry D.
Pairgain Technologies, Inc.
Slusher Jeffrey
Tra Quan
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