Voltage generator in a MOS integrated circuit

The present invention relates to integrated circuits and, more particularly, to a voltage generator for use in a MOS integrated circuit.

As is known, during the manufacture of integrated circuits, some electrical quantities cannot be determined precisely at the design stage because they depend on manufacturing parameters which may vary randomly, although within statistically predictable limits. In other words, the same electrical quantity, for example, the gate-source voltage of a conducting MOS transistor, may have different values in different specimens of the same integrated circuit. This variability is a problem which becomes more serious with lower supply voltages of the integrated circuit. Since, in the design of integrated circuits, there is a tendency to reduce the dimensions of the transistors and consequently to reduce the supply voltage, the problem is experienced to an ever greater extent.

In integrated circuits, it is sometimes necessary to provide reference and/or biasing voltages which take account of this variability of the manufacturing process, that is, which are not a predetermined fraction of the supply voltage of the integrated circuit but which depend on the actual value of one or more electrical quantities dependent on the manufacturing parameters. According to the prior art, it is possible to produce current generators which are substantially independent of variations of the manufacturing parameters, but it is not possible to produce voltage generators having the same independence.

An object of the present invention is to provide voltage generators which are substantially independent of variations of the manufacturing parameters.

This and other objects and advantages are achieved by the generator including complementary MOS transistors interconnected in four circuit branches one of which contains a constant-current generator. Voltages picked up at various nodes of the circuit can be used as reference and/or biasing voltages of the integrated circuit, which account for the variability of the manufacturing parameters.

The invention will be understood better from the following detailed description of a non-limiting embodiment thereof, given with reference to the appended drawings, in which:

FIG. 1 is a circuit diagram of a generator according to the present invention; and

FIG. 2 is a circuit diagram of a current generator for use with the circuit of FIG. 1 .

The circuit of FIG. 1 substantially includes four interconnected branches. In particular, a first branch comprises an n-channel MOS transistor M 1 having its drain terminal connected, via a constant-current generator Go, to a first supply terminal, indicated VDD, its source terminal connected to the second supply terminal, indicated by the ground symbol, and its gate terminal connected to another branch of the circuit which will be described below. The generator Go is preferably a generator of a current of predetermined value independent of the process variations and of temperature, for example, such as that shown in FIG. 2, which will be described below.

A second branch comprises a p-channel MOS transistor M 2 and an n-channel MOS transistor M 3 having their drain terminals connected to one another and their source terminals connected to the first supply terminal (VDD) and to the second supply terminal (ground), respectively. The transistor M 2 has its gate terminal connected to a junction node A between the transistor M 1 and the current generator Go and the transistor M 3 has its gate terminal connected to its drain terminal and to a third circuit branch.

The third branch comprises a p-channel MOS transistor M 4 and an n-channel MOS transistor M 5 having their drain terminals connected to one another via two resistors R 1 and R 2 in series with one another, and their source terminals connected to the first supply terminal (VDD) and to the second supply terminal (ground), respectively. Moreover, the transistor M 4 has its gate terminal connected to the node A and the transistor M 5 has its gate terminal connected to the gate terminal of the transistor M 3 .

A fourth circuit branch comprises a p-channel MOS transistor M 6 and an n-channel MOS transistor M 7 having their drain terminals connected together to a junction node B between the two resistors R 1 and R 2 , their source terminals connected to the first supply terminal (VDD) and to the second supply terminal (ground), respectively, and their gate terminals connected to the drain terminal of the transistor M 4 and to the drain terminal of the transistor M 5 , respectively.

The terminals of the circuit at which a reference and/or biasing voltage can be picked up are the node B (voltage Vr), the gate terminals of the transistors M 7 and M 6 (voltages Vb 1 , Vb 2 ) and the gate terminals of the transistors M 3 and M 2 (voltages (Vb 3 , Vb 4 ).

The operation of the circuit will now be considered. If the gate currents are ignored, the current Ip forced by the generator Go at the node A is equal to the drain current of the transistor M 1 . The voltage of the node A adopts a constant value (determined by the voltage VgsM 1 between the gate and the source of the transistor M 1 ) which is applied to the gate terminals of the transistors M 2 and M 4 . Since the transistors M 2 and M 4 have the same voltage Vgs between their gates and their sources, and assuming they have the same dimensions, the same current I passes through them. Clearly, the current I also passes through the transistor M 3 . Since the transistor M 5 has the same Vgs voltage as the transistor M 3 , and assuming it has the same dimensions as M 3 , the current I also passes through the transistor M 5 .

Again ignoring the gate currents, it can be seen that the current I also passes through the resistor R 2 . Since the same current I passes through the resistor R 1 , no current passes from the node B to the drain terminals of the transistors M 6 and M 7 . Moreover, equal currents pass through the latter transistors M 6 and M 7 .

The voltage Vr at the node B will now be examined. It can be regarded as the voltage between the drain and the source of the transistor M 7 , that is:

Vr=VdsM 7 =I·R 2 +VgsM 7   (1)

or as the difference between the supply voltage VDD and the voltage between the drain and the source of the transistor M 6 , that is:

Vr=V DD −|VdsM 6 |=V DD −I·R 1 +VgsM 6   (2)

From expressions (1) and (2):

I(R 1 +R 2 )=V DD +VgsM 6 −VgsM 7 ,

from which: I = V DD + VgsM6 - VgsM7 R1 + R2     and ( 3 ) Vr = R2 R1 + R2  ( V DD + VgsM6 - VgsM7 ) + VgsM7 ( 4 )

As can be seen, the current I of expression (3) and the voltage Vr of expression (4) depend on the gate-source voltages of two MOS transistors, one with an n channel and the other with a p channel, and therefore take account of the deviations from the design values due to the variability of the manufacturing parameters. The voltage Vr can be used, in the integrated device which includes the above-described voltage-generator circuit, as a reference or biasing quantity which adapts automatically to the characteristics of each specimen of the integrated device.

The voltages Vb 1 , Vb 2 picked up at the gate terminals of the transistors M 7 and M 6 can be used as biasing voltages for two MOS transistors with an n channel and a p channel, respectively, having load resistors. The drain current of each of the two transistors brings about a voltage drop in the load resistor which also follows the process variations. This voltage will in fact depend substantially on the term VDD−(VgsM 7 +VgsM 6 ) and on the scale ratios which, as is known, can be determined by modifying the width/length (W/L) ratios of the channels of the transistors and the values of the load resistors used.

The voltages Vb 3 and Vb 4 picked up at the gate terminals of the transistors M 3 and M 2 can be used as biasing voltages for n-channel and p-channel MOS transistors, respectively, to generate currents proportional to the current I of expression (3).

Various voltages are thus obtained, which can be used as reference and/or biasing voltages in the integrated device in which the generator according to the invention is formed and which adapt automatically to the actual values of the electrical parameters that are dependent on the manufacturing process. It will be noted that these voltages also adapt to temperature variations since the Vgs voltages on which they depend vary with temperature.

As shown in FIG. 2, the generator Go may include an operational amplifier OP having its non-inverting input terminal connected to a source (not shown) of a reference voltage Vbg which is constant and independent of temperature, for example, a source referenced to the conduction band of silicon (a band-gap referenced voltage source). The output of the operational amplifier OP is connected to the gate terminal of an n-channel MOS transistor M 8 . The transistor M 8 has its source terminal connected to the inverting input terminal of the amplifier OP and, via a resistor Rext, to the ground terminal, and has its drain terminal connected to the drain terminal of a p-channel MOS transistor M 9 . The transistor M 9 has its source terminal connected to the supply terminal VDD, and its gate terminal connected to its own drain terminal and to the gate terminal of another p-channel MOS transistor M 10 . The latter has its source terminal connected to the supply terminal VDD and its drain terminal connected to the node A of the circuit of FIG. 1 .

In operation, the voltage at the inverting input of the amplifier OP is brought to the voltage of the non-inverting input. The resistor Rext therefore has the voltage Vbg at its terminals and a current Io=Vbg/Rext passes through it. This resistor is preferably a very precise, discrete component so that the current Io can be set correspondingly precisely. Moreover, it is independent of temperature, as is the voltage Vbg. The transistors M 9 and M 10 are connected as a current mirror so that the current Ip which passes through the transistor M 10 and is forced at the node A is equal to the current Io if the transistors M 9 and M 10 have the same dimensions, or is proportional thereto in accordance with the dimensional ratios between the two transistors if they have different dimensions. In any case, the current Ip is a current which can be set very precisely and is independent of temperature.

Although only one embodiment of the invention has been described and illustrated, naturally many modifications and variations are possible within the scope of the same inventive concept. For example, p-channel MOS transistors could be used instead of the n-channel MOS transistors and vice versa, with the polarity of the supply voltage reversed, or the dimensional ratios between the transistors could be different so that the currents in the second and third circuit branches would differ from one another although they would be proportional to the current I of expression (3).