Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2000-02-18
2001-07-31
Zweizig, Jeffrey (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S077000, C327S143000
Reexamination Certificate
active
06268764
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor integrated circuits having low voltage detect and power-on reset, and, more particularly, to an integrated circuit using a bandgap voltage comparator having a low voltage threshold with long term voltage stability over a wide temperature range.
BACKGROUND OF THE RELATED TECHNOLOGY
Electronic devices, such as, cellular telephones, laptop computers, keyless and wireless entry devices, and other integrated circuit based products may be powered by, for example, a battery, a fuel cell, a solar cell, a generator and the like require a stable and accurate voltage reference for effective and predictable operation. It is important that an accurate and reliable way of initiating the start-up and shut-down of electronic circuits in these battery powered devices over all possible operating conditions be used. During start-up (power-up) a minimum voltage value must be reached before beginning operation thereof, and during operation whenever a voltage goes below a critical value the device must be stopped or inhibited from further operation. Typical applications that perform these functions are power-on reset (POR) and power-low reset (or brown-out reset (BOR)).
POR and BOR circuits typically use a precision voltage reference in combination with a voltage comparator circuit(s) for determining if the critical voltage value has been reached at which a device may properly operate. Typical voltage references used in integrated circuits have been buried Zener and bandgap references. The buried Zener is a very stable and accurate voltage reference, however, it typically operates at about 5 volts or more and draws several hundred microamperes for optimum operation. The newer battery powered electronic systems may run at a battery voltage of 2 volts or less, thus, the buried Zener technique is not suitable as a voltage reference which must run from such a low voltage and also have low power consumption. For such applications a “bandgap reference” may be utilized.
A bandgap voltage reference, typically 1.2 volts, may be generated with a semiconductor circuit. This reference may be hereinafter referred to as a bandgap voltage and may be compensated for variations over changes in temperature by combining a negative temperature coefficient voltage circuit with a positive temperature coefficient voltage circuit to produce a substantially zero temperature coefficient voltage circuit, i.e., the bandgap voltage value remains substantially the same over a wide temperature range. A precision bandgap voltage reference circuit is more fully described in U.S. Pat. No. 5,900,773 by David M. Susak, and is incorporated by reference herein for all purposes.
A bandgap reference in combination with a voltage comparator may be used to sense operating voltage levels for POR and BOR circuits. The bandgap reference and comparator may be combined into one circuit such as disclosed in U.S. Pat. No. 5,781,043 by Willam Slemmer, and entitled “DIRECT CURRENT SUM BANDGAP VOLTAGE COMPARATOR” (referred to hereinafter as “Slemmer”). The Slemmer patent discloses a direct current sum bandgap voltage comparator for detecting voltage changes in a power supply. Upon detecting a power supply voltage level below a certain value, the Slemmer comparator will cause a transfer switch to change the power source to a backup battery. The Slemmer bandgap voltage comparator uses four current sources summed together to produce a summing node voltage level, and generates a logic signal that indicates when the summing node voltage is greater than or equal to, or less than a predetermined value. The predetermined value corresponds to a desired power supply voltage switchover value.
The prior art voltage references and comparator circuits are too complex, draw too much current, require operating voltage levels higher than are available in the newer battery operated electronic systems, and suffer from temperature and voltage stability variations. Thus, a need exists to provide an improved voltage comparator having low operating current, a stable voltage reference over a wide range of operating temperatures, and simple and reliable implementation in an integrated circuit.
SUMMARY OF THE INVENTION
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing an improved bandgap voltage comparator which may be utilized with other circuits in integrated circuit devices as a low voltage detection circuit. The integrated circuit devices may be, for example but not limited to, the complementary metal oxide semiconductor (CMOS) PIC Microcontroller family manufactured by Microchip Technology Inc., more fully described at http://www.microchip.com/ and incorporated by reference herein for all purposes. The present invention may operate at one volt and above, and only draw a relatively small amount of current in operation. Implementation of the embodiments of the invention in an integrated circuit is efficient and only requires a small area of the die. Accurate tip points are preferably plus or minus 50 millivolts over all process comers and industrial temperature ranges. In embodiments of the invention, power up and power down trip points can be set independently, i.e., programmable hysteresis. These trip points are insensitive to VDD rise and fall times. Embodiments of the invention may have immediate POR or BOR generation on a low voltage condition, i.e., falling VDD. A hold-off time, for example but not limitation, may be 1-10 microseconds (programmable) for a POR signal after detection of a rising trip point. The embodiments of the invention may draw no current when placed in a standby mode (sleep mode), and can evaluate voltage levels when taken out of the standby mode. A low voltage condition signal will be generated if a low voltage condition exists upon activation from the standby mode. The present invention is thus well suited for both power-on reset (POR) and power-low reset (brown-out reset BOR) modes. The predictable and efficient operation of this circuit will ensure reliable operation for all battery (2 volts or more) operated applications.
In accordance with the embodiments of the present invention, an improved bandgap voltage comparator circuit may be implemented on a semiconductor integrated circuit substrate using standard fabrication processes. Standard PNP transistors such as, for example but not limitation, lateral PNP (LPNP) transistors may be used in a circuit needing only two current mirrors to form a bandgap voltage comparator. Using only two current sources greatly simplifies the bandgap circuit and improves trip voltage stability with changes in temperature. The areas of the PNP transistors used for the second current mirror circuit may be from about four to forty eight times the area of the PNP transistors used for the first current mirror circuit, depending on the gain desired for the comparator circuit. An exemplary schematic diagram of the basic bandgap comparator of the present invention is illustrated in FIG.
4
A and is described in more detail hereinbelow.
A bandgap comparator circuit, according to the embodiments of the invention, having an adjustable trip point is illustrated in the schematic diagram of FIG.
4
and is described in more detail hereinbelow. A graphical representation of an operational simulation of the embodiment of the bandgap comparator low voltage detection system of
FIG. 2
is illustrated in FIG.
11
. An output buffer circuit that may be used in combination with the bandgap comparator circuit of
FIG. 4
is illustrated in
FIG. 5. A
power-on reset delay circuit that may be used with the bandgap comparator of
FIG. 4
is illustrated in FIG.
6
and is described in more detail hereinbelow. An output latch with power down circuit that also may be used with the bandgap comparator circuit of
FIG. 4
is illustrated in FIG.
7
and is described in more detail hereinbelow.
Bandgap voltage operation in integrated circuit devices is more fully described in “Analys
Eagar Layton
Smit Willem
Baker & Botts L.L.P.
Microchip Technology Incorporated
Zweizig Jeffrey
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