Voltage reference with linear, negative, temperature coefficient

Abstract: A bandgap voltage reference circuit employs a Vbe voltage multiplier network in a feedback line of an output amplifier of the bandgap reference circuit, thus permitting to independently fix the output voltage that is produced and the temperature coefficient thereof. A voltage reference having a linear negative temperature coefficient in an extended temperature variation range may be obtained, starting from a bandgap reference voltage with a positive temperature coefficient.

Description: FIELD OF THE INVENTION The present invention relates to a circuit capable of generating a reference voltage having a negative temperature coefficient, starting from a bandgap reference with a positive temperature coefficient. BACKGROUND OF THE INVENTION In a large variety of electronic circuits, it is often required that certain control parameters maintain always the same preset values, independently of the variation of temperature to which the circuit may be subject. For example a parameter to be so controlled may be the maximum limiting current that can circulate through a load, that is, for example, through a power transistor driving an external load. Commonly, such a temperature stabilization is implemented by comparing the voltage drop on a sensing resistance through which the current to be controlled flows (which voltage drop signal is normally used for driving a control and regulation feedback loop) with a reference voltage. For example, if the control and regulation loop must intervene always upon the reaching of the same output current value, it is necessary that the reference voltage vary with the temperature with the same law of the sensing resistance, in view of the fact that a resistor (in an integrated or discrete form) notably has a nonnegligeable temperature coefficient. A circuit that is widely used for generating a voltage that varies according to a precise law with the temperature, is the so-called bandgap reference circuit, a functional diagram of which is depicted in Fig. 1. A bandgap reference circuit as the one shown in Fig. 1, is based on the principle of exploiting variations of opposite sign with the temperature of two parameters, namely the base-emitter voltage Vbe (≈-2mV/°C) and the so-called thermal voltage: Vt (≈+0.085mV/°C). By referring to the diagram of Fig. 1, the bandgap voltage (Vbg) provided by the circuit is given by: Vbg = Vbe1 + K*Vt wherein: Vt = k T q and K is a constant that depends from the values of the resistances RA and RB and by the ratio n2/n1 between the emitter areas of the respective transistors Q1 and Q2. By expanding the formula (1): From the above formula (2) it may be observed that by varying the ratio RB/RA and/or n2/n1, a temperature coefficient of the Vbg that extends from -2mV/°C to positive values may be obtained. The documents EP-A-0 216 265 discloses a band-gap circuit for generating reference voltage of a predeterminate temperature coefficient, which may also be negative, starting from a band-gap voltage generated by a band-gap reference circuit and an amplifier. The circuit includes a network (NW) consisting of a Vbe voltage multiplier circuit (T4, R5, R6), functionally connected between the band-gap reference circuit and a regulated supply node. The document GB-A- 2 121 629 discloses a band-gap circuit generating a positive temperature coefficient voltage across a resister which is part of a voltage divider, whereby the positive temperature coefficient scaled up by the voltage divider ratio is added to the negative temperature coefficient of the base-emitter voltage of transistor to provide a zero temperature coefficient output voltage Vreg2. The document US-A-4 636 710 discloses a temperature stabilized stacked band-gap voltage reference regulator, whereby a pair of ratioed emitter size transistors is operated to produce a delta Vbe. This voltage is combined with a negative temperature coefficient voltage produced by forward biased series connected diodes to produce a combined voltage that is a multiple of the semiconductor band-gap extrapolated to absolute zero. The document US-A-4 683 416 discloses a voltage supply circuit for providing a regulated output voltage the magnitude and temperature coefficient of which can be independently controlled. An intrinsic limitation of this solution, consists in the fact that the variation of the bandgap voltage (Vbg) that is generated, does not remain linear for all possible values of T, but it may be considered linear only within a restricted range of variation of temperature that becomes wider with an increase of the coefficient K. In other words, the equation (2) ceases to be valid beyond a certain temperature and the range of linearity that is associated with the bandgap circuit of Fig. 1, becomes relatively small if a negative temperature coefficient is desired for the produced bandgap voltage Vbg. On the other hand, in many practical applications, it is required that the voltage variation remain linear for a relatively broad range of variation of temperature for example: -40°C < T < 150°C A further drawback of the known bandgap reference circuits, is that the choice of the termal coefficient and of the voltage Vbg that is generated are tied together in the sense that, once the value of one of these two parameters is fixed, the other is also automatically fixed. Therefore, there is a necessity or utility for a circuit capable of generating a reference voltage with a negative temperature coefficient, starting from a bandgap reference voltage having a positive temperature coefficient, in order to obtain a broad range of linear variation with a negative temperature coefficient. This and other objectives and advantages are obtained by the circuit for generating a reference voltage with a negative temperature coefficient, object of the present invention. Basically, the circuit of the invention permits to generate a reference voltage with a negative temperature coefficient, starting from a bandgap voltage having a positive temperature coefficient. Moreover, the selection of a certain temperature coefficient does not restrain the definition of the value of the reference voltage that is produced, thus allowing to associate with a certain selected temperature coefficient a generated reference voltage of any desired level. Basically, the circuit of the invention comprises a common, bandgap voltage generating network and an output amplifier, that, according to the invention, is provided with a feedback network which comprises a multiplier of a Vbe voltage. A Vbe multiplier circuit is functionally connected between an output node of the amplifier and a node of the bandgap voltage generating network onto which the bandgap voltage is generated, which is connected to ground through a resistance that fixes the current that circulates through the Vbe multiplier circuit. A resistive output voltage divider is functionally connected between the output node of the amplifier and ground. The different aspects and advantages of the circuit of the invention will become more evident through the following description of several important embodiments and by referring to the attached drawings, wherein: Figure 1 is a functional diagram of a bandgap reference voltage generating circuit according to the prior art; Figure 2 is a functional block diagram of a reference voltage generating circuit according to the present invention; Figure 3 is a circuit diagram of a Vbe multiplier that may be employed in the circuit of the invention; Figure 4 is a circuit diagram of an embodiment of the circuit of the invention. With reference to Fig. 2, the circuit of the invention may employ a common, bandgap reference voltage generating circuit, as the one depicted in Fig. 1, here schematically identified as a block. Of course, the bandgap voltage generating circuit may have any of the known architectures, it may be realized with junction bipolar transistors, as shown in some of the figures, but may also be realized with field effect transistors. Between the output node A of the amplifier and the bandgap node (Vbg) of the bandgap voltage generating network, is connected a Vbe voltage multiplier circuit (K'*Vbe) through which circulates a current that, may be suitably stabilized against variations of the supply voltage. A load resistance R is connected between the Vbg node and ground. The reference voltage Vout that is produced by the circuit may be tapped from an intermediate node of a resistive output voltage divider R1-R2, connected between the output node A of the amplifier and ground. By analysing the circuit of Fig. 2, Vout = R2 R1 + R2 (Vbg + K'Vbe) wherein K' is the multiplication factor of a Vbe voltage of the relative multiplier circuit. By differentiating in terms of temperature the equation (3): dVout dT = K1 dVbg dT + K1*K' dVbe dT wherein K1 = R2 R1 + R2 Of course, for obtaining a negative temperature coefficient of the reference voltage Vout that is generated, starting from a positive temperature coefficient of the bandgap voltage Vbg, the following disequality must hold: Solution of the system of equations formed of the equations (3) and (4) permits to obtain the values of the resistive voltage divider R1-R2, as well as of the multiplication factor K' of the Vbe multiplier circuit, that are required for producing an output voltage Vout having the desired negative temperature coefficient. The Vbe multiplier circuit (K'Vbe) may have any suitable circuit form. In Fig. 3 a circuit suitable to implement the Vbe multiplier circuit is shown. The circuit is composed of a bipolar transistor Q, the base of which is connected to an intermediate node of a resistive voltage divider RK-RH of the voltage present between the collector and the emitter of the transistor. The multiplication factor is given by the ratio between the two resistances RK and RH that compose the voltage divider, plus 1. An alternative embodiment of a Vbe multiplier circuit is depicted in the circuit diagram of Fig. 4, which shows an embodiment of the whole circuit. The bandgap voltage generating network is composed of Q6, Q7, Q8 and Q9, RA and RB, and is indicatively confined within a dash line perimeter 1. The output amplifier of the bandgap circuit is constituted by a first amplifying stage, composed of a common-collector configured transistor Q10, having a load constituted by a current generator Q4. Q10 "sees" as a total load, the current generator Q4 and the base of the transistor Q5, also in a common-collector configuration, which constitutes a second amplifying stage. Through the output node A of the second amplifying stage, constituted by the transistor Q5, current is delivered to the network that characterize the circuit of the invention and which is indicatively confined in the dash line perimeter 2 of Fig. 4. Through the output network 2, current is injected into the bases of the transistors Q8 and Q9 of the bandgap voltage generating network, thus implementing a stabilization feedback loop of the output voltage. By assuming that on the Vbg node that corresponds to the bases of the transistors Q8 and Q9, the voltage tend to rise, the collector voltage of the transistor Q9 will tend to decrease, thus forcing Q10 to conduct more current and to subtract current from the base of Q5. As a consequence, also the emitter current of the transistor Q5 and therefore the voltage drop on R10 will tend to decrease, thus stabilizing the output voltage Vout. In the embodiment shown in Fig. 4, the Vbe voltage multiplier circuit is constituted by a chain of directly biased diodes, D1 ... Dn. Advantageously, the bandgap voltage generating network, that is the emitters of transistors Q6 and Q7 that constitute the biasing current mirror of the pair of transistors Q8 and Q9, are not direclty connected to Vcc, but to the output node A of the second amplifier stage onto which is intrinsically present a stabilized voltage in respect of possible variations of the supply voltage Vcc. Also the currents in the two branches of the current mirror composed of Q3 and Q4 (the latter forcing a bias current on the amplifying stage Q10) may advantageously be fixed by Q2 and R8 at a stabilized level, by driving the transistor Q2 with the stabilized voltage present on the node A. The diode D5 has the function of making symmetrical the operating conditions of the two branches (Q6-Q8 and Q7-Q9) of the mirror. In fact: Vc Q6 + Veb Q6 + Vbe Q5 - Vd 6 - Veb Q10 = Vc R7 therefore: Vc Q6 ≈ Vc Q7 Finally, the circuit may be completed by a "start-up" network composed of R7, D3 ... D4 and Q1. By analysing again equation (2), the following relationship may be derived: dVbg dT = dVbe dT +2 RB RA k q ln (n) where n: A8 A9 A8 and A9 being the emitter areas of the respective transistors Q8 and Q9. In view of the equation (6), equation (4) becomes: dVout dT = (K'+1) R2 R1 + R2 dVbe dT + 2 RB*R2 RA(R1+R2) k q ln(n) From this last equation, it is easily observed that, for- obtaining a negative temperature coefficient, it will be sufficient to verify the following disequality: By establishing a certain value of Vout, at room temperature, the values of R1, R2, RA, RB and K' may be easily calculated, in order to obtain the desired temperature coefficient of the reference voltage Vout generated by the circuit. From what has been described above, it is clear that all the stated objectives are fully met by the circuit of the invention, in particular a certain output voltage Vout at room temperature may be fixed according to need and on the other hand, a precise temperature coefficient may be implemented according to what required by the particular compensating circuit that will utilize the reference voltage (Vout) produced by the circuit of the invention.

Claims: A circuit for generating a reference voltage with a negative temperature coefficient from a bandgap voltage with a positive temperature coefficient as generated by a bandgap reference circuit comprising a bandgap voltage generating network (Q6,Q7,Q8,Q9,RA,RB) and an amplifier (Q10,Q4,Q5) characterized by comprising a network consisting of at least a Vbe voltage multiplier circuit, (D1,D2,...Dn) functionally connected between an output node (A) of said amplifier and a node at said bandgap voltage (Vbg) of said bandgap voltage generating network, at least a resistance (R10) connected between said node at bandgap voltage (Vbg) and ground and a resistive voltage divider (R1,R2) connected between said output node (A) of said amplifier and ground. A circuit as defined in claim 1, characterized by the fact through said bandgap voltage generating network circulates a biasing current that is stabilized against variations of the supply voltage. A circuit as defined in claim 1, characterized by the fact that said amplifier comprises at least a first (Q10,Q4) and a second (Q5) amplifying stages. A circuit as defined in claim 3, wherein each of said first and second amplifying stages is constituted by a common-collector configured bipolar transistor (Q10,Q5). A circuit as defined in claim 4, wherein said Vbe voltage multiplier circuit comprises a bipolar transistor (Q) having a base connected through a first resistance (RK) to said output node (A) of said second amplifying stage, to which a collector of the transistor (Q) is also connected, said base being further connected through a second resistance (RH) to said bandgap voltage node (Vbg) to which an emitter of the transistor (Q) is also connected; the multiplication factor being given by the ratio between said first resistance (RK) and said second resistance (RK) plus 1. A circuit as defined in claim 1, wherein said Vbe voltage multiplier circuit comprises a plurality of directly biased diodes (D1,D2,...Dn), connected in series between said output node (A) of said amplifier and said bandgap voltage node (Vbg). A circuit as defined in claim 1, wherein said bandgap voltage generating network (Q6,Q7,Q8,Q9,RA,RB) is supplied with the voltage present on said output node (A) of said amplifier; a biasing current, defined by a transistor (Q2) driven by the voltage present on said output node of said amplifier and by the value of a resistance (R8) connected between said transistor (Q2) and ground, being mirrored (Q3) on a load element (Q4) of the amplifier Q10.

Inventors: AIELLO NATALE, PALARA SERGIO, SCACCIANOCE SALVATORE, AIELLO, NATALE, PALARA, SERGIO, SCACCIANOCE, SALVATORE
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO - CORIMME, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: G05F3
IPC codes: G05F3