Abstract:
A start-up circuit for supplying current to an analog circuit. The start-up circuit comprises a capacitor connected to a current mirror. A power-up signal input to the start-up circuit causes the capacitor to discharge to the current mirror thereby causing the current mirror to provide a current to the analog circuit.

Description:
FIELD OF THE INVENTION 
     This invention relates to a start-up circuit particularly useful in conjunction with a low power analog circuit. 
     BACKGROUND OF THE INVENTION 
     Low power systems having relatively small currents flowing therein, typically use sleep states in which circuit portions are powered down when not needed to conserve battery charge. Charging capacitors in the circuit up to operating voltage using these small currents requires long periods of time. To overcome the problem of slow power-up, a node to be charged may be brought to power supply voltage through a sufficiently large transistor for an amount of time dictated by a clock. Upon expiration of the appropriate time period, the charging is ceased and the circuit is allowed to settle back to the operating level. The disadvantage of pulling the node to a supply voltage to power-up a circuit is that it has to settle down afterwards. This may take considerable time if the currents available are relatively small. A further disadvantage is that the circuit requires a clock adding additional circuitry and hence consuming additional power. 
     Another method known to increase power-up speed includes using a kick-start circuit to pump current into a circuit to be powered up. The kick-start circuit provides current to transistors in the circuit being powered up. When the transistors are charged sufficiently, a transistor that produces a logic signal is turned on. The signal then turns the kick-start circuit off, leaving the attached circuitry in a powered-up state. 
     The disadvantage of a kick-start circuit is that charge pumped into the circuit to be powered up is not related to the amount of charge required to charge the capacitor in the kick-start circuit. Therefore, the kick-start circuit may overshoot the desirable level of charge, and hence, a period of settling down may be necessary. 
     FIG. 1 depicts a known start-up circuit  100  used in conjunction with a voltage reference circuit  102 . Start-up circuit  100  is shown by dotted lines. Voltage reference circuit  102  has two possible equilibrium points, one of which corresponds to zero voltage and zero current, and a second, non-zero equilibrium point, which corresponds to a useful reference voltage. Therefore, voltage reference circuit  102  must be designed to choose only the non-zero equilibrium point to establish the reference voltage. Start-up circuit  100  is provided to allow voltage reference circuit  102  to utilize only the desired equilibrium point. If voltage reference circuit  102  is at the undesired equilibrium point, the voltage is zero and therefore, I 1  and I 2  are zero. Consequently, transistor  104  provides current in transistor  106  which then moves voltage reference circuit  102  to the non-zero equilibrium point. Transistor  104 &#39;s source voltage increases as the desired equilibrium point is approached. This causes the current through transistor  104  to decrease. When voltage reference circuit  102  reaches the non-zero equilibrium point, the current through transistor  106  will be substantially the same as the current through transistor  108 . Transistor  110  and resistor  112  set the gate bias voltage for transistor  104 . Voltage reference circuit  102  is on within a gate bias voltage window. Therefore, the gate bias voltage must be high enough to turn voltage reference circuit  102  on but must not exceed the upper limit of the voltage window. 
     FIG. 2 depicts a kick-start circuit. When current flows in the transistors of the main part of the circuit or band gap reference, the kick-start circuit is turned off. This occurs because MP 4  mirrors the current into MN 6  which drives the gate of MN 3  high and pulls down the drain node of MN 3 . Driving this node low turns off the current mirrors in the kick-start circuit, so it stops sourcing and sinking current to the band gap reference circuit. R 3  ensures that current flows in the kick-start circuit when the band gap reference circuit is powered down. 
     Conventional circuits do not provide the accuracy and speed desirable to power-up low power systems. Accordingly, there is a need for a start-up circuit that provides a targeted current quickly without significantly overshooting or falling short of the targeted value. 
     SUMMARY OF THE INVENTION 
     A start-up circuit is disclosed for supplying current to an analog circuit. The start-up circuit provides current to an analog circuit quickly and accurately. The start-up circuit comprises a capacitor connected to a current mirror. Upon a power-up signal input to the start-up circuit the capacitor discharges through the reference transistor of the current mirror. The capacitor discharge causes the current mirror to provide a current to the analog circuit. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a prior art start-up circuit. 
     FIG. 2 depicts another prior art start-up circuit. 
     FIG. 3 depicts one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention provide a start-up circuit that powers-up an analog circuit more quickly and accurately than conventional methods. The start-up circuit includes a capacitor, preferably in the form of a transistor, one plate of which is connected to a positive terminal of a power supply, the other to a negative terminal of the power supply. The capacitor begins charging to the power supply voltage upon input of a power-down signal to the start-up circuit. When the power-down signal is withdrawn, the capacitor is discharged through a diodeconnected transistor. The diode-connected transistor forms the reference half of a current mirror. Current mirrored in a second transistor is used to charge one or more internal nodes of the analog circuit being powered up. The current mirror produces a high current relative to that which is input to the current mirror. The current output from the current mirror trails off to zero, thus charging internal nodes quickly, generally without long-term current drain. 
     In one embodiment of the start-up circuit a means for receiving a power-down signal is provided. The receiving means charges to a power supply voltage and discharges to a means for providing a reference current. The current of the reference means is mirrored by a current mirroring means. 
     The current mirroring means then provides current to charge one or more nodes of the analog circuit. 
     The receiving means is preferably a transistor and the current mirror reference means is preferably a diode-connected transistor. 
     FIG. 3 depicts one embodiment of start-up circuit  300  for providing current to analog circuit  302  in response to a power-down signal. Start-up circuit  300  comprises a plurality of transistors. The particular embodiment depicted in FIG. 3 comprises five transistors  304 ,  306 ,  308 ,  310  and  312 , each having a gate, a source and a drain, and capacitor  314  having a first electrode  316  and a second electrode  318 . First capacitor electrode  316  receives an input voltage and second capacitor electrode  318  is connected in series to the drain of first transistor  304 . The source of first transistor  304  is connected to ground and the gate of first transistor  304  receives a power down signal input. Second capacitor electrode  318  is further connected to the drain of second transistor  306  and the drain of second transistor  306  is further connected to the drain of third transistor  308 . The gate of second transistor  306  receives the power-down signal. The source of third transistor  308  is connected to the source of fourth transistor  310  and the source of fourth transistor  310  receives a voltage input. The drain of fourth transistor  310  is connected to the gates of transistors  308  and  312 . The gate of fourth transistor  310  receives an inverted power-down signal. The gate of fifth transistor  312  is further connected to the gate of third transistor  308  and the source of fifth transistor  312  receives a voltage input. The drain of fifth transistor  312  provides a start-up current to circuit  302  being powered-up. 
     In the power-down mode, node  320  is pulled to ground while  322  is pulled to VDD, so no current flows in the circuit. Upon power-up, transistor  304  turns off so that node  320  is disconnected from ground and transistor  306  turns on, connecting node  320  to node  322 . This causes the charge C on capacitor  314  to be discharged through transistor  308  and the current flowing through transistor  308  to be mirrored in transistor  312 . Current flowing through transistor  312  is larger than that flowing through transistor  308  by a factor of A. A is equal to the differences in the channel-width-to-channel-length ratios (W/L ratios) of transistors  308  and  312 . The current from transistor  312  causes node  324  to be pulled up. By using a transistor for the capacitor and adjusting the current ratio of transistors  308  and  312 , a charge can be established to power-up the analog circuitry more quickly and accurately than in conventional circuits. The transistor&#39;s capacitance is set by the oxide thickness of the transistor gate which matches the capacitance of the other current mirror transistor gate, allowing the charge to be more precisely mirrored into the nodes to be powered-up than if a non-transistor capacitor is used. 
     It is also possible to mirror several currents by using other transistors apart from transistor  312  to power-up several different parts of an analog circuit or several circuits at the same time. 
     Embodiments of the start-up circuit may be used in conjunction with analog circuits in which it is desirable to power-up the circuit more quickly and accurately than is possible with conventional circuits. In one embodiment the start-up circuit is used to power a band gap reference circuit and in another embodiment it is used to power a current steering circuit for a digital to analog conversion circuit. Embodiments of the start-up circuit may be incorporated into a semiconductor device. 
     The start-up circuit is simple, consumes substantially no power in its quiescent state, and because it generates current to charge the capacitors of an analog circuit based on the charge on the capacitor, the circuit can, with careful rationing, transfer the desired amount of charge to bring the circuit up but not overshoot the desired level of current. 
     While the invention has been described in what is presently considered to be preferred embodiments, many variations and modifications will become apparent to those skilled in the art. Accordingly, it is intended that the invention not be limited to the specific illustrative embodiments but be interpreted within the full spirit and scope of the appended claims.