Abstract:
A voltage regulator has a device for regulating an output voltage, having an input to receive an input voltage and an output to deliver an output voltage of a constant level, and a device for correcting a drop-out voltage violation, coupled to the device for regulating, to determine an occurrence of a drop-out voltage violation and to cause the device for regulating to change the level of the output voltage upon detection of the drop-out voltage violation. A method for regulating an output voltage has the steps of receiving an input voltage, generating and outputting a regulated output voltage of a constant level, monitoring occurrence of a drop-out voltage violation, and causing a change of the level of the output voltage upon detection of the drop-out voltage violation.

Description:
TECHNICAL FIELD 
     The invention relates to a voltage regulator and a voltage regulation method. 
     BACKGROUND 
     The function of a voltage regulator circuit is basically to maintain a precise voltage regardless of the current drawn by a load.  FIG. 1  shows an example of a voltage regulator. This example of a voltage regulator comprises three basic components: a reference voltage source  3  (e.g. a band-gap reference) for generating a reference voltage Vref, an error amplifier circuit  4  which compares an output voltage Vout with the reference voltage Vref via the feedback resistors R 1 , R 2  so as to determine the error in the output voltage, and an output stage  5  which regulates the output voltage according to the error amplifier output. A first resistor R 1  is coupled to a node between the output stage  5  and the output port  2  of the voltage regulator and a second resistor R 2  is serially coupled to the first resistor R 1 . An electric path is coupled to a node between the resistors R 1  and R 2  to tap-off a feedback voltage Vfb. The electric path and the two resistors R 1  and R 2  thus form a potential divider. The feedback voltage Vfb is always a particular fraction of the regulator output Vout. The error amplifier circuit further comprises an operational amplifier  4 . 1 . A negative input of the operational amplifier  4 . 1  is supplied with the reference voltage Vref and a positive input of the operational amplifier  4 . 1  is supplied with the feedback voltage Vfb. The output of the operational amplifier  4 . 1  is coupled to a gate of the output stage  5 . 
     Linear voltage regulators may require an input voltage at least some minimum amount higher than the desired output voltage. This minimum amount is called the “drop-out” voltage which is thus an important parameter of a voltage regulator. For example, a common voltage regulator has an output voltage of 5 V, but can only maintain it if the input voltage remains above about 7 V. Its drop-out voltage is therefore 7 V-5 V=2 V. When the supply voltage is less than about 2 V above the desired output voltage, the supply voltage to the regulator becomes too close to the output voltage such that the regulation properties start to degrade and the regulator can no longer hold the output voltage stable against changes in the input voltage. 
     There can be typically certain inaccuracies in the output voltage. An important contributing factor to this is the inaccuracy of the reference voltage Vref, particularly when using ultra low power band-gap references. Therefore, in a conventional linear regulator the minimum input voltage Vin is defined by the maximum output voltage Vout plus the drop-out voltage. 
     It is desirable to minimize the minimum allowable input voltage Vin. This allows a device where the regulator is connected to the battery to be operated for a longer portion of the battery discharge curve. In a system where a switched DC-DC converter supplies the input Vin, it allows the losses in the linear regulator to be minimized by minimizing the voltage drop across the regulator. This may imply that the drop-out voltage of the regulator must be kept as low as possible. However, a very low drop-out voltage may require an extremely low-resistance and physically large output device, which is undesirable in a low-cost circuit. 
     SUMMARY 
     A voltage regulator may comprise an output voltage regulation circuit, comprising an input to receive an input voltage and an output to deliver an output voltage of a constant level, and a drop-out voltage violation correction circuit, coupled to the output voltage regulation circuit, to detect an occurrence of a drop-out voltage violation and to cause the output regulation circuit to change the level of the output voltage upon detection of the drop-out voltage violation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are better understood with reference to the following drawings. 
         FIG. 1  is a block diagram of a conventional voltage regulator; 
         FIG. 2  is a block diagram of an embodiment of a voltage regulator; 
         FIG. 3  is a further embodiment of a voltage regulator; 
         FIG. 4  is a further embodiment of a voltage regulator; 
         FIG. 5  is a further embodiment of a voltage regulator showing with dashed and chain-dotted lines the circuits of the embodiment of  FIG. 2  with further details; and 
         FIG. 6  the same embodiment of a voltage regulator as shown in  FIG. 5  showing within dashed line the circuit of the embodiment of  FIG. 4  with further details. 
     
    
    
     DETAILED DESCRIPTION 
     Different aspects and embodiments are described with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects of embodiments of the invention. It may be evident, however, to one skilled in the art that one or more aspects of the embodiments of the invention may be practiced with a lesser degree of the specific details. In other instances, known structures and devices are shown in block diagram form in order to facilitate describing one or more aspects of the embodiments of the invention. The following description is therefore not to be taken in a limiting sense, and the scope of the invention is defined by the appended claims. 
     Referring to  FIG. 2 , there is shown an embodiment of a voltage regulator. The voltage regulator  20  comprises an input port  1  for supplying an input voltage Vin to the voltage regulator  20  and an output port  2  for delivering an output voltage Vout. The input voltage Vin is supplied to the input of an output voltage regulation circuit  22  the purpose and function of which is to generate and maintain a precise and constant output voltage Vout. The output voltage Vout is delivered to an output of the output voltage regulation circuit  22  from where it is delivered to the output port  2  of the voltage regulator  20 . 
     The output voltage regulation circuit  22  fulfills its function to generate and maintain a precise and constant output voltage Vout as long as the drop-out voltage, in particular the difference between the input voltage Vin and the output voltage Vout, is above a predetermined level. If, for reasons like degradation of the input power of a battery, the input voltage Vin decreases, then as a result also the drop-out voltage decreases. When the drop-out voltage decreases below the above-mentioned predetermined drop-out voltage level, the voltage regulator  20  can no longer hold the output voltage Vout stable and constant. 
     Therefore, the output voltage regulation circuit  22  is coupled to a drop-out voltage violation correction circuit  23  the purpose and function of which is to detect the occurrence of a drop-out voltage violation and to cause the output regulation circuit  22  to change the level of the output voltage Vout upon detection of a drop-out voltage violation. A drop-out voltage violation is essentially the decrease of the drop-out voltage below the above-mentioned predetermined level. Therefore, the function of the drop-out violation correction circuit aims to correct the drop-out voltage violation and to bring the drop-out voltage back into an allowable range, namely above the predetermined level. In particular, this is accomplished by causing the output regulation circuit  22  to reduce the level of the output voltage Vout for an amount so as to bring back the drop-out voltage into the allowable range, namely above the predetermined level. 
     Referring to.  FIG. 3 , there is shown a further embodiment of a voltage regulator  30  which is similar to the embodiment of  FIG. 2 . In this embodiment the drop-out voltage violation correction circuit  33  has a first input to receive the input voltage Vin and a second input to receive the output voltage Vout. In the drop-out voltage violation correction circuit  33  it can be determined on the basis of the supplied voltage values whether a drop-out voltage violation occurs. In the situation of a drop-out voltage violation the output regulation circuit  32  is caused to change the level of the output voltage. 
     With the embodiments of voltage regulators of  FIGS. 2 and 3  a method for regulating an output voltage can be performed. In this method an input voltage Vin is received by the output voltage regulation circuit  22  or  32 , respectively. A regulated output voltage Vout of a constant level is generated and outputted by the output voltage regulation circuit  22  or  32 , respectively. The occurrence of a drop-out voltage violation is monitored by the drop-out violation correction circuit  23  or  33 , respectively, and a change of the level of the output voltage upon detection of a drop-out voltage violation is caused by the drop-out voltage violation correction circuit  23  or  33 , respectively. 
     Referring to  FIG. 4 , there is shown a further embodiment of a voltage regulator. The voltage regulator  40  comprises an output stage  41  coupled between an input port  1  and an output port  2 . With the output stage  41  the current flow between the input port  1  and the output port  2  can be controlled. The voltage regulator  40  further comprises a potential divider  42  which is coupled to a first node  40 . 1  between the output stage  41  and the output port  2 . The potential divider  42  allows to tap-off a feedback voltage Vfb which is to be used for the regulation of the output voltage Vout. A control unit  43  is coupled to the potential divider  42  to control the potential divider  42  upon receipt of a signal indicating a drop-out voltage violation to change the level of the feedback voltage Vfb and the level of the output voltage Vout. The feedback voltage Vfb is normally a particular fraction of the output voltage Vout and is thus representative of the output voltage Vout. Therefore, it can be used to stabilize the output voltage Vout by generating a control voltage derived from the feedback voltage Vfb and to drive the output stage  41  with the control voltage. A signal indicating a drop-out voltage violation is received by the control unit  43  and the control unit  43  thereupon controls the potential divider  42  to change the level of the feedback voltage Vfb and as a consequence also the level of the output voltage Vout. In particular, the potential divider  42  acts to reduce the level of the feedback voltage Vfb so that also the level of the output voltage Vout is reduced. As a consequence, the drop-out voltage is brought back into a range above a predetermined value. 
     With the embodiment of a voltage regulator according to  FIG. 4 , the following method for regulating an output voltage can be performed. An input voltage Vin is received by the output stage  41  and a regulated output voltage Vout of a constant level is generated and outputted by the output stage  41 . From the output voltage Vout a feedback voltage Vfb is generated by the potential divider  42 . The feedback voltage Vfb is used for regulating the output voltage Vout. Upon receiving a signal indicating a drop-out voltage violation by the control unit  42 , the level of the feedback voltage Vfb is changed and thus the level of the output voltage Vout is also changed. In particular, the levels of the feedback voltage Vfb and the output voltage Vout are reduced. 
     Referring to  FIG. 5 , there is shown a further embodiment of a voltage regulator. The embodiment of  FIG. 5  is a further development of the embodiment of  FIG. 3 . In particular, the output voltage regulation circuit  32  as shown in  FIG. 3  corresponds to the output voltage regulation circuit  52  (surrounded by the dashed line) in  FIG. 5  and the drop-out voltage violation correction circuit  33  of  FIG. 3  corresponds to the drop-out voltage violation correction circuit  53  (surrounded by the chain-dotted line) of  FIG. 5 . However, as compared to  FIG. 3 , the output voltage regulation circuit  52  and the drop-out voltage violation correction circuit  53  are depicted with some more details of their circuit configuration. 
     The output voltage regulation circuit  52  comprises an input which is coupled to the input port  1  to supply the input voltage Vin to the output voltage regulation circuit  52 . The input voltage Vin is then provided to an output stage  52 . 1  the function of which is to control the current flow between the input and the output of the output voltage regulation circuit  52 . The output stage  52 . 1  may be implemented as a metal oxide semiconductor field-effect transistor (MOSFET), in particular a self-blocking MOSFET, as shown in  FIG. 5  in a preferable implementation. Between the output of the output stage  52 . 1  and the output of the output voltage regulation circuit  52  there is provided a first node  52 . 2 . A first resistor R 1  is coupled to the first node  52 . 2  with one of its terminals. The other terminal of the resistor R 1  is coupled to a second node  52 . 3 . Also coupled to the second node  52 . 3  is an electric line to tap-off a feedback voltage Vfb which is a fraction of the output voltage Vout. The electric line is coupled to the positive input of an operational amplifier  52 . 4  to supply the positive input with the feedback voltage Vfb. The output voltage regulation circuit  52  also comprises a band-gap reference voltage source  52 . 5  which outputs a reference voltage Vref. The reference voltage Vref is supplied to the negative input of the operational amplifier  52 . 4 . The output of the operational amplifier  52 . 4  is coupled to the gate of the output stage  52 . 1 . 
     The output voltage regulation circuit  52  is coupled to a drop-out voltage violation correction circuit  53 . The drop-out voltage violation correction circuit  53  comprises a second resistor R 3  comprising a variable and programmable resistance value. The second resistor R 3  is coupled with one of its terminals with the second node  52 . 3  of the output voltage regulation circuit  52 . The other terminal of the second resistor R 3  is coupled to one of the terminals of a third resistor R 2 . The other terminal of the third resistor R 2  is coupled to ground. The first resistor R 1 , the second node  52 . 3 , the electric line coupled to the second node  52 . 3  and the second resistor R 3  form together a potential divider. Moreover, the potential divider has variable properties as one of its constituents, namely the second resistor R 3  has a variable and programmable resistance value. In particular, if the resistance value of the second resistor R 3  is increased, then also the potential at the second node  52 . 3  is increased so that the feedback voltage Vfb which is tapped off at the second node  52 . 3  and supplied to the positive input of the operational amplifier  52 . 4  is also increased. An increase of the feedback voltage Vfb is considered by the output voltage regulation circuit  52  as a respective increase of the output voltage Vout which is in fact not the case as the output voltage Vout has remained constant. As a result, the output voltage regulation  52  reacts so as to decrease the output voltage Vout in order to have the same feedback voltage Vfb as before. Hence, an increase of the resistance value of the second resistor R 3  leads to a decrease of the output voltage Vout and thus to an increase of the drop-out voltage. The second resistor R 3  may comprise a network of resistors which may be programmed with a digital bit word from the control unit  53 . 6  to obtain a desired resistance value. 
     The drop-out voltage violation correction circuit  53  comprises two inputs one of which is for supplying the input voltage Vin and the other one is for supplying the output voltage Vout. The voltage values Vin and Vout are supplied to a comparator circuit comprising a comparator  53 . 1  and four resistors  53 . 2 ,  53 . 3 ,  53 . 4 , and  53 . 5 . A fourth resistor  53 . 2  comprising a resistance value R+r and a fifth resistor  53 . 3  comprising a resistance value R are connected in series wherein the fourth resistor  53 . 2  receives the input voltage Vin and the fifth resistor  53 . 3  is connected with one of its terminals to the fourth resistor  53 . 2  and with its other terminal to ground. Between the fourth resistor  53 . 2  and the fifth resistor  53 . 3  there is provided a node which is connected to the negative input of the comparator  53 . 1 . A sixth resistor  53 . 4  comprising a resistance value R and a seventh resistor comprising a resistance value R are connected in series wherein the sixth resistor  53 . 4  receives the output voltage Vout and the seventh resistor  53 . 5  is connected with one of its terminals to the sixth resistor  53 . 4  and with its other terminal to ground. Between the sixth resistor  53 . 4  and the seventh resistor  53 . 5  there is provided a node which is connected to the positive input of the comparator  53 . 1 . The comparator  53 . 1  thus compares a voltage which is representative of the input voltage Vin with another voltage which is representative of the output voltage Vout. 
     If the comparator  53 . 1  detects that the difference between the voltages input to its positive and negative inputs is below a certain predetermined threshold, it outputs a signal “too_low”. This signal “too_low” is transmitted to a control unit  53 . 6 . The control unit  53 . 6  then generates and outputs a trim signal to the second variable resistor R 3 . 
     The voltage regulator thus allows a lower minimum input voltage Vin for a given voltage regulator drop-out voltage. The comparator  53 . 1  compares the input voltage Vin and the output voltage Vout (or voltages representative to the input and output voltages) and indicates with the signal “too_low” to the control unit  53 . 6  if the condition is detected that the input voltage Vin is too low so that the drop-out voltage would be violated. If this condition is detected, the second variable resistor R 3  is programmed by the control unit  53 . 6  to set the output voltage to a lower value as described above. Typically, this procedure would only be enabled at specific instances and be semi-static so as to avoid any dynamic instability in the calibration mechanism. 
     In the embodiment of  FIG. 5 , as explained above, a resistive divider network of fourth to seventh resistors  53 . 2  to  53 . 5  is used to compare the input and output voltages. The ratios of the resistive dividers may be chosen such that, with the second variable resistor R 3  at its minimum value, the comparator generates the signal “too_low” when the input voltage Vin is at the lowest allowable value and the drop from Vin to Vout is just crossing the lowest safe value. Other ratios may be chosen as an alternative. 
     This procedure does guarantee that, as long as the input voltage Vin is within its specified range, inaccuracies in the reference voltage Vref do not cause the drop-out voltage to be exceeded. In the event that Vref is instead too low, the signal “too_low” and the reduction of the output voltage will never be activated, thereby not effecting the accuracy at the lower limit of the output voltage. 
     In order to deal with variations of the reference and input voltages with time, it is desirable to periodically detect if the trimming can be reduced. This can be done by occasionally attempting to back-off the trimming until the signal “too_low” is indicated, and/or by including a separate comparator that detects when the input voltage is much higher than the output voltage. 
     Referring to  FIG. 6 , there is shown an embodiment of a voltage regulator which is virtually identical to the embodiment as shown in  FIG. 5 . In fact,  FIG. 6  shows the same circuit configuration as  FIG. 5 , but  FIG. 6  shows another way of grouping particular circuit elements together in order to compare it with the embodiment as depicted in  FIG. 4 . The voltage regulator  40  as shown in  FIG. 4  comprises an output stage  41  which is comparable to the output stage  61  of the voltage regulator  60  in  FIG. 6 . In  FIG. 4  there are shown a potential divider  42  and a control unit  43 , both in block form. In  FIG. 6  the respective circuit parts are represented in more detail. The potential divider  62  of the voltage regulator  60  is comparable to the potential divider  42  of the voltage regulator  40  in  FIG. 4 . In  FIG. 6  the potential divider  62  is coupled to a first node  60 . 1  provided between the output of the output stage  61  and the output port  2 . The potential divider  62  comprises a first resistor R 1 , a second node  62 . 1 , an electric line coupled to the second node  62 . 1  and a second variable resistor R 3 . The control unit  63  which is comparable to the control unit  43  in  FIG. 4  is coupled with the second variable resistor R 3  in order to program the second resistor R 3  to a higher value for programming a higher output voltage Vout in case of a drop-out voltage violation.