Abstract:
A circuit includes a current generator, a start-up circuit coupled to provide a start-up current to the current generator during a start-up phase of the current generator, and a cut-off circuit coupled to both the current generator and to the start-up circuit to provide a control signal that reduces the start-up current when an output current from the current generator exceeds a threshold value.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to electronic circuits, and more particularly, to a start-up circuit for a current generator.  
         [0003]     2. Description of the Related Art  
         [0004]     Current generators using internal feedback often require some type of start-up circuit to get the current generator started. Start-up circuits are needed because most such current generators have two stable states: one of them being the operating state at which the desired amount of current flows, and the other being a zero-current or off state. When power is first applied to a current generator, it is sometimes necessary to provide a separate input current to move them from the off state towards the correct current flow state. Start-up circuits typically supply a small amount of start-up current to the current generator in order to eliminate the zero-current state so that the current generator can get started and stabilize at the desired operating state.  
         [0005]     Typical start-up circuits, however, continue to supply the start-up current to the current generator even after the desired operating state has been achieved. The presence of the start-up current after the current generator has stabilized to the desired operating state can, in many situations, have a detrimental effect on the current generator&#39;s performance. This is because the start-up current is now an unwanted element that unnecessarily influences the stable operation of the current generator, and can cause a significant change or variation in the generated currents. This is especially true when the current generator is designed to operate at low current.  
         [0006]      FIG. 1  is a circuit diagram of a prior art start-up circuit  10  for a current generator  12 . Start-up circuit  10  is coupled to appropriate voltage supply sources Vs and Vss, for example 1.8 volts and ground, respectively, and includes transistors M 1 , M 2  and D 1 -D 3 . Transistors D 1 -D 3  are each diode-connected n-channel MOSFETs having a drain, a source and a gate, and each having its gate connected to its drain. Transistor D 1  has its source connected to voltage source Vss, transistor D 2  has its source connected to the drain of transistor D 1 , and transistor D 3  has its source connected to the drain of transistor D 2  and its drain connected to node  325 , thus forming a series string of diode-connected MOSFETs having an equivalent resistance from node  325  to Vss.  
         [0007]     Transistors M 1  and M 2  are each p-channel MOSFETs having a source, a drain and a gate. Transistor M 2  has its source connected to voltage source Vs, and has its drain connected to node  325 , which is also the drain of transistor D 3 . Transistor M 1  has its source connected to voltage source Vs, and has its gate connected to node  325 . The drain of transistor M 1  is coupled to an input node  13  of the current generator to provide the start-up current to current generator  12 .  
         [0008]     Current generator  12  includes transistors Q 1  and Q 2 , resistors R 2  and R 3 , and a current mirror consisting of transistors M 3  and M 4 . Transistors M 3  and M 4  are each p-channel MOSFETs having a source, a drain and a gate. Transistor M 4  has its drain connected to its gate, and its gate connected to the gate of transistor M 3  forming node  436 . The sources of transistors M 3  and M 4  are connected to voltage source Vs. The gate of transistor M 2  is coupled to node  436 .  
         [0009]     Transistors Q 1  and Q 2  are each npn bipolar junction transistors having a collector, an emitter and a base, where transistors Q 2  and Q 1  have a size ratio difference of a desired value, for example, 6:1. Transistor Q 1  has its emitter connected to voltage source Vss, and its base connected to the drain of transistor M 3 . Resistors R 2  and R 3  are connected in series between the drain of transistor M 3  and the collector of transistor Q 1 . Transistor Q 2  has its emitter connected to voltage node Vss, its base connected to the collector of transistor Q 1 , and its collector connected to the drain of transistor M 4 . The base of transistor Q 2  is connected to the drain of transistor M 1  so that the start-up current from start-up circuit  10  is received at the base of transistor Q 2 .  
         [0010]     It is assumed that the voltage at voltage source Vs is initially 0 volts, resulting in no current flowing in the circuit. When the circuit is first powered up and the voltage level rises from zero volts toward a stable Vs, transistors M 1 -M 4  will be turned on, and transistors D 1 -D 3 , Q 1  and Q 2  remain off for a short time. A start-up current Is is provided through transistor M 1  to node  13  to start operation of the current generator  12 . As the voltage at voltage source Vs continues to increase to 1.8 volts, for example, the voltage across diode-connected transistors D 1 -D 3  also increases. When the voltage at node  325  is high enough to turn on transistors D 1 -D 3 , current flows through transistors D 1 -D 3 . Current continues to flow through transistor M 1 , which provides the start-up current Is to current generator  12 . The amount of start-up current provided by transistor M 1  is controlled by the voltage at node  325 , which is determined by the equivalent resistance across diode-connected transistors D 1 -D 3  as compared to M 2 .  
         [0011]     Upon receiving the start-up current Is from transistor M 1 , transistor Q 2  turns on and starts operation of the current generator. The current generator quickly reaches its designed operating state, producing the present output current Io through line  14 . The start-up current Is continues to be provided via transistor M 1  at a value determined by the combination of the voltage at node  436  and node  325  under the control of transistor M 2  and diodes D 1 -D 3 .  
         [0012]     The start-up current Is, even though it is small, continues to affect operation of the current generator  12 . Any noise present on voltage source Vs will affect the amount of current supplied to node  13 , thus causing a variation in the output current Io on line  14 . The goal of a current generator is to provide a stable, constant current value even if the power supply voltage fluctuates or has noise on the line. The continued application of some value of current to node  13  from the start-up circuit causes unwanted fluctuations and noise in the output current. This has an even greater detrimental effect in very low voltage and low current circuits.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     An embodiment of the present invention provides a circuit comprising: a current generator, a start-up circuit coupled to provide a start-up current to the current generator during a start-up phase of the current generator, and a cut-off circuit coupled to both the current generator and to the start-up circuit to provide a control signal that reduces the start-up current when an output current from the current generator exceeds a threshold value.  
         [0014]     Another embodiment of the present invention provides a circuit comprising: current generating means for generating an output current, start-up means for providing a start-up current to the current generating means during a start-up phase, and cut-off means for reducing the start-up current when the output current exceeds a threshold value.  
         [0015]     Another embodiment of the present invention provides a method of starting a current generator, comprising: providing a start-up current to the current generator during a start-up phase of the current generator, receiving a feedback signal from the current generator as a function of an output current of the current generator, and reducing the start-up current in response to the feedback signal.  
         [0016]     Another embodiment of the present invention provides a start-up circuit for a current generator, comprising: first and second power supply nodes for connection to an electrical power supply, a feedback node for receiving a feedback signal from the current generator, an output node for applying a start-up current to the current generator, a first transistor connected to the feedback node for drawing a first current, a second transistor connected to the first transistor for drawing a second current, a current mirror connected to the first and second transistors for regulating the first and second currents and providing a control signal, and a third transistor connected to the current mirror and the output node for drawing the start-up current in response to the control signal. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0017]      FIG. 1  is a circuit diagram of a prior art start-up circuit.  
         [0018]      FIG. 2  is a circuit diagram of a first embodiment of a start-up circuit according to the present invention.  
         [0019]      FIG. 3  is a circuit diagram of a second embodiment of a start-up circuit according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]      FIG. 2  is a circuit diagram of a first embodiment of a circuit  20  according to the present invention. The circuit  20  includes a current generator  12  similar to that in  FIG. 1  and also a cut-off circuit  22  and a start-up circuit  24 . Circuit  20  is coupled to appropriate voltage sources Vs and Vss, for example 1.8 volts and ground, respectively, and includes resistors R 2 -R 7 , transistors M 3 -M 7 , a current mirror consisting of transistors Q 1  and Q 2 , and a current mirror consisting of transistors Q 3  and Q 4 . Transistors Q 3  and Q 4  are each npn bipolar junction transistors having a collector, an emitter and a base, where transistors Q 4  and Q 3  have a selected size ratio, for example 2:1, 3:1 or some other value. Transistor Q 4  has its collector connected to its base, and its base connected to the base of transistor Q 3 . Resistors R 4  and R 5  are connected in series between the emitter of transistor Q 3  and voltage source Vss, and resistors R 6  and R 7  are connected in series between the emitter of transistor Q 4  and voltage source Vss. Resistors R 4  and R 6  have a selected resistance ratio, for example 2:1, 3:1 or some other value. Resistors R 5  and R 7  have a similar resistance ratio.  
         [0021]     Transistors M 5 -M 7  are each p-channel MOSFETs having a source, a drain and a gate. Transistor M 7  has its source connected to voltage source Vs, its drain connected to the collector of transistor Q 4 , and its gate connected to node  436 . Transistor M 6  has its source connected to voltage source Vs, its gate connected to the gate of transistor M 7 , and its drain connected to node  433 , which is also the collector of transistor Q 3 . Transistor M 5  has its source connected to voltage source Vs, and its gate connected to node  433 . The drain of transistor M 5  is coupled to input node  25  of the current generator to provide the start-up current to current generator  12 .  
         [0022]     It is assumed that the voltage at voltage source Vs is initially 0 volts, resulting in no current flowing in the circuit. When the circuit is first powered up and the voltage level rises from zero volts toward a stable Vs, transistors M 3 -M 7  will be turned on, and transistors Q 1 -Q 4  remain off for a short time. A start-up current Is is provided through transistor M 5  to node  25  to start operation of the current generator  12 . If necessary, to turn on transistors Q 3  and Q 4  and start operation of cut-off circuit  22 , current can be injected into the base of transistor Q 4 . For example, cross-coupled NAND gates can be used to provide a one-shot into the base of transistor Q 4 .  
         [0023]     Upon receiving the start-up current Is from transistor M 5 , transistor Q 2  turns on and starts operation of the current generator. The current generator quickly reaches its designed operating state, producing the present output current Io through line  26 . Because transistor M 4  is turned on and connected as a diode, the voltage at node  436  is held at a diode-drop below voltage source Vs. The gates of transistors M 6  and M 7  are connected to node  436 , and as a result, the voltage at node  436  ensures that transistors M 6  and M 7  remain on. In this way, node  436  provides a feedback signal to cut-off circuit  22 .  
         [0024]     The current mirror consisting of transistors Q 3  and Q 4  controls the current flow through transistors M 6  and M 7 . In this particular example, transistors Q 4  and Q 3  have a size ratio of 2:1. As a result, transistor Q 4  will draw twice as much current as transistor Q 3 . All that remains to determine the voltage at node  433  is the resistance values of resistors R 4 -R 7 . In this particular example, the resistance of resistors R 4  and R 5  is twice the resistance of resistors R 6  and R 7 . This creates a high voltage at node  433  that approaches voltage source Vs. Because the gate of transistor M 5  is connected to node  433 , the voltage at node  433  controls the start-up current Is drawn by transistor M 5 . In this way, node  433  provides a control signal to start-up circuit  24 .  
         [0025]     As the voltage at node  433  approaches voltage source Vs, the voltage between the source and the gate of transistor M 5  becomes less than the threshold voltage of the p-channel MOSFET, thus turning off transistor M 5 . As a result, the start-up current Is drawn by transistor M 5  is reduced to approximately zero. Therefore, the start-up current Is will no longer affect the operation of current generator  12 .  
         [0026]      FIG. 3  is a circuit diagram of a second embodiment of a circuit  30  according to the present invention. Circuit  30  is a high voltage version of circuit  20  and is coupled to an input voltage of approximately 20 to 100 volts, preferably 60 volts. Circuit  30  operates on the same basic principles as circuit  20 , except circuit  30  utilizes pnp bipolar junction transistors and n-channel MOSFETs.  
         [0027]     Circuit  30  includes a current generator  32 , a cut-off circuit  33  and a start-up circuit  34 . Circuit  30  is coupled to appropriate voltage sources Vs and Vss, for example 60 volts and ground, respectively, and includes resistors R 8 -R 15 , transistors M 8 -M 12 , diode D 4 , a current mirror consisting of transistors Q 5  and Q 6 , and a current mirror consisting of transistors Q 7  and Q 8 . Transistors Q 5  and Q 6  are each pnp bipolar junction transistors having an emitter, a collector and a base, where transistors Q 6  and Q 5  have a selected size ratio, for example 2:1, 3:1 or some other value. Transistor Q 6  has its collector connected to its base, and its base connected to the base of transistor Q 5 . Resistors R 10  and R 11  are connected in series between the emitter of transistor Q 6  and voltage source Vs, and resistors R 8  and R 9  are connected in series between the emitter of transistor Q 5  and voltage source Vs. Resistors R 8  and R 10  have a selected resistance ratio, for example 2:1, 3:1 or some other value. Resistors R 9  and R 11  have a similar resistance ratio.  
         [0028]     Transistors M 8 -M 12  are each n-channel MOSFETs having a drain, a source and a gate. Transistor M 10  has its source connected to voltage source Vss, its drain connected to the collector of transistor Q 6 , and its gate connected to the gate of transistor M 9 . Transistor M 9  has its source connected to voltage source Vss, and its drain connected to node  541 , which is also the collector of transistor Q 5 . Transistor M 8  has its source connected voltage source Vss, and its gate connected to node  541 . Diode D 1  has its anode connected to voltage source Vss, and its cathode connected to the gate of transistor M 8 . The drain of transistor M 8  is coupled to an input node  35  of the current generator to draw the start-up current from current generator  32 .  
         [0029]     Current generator  32  includes transistors Q 7  and Q 8 , resistors R 12 -R 15 , and a current mirror consisting of transistors M 11  and M 12 . Transistor M 12  has its drain connected to its gate, and its gate connected to the gate of transistor M 11  forming node  547 . The sources of transistors M 11  and M 12  are connected to voltage source Vss. The gate of transistor M 10  is coupled to node  547 .  
         [0030]     Transistors Q 8  and Q 7  have a size ratio difference of a desired value, for example, 6:1. Transistor Q 7  has its base connected to the drain of transistor M 11 . Resistor R 12  is connected between the emitter of transistor Q 7  and voltage source Vs, and resistors R 14  and R 15  are connected in series between the collector of transistor Q 7  and the drain of transistor M 11 . Transistor Q 8  has its collector connected to the drain of transistor M 12 , and its base connected to the collector of transistor Q 7 . Resistor R 13  is connected between the emitter of transistor Q 8  and voltage source Vs. The base of transistor Q 8  is connected to the drain of transistor M 8  so that the start-up current is drawn by start-up circuit  34  from the base of transistor Q 8 .  
         [0031]     It is assumed that the voltage at voltage source Vs is initially 0 volts, resulting in no current flowing in the circuit. When the circuit is first powered up and the voltage level rises from zero volts toward a stable Vs, transistors M 8 -M 12  will be turned on, and transistors Q 5 -Q 8  remain off for a short time. A start-up current Is is drawn by transistor M 8  from node  35  to start operation of the current generator  32 . If necessary, to turn on transistors Q 5  and Q 6  and start operation of cut-off circuit  33 , current can be drawn from the base of transistor Q 6 .  
         [0032]     Upon the start-up current Is being drawn by transistor M 8 , transistor Q 8  turns on and starts operation of the current generator. The current generator quickly reaches its designed operating state, producing the present output current Io through line  36 . Because transistor M 12  is turned on and connected as a diode, the voltage at node  547  is held at a diode-drop above voltage source Vs. The gates of transistors M 9  and M 10  are connected to node  547 , and as a result, the voltage at node  547  ensures that transistors M 9  and M 11  remain on. In this way, node  547  provides a feedback signal to cut-off circuit  33 .  
         [0033]     The current mirror consisting of transistors Q 5  and Q 6  controls the current flow through transistors M 9  and M 1 . In this particular example, transistors Q 6  and Q 5  have a size ratio of 2:1. As a result, transistor Q 6  will draw twice as much current as transistor Q 5 . All that remains to determine the voltage at node  541  is the resistance values of resistors R 8 -R 11 . In this particular example, the resistance of resistors R 8  and R 9  is twice the resistance of resistors R 10  and R 11 . This creates a low voltage at node  541  that approaches voltage source Vss. Because the gate of transistor MB is connected to node  541 , the voltage at node  541  controls the start-up current Is drawn by transistor M 8 . In this way, node  541  provides a control signal to start-up circuit  34 .  
         [0034]     As the voltage at node  541  approaches voltage source Vss, the voltage between the gate and the source of transistor M 8  becomes less than the threshold voltage of the n-channel MOSFET, thus turning off transistor M 8 . As a result, the start-up current Is drawn by transistor M 8  is reduced to approximately zero. Therefore, the start-up current Is will no longer affect the operation of current generator  32 .  
         [0035]     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0036]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.