Abstract:
A linear regulator integrated circuit may be formed having four external terminals including a voltage input (Vin) terminal, a voltage output (Vout) terminal, a Set terminal, and an operational amplifier (op amp) power terminal. A user connects an external resistor to the Set terminal for creating a reference voltage. An op amp controls a pass (or series transistor) to cause an output voltage at the Vout terminal to equal the reference voltage. The op amp has a first power supply terminal internally coupled to the Vin terminal and a second power supply terminal coupled to the op amp power terminal. The op amp power terminal allows a user to externally couple the op amp second power supply terminal to either the Vout pin (for high voltage applications), system ground (for medium voltage applications), or another voltage (to provide additional headroom in very low voltage applications).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 61/813,789, filed Apr. 19, 2013, by Robert Dobkin et al., assigned to the present assignee and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to linear voltage regulator integrated circuits and, in particular, to such an IC that provides a versatile operational amplifier ground pin connection. 
     BACKGROUND 
       FIG. 1  illustrates one representative prior art linear voltage regulator  10 , which is an LT3080 low dropout (LDO) regulator manufactured by Linear Technology Corp. An LDO regulator is generally synonymous with a linear voltage regulator, and the term “low dropout” refers to the small minimum voltage differential that can occur between the input voltage terminal and the regulated output voltage terminal while still achieving regulation. 
     LDO regulators operate by varying the conductivity of a pass (or series) transistor, connected between the input terminal and output terminal, to achieve a predetermined output voltage. The output level of an operational amplifier (op amp), which is a type of differential amplifier, controls the conductivity of the pass transistor. Typically, the regulator&#39;s output voltage is fed back into one input terminal of the op amp, and the conductivity of the pass transistor is controlled to match the output voltage to a reference voltage applied to the other input of the op amp. The user selects the reference voltage. Alternatively, a divided output voltage is fed back and matched to a fixed reference voltage, where the user selects resistors for the divider to achieve the desired output voltage. 
     In  FIG. 1 , the power supply voltage may be applied to both the Vin pin  12  and the Vcontrol pin  14  of the IC package. The user connects an Rset resistor  15  between a Set pin  16  and system ground to set the output voltage Vout provided at the Vout pin  18 . A fixed precision current source  20  supplies a fixed current through the Rset resistor  15  to generate a reference voltage Vref at the non-inverting input of the op amp  22 . The output voltage Vout is applied to the inverting input of the op amp  22 . The terms inverting and non-inverting simply refer to the two branches of the differential amplifier in the op amp  22 , shown in  FIG. 2 . It is assumed that the op amp  22  includes a driver supplying the required current for driving the base of the pass transistor  24 . 
     Using an internal current source  20  and Rset resistor  15  to set the reference voltage is preferred to dividing the output voltage and matching the divided voltage to a fixed bandgap reference voltage source (typically about 1.2 volts), since, by using the current source, the loop gain and bandwidth of the regulator are not affected by the output voltage, as the regulator will always be in a unity gain configuration. 
     The op amp  22  controls the conductivity of the pass transistor  24  so that Vout matches Vref. Such an op amp in a regulator application is also referred to as an error amplifier. 
     In one application of the regulator  10 , the op amp  22  has its power terminals connected to the Vout pin  18  and to the power supply voltage Vin, by externally shorting the Vcontrol pin  14  to the Vin pin  12 . This allows the regulator to be “floating” and used in high voltage applications. 
     In a typical example, the op amp  22  needs about 1.4 volts across its power terminals in order to operate properly. Accordingly, when the Vcontrol pin  14  is tied to the Vin pin  12 , the LDO regulator  10  (a positive voltage regulator) can only regulate Vout to within 1.4 volts of Vin in order for the op amp  22  to be adequately powered. It would be desirable for various reasons, including efficiency and battery life, to enable regulation within 1.4 volts of Vin. Accordingly, for non-high voltage applications, the data sheet for the LT3080 ( FIG. 1 ) describes the option of connecting the Vcontrol pin  14  to a voltage higher than Vin so that the differential between Vin and Vout can go down to the Vce saturation voltage of the pass transistor  24  (typically 100-500 mV, depending on the load current), while still providing at least 1.4 volts to power the op amp  22 . However, most applications do not already have a separate voltage source higher than Vin. 
     The LT3080 does not allow any power terminal of the op amp  22  to be tied to system ground. Being able to ground the op amp  22  provides various advantages, including low dropout voltage and independence from Vout. 
     Other types of voltage reference-based regulators always require the ground pin to be tied to the system ground, so there is no versatility in the ground pin coupling. 
     What is needed is an LDO regulator IC that has more versatility in the connection of its ground pin. This would allow the regulator&#39;s ground pin to be connected in a way that is most optimal for the particular application, such as low input voltage, high input voltage, low output voltage, regulation close to Vin, regulation close to ground, etc. Further, the concept should be applicable to both positive voltage regulators and negative voltage regulators. 
     SUMMARY 
     In one embodiment, a positive voltage LDO regulator IC uses an on-chip current source which, along with a user-selected Rset resistor connected between a Set pin and system ground, sets the reference voltage for the op amp. The op amp controls a driver for driving a pass transistor connected between the input voltage (Vin) pin and the output voltage (Vout) pin. 
     The reference voltage is tied to the inverting input of the op amp, and the non-inverting input of the op amp is tied to the Vout pin. The regulator controls the pass transistor to cause Vout to be substantially equal to the reference voltage. 
     A positive supply input terminal of the op amp is tied, on-chip, to the Vin pin and to the positive terminal of the current source. The Vin pin will usually be tied to the positive rail voltage, resulting in the current source and op amp to be tied to the maximum positive voltage. 
     The negative supply input terminal of the op amp is connected to its own IC pin. This allows the negative supply input terminal of the op amp to be connected to either system ground, Vout (a positive voltage), or a negative voltage. Each of the connections provides a different advantage, and the optimal connection will be based on the particular application. Thus, the product&#39;s increased flexibility allows it to be used in a wider variety of applications, resulting in increased sales. 
     The invention is equally applicable to a negative LDO regulator, where the positive supply input terminal of the op amp is connected to its own IC pin, allowing it to be externally connected to either system ground, Vout (a negative voltage), or a positive voltage. 
     The op amp used is a rail-to-rail type that can operate to control the pass transistor even when the output voltage (or set voltage) is close to either the upper rail voltage or the lower rail voltage. 
     The flexibility of the connections also allows the product to be interconnected with another LDO regulator to form a four quadrant power supply that can either source current to a load or sink current from the load. Such a power supply finds use in AC systems, Class AB amplifiers, and other applications. 
     In another embodiment, the op amp with the flexible supply input terminal can be an error amplifier in a switching voltage regulator. 
     Various other embodiments are described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art LT3080 positive voltage LDO regulator. 
         FIG. 2  illustrates a rail-to-rail op amp that may be used in a linear regulator in accordance with one embodiment of the present invention. 
         FIG. 3  illustrates a positive voltage LDO regulator IC in accordance with one embodiment of the present invention. 
         FIG. 4  illustrates a negative voltage LDO regulator IC in accordance with one embodiment of the present invention. 
         FIG. 5  illustrates a four quadrant power supply chip or system using the positive voltage LDO regulator of  FIG. 3  in conjunction with the negative voltage LDO regulator of  FIG. 4 . 
     
    
    
     Elements that are the same or equivalent are labeled with the same numeral. 
     DETAILED DESCRIPTION 
     A rail-to-rail op amp is used in one embodiment of a linear regulator to drive a pass transistor to regulate Vout anywhere within substantially the full range between the upper and lower voltage rails of the system. 
       FIG. 2  is an example of a suitable rail-to-rail op amp  25 , although many other configurations of rail-to-rail op amps can be used in the regulator. On the left side of the op amp  25  circuit, the Set voltage from the Set pin of the regulator is applied to the base of the PNP transistor  26 , and the regulator&#39;s output voltage Vout is applied to the base of the PNP transistor  28 . On the right side of the op amp  25  circuit, the output voltage Vout is applied to the base of the NPN transistor  30 , and the Set voltage is applied to the base of the NPN transistor  32 . 
     A current source  33  sources a fixed current to the tied emitters of the PNP transistors  26 / 28 , and a current source  34  sinks a fixed current from the tied emitters of the NPN transistors  30 / 32 . The op amp&#39;s power supply terminals (labeled V+ and V−) are tied between any two voltages, depending on the application. As more fully explained with respect to  FIGS. 3-5 , these two voltages may be the positive and negative rail voltages, positive and ground rail voltages, a positive rail voltage and Vout, ground and negative rail voltages, or Vout and a negative rail voltage. 
     The Set and Vout inputs to the op amp  25  are configured for driving an NPN pass transistor, such as for the negative voltage regulator of  FIG. 4 . The op amp  25  inputs will be opposite for a linear regulator using a PNP pass transistor. Many other configurations of the op amp are suitable. 
     A summing circuit  35  receives the output signals from both sides of the op amp  25  circuit and generates a signal at node  36  for application to a conventional driver  37  (a buffer), where the driver  37  provides the necessary current to drive the base of the pass transistor. The signal at node  36  may be substantially anywhere between the upper and lower voltages applied to the V+ and V− terminals of the op amp  25 . The pass transistor is connected across the Vin and Vout pins of the regulator IC. If the Vout/Set voltage is close to V+, then the differential amplifier on the right side of the circuit controls the drive signal for the pass transistor. For all other Vout/Set voltages, the differential amplifier on the left side of the circuit controls the drive signal. 
     The present invention applies to regulators using a wide variety of op amp configurations. 
     Op amps, such as the op amp  25 , require a minimum voltage across its power terminals V+ and V− in order for them to operate. For example, the op amp  25  may require at least 1.4 volts between V+ and V−. 
       FIG. 3  illustrates the invention incorporated in a positive voltage LDO regulator  40 , where the Vin pin  42  is connected to a positive power supply voltage Vin, and the Vout pin  44  is settable to provide virtually any voltage between approximately 300 mV below Vin and ground. In  FIGS. 3-5 , it is assumed that the op amp includes a driver circuit supplying the necessary current for driving the pass transistor. 
     The positive supply input terminal  46  of the op amp  25  is tied, on-chip, to the Vin pin  42  and the positive voltage input of the precision current source  50 . In one embodiment, the current source  50  generates 50 microamps. The op amp  25  may be any type of op amp, such as shown in  FIG. 2  or other types. 
     The inverting input of the op amp  25  is tied, on-chip, to the Set pin  52 , and the non-inverting input of the op amp  25  is tied, on-chip, to the Vout pin  44 . Tying terminals together on-chip, when possible, is important to reduce pin count and to make the chip easier to use. 
     The user connects an off-chip Rset resistor  54  between the Set pin  52  and ground to create a desired reference voltage Vref applied to the inverting input of the op amp  25 . Generating the reference voltage using an on-chip current source  50  and an Rset resistor  54  is preferable over comparing a divided output voltage to a fixed bandgap reference, since operating characteristics of the feedback loop are not affected by the output voltage. 
     All components other than the Rset resistor  54  are on a single chip, which may be packaged in a 4-pin package. 
     A PNP pass (or series) bipolar transistor  56  is connected, on chip, between the Vin pin  42  and the Vout pin  44 . The transistor  56  is controlled by the op amp  25  and feedback loop to cause Vout to be substantially equal to Vref. 
     The negative supply input terminal  57  of the op amp  25  is connected to its own pin  58  of the chip. Users of the circuit and the data sheet for the product may refer to the versatile pin  58  as a versatile “ground pin,” even though the pin  58  may be connected to voltages other than ground, since a “ground pin” is a familiar term to users and is substituted by the versatile pin  58 . Accordingly, the chip may use an inexpensive and small four pin package. 
     In applications where the regulator is operating at high voltages, such as 120 volts or higher, the pin  58  may be externally tied by the user to the Vout pin  44 , so the regulator is floating. Hence, the op amp  25  only has to withstand the voltage differential Vin-Vout. A disadvantage of this connection is that the voltage differential cannot go below about 1.4 volts in order for the op amp  25  to operate properly. 
     At low operating voltages, the user can tie the pin  58  to the system ground. In this case, Vout can be closer to Vin without affecting the operation of the op amp  25 , and the only drawback is that Vin must be above 1.4 volts in order for the op amp  25  to operate properly. 
     In some applications, a Vout of 1.2 volts and lower is used and, if a battery generates Vin, it would be beneficial to allow Vin to go below 1.4 volts to prolong useful battery life. 
     In a third option, the user may tie the pin  58  to a voltage more negative than Vout, such as generated by another power supply or a charge pump. In one example, the pin  58  is coupled to a negative voltage relative to ground. If such is the case, the power supply voltage for the op amp  25  will always be sufficient, independent of Vout or Vin, so the only limit for regulation is the Vce saturation voltage of the transistor  56 , which may be as low as 100 mV for a low load current. Very little current is used by the op amp  25 , so a low power source may be used to supply the negative voltage. In some applications, the system uses a variety of voltages, and a suitable voltage source may be already available. 
     Accordingly, the regulator  40  has greater flexibility than prior art regulators and can potentially operate more efficiently, depending on the application and how the user connects the pin  58 . 
       FIG. 4  illustrates the concept being applied to a negative voltage LDO regulator  70 . All components may be the same as in  FIG. 3 , but their configurations are different. In  FIG. 4 , Vin is more negative than Vout, such as a negative voltage with respect to ground. The current source  72  negative terminal is tied to the Vin pin  74  and sinks a fixed current (e.g., 50 microamps) through the Rset resistor  76 , connected between the Set pin  78  and ground, to create the reference voltage Vref. The Vout pin  80  is tied to the non-inverting input terminal of the op amp  25 . The op amp  25  controls the NPN transistor  84  to cause Vout to substantially equal Vref. 
     The negative supply input terminal of the op amp  25  is tied to the Vin pin  74 , and the positive supply input terminal is connected to its own pin  86 . 
     In applications where the regulator is operating at high negative voltages, such as −120 volts or higher, the pin  86  may be externally tied by the user to the Vout pin  80 , so the regulator is floating. Hence, the op amp  25  only has to withstand the voltage differential Vin-Vout. A disadvantage of this connection is that the voltage differential cannot go below about 1.4 volts in order for the op amp  25  to operate properly. 
     At low operating voltages, the user can tie the pin  86  to the system ground. In this case, Vout can be closer to Vin without affecting the operation of the op amp  25 , and the only drawback is that Vin must be more negative than −1.4 volts in order for the op amp  25  to operate properly. 
     In a third option, the user may tie the pin  86  to a voltage more positive than Vout, such as generated by another power supply or a charge pump. In one example, the pin  58  is coupled to a positive voltage relative to ground. If such is the case, the power supply voltage for the op amp  25  will always be sufficient, independent of Vout or Vin, so the only limit is the Vce saturation voltage of the transistor  84 , which may be as low as 100 mV for a low load current. Very little current is used by the op amp  25 , so a low power source may be used to supply the positive voltage. In some applications, the system uses a variety of voltages, and a suitable voltage source may be already available. 
     With the regulators of  FIGS. 3 and 4 , a load may be connected between the Vout pin and ground, and the regulator provides the regulated Vout for a wide range of load currents. Accordingly, the negative LDO voltage regulator  70  of  FIG. 4  sinks current through the load, and the positive LDO regulator  40  of  FIG. 3  sources current through the load. 
     The positive and negative LDO regulators utilizing the present invention can be connected to create a four quadrant power supply, as shown in  FIG. 5 , that can either sink current or source current to a load connected between the Vout pin and ground. The input voltages may be a positive Vcc voltage and a negative Vee voltage. 
       FIG. 5  illustrates how the regulators  40  and  70  may be interconnected, such as on a printed circuit board or within a single package, to generate either a positive voltage Vout or a negative voltage Vout, depending on the application. Also, depending on the application, the user can connect any of the op amp supply voltages described above to the pins  58  and  86 . In one embodiment, the load requirements are variable, and the Rset resistor  90  can be automatically switched to a different resistance (such as by adding resistors in parallel or using a MOSFET) and different supply voltages are connected to the pins  58  and  86 , depending on the optimal requirements for the application. 
     Many other uses of the inventive LDO voltage regulators are envisioned. 
     In other embodiments, the amplifiers do not need to be op amps and do not need to be differential. Further, the invention may be used in non-regulator circuits, such as a circuit that provides a reference voltage. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications that are within the true spirit and scope of this invention.