Voltage Detection Circuit Composed of at Least Two MOS Transistors

The present invention relates to an improvement in a voltage level detection circuit which senses whether a power source voltage drops below a given voltage level.

A typical prior art voltage level detection circuit as shown in FIG. 1 is well known. In the drawing T.sub.1 through T.sub.4 designate MOS transistors, R.sub.1 and R.sub.2 designate resistors, P designates a P-channel type MOS transistor, and N designates an N-channel type MOS transistor. T.sub.2 and T.sub.4 form an inverter circuit. In addition, E.sub.1 represents a power source voltage such as a battery with the cathode connected with a utility circuit and the anode grounded, and E.sub.2 and E.sub.3 represents potentials at respective nodes.

When the power voltage E.sub.1 is increased above the threshold voltage VH.sub.3 of the MOS transistor T.sub.3, the transistor T.sub.3 is turned ON and E.sub.2 is increased to the ground potential. This means that the transistor T.sub.1 is turned ON too and the drain potential E.sub.3 of T.sub.1 is equal to the power source voltage E.sub.1 . In other words, the illustrated circuit compares the power source voltage E.sub.1 with the threshold voltage of the P channel or N channel MOS transistor and then detects a drop in the power source voltage. FIGS. 4(a) and 4(b) are voltage characteristic charts showing a relationship between the voltages E.sub.1 and E.sub.2. However, design of such a circuit arrangement is relatively difficult because the MOS transistor should be driven in a normal mode up to near the threshold voltage level. Therefore, the resistor R.sub.1 is smaller and the decision level E.sub.1 is higher in practical use as depicted by the broken line in FIGS. 4(a) and 4(b ). This approach, however, results in increase of current flowing through the resistor R.sub.1 and the transistor T.sub.3 and in other words, increase of consumption current. The battery is often renewed.

Accordingly, it is an object of the present invention to provide a voltage level detection circuit with less power consumption.

In accordance with the voltage level detection circuit of the present invention, a power source voltage is compared with reference to the threshold voltage of an MOS transistor and the results of such comparison are developed as voltage level detection outputs. The level detection circuit is comprised of a first MOS transistor having the gate and the drain connected together with a power source voltage V.sub.DD (E.sub.1) via a resistor and the source grounded and a second MOS transistor having the gate connected with the drain of the first MOS transistor and the drain grounded via a resistor.

Other objects and novel features of the present invention as set forth in the appended claims and the present invention as to its organization and its mode of operation will best be understood from a consideration of the following detailed description of the preferred embodiments taken in connection with the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of a prior art voltage level detection circuit;

FIG. 2 is a circuit diagram of an embodiment of a voltage level detection circuit embodying the present invention;

FIG. 3 is a circuit diagram of another embodiment of the present invention;

FIGS. 4(a), 4(b), 5(a) and 5(b), 6(a) through 6(c) are diagrammatic charts showing respective node potentials against a power voltage E.sub.1 ;

FIG. 7 is a circuit diagram of an example of application of the present invention;

FIG. 8 is a diagrammatic chart showing a node potential V.sub.out against the power source voltage V.sub.DD (E.sub.1); and

FIGS. 9 and 10 are circuit diagrams using modifications in the embodiment of FIG. 7.

FIG. 2 illustrates a circuit diagram of an embodiment of the present invention whih includes a first MOS transistor T.sub.7 having the gate and the drain connected together with a power supply voltage E.sub.1 via a resistor R.sub.3 and the source grounded and a second MOS transistor T.sub.5 having the gate connected with the drain of the first MOS transistor T.sub.7 and the drain grounded via a resistor R.sub.4. The drain of the second MOS transistor T.sub.5 is connected with the gate of an inverter composed of a couple of MOS transistors T.sub.6 and T.sub.8. The inverter drives voltage detection outputs.

FIGS. 5(a) and 5(b) show the drain voltage E.sub.4 of the MOS transistor T.sub.7 and the drain voltage E.sub.5 of the MOS transistor T.sub.5 against the power supply voltage E.sub.1. If the power supply voltage E.sub.1 is below the threshold voltage VH.sub.7 of the MOS transistor T.sub.7, then the MOS transistor T.sub.7 is turned OFF with E.sub.4 being the same as the potential E.sub.1.

When E.sub.1 is increased above the threshold voltage of T.sub.7, this makes the gate voltage E.sub.4 of T.sub.5 fixed near the threshold voltage VH.sub.7 as shown in FIG. 5(a). If a potential difference between E.sub.1 and E.sub.4 is below the threshold voltage VH.sub.5 of T.sub.5, T.sub.5 remains in the OFF state and E.sub.5 remains at the ground potential.

Thereafter, the difference between E.sub.1 and E.sub.4 exceeds the threshold voltage VH.sub.5 of T.sub.5 so that T.sub.5 is turned ON and E.sub.5 is equal to E.sub.1 as shown in FIG. 5(b). It is obvious from FIG. 5(a) that E.sub.4 is kept at a fixed level as long as the threshold voltage of T.sub.7 is reached and that a potential variation in E.sub.5 is developed at a sum of the P channel and N channel threshold voltages. This voltage value corresponds to a detection level. Therefore, by corresponding a desired driving voltage to the threshold voltages of the respective transistors it is able to sense drop in the power supply voltage.

The above-mentioned resistors R.sub.3 and R.sub.4 are load resistors for the MOS transistors T.sub.7 and T.sub.5. These resistance values may be high enough to avoid unstable operation by noise, etc., so that current flowing through T.sub.7 and T.sub.5 is remarkably small. As a result, less power consumption voltage detection is achieved.

FIG. 3 shows a circuit diagram of another embodiment of the present invention wherein a series circuit of resistors R.sub.5 and R.sub.6 is provided instead of the resistor R.sub.3 of FIG. 2 and the junction of R.sub.5 and R.sub.6 is connected with the gate of an MOS transistor T.sub.9. The remaining portion is identical with the implementation shown in FIG. 2.

When a potential difference between the power supply voltage E.sub.1 and the gate voltage E.sub.7 of the MOS transistor T.sub.9 exceeds the threshold voltage VH.sub.9 of the MOS transistor T.sub.9, voltage detection is achieved. The gate potential E.sub.7 is a potential divided by the resistors R.sub.5 and R.sub.6 and more particularly a portion of the potential difference between the power supply voltage E.sub.1 and the drain voltage E.sub.6 of a MOS transistor T.sub.11 which is determined by a division ratio of the resistors R.sub.5 and R.sub.6. The gate potential E.sub.7 is therefore higher than the potential E.sub.4 as shown by FIG. 2. This implies that the detection level is higher than that in the circuit of FIG. 2.

FIGS. 6(a) through 6(c) show variations of potentials E.sub.6, E.sub.7 and E.sub.8 at respective nodes as a function of the power supply voltage E.sub.1. Although in FIG. 6(c) E.sub.8 is varied from the zero level at a much higher level E.sub.1 than that in FIG. 5(b), the amplitude of E.sub.1 is optionally selectable by proper choice of the ratio of the resistor R.sub.5 and R.sub.6. Therefore, the detection level is also selectable. It will be noted that an MOS transistor T.sub.12 is the substitution for the resistor R.sub.4 of FIG. 2. In this case, such MOS resistor occupies a reduced area in implementating the circuit with the LSI technology.

The voltage level detection outputs can be utilized in a wide range of applications. An example of an application to an initial condition setting circuit is illustrated in FIG. 7 wherein T.sub.21 through T.sub.29 are C-MOS transistors and P designates P-channel transistors and N designates N-channel transistors. This includes a first MOS transistor T.sub.25 having the drain and the gate connected together with a power supply voltage V.sub.DD via a resistor R.sub.22 and the source grounded and a second MOS transistor T.sub.21 having the gate connected with the drain of the first MOS transistor T.sub.25, the source connected with the power supply voltage V.sub.DD and the drain grounded via a resistor R.sub.23 and connected with the gates of T.sub.22 and T.sub.26 forming a C-MOS inverter. The output of the inverter is coupled with a set input or a reset input of a directly coupled flip-flop composed of T.sub.23, T.sub.24, T.sub.28 and T.sub.29. The output of the flip-flop serves as an initial condition setting signal ACL. A gate signal R.sub. 5 of T.sub.27 is to reset the flip-flop. The power supply potential V.sub.DD is negative.

A relationship between V.sub.DD and the drain potential of T.sub.1 is shown in FIG. 8. T.sub.22 and T.sub.26 are inverted when V.sub.DD is increased above a sum of the threshold voltage V.sub.TH(P) of the P-channel transistor T.sub.25 and that V.sub.TH(N) of the N-channel transistor T.sub.21. Since the drain potential of T.sub.21 is utilized as the set input of the flip-flop, the output ACL of the flip-flop bears the ground potential when the drain of T.sub.1 is at the ground potential. If the drain of T.sub.1 is at the potential V.sub.DD, no set input is developed so that the flip-flop remains in the previous condition until the reset signal R.sub.5 is applied to the gate of the transistor T.sub.27. Therefore, when the power supply voltage V.sub.DD is below the sum of the threshold voltages, the ACL signal bears the ground potential and is impressed on other flip-flops or a register in order to establish an initial condition.

FIG. 9 is a modification of the embodiment shown in FIG. 7 wherein the resistor R.sub.22 of FIG. 7 is divided into two resistors R.sub.24 and R.sub.25 and the junction of the resistors R.sub.24 and R.sub.25 is connected with the gate of a transistor T.sub.30. A transistor T.sub.35 is the substitution for the resistor R.sub.23 of FIG. 7. FIG. 10 shows MOS transistors T.sub.40 and T.sub.41 provided instead of the resistors R.sub.24 and R.sub.25 of FIG. 9. In design of an LSI, a ratio of T.sub.40 and T.sub.41 can be varied to change the detection level.

While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as claimed.