Low Voltage Inhibit Circuit and Integrated Circuit Using Same

This invention relates generally to electronic circuits, and more particularly, to electronic circuits which are subject to unexpected losses in power.

Generally, it is desirable for monolithic circuits to be designed for minimum power consumption. The need for low power consumption is normally traded off with other factors such as integrated circuit cost and design time. However, recently integrated circuits are being used in a wide variety of products powered by batteries. In these products low power consumption is even more important because the higher the power consumption, the shorter the amount of time the products may be operated without replacing or recharging the battery.

Frequently, the circuits are in the form of microcontrollers. Microcontrollers are data processors having not only a central processing unit (CPU), but also other circuitry such as volatile and nonvolatile memory, timers, event counters, and input/output circuitry normally associated with computer systems. Microcontrollers may be affected by a drop in the power supply voltage. Not only will the microcontroller cease to function entirely when the power supply voltage falls too far, but they may also function incorrectly as the battery voltage falls, causing deleterious effects in the system.

In an effort to avoid this harmful operation, microcontroller designers have included circuitry to detect that the battery voltage has fallen to a critical level, and place the microcontroller into reset. These circuits are variously known as low voltage inhibit (LVI) or low voltage reset (LVR) circuits. Typically, the LVI circuits are analog circuits based on a bandgap voltage reference. A comparator detects when a fraction of the battery voltage drops below the bandgap voltage and takes some action in response, such as activating a reset signal to reset the microcontroller.

One difficulty faced by integrated circuit designers is that because the LVI circuit is an analog circuit, it consumes a significant amount of power itself, and it contributes to battery drain. Known techniques for reducing the LVI circuit's power consumption, furthermore, require an increase in integrated circuit area. What is needed, then is a low voltage inhibit circuit which is compact in terms of circuit area, and which has a minimum of current drain so as not to reduce battery life even further. Such a low voltage inhibit circuit and an integrated circuit using it are provided by the present invention, whose features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 illustrates in partial block, partial logic, and partial schematic form an integrated circuit with a low voltage inhibit (LVI) circuit according to the present invention.

FIG. 2 illustrates in schematic form a first complementary metal-oxide-semiconductor (CMOS) Schmitt trigger suitable for use in the low voltage detection circuit of FIG, 1.

FIG. 3 illustrates in schematic form a second CMOS Schmitt trigger suitable for use in the low voltage detection circuit of FIG. 1.

FIG. 4 illustrates in partial logic and partial schematic form the primary low voltage detection circuit of FIG. 1.

FIG. 5 illustrates a graph of the current required by the low voltage detection circuit of FIG. 1 as the power supply voltage varies.

FIG. 1 illustrates in partial block, partial logic, and partial schematic form an integrated circuit 20 with a low voltage inhibit (LVI) circuit 21 according to the present invention. Integrated circuit 20 also includes an internal circuit 22 which is connected between a more positive power supply voltage terminal supplied from a battery, not shown, labelled "V.sub.BATT " and a ground power supply voltage terminal labelled "VSS". V.sub.BATT has a particular nominal voltage but as the battery nears the end of its life, V.sub.BATT compresses and eventually falls below a point at which internal circuit 22 can no longer function properly. In the illustrated embodiment, V.sub.BATT has a nominal voltage of three volts, and internal circuit 22 is capable of proper operation until V.sub.BATT falls below 1.8 volts. However, it should be apparent that these voltages are merely illustrative and will vary from embodiment to embodiment.

In the illustrated embodiment, internal circuit 22 is a microcontroller, and thus has both volatile and nonvolatile memory and peripheral circuits. The function of these circuits is well known in the art and it should be appreciated that LVI circuit 21 may be used with internal circuits other than microcontrollers.

LVI circuit 21 includes generally a primary LVI circuit 30, a secondary LVI circuit 35, and a disabling circuit 40. Primary LVI circuit 30 has a sense input terminal for receiving the battery voltage V.sub.BATT, an enable input terminal for receiving a signal "LVI2", and an output terminal for providing a signal labelled "LVIA". Secondary LVI circuit 35 includes a Schmitt trigger 36 and an inverter 37. Schmitt trigger 36 has an input terminal for receiving V.sub.BATT, and an output terminal. Inverter 37 has an input terminal coupled to the output terminal of Schmitt trigger 36, and an output terminal. Disabling circuit 40 includes an SR flip-flop 41 and an AND gate 42. Flip-flop 41 has a set input terminal labelled "S" connected to the output terminal of primary LVI circuit 20 for receiving signal LVIA, a reset input terminal labelled "R" connected to the output terminal of Schmitt trigger 36, a true output terminal labelled "Q" for providing a signal labelled "RESET" to internal circuit 22, and a complementary output terminal labelled "Q". AND gate 42 has a first input terminal connected to the output terminal of inverter 37, a second input terminal connected to the Q output terminal of flip-flop 41, and an output terminal connected to the EN input terminal of analog LVI circuit 30 for providing signal LVI2.

LVI circuit 21 includes both a highly-accurate analog LVI circuit (primary LVI circuit 30), and less accurate but lower power LVI circuit (secondary LVI circuit 35). When the battery voltage is near its nominal value, secondary LVI circuit 35 keeps primary LVI circuit 30 disabled. Secondary LVI circuit 35 is able to detect a drop in V.sub.BATT, although less accurately then primary LVI circuit 30. After detecting such a drop, secondary LVI circuit 35 activates signal LVI2, which enables primary LVI circuit 30, which may now more accurately track reductions in V.sub.BATT until at last V.sub.BATT falls below the critical voltage. At this point, primary LVI circuit 30 activates signal LVIA, which causes flip-flop 41 to be set and to activate signal RESET at its Q output terminal. At this point flip-flop 41 also deactivates signal Q, causing the output of AND gate 42 to switch to a logic low, which inactivates primary LVI circuit 30.

LVI circuit 21 reduces power consumption in two important ways compared to known LVI circuits. First, when the battery is at or near its nominal voltage, only secondary LVI circuit 35 is enabled and power consumption is negligible. Second, after secondary LVI circuit 35 enables primary LVI circuit 30 and primary LVI circuit 30 detects a drop in battery voltage below the minimum threshold, disabling circuit 40 again disables primary LVI circuit 30 to prevent further power consumption.

FIG. 2 illustrates in schematic form a first complementary metal-oxide-semiconductor (CMOS) Schmitt trigger 50 suitable for use in low voltage detection circuit of FIG. 1. Schmitt trigger 50 includes resistors 51 and 52 connected in series between V.sub.BATT and V.sub.SS. The interconnection point forms a voltage labelled "V.sub.IN " which is proportional to V.sub.BATT referenced to V.sub.SS. This proportional voltage is provided to an input of a CMOS inverter formed with a P-channel transistor 53 and an N-channel transistor 54. Transistor 53 has a source connected to V.sub.BATT, a gate for receiving signal V.sub.IN, and a drain. Transistor 54 has a drain connected to the drain of transistor 53, a gate for receiving signal V.sub.IN, and a source connected to V.sub.SS. Schmitt trigger 50 also includes an inverter 55 and a P-channel transistor 56. Inverter 55 has an input terminal connected to the drains of transistors 53 and 54, and an output terminal. Transistor 56 has a source connected to V.sub.BATT, a gate connected to the output terminal of inverter 55; and a drain connected to the input terminal of inverter 55. Schmitt trigger 50 scales V.sub.BATT to form proportional voltage V.sub.IN which causes the inverter formed by transistors 53 and 54 to switch when V.sub.BATT approaches a critical range which is above the final inhibit voltage. Resistors 51 and 52 are preferably high-valued to minimize steady-state (DC) current flow. The only other current flow occurs when the inverter switches.

Because of its extremely low power consumption, Schmitt trigger 50 is preferably used when the manufacturing process ensures adequate headroom when V.sub.BATT approaches the critical range. If this condition is not met, another Schmitt trigger which draws more current but also is able to operate with less headroom is preferred. FIG. 3 illustrates this second CMOS Schmitt trigger 60 in schematic form. Like Schmitt trigger 50, Schmitt trigger 60 includes two resistors 61 and 62 whose interconnection point provides a voltage V.sub.IN proportional to V.sub.BATT. Schmitt trigger 60 also includes a P-channel transistor 63, an N-channel transistor 64, an inverter 65, and a P-channel transistor 66. Inverter 65 and transistor 66 function similarly to corresponding inverter 55 and transistor 56 of FIG. 2, respectively, and will not be further described. Transistor 63 has a source connected to V.sub.BATT, a gate connected to V.sub.SS, and a drain connected to the input terminal of inverter 65. Transistor 64 has a drain connected to the drain of transistor 63, a gate for receiving signal V.sub.IN, and a source connected to V.sub.SS. Since the gate of transistor 63 is connected to VSS rather than receiving signal V.sub.IN, Schmitt trigger 60 avoids the headroom problems associated with Schmitt trigger 50, but at the cost of some DC current flow caused by transistor 63 being continuously conductive.

FIG. 4 illustrates in partial logic and partial schematic form primary low voltage inhibit (LVI) circuit 30 of FIG. 1. While many LVI circuit configurations would be apparent to those of ordinary skill in the art, primary LVI circuit 30 additionally includes an enable input terminal which allows it to be used in LVI circuit 20 of FIG. 1. Primary LVI circuit 30 includes generally a bandgap voltage reference circuit 70, a proportional voltage circuit 80, and an output comparator 90. Bandgap voltage reference circuit 70 includes resistors 71-73, PNP bipolar transistors 74 and 75, and an operational amplifier 76. Resistor 71 has a first terminal connected to V.sub.BATT, and a second terminal. Resistor 72 has a first terminal connected to the second terminal of resistor 71, and a second terminal. Resistor 73 has a first terminal connected to V.sub.BATT, and a second terminal. Transistor 74 has an emitter connected to the second terminal of resistor 72, and a base and a collector each connected to V.sub.SS. Transistor 75 has an emitter connected to the second terminal of resistor 73, and a base and a collector each connected to V.sub.SS. Operational amplifier 76 has a positive input terminal connected to the second terminal of resistor 73, a negative input terminal connected to the second terminal of resistor 71, an enable input terminal for receiving signal LVI2, and an output terminal. Resistors 71 and 73 each have equal resistances designated "R", whereas the value of resistor 72, designated "R'", depends on the manufacturing process. Transistor 74 has an emitter area labelled "8e", which is eight times the emitter area of transistor 75, labelled "1e". Bandgap voltage reference circuit 70 is very low power by the inclusion of an operational amplifier 76 which is selectively disabled. Otherwise, bandgap voltage reference circuit 70 functions as a conventional bandgap voltage reference circuit.

Proportional voltage circuit 80 includes resistors 81 and 82 and an N-channel transistor 83. Resistor 81 has a first terminal connected to V.sub.BATT, and a second terminal. Resistor 82 has a first terminal connected to the second terminal of resistor 81, and a second terminal. Transistor 83 has a drain connected to the second terminal of resistor 82, a gate for receiving signal LVI2, and a source connected to V.sub.SS. Proportional voltage circuit 80 generates a voltage which is proportional to V.sub.BATT, but is disabled from drawing DC current when transistor 83 is made nonconductive by signal LVI2 being inactive. Comparator 90 has a positive input terminal connected to the second terminal of resistor 81, a negative input terminal connected to the output terminal of operational amplifier 76, an enable input terminal for receiving voltage LVI2, and an output terminal for providing voltage LVI1. The values of resistors 81 and 82 set the proportional voltage to equal the output of bandgap voltage reference circuit 70 when V.sub.BATT reaches its critical voltage, which when signal LVI2 is active causes comparator 90 to activate signal LVI1. In the illustrated embodiment, the critical voltage is about 1.8 volts.

FIG. 5 illustrates a graph of the relative current drawn by the low voltage detection circuit of FIG. 1 as the power supply voltage varies. In FIG. 5, the horizontal axis represents V.sub.BATT, and the vertical axis represents relative total current consumption. The dashed line represents current consumption of known LVI circuits, i.e., ones which use an analog LVI circuit only. Down to approximately the critical voltage of about 1.8 volts, current consumption of these known LVI circuits is fixed. Conventional circuit design techniques such as increasing the values of the resistors lowers this level but at a substantial cost of circuit area. Below a certain voltage, current consumption compresses.

The solid line represents current consumption using LVI circuit 21. Above a critical range of voltage, between 1.8 volts and a voltage labelled V1, only secondary LVI circuit 35 is active, reducing current consumption to substantially zero. When V.sub.BATT falls below V1, secondary LVI circuit 35 activates signal LVI2, enabling primary LVI circuit 30. Only within this reduced range of voltage does LVI circuit 21 draw as much current as conventional LVI circuits. Below 1.8 volts, disabling circuit 40 again disables primary LVI circuit 30. Thus, the average current drain over the life of the battery is significantly reduced. Note that while the value of V1 controls when primary LVI circuit becomes active and thus affects average current consumption, it may vary somewhat without affecting the overall operation of LVI circuit 21.

While the invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. For example, different primary and secondary LVI circuits may be used. The primary LVI circuit may be, for instance, a bandgap voltage reference circuit, a zener diode, or the like. Also many other transistor technologies besides CMOS are possible. While in the illustrated embodiment the internal circuit was a microcontroller, the invention is applicable to other types of circuits such as microprocessors, memories, peripherals such as timers, frequency synthesizers, and the like. Also in some embodiments it may be desirable to provide the RESET signal as an output signal from the integrated circuit for use by other circuits. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.