Abstract:
A voltage regulator includes a regulation MOS transistor and an amplifier providing an output for driving a gate of the regulation MOS transistor. The amplifier drives the gate based upon a difference between a reference voltage and a feedback voltage. The voltage regulator may further include a circuit for making the amplifier switch to a standby mode with low current consumption when the difference between the supply voltage and the output voltage of the regulator is below a first threshold. This is done while maintaining, at the gate of the regulation transistor, a voltage that keeps the regulation transistor on. The present invention is particularly applicable to the management of power supplies in portable telephones.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of electronic circuits, and, more particularly, to low drop-out (LDO) type linear voltage regulators, namely low serial voltage drop-out regulators. 
     BACKGROUND OF THE INVENTION 
     Low drop-out (LDO) linear voltage regulators, such as low serial voltage drop-out regulators, may be used in several applications. In particular, such regulators may be used in mobile telephones to deliver a regulated voltage to radio transmission/reception circuits from a battery. 
     By way of example, a standard linear regulator  10  according to the prior art whose output delivers a regulated voltage Vout to a load Z is shown in FIG.  1 . The load Z represents, for example, several radio circuits in a mobile telephone. The regulator  10  is electrically powered by a voltage Vbat delivered by a battery  1  and includes a differential amplifier  2  whose output drives a gate G of a P-channel metal oxide semiconductor (PMOS) regulation transistor  3 . The transistor  3  is generally a transistor with low serial resistance in the conductive or on state (drain-source resistance RdsON), and it receives the voltage Vbat at its source S. A drain D of the transistor  3  is connected to the output of the regulator  10  and to the anode of a capacitor Cst for filtering and stabilizing the voltage Vout. This capacitor Cst is parallel-connected with the load Z. 
     The amplifier  2  receives a reference voltage Vref at its negative input and a feedback voltage Vfb at its positive output. The voltage Vfb is a fraction of the voltage Vout provided to the input of the amplifier  2  by a divider bridge including two resistors R 1 , R 2 . 
     Operation of a regulator of this kind, which is well known to those skilled in the art, includes modulation of the gate voltage Vg of the transistor  3  using the amplifier  2 . This is done based upon a difference between the voltage Vfb and the reference voltage Vref, which the amplifier maintains at about 0 V. When the voltage Vg is smaller than the value Vbat−Vtp, the transistor  3  is on because its gate-source voltage Vgs is higher than the threshold voltage Vtp. When the voltage Vg is higher than Vbat−Vtp, the transistor is off. In a stabilized mode, the voltage Vout is regulated in the neighborhood of its nominal value Voutnom, which is equal to (R 1 +R 2 ) Vref/R 2 . 
     A typical embodiment of the amplifier  2  according to the prior art is shown in FIG.  2 . The amplifier  2  includes a differential stage  5  which receives the voltages Vref and Vfb as inputs and is biased by a current generator  6 . The output of the differential stage  5  drives the gate of an N-channel MOS (NMOS) transistor  7  connected between the output node of the amplifier  2  and ground. The transistor  7  is biased at its drain D by a current generator  8 . The output node of the amplifier is connected to the power supply voltage Vbat by a gate resistor Rg, which determines the gain of the amplifier and the maximum current that can be delivered at the output. Thus, according to the value of the signal delivered by the differential stage  5 , the transistor  7  draws the output of the amplifier  2  to ground or the resistor Rg draws the output upwards, namely toward the voltage Vbat. 
     In an application such as supplying power in a mobile telephone, it is important that the regulation amplifier consume as little electricity as possible to maintain the charge of the battery. To this end, the gate resistance Rg is chosen such that it has a high value (e.g., 100 KΩ) to limit the current flowing in the output stage. At the same time, the currents delivered by the generator  6 ,  8  are calibrated appropriately. Generally, the choice of the resistance Rg and of the bias currents is the result of a compromise between the need to efficiently drive the transistor  3 , which generally has a high parasitic gate capacitance, and the need for low consumption. 
     Such consumption is typically in the range of 50 to 200 microamperes, i.e., it is acceptable per se when the battery is properly charged, and allows the regulator to work in a stabilized mode. Yet, the present invention is based on the assumption that this consumption is too high when the battery voltages Vbat become low and are below the nominal value Voutnom of the output voltage. Such a drop in the voltage Vbat below the nominal voltage Voutnom may be temporary and due to high current consumption, or it may be due to the fact that the battery has become discharged. 
     Turning now to FIGS. 3A,  3 B, and  3 C, according to observations and conclusions that form an integral part of the present invention, the passage of the voltage Vbat below the value Voutnom at an instant tA (FIG. 3A) results in the feedback voltage Vfb being lower than Vref at the input of the amplifier  2 . This voltage is unbalanced and makes the gate voltage Vg drop to ground to compensate for the imbalance (FIG.  3 B). The regulation transistor  3  is continually on, the voltage Vout becomes substantially equal to the voltage Vbat (FIG.  3 ), and the regulator  10  works in the follower mode. Since the output node of the amplifier  2  is grounded, it can be seen in FIG. 2 that the consumption in the gate resistor Rg is at the maximum. 
     Thus, the amplifier consumes current unnecessarily when the regulator works in the follower mode. This is because the regulation transistor is continually on and the output voltage Vout can no longer be regulated. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a voltage regulator which overcomes the above drawback, for example, by switching the regulation amplifier into a low consumption standby mode while keeping the regulation transistor in the on state. 
     This and other objects, features, and advantages according to the present invention are provided by a, voltage regulator including a regulation MOS transistor and an amplifier whose output drives a gate of the regulation MOS transistor based upon a difference between a reference voltage and a feedback voltage. The voltage regulator may also include a circuit or means to make the amplifier change over into a standby mode with low current consumption when the difference between the supply voltage and the output voltage of the regulator is below a first threshold. This is done while keeping an electrical potential at the gate of the regulation MOS transistor at a value that keeps the regulation MOS transistor in the on state. 
     More specifically, the voltage regulator may include a comparator for comparing the supply voltage and the output voltage of the regulator and delivering a standby signal to the amplifier when the difference between the supply voltage and the output voltage of the regulator is below the first threshold. Also, the comparator provides a switch-over hysteresis and cancels the signal for putting the amplifier on standby when the difference between the supply voltage and the output voltage of the voltage regulator is higher than a second threshold, where the second threshold is higher than the first threshold. 
     Additionally, the amplifier may include a resistor connecting the output of the amplifier to the supply voltage. Further, a switch may be series-connected with the resistor and may be open when the amplifier is put on standby. Otherwise, this switch is closed. The amplifier may also include current sources that switch to low current mode when the amplifier is put on standby. 
     In addition, the amplifier may include a switch driven by a standby-setting signal to connect the gate of the regulation MOS transistor to an electrical potential making the regulation MOS transistor conductive when the amplifier is put on standby. The amplifier may also include a stage for biasing the gate of the regulation MOS transistor when the amplifier is on standby. The stage biases the gate with a voltage that is set so that the gate-source voltage of the regulation MOS transistor is close to the threshold voltage of the regulation MOS transistor. The electrical supply of the amplifier may be eliminated in the standby mode by a switch. 
     A mobile telephone according to the invention includes at least one radio circuit, a battery, and a voltage regulator as described above for powering the at least one radio circuit from the battery. 
     A method aspect of the invention is for managing the power available in a battery powering a load using a voltage regulator. The voltage regulator includes a regulation MOS transistor and an amplifier whose output drives a gate of the regulation transistor based upon a difference between the reference voltage and a feedback voltage. The method includes monitoring the difference between the supply voltage and the output voltage of the regulator and switching the amplifier to a standby mode providing low current consumption when the difference between the supply voltage and the output voltage of the regulator is below a first threshold. This is done while keeping the gate of the regulation MOS transistor at a potential that keeps the regulation MOS transistor in the on state. 
     More specifically, the amplifier may be reactivated when the difference between the supply voltage and the output voltage of the regulator is higher than a second threshold, where the second threshold is higher than the first threshold. The consumption of the amplifier may be reduced in standby mode by disconnecting the output node of the amplifier from the supply voltage, diminishing the current delivered by current sources internal to the amplifier, or disconnecting the supply voltage. Further, when the amplifier is put on standby, it is advantageous to apply a gate voltage to the gate of the regulation MOS transistor where the gate voltage is set so that the gate-source voltage of the regulation transistor is close to its threshold voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, characteristics and advantages of the present invention will be explained in greater detail in the following description of an exemplary embodiment of a voltage regulator according to the invention, given by way of non-limitative example, with reference to the appended drawings, in which: 
     FIG. 1 (described above) is a schematic diagram of a voltage regulator according to the prior art; 
     FIG. 2 is a more detailed schematic diagram of the amplifier of the voltage regulator of FIG. 1; 
     FIGS. 3A to  3 C are graphs of electrical signals illustrating operation of the voltage regulator of FIG. 1 when the supply voltage drops below the nominal value of the output voltage; 
     FIG. 4 is a schematic diagram of a voltage regulator according to the invention; 
     FIGS. 5A to  5 C are graphs of electrical signals illustrating operation of the voltage regulator of FIG. 4 in a follower mode; and 
     FIGS. 6 to  9  are schematic diagrams of four alternative embodiments of an amplifier of the voltage regulator of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to FIG. 4, a regulator  20  according to the invention is supplied with a voltage Vbat provided by a battery  21 . The regulator  20 , like the regulator described with reference to FIG. 1, includes a differential amplifier  22  whose output controls the gate of a PMOS regulation transistor  23 . The drain D of the transistor  23  is connected to the output of the regulator  20  and is connected to a stabilizing capacitor Cst, which is parallel-connected with the load Z. These elements are arranged as described above. 
     The output voltage Vout is brought to the positive input of the amplifier  2  by a divider bridge including two resistors R 1 , R 2 . The resistor R 1  may be zero in the case of a direct feedback of the output voltage Vout at the input of the amplifier  22 , and the resistor R 2  is, in this case, mathematically infinite. The reference voltage Vref applied to the negative input of the amplifier  2  is, for example, a voltage known as a bandgap voltage having high stability as a function of temperature and generated by PN junction diodes and current mirrors. The voltage Vref is thus independent of the voltage Vbat provided that it is smaller than the lowest value of the voltage Vbat. 
     The working of the regulator  20  in a stabilized operation is as previously described and will therefore not be discussed further herein. The amplifier  2  keeps the feedback voltage Vfb at a level equal to the reference voltage Vref, and the nominal output voltage Voutnom is equal to (R 1 +R 2 ) Vref/R 2 . 
     According to the invention, the amplifier  22  has a “normal” operation mode and a “standby” mode and changes from one to the other according to the value of the signal Vlc applied to an input LCIN, which is designed for this purpose. Placing the amplifier  22  into standby includes placing the amplifier in a state of low electrical consumption while keeping the gate voltage Vg at a potential that keeps the regulation transistor  23  on. Various exemplary embodiments of the amplifier  22  shall be described further below. It will be assumed hereinafter that the amplifier changes over into standby mode when the signal Vlc goes to 1. 
     The signal Vlc is delivered by a comparator  24  receiving the output voltage Vout at its positive input and the supply voltage Vbat at its negative input, where the comparator  24  is supplied with the voltage Vbat. The comparator  24  is a threshold comparator Vd 1  and places its output at 1 (signal Vlc) when the differential voltage Vd at one of its inputs (which is equal to the difference between the voltage Vbat and the voltage Vout) becomes lower than the threshold Vd 1 . For reasons of output stability, the comparator  24  also preferably has a switch-over hysteresis and resets its output to 0 when the differential voltage Vd rises and becomes greater than a threshold Vd 2  greater than Vd 1 . The thresholds Vd 1 , Vd 2  are equal, for example, to 100 mV and 120 mV respectively. 
     Thus, as shall be seen in greater detail hereinafter, the amplifier  22  changes into standby mode while keeping the regulation transistor  23  on when the regulator  20  works in follower mode. This is due to a drop in the supply voltage Vbat below the nominal value Voutnom of the output voltage. 
     Turning to FIGS. 5A,  5 B,  5 C, the working of the regulator  20  in follower mode and the voltages Vbat and Vout, the differential voltage Vd and the signal Vlc are respectively illustrated. FIGS. 5A and 5B show a reduction of the voltage Vbat from its nominal value Vbatnom, which has no effect on the regulated voltage Vout, which remains equal to Voutnom so long as the voltage Vbat remains greater than Voutnom. The differential voltage Vd diminishes proportionally to the voltage Vbat up to a time t 2  when the voltage Vbat becomes substantially equal to Voutnom and drives the voltage Vout, in its drop. The regulator is then unbalanced and working in follower mode. 
     At the time t 2 , the differential voltage Vd reaches a minimum value Vdmin that corresponds to the drop in voltage at the terminals of the regulation transistor  23 . This drop in voltage Vdmin is very low, e.g., 50 mV, because the regulation transistor of an LDO type regulator generally has a very low drain-source resistance VdsON when it is on. Starting from the time t 2 , the voltage Vout starts diminishing and follows the voltage Vbat minus the voltage difference Vdmin. 
     The passage into follower mode is detected by the comparator  24  at a point in time t 1  that precedes t 2  but is, however, very close to t 2  when the differential voltage Vd reaches its threshold Vd 1  discussed above, which is chosen to be very close to the minimum Vdmin. Thus, at the time t 1 , the signal Vlc goes to 1 (FIG. 5C) and the amplifier  2  is put on standby. The “ 1 ” logic of the signal Vlc herein is the voltage Vbat which supplies the comparator  24 . 
     FIGS. 5A to  5 C show that the voltage Vbat then rises to its nominal value, for example, after the recharging of the battery  21  or the natural regeneration of the battery when the consumed current diminishes. At a time t 3 , the voltage Vbat exceeds the value Voutnom. At a time t 4 , the differential voltage Vd crosses the threshold Vd 2  and the amplifier  22  changes over into its normal mode of operation. The voltage Vout returns to its nominal value Voutnom. 
     A non-limiting description will now be given of various embodiments of the amplifier  22  provided by the structure of the amplifier  2  described in the introduction with reference to FIG.  2 . The amplifier  22   a  illustrated in FIG. 6 has a structure similar to that of the amplifier  2 . The differential stage  5  is biased by the current generator  6  whose output drives the NMOS transistor  7 , which is biased at its drain by the current generator  8 . A resistor Rg connects the output node of the amplifier  22   a  to the voltage Vbat. A switch  25 , e.g., a PMOS transistor, is series-connected with the resistor Rg. The transistor  25  receives the signal Vlc at its gate and is thus permanently on when the regulator works in stabilized operation, the signal Vlc being at 0 as indicated further above. 
     When the regulator works in follower mode and the voltage Vg at the output node is drawn toward ground by the NMOS transistor  7 , the signal Vlc goes to  1 , the transistor  25  turns off, and no current flows into the resistor Rg. Cutting off the path connecting the output node of the amplifier  22   a  to the voltage Vbat in this way provides a savings in current consumption that may be substantial, e.g., in the range of 80%. 
     The amplifier  22   b  shown in FIG. 7 differs from the amplifier  22   a  in that the current generators  6 ,  8  have been replaced by current generators  6 ′,  8 ′ that are controlled by the signal Vlc. The current generators  6 ′,  8 ′ deliver different currents depending on the value Vlc. The respective currents I 1 ′, I 2 ′ delivered when the signal Vlc is at 1 are, for example, equal to one half of the currents I 1 , I 2  delivered when the signal Vlc is at 0. The currents I 1 ′, I 2 ′ may be, for example, equal to 10 microamperes and the currents I 1 , I 2  are equal to 20 microamperes. Those skilled in the art will be capable of making such generators  6 ′,  8 ′. For example, they may be made by the parallel connection in current mirrors of transistors having the same structure and by turning-off of one transistor in two when the signal Vlc is at 1. Thus, this arrangement may save several tens of additional microamperes. 
     The amplifier  22   c  of FIG. 8 has the same internal structure as the amplifier  2  of FIG.  2 . However, the voltage Vbat is applied to the supply input of the amplifier  2  by a PMOS transistor  26  driven by the signal Vlc. Furthermore, an NMOS transistor  27  driven by the signal Vlc is added between the output of the amplifier  2  and ground. Thus, when the signal Vlc is at 0 (balanced regulator), the transistor  26  is on and the transistor  27  is off. The amplifier  2  works as if these two elements did not exist. When the signal Vlc is at 1 (with the regulator in follower mode), the transistor  26  is off and the transistor  27  is on. The amplifier  2  no longer receives the supply voltage Vbat and is completely powered down. 
     The transistor  27  draws the output of the amplifier to 0 (voltage Vg) to keep the regulation transistor  23  in the on state. This embodiment  22   c  is therefore distinguished from the above embodiments  22   a ,  22   b  in that, in standby mode, the gate voltage Vg is not drawn to ground by the NMOS transistor of the output stage of the amplifier  2 , which is out of operation, but rather by the additional transistor  27 . 
     The amplifier  22   d  shown in FIG. 9 also has the amplifier  2  and the transistor  26  for powering-off of the amplifier  2  when the signal Vlc is at 1. The pull-down transistor  27  at the output of the amplifier  22   c  is replaced by a further improved biasing stage  30  that maintains the output of the amplifier  2  at a voltage Vg greater than ground when the amplifier  2  is powered off. The voltage Vg is chosen so that the gate-source voltage Vgs of the regulation transistor  23  is held to about the threshold voltage Vtp of the transistor  23 . 
     The biasing stage  30  has, for example, a first PMOS transistor  31  receiving the voltage Vbat on its source. The first PMOS transistor  31  is connected by its drain to the source of a second PMOS transistor  32 , whose drain is connected to the output node of the amplifier  22   d.  The transistors  31 ,  32  are diode-mounted, each having its gate connected to its drain. Between the output node and ground, the biasing stage  30  includes a high-value resistor  33  series-connected with an NMOS transistor  34  driven by the signal Vlc. 
     When the regulator works in a stabilized mode, the voltage Vg is held around the value Vbat−Vtp by the output of the amplifier  2 , Vtp being the threshold voltage of a PMOS transistor. Thus, the two diode-mounted transistors  31 ,  32  are turned off. Furthermore, the signal Vlc is at 0 and the transistor  34  is also off. The amplifier  2  works as if the bias stage  30  did not exist. When the regulator is unbalanced, the voltage Vg tends towards 0, the signal Vlc goes to 1, and the amplifier  2  is powered off. The two diode-mounted transistors  31 ,  32  come on and each of them imposes a voltage Vtp at their terminals so that the gate voltage Vg is equal to Vbat−(2Vtp). The voltage Vgs of the regulation transistor  23  is thus equal to 2Vtp in absolute value and is close to Vtp (plus or minus the value Vtp, in the range of 0.7 V). Of course, other methods may be used to keep the voltage Vg even closer to the threshold voltage Vtp without departing from the scope of the present invention. 
     An advantage of this embodiment is that it does not entirely discharge the parasitic gate capacitance Cg of the regulation transistor  23  (shown in dashes). A value of the capacitance Cg is generally high (100-200 picofarads) in a regulation transistor with a low serial resistance RdsON. Indeed, when the voltage Vg is grounded, the capacitance Cg is entirely discharged during the standby mode. If the voltage Vbat rises sharply, a delay in the closing of the transistor  23  (turning off the transistor) occurs during the return to regulated mode due to the capacitance Cg charging time. A delay of this kind in closure causes a voltage overshoot at the output of the regulator because the voltage Vout continues to follow the voltage Vbat beyond its nominal value Voutnom. By keeping the voltage Vg non-zero during the standby mode, the gate capacitance Cg does not get entirely discharged, and changing from the standby mode to the regulated mode is done at a high speed with a sharp attenuation of the voltage overshoot phenomenon. 
     Naturally, various combinations of the characteristics of each of the amplifiers  22   a  to  22   d  may be used to make other alternative embodiments. In particular, the biasing stage  30  of the amplifier  22   d  may be incorporated into the amplifiers  22   a ,  22   b . It is also within the scope of those skilled in the art to apply the principles and approaches explained herein to known amplifier structures other than that of the amplifier  2 , which has been discussed herein as one possible example. Furthermore, although the above examples refer to a regulator having a PMOS type regulation transistor, it is within the scope of the invention and within the scope of those skilled in the art to apply the teachings of the present invention to regulators having an NMOS type regulation transistor. 
     Furthermore, although the problems resolved by the present invention have been described with reference to mobile or portable telephones, it goes without saying that a regulator according to the invention may be used in various other applications. This is especially true in applications where a supply voltage is provided by a battery whose autonomy needs to be preserved.