DE3788033T2 - Voltage regulator with precision thermal current source. - Google Patents

Description

The invention relates to power supply circuits and, in particular, to integrated circuits (ICs) capable of generating currents of controlled magnitude and with predetermined temperature characteristics suitable for use in generating a voltage regulator voltage whose magnitude and temperature coefficient can be adjusted.

There are many circuit and system applications which require power supplies or sources for supplying currents having predetermined temperature coefficients (TC) and regulated magnitudes, and which are independent of the supply voltage. In particular, it is sometimes desirable to use a power supply circuit which provides a current of a magnitude having a positive TC which varies directly with absolute temperature. The current may be used to cancel the negative TC present at the PN junctions of a differential transistor pair, e.g. to enable the gain of a differential amplifier comprising a differential transistor pair to remain substantially constant with temperature changes. Since ICs may generally contain many such differential amplifiers, a current source circuit of the type described above may be required which provides a plurality of such currents, each having a predetermined magnitude and an associated temperature coefficient. One use for such thermal current sources is in conjunction with other circuits in producing a regulated output voltage having a known TC. This thermal current can be used to create a voltage with a positive TC across a resistor, which is then placed in series with the base/emitter voltage with a negative TC of an NPN transistor to create an output voltage with a zero TC. These types of voltage regulators are sometimes referred to by those skilled in the art as bandgap voltage regulators.

State of the art voltage regulators typically include a pair of transistors operating at different current densities. The two transistors are connected with associated circuitry to develop a voltage between them that is proportional to the difference in their respective base/emitter voltages (ΔVBE). This differential voltage is used to adjust the current at the emitter of one of the transistors and has a positive temperature coefficient (TC). The thermal emitter current is used to generate a voltage that varies directly with absolute temperature, which in turn is combined with a voltage with negative TC to produce a combined output voltage with essentially zero TC.

Although the prior art regulators have considerable advantages, most if not all suffer from serious limitations. For example, to prevent thermal current errors caused by differences in the collector/emitter voltages of the two transistors, prior art regulators require complicated feedback schemes to prevent mismatch of the two devices. These schemes are undesirable in integrated circuit design because they require an undue amount of chip area. In addition, the voltage level and temperature coefficient of the regulated output voltage of these prior art regulators cannot be independently adjusted, but are determined by the magnitude of the differential voltage ΔVBE. In addition, prior art regulators cannot produce regulated voltages with adjustable TC that are below the VBE voltage of a transistor.

Thus, there is a need for an improved integrated thermal current source circuit that overcomes the problems of the state-of-the-art thermal current source circuits.

An additional need exists for a regulator circuit that does not suffer from the aforementioned limitations of the prior art regulators and does not require a complex feedback circuit to provide an output voltage that can be tuned to any voltage and temperature coefficient using a precision thermal current source.

Summary of the invention

It is accordingly an object of the present invention to provide an improved thermocurrent source circuit.

It is a further object of the present invention to provide a circuit for generating a current having a controlled magnitude and a controlled temperature coefficient, which is suitable for manufacture in the form of an integrated circuit.

In accordance with the above and other objects, there is provided a thermo-power supply circuit comprising:

Thermal power supply circuit comprising:

first and second transistors having their bases coupled together and operating at different predetermined current densities to produce a differential voltage having a predetermined positive temperature coefficient between their emitters;

a first resistor coupled to the emitter of the first transistor to sink current therefrom;

a second resistor coupled to the emitter of the second transistor to sink a portion of the current therefrom;

a third transistor whose collector/emitter path is coupled between the emitter of the second transistor and a circuit node (Iout); and

Feedback circuit means coupled to the collector of the second transistor and responsive to the second transistor for providing a feedback signal to the base of the third transistor such that the latter sinks current from the emitter of the second transistor of a predetermined magnitude proportional to the differential voltage and inversely proportional to the value of the second resistor, which current flows through the third transistor to the circuit node and has the predetermined positive temperature coefficient.

An improved voltage regulator is also described which includes a thermocurrent source according to the invention. The voltage regulator provides an output voltage which can be set to a predetermined voltage level and a predetermined temperature coefficient.

Short description of the drawing

Fig. 1 is a schematic diagram illustrating a thermal power supply according to the present invention;

Fig. 2 is a schematic diagram illustrating a second thermal power supply according to the invention;

Fig. 3 is a schematic diagram showing a third thermal power supply according to the invention; and

Fig. 4 is a schematic diagram illustrating a voltage regulator incorporating the thermal power supply of Fig. 3.

Detailed description of preferred designs

Turning now to the figures, there are shown various embodiments of thermal current sources in accordance with the present invention which are suitable for manufacture in integrated circuit form and used to establish a regulated voltage. It is to be understood that like components described with respect to the figures are designated by like reference numerals. Fig. 1 illustrates the basic components and a connection of a reference cell 12 of thermal current source 10. Current source 10 is adapted to provide a multiple output to multiple current sources such as NPN transistors 14, 16 and 18 coupled to it at terminal 20. The collectors of the current source transistors are connected to respective current utilization circuits 22, 24 and 26, each of which requires a current with a predetermined temperature characteristic that varies with the absolute temperature.

The reference cell 12 of the thermal current source 10 includes a pair of NPN transistors 28 and 30 having their emitters connected to the base of an NPN transistor 36 through resistors 32 and 34, respectively. The collector/emitter path of transistor 36 is coupled between the emitter of transistor 30 and the negative power supply conductor 38 to which a negative or ground reference voltage -V is applied. Transistor 28 is diode-connected with its collector and base terminals connected to the base terminal of transistor 30. A pair of current sources 40 and 42 supply currents I₁ and I₂ to the collectors of transistors 28 and 30, respectively, and are connected to a supply conductor 44 to which a positive supply voltage Vcc is applied. Feedback to the base of transistor 36 is provided by a buffer NPN transistor is created, the base of which is coupled to the collector of transistor 30 and the collector/emitter path between conductor 44 and output node 20 (to the base of transistor 36) is connected in series with resistor 48 to the negative supply conductor 38.

The concept of the present invention consists of (1) developing a differential voltage with a positive temperature coefficient (TC), (2) using the differential voltage to adjust the current flowing into the collector of transistor 36, the collector current having a magnitude that varies with absolute temperature, (3) exploiting the base-emitter voltage drop VBE of the negative TC transistor to develop a current with a negative TC through resistor 56, and (4) summing the two currents at node 62 to create a combined voltage whose value and voltage coefficient are controllable.

A differential voltage is produced in the present invention by operating transistors 28 and 30 at different current densities which, as will be understood, produce a positive differential voltage VBE between the emitters of the two transistors. In the present invention, transistor 28 is operated at a lower current density than transistor 30 by making its emitter area N times larger than the emitter area of transistor 30 (where N is a positive number) and setting I1 equal to I2. If resistor 32 is equal to resistor 34, the voltage developed across the base/emitter junction of transistor 28 and resistor 32 will be equal to the voltage developed across the base/emitter junction of transistor 30 and resistor 34. However, since the transistor 28 is operated with the lower current density, its base/emitter voltage will be lower than the base/emitter voltage of the transistor 30, thereby maintaining a rest position of the aforementioned differential voltage between their emitters Initially, however, transistor 28, sinking all of the current I₁ and operating as a diode, will set the voltage to bias transistor 30. Since the emitter of transistor 30 is (1/N) times smaller than the emitter of transistor 28, the former will initially sink a smaller collector current than the magnitude of I₂. This will cause the collector voltage of transistor 30 to rise, which in turn will turn on feedback transistor 46. Transistor 46 will then supply base drive current to transistor 36, thereby rendering it conductive to sink a current IT at its collector from the emitter of transistor 30 until the current flow through the latter is equal to current I₂, which is equal to I₁. By impressing the current through transistor 30 to the same magnitude as the current flowing through transistor 28, the circuit feedback effect will produce the differential voltage VBE between their emitters. This establishes the current IT sunk by transistor 36. Thus, from the above, it can be shown that the quiescent operating condition is:

I₁R₃₂ + VBE28 = VBE30 + (I₂ - IT)R₃₄ and

VBE30 - VBE28 = ΔVBE, and then,

since I₁R₃₂ = I₂R₃₄,

IT = ΔVBE/R₃₄ :

where ΔVBE = (KT/q)1n N;

K = Boltzmann constant

T = absolute temperature

q = electron charge.

Therefore, IT is a thermal current having a magnitude that can be controllably adjusted by the value of R34 and that varies in direct relation to absolute temperature. The NPN transistor 46 provides a feedback current to bias the base of transistor 36 to ensure that it sinks the correct collector current. The transistor 46 buffers the radiating base currents of the power supply Transistors 14, 16 and 18 from interfering with the operation of transistors 28 and 30. Resistor 48 is selected to sink a current greater than the sum of the currents flowing through resistors 32 and 34 to insure the proper bias current in transistor 46. By grounding the emitter of transistor 36 and coupling the bases of current source transistors 14, 16 and 18 to terminal 20, all of the collector currents of the latter become thermocurrents which vary as IT varies. These currents can be ratioed to be of any desired magnitude, for example by using emitter resistors or an emitter area ratio. Thus transistor 16 has resistor 46 in its emitter path and transistor 18 is shown with multiple emitters. The thermal current source cell 12 is relatively independent of changes in the power supply voltage because the collector/emitter voltages of the transistors 28 and 30 are well matched because the collector-base voltage of the two transistors is essentially zero.

In Fig. 2, a pair of NPN transistors 50 and 52 are shown which improve the accuracy of the thermal current source 10. The transistor 50, with its collector/emitter connected between the power supply conductor 44 and the base terminals of the transistors 28 and 30 and its base connected to the current source 40, buffers the base currents for the latter transistors to reduce an error between I1 and I2. Similarly, the transistor 52, with its collector/emitter connected between the power supply conductor 44 and the base of the transistor 46 and its base connected to the current source 42, buffers the base current of the transistor 46.

Fig. 3 shows a thermal current source 54 which provides an output current Iout having an adjustable temperature coefficient using concepts described above with respect to the current source 10. The thermal Current source 54 contains an additional resistor 54 which is connected between the base and emitter of transistor 36 of reference cell 12. Iout is therefore:

Iout = TT + VBE36/R₅₆; and

Iout = ΔVBE/R₃₄ + VBE36/R₅₆,

where VBE36 is the base/emitter voltage of transistor 36 ; and R56 is the resistance of resistor 56.

Since VBE has a positive TC and VBE36 has a negative TC, the selection of the ratio of R34 to R56 can set the TC of Iout either positive, negative, or even zero. It can be understood that VBE of transistor 36 is well controlled since its collector current is known as VBE/R34.

For example, resistors 32 and 34 were shown above as being connected to the base of transistor 36. However, it will be appreciated from the present description that resistors 32 and 34 may also be connected at a common node to any reference voltage source as long as transistor 30 is protected from going into saturation. It is also to be understood that transistor 52 may be used to buffer transistor 46 as shown in Figure 2.

Fig. 4 illustrates the voltage regulator 60 of the present invention incorporating the thermal current source 54 described above. In the preferred embodiment, the output node 62 is connected in series with an additional resistor 64. The transistor 52, whose base/emitter path is connected between the collector of the transistor 30 and the base of the transistor 46 and whose collector is coupled to the conductor 44, further buffers the collector of the transistor 30 against the effects of load currents drawn at node 66 to a load connected thereto. In addition, the transistor 52 also ensures that the collector voltage of transistor 30 is equal to the collector voltage of transistor 28 to prevent mismatch between the two transistors. Resistor 68 is connected between the emitter of transistor 52 and output terminal 62 at which a regulated output voltage Vout is produced.

A voltage is developed across resistor 64 that is proportional to current Iout, combined with the VBE of transistor 36 to give the combined voltage Vout. This makes Vout equal to:

Vout = VBE36 (1 + R64/R56) + ΔVBE R64/R34, (4)

where R64 is the resistance value of resistor 64.

Thus, by properly selecting the resistance ratio values, Vout can be set to any desired voltage and to any temperature coefficient, independently of each other.

It will be understood that although Vout is taken from output 66 in the preferred embodiment, a regulated output voltage that can be used as the output voltage of the regulator is also produced at node 62.

Thus, what is described above is a novel voltage regulator comprising a thermal current source for providing a thermal current having adjustable temperature coefficients and means for developing a voltage proportional to the thermal current and combining the voltage with another voltage having a different temperature coefficient to produce a combined voltage whose magnitude and temperature coefficient can be independently controlled.

Claims (9)

1. Thermal power supply circuit comprising: first (28) and second (30) transistors having their bases coupled to one another and operating at different predetermined current densities to produce a differential voltage having a predetermined positive temperature coefficient between their emitters; a first resistor (32) coupled to the emitter of the first transistor to act as a sink for the current therefrom; a second resistor (34) coupled to the emitter of the second transistor to act as a sink for a portion of the current therefrom; a third transistor (36) whose collector/emitter path is coupled between the emitter of the second transistor and a circuit node (Iout); and Feedback circuit means (46) coupled to the collector of the second transistor and responsive to the second transistor for providing a feedback signal to the base of the third transistor such that the latter sinks current from the emitter of the second transistor of a predetermined magnitude proportional to the differential voltage and inversely proportional to the value of the second resistor, which current flows through the third transistor to the circuit node and has the predetermined positive temperature coefficient. 2. Thermal power supply circuit according to claim 1, further comprising: a third resistor (56) connected between the base and emitter of the third transistor for providing a current to the circuit node (Iout) having a predetermined magnitude and a negative temperature coefficient which is summed with the current flowing through the third transistor, the summed current developed at the circuit node having a controllable magnitude and a controllable temperature coefficient which are dependent on the predetermined magnitude of the current flowing through the third transistor and the predetermined magnitude of the current provided by the third resistor. 3. Thermal power supply circuit according to claim 1 or 2, wherein the feedback circuit means (46) comprises a fourth transistor (46) having an emitter coupled to the base terminal of the third transistor, a collector coupled to a first power supply conductor (44) and a base terminal coupled to a collector of the second transistor. 4. A thermal power supply circuit according to claim 3, further comprising: first conductive means (50) for coupling the collector of the first transistor to its base terminal, the base terminals of the first and second transistors being coupled together; and Current source means (40, 42) coupled between the first power supply conductor (44) and the collectors of the first and second transistors for supplying current thereto. 5. A thermal power supply circuit as claimed in claim 4, wherein the emitter area of the first transistor is N times the emitter area of the second transistor, where N is a positive number and the feedback circuit means forces the current conducted by the first and second transistors to be substantially equal. 6. A thermal power supply circuit according to claim 5, further comprising: a fifth transistor (52) having a base terminal coupled to the collector of the second transistor, a collector coupled to the first power supply conductor (44), and an emitter coupled to the base terminal of the fourth transistor (46), the first conductive means including a sixth transistor (50) having a base terminal coupled to the collector of the first transistor, a collector coupled to the first power supply conductor (44), and an emitter coupled to the base terminal of the first transistor. 7. A thermal power supply circuit according to claim 3, 4 or 6, wherein the feedback circuit means further comprises circuit means (48) coupled between the emitter of the fourth transistor and a second power supply conductor (8) for sinking a predetermined current from the fourth transistor. 8. An integrated voltage regulator (60) comprising a thermal power supply circuit according to claim 2, 3, 4, 5, 6 or 7 and a fourth resistor (64) connected to the circuit node (Iout) by which the currents flowing through the third transistor and the second transistor (56) are summed so as to develop a regulated voltage across it, the magnitude and temperature coefficient of which can be independently adjusted. 9. The voltage regulator of claim 8, further comprising means for coupling the base terminal of the third transistor to an output of the voltage regulator.