Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-11-27
2003-01-14
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S315000
Reexamination Certificate
active
06507179
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to electrical reference voltage circuits and more particularly to methods and apparatus for reducing output voltage ripple in bandgap reference circuits.
BACKGROUND OF THE INVENTION
Reference voltages are used in a wide variety of analog circuits, including wireless communications devices, memory devices, voltage regulators, conversion circuits, and others, to provide steady DC reference voltages. The reference voltages are used for biasing various circuit components, providing references to comparator circuit inputs, and for calibration circuits, and the like. For instance, in designing various analog circuits, such as digital to analog converters, voltage regulators, or low drift amplifiers, it is necessary to establish an independent, stable bias reference. Typically this reference is a voltage, which provides a substantially constant output voltage regardless of changes in input voltage, output current, or temperature, although current references are sometimes used.
Voltage reference circuits are sometimes designed using reference diodes such as Zener diodes, where a reference voltage is established across a biased diode and buffered for use in other circuitry. Over the past several decades, however, so-called “bandgap” voltage reference circuits have been predominantly used rather than Zener diode type approaches, due to superior reference voltage stability with changing temperature. Bandgap voltage reference circuits take advantage of temperature coefficients associated with semiconductor device physical properties so as to provide a reference voltage generally insensitive to thermal variations, at least with respect to first order effects. Such bandgap circuits are well known and many variations are in use, for example, in which second order non-linear effects are addressed or otherwise compensated for in providing stable reference voltages. The physical characteristics of semiconductor devices used to implement bandgap circuit designs are derived from the voltage gap between the conduction band and the valence band of the semiconductor material (e.g., silicon), and hence the term “bandgap” reference.
In a bandgap reference circuit, a signal corresponding to a base-emitter voltage (V
BE
) is summed with a signal corresponding to the difference in base-emitter voltages of two diode-connected transistors (&Dgr;V
BE
) of different emitter sizes in producing a bandgap output reference voltage. The first component V
BE
is known to have a negative temperature coefficient, whereas the latter component &Dgr;V
BE
has a positive temperature coefficient. Thus, the bandgap type reference circuit utilizes predictable temperature drift properties of opposite polarities with appropriate scaling, by which the effects of the two opposite-polarity drifts are made to cancel, resulting in a nominally zero temperature coefficient output voltage level.
In a bipolar transistor, the temperature dependence of the base-emitter voltage drop V
BE
exhibits a negative temperature coefficient of about −2 mV per degree C. Conversely, the temperature dependence of &Dgr;V
BE
between two transistors is proportional to the absolute temperature through the thermal voltage V
T
, with V
T
equal to kT/q, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and q is the electron charge. The &Dgr;V
BE
term accordingly exhibits a positive temperature coefficient, and is sometimes referred to as a Proportional T o Absolute Temperature (PTAT) component. In typical bandgap circuits, one or both of these components, usually voltage signals, are scaled and the scaled signals are then subtracted in order to provide a temperature independent bias voltage, with the opposite polarity temperature coefficients canceling one another. In this manner, bandgap reference circuits compensate the negative temperature coefficient of a bipolar transistor's base-emitter voltage, V
BE
, with the positive temperature coefficient of the thermal voltage V
T
associated with the difference in base-emitter voltages of two diode-connected transistors &Dgr;V
B
.
In addition to being temperature independent, voltage reference circuits should also provide a substantially constant output voltage in the presence of changing supply voltage levels and/or changing loading conditions. In this regard, the basic bandgap reference circuit designs suffer from output noise or ripple voltages caused by ripple or noise components in the power source supplying the bandgap circuit. One measure of the ability of a reference circuit to suppress or reject such supply ripple voltages is referred to as the power supply ripple rejection (PSRR). Within the context of modem high-speed digital devices, noise immunity or suppression is becoming more and more important, where fast switching of digital circuitry (e.g., in wireless communications and/or portable computational devices) may impart noise onto a supply voltage (e.g., such as a battery) providing power to the voltage reference circuit. Cascode devices are sometimes added to bandgap circuits to increase the PSRR (e.g., by reducing the amount of output ripple). However, cascode devices, if so employed, must be connected in series with other reference circuit components, between the supply voltage and ground. As a result, such cascode techniques reduce the voltage headroom available in the circuit as a whole. Another approach is to provide a pre-regulated power supply for the bandgap circuit. However, the circuit associated with the pre-regulation will consume more power, area and increase the complexity of the whole system.
In this regard, there is a continuing trend toward low power, low voltage systems, for example, such as wireless communications devices, portable computational devices, and the like, in which stable reference voltage circuits are needed. For instance, many modem wireless systems are being designed for operation using batteries supplying as low as 1.3 volts DC. In such applications, therefore, ripple reduction techniques involving cascode circuitry may be impractical or unworkable, such as where the bandgap reference output voltage is about 1.2 volts DC. Thus, there is a need for improved bandgap voltage reference circuits and techniques by which output ripple can be reduced without adversely impacting current and future supply voltage headroom requirements. Furthermore, as the power consumption constraints become more stringent, it is also desirable to provide reference circuits, such as bandgap systems with improved noise immunity, without significantly increased power consumption.
SUMMARY OF THE INVENTION
The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The invention involves reducing output ripple voltages in bandgap voltage reference circuit using ripple rejection circuitry. The ripple rejection circuitry represents a subtractor that has two input control signals (the first one is from the output of an error amplifier and the second one is from the power supply). The output of the ripple rejection circuitry is simply the subtraction of these two control signals. The inclusion of the supply voltage component in the control signal advantageously provides for improved power supply ripple rejection (PSRR). The ripple rejection circuitry, moreover, does not adversely affect the supply voltage headroom in the bandgap circuitry, and further does not significantly increase power consumption, thus being particularly applicable in low power, low voltage applications. The invention thus represents an advancement over conventional bandgap reference circuits, findin
Jun Chen
Kuok Hoon Siew
Berhane Adolf Deneke
Brady III Wade J.
Neerings Ronald O.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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