Abstract:
The invention relates to a device for transitioning a bandgap reference circuit to its operational mode, of supplying a reference voltage, when power is supplied to the combined circuits. The device is comprised of a supply-independent biasing circuit that is electrically connected to a start-up circuit and supports the start-up circuit&#39;s ability to cause a bandgap reference circuit to transition to its operational mode for any supply voltage that supports the bandgap reference circuit&#39;s operational mode.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to bandgap reference circuits which provide a reference voltage independent of temperature. Specifically, a start-up circuit for a bandgap reference circuit is described whose operation is independent of power supply variations, which is an especially useful feature in battery operated circuits. 
     BACKGROUND OF THE INVENTION 
     Bandgap reference circuits are used in numerous RF analog circuit applications to provide a very stable reference voltage. For example, a wireless, radio frequency (RF) telephone is typically operated with a battery capable of generating a raw voltage between 2.7 to 5.5 volts, depending upon its state of discharge. Many of the circuit elements comprising the telephone, such as a voltage controlled oscillator, cannot operate properly without a stable and accurate reference voltage. Although a bandgap reference circuit can provide an accurate and stable reference, it has its own operational deficiencies. 
     A bandgap reference circuit has two stable states, an off state, that does not provide a reference voltage, and an operational state, that does. When power is applied to a bandgap circuit, the circuit enters the off state, in which no current flows through the bandgap reference circuit. The bandgap reference circuit remains in the off state until an external circuit forces it to transition to the operational state. Once the operational state has been established, the external start-up circuit must be electrically disconnected from the bandgap reference circuit so that the start-up circuit no longer influences the circuit operation. 
     SUMMARY OF THE INVENTION 
     The invention provides a start-up circuit for a bandgap reference circuit that will perform its intended function irrespective of substantial voltage variations of a power supply. The invention is comprised of a bandgap reference circuit, a start-up circuit that is electrically connected to the bandgap reference circuit, and a supply-independent biasing circuit that is electrically connected to the start-up circuit. The start-up circuit forces the bandgap reference circuit to transition to its operational mode when power is supplied to the two circuits. A supply-independent biasing circuit is integrated with the start up circuit to provide the proper operation of the start-up circuit for any supply voltage that supports the bandgap reference circuit&#39;s operational state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be further described in conjunction with drawing, in which: 
     FIG. 1 illustrates the circuit schematic of the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principal purpose of the combined circuits illustrated in FIG. 1 is to support the ability of the bandgap reference circuit  26  to produce a stable and accurate reference voltage to one or more circuits (not shown) of an electronic device. A bandgap reference circuit  26  provides a substantially invariant voltage when subjected to variations of temperature and power supply voltage. Although the bandgap concept is known in the prior art, the present invention provides a reliable means to get the bandgap reference circuit  26  started when a power source, having an inconsistent or a variable supply voltage VDD, is applied to the circuit through terminals  22 ,  21 . 
     The start-up circuitry  25  shown in the embodiment of FIG. 1 can perform its function substantially independent of the particular supply voltage applied to the circuit. The bandgap voltage is created by the arrangement of NPN transistors  16  and  20 , along with resistor  17 . Output terminal  23  is connected to the collector of transistor  20 . The voltage across resistor  17  equals the potential difference between the base-emitters of transistors  16  and  20 . This potential difference, termed delta-Vbe, has a positive temperature coefficient, is affected by designing the emitter area and/or current of transistor  16  to be greater than that of the transistor  20 . The potential across the base emitter junction of transistor  16  has a negative temperature coefficient. By proper selection of resistor  17 , an emitter area ratio between that of transistors  16  and  20  together with a correct choice of the aspect ratio of PMOS transistors  15  and  19 , the positive and negative temperature coefficients of resistor  17  and Vbe of transistor  16 , respectively, cancel each other to induce a temperature independent voltage across terminal  23 , the bandgap voltage. 
     P-type, MOS transistors  15  and  19  provide current to transistors  16  and  20 , respectively. Current is first established through transistor  19 , which causes transistor  20  and then transistor  16  to turn on. Once transistor  16  is turned on, transistor  15  begins conducting current and the combined currents, flowing through the drain-source junctions of transistors  15  and  19 , are conducted to ground potential. The collector-emitter junction currents flowing through transistors  20  and  16  will be substantially equal when these transistors reach an operationally steady state. For transistors  16  and  20  to reach their steady state operating point, transistor  19  must minimally conduct a threshold level of current to transistors  16  and  20 , to fully turn them on. This threshold level of current is reached only after the supply voltage  21  ramps up to its operational level. 
     Without a start-up circuit, the bandgap circuit will not operate because it may remain in a stable state where the current is zero. Under such a stable condition, the gates of PMOS transistors  15  and  19  are at the supply potential, and the bases of transistors  16  and  20 , which is the bandgap output terminal  23 , is at ground potential. The proper operating condition of the bandgap circuit occurs when the current through the drain of PMOS transistor  15  equals (delta-Vbe)/R. This condition is achieved when power is applied, and the start-up circuit pulls the gates of PMOS transistors  15  and  19  to ground, thereby allowing for current to flow. When the current through PMOS transistor  15  is (delta-Vbe)/R, NMOS transistor  10  is electrically disconnected from the bandgap, i.e. it ceases to provide drain current and the start-up circuit does not interfere with the proper operation of the bandgap circuit. 
     The electrical disconnection is achieved by the arrangement of transistors  11 ,  12  and  9 . If the PMOS transistor  11  equals the size of PMOS transistor  15 , and NMOS transistor  12  equals the size of NMOS transistor  9 , when (delta-Vbe)/R, current will flow through PMOS transistor  15 , PMOS transistor  11 , NMOS transistor  12  and NMOS transistor  9 . If the current in the drain of PMOS transistor  7  is smaller or equal to (delta-Vbe)/R, then all of the current from PMOS transistor  7  flows through NMOS transistor  9 , and none flows through NMOS transistor  8 . If no current flows in NMOS transistor  8 , its gate to source potential becomes zero. Since NMOS transistor  8  and NMOS transistor  10  forms a current mirror, no current will flow in NMOS transistor  10 . 
     The supply independent biasing assures that under wide variations in the power supply voltage, the current through the drain of PMOS transistor  7  will not exceed a particular value. The current in PMOS transistor  7  is set by the current NPN transistor  5 , times the current mirror ratio between MPOS transistor  4  and PMOS transistor  7 . Therefore, the current in NPN transistor  5  should be independent of supply variations. When power is applied, the Vbe of NPN transistor  3  will increase through the current provided by resistor  1 . The base-emitter diode of NPN transistor  3  becomes forward biased, collector current I3 will flow. With supply variations, I3 will vary. However, if the value of resistor  2 , R 2 , is such that R 2 =(kT)/q*I3) then the collector voltage of NPN transistor  3  will remain largely unaffected and consequently, the current in NPN transistor  5  will also be insensitive to variations in supply voltage. The reason that the collector voltage in NPN  3  is insensitive to variations in supply voltage is because the Vbe of transistor  5  equals the Vbe of transistor  3 -kT/q. For small changes in I3 due to supply variations, the Vbe of transistor  5  remains fixed, because as I3 increases, raising Vbe of transistor  3 , and the voltage drop across resistor  2  also increases by an equal amount resulting in a zero net change in the Vbe of transistor  5 . 
     The start-up circuit  25  forces an initial current in the bandgap reference circuit by pulling down the gate voltage of transistors  15  and  19 . The gate pull down is performed by N type, MOS transistor  10 . When power is applied to the start-up circuit, a current is established in N-type, MOS transistor  8  and is mirrored to transistor  10 . The drain of transistor  10  pulls down the gate voltages of transistors  15  and  19 , establishing a current in the bandgap reference circuit through transistors  19  and  20 , which is configured as a diode-connected device. The current in the bandgap reference circuit transistors  19  and  20  is mirrored to P-type, MOS transistor  11  and N-type, MOS transistors  9  and  12 . Once the current has reached a threshold value in the bandgap reference circuit, such that the circuit is stabilized in the operational mode, the mirrored current conducted by transistors  11  and  12  generates a voltage potential across the gate-source junctions of transistors  9  and  12 . This voltage is adequate to turn on transistor  9 , thereby pulling down the voltage applied to the gates of N-type, MOS transistor  8  and transistor  10 , thereby turning them off. With transistor  10  turned off, it no longer influences the current flowing through transistors  15  and  19  and has no further affect on the operation of the bandgap reference circuit. 
     Just as transistor  10  is turned off by the conduction of current through transistor  9 , so too is transistor  8 . The current previously conducted through the transistor  8  is subsequently conducted to ground potential through transistor  9 . 
     The supply-independent, biasing circuit  24  ensures that the current through P-type, MOS transistor  7  and conducted to ground potential by either transistor  8  or  9 , depending upon which one is alternatively turned on, remains constant even if the supply voltage drifts. Without this circuit, the start-up circuit may perform erratically when subjected to voltage variations of the power supply. For example, suppose the power supply voltage drifts upwards. As the supply voltage increases, the current conducted by transistor  7  may concomitantly rise. The increased current may increase the voltage potential developed across the drain-source junction of transistor  8  and thereby increase the voltage applied to the gates of transistors  8  and  10 . A rise of the voltage potential applied to these gates may cause the transistors to remain on, when they should turn off, or turn on when they should remain off. 
     The supply-independent, biasing circuit  24  eliminates the potential for a drifting voltage supply to adversely affect the operation of the start-up circuit. It does this by regulating the current flow through transistor  7  as set forth above. As the supply voltage drifts up, the base voltage of NPN transistor  3  drifts up, as well. An increased base-emitter junction voltage on transistor  3  causes the collector current through transistor  3  to increase, allowing the base-emitter junction of NPN transistor  5  to remain at the same voltage potential. The increase in collector current is controlled by the selection of resistor  2  and resistor  1 , so as to offset voltage variations produced by the power supply. The potential at the base of transistor  5  results in fixed current through P-type, MOS transistor  4  that is mirrored to transistor  7 . Therefore, the voltage potential developed at the gate-source junctions of transistors  8  and  10  is independent of voltage variations of the power supply and is only influenced by the operation of the startup circuit. 
     The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention but, as mentioned above, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.