1. Field of the Invention
The present invention relates to the design of electronic circuits that are useful in low power, low current electronic systems. In particular, the present invention relates to electronic circuits that utilize start up circuits, the start up circuits consuming no stand by current upon the completion of a start up sequence.
2. Description of the Prior Art
Typical microelectronic systems have various electronic components that often share one or more common biasing circuits. Examples of circuits that have common biasing arrangements include operational amplifiers, comparators, and other analog components such as level detectors.
MOS type transistors function as voltage controlled devices where the conduction channel is activated by applying a voltage field across the conduction channel. Because MOS devices are field effect devices with an insulated gate structure, current does not flow through the devices"" gate terminals. Since MOS transistors only consume power when biased in their active region, MOS transistors are useful in low power circuits. For example, a typical CMOS digital inverter circuit does not use any power unless it is transitioning from one logic state to another. MOS transistors are thus widely used in digital electronic systems for their reduced power consumption characteristics.
Analog circuits tend to use more power than digital circuits in part due to the active nature of the circuits. Linear amplifiers, comparators and other analog circuits normally require biasing circuits for proper operation. Unlike many digital circuits, analog circuits tend to consume xe2x80x9cstand-byxe2x80x9d current. For this reason, it is often desired to consolidate biasing circuits from various analog circuits into one common biasing circuit in order to reduce overall system power.
A simple biasing arrangement using MOS devices includes a diode-connected transistor that is series connected to a resistor, the combination connected across the power supply. The diode-connected transistor will conduct current as soon as the power supply levels exceed the threshold voltage of the transistor. Once the power supply has reached its full potential, the current through the diode-connected device is inversely proportional to the resistor value. The gate connection of the diode connected device functions as a biasing voltage for other transistors. The gate connections of the other transistors are connected to the common biasing voltage in such a way that the other transistors will also conduct a current level that is inversely proportional to the resistor in the bias circuit.
The diode/resistor combination discussed above is not well suited for ultra low power microchip applications. In order to keep the overall power consumption down, the resistor must have a relatively high value ( greater than 1Mxcexa9). For Example, when the resistor in the bias circuit has a value of 5Mxcexa9, the diode-connected device will consume roughly 860 nA on a 5 Volt supply (4.3 v/5Mxcexa9=860 nA). Thus, where the microchip power budget is on the order of 1 or 2 xcexcA, almost an entire xcexcA of current will be used up on the biasing arrangement alone (approximately 40% of the power budget). The diode/resistor arrangement suffers from poor regulation over varied power supply voltages. In addition, the simple diode/resistor circuit tends to have poor regulation performance over varying temperature ranges.
Another example of a conventional biasing arrangement is known as a xe2x80x9cVt generatorxe2x80x9d. Many modern CMOS processes are based on P-type doped substrates. Parasitic PNP transistors are inherently formed in the substrate of p-type CMOS circuits, with a fixed collector connected to the substrate. The parasitic PNP devices can be configured as diodes. A xe2x80x9cVt generatorxe2x80x9d uses the temperature dependent characteristics of diode connected parasitic PNP devices to generate a voltage proportional to absolute temperature (VPTAT). A circuit is arranged using current mirrors with diode connected devices in such a way as to form a xe2x80x9cvptat-loopxe2x80x9d, which generates a voltage drop across a resistor. The voltage drop is normally very small (Vt is on the order of 26 mV).
The VPTAT generator biasing arrangement requires a smaller resistor value as compared to the diode/resistor circuit previously discussed. The current density of each parasitic diode connected device in a VPTAT generator is proportional to the current mirror ratio and the area of the diode. The voltage across the resistor is given by VR=Vtxc2x7ln((IC1xc2x7IS2/(IC2xc2x7IS1)), where IC1, IC2 are the currents in the mirrors and IS1, IS2 are proportional to the emitter areas (A1, A2) of the diodes. The voltage across the resistor is also given as VR=IC2xc2x7R. Thus, R=VR/IC2=(Vt/IC2)xc2x7ln((IC1xc2x7IS2)/(IC2xc2x7IS1)). For example, if the ratio of currents in the mirror are 1:1 (IC1:IC2), the bias current is 860 nA, the ratio of the diode emitter areas is 4:1 (IS2:IS1), and the Vt is 26 mV, then the resistor value which is required in the VPTAT generator is given by: R=(26 mV/860 nA)*ln(4)=42Kxcexa9. This is substantially less than the 5Mxcexa9 resistor which is required in the simple diode/resistor biasing circuit previously discussed. In addition, the VPTAT generator provides a supply voltage independent bias current which consumes less die area than the diode/resistor arrangement. However, the VPTAT generator produces a bias current that is dependent upon temperature.
A further conventional biasing arrangement counteracts temperature effects by using a so-called xe2x80x9cband-gapxe2x80x9d reference circuit. Band-gap reference circuits use the inherent characteristics of bipolar transistors (often connected as diode devices by shorting the collector and base together) to compensate for detrimental temperature effects. The energy band-gap of Silicon is on the order of 1.2 V, and is independent from temperature and power supply variations. Bipolar transistors have a negative temperature drift with respect to the base-emitter voltage (Vbe decreases as operating temperature increases). However, the thermal voltage of a bipolar transistor has a positive temperature drift (Vt=kT/q, thus Vt increases as temperature increases). Thus, the negative temperature drift in a bipolar transistor base-emitter voltage is counteracted by the positive temperature drift in the thermal voltage (Vt).
One typical problem associated with band-gap type reference circuits, as well as other electronic circuits, is that there is a possibility that during the power up sequence, the transistors will find a state where they will not turn on. VPTAT and bandgap reference circuits are circuits that are configured as feedback circuits. When the voltage gain in the feedback loop is too high, an unstable operating condition results. Typically the gain in the feedback loop must be maintained at a lower level to permit stable operation. As discussed previously, it is likely that the reference circuit is being used as a biasing reference for other circuits. To ensure proper functionality of circuits that share the common biasing connection, it is crucial that the reference circuit properly starts-up when the power is turned on and simultaneously finds a stable operating condition.
One way to ensure start-up of a circuit is to form a conduction path from one supply through a transistor in the problematic circuit. This in turn causes the problematic circuit to enter a known state of active operation. For example, a resistor can be connected between one of the respective power supplies and a diode connected device in the reference circuit. The resistor and diode device form the conduction path in the reference circuit. The diode device will begin to conduct and in turn the circuit will begin normal operation.
A problem with the above described resistor conduction path approach is that the resistor will remain connected to the reference circuit even though the reference circuit has reached normal operating levels. For ultra low power circuits the total power budget is on the order of 1 or 2 xcexcA. The continued conduction through the resistor has a disadvantage in that the start-up path consumes constant power. Thus, in order to reduce the loss of power due to the start-up resistor, the resistor needs to have a very high ohmic value. The continued conduction through the resistor may also introduce an offset in the reference circuit.
In integrated circuits, high value resistors require large sheet resistance materials to be integrated onto the chip. Typical resistivity values for readily available materials such as polysilicon are on the order of 1 kxcexa9/xe2x96xa1. Well material typically has a sheet resistance on the order of 10 kxcexa9/xe2x96xa1. For resistance values over 1Mxcexa9, low doped, high ohmic integrated resistors consume substantial amounts of die area. Additionally, dioderesistor arrangements suffer from temperature instability as the resistance varies widely due to high temperature coefficients. High value integrated circuit resistors also suffer from junction leakage, high junction capacitance and are often subject to poor process control.
Some semiconductor manufacturing processes offer resistors called xe2x80x9cEpi-FETxe2x80x9d or xe2x80x9cpinchedxe2x80x9d resistors. These resistors are higher in sheet resistivity, where resistivity increases as the supply voltage increases. However, these resistors are subject to poor process control and often have reliability problems at cold temperatures.
Simple capacitive circuits are also used as start-up circuits. Capacitive circuits assume that a known or well-defined charge condition exists on the capacitors both before and during start-up. If the start up circuit does not reach a known or well defined condition (i.e., completely discharged or fully charged), there is a possibility that the reference circuit will not properly initialize and start-up.
The present invention relates to a start-up circuit and method for securing a stable start-up behavior for a bias generator circuit. Typical bias circuit arrangements will successfully start-up under normal operating conditions. Under extreme operating conditions such as very cold temperatures and low xe2x80x9cbetaxe2x80x9d process corners, bias circuits often behave unpredictably and may not find a conductive operating condition. Start-up circuits and methods according to the invention cooperate with the xe2x80x9cnaturalxe2x80x9d start up of most bias circuits to enhance the predictability and reliability of the bias circuit achieving proper conduction.
The present invention is related to a start-up circuit for forcing a bias generator circuit into a known steady-state condition during a power-up sequence. According to one aspect of the invention, cross-currents and capacitive gate charging currents of a ring oscillator are used to ensure proper start-up of the bias circuit. A detector circuit monitors potential changes in the power supply during power up. As the power supply is brought up to a full potential, the detector circuit detects the potential changes in the power supply and monitors activity in the ring oscillator. The detector circuit controls a current source circuit. The detector circuit together with the current source circuit monitor activity of the oscillator, and feeds a start-up current into a section of the bias generator circuit. The start-up current forces the bias generator circuit into an active state of operation. After the biasing currents become active, a shut down circuit cuts off the current flow into the ring-oscillator, thereby minimizing total power consumption.
In a voltage proportional to absolute temperature (VPTAT) type bias circuit, there is possibility that the bias circuit will not find a conductive operating point after the power supply reaches its operating potential. A capacitive voltage divider is used detect the power supply power-up. An output of the capacitive voltage divider is coupled to an input to a ring oscillator. The ring oscillator has an extremely high voltage gain such that the ring oscillator will easily achieve an oscillating condition with very little input voltage (due to its high instability). A current mirror detects current flowing through the ring-oscillator and generates a start-up current for the VPTAT generator. The start-up current activates the biasing transistors in the VPTAT generator. Once the VPTAT generator turns on, the biasing transistors become active and biasing currents begin to flow. The biasing currents in turn activate a transistor switch that deactivates the ring oscillator, thereby reducing the total current flow in the electronic system.
According to a feature of the invention, an apparatus forces a startup current into a selected node in a current path of a bias circuit when power is activated, comprising: an oscillator; wherein the oscillator oscillates when active, a monitor, wherein the monitor produces a control signal when the oscillator is active, and a current source that includes a current control input coupled to the control signal, wherein the current source provides the start-up current at the selected node in response to the control signal such that the current path in the bias circuit is activated into a conducting state when the oscillator consumes power.
It is a further feature of the invention to provide for an apparatus as described above, wherein the oscillator includes an oscillator control input that disables the oscillator when activated, whereby power consumption is reduced. The oscillator includes an input node, an output node, and a feedback circuit coupled between the input node and the output node, whereby the feedback circuit forms a feedback loop in the oscillator. The feedback circuit may include a capacitor.
According to another feature of the invention, the apparatus further includes a switch circuit that couples the input node of the oscillator to a potential when activated, wherein the switch circuit is activated when the bias circuit actively conducts such that and the potential is coupled to the input node of the oscillator and the oscillator is disabled to reduce power consumption. The switch circuit may include any variety of switching elements such as a MOS transistor, a bipolar transistor, as well as other switching elements. The activation of the switch is controlled.
According to yet another feature of the invention, the apparatus further provides for the oscillator including an inverting circuit and the feedback circuit includes a capacitor to ac couple the output node to the input node of the oscillator when the oscillator is active. The capacitor isolates the input node from the output node of the oscillator when the oscillator is disabled.
According to a further feature of the invention, the apparatus further provides for a noise injector that couples noise from a power supply line to the input node of the oscillator, wherein the noise coupled to the input node of the oscillator initiates oscillation of the oscillator. The noise injector may include a capacitor that couples noise from the power supply line to the input node of the oscillator. Alternatively, the noise injector may include a first capacitor connected between a power supply line and the input of the oscillator, and a second capacitor connected between another power supply line and the input of the oscillator such that the first and second capacitors inject charge into the oscillator.
According to still another feature of the invention, the apparatus further provides that the monitor includes a transistor that operates as a diode and the transistor is forward biased when the oscillator is active. The current source may include another transistor that conducts the start-up current when the transistor for the monitor conducts. The operation of the current source is controlled. The monitor may include a first transistor and the current source include a second transistor such that the first and second transistors form a mirror pair where current flowing in the second transistor is proportional to current flowing in the oscillator when the oscillator is active. The current source may also include a current amplifier circuit. The monitor senses one of a current flow and a voltage change in the oscillator when the oscillator is active.
According to a feature of the invention, a start-up circuit is provided that couples a start-up current into a selected node in a current path of a bias circuit when power is activated, comprising: an oscillator, wherein the oscillator consumes power when active, a control means produces a control signal proportional to power consumption in the oscillator, a current source includes a control input coupled to the control signal such that the current source provides the start-up current proportional to the power consumption of the oscillator, and coupling means couples the start-up current to the selected node such that the current path in the bias circuit is forced into a conducting state, whereby and said bias circuit is activated. The start-up circuit may further provide for detection means produces a bias active signal when the bias circuit is in active operation, and disable means to disable the oscillator in response to the active signal such that the oscillator is disabled when the bias circuit is active and power consumption is reduced. Furthermore, injection means couples a noise signal into the oscillator circuit to initiate oscillations in the oscillator.
According to another feature of the invention, a method of using an oscillator to force a bias circuit into an active state of operation, provides for: detecting an oscillation in the oscillator to produce an oscillator active signal, producing a start-up current in response to the oscillator active signal, feeding the start-up current into the bias circuit to force the bias circuit into the active state of operation, detecting the active state of operation to produce a bias active signal, and disabling the oscillator circuit in response to said bias active signal such that the oscillator is inactive when the bias circuit is active and power consumption is reduced. The method may further include injecting noise into the oscillator circuit to initiate oscillations in the oscillator.
Briefly stated, the invention provides a start-up circuit for a bias generator circuit including an oscillator, a power monitoring circuit and a controllable current source. As the power supplies begin to ramp up to their final voltage, the power monitoring circuit monitors power consumed by the oscillator. As the oscillator begins to ring, the power monitoring circuit couples a control signal to the controllable current source, which generates a current used to start-up the bias generator circuit. Once the bias generator circuit has achieved an active operating condition, the startup circuit is disengaged from the bias circuit by disabling the oscillator. The start-up circuit does not consume any standby current when disabled, and does not effect the operation of the bias generator circuit.