Abstract:
A startup circuit operating with a bandgap circuit having a predetermined node with a current change proportional to temperature change and a current source connected to the predetermined node comprising: a controllable current switch connected between the predetermined node and a control node of the current source; wherein when the voltage at the predetermined node is floating when starting the bandgap circuit, the controllable current switch biases the current source at the control node whereby the voltage at the predetermined node changes based on the current provided by the current source causing the bandgap circuit to start its normal operation.

Description:
BACKGROUND 
   The present invention is related to a device and method for starting a low supply bandgap reference circuit. 
   The objective of a bandgap reference circuit is to provide a voltage that remains constant when the temperature changes. The bandgap reference circuit generates a stable voltage over a temperature range by utilizing two semiconductor circuits, one for providing a voltage that is proportional to absolute temperature (PTAT) and a second for providing a voltage that is complementary to absolute temperature (CTAT). Conventionally the sum of the two circuits is used to provide a temperature-stabilized voltage reference. 
     FIG. 1  shows a simplified circuit of a conventional bandgap reference circuit  100 . Node A provides a voltage, which is complementary to absolute temperature (CTAT) based on the negative temperature dependent junction voltage of a PN diode, which is about −1.5 mV/° C. Node B provides a large area PN-type device  104  in series with resistor  102  to ground. The feedback loop comprising an opamp  106  and a pair of matched controlled current sources P 1  and P 2  forces the voltages at node A and node B to be equal. According to the I-V equation of a PN diode, the voltages at nodes A and B, Va and Vb are:
   Ia=Aa*I 0*exp( qVa/kT )   Ib=N*Aa*I 0*exp( q ( Vb−Ib*Rb )/ kT ) 
If Va is set to be equal to Vb, and Ia=Ib, the above two equations can be simplified as
   Ib*Rb=kT/q *ln( N ), 
so that the current Ib flowing through node B is proportional to absolute temperature.
 
   The output voltage Vbg developed across a resistor in the output stage  110 , is a PTAT current Ic, mirrored from Ib, on Rc in series with a negative-temperature-coefficient diode voltage. The Vbg could be designed to be temperature independent if the magnitudes of Ic and Rc are proper to compensate the negative temperature coefficient of a diode. 
   To reduce power consumption, the feedback loop is generally self-biased. Like other self-biasing circuits, the bandgap reference circuit  100  may have two stable states. The first stable state is when it begins normal operation as designed, and the second stable state is when all the currents are zero (or floating). The circuit can be at zero current when the bandgap circuit initially powers up or as a result of power interruptions. When this zero current state occurs, the bandgap circuit is in a non-started state and the bandgap voltage (Vbg) is improper. A “startup” circuit may be employed to ensure the bandgap reference circuit starts. The purpose of a startup circuit is to ensure the proper operational state can be set during power up without interfering with normal operation of the bandgap circuit once it is started. 
     FIG. 2  shows a simplified schematic of a conventional bandgap reference circuit  200  with a startup circuit. The bandgap reference circuit  200  is comprised of a diode  108  for providing a CTAT voltage at the node A, a plurality of diodes  104  in series with resistor  102  for providing a PTAT voltage at the node B, and an opamp  106  for controlling two PMOS devices P 1  and P 2  and providing proper biasing to the PMOS device P 3  in the output stage. In the figure the opamp is shown as discrete devices, however, other opamp circuitry may be used. The PMOS devices P 1  and P 2  provide currents to the nodes A and B. The startup circuit comprises an NMOS device N 1  coupled between a complementary power supply such as a ground or VSS, and the gates of the PMOS devices P 1  and P 2 . The NMOS device N 1  is controlled by power on a reset signal PONRST generated externally to the circuit. 
   To operate the power on reset of the bandgap reference circuit  200 , the signal PONRST is controlled to a logical “high” such that the NMOS device N 1  is turned on. Turning on the NMOS device N 1  biases the PMOS devices P 1  and P 2  into conduction such that they provide a current to the nodes A and B. Once the current passes through either the node A or the node B, the voltages at the nodes A and B are established and the bandgap circuit is brought out of the non-started state and begins normal operation. The drawback to the power on a reset circuit is that is depends on an external signal PONRST to start the bandgap reference circuit. 
   The deficiencies of the conventional circuitry and methods for starting a bandgap circuit show that a need still exists for improvement. To overcome the shortcomings of the conventional circuitry, new circuitry and method for starting a bandgap circuit is needed. 
   SUMMARY 
   This invention is for a startup circuit operating with a bandgap circuit having a predetermined node with a current change proportional to temperature change and a current source connected to the predetermined node comprising: a controllable current switch connected between the predetermined node and a control node of the current source; wherein when the voltage at the predetermined node is floating when starting the bandgap circuit, the controllable current switch biases the current source at the control node whereby the voltage at the predetermined node changes based on the current provided by the current source causing the bandgap circuit to start its normal operation. 
   The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a conventional bandgap circuit. 
       FIG. 2  illustrates a circuit schematic for a conventional bandgap circuit and startup circuitry. 
       FIG. 3  illustrates one embodiment of the current invention. 
       FIG. 4  illustrates another embodiment of the current invention. 
       FIG. 5  illustrates yet another embodiment of the current invention. 
   

   DESCRIPTION 
   This invention relates generally to bandgap reference circuits and more specifically to a system and method of starting a bandgap reference circuit reliably and without interfering with normal circuit operation. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
   References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure or characteristic, but every embodiment may not necessarily include the particular feature, structure or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one of ordinary skill in the art to affect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. Parts of the description are presented using terminology commonly employed by those of ordinary skill in the art to convey the substance of their work to others of ordinary skill in the art. 
     FIG. 3  shows one embodiment of the present invention  300 . The bandgap reference circuit  300  is comprised of a node A for providing a CTAT voltage (Va) based on a negative temperature dependent junction voltage of a PN diode  108 . And a node B for providing a PTAT current (Ib) based on a plurality of semiconductor junctions  104  in series with a resistor  102 , an opamp  106  for controlling the PMOS device P 1  and the PMOS device P 2  and providing proper biasing to the PMOS device P 3  in the output stage. The PMOS devices P 1  and P 2  provide a current to the nodes A and B and the PMOS device P 3  provides a current to a resistor in the output stage for generating an output voltage Vbg. This embodiment has a PMOS device Mx coupled between the node B and a node C. The node C is the gate of the PMOS devices P 1 , P 2  and P 3 . The PMOS device Mx is configured with the gate and drain connected to node B and the source connected to node C. 
   In view of the foregoing, when the Vb and Va are floating, such as when the bandgap reference  300  does not start, the PMOS device Mx controls the controlled current sources P 1  and P 2  to supply a current through the nodes A and B. Once the current passes through either node A or B, the voltages Va and Vb are established and the bandgap circuit is brought out of the non-started state and begins normal operation. 
   It will be appreciated by those having skill in the art that in this embodiment Mx should be designed so that the current flowing through Mx is much smaller than through P 2 . In a bandgap reference circuit operating with a supply voltage of about 1 volt, Vb should be set to a voltage very close to the voltage at node C (Vc) such that P 2  is operating in the saturation region. To ensure proper operation, the initial voltage Vb should be less than Vc−|V th | but the final voltage greater than Vc−|V th | where V th  is the threshold voltage of Mx. 
   It is understood that one skilled in the art of integrated circuit design could affect different means to create a bandgap reference than the one shown. It will also be appreciated by those having ordinary skill in the art that this invention can be practiced using other devices or PN junction modules including but not limited to diodes, cascaded PMOS devices connected as diodes or other PN junction module configurations. 
   One aspect of the present invention is that it provides circuitry and a method for starting a bandgap circuit during initial power-up or following a power interruption. Some of the advantages of the present invention are simplicity, reliability and, that it does not affect normal bandgap circuit operation once started and would only require a small area if implemented monolithically. 
     FIG. 4  shows another embodiment of this invention  400 . The bandgap reference circuit  400  is comprised of a node A for providing a CTAT voltage (Va) based on a negative temperature dependent junction voltage of a PN diode  108 . And a node B for providing a PTAT current (Ib) based on a plurality of semiconductor junctions  104  in series with a resistor  102 , an opamp  106  for controlling the PMOS device P 1  and the PMOS device P 2  and providing proper biasing to the PMOS device P 3  in the output stage. The PMOS devices P 1  and P 2  provide a current to the nodes A and B and the PMOS device P 3  provides a current to a resistor in the output stage for generating an output voltage Vbg. In this embodiment, a PMOS device Mx is connected having source at node C, the gate at node B and the drain connected to a complementary supply such as ground or VSS. 
   In view of the foregoing, if the bandgap circuit fails to start, Vb could be floating causing Mx to control the controlled current sources P 1  and P 2  to supply current through the nodes A and B. Once current passes through either node A or B, the voltages at the nodes A and B are established and the bandgap circuit is brought out of the non-started state and begins normal operation. 
   It is understood by those having ordinary skill in the art that in this embodiment Vb should be set to a voltage very close to the voltage at node C (Vc) such that P 2  is operating in the saturation region. During normal operation Vb should meet the condition Vb+|Vtp|&gt;Vc to cutoff Mx where Vtp is the threshold voltage of Mx. 
     FIG. 5  shows another embodiment of the current invention  500 . The bandgap reference circuit  500  is comprised of a node A for providing a CTAT voltage (Va) based on a negative temperature dependent junction voltage of a PN diode  108 . And a node B for providing a PTAT current (Ib) based on a plurality of semiconductor junctions  104  in series with a resistor  102 , an opamp  106  for controlling the PMOS device P 1  and the PMOS device P 2  and providing proper biasing to the PMOS device P 3  in the output stage. The PMOS devices P 1  and P 2  provide a current to nodes A and B and the PMOS device P 3  provides a current to a resistor in the output stage for generating an output voltage Vbg. In this embodiment an NMOS device Mx is connected with the source at node C, the drain is connected to a complementary power supply such as ground or VSS and the gate is coupled to node B through an inverter  502 . 
   In this embodiment, if the bandgap circuit does not start, Vb could be floating near Vss. This could be seen as a logical zero on the input of inverter  502  causing a logical 1 at the output of the inverter  502 . The logical 1 output of the inverter  502  drives the NMOS device Mx into conduction, thereby lowering the voltage at the gates of the controlled current sources P 1  and P 2 . The devices P 1  and P 2  will then supply current through nodes A and B. Once current passes through either node A or B, the voltages at nodes A and B are established and the bandgap circuit is brought out of the non-started state and begins normal operation. Once in normal operation, Vb will be seen as a logical 1 at the input of inverter  502  causing a logical 0 at the output of inverter  502  thus shutting off NMOS device Mx. 
   These embodiments show one of the advantages of the current invention. Once the bandgap circuit is operating properly, device Mx does not affect the normal operation of the bandgap reference. Other advantages include its simplicity, that it does not draw any current from the output stage of the bandgap circuit and that it only operates if the bandgap circuit fails to start properly. 
   The above illustration provides many different embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.