Reference current generator for low voltage integrated circuit, uses P and N type transistors one of which operates in saturated mode

CURRENT GENERATOR FOR LOW SUPPLY VOLTAGE

The invention relates to a reference current generator, particularly useful for integrated circuits using low supply voltages. A generator according to the invention produces a current independent of the supply voltage.

To create currents independent of the supply voltage, it is known to use reference current generators of the bootstrap type, a simplified example of which is shown in FIG. 1.

The generator of FIG. 1 essentially comprises two transistors T1, T2 of type P, two transistors T3, T4 of type N and a resistor RI. The drain of transistor T1 and the drain of transistor T3 are connected together; a supply voltage VDD is applied to the source of transistor T1 and a reference voltage VSS is applied to the source of transistor T3. The source of transistor T2 is connected to the source of transistor T1, and the gate and drain of T2 are connected together to the gate of T1 and the drain of T4. Finally, one pole of the resistor RI is connected to the source of T4 and the reference voltage VSS is applied to another pole of the resistor RI.

The generator of Figure 1 operates as follows. Of the currents respectively the transistors T1, current mirror. The proportional currents, possibly equal: Il = a * I2.

Current II passes through transistor T3, imposing a voltage VTN3 between the gate and the source of T3. VTN3 is the threshold voltage of transistor T3, VTN3 is independent of the supply voltage VDD.

Current 12 flows through resistor RI and a voltage RI * 12 appears across it. As of the cross form a 12 are which

II,

II,

T2 resistor RI is connected between the gate and the source of transistor T2, we have at equilibrium RI * 12 = VTN3 or = VTN3 / R1

The current 12 is thus independent of the supply voltage VDD, it depends only on the threshold voltage of the transistor T3 and of the resistor RI.

The current 12 obtained is copied in order to be used elsewhere, by using a copying transistor T5, the gate and the source of which are respectively connected to the poles of the resistor RI. The drain of transistor T5 is connected to an auxiliary circuit which uses the reference current flowing in transistor T5. The current 15 is directly proportional to the current 12 flowing in the resistor RI.

It will be noted that the current 12, if it is independent of the supply voltage VDD, is on the other hand dependent on the temperature of the circuit because the threshold voltage VTN3 is itself dependent on the temperature, in a linear manner. We have effect:

= (VTN3 (T0) - K (T-TO)) / RI, with

T, the temperature, T0 a reference temperature, and VTN3 (T0), the threshold voltage of T3 at the temperature T0.

The variation, as a function of the temperature, of the current produced by a generator, is not necessarily a disadvantage. In fact, certain circuits use reference currents of variable value as a function of the temperature.

Otherwise, it is quite easy to come to terms with a variable current such as that produced by a generator according to FIG. 1, insofar as the variations of the threshold voltage VTN3 as a function of the temperature T are known and are of simpler: the threshold voltage VTN3, and therefore the current 12 which crosses the resistor RI, varies linearly as a function of the temperature: 12 is in fact equal to 12 = I0 * (lb * T). If a constant current is required, it is known to combine a generator producing a type current I = I0 * (l + b * T) with a generator producing a type current I = I0 * (lb * T) to obtain a current independent of temperature.

To create currents, it is also known to use reference current generators using a bipolar transistor. A simplified example of such a reference generator is shown in Figure 2.

Compared to the generator of FIG. 1, the circuit of FIG. 2 additionally comprises a bipolar transistor T6. An emitter of transistor T6 is connected to the source of T3 and the reference voltage VSS is applied to a collector and a base of T6 which are connected together. Finally, the grid of T3 is no longer connected to the source of T4 but to its grid.

the generator of figure 2 works in a similar way to that of figure 1. The current 12 flowing in the resistor RI is simply equal in this case to:

= VBE6 / R1,

VBE6 being a threshold voltage between the base and the emitter of transistor T6, independent of the supply voltage VDD. VBE6, on the other hand, depends on the temperature in a linear fashion.

Additional information concerning the production of generators such as those shown schematically in FIG. 1 or FIG. 2 can be found in the document CMOS Analog Circuit Design, Editions Holt Rinehart and Winston 1987.

The generators according to FIG. 1 or FIG. 2 are used whenever it is desired to obtain a reference current independent of the supply voltage. This is frequent because often, the supply voltage of a circuit can vary: it often depends on the power supplied to the circuit.

However, the generators according to FIG. 1 or FIG. 2 have a significant drawback, namely the value of the minimum supply voltage VDDmin to be used to supply such generators. Indeed, the supply voltage VDD applied must be sufficient to turn on, or even saturate all the transistors of the generators, so that a current flows in these transistors.

For example, for the generator of figure 1, the minimum voltage VDDmin to be applied is equal to:

VDDmin = VTN3 + VDS4 + VGS2, with:

VTM3, threshold voltage of T3, of the order of 0.60 V

VDS4, voltage between the drain and the source of transistor T4, of the order of 0.15 V, and

VGS2, voltage between the gate (or the drain, since they are connected together) and the source of T2, of the order of 0.70 V

Consequently, the voltage VDDmin for the circuit of FIG. 1 is of the order of 1.5 V.

In the same way and for the same reasons, for the circuit of figure 2, the minimum supply voltage VDDmin to be used is equal to:

VDDmin = VBE5 + VGS3 + VDS1, with:

VBE5, voltage between the emitter and the base of T5, of the order of 0.7 V,

VGS3, voltage between the gate and the source of transistor T3, of the order of 0.65 V, and

VDS1, voltage between the drain and the source of T1, of the order of 0.15 V

Consequently, the voltage VDDmin necessary to supply the circuit of FIG. 2 is of the order of 1.5 V.

Also, whatever the known current generator used, the minimum supply voltage VDDmin to be used is of the order of 1.5 V.

However, such a minimum voltage can be prohibitive, in particular for circuits made in the weakest submicronic technologies, such as 0.25μτη or less technologies, which can only use voltages lower than 1.5 V, or even 1, 2 V for 0.13μτη technologies.

An object of the invention is to provide a new current generator, producing a current independent of the supply voltage.

Another object of the invention is to provide a current generator which uses a low supply voltage, for example less than 1.5 V.

<td>With</td><td>these</td><td>Goals</td><td>in view, the invention</td><td>concerned</td><td>a</td>

<td>generator</td><td>of</td><td>current</td><td>to produce a</td><td>current</td><td>of</td>

<td>reference.</td><td></td><td></td><td></td><td></td><td></td>

<td>According to</td><td>l¹</td><td>'invention</td><td>, the generator</td><td>includes</td><td>a</td>

<td colspan="3">first transistor of</td><td>type P, one of which</td><td>source</td><td>is</td>

<td>connected to</td><td>a</td><td colspan="2">first pole of a resistor</td><td>and which</td><td>a</td>

gate is connected to a second pole of the resistor, the reference current, flowing in the resistor, being variable as a function of a threshold voltage of the first transistor, and a second N-type transistor, including a drain, a gate and a source are connected respectively to the second pole of the resistor, to the first pole of the resistor and to the drain of the first transistor, the second transistor operating in saturation mode.

The reference current produced is thus fixed by the voltage between the gate and the source of the first transistor, which is itself equal to the threshold voltage of the transistor. The produced reference current therefore does not depend on the supply voltage.

The above generator is advantageously completed by a current source comprising a first pole to which a supply voltage is applied and a second pole connected to the first pole of the resistor.

The current source used supplies current and energy to the resistor and the first transistor, it supplies in particular the reference current flowing in the resistor and the current in the first transistor. The first transistor and the second transistor are chosen of adequate size (in terms of gate length / width) so that they are saturated in normal operation of the generator. Thus, as will be seen better below, the current flowing in the transistor is very low and the current supplied by the current source is very close to the reference current produced by the current generator according to the invention.

A reference voltage is applied to the drain of the first transistor.

For a generator according to the invention, the minimum supply voltage to be applied is equal to the sum of the voltage between the gate and the source of the second transistor and of the voltage between the poles of the current source. It is therefore lower (of the order of 1 to 1.2 V) than that to be applied for known generators, as will be seen better below in the examples.

A current generator according to the invention can therefore be used even for producing circuits in the finest technologies having a low supply voltage.

The generator according to the invention is advantageously completed by a third transistor, of type N, of which a gate and a source are respectively connected to the gate and to the source of the second transistor.

The third transistor and the second transistor thus form a current mirror: the third transistor copies the reference current flowing in the second transistor and a current proportional (or equal) to the reference current (and therefore independent of the supply voltage) is thus accessible on the drain of the third transistor and can be used by an external circuit.

According to one embodiment, the current source, used in a generator according to the invention, comprises a fourth transistor and a fifth transistor, the supply voltage being applied to the common source of the fourth transistor and of the fifth transistor, the gate of the fourth transistor and the gate of the fifth transistor being connected together to the drain of the fifth transistor and to the drain of the third transistor and the drain of the fourth transistor being connected to the first pole of the resistor.

The invention will be better understood and other characteristics and advantages will appear on reading the following description of examples of implementation of current generators according to the invention. The description should be read in conjunction with the accompanying drawings in which:

- Figures 1 and 2, already described, are diagrams of known current generators, and

- Figures 3, 4 are diagrams of current generators according to the invention.

In a first exemplary implementation, a current generator according to the invention comprises (FIG. 3) a current source SI, a P-type transistor T7, an N-type transistor T8, and a resistor R2. The transistors T7, T8 are dimensioned so that they are saturated in normal operation.

A supply voltage λ / DD is applied to a first terminal of the current source SI, which produces a current 10 on a second terminal. The current source SI is not necessarily perfect, and in particular, the current 10 may depend on the voltage

VDD as well as any other parameter.

Resistor R2 has a first pole connected to the source of transistor T7, to the gate of transistor T8 and to the second terminal of current source SI; resistor R2 has a second pole connected to the gate of T7 and to the drain of T8.

Finally, a reference voltage VSS is applied to the source of T8 and to the drain of T7 which are connected together. The reference voltage VSS is lower than the supply voltage VDD, the voltage VSS corresponds for example to a ground voltage of the circuit.

The operation of the generator of FIG. 3 is as follows. The source SI produces the current 10, which may be variable, which is divided into a current 18, passing through resistor R2, and into a current 17, flowing between the source and the drain of transistor T7.

The transistor T7 is on and saturated (it was dimensioned for this). Consequently, the current 17 is very low, and in particular much lower than the current 18. The voltage between the gate and the source of the transistor T7 is equal to:

VGS7 = VTP7 + VDsat, with

VGS7, the voltage between the gate and the source of transistor T7,

VTP7, the threshold voltage of P-type transistor T7, of the order of 0.60 V,

VDsat, the voltage between the drain and the source of T7, at the saturation point, VDsat is very low, for example of the order of 0.05 V.

It is deduced from this that the voltage between the gate and the source of T7 is approximately equal to the threshold voltage of transistor T7. As the voltage across resistor R2 is equal to the voltage between the gate and the source of transistor T7, current 18 flowing in resistor R2 is finally equal to:

= VTP7 / R2.

As the threshold voltage VTP7 and the resistor R2 are independent of the supply voltage VDD, we obtain, as for the generators of the prior art, a current 18 independent of the supply voltage VDD. The current 18 depends on the other hand on the temperature T because the threshold voltage VTP7 depends on it according to the relation

VTP7 (T) = VTP7 (T0) -K (T-TO), where T0 is a reference temperature, K is a constant, and VTP7 (T0) is a reference value of the threshold voltage associated with the temperature T0 .

Another example of a current generator according to the invention is shown in FIG. 4. Compared to that of FIG. 3, the generator of FIG. 4 additionally comprises two transistors T9, T10 of type P and a transistor Tll of type. N. The transistors T9, T10 in this example form the current source SI.

The supply voltage λ / DD is applied to the common source of transistors T9, T10, the gates of which are connected together, the gate of T10 also being connected to its drain. The drain of T9 is connected to the first pole of resistor R2 and to the source of transistor T7; the transistor T9 produces the current 10.

The transistor T7 being saturated, as in the previous example, the current 17 flowing in the transistor T7 is very low and the current 18 flowing in the resistor R2 is little different from the current 10. The current 18 is copied by the transistors T10, Tll, consequently, a current 111 flows in the transistors T10, Tll, the current 111 being directly proportional to the current 18. The current 111 is therefore independent of the supply voltage λ / DD, but it varies linearly with the temperature.

In the example of figure 4, the transistors T9, ίο

T10 form the current source SI. However, other types of current source can of course be used to produce the source SI, the main thing being to have a source capable of supplying sufficient current on the one hand to supply and saturate the transistor T7 and on the other hand to supply resistor R2.

It will be noted that with the generator of FIG. 3 or that of FIG. 4, the minimum voltage VDDmin to be applied to the generator is equal to:

VDDmin # VGS8 + VSI, with

VGS8, the voltage between the gate and the source of transistor T8, of the order of 0.6 to 0.9 V, and

VSI, the voltage between the terminals of the current source SI; if the source SI is produced (in accordance with FIG. 4) using the transistors T9, T10, then the voltage VSI is equal to the voltage between the drain and the source of the transistor T9, it is therefore of the order of

0.2 V.

The minimum supply voltage to be used is thus of the order of VDDmin # 0.8 to 1.1 V, which is much lower than the voltage VDDmin which it is necessary to use in known generators such as those of Figure 1 or of Figure 2. A current generator according to the invention can therefore be used for any type of integrated circuit, including integrated circuits produced according to the finest technologies, for example 0.13μιη which use a low supply voltage.

It will be noted that, as in the known circuits, the current produced by a generator according to the invention is dependent on the temperature since the threshold voltage VTP7 of the transistor T7 itself depends on it. However, this is no more troublesome than for known circuits: the variations of the threshold voltage VTP7, and therefore of the current 18, as a function of the temperature are known, they are moreover linear (18 = I80 * (the .T)) and the consequences can be easily taken into account.