GB2308453A

GB2308453A - Voltage detector

Info

Publication number: GB2308453A
Application number: GB9526318A
Authority: GB
Inventors: Alexis Marquot; Alain Duret
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1995-12-22
Filing date: 1995-12-22
Publication date: 1997-06-25
Anticipated expiration: 2015-12-22
Also published as: GB9526318D0; GB2308453B

Abstract

A voltage detector is provided with a voltage sensing circuit (1) having first (P1) and second (P2) FETs connected to an input voltage (V DD ). The first FET is connected as a diode and the second FET has a reference voltage (V BIAS ) applied by a bias circuit (2) to its gate electrode by biasing circuit (N3,P3). The output of the voltage detector is derived from a switching circuit (N1,N2) which compares the conductances of the first and second FETs and produces a different output depending on whether the input voltage is less than or greater than a predetermined threshold voltage (V DDT ). In order to reduce the overall temperature dependence of the voltage detector, the biasing circuit is provided to have a temperature dependence that compensates for the temperature dependence of the rest of the voltage detector circuit.

Description

VOLTAGE DETECTOR Field of the Invention This invention relates to a voltage detector.

Background of the Invention Voltage detectors are commonly employed in integrated circuit chips and typically provide an output that indicates whether a given input voltage is less than or greater than a predetermined value.

In bipolar integrated circuit chips, voltage detectors are usually based on circuits utilising the bandgap of the semiconductor material in order to provide a reference voltage that is very stable with respect to temperature change.

However, these bipolar voltage detection circuits are rather sophisticated and occupy a relatively large surface area, which may be undesirable in applications where a high level of integration is required.

Moreover, voltage detection circuits based on bipolar technology may be difficult to implement using MOS integrated circuit technology and therefore the need exists for a voltage detection circuit that is simple in construction and may be easily employed in MOS integrated circuits.

This invention therefore seeks to provide a voltage detector which mitigates the above-mentioned disadvantages of prior-art voltage detectors.

Summarv of the Invention According to the present invention there is provided a voltage detector comprising voltage sensing means connected to an input terminal of said voltage detector, said voltage sensing means comprising first conduction means connected to said input terminal of said voltage detector and having a first conductance; second conduction means connected to said input terminal of said voltage detector and having a second conductance dependent on an input voltage of said voltage detector; and switching means connected to said first and second conduction means, said switching means providing a first output when said input voltage is less than a predetermined value and a second output when said input voltage is greater than said predetermined value, said predetermined value being determined by a comparison of said first and second conductances by said switching means; wherein said predetermined value is dependent on the temperature of said voltage detector; said voltage detector further comprising: biasing means for biasing said first conduction means with a reference voltage, said first conductance being dependent on said reference voltage; whereby said reference voltage supplied by said biasing means has a predetermined temperature dependence that compensates for said temperature dependence of said predetermined value of said voltage sensing means.

Brief Descnption of the Drawings An exemplary embodiment of the invention will now be described with reference to the drawings in which: FIG. 1 shows a circuit diagram of a voltage detector according to a preferred embodiment of the invention.

FIG. 2 shows the voltage characteristic of the circuit shown in FIG. 1.

FIG. 3 shows the shift in the threshold voltage versus temperature for the voltage detector of FIG. 1 both with and without a temperature-dependent reference voltage.

FIG. 4 shows the temperature dependence of the reference voltage in the circuit of FIG. 1.

Detailed Description of a Preferred Embodiment Referring to FIG. 1, there is shown a circuit diagram of a voltage detector having an input terminal to which an input voltage VDD is applied in operation. Connected to the input terminal is a voltage sensing circuit 1 formed by a current mirror formed with two circuit arms, with the first arm containing an NMOS field effect transistor (FET) N1 in series with a PMOS transistor P1 and the second arm containing an NMOS transistor N2 in series with a PMOS transistor P2. The gates of the NMOS transistors N1 and N2 are connected together, and the gate of transistor N1 is strapped to the drain electrode.

In the first arm of the current mirror the gate electrode of PMOS transistor P1 is tied electrically to the drain electrode. Consequently on application of an input voltage VDD transistor P1 is saturated and functions as a diode.

In the second arm of the current mirror, the gate electrode of PMOS transistor P2 is biased with a reference voltage VBIAS; thus during operation transistor P2 is biased by a voltage VDD - VBIAS between its source and gate electrodes and - for small values of the input voltage VDD - has a linear output response to the input voltage VDD.

The output of the voltage detector of FIG. 1 is shown in FIG. 2. As will be evident to the skilled reader, the output voltage is determined by a comparison of the conductances of the two PMOS transistors P1 and P2 carried out by NMOS transistors N1 and N2: When the input voltage VDD is small, the conductance of P2 is higher than the conductance of P1; hence transistor N2 is effectively switched off and the output voltage VOUT is approximately equal to the input voltage VDD. As the input voltage increases, however, the conductance of transistor P2 falls in comparison to that of transistor P1. Above a certain threshold voltage VDDT at the input, transistor N2 is switched on, so that the output voltage VOUT is clamped to approximately the supply voltage Vss.

The threshold voltage VDDT at which the voltage detector switches output state is determined by the operating characteristics of the transistors N1, N2, P1 and P2, as well as the level of the reference voltage VBIAS. However, the characteristics of the transistors are temperature dependent, with the result that the value of the threshold voltage is also found to vary with temperature. In the example shown in FIG. 1, if the reference voltage VBIAs is fixed, an increase in temperature of the voltage detector results in a downward drift in the value of the threshold voltage VDDT, as shown in curve (i) of FIG. 3.

In order to improve the overall temperature stability of the voltage detector, the reference voltage VBIAS is supplied by a biasing circuit 2 that is selected to display a variation in VBIAS with temperature that compensates for the shift in the threshold voltage VDDT of the rest of the circuit.

In the circuit of FIG. 1 this is achieved by providing a further NMOS transistor N3 connected as a diode in series with a weak load represented by PMOS transistor P3. The threshold voltage VBIAS therefore follows the shift with temperature of the characteristic of transistor N3 with the result that as the temperature of the voltage detector increases, the value of VBLAS decreases, as shown in FIG. 4.

The decrease in VBIAS with increasing temperature compensates for the temperature dependence of the threshold voltage VDDT in the following manner: as VBIAS decreases, the conductance of P2 is caused to increase with temperature for any given input voltage VDD. Since the threshold voltage VDDT is determined by a comparison between the conductances of transistors P1 and P2, any increase in the value of P2 will cause a corresponding increase in the level of the threshold voltage VDDT. Hence the effect of a reference voltage VBIAs that decreases with increasing temperature is a threshold voltage that increases with temperature, thereby compensating for the downward drift in the threshold voltage shown in curve (i) of FIG. 3, obtained with a fixed reference voltage VBIAS.

The effect of providing a temperature dependent reference voltage VBIAS is shown in curve (ii) of FIG. 3, which indicates the reduced dependence of the threshold voltage VDDT on the temperature of the voltage detector.

Since the circuit shown in FIG. 1 is based on CMOS technology, it may be utilised on CMOS integrated circuit chips to provide a simple, relatively temperature insensitive voltage detector.

Claims

1. A voltage detector comprising: voltage sensing means (1) connected to an input terminal of said voltage detector, said voltage sensing means (1) comprising: first conduction means (P1) connected to said input terminal of said voltage detector and having a first conductance; second conduction means (P2) connected to said input terminal of said voltage detector and having a second conductance dependent on an input voltage (VDD) of said voltage detector; and switching means (N1,N2) connected to said first (P1) and second (P2) conduction means, said switching means (N1,N2) providing a first output (-VDD) when said input voltage (VDD) is less than a predetermined value (VDDT) and a second output (-Vss) when said input voltage (VDD) is greater than said predetermined value (VDDT), said predetermined value (VDDq) being determined by a comparison of said first (P1) and second (P2) conductances by said switching means (N1,N2); wherein said predetermined value (VDDT) is dependent on the temperature of said voltage detector; said voltage detector further comprising: biasing means (2) for biasing said first conduction means (P1) with a reference voltage (VBIAS), said first conductance being dependent on said reference voltage (VOLES); whereby said reference voltage (BIAS) supplied by said biasing means (2) has a predetermined temperature dependence that compensates for said temperature dependence of said predetermined value (VDDT) of said voltage sensing means (1).

2. A voltage detector according to claim 1, wherein: said first conduction means (P1) comprise a first FET connected as a diode; and said second conduction means (P2) comprise a second FET having said reference voltage (VBIAS) connected to the gate electrode thereof.

3. A voltage detector according to claim 1 or 2, wherein said switching means comprises third (N1) and fourth (N2) FETs connected in series with said first (P1) and second (P2) FETs, respectively.

4. A voltage detector according to any one of the preceding claims, wherein said biasing means comprises fifth (P3) and sixth (N3) FETs connected in series.

5. A voltage detector according to any one of the preceding claims, wherein said sixth FET is connected as a diode.

6. A voltage detector according to any one of the preceding claims,wherein said voltage detector is a part of a CMOS integrated circuit.