BiCMOS/CMOS low drop voltage regulator

Abstract: The invention relates to a low-drop type of voltage regulator (1) formed with BiCMOS/CMOS technology and being of the type which comprises: an input terminal (IN), receiving a stable voltage reference (Vrif) and being connected to one input (-) of an operational amplifier (2) through a switch controlled by a power-on enable signal (CE); a supply voltage reference (Vpos) powering the operational amplifier (2); an output transistor (M1) connected to an output (U) of the amplifier (2) to generate a regulated voltage value (Vreg) to be fed back to the amplifier (2) input; a second transistor (M2) connected in series between the output transistor (M1) and the supply voltage reference (Vpos). The regulator of this invention comprises a control circuit portion (7) connected between the control terminal of the second transistor (M2) and the supply voltage reference (Vpos) to prevent the breakdown of the output transistor (M1) from occurring.

Description: Field of the Invention This invention relates to a low-drop type of voltage regulator formed with BiCMOS/CMOS technology. The invention specifically concerns a regulator as above which comprises: an input terminal, receiving a stable voltage reference and being connected to one input of an operational amplifier through a switch controlled by a power-on enable signal; a supply voltage reference powering the regulator; an output transistor connected to an output of the amplifier to generate a regulated voltage value to be fed back to the amplifier input; a second transistor connected in series between the output transistor and said supply voltage reference. As is well known, the technical field of radiofrequency signal transmission and reception requires that GSM or DCS devices be provided which can even operate on varying supply voltages, generally between 3V and 5V. This demand is made today more pressing by the availability of portable telephone sets of the dual band type, which can be operated at both frequency standards. To that aim, it becomes necessary to provide such devices with voltage regulators effective to produce a stable working voltage and capable of accommodating variations in the supply voltage or disturbance of any kind. Further, to provide electronic devices, integrated monolithically in a single chip, which can operate on signals at frequencies in the GHz range, a technology is required which allows of the integration of components active at very high cut-off frequencies on the order of a few tens of GHz. This involves of necessity the minimization of any parasitic capacitances which, if allowed to appear in the device, could depress the working frequency substantially. In the instance of circuits integrated with CMOS technology, minimizing the parasitic capacitances is to reduce the thickness of the gate oxide layer of MOS transistors to a minimum. While this can make the transistors extremely fast, it has the disadvantage of lowering their maximum sustainable working voltage. MOS transistors so constructed would exhibit low gate-source or gate-drain breakdown voltages. In the instance of circuits integrated with bipolar technology, reducing the parasitic capacitances is to reduce the width of the base region as well as the time allowance for the carriers passage through the base region. Here again, the transistor capacity to sustain high working voltages is concurrently reduced. Bipolar transistors with this construction would have a low collector-emitter breakdown voltage. The problem of how to provide GSM-DCS dual band devices operating on varying supply voltages has been addressed by using a combined BiCMOS technology which allows of breakdown voltages up to 3.5V for both bipolar and MOS transistors. Prior Art A prior art voltage regulator constructed with BiCMOS/CMOS technology is shown by way of example in Figure 1 herewith. This regulator comprises an operational amplifier OPAMP having an output connected to the control terminal of a PMOS transistor M1 to produce a regulated voltage value Vreg. An input terminal In of the regulator receives a voltage reference Vrif which is applied to the inverting input of the amplifier through a switch controlled by a signal CE (Chip Enable); this signal being a CMOS digital signal arranged to control the turning on/off of the whole device. The regulated output terminal is fed back to the amplifier inputs through a resistive divider formed of a resistor pair R1, R2. This divider is connected in parallel with an output capacitor C. In essence, upon the occurrence of a variation in the supply, the output voltage value Vreg is led back to the input of an error amplifier OPAMP at a ratio of R1/(R1+R2) for comparison with a reference voltage Vrif. The regulated voltage Vreg is given by the following relation: Vreg = Vrif(1 + R1/R2) The output PMOS transistor should be of such dimensions as to ensure operation in the saturation range at the largest delivered current. In addition, the output capacitor C allows a dominant pole compensation to be carried out and affords good rejection of supply disturbance at all the frequencies. While being advantageous in many ways, this prior solution has a drawback in that, with the regulator in the "off" state, the voltage Vgd across the gate and drain terminals of the transistor M1 and the voltage Vsd across the source and drain terminals of the transistor M1 are equal to the supply voltage Vpos of the device. Where this voltage Vpos is higher than the gate-drain and source-drain breakdown voltages, the condition becomes unacceptable for the device operation because it would cause the output PMOS transistor M1 to fail. A viable prior solution to this problem is illustrated schematically by Figure 2. Unlike the example of Figure 1, a cascode structure is shown in Figure 2, wherein a series of PMOS transistors M1, M2 are employed, with the gate terminal of the transistor M2 being held at a voltage reference Vg2. This solution has a drawback in that it cannot be applied to low drop regulators, since large-size transistors would be needed which occupy a large circuit area and make compensation difficult from the presence of high parasitic capacitances. A stabilized low dropout voltage regulator circuit is described in the US patent No. 4,928,056 to Pease. The underlying technical problem of this invention is to provide a voltage regulator of the low drop type, for construction with BiCMOS/CMOS technology, which has such structural and functional features as to be usable with higher supply voltages than the breakdown voltage of active components, thereby overcoming the limitations of prior art circuits. Summary of the Invention The concept behind this invention is one of having a circuit portion connected between the output of the operational amplifier in the regulator and the supply thereto, which is effective to prevent breakdown of the output PMOS transistor when the regulator is in the "off" state. Based on this concept, the technical problem is solved by a voltage regulator as previously indicated and defined in the characterizing portion of Claim 1. The features and advantages of a regulator according to the invention will become apparent from the following description of an embodiment thereof, given here by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a diagramatic view of one prior art voltage regulator; Figure 2 is a diagramatic view of another prior art voltage regulator; Figure 3 is a diagramatic view of a low drop voltage regulator according to this invention; Figure 4 is a diagramatic view showing the voltage regulator of Figure 3 in greater detail. Detailed Description Referring to the drawing views, specifically to the example of Figure 3, a voltage regulator formed with BiCMOS/CMOS technology, according to the invention, is generally shown schematically at 1 and useful in integrated electronic devices which are operated at higher supply voltages than the device breakdown voltages. The regulator 1 is intended, particularly but not exclusively, for incorporation to an integrated telephone circuit for dual band applications in conformity with the GSM and/or DCS standards for radiofrequency transmission. The regulator 1 includes an operational amplifier 2 having an output U, and having an inverting (-) first input and a non-inverting (+) second input. The regulator 1 has an input terminal IN connected to the inverting (-) input of the amplifier 2 through a switch which is controlled by an enable signal CE. The signal CE (Chip Enable) represents the activating signal for the whole integrated circuit whereto the regulator 1 is incorporated. The input terminal IN is applied a reference potential Vrif. The non-inverting (+) of the amplifier 2 is also connected to a supply reference, such as a ground GND, through a second switch which is controlled by a signal NCE. This signal NCE represents the logic negation of the signal CE. The output U of the amplifier 2 is connected to the control terminal of an output PMOS transistor M1 having its drain terminal D linked to the ground reference GND by a resistive divider 3 which comprises first R1 and second R2 resistors. The interconnecting node between the resistors R1 and R2 is feedback connected to the non-inverting (+) input of the amplifier 2. An output capacitor C is in parallel with the divider 3. The drain terminal of the transistor M1 also represents an output terminal OUT for the regulator 1 whence a regulated voltage value Vreg will be extracted. Advantageously in this invention, the regulator 1 further comprises a second MOS transistor M2 connected in series with the MOS transistor M1. Although this transistor is again of the PMOS type, both transistors M1, M2 could well be of the NMOS type, for a negative regulator. The drain terminal of the transistor M2 is connected to the source terminal of the transistor M1 and also represents the virtual supply to the amplifier 2 of the regulator 1. Further, the source terminal of the second transistor M2 is connected to a supply voltage reference Vpos. Advantageously, a control circuit portion 7 is connected between the output U of the operational amplifier 2 and the supply voltage reference Vpos of the regulator 1, and is operative to turn on/off the transistor M2. More particularly, the circuit portion 7 comprises a switch 4 connected between the gate terminal of the second transistor M2 and a reference of potential Vg2. The switch 4 is controlled by a signal CE_1. The signal CE_1 is suitably timed relative to the signal CE such that the transistor M2 is never turned on ahead of the transistor M1 and overvoltages at the source terminal of the output transistor M1 are prevented from occurring. Provided downstream of the switch 4 is a second switch 5 which is connected between the gate terminal of the transistor M2 and the supply voltage reference Vpos. This second switch 5 of the circuit portion 7 is controlled by a signal NCE_1 being the logic negation of the signal CE_1. The operation of the voltage regulator according to this invention will now be described. The transistor M2 functions as a switch, and in normal operating conditions, with the signal CE having a high logic value, the transistor M2 will be in the "on" state. As the regulator 1 is turned off by the signal CE going to a low logic value, the whole circuit is in the "off" state and the regulator structure is equivalent to the cascode structure shown in Figure 2. This removes the risk of breakdown of the transistor M1 in the "off" condition, since the structure comprising M1 and M2 is the equivalent of a cascode, but without the need for increased area availability since it is no longer necessary to ensure operation of the transistor M2 in the saturation range at the largest delivered current. The circuit portion 7 will be cut off upon the enable signal CE being restored to a high logic value. Shown in Figure 4 by way of non-limitative example is a possible circuit embodiment of the electric diagram of Figure 3 using a BiCMOS technology. The example of Figure 4 includes a bandgap cell 8 for producing the reference potential Vrif to be applied to the regulator 1 input. The regulator includes an amplifier 2 in a feedback loop which is effective to return the bandgap voltage to the resistive divider 3, where this reference will be amplified and brought back to a regulated voltage value Vreg. The regulated voltage obeys the following relation: Vreg = Vbg * (1 + R1/R2) This embodiment has been tested by the Applicant using a supply voltage of 5V and a breakdown voltage of 3.5V. The switches 4 and 5 were, by way of example, formed of a series of diodes D1, D2, D3 connected in parallel to a resistor R3 and driven from a control circuitry 9. The diodes were connected in series with one another between the gate terminal of transistor M2 and the supply voltage reference Vpos. This circuitry comprised a pair of bipolar NPN transistors Q1, Q2 having their respective base and emitter terminals connected together, the collector terminal of the transistor Q2 being connected to drive the gate terminal of the transistor M2. In normal operating conditions, with the signal CE high, the diodes D1, D2, D3 are "on" and function to supply a high voltage Vsg to the transistor M2, with an attendant voltage Vsd low. In this way, the regulator 1 of this invention operates properly in normal operating conditions. Conversely, in the "off" state, with the signal CE low, the circuit ensures that the source or the gate terminal of the output transistor M1 never attains a voltage level which can bring it to breakdown, since an equivalent structure of the cascade structure is created. The regulator of this invention does solve the technical problem, and affords a number of advantages, foremost among which is the capability of this regulator to operated on higher supply voltages then the breakdown voltage of the active components incorporated to the regulator. In essence, the structure according to this invention is the equivalent of a cascode structure in the "off" condition, but in normal conditions of operation it is as if it did not interfere at all with the activity of the regulator, even at a low supply voltage (low drop), since the transistor M2 is the equivalent of a short circuit.

Claims: A low-drop type of voltage regulator (1) formed with BiCMOS/CMOS technology and being of the type which comprises: an input terminal (IN), receiving a stable voltage reference (Vrif) and being connected to the inverting input of an operational amplifier (2) through a switch controlled by a power-on enable signal (CE); a supply voltage reference (Vpos) powering the regulator (1) ; an output transistor (M1) connected to the output (U) of the amplifier (2) to generate a regulated voltage value (Vreg) to be fed back to the non-inverting input of the amplifier (2); a second transistor (M2) connected in series between the output transistor (M1) and said supply voltage reference (Vpos), characterized in that it comprises a control circuit portion (7) connected between the control terminal of the second transistor (M2) and said supply voltage reference (Vpos) to prevent the breakdown of the output transistor (M1) from occurring. A regulator according to Claim 1, characterized in that said circuit portion (7) comprises a first controlled switch (4), connected between a reference of potential (Vg2) and said control terminal of the second transistor (M2), and a second controlled switch (5), connected between said control terminal of the second transistor (M2) and said supply voltage reference (Vpos). A regulator according to Claim 2, characterized in that said first switch is controlled by an enable signal (CE_1) which is offset in time from said regulator power-on enable signal (CE). A regulator according to Claim 1, characterized in that said circuit portion (7) comprises a series of diodes (D1,D2,D3) connected between the control terminal of the second transistor (M2) and said supply voltage reference (Vpos). A regulator according to Claim 4, characterized in that it comprises a resistor (R3) in parallel with said series of diodes (D1,D2,D3). A regulator according to Claim 1, characterized in that said circuit portion (7) creates an equivalent structure of a cascode structure when the regulator (1) is in the "off" state. A regulator according to Claim 1, characterized in that said circuit portion is cut off upon the enable signal (CE) being restored to a high logic value. An integrated telephone circuit of the dual band type, incorporating at least one voltage regulator as claimed in Claim 1.

Inventors: CALI GIOVANNI, PAPARO MARIO, PELLERITI ROBERTO, CALI', GIOVANNI, PAPARO, MARIO, PELLERITI, ROBERTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1999
Main classification: G05F1
IPC codes: G05F1