VOLTAGE STABILIZER

Abstract: A voltage stabilizer with a very low voltage drop-out comprises a voltage regulator circuit of the series type, with a first p-n-p transistor (Q1) connected to the input terminal (IN″) of the stabilizer via a second p-n-p transistor (Q2) and connected to earth via a capacitor (C″). The stabilizer comprises biasing and switching circuit means (Q3, Q4) connected to the input terminal (IN"), the base terminal of the second transistor (Q2) and earth.

Description: The present invention relates to voltage stabilizers such as monolithically integrated voltage stabilizers which can be used in automobile applications or for apparatus of portable type. Voltage stabilizers supply a voltage having a well-specified and constant value or values from a voltage having an indeterminate value which is supplied to them. Voltage stabilizers can be advantageously used as supply devices for other devices: as a function of the load connected to them they in fact supply the current required so that the voltage supplied to this load always remains constant. At present, for reasons of compactness, ease of use and economic viability, the voltage stabilizers produced for all fields of application tend to be of the electronic integrated circuit type. 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 comprising feedback circuit means connected to the output terminals and sensitive to the instantaneous value of these electrical magnitudes. The lower limit of the field of correct operation of an electronic voltage stabilizer is ascertained from a parameter which is generally known in the technical literature by the term "drop-out", which is the difference between the minimum value of the input voltage required for correct operation of the stabilizer and the value of the constant voltage which the stabilizer has to supply as output and thus indicates the voltage drop across the device. For instance, the voltage stabilizers used in automobile applications have to meet very severe requirements as a result of operating conditions which may involve both substantial temperature and humidity variations and substantial, occasionally abrupt, variations in the supply voltage supplied by the motor vehicle battery. These stabilizers must therefore be very reliable, accurate and stable, while at the same time being economically viable, and must in particular have a low drop-out since the supply voltage supplied by the battery of a motor vehicle may normally drop, during cold starting, from the typical 14.4 V at full charge to some 6 V. Account must also be taken of the positive and negative voltage peaks having a maximum amplitude of up to 150 V which may be present on the supply line of a motor vehicle as a result of the switching transients of inductive loads (ignition coils, relays, etc.) or electrical connection cable detachments or breakages. The monolithically integrated voltage stabilizer circuits most commonly used for automobile applications are those with so-called "series"-type regulation, in which the output voltage is regulated to a constant value by a bipolar power transistor connected in series to an output terminal and suitably base-controlled to cause it to conduct as a function of the load. A suitably dimensioned power transistor may even withstand, with no drawbacks, positive voltage peaks having a high amplitude and thus continue to ensure the regulation of the output voltage. The negative peaks of the input voltage could, however, cause the transistor to be cut off, thereby causing interruptions, albeit brief, in the supply to the consumer circuits connected to the voltage stabilizer, with serious drawbacks when these comprise integrated memories and 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, so that a sufficient supply to the power transistor can be maintained during very short negative transients in the input voltage. Fig. 1 of the drawings shows the diagram of a voltage stabilizer as known from EP-A-0,280,514 with "series"-type regulation obtained by p-n-p power transistors. The circuit diagram of Fig. 1 comprises a bipolar p-n-p transistor T having its emitter terminal 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 is connected via a second resistor R2 to earth. The inverting input of the amplifier is, in contrast, connected to a voltage reference VR. The part of the diagram of Fig. 1 which represents the voltage regulator circuit which can be monolithically integrated is enclosed in a rectangular block of broken lines. The capacitor C is charged via the diode D to the typical value of the battery voltage less the voltage drop across the diode, during normal charging conditions. However, during negative voltage transients the diode D prevents the capacitor C from discharging via the input terminal with the result that this capacitor can discharge only via the transistor of the regulation circuit, thus allowing it to conduct during the transient. In the case of a stabilizer comprising a power transistor of p-n-p type, there is a drop-out: V DRP = V D + V CE sat in which V D is the voltage drop across the diode D when conducting and V CE sat is the collector-emitter voltage of the transistor T when it is at saturation. Using a power transistor of n-p-n type it is possible to achieve, with the same drop-out, an integration area occupation on the part of the regulation circuit which is lower than that which can be obtained with a power transistor of p-n-p type. Fig. 2 shows the diagram of a voltage stabilizer known from EP-A-0,280,514 comprising a bipolar power transistor of n-p-n type T′1 whose collector terminal is connected to the cathode of a diode D′ and to a first terminal of a capacitor C′, the second terminal of which is connected to earth. The stabilizer comprises first and second bipolar p-n-p transistors T′2 and T′3 both having their collector terminals 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′ in 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 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 a voltage reference 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. The regulation circuit which can be monolithically integrated is also enclosed in a rectangular block of broken lines in Fig. 2. The drop-out of the voltage stabilizer described here has a value: V DROP = V BE + V CE sat , 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 at a diode and V CE sat is the collector-emitter voltage of the transistor T′3 when it is at saturation, this drop-out consequently being equal to that of the stabilizer shown in Fig. 1. According to the invention, there is provided a voltage stabilizer as defined in the appended Claim 1. Preferred embodiments of the invention are defined in the other appended claims. It is thus possible to provide a voltage stabilizer device which has a drop-out lower than that of known devices. The invention will be further described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is the above-described circuit diagram of a voltage stabilizer with a "series"-type regulation circuit, comprising a power transistor of p-n-p type; Fig. 2 is the above-described circuit diagram of a voltage stabilizer with a low drop-out comprising a power transistor of n-p-n type; and Fig. 3 is the circuit diagram of 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 Fig. 3 comprises first and second bipolar p-n-p transistors Q1 and Q2. The emitter terminal of the transistor Q2 forms an input terminal IN″ of the stabilizer; the collector terminal of the transistor Q1 forms an output terminal OUT″. The collector terminal of the transistor Q2 is connected to the emitter terminal of the transistor Q1 at a circuit node B which is connected to earth via a capacitor C″. The base terminal of the first transistor Q1 is connected to the output terminal of a differential amplifier A″ whose inverting input is connected via a first resistor R″1 to the collector terminal of the transistor Q1 and via a second resistor R″2 to earth. The voltage stabilizer of Fig. 3 also comprises biasing and switching circuit means connected to the base terminal of the transistor Q2, the input terminal IN″ and earth. The switching circuit means comprise third and fourth transistors Q3 and Q4 both of n-p-n type. The emitter terminals of Q3 and Q4 are both connected to earth. The base and collector terminals of the transistor Q4 are both connected to the base terminal of the transistor Q3 in a current mirror circuit configuration. The collector terminal of the transistor Q3 is connected to the base terminal of the transistor Q2, while the collector terminal of the transistor Q4 is connected to the input terminal IN″ via third and fourth resistor R3 and R4 connected in series. The connection node between these resistors is connected to a second voltage reference VR2. As is immediately evident, the drop-out of the voltage stabilizer has a value: V DROP = V CE sat Q1 + V CE sat Q2 , i.e. it is equal to the sum of the collector-emitter voltages of the transistors Q1 and Q2 when they are at saturation, and is thus lower than the drop-out of the known voltage stabilizers described above. The biasing and switching circuit means are designed to supply the base of the transistor Q2 when the voltage supplied to the input terminal of the stabilizer remains higher than or equal to the sum of the voltage actually regulated as output and the voltage drop V DROP of the stabilizer, i.e. V IN″ > V OUT″ + 2 V CE sat , and to discontinue the connection between the base terminal of the transistor Q2 and earth when the voltage supplied to the input no longer allows normal operation of the stabilizer. In this way, during negative peaks of the input voltage, the opening of the connection between the base terminal of the transistor Q2 and earth prevents the base-collector junction of the transistor Q2 from being forwardly biased and forming a path for the discharging to earth of the capacitor C″. Thus, the capacitor C″ can discharge only via Q1, keeping the node B at a potential sufficient for normal operation of the output regulation circuit during the entire period of the input voltage transient. The current mirror circuit structure contained in the diagram of Fig. 3 embodies biasing and switching circuit means controlled by the input voltage by means of the resistors R3 and R4 which, together with the voltage reference VR2, establish the value of the current flowing in the transistor Q4. The transistor Q3 is simultaneously a current generator for the supply of the transistor Q2 and an electronic switch for the opening of the connection between the base terminal of the transistor Q2 and earth when the supply to this transistor is cut off. These biasing and switching circuit means can obviously be embodied in other ways known to persons skilled in the art, for instance by selecting and dimensioning the circuit components so that the switch opens automatically at specific input potential values in the presence of both negative and positive voltage peaks. In this case, a voltage stabilizer circuit structure comprising two p-n-p transistors in series between the input and output, in addition to having a drop-out which is lower than that of known stabilizers, is particularly suited to technological implementing solutions which allow the device to be very reliable under all conditions of use without substantial cost increases. If the transistor Q2 is cut off at both positive and negative input voltage peaks and its base connection is opened on cut-off, the transistor Q1, as a result of the capacitor C, is subjected to voltages having normal values whatever the input voltage. The transistor Q1 can consequently be embodied as a p-n-p transistor with an isolated vertical collector which does not withstand high voltage transients but occupies a limited integration area. The transistor Q2 can in contrast be a normal lateral p-n-p transistor as a result of the fact that the base contact is opened precisely at the negative input peaks which thus avoids both inverse conduction of the transistor and conduction via the collector-emitter junction. The increased integration area occupation by the lateral p-n-p transistor may entail increased costs, but these are offset by the very low drop-out which can be obtained.

Claims: A voltage stabilizer comprising a first bipolar transistor (Q1) having an emitter terminal which is arranged to be connected to earth via a capacitor (C″), a collector terminal which forms an output terminal (OUT″) of the stabilizer, and a base terminal which is connected to an output terminal of a differential amplifier (A″) having a first input terminal which is connected to a potential divider (R″1, R″2) connected between the output terminal (OUT″) of the stabilizer and earth and a second input terminal which is connected to a first voltage reference (VR1), characterized by comprising a second bipolar transistor (Q2) having an emitter terminal which forms an input terminal (IN″) of the stabilizer, a collector terminal which is connected to the emitter terminal of the first transistor (Q1) and a base terminal which is connected to biasing and switching circuit means which is also connected to earth, and to the input terminal of the stabilizer (IN″). A voltage stabiliser as claimed in Claim 1, characterised in that the first and second bipolar transistors (Q1, Q2) are of p-n-p type. A voltage stabilizer as claimed in claim 1 or 2, characterized in that the biasing and switching circuit means comprise a switch connected between the base terminal of the second transistor (Q2) and earth. A voltage stabilizer as claimed in claim 3, characterized in that the switch is formed by a transistor of a current mirror circuit (Q3, Q4, R3, R4) having an input branch connected to a second voltage reference (VR2) and to the input terminal of the stabilizer (IN″) and an output branch connected to the base terminal of the second transistor (Q2). A voltage stabilizer as claimed in claim 3, characterized in that the biasing and switching circuit means comprise a threshold comparator circuit arranged to open the switch when the potential value at the input terminal (IN″) of the stabilizer is lower than a first predetermined value or greater than a second predetermined value greater than the first predetermined value. A voltage stabilizer as claimed in any one of the preceding claims, characterised by being monolithically integrated. A monolithically integrated voltage stabilizer as claimed in claim 6, characterized in that the first transistor (Q1) is a p-n-p transistor with an isolated vertical collector and the second transistor (Q2) is a lateral p-n-p transistor.

Inventors: MAGGIONI, GIAMPIETRO, MORELLI, MARCO, MURARI, BRUNO
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1989
Main classification: G05F1
IPC codes: G05F1