Abstract:
A voltage regulator having an output terminal provided for being connected to a load, including an amplifier having its inverting input connected to a reference voltage, and its non-inverting input connected to the output terminal, a charge capacitor arranged between the output terminal and a first supply voltage, first and second voltage-controlled switches each arranged to connect a second supply voltage and the output terminal, and a control means adapted to providing a voltage depending on the output voltage of the amplifier, on the one hand, to the gate of the first switch and, on the other hand, when the current flowing through the first switch reaches a predetermined threshold, to the gate of the second switch.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of voltage regulators and in particular to that of regulators with a low drop out. 
     2. Description of the Related Art 
     A low drop out (LDO) regulator made in the form of an integrated circuit may be used to provide, with low noise, a predetermined voltage to a set of electronic circuits from a supply voltage provided by a rechargeable battery. Such a supply voltage decreases in time and is likely to include noise due for example to the action of neighboring electromagnetic radiations on the battery-to-regulator connections. The regulator is said to have a low drop out since it provides a voltage close to the supply voltage. 
     FIG. 1 schematically shows a conventional low-drop out regulator. The regulator includes an output terminal O intended for being connected to a load R. Load R, essentially resistive, represents the input impedance of the set of the circuits supplied by the regulator. For simplicity, it is considered hereafter that load R is a resistor. The regulator includes an operational amplifier  2  having an inverting input IN −  connected to a positive reference voltage Vref and having a non-inverting input IN +  connected to terminal O by a feedback loop. Voltage Vref is generated in a known manner by a constant voltage source (not shown) with a high output impedance. Amplifier  2  is supplied between a positive supply voltage Vbat provided by the battery and a ground voltage GND. The output of amplifier  2  is connected to the gate of a P-channel MOS power transistor T 1  having its drain connected to output terminal O and its source connected to voltage Vbat. Transistor T 1  is of MOS rather than bipolar type, especially to minimize the difference between output voltage Vout of terminal O and supply voltage Vbat. A charge capacitor C is arranged between output terminal O and voltage GND. 
     The regulator maintains voltage Vout of output terminal O to a value equal to reference voltage Vref. Any variation in voltage Vbat translates as a variation in voltage Vout, which is transmitted by the feedback loop on input IN + . When the regulator operates properly, the variation in the voltage of terminal IN +  causes the return of voltage Vout to voltage Vref. For this purpose, the regulator circuit, which forms a looped system between input IN +  and terminal O, must form a stable system. For this system to be stable when looped, its open-loop gain must not exceed 1 when the phase shift is smaller than −180° (phase opposition between the system input and output). 
     FIG. 2 illustrates, according to frequency f, the variation in gain G and in phase shift φ of the open-loop regulator between input IN +  and terminal O. For low frequencies f, gain G is equal to static gain Gs of the open-loop regulator. The elements forming the regulator each have a gain which varies according to frequency. The cut-off frequency of an element having a gain that decreases as the frequency increases corresponds to a “pole” of the transfer function of the open-loop regulator. Each pole of the transfer function of the open-loop regulator introduces a drop by 20 dB per decade in gain G. Further, each pole of the transfer function of the open-loop regulator introduces a phase shift φ by −90°. For simplicity, it is considered hereafter that the transfer function of the open-loop regulator only includes one main pole P 0  and one secondary pole P 1 . The frequency of main pole P 0  especially depends on the inverse of the product of the values of load resistance R and of capacitance C. The frequency of secondary pole P 1  especially depends on the gate impedance of transistor T 1 . The features of the elements forming the regulator are chosen so that, when phase shift φ becomes equal to −180°, gain G is smaller than 1 (0 dB). In FIG. 2, pole P 0  is at a low frequency, pole P 1  is at a greater frequency than pole P 0 . For a frequency smaller than the frequency of pole P 0 , the gain is equal to static gain Gs of the open-loop regulator. Between poles P 0  and P 1 , the gain drops by 20 decibels per decade. Beyond pole P 1 , the gain drops by 40 decibels per decade. The phase shift drops from 0 to −90° at pole P 0  and from −90° to −180° at pole P 1 . 
     The voltage regulator provides a current IO to load R, while maintaining output terminal O to reference voltage Vref. For the regulator to be able to provide a strong current IO, transistor T 1  must be large. As a result, the gate capacitance of transistor T 1  is high. The output impedance of amplifier  2  is small to be able to control the gate of transistor T 1 . The current IA consumed by amplifier  2  depends on the output impedance of amplifier  2  and is high. The efficiency of the voltage regulator depends on ratio IO/(IA+IO). Thus, the efficiency of a conventional regulator is low when current IO is low, for example, when the circuits supplied by the regulator are in a stand-by mode. Many electronic appliances supplied by a rechargeable battery, such as cellular phones, must be able to remain in stand-by mode for a long time. A conventional voltage regulator is poorly adapted to such appliances. 
     A conventional way of increasing the regulator efficiency consists of increasing the output impedance of amplifier  2  to reduce current IA consumed by amplifier  2 . However, the value of static gain Gs of the regulator is in particular proportional to output impedance Zout of the amplifier. A strong output impedance Zout makes static gain Gs high and shifts the secondary pole towards low frequencies, which respectively shifts the gain curve upwards and the phase curve to the left and makes the regulator stability difficult to obtain. FIG. 2 illustrates as an example gain and phase curves G′ and φ′ of an open-loop regulator having previous pole P 0 , having a secondary pole at a frequency P 1 ′ smaller than previous frequency P 1 , and having a static gain Gs′ greater than previous static gain Gs. Gain G′ is greater than 1 (0 dB) when phase shift φ′ reaches −180°, which makes the regulator unstable. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention is to provide a voltage regulator having a high efficiency. 
     To achieve this embodiment, as well as others, the present invention provides a voltage regulator having an output terminal connected to a load, including an amplifier having its inverting input connected to a reference voltage, and its non-inverting input connected to the output terminal, a charge capacitor arranged between the output terminal and a first supply voltage, first and second voltage-controlled switches each arranged to connect a second supply voltage and the output terminal, and a control means adapted to provide a voltage depending on the output voltage of the amplifier to the gate of the first switch and, when the current flowing through the first switch reaches a predetermined threshold, to the gate of the second switch. 
     According to an embodiment of the present invention, the current running through the first switch is smaller than or equal to said predetermined threshold. 
     According to an embodiment of the present invention, the amplifier is supplied between the first supply voltage and the second supply voltage. 
     According to an embodiment of the present invention, the first and second voltage switches are MOS transistors of a first type, the gate of the second switch being wider than the gate of the first switch. 
     According to an embodiment of the present invention, the control means includes first and second impedances, a first terminal of each impedance being connected to the second supply voltage, first and second bipolar transistors having their collectors connected to a second terminal respectively of the first and second impedances, and to the gates respectively of the first and second switches, the base and the collector of the first transistor being interconnected, the base of the second transistor being connected to the first supply voltage via a current source, a third MOS transistor of a second type arranged to connect the emitters of the first and second transistors to the first supply voltage, the gate of the third transistor being connected to the amplifier output, and a fourth diode-connected MOS transistor of the first type, arranged to connect the base of the second transistor to the second supply voltage. 
     According to an embodiment of the present invention, the first and second switches and the fourth transistor are P-channel MOS transistors, the first and second bipolar transistors are of NPN type, and the third transistor is an N-channel MOS transistor. The foregoing embodiments, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 previously described, schematically shows a conventional voltage regulator; 
     FIG. 2 previously described, illustrates the operation of the voltage regulator of FIG. 1; 
     FIG. 3 schematically shows a voltage regulator according to the present invention; 
     FIG. 4 schematically shows the control means of the voltage regulator of FIG. 3; 
     FIG. 5 illustrates the operation of the power transistors of the voltage regulator of FIG. 3; and 
     FIG. 6 illustrates the operation of the voltage regulator of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 schematically shows an embodiment of a voltage regulator according to the present invention. The regulator includes an output terminal O connected to a load R and to a capacitor C. The regulator includes an amplifier  2  supplied between a voltage Vbat and a voltage GND. The inverting and non-inverting inputs IN −  and IN +  of amplifier  2  are respectively connected to a positive reference voltage Vref and to terminal O. According to the present invention, the voltage regulator includes P-channel power MOS transistors T 2  and T 3 , having their respective drains connected to output terminal O and their respective sources connected to supply voltage Vbat. The output current IO is equal to the sum of the currents through the transistors, T 2  and T 3  so that: I 2 +I 3 =IO. The voltage regulator further includes a control means  4  having an input terminal COM connected to the output of amplifier  2 , and first and second output terminals COM 2 , COM 3  respectively connected to the gates of transistors T 2  and T 3 . 
     FIG. 4 schematically shows an embodiment of control means  4  of FIG.  3 . Two impedances Z 2  and Z 3  each have a first terminal connected to supply voltage Vbat. Two bipolar transistors T 4  and T 5 , of type NPN, have their collectors each connected to a second terminal respectively of impedances Z 2  and Z 3 . The collectors of transistors T 4  and T 5  further are respectively connected to output terminals COM 2  and COM 3  of control means  4 . The collector and the base of transistor T 4  are interconnected. The base of transistor T 5  is connected to a biasing means. The biasing means includes a P-channel MOS transistor T 7  having its drain connected to the base of transistor T 5 . The source of transistor T 7  is connected to voltage Vbat. The gate and the drain of transistor T 7  are interconnected so that transistor T 7  forms a diode. The drain of transistor T 7  is further connected to voltage GND via a current source CS. In an alternative embodiment, the power supply voltage to the transistors T 2  and T 3  is different from the power supply voltage to the control means  4  and amplifier  2 . The control circuit and amplifier may be connected to a Vdd, a regulated voltage output or a different value than Vbat. Similarly, the transistors T 2  and T 3  may be coupled to Vdd or some other value. The ground voltage reference for the amplifier  2  and supply  4  may be the same ground reference as for the load R, or alternatively, the ground reference voltage can be different, relative to each other. 
     The base of transistor T 5  is submitted to a reference voltage VB 5 =Vbat−|VGS 7 |, where VGS 7  is the voltage drop in the diode formed by transistor T 7 . Diode-mounted transistor T 4  is run through by a current I 4  when transistor T 6  is on. The emitter of transistor T 5  is submitted to a voltage VE 5 =Vbat−(Z 2 )×(I 4 )−VBE 4 , where VBE 4  is the voltage drop in the diode formed by transistor T 4  and (Z 2 )×(I 4 ) is the voltage drop across the load Z 2 , which is the impedance multiplied by the current. The base-emitter voltage of transistor T 5  is VBE 5 =(Z 2 )×(I 4 )+VBE 4 −|VGS 7 |. When current I 4  is low, voltage VBE 5  is smaller than the threshold voltage of transistor T 5 , and transistor T 5  is off. When the regulator provides a low current IO, transistor T 6 , controlled by amplifier  2 , is run through by a low current I 6 . Transistor T 5  is off and I 4 =I 6 . When current IO provided by the regulator increases, currents I 6  and I 4  increase. When current I 4  increases, voltage (Z 2 )×(I 4 ) increases and voltage VBE 5  increases to turn transistor T 5  on. A current I 5  then runs through transistor T 5 , with I 6 =I 4 +I 5 . 
     Impedance Z 2  is chosen so that gate/source voltage VGS 2  of transistor T 2 , where |VGS 2 |=Vbat−(Z 2 )×(I 4 ), is adapted to turning transistor T 2  on for a low value of current I 4 . Impedance Z 3  is chosen so that gate/source voltage VGS 3  of transistor T 3 , where |VGS 31 =Vbat−(Z 3 )×(I 5 ), is adapted to turning transistor T 3  on for a current I 5  corresponding to a threshold value, IO s , of current IO where IO s  equals the standby current. 
     FIG. 5 illustrates the value of currents I 2  and I 3  running through transistors T 2  and T 3  as a function of current IO. When IO is smaller than IO s , only transistor T 2  is on and current I 2  equals IO As IO increases, the current I 2  increases proportionally toward current IO s . When current IO is greater than threshold current IO s , transistor T 3  comes on so that both transistors T 2  and T 3  are on. The elements of control means  4  are chosen so that current I 2  remains substantially equal to IOS while current I 3  increases proportionally to supply the needed current IO. The total current IO is equal to the sum of I 2  and I 3 . 
     According to the present invention, current IO s  is a sufficient current to power load R, for example when the circuits represented by load R are in a stand-by mode. Transistor T 2 , which is run through by a current smaller than or equal to IO s , has a reduced size and a low gate capacitance as compared to transistor T 3 . When the regulator provides a low current, the current IA consumed by the amplifier to bias transistor T 2  is reduced, which obtains a good efficiency of the regulator. Thus, when IO is smaller than IO s , the response time is increased and power consumption of the regulator is reduced as a result of the reduced size and lower gate capacitance of T 2 . Transistor T 3  carries a current smaller than or equal to IOmax−IO s , where IOmax is the maximum current provided to load R by the regulator. Transistor T 3  is larger and provides greater drive current capability than transistor T 2 , and thus has a larger gate capacitance. Transistor T 3  is such that the current IA consumed by the amplifier to bias both transistors T 2  and T 3  is substantially equal to the current necessary to bias transistor T 1  of FIG.  1 . Thus, when IO is greater than IO s , the regulator efficiency is substantially equal to the efficiency of a conventional regulator. The present circuit thus has the advantage of providing a high current capability according to the needs of the load during heavy operational use, while providing increased circuit efficiency, improved stability and faster response time during low current consumption. 
     FIG. 6 schematically shows open-loop gain G and phase φ of the regulator according to the present invention when transistors T 2  and T 3  are on. The gates of transistors T 2  and T 3  are controlled in parallel. Transistors T 2  and T 3  are selected so that the sum of their gate capacitances introduces a secondary pole having substantially the same frequency P 1  as in FIG.  2 . Further, the gains of transistors T 2  and T 3  depending on the W/L ratios of transistors T 2  and T 3 , are selected so that the open-loop regulator static gain is substantially equal to static gain Gs of FIG.  2 . When transistors T 2  and T 3  are on, the regulator according to the present invention has the same stability as a conventional regulator. FIG. 6 also illustrates open-loop gain G″ and phase φ″ of the regulator according to the present invention when only transistor T 2  is on. The gate capacitance of transistor T 2  is low, which results in shifting the secondary pole towards a frequency P 1 ″ greater than previous frequency P 1 , which improves the regulator stability. Further, the static gain of the open-loop regulator, which depends on the W/L ratio of transistor T 2 , has a value Gs″ smaller than previous gain Gs, which also improves the regulator stability. A regulator according to the present invention thus exhibits an improved stability when it provides a low current. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. As an example, the present invention has been described in relation with a voltage regulator using MOS power transistors T 2  and T 3 , but those skilled in the art will easily adapt the present invention to a voltage regulator using another type of voltage-controlled power switch. 
     The present invention has been described in relation with a specific biasing means of transistor T 5 , but those skilled in the art will easily adapt the present invention to other biasing means, for example, a conventional reference voltage source. 
     The present invention has been described in relation with positive voltages Vbat and Vref, but those skilled in the art will easily adapt the present invention to negative voltages Vbat and Vref, by inverting the types of the described MOS transistors and by replacing the NPN-type bipolar transistors by PNP-type transistors. 
     For simplicity, the present invention has been described in relation with a resistive load R, but those skilled in the art will easily adapt the present invention to a complex load. 
     For simplicity, the present invention has been described in relation with a voltage regulator using a non-resistive feedback loop and providing a voltage equal to a received reference voltage Vref. However, those skilled in the art will easily adapt the present invention to a voltage regulator in which the feedback loop includes a resistive bridge, and which outputs a voltage different from received voltage Vref. 
     The present invention has been described in relation with an open-loop regulator, the open-loop transfer function of which includes a main pole and a secondary pole, but those skilled in the art will easily adapt the present invention to an open-loop regulator having a different open-loop transfer function, for example including zeros and having a greater number of poles. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.