Abstract:
Bandgap voltage reference circuitry capable of operating at very low power supply voltages. The current source for driving the core bandgap voltage reference is implemented with insulated gate field effect transistors having low threshold voltages. Voltage clamp circuitry protects the transistors from power supply voltage variations rising above a predetermined clamp voltage. An output amplifier with output biasing circuitry having a circuit structure similar to that of the core bandgap voltage reference ensures that the bandgap reaches the intended steady state of operation.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to bandgap voltage reference circuits, in a particular, to bandgap voltage reference circuits capable of operating at low power supply voltages such as within a range of 1.5-5.5 volts. 
     2. Related Art 
     As is well known in the art, reliable voltage references are required for many types of circuits an systems. In particular, such voltage references are often required to be consistent over temperature. Perhaps the most common voltage reference circuitry relies upon the bandgap of silicon. Various forms of such circuits have been designed and implemented to generate a reference voltage of 1.2 volts that is substantially constant over temperature. However, if circuits are required to operate at lower voltages, such as 1.5 volts, a bandgap voltage of 1.2 volts leaves only 0.3 volt headroom. Such little voltage headroom is often inadequate to maintain proper circuit operation. 
     Referring to  FIG. 1 , when operating at such low power supply voltages, where headroom becomes a significant problem, most existing bandgap reference circuits use a parallel architecture where a proportional to absolute temperature (PTAT) current and a base-emitter voltage (VBE), or a portion of VBE, are generated separately and combined together to produce the 1.2 volt bandgap voltage, or a divided-down voltage based on such bandgap. For example, as shown, a differential amplifier A 1 , in conjunction with current mirror circuitry formed by PMOS devices M 0 , M 1 , M 2 , M 3 , bipolar junction transistors Q 0 , Q 1  and a resistor R 0  provide a PTAT current via the drain electrode of PMOS device M 0 . Another differential amplifier A 2 , in conjunction with current mirror circuitry formed by PMOS devices M 4 , M 5 , M 6 , M 7 , a bipolar junction transistor Q 2  and resistor R 2  provide a current based on the VBE of transistor Q 2  via the drain electrode of PMOS device M 4 . These currents combine and generate the bandgap voltage VBG across an output resistor R 1 . 
     While such a circuit architecture allows for operation at a low power supply voltage VDD, errors in the bandgap voltage VBG over temperature are nonetheless generated from the input offsets of the two amplifiers A 1 , A 2 , and mismatches within the current mirror circuits. Further, such an architecture is relatively large in size and has two separate closed loop systems (about the differential amplifiers A 1 , A 2 ) that require separate compensation. While it is possible to use bandgap trimming to improve the bandgap accuracy, the circuit size will become even larger as a result and test times increase due to the trimming needed. When using low voltage devices (e.g., maximum VDS of 1.8 volts), this circuit architecture also limits the maximum power supply voltage (VDD), since PMOS devices M 0 , M 2 , M 3 , M 4 , M 6  and M 7  are exposed to nearly the entire VDD voltage level. Adding voltage protection circuitry in series with these devices will then add circuit complexity and limit the operation at low VDD power supply levels. 
     Accordingly, it would be desirable to have an improved bandgap reference circuit Architecture capable of operating at significantly reduced power supply voltages, while minimizing the number of offsets and trimming requirements. 
     SUMMARY 
     In accordance with the presently claimed invention, bandgap voltage reference circuitry capable of operating at very low power supply voltages is provided. The current source for driving the core bandgap voltage reference is implemented with insulated gate field effect transistors having low threshold voltages. Voltage clamp circuitry protects the transistors from power supply voltage variations rising above a predetermined clamp voltage. An output amplifier with output biasing circuitry having a circuit structure similar to that of the core bandgap voltage reference ensures that the bandgap reaches the intended steady state of operation. 
     In accordance with one embodiment of the presently claimed invention, bandgap voltage reference circuitry includes:
         first and second power supply electrodes to convey a power supply voltage;   current mirror circuitry coupled to the first power supply electrode and responsive to the power supply voltage and a first clamped voltage by providing first and second currents;   bandgap reference circuitry coupled between the current minor circuitry and the second power supply electrode, and responsive to the power supply voltage, the first and second currents and the first clamped voltage by providing a bandgap reference voltage; and   first voltage clamp circuitry coupled to the first power supply electrode, the current mirror circuitry and the bandgap reference circuitry, and responsive to the power supply voltage and the first clamped voltage by preventing the first clamped voltage from exceeding a first predetermined value.       

     In accordance with another embodiment of the presently claimed invention, a method of providing a bandgap voltage reference includes:
         generating first and second currents in response to a power supply voltage and a first clamped voltage;   generating a bandgap reference voltage in response to the power supply voltage, the first and second currents and the first clamped voltage; and   preventing, in response to the power supply voltage and the first clamped voltage, the first clamped voltage from exceeding a first predetermined value.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional bandgap reference circuit using a parallel circuit architecture. 
         FIG. 2  is a schematic diagram of a bandgap voltage reference circuit in accordance with one embodiment of the presently claimed inventions. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
     Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, or one or more voltages. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. 
     As discussed in more detail below, bandgap voltage reference circuitry in accordance with the presently claimed invention provides a precise bandgap voltage reference for a wide range of power supply voltages in common use today, such as 1.5-5.5 volts. Such applications include Portable System Battery Chargers with a termination voltage requirement of +/−1%, low dropout (LDO) voltage regulators, switching power supplies, and other precision systems that must operate over wide ranges of power supply voltages. Such reference circuitry uses the Brokaw Architecture which allows for simple implementation and a small number of components to optimize component matching. Further, and perhaps most advantageously, such voltage reference circuitry takes advantage of low voltage threshold PMOS devices (e.g., VTP=0.44 volt, VDS=1.8 volts) to address the low voltage headroom issue. Component matching is included and circuit startup is reliable and operates over a wide range of power supply voltages and rise times (e.g., 1 microsecond-10 milliseconds). 
     Referring to  FIG. 2 , a bandgap voltage reference circuit in accordance with an exemplary embodiment of the presently claimed invention can be implemented as shown and described herein. In accordance with the Brokaw Architecture, bipolar junction transistors Q 6  and Q 7 , with an emitter area ratio of Q 6 :Q 7 =14:1, establish the differential base-emitter voltage Vbe, with their respective emitter currents IQ 6  and IQ 7  conducted through the parallel combination of resistors R 1  and R 2  and resistor R 0 . Dual emitter resistors R 1 , R 2  for transistor Q 6  are used to allow smaller size resistors to be used while still achieving the same equivalent resistance needed for the proper ratio as compared to resistor R 0 . 
     The magnitudes of these currents IQ 6 , IQ 7  are ensured as being equal by the current mirror action of PMOS transistors M 12  and M 15 . In accordance with an exemplary embodiment, these transistors M 12 , M 15  have channel width-to-length ratios of 55:8 microns, and are biased at approximately 150 millivolts overdrive voltage for optimal matching. The operating voltage Vds across the drain and source electrodes of these transistors M 12 , M 15  is limited to their maximum safe operating voltage of 1.8 volts by a voltage clamp circuit formed by diode-connected PMOS transistors M 21 , M 22 , M 24  connected between the positive power supply voltage VDD and the drain electrode of current mirror transistor M 15 . 
     While unnecessary when the circuit is operating at a very low power supply voltage (e.g., VDD=1.5 volts), such voltage clamp circuitry prevents the drain-source-to voltages Vds across current mirror transistors M 15  and M 12  from exceeding their maximum operating voltage (e.g., 1.8 volt) when the circuit is operating at a higher power supply voltage (e.g., 1.8-5.5 volts). 
     Transistor Q 5 , diode-connected transistors Q 13  and Q 14 , resistors R 4  and R 7  and a current source I 1  form a startup circuit which initiates current flow through the current mirror circuit M 12 , M 15 . This start-up circuit shuts down once circuit operation has begun, due to the resulting inadequate base-emitter drive voltage for transistor Q 5  (Vbe=1.4 volts-1.2 volts=0.2 volts). 
     Transistor Q 16 , biased by the power supply voltage VDD and current source I 1 , prevents a parasitic PNP transistor formed by the base, collector and P-substrate of transistor Q 6  from turning on during circuit startup with a low power supply ramp rate. 
     The resulting output voltage at the drain electrode of transistor M 15  is driving the output stage formed by transistors M 23 , M 1  and Q 4 , and resistor R 6 . Diode-connected PMOS transistor M 0 , biased by a current source I 2 , provides a gate drive voltage level-shifted down from the power supply voltage VDD for output transistor M 1 . 
     A second voltage clamp circuit in the form of diode-connected PMOS transistors M 27 , M 26 , M 25  and M 57  clamp the maximum voltage Vds across the drain and source electrodes of output transistor M 23  to prevent it from exceeding its maximum operating voltage (e.g., &lt;1.8 volt). Further, the biasing action of transistor M 1  maintains a substantially constant drain-to-source voltage VDS across transistor M 23 , thereby preventing channel modulation. 
     Diode-connected transistor Q 4  and resistor R 6  serve as the output load for output transistor M 23 , and simulate the serial connection of transistors Q 6  and Q 7  and resistors R 1 , R 2  and R 0 . This provides matching for the respective loads of current mirror transistors M 12  and M 15 , and output transistor M 23 . 
     The resulting bandgap reference voltage VBG is provided at the base electrodes of transistors Q 6  and Q 7 . 
     Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.