VOLTAGE REGULATOR AND VOLTAGE STABILIZER

Abstract: A voltage stabilizer with a minimal voltage drop designed to withstand high voltage transients comprises a "series" type voltage regulator circuit with a power transistor of n-p-n type (T′1). The collector terminal of this transistor (T′1) is connected to earth via a capacitor (C′) and to the cathode of a diode (D′) whose anode forms an input terminal (IN′) of the stabilizer. The base terminal of the power transistor (T′1) is connected to the collector terminals of first and second transistors (T′2, T′3) of p-n-p type which have their emitter terminals connected to the cathode and the anode, respectively, of the diode (D′) and their base terminals connected to circuit biasing means (D′2, D′3, G′2, G′3), Feedback control of output voltage is provided by a differential amplifier (A′) and a potential divider (R′1, R′2).

Description: The present invention relates to a voltage regulator, for instance for a voltage stabilizer comprising a monolithically integrated regulator circuit for use in motor vehicle applications, and to a voltage stabilizer. Voltage stabilizers supply, from a supply voltage whose value is not defined, a voltage with a well defined and constant value or values. Voltage stabilizers may be advantageously used as supply devices for other devices: as a function of the load connected thereto they in fact supply the current needed to keep the voltage applied to this load constant. At present, for reasons of compactness, ease of use and economic viability, integrated circuit electronic voltage stabilizers are tending to be constructed for all types of application. In general, the electrical magnitudes of voltage and current at the output terminals of these electronic voltage stabilizers are determined by an internal regulation circuit which is monolithically integrated and comprises circuit feedback means connected to the output terminals and sensitive to the instantaneous value of these electrical magnitudes. The lower limit of the correct operating range of an electronic voltage stabilizer is pinpointed by a parameter known in general in the technical literature by the term "drop-out", i.e. the difference between the minimum value of the input voltage required for the correct operation of the stabilizer and the value of the constant voltage which the stabilizer has to supply as output, which is thus the voltage drop across the device. Voltage stabilizers used in motor vehicle applications must satisfy particularly strict requirements as a result of operating conditions which entail major variations in temperature and humidity as well as considerable, and in some cases abrupt, variations in the supply voltage generated by the battery of the vehicle. These stabilizers must therefore be extremely reliable, accurate and stable, while still being economically viable, and must in particular have a low drop-out since the supply voltage supplied by the battery of a vehicle may normally drop, during cold starting, from a typical 14.4 V at full charge to approximately 6 V. Account must also be taken of the positive and negative voltage peaks with a maximum amplitude of up to 150 V which may be present on the supply line of a vehicle as a result of the switching transients of the inductive loads (starter solenoids, relays, etc.) or of disconnections or breakages of electrical connection cables. The monolithically integrated voltage regulator circuits most commonly used in voltage stabilizers for motor vehicle applications are those with "series" type regulation, in which the output voltage is regulated to a constant value by a bipolar power transistor connected in series between input and output terminals of the stabiliser. The base of the transistor is appropriately controlled to cause it to conduct as a function of the load. A suitably dimensioned power transistor can also withstand, without drawbacks, positive voltage peaks having a high amplitude and may thus continue to ensure the regulation of the output voltage. Negative peaks in the input voltage could, however, cause the transistor to be cut off, with interruptions, albeit brief, in the supply to the user circuits connected to the voltage stabilizer, with serious drawbacks when these comprise memories and integrated logic circuits which require a constant supply. For this reason voltage stabilizers comprising "series" type regulation circuits also comprise a capacitor and an input diode, which are not integrated, in order to maintain, during very short negative transients in the input voltage, an adequate supply to the power transistor. Figures 1 and 2 of the accompanying drawings show a known voltage stabilizer with "series" type regulation by means of a p-n-p transistor and a known voltage stabilizer with "series" type regulation by means of an n-p-n power transistor, respectively. The stabilizer shown in Figure 1 comprises a bipolar transistor T of p-n-p type whose emitter terminal is connected to the cathode of a diode D, whose anode forms an input terminal IN, and to a first terminal of a capacitor C whose second terminal is connected to earth. The collector terminal of the transistor T forms an output terminal OUT. The base terminal of the transistor T is connected to the output terminal of a differential amplifier A whose non-inverting input is connected via a first resistor R1 to the terminal OUT and via a second resistor R2 to earth. The inverting input of the amplifier is connected to receive a reference voltage V R . The part of Figure 1 which shows the voltage regulator circuit which can be monolithically integrated has been enclosed in a rectangular block shown in broken lines. In the stabilizer shown in Figure 2, the transistor T of p-n-p type is replaced by a bipolar transistor T1 of n-p-n type. The output terminal of the differential amplifier A is not in this case connected directly to the base terminal of the transistor T1 but to the base terminal of a second bipolar transistor T2 of p-n-p type. The emitter and collector terminals of the transistor T2 are connected to the collector terminal and to the base terminal, respectively, of the transistor T1. All the other components of the stabilizer are identical to those of Figure 1. In both cases the capacitor C is charged via the diode D to the typical value of the batter voltage less the voltage drop across the diode, during normal charging conditions of the battery. During negative voltage transients, however, the diode D prevents the discharge of the capacitor C via the input terminal with the result that this capacitor can be discharged only via the transistor of the regulation circuit, enabling its conduction during the transient. The two types of voltage stabilizer will now be compared, calculating their drop-out. In the case of the stabilizer comprising a power transistor of p-n-p type, the drop-out is: V DROP = V D + V CEsat in which V D is the voltage drop across the doide D when it is conducting a V CEsat is the collector-­ emitter voltage of the transistor T when it is saturated. In the case, however, of the stabilizer comprising a power transistor of n-p-n type, it is: V DROP = V D + V CEsat + V BE in which V D is again the voltage drop across the input diode, V CEsat is the collector-emitter voltage of the transistor T2 when it is saturated, and V BE is the base-emitter voltage of the transistor T1 when it is conducting. It is thus possible to construct a voltage stabilizer with minimal drop-out using, as shown in Figure 1, a regulation circuit comprising a power transistor of p-n-p type. A voltage stabilizer of the type shown in Figure 2 is, however, economically advantageous since, by using an n-p-n power transistor, it is possible to achieve an overall occupation of integration area of the regulation circuit which is lower than that which can be achieved with a p-n-p power transistor. It should be noted that, with the voltage stabilizers examined above and with equal current supplied to the load, it is generally necessary to insert an external capacitor between the output terminal OUT and earth in order to stabilize the regulation loop during operation. If use is made of an n-p-n power transistor, this provides a regulation loop with a gain which is lower than that which can be achieved with a p-n-p power transistor so that it is possible to use an output capacitor having a lower capacitance which is thus less costly. Since the production level of devices for use with motor vehicles is very high, these economic advantages are fairly substantial. According to the invention, there is provided a voltage regulator circuit as defined in the appended Claim 1. Preferred embodiments are defined in the other appended claims. It is thus possible to provide a voltage stabilizer device with minimal drop-out designed to withstand high voltage transients, which is economically advantageous with respect to similar known voltage stabilizers. The invention will be further described, by way of non-limiting example, with refernce to the accompanying drawings, in which: Figure 1 is a diagram, described above, of a known voltage stabilizer circuit with a "series" type regulation circuit comprising a p-n-p power transistor; Figure 2 is a diagram, described above, of a known voltage stabilizer circuit with a "series" type regulation circuit comprising an n-p-n power transistor; and Figure 3 is a circuit diagram showing a voltage stabilizer constituting a preferred embodiment of the invention. The same reference letters and numerals are used in the figures for corresponding components. The voltage stabilizer shown in Figure 3 comprise a bipolar transistor T′1 of n-p-n type whose collector terminal is connected to the cathode of a diode D′ and to a first terminal of a capacitor C′ whose second terminal is connected to earth. The stabilizer also comprises first and second bipolar transistors T′2 and T′3 of p-n-p type whose collector terminals are connected to the base terminal of the transistor T′1. The emitter terminal of the transistor T′2 is connected to the cathode of the diode D′ and the emitter terminal of the transistor T′3 is connected to the anode of the diode D′ at a circuit node which forms an input terminal IN′ of the stabilizer. The emitter terminal of the transistor T′1 forms an output terminal OUT′. The transistor T′1 is a power transistor; the diode and the capacitor are discrete components. The base terminal of the transistor T′1 is connected to the output terminal of a differential amplifier A′ whose inverting input is connected to the output terminal OUT′ via a first resistor R′1 and to a common terminal GND′ via a second resistor R′2. The common terminal GND′ is connected to earth. The non-inverting input of the differential amplifier is connected to receive a reference voltage V′ R . The base terminal of the transistor T′2 is connected to the common terminal GND′ via a first constant current generator G′2 and to the cathode of a diode D′2 whose anode is connected to the emitter terminal of the transistor T′2. The base terminal of the transistor T′3 is connected to the common terminal GND′ via a second constant current generator G′3 and to the cathode of a diode D′3 whose anode is connected to the emitter terminal of the transistor T′3. As in the other figures, the regulation circuit which can be monolithically integrated has been enclosed in a rectangular block shown in broken lines in Figure 3, and can be constructed and marketed as a monolithically integrated voltage regulator device. The transistors T′2 and T′3 may be electrically and physically identical. In normal operating conditions, when the voltage supplied to the input terminal is sufficiently high, both transistors T′2 and T′3 can conduct. The drop-out of the voltage stabilizer therefore has the value: V DROP = V BE + V CEsat′ in which V BE is the base-emitter voltage of the transistor T′1 in conduction, with a value approximately equal to the voltage drop V D across a diode, and V CEsat is the collector-emitter voltage of the transistor T′3 when it is saturated and is thus equal to the minimum drop-out which can be obtained with a stabilizer of the type shown in Figure 1. However, when an input negative voltage peak occurs, the transistor T′3 automatically cuts off as a result of the biasing conditions at its terminals and the supply of the transistor T′1 comes exclusively from the transistor T′2 which continues to conduct as a result of the presence of the capacitor C1 and the diode D′ which prevents its discharge at the input. Since it remains cut off during the voltage transient, the p-n-p transistor T′3 can withstand the inverse overvoltage at its terminals without the need for any form or protection, particularly if it is constructed as a "lateral" p-n-p transistor. Only the transistors T′2 and T′3 have to supply the base current of the power transistor T′1 so that the overall occupation of integration area for the voltage regulator circuit of the voltage stabilizer is undoubtedly less than that of the regulation circuit of the known stabilizer with minimal drop-out shown in Figure 1. The voltage stabilizer is thus economically advantageous, since it also makes it possible to use a smaller and less costly output stabilizing capacitor as the power transistor of the regulation loop is of n-p-n type. Although a single embodiment of the invention has been described and illustrated, many variations are possible without departing from the scope of the invention. For example, the circuit biasing means formed by the diodes D′2 and D′3 and by the current generators G′2 and G′3 could be replaced by more complex circuit biasing means designed to keep the transistor T′2 in conduction exclusively during the negative transients of the input voltage so as to prevent useless current absorption. These circuit means can be constructed in a manner known to persons skilled in the art.

Claims: 1. A voltage regulator circuit comprising a first bipolar transistor (T′1) of n-p-n type, having a collector terminal and an emitter terminal which form a first input terminal and an output terminal (OUT′), respectively, and having a base terminal which is connected to an output terminal of a differential amplifier (A′) having a first input terminal, connected via a first resistor (R′1) to the output terminal (OUT′) and connected via a second resistor (R′2) to a common terminal (GND′) for connection to earth, and a second input terminal for receiving a reference voltage (V′R), characterized by comprising second and third bipolar transistors (T′2, T′3), of p-n-p type, each having a collector terminal connected to the base terminal of the first transistor (T′1) and a base terminal connected to circuit biasing means (D′2, D′3, G′2, G′3), the second transistor (T′2) having an emitter terminal connected to the first input terminal and the third transistor (T′3) having an emitter terminal which forms a second input terminal of the circuit. 2. A voltage regulator circuit as claimed in Claim 1, characterized in that the circuit biasing means comprise a first diode (D′2) whose cathode and anode are connected to the base terminal and the emitter terminal, respectively, of the second transistor (T′2), a second diode (D′3) whose cathode and anode are connected to the base terminal and the emitter terminal, respectively, of the third transistor (T′3), a first constant current generator (G′2) connected between the base terminal of the second transistor (T′2) and the common terminal (GND′) of the circuit and a second constant current generator (G′3) connected between the base terminal of the third transistor (T′3) and the common terminal (GND′) of the circuit. 3. A voltage regulator circuit as claimed in Claim 1 or 2, characterized by being monolithically integrated. 4. A voltage regulator circuit as claimed in Claim 3, characterized in that the third transistor (T′3) is a bipolar transistor of lateral p-n-p type. 5. A voltage stabilizer device characterized by comprising a voltage regulator circuit as claimed in any one of the preceding claims, the first input terminal of the regulator circuit being connected to the cathode of a third diode (D′) and to a first terminal of a capacitor (C′), the second input terminal of the regulator circuit being connected to the anode of the third diode (D′) at a node which forms an input terminal (IN′) of the stabilizer device, the output terminal of the regulator circuit constituting an output terminal (OUT′) of the stabilizer device, and the second terminal of the capacitor (C′) and the common terminal of the regulator circuit being connected together. 6. A voltage stabilizer device as claimed in Claim 5, characterized in that the circuit biasing means are arranged to keep the second transistor (T′2) in conduction at least during a negative transient in a voltage supplied to the input terminal of the stabilizer device.

Inventors: MORELLI, MARCO, MURARI, BRUNO
Applicants: SGS-THOMSON MICROELECTRONICS S.P.A.
Application year: 1988
Main classification: G05F1
IPC codes: G05F1