EP0264563B1 - Voltage regulator having a precision thermal current source - Google Patents

Description

• This invention relates to current supply circuits and, more particularly, to integrated circuits (IC) capable of producing currents having regulated magnitudes and predetermined temperature characteristics which are suitable to be used to produce a voltage regulator voltage the magnitude and temperature coefficient of which can be set.
• There are many circuit and system applications that require current supplies or sources for providing currents having predetermined temperature coefficients (TC) and regulated magnitudes which are independent of supply voltage. More particularly, it is sometimes desirable to utilize a current supply circuit providing a current with a magnitude that has a positive TC that varies directly with absolute temperature. The current can be exploited to cancel the negative TC inherent in the PN junctions of a differential pair of transistors, for instance, so as to enable the gain of a differential amplifier comprising the differential pair of transistors to remain substantially constant with temperature changes. Since IC's generally can include many such differential amplifiers, it may require a current supply circuit of the above described type that can provide a plurality of such currents each having a predetermined magnitude and temperature coefficient associated therewith. A use for such thermal current sources is in conjunction with other circuitry to provide a regulated output voltage having a known TC. This thermal current can be utilized to produce a voltage across a resistor having a positive TC which is then placed in series with the negative TC base-to-emitter voltage of a NPN transistor to provide a zero TC output voltage. These types of voltage regulators are sometimes referred to by those skilled in the art as bandgap voltage regulators.
• Prior art voltage regulators commonly include a pair of transistors operated at different current densities. The two transistors are interconnected with associated circuitry so as to develop a voltage therebetween that is proportional to the difference in the respective base-to-emitter voltages (ΔV_be). This difference voltage is used to set the current in the emitter of one of the transistors and has a positive temperature coefficient (TC). The thermal emitter current is utilized to produce a voltage that varies directly with absolute temperature which, in turn, is combined with a negative TC voltage to produce a combined voltage having a substantially zero TC.
• Although such prior art regulators have significant advantages most, if not all, suffer from serious limitations. For instance, to prevent errors in the thermal current that may be caused by differences in the collector-to-emitter voltages of the two transistors, prior art regulators require complex feedback schemes to inhibit mismatch of the two devices. These schemes are not desirable in the design of integrated circuits as undue chip area is required. Additionally, the voltage level and temperature coefficient of the output regulated voltage of these prior art regulators can not be independently set but rather are determined by the magnitude of the difference voltage ΔV_BE. Moreover, prior art regulators can not generate adjustable TC regulated voltages less than the value of a transistor V_BE voltage.
• Hence, a need exists for an improved integrated thermal current source circuit that overcomes the problems of prior 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 which does not require complex feedback circuitry to provide an output voltage that can be set to any voltage and temperature coefficient utilizing a precision thermal current source.
Summary of the Invention
• Accordingly, it is an object of the present invention to provide an improved thermal current source circuit.
• It is another object of the present invention to provide a circuit for producing a current having a regulated magnitude and temperature coefficient and which is suited to be fabricated in integrated circuit form.
• In accordance with the above and other objects there is provided a thermal current supply circuit comprising:
     first and second transistors having their bases coupled together and being operated at different predetermined current densities to produce a difference voltage between the emitters thereof having a predetermined positive temperature coefficient;
     a first resistor coupled to the emitter of said first transistor for sinking the current therefrom;
     a second resistor coupled to the emitter of said second transistor for sinking a portion of the current therefrom;
     a third transistor having its collector-emitter path coupled between the emitter of said second transistor and a circuit node (I_out); and
     feedback circuit means coupled to the collector of said second transistor and being responsive to said second transistor to provide a feedback signal to the base of said third transistor such that the latter sinks current from the emitter of said second transistor of a predetermined magnitude which is proportional to the difference voltage and inversely proportional to the value of said second resistor, said current flowing through said third transistor to the circuit node having said predetermined positive temperature coefficient.
• An improved voltage regulator is also disclosed comprising a thermal current source in accordance with the invention. The voltage regulator provides an output voltage that can be set to a predetermined voltage level and temperature coefficient.
Brief Description of the Drawings
• Fig. 1 is a schematic diagram illustrating a thermal current supply of the present invention;
• Fig. 2 is a schematic diagram illustrating a second thermal current supply of the invention;
• Fig. 3 is a schematic diagram illustrating a third thermal current supply of the invention; and
• Fig. 4 is a schematic diagram illustrating a voltage regulator which includes the thermal current supply of Fig. 3.
Detailed Description of the Preferred Embodiments
• Turning now to the Figures there are shown several embodiments of thermal current source of the present invention which are suited to be manufactured in integrated circuit form and utilized to establish a regulated voltage. It is understood that corresponding components described in relation to the Figures are designated by the same reference numerals. Fig. 1 illustrates the basic components and interconnection of reference cell 12 of thermal current source 10. Current source 10 is suited for providing fan out to multiple current sources such as NPN transistors 14, 16 and 18 coupled thereto 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 having a predetermined temperature characteristic that varies with absolute temperature.
• Reference cell 12 of thermal current source 10 includes a pair of NPN transistors 28 and 30 the emitters of which are respectively coupled via resistors 32 and 34 to the base of NPN transistor 36. The collector-emitter path of transistor 36 is coupled between the emitter of transistor 30 and negative supply conductor 38 to which negative or ground reference voltage -V is supplied. Transistor 28 is connected as a diode having its collector and base interconnected to the base 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 power supply conductor 44 to which a positive operating voltage V_cc is supplied. Feedback is provided to the base of transistor 36 by buffer NPN transistor 46 which has its base coupled to the collector of transistor 30 and its collector-emitter path coupled between conductor 44 and output node 20 (to the base of transistor 36) in series with resistor 48 to negative supply conductor 38.
• The concept of the present invention consists of (1) developing a difference voltage having a positive temperature coefficient (TC), (2) utilizing the difference voltage to set the current that flows in the collector of transistor 36 wherein the collector current has a magnitude that varies with absolute temperature, (3) utilizing the negative TC base-emitter voltage drop, V_BE, of transistor 36 to develop a current having a negative TC through resistor 56, and (4) summing the two currents at node 62 to produce a combined voltage the value and temperature coefficient of which is controllable.
• A difference voltage is produced in the present invention by operating transistors 28 and 30 at different current densities, which as understood, generates a positive difference voltage ΔV_BE between the emitters of the two transistors. In the subject 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 equals resistor 34, the voltage developed across the base-emitter of transistor 28 and resistor 32 will equal the voltage developed across the base-emitter of transistor 30 and resistor 34. However, since transistor 28 is operated at the lower current density its base-emitter voltage will be less than the base-emitter voltage of transistor 30 wherein at quiescence the aforementioned difference voltage is established between the emitters thereof. Initially, however, since transistor 28 sinks all of the current I₁ and is operated as a diode it will set the voltage to bias transistor 30. As the emitter of transistor 30 is (1/N) times smaller than the emitter of transistor 28 the former will initially sink a collector current less than the magnitude of I₂. This causes the collector voltage of transistor 30 to rise which turns on feedback transistor 46. Transistor 46 will then source base current drive to transistor 36 thereby rendering it conductive to sink a current, I_T, at its collector from the emitter of transistor 30 until the current flow through the latter equals the current I₂, which is equal to I₁. By forcing the current through transistor 30 to be equal to the current flow through transistor 28 the circuit feedback action produces the difference voltage ΔV_BE between the emitters thereof. This establishes the current I_T sank by transistor 36. Thus, from the above it can be shown that in the quiescent operating condition:
  
  ${\text{I₁R₃₂ + V}}_{\text{BE28}}{\text{=V}}_{\text{BE30}}{\text{+ (I₂-I}}_{\text{T}}{\text{)<figure-callout id="34" label="R" filenames="imgf0001.png" state="{{state}}">R</figure-callout>}}_{\text{34}}\text{and}$

  

  
  ${\text{V}}_{\text{BE30}}{\text{- V}}_{\text{BE28}}{\text{= ΔV}}_{\text{BE}}\text{, then}$

  

  
  $\text{since I₁R₃₂= I₂R₃₄}$

  

  
  ${\text{I}}_{\text{T}}{\text{= ΔV}}_{\text{BE}}\text{/R34 :}$

  

  
  
  where

ΔV_BE =

(KT/q)1n N;

K =

Boltzman's constant

T =

Absolute temperature

q =

electron charge

• Hence I_T is a thermal current having a magnitude which can be controllably set by the value of R34 and which varies in direct relation to absolute temperature. NPN transistor 46 provides feedback current to bias the base of transistor 36 to ensure that it sinks the correct collector current. Transistor 46 also buffers the fan out base currents of current supply transistors 14, 16 and 18 from affecting 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 assure 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 will be thermal currents that vary as I_T varies. These currents can be ratioed to have any desired magnitude by, for instance, utilizing emitter resistors or by emitter area ratioing. Thus, transistor 16 has resistor 49 in its emitter path and transistor 18 is shown as having multi-emitters. Thermal current source cell 12 is relatively independent to variations in the power supply voltage as the collector-emitter voltages of transistors 28 and 30 are well matched since the collector-base voltage of both transistors is substantially equal to zero.
• Referring to Fig. 2, a pair of NPN transistors 50 and 52 are shown which improve the precision of thermal current source 10. Transistor 50, which has its collector emitter path coupled between power supply conductor 44 and the bases of transistors 28 and 30 and its base connected to current source 40, buffers the base currents to the latter transistors to reduce error between I₁ and I₂. Similarly, transistor 52, with its collector-emitter path connected between power supply conductor 44 and the base of transistor 46 and its base connected to current source 42, buffers the base current of transistor 46.
• Fig. 3 shows a thermal current source 54 which provides an output current I_out that has an adjustable temperature coefficient using the concepts disclosed above with respect to current source 10. Thermal current source 54 includes an additional resistor 56 coupled between the base and emitter of transistor 36 of reference cell 12. Iout is therefore equal to:
  
  ${\text{I}}_{\text{out}}{\text{= I}}_{\text{T}}{\text{+ V}}_{\text{BE36}}\text{/R56; and}$

  

  
  ${\text{I}}_{\text{out}}{\text{= ΔV}}_{\text{BE}}{\text{/R34 + V}}_{\text{BE36}}\text{/R56}$

  

  ,
  
     where V_BE36 is the base-to emitter voltage of transistor 36; and
     R56 is the value of resistor 56.
• Since V_BE has a positive TC and V_BE36 has a negative TC, selection of the ratio of R34 to R56 can set the TC of I_out either positive, negative or even zero. It is understood that V_BE of transistor 36 is well controlled as the collector current thereof is known to be V_BE/R34.
• By way of example, resistors 32 and 34 have been illustrated above as being interconnected to the base of transistor 36. However, it is apparent from the present disclosure that resistors 32 and 34 could also be interconnected at a common node to any source of reference potential as long as transistor 30 is inhibited from becoming saturated. It is also understood that transistor 52 could be used to buffer transistor 46 as illustrated in Fig. 2.
• Fig. 4 illustrates voltage regulator 60 of the present invention which includes thermal current source 54 described above. In the preferred embodiment output node 62 is connected in series with additional resistor 64. Transistor 52, which has its base-emitter coupled between the collector of transistor 30 and the base of transistor 46 and its collector coupled to conductor 44 further buffers the collector of transistor 30 from the effects of load currents sourced at node 66 to a load means connected thereto. Additionally, transistor 52 also ensures that the collector voltage of transistor 30 equals 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 66 at which is produced regulated output voltage V_out.
• A voltage is developed across resistor 64 that is proportional to the current I_out combined with the V_BE of transistor 36 to produce combined voltage V_out. Thus, V_out is equal to:
  
  ${\text{V}}_{\text{out}}{\text{= V}}_{\text{BE36}}{\text{(1 + R64/R56 ) + ΔV}}_{\text{BE}}\text{R64/R34 (4)}$

  

  
  
     where R64 is the value of resistor 64.
• Hence, by proper selection of resistor ratios, V_out can be set to any desired voltage and any temperature coefficient independently of one another.
• It is understood that although V_OUT is taken at output 66 in the preferred embodiment, a regulated output voltage is also produced at node 62 which could be used as an output voltage of the regulator.
• Thus, what has been described above is a novel voltage regulator comprising a thermal current source for providing a thermal current having an adjustable temperature coefficient and means for developing a voltage proportional to the thermal current and combining the voltage with another voltage of a different temperature coefficient to produce a combined voltage the magnitude and temperature coefficient of which can be independently controlled.

Claims (9)

1. A thermal current supply circuit comprising:
      first (28) and second (30) transistors having their bases coupled together and being operated at different predetermined current densities to produce a difference voltage between the emitters thereof having a predetermined positive temperature coefficient;
      a first resistor (32) coupled to the emitter of said first transistor for sinking the current therefrom;
      a second resistor (34) coupled to the emitter of said second transistor for sinking a portion of the current therefrom;
      a third transistor (36) having its collector-emitter path coupled between the emitter of said second transistor and a circuit node (I_out); and
      feedback circuit means (46) coupled to the collector of said second transistor and being responsive to said second transistor to provide a feedback signal to the base of said third transistor such that the latter sinks current from the emitter of said second transistor of a predetermined magnitude which is proportional to the difference voltage and inversely proportional to the value of said second resistor, said current flowing through said third transistor to the circuit node having said predetermined positive temperature coefficient.
2. The thermal current supply circuit according to claim 1 further comprising:
      a third resistor (56) coupled between the base and emitter of the third transistor for providing a current to the circuit node (I_out) having a predetermined magnitude and a negative temperature coefficient that is summed with the current flowing through said third transistor, wherein the summed current developed at the circuit node has a controllable magnitude and controllable temperature coefficient which are dependent on the predetermined magnitude of the current flowing through said third transistor and the predetermined magnitude of the current provided by the third resistor.
3. The thermal current supply circuit according to claim 1 or 2 wherein said feedback circuit means (46) comprises a fourth transistor (46) having an emitter coupled to said base of said third transistor, a collector coupled to a first power supply conductor (44), and a base coupled to a collector of said second transistor.
4. The thermal current supply circuit according to claim 3 further comprising:
      first conductive means (50) for coupling the collector of said first transistor to said base thereof, said bases of said first and second transistors being coupled together; and
      current source means (40, 42) coupled between said first power supply conductor (44) and the collectors of said first and second transistors for supplying current thereto.
5. The thermal current supply circuit according to claim 4 wherein the emitter area of said first transistor is N times the emitter area of said second transistor, where N is a positive number and said feedback circuit means forces the current conducted by said first and second transistors to be substantially equal.
6. The thermal current supply circuit according to claim 5 further comprising:
      a fifth transistor (52) having a base coupled to said collector of said second transistor, a collector coupled to said first power supply conductor (44) and an emitter coupled to said base of said fourth transistor (46), said first conductive means includes a sixth transistor (50) having a base coupled to said collector of said first transistor, a collector coupled to said first power supply conductor (44) and an emitter coupled to said base of said first transistor.
7. The thermal current supply circuit according to claim 3, 4, 5 or 6 wherein said feedback circuit means further comprises circuit means (48) coupled between said emitter of said fourth transistor and a second power supply conductor (38) for sinking a predetermined current from said fourth transistor.
8. An integrated voltage regulator (60) comprising a thermal current supply circuit according to claim 2, 3, 4, 5, 6 or 7 and a fourth resistor (64) connected to said circuit node (I_out) through which said currents flowing through said third transistor and said second resistor (56) are summed such that a regulated voltage is developed thereacross, the magnitude and temperature coefficient of which can be independently set.
9. The voltage regulator according to claim 8 further including means for coupling the base of said third transistor to an output of the voltage regulator.

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