Patent Publication Number: US-7589510-B2

Title: Voltage regulator having variable threshold voltage switch

Description:
TECHNICAL FIELD 
   The present invention relates to voltage regulators. 
   BACKGROUND 
   A low voltage drop over the voltage regulator is achieved by the use of a MOSFET as a voltage regulating element together with a charge pump providing a sufficiently high gate potential which has to be higher than the output voltage of the voltage regulator, in the case of a low drop regulator even higher than the input voltage of the voltage regulator. 
   In order to guarantee a substantially constant output voltage, the gate of the voltage regulating MOSFET (e.g. a power MOSFET) is supplied with a bias current provided by a charge pump and controlled by a closed loop control system. That is, the output voltage of the voltage regulator is received by a controller which controls the gate current (and therefore the gate voltage) of the voltage regulating MOSFET such, that the output voltage of the voltage regulator remains substantially constant. 
   In response to an upward step of the load current (i.e. the output current) the output voltage will slightly drop due to the higher voltage drop over the voltage regulating MOSFET. Triggered by this voltage drop the controller will increase the gate current for charging the gate-source-capacitance of the voltage regulating MOSFET in order to increase the conductivity of the voltage regulating MOSFET thus re-adjusting the output voltage to its desired value. 
   The time which is needed to compensate for the disturbance in the output voltage induced by the step in a load current is determined by the loop bandwidth of the closed loop control system and especially dependent on the value of the gate-source-capacitance of the voltage regulating MOSFET. 
   With a given value of the gate-source-capacitance of the voltage regulating MOSFET the speed of the closed loop control system can only be increased by increasing the gate current which charges the gate of the MOSFET. This gate current is supplied by a charge pump, as explained before, and, in order to minimize power consumption, an increase of the maximum gate current which would entail a more costly charge pump is not desirable. 
   SUMMARY 
   In one embodiment of the invention the inventive voltage regulator comprises a power filed effect transistor having a threshold voltage, a drain terminal receiving an input voltage, a source terminal providing an output voltage and a load current, a gate terminal responsive to a control signal, and a bulk terminal. The voltage regulator further comprises a control loop circuits responsive to the output voltage and providing the control signal. The control loop circuit is adapted for adjusting said control signal to such a value that the output voltage is regulated to a desired (constant) value. Additionally the threshold voltage of the power field effect transistor is modified dependent on the load current. Alternatively the threshold voltage can be modified dependent on the output voltage or on both, the output voltage and the load current. 
   In another embodiment of the invention the voltage regulator additionally comprises a switching circuit for modifying the threshold voltage. The switching circuit is responsive to the output voltage and/or to the load current and it is adapted for connecting the bulk terminal of the field effect transistor with either the source terminal or a constant potential dependent on the load currents and/or the output voltage. 
   Another aspect of the invention also comprises a method for controlling the power field effect transistor which was defined above. In one embodiment the method comprises the step of modifying the threshold voltage dependent on the load current and/or the output voltage. This can be done, for example, by a connecting the bulk terminal of the field effect transistor with either the source terminal or a constant potential dependent on the load current and/or the output voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, instead emphasis being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts. In the drawings: 
       FIG. 1  shows a conventional voltage regulator with a power MOSFET as a regulating element and a feedback circuit for regulating the output voltage to a desired constant value. 
       FIG. 2  shows one embodiment of the inventive voltage regulator comprising a switching circuit for modifying the threshold voltage of the voltage regulating power MOSFET. 
       FIG. 3  shows the embodiment of  FIG. 2  with the switching circuit being illustrated in more detail. 
       FIG. 4  shows timing diagrams of the load current, the output voltage and the gate voltage illustrating the step response of a voltage regulator according to  FIG. 1 . 
       FIG. 5  shows timing diagrams of the load current, the output voltage, the gate voltage and the bulk voltage illustrating the step response of an voltage regulator according to  FIG. 2  or  3 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a simple voltage regulator using a power MOSFET Mp as a voltage regulating element. In the embodiment shown in  FIG. 1  a n-MOS transistor is used whose drain terminal D is connected to a first supply terminal receiving an input voltage Vin and whose source terminal S is connected to an output terminal providing an output voltage Vout and a load current Iload. For compensating for high frequency current spikes a capacitance Cout is connected between the source terminal S and a second supply terminal, e.g. a ground terminal GND. The voltage regulator further comprises a feedback circuit  10  for regulating the output voltage Vout, i.e. the source potential of the power MOSFET, to a desired (e.g. constant) value. 
   The feedback circuit  10  comprises a controller  13  whose input is connected to the source terminal S and responsive to the output voltage Vout. The output of the controller  13  provides a controller voltage Vc received by the gate of a controlling transistor  12  whose source terminal is connected to the ground terminal GND and whose drain terminal is connected to the gate G of the voltage regulating power MOSFET Mp and to a current source  11  providing a bias current Ibias to the gate G and to the controlling transistor  12 . The current source  11  is connected to a third supply terminal receiving a supply voltage Vcp provided by a charge pump (not shown). 
   The function of the feedback circuit can be easily understood with the help of the timing diagrams shown in  FIG. 4 .  FIG. 4  illustrates the step response of the output voltage Vout, the controller voltage Vc, and the gate voltage Vg to an upward step of the load current Iload. In the circuit of  FIG. 1  with a given output voltage Vout, a given load current Iload, and a given supply voltage Vin the drain-source voltage Vds of the power MOSFET Mp has been adjusted by the feedback circuit  10  such, that drain-source voltage Vds (i.e. the product RDS×Iload of the drain-source resistance RDS and the load current Iload) is equal to the difference between the supply voltage Vin and the output voltage Vout. An upward step of the load current Iload firstly results in a drop of the output voltage Vout. Triggered by this voltage drop the controller  13  reduces the controller voltage Vc, i.e. the gate voltage of the controlling transistor  12 , thus increasing the fractional part of the bias current Ibias used for charging the gate (i.e. the gate-source-capacitance) of the power MOSFET Mp. An increased gate charge results in a higher gate voltage Vg of the power MOSFET and in a lower drain-source voltage Vds (i.e. in a lower drain-source resistance RDS) which compensates for the higher load current Iload, thus readjusting the output voltage to its desired (constant) value. 
   The time which is needed to readjust the drop in the output voltage Vout to its desired constant value depends on the time the feedback circuit  10  needs to react to a drop in the output voltage, i.e. the loop delay time tL, the time which is needed to charge the gate-source capacitance of the power MOSFET Mp, i.e. the charging time tC. The loop delay time tL depends on the bandwidth of the feedback circuit  10  and is usually much smaller than the charging time tC. To decrease the overall delay time tD (TD=tL+tC) it is necessary to reduce the charging time tC, which could be done by increasing the bias current Ibias which would entail higher costs for the current source  11  and the charge pump. 
   Another possibility to improve the overall delay tD time without the need for increasing the bias current Ibias is shown in  FIG. 2 . compared to the circuit of  FIG. 1  a current measurement means  30  is connected in series to the drain-source path of the power MOSFET Mp. In the case of  FIG. 2  the current measurement means is connected between the drain terminal D of the power MOSFET Mp and the supply terminal receiving Vin. The current measurement means  30  provides a measurement signal S 30  which depends on the load current Iload. The voltage regulator further comprises a switching circuit  20  being responsive to the load current Iload (or, strictly speaking, to the measurement signal S 30 ). The switching circuit  20  is connected to the output terminal providing the output voltage Vout (i.e. the source potential) and with the bulk terminal B of the power MOSFET Mp. The switching circuit comprises a switch SW responsive to the measurement signal S 30 . The switch SW is adapted for connecting the bulk terminal B of the power MOSFET Mp with either the source terminal S or a constant potential V 2  dependent on the value of the load current Iload or the measurement signal S 30  respectively. 
   The constant potential V 2  is preferably lower than the output voltage Vout and can also be equal to ground potential GND. An “ordinary” MOSFET would have its bulk terminal B connected to its source terminal S. Compared to this switching state (a first switching state) the threshold voltage of the power MOSFET Mp increases, if the switch SW connects the bulk terminal B of the power MOSFET Mp with the constant potential V 2  being lower than the source potential (Vout) of the power MOSFET Mp. This state of the switch SW is further referred to as the second switching state. The function of the circuit is explained in more detail by reference to  FIGS. 3 and 5 . 
     FIG. 3  shows the embodiment of  FIG. 2  wherein the measurement means  30  and the switching circuit  20  are illustrated in more detail. The measurement circuit  30  comprises a shunt resistor R, a voltage source providing the offset voltage Vos and a comparator  31 . The shunt resistor is connected to the drain terminal D of the power MO8FET Mp with its first terminal in series to the drain-source path of the power MOSFET. A second terminal of the shunt resistor R is connected to a non-inverting input of the comparator  31  and the first terminal of the shunt resistor R is also connected to the inverting input of the comparator  31  via the voltage source providing the offset voltage Vos. The output signal of the comparator assumes a first logic level, e.g. a high level, if the load current Iload is higher than a reference current defined by the quotient Iref=Vos/R of the shunt resistor R and the offset voltage Vos. Of course any other method for measuring the load current Iload and comparing it with a reference current is applicable (e.g. a sense-FET). 
   Additionally to the embodiment shown in  FIG. 2  the switching  20  circuit comprises a comparator  23 , an AND-gate  22  with an inverting and a non-inverting input, and transistors M 1 , M 2  provide the functionality of the switch SW. The comparator  23  is adapted for comparing the output voltage Vout with a reference voltage Vref and for providing an output signal which assumes a first logic level, e.g. a high level, if the output voltage is higher than the reference voltage. The output of the comparator  23  is connected with the non-inverting input of the AND-gate  22 . The inverting input of the AND-gate  22  is connected with the output of the comparator  31  which has been described above. The AND-gate  22  provides a switching signal S 22  controlling the switching states of the transistors M 1 , M 2 . 
   In the current embodiment the switching signal S 22  assumes a first logic level, e.g. a high level, if the load current Iload is lower than a reference current defined by the quotient Vos/R and the output voltage is higher than the reference voltage Vref. Then the first p-MOS transistor M 1  is switched to an off-state and the n-MOS transistor M 2  is switched to an on-state, thus isolating the bulk terminal B of the power MOSFET Mp from the output terminal providing the output voltage Vout (and also from its source terminal S) and connecting the bulk terminal B of the power MOSFET Mp with the constant potential V 2  which is—in the current case—equal to the ground potential. 
   If either the output voltage drops below the reference voltage Vref or the load current rises above the reference current defined by the quotient Vos/R the output logic level of one of the comparators  23 ,  31  will change and the output signal S 22  of the AND-gate  22  will switch to a second logic level, e.g. a low level, thus switching on the p-MOS transistor M 1  and switching off the n-MOS transistor M 2  and the p-MOS transistor M 3 . The bulk terminal B of the power MOSFET Mp is than connected to the source terminal S of the power MOSFET Mp and isolated from the constant potential V 2 . 
   Connecting the bulk terminal either with a constant potential V 2  or with the source terminal S will change the threshold voltage of the voltage regulating power MOSFET Mp. The effect of this change of the threshold voltage on the speed of the feedback circuit can easily be explained by the help of  FIG. 5 .  FIG. 5  shows, like  FIG. 4 , timing diagrams of the load current Iload, the output voltage Vout, the control voltage Vc, the gate voltage Vg, and the bulk voltage Vb. The left hand side of the timing diagram of Iload shows the load current Iload dropping below the reference current Iref=Vout/R. As a consequence the bulk terminal B is isolated from the source terminal S and connected with a constant potential V 2 . This results in an increase of the threshold voltage of the power MOSFET Mp and the controller  13  (via the controlling transistor  12 ) has to adjust the gate voltage Vg to a higher value, i.e the gate G of the power MOSFET Mp is pre-charged during the second switching state when the load current Iload is below the reference current and the output voltage Vout is above the reference voltage Vref. In response to an upward step in the load current Iref a drop in the output voltage Vout will be observed. Due to the rise of the load current Iload the bulk terminal B of the power MOSFET Mp will again be connected with the source terminal S and therefore the threshold voltage of the power MOSFET Mp is decreased again. Due to the fact, that the gate G of the power MOSFET Mp was precharged before, less charge is necessary to increase the gate voltage to a value necessary for compensating for the increase load current. As a consequence the feedback circuit  10  can react much faster for regulating the output voltage Vout to its desired constant value and the charging time tC is greatly reduced, thus improving the overall performance of the voltage regulator.