Patent Abstract:
Systems and methods to achieve a startup circuit of bandgap voltage reference generator circuits monitoring a current flow in the bandgap voltage reference generator circuit have been achieved. The startup circuit can operate at supply voltages of about one threshold voltage and is therefore appropriate for low voltage applications. The monitoring of a current through an electrical component inside the bandgap voltage reference generator circuit by replication the component branch in a scaled version saves power and does not disturb the normal operation of the current-mode bandgap voltage reference generator. The startup circuit invented can be applied for current-mode bandgap voltage reference generator circuits as well as for voltage-mode bandgap voltage reference generator circuits.

Full Description:
BACKGROUND OF THE INVENTION 
       [0001]    (1) Field of the Invention 
         [0002]    This invention relates generally to bandgap voltage reference generators and relates more specifically to a startup circuits for bandgap voltage reference generators. 
         [0003]    (2) Description of the Prior Art 
         [0004]    Typical bandgap voltage reference generator circuits are self-biased and have two stable bias-points, a desired operational bias point for which the circuit has been designed for and an undesired bias-point where the bandgap output voltage reference remains at 0 V. A dedicated startup circuit is required to ensure that the bandgap voltage reference generator is brought to an operating range where it can obtain the desired bias-point. 
         [0005]    Current practice is to monitor a voltage inside the bandgap voltage reference generator and to initiate a current flow into the bandgap reference voltage generator circuit as long as the voltage monitored does not have the operation condition required. The disadvantage of monitoring a voltage is that the voltage monitoring is performed by a voltage comparator, which requires a specific voltage for its operation and a reference voltage. 
         [0006]    It is a challenge for the designers of startup circuits for bandgap voltage reference generators to design a circuit wherein the startup condition is process independent because a threshold voltage of NMOS or PMOS devices is used as reference and wherein no reference voltage is required for the startup. 
         [0007]    There are known patents or patent publications dealing with startup circuits for bandgap voltage reference generators. 
         [0008]    (U.S. Pat. No. 6,972,550 to Hong proposes a bandgap voltage reference generator including a bandgap voltage reference circuit and a fast startup circuit. The fast start-up circuit, which is cost-efficient and saves power consumption, can rapidly start up the bandgap reference voltage circuit coupled thereto. 
         [0009]    U.S. Patent Publication (US 2007/0241735 to Rabeyrin et al.) discloses a power efficient startup circuit for activating a bandgap reference circuit. The startup circuit uses a voltage supply having a voltage level to initiate the flow of a startup current used to activate the bandgap reference circuit. 
         [0010]    U.S. Patent Publication (US 2008/0018317 to Chen et al.) describes a bandgap reference circuit having a low sensitivity to temperature and supplied voltage installs a compensation circuit on a bandgap reference circuit to substitute a prior art that uses a resistor to match the circuit startup purpose and solve the problem of startup error caused by the manufacturing error. 
         [0011]    Furthermore there are two more patents dealing with startup circuits for bandgap voltage reference generators: 
         [0012]    (U.S. Pat. No. 6,784,652 to Aude) describes a startup circuit for a bandgap voltage reference generator circuit. Monitoring an internal reference voltage of the bandgap voltage reference generator circuit, current flow for the bandgap circuit diodes is initiated following circuit startup, e.g., initial application or DC power 
         [0013]    (U.S. Pat. No. 6,784,652 to Yu) discloses a circuit including a band-gap reference circuit and a start-up circuit coupled between an output and an input of the band-gap reference circuit. When the output of the band-gap reference circuit is below a start-up voltage threshold, the start-up circuit provides a first voltage at the input of the band-gap reference circuit, which, in turn, causes the band-gap reference circuit to produce a desired voltage at the output. 
       SUMMARY OF THE INVENTION 
       [0014]    A principal object of the present invention is to achieve methods and systems for a startup circuit of a bandgap voltage reference generator that monitors a current flow through a bandgap diode and therefore ensures that the bandgap voltage reference generator is in the correct operating range. 
         [0015]    A further object of the invention is to achieve a startup circuit for a bandgap voltage reference generator, which does not require a reference voltage. 
         [0016]    A further object of the invention is to achieve a startup circuit for a bandgap voltage reference generator that operates at supply voltages of about a transistor threshold voltage and is therefore appropriate for low voltages applications 
         [0017]    A further object of the invention is to achieve a startup circuit for a bandgap voltage reference generator wherein the behavior of the startup circuit is controlled by a current source. 
         [0018]    A further object of the invention is to achieve a startup circuit for a bandgap voltage reference generator, wherein the current through an electrical component of the bandgap voltage reference generator is monitored by replicating the component branch in a scaled version to save power without disturbing the normal operation of the bandgap voltage reference generator. 
         [0019]    Moreover another object of the invention is to achieve a bandgap voltage reference generator having a startup condition being independent of a semiconductor process used with the components. 
         [0020]    In accordance with the objects of this invention a method for startup circuits of bandgap voltage reference generator circuits monitoring a current flow in the bandgap voltage reference generator circuit has been achieved. The method invented comprises, first, the following steps of: (1) providing a current-mode bandgap voltage reference generator circuit and a startup circuit comprising a current detection unit, (2) monitoring a current flow in the bandgap voltage reference generator circuits by the current detection circuit, and (3) injecting a startup current to the bandgap voltage reference generator circuit by the startup circuit as long as said current monitored is below a limit defined. Furthermore the method comprises the steps of (4) disengage functionally the startup circuit after said current monitored has reached said limit defined and the bandgap voltage reference generator circuit has reached its operating range, and (5) regulating, right after the bandgap voltage reference generator circuit has reached its operating range, an internal voltage of the bandgap voltage reference generator in order to reach the bias point desired of the bandgap voltage reference generator. 
         [0021]    In accordance with the objects of this invention a startup circuit of bandgap voltage reference generator circuits monitoring a current flow in the bandgap voltage reference generator circuit has been disclosed. The startup circuit invented comprises, firstly, a first circuit branch, comprising a first transistor and a first diode, which has been replicated from a branch of the bandgap voltage reference generator circuit, wherein the current through said branch of the bandgap voltage reference generator circuit is mirrored to said first branch, and wherein said first branch is connected between Vdd voltage and ground, a current detection unit having inputs and an output, wherein a first input is a current through said first diode, a second input is a reference current, and the output is a voltage to a gate of second transistor initiating a startup current to the bandgap voltage reference generator circuit if said current of the first input is smaller than said reference current, and a second circuit branch comprising a current source, said second transistor and a current mirror configuration mirroring the current of the second circuit branch to a third circuit branch, wherein said second branch is connected between Vdd voltage and ground. Furthermore the startup circuit invented comprises said third branch comprising a transistor of said current mirror configuration connecting the second and the third branch wherein said third branch is connected between a gate of said first transistor and ground. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    In the accompanying drawings forming a material part of this description, there is shown: 
           [0023]      FIG. 1  illustrates a principal schematic of a current-mode bandgap voltage reference generator driven by startup circuit of the present invention. 
           [0024]      FIG. 2  shows principal voltage characteristics of the interaction between a bandgap voltage reference generator circuit and a startup circuit invented. 
           [0025]      FIG. 3  shows a principal schema of a current-mode bandgap voltage reference generator driven by a startup circuit of the present invention including a more detailed implementation of the startup current detection. 
           [0026]      FIG. 4  illustrates a flowchart of a method invented for startup circuits of bandgap voltage reference generator circuits monitoring a current flow in the bandgap voltage reference generator circuit. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Circuits and methods for a startup bandgap voltage reference generator are disclosed. Instead of monitoring a voltage as usually performed in prior art, the startup circuit of the present invention monitors a current flow in the bandgap voltage reference generator circuit. The startup circuit operates using supply voltages in the order of about a threshold voltage of a PMOS or NMOS device and is therefore appropriate for low voltage applications. 
         [0028]    In addition the need for a voltage reference is removed. The advantage of this is that the startup condition is semiconductor process independent because in prior art often the threshold voltage of MOS devices is used as reference voltage. 
         [0029]      FIG. 1  illustrates a principal schematic of a current-mode bandgap voltage reference generator driven by startup circuit of the present invention. The circuit of  FIG. 1  has two parts, a startup circuit  1  invented and a current-mode bandgap voltage reference generator  2 . It should be noted that the invention could be applied to voltage-mode bandgap voltage reference generators as well. 
         [0030]    As it is well known, a bandgap voltage reference generator circuit provides an output voltage with a low sensitivity to temperature with the help of diodes. In a conventional bandgap voltage reference generator, the voltage is obtained as a sum of a voltage complementary to absolute temperature (CTAT) and a voltage, which is proportional to absolute temperature (PTAT). This yields to a reference voltage, which is approximately 1.2 V. Thus, the supply voltage must be somewhat higher than 1.2 V. For lower supply voltages a current-mode bandgap voltage reference generator can be used which sums up a CTAT and PTAT current rather than voltages and forces this current through a resistor. 
         [0031]    The operational amplifier OP 1  in the bandgap voltage reference generator  2  shown in  FIG. 1  controls the currents through transistors P 1  and P 2  and hence the node voltages Vn and Vp. 
         [0032]      FIG. 2  shows the principal voltage characteristics of V n , V p  and the resulting differential input voltage V in =V p −V n  of operational amplifier OP 1  versus the current of transistors P 1  and P 2 . The differential input voltage Vin of OP 1  remains low as long as the voltage drops on the resistors R 1  and R 2  are below the forward voltage of diodes D 1  and D 2 . This could cause the bandgap voltage reference generator circuit  2  to stay in a wrong stable bias point of OP 1  has not enough gain or if OP 1  has an offset voltage higher than V in . The output port of the bandgap voltage reference generator circuit is signified by V out . 
         [0033]    If the diode forward voltage has been reached significant current is flowing into the diodes D 1  and D 2  producing an input voltage difference V in  for OP 1  because of their different size, which is allowing OP 1  to bring up the bandgap voltage reference generator circuit to its correct bias-point. Therefore the basic idea of the startup circuit  1  is to monitor the current through the diode and to inject a startup current I start  to the bandgap voltage reference generator  2  as long as the diode current is below a certain limit. This always brings the bandgap voltage reference generator  2  into operation with a sufficiently high input voltage V in  and the bandgap voltage reference generator  2  circuit will reach its bias-point desired due to the regulation of OP 1 . 
         [0034]    The monitoring of the diode current by the startup circuit is performed by using a scaled matching replication of the diode/resistor/transistor structure of diode D 1 , resistor R 1 , and transistor P 1  of the bandgap voltage reference generator  2 . This diode/resistor/transistor structure has been replicated in the startup circuit  1  to a matched diode/resistor/transistor structure of diode D 3 , resistor R 4 , and transistor P 5 . 
         [0035]    It should be noted that another branch of the bandgap voltage reference generator, such as e.g. the branch comprising transistor P 2 , diode D 2 , and resistor R 3  could be replicated alternatively as well. In the preferred embodiment the P 1 , D 1 , and R 1  branch has been selected for replication because diode D 1  is smaller than diode D 2  and therefore current will flow through D 2  before flowing through diode D 1 . For startup purposes it is thus better to use diode D 1  for replication because it represents a “harder” condition. 
         [0036]    The current detection circuit  3  detects if the current I 3  is lower than current I off , which is another input to the current detection circuit  3 . As long as the current I 3  is lower than the current I off , the current detection circuit  3  turns on transistor P 4  and enables the current I start  to pull down the voltage at node V op . Therefore transistors P 1 , P 2 , and P 5  are on during the startup period and initiate a current flow. At first the current through transistor P 5  is low and while voltage V 3  is lower than the forward voltage of diode D 3 , most of the current through transistor P 5  is flowing to the resistor R 4 . The current I 3  is then still lower than current I off  and transistor P 4  remains on and this causes current I start  to pull the node voltage V op  further down, forcing more current through transistor P 5 . 
         [0037]    When the voltage drop on resistor R 4  reaches the diode forward voltage of diode D 3 , the current I 3  increases and reaches current I off . The current detection circuit  3  turns transistor P 4  off, stopping to pull the voltage V op  further down. Now, the startup circuit  1  has no influence on the bandgap voltage reference generator  2  anymore, i.e. the startup circuit is functionally disengaged. At this moment the bandgap voltage reference generator  2  is in the operating range (as shown in  FIG. 2 ), because current is flowing through diodes D 1  and D 2  as well now. The operational amplifier OP 1  regulates the voltage V op  further down until the bandgap voltage reference generator  2  has reached its bias point desired. 
         [0038]    An implementation of the current detection circuit is illustrated in  FIG. 3 .  FIG. 3  shows a principal schematic of a current-mode bandgap voltage reference generator driven by a startup circuit of the present invention including a more detailed implementation of the startup current detection. A preferred embodiment of the current detection circuit implementation  3  is shown in  FIG. 3  inside of the dotted line in the startup circuit  1 . 
         [0039]    First, when the startup of the bandgap voltage reference generator  2  begins, no current is flowing through transistors P 2 , P 1 , and P 5 , and the current I 3  through diode D 3  is zero. A current I in  is mirrored by transistors P 7 , P 8 , and P 9 . Transistor P 7  is sourcing transistor N 3 , which means that transistors N 4  and N 5  are conductive. 
         [0040]    The current capability of N 5  is much higher than the current capability of transistor N 4 , i.e. transistor N 5  can conduct a much higher current than transistor N 4 . Transistors N 4  and N 5  are able to sink more current than transistor P 8  can provide, hence pulling the gate of transistor P 4  to ground. In turn, this allows transistor N 2  to pull down the node voltage V op , hence initiating a current flow through transistors P 2 , P 1 , P 3  and P 5 , thus injecting a startup current to the bandgap voltage reference generator  2 . Other circuit designs than the preferred circuit to inject a startup current using a current source described above are possible as well. 
         [0041]    Controlling the startup of the bandgap voltage reference generator behavior by a current source as I in  shown in  FIG. 3  avoids an overshoot of the output voltage of the bandgap voltage reference generator. 
         [0042]    As long as current I 5  is higher than the current capability of transistor P 8 , the gate of transistor P 4  is held at ground allowing the current through transistor P 5  to increase during startup. The current I 3  increases after the node voltage V 3  reaches the diode forward voltage of diode D 3 . Therefore the source voltage of transistor N 5  increases, hence switching off transistor N 5 . The gate of transistor P 4  is pulled up by transistor P 8  that turns transistor N 2  off. This means that no startup current is flowing to the bandgap voltage reference generator anymore. Now, the startup circuit  1  has no influence on the bandgap voltage reference generator  2 , i.e. the startup circuit is functionally disengaged. Other circuit designs than the preferred circuit to compare a current in the bandgap voltage reference generator with a reference current and to stop the injection of a startup current if the current in the bandgap voltage reference generator has reached a current limit defined described above are possible as well. 
         [0043]    The current detection circuit disclosed can operate well below 1 V supply voltage and is therefore suitable for low voltage applications. The current I off  for the startup condition can be set well below the diode bias-current, which means that the bandgap voltage reference generator has a wide operating range. The startup circuit  1  operates as well for conventional bandgap voltage reference generators without resistors in parallel to the diodes. For those circuits resistor R 4  in the startup circuit  1  can be removed as well. 
         [0044]    The startup behavior of the bandgap reference circuit can be controlled by the current I start , which is set by the current source transistor N 2 . 
         [0045]    It should be noted again that the startup circuit of the present invention could be used with voltage-mode bandgap reference circuits as well, if the resistors R 1 , R 2 , and replicated resistor R 4  as shown in  FIG. 4  are omitted. 
         [0046]      FIG. 4  illustrates a flowchart of a method invented for startup circuits of bandgap voltage reference generator circuits monitoring a current flow in the bandgap voltage reference generator circuit. A first step  40  describes the provision of a current-mode bandgap voltage reference generator circuit and a startup circuit comprising a current detection unit. A next step  41  illustrates monitoring a current flow in the bandgap voltage reference generator circuits by the current detection circuit. Step  42  describes injecting a startup current to the bandgap voltage reference generator circuit by the startup circuit as long as said current monitored is below a limit defined. Step  43  discloses the functionally disengagement of the startup circuit after said current monitored has reached the limit defined and the bandgap voltage reference generator circuit has reached its operating range. 
         [0047]    Step  44  illustrates regulating, right after the bandgap voltage reference generator circuit has reached its operating range, an internal voltage of the bandgap voltage reference generator in order to reach the bias point desired of the bandgap voltage reference generator. 
         [0048]    In summary, key items of the present invention are: 
         [0000]    1. Instead of monitoring a voltage, the startup circuit invented monitors a current flow in the bandgap voltage reference generator circuit. The invention is not using a MOS threshold to accomplish a startup function.
 
2. Controlling the behavior of the startup circuit by a current source avoids an overshoot of the output voltage.
 
3. The invention allows a low voltage operation of the startup circuit.
 
4. A process independent startup condition has been achieved because no voltage reference is required.
 
5. The bandgap generator and the startup circuit invented can operate with supply voltages in the 1 V range.
 
         [0049]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

Technology Classification (CPC): 6