Patent Application
20090309569
The present invention relates generally to multi-stable self-biasing electronic circuits and, in particular, to arrangements for avoiding undesired states of such circuits.
Multi-stable electronic circuits are quite common, and are used for various purposes. Certain types of bandgap reference circuits, for example, have two stable operating states, including a normal operating state in which a reference voltage is provided at an output and an off state in which the output level is zero. On power up, the output level of such a bandgap reference circuit initially starts at 0 Volts, which is the correct voltage for one of the stable operating states. The circuit may actually remain in its latched off state after being powered up.
This startup problem might not be encountered where there are large amounts of circuit noise or some sort of startup circuit to bias the bandgap reference circuit toward its normal operating state. One conventional way to start up a bandgap reference circuit is to use a pull-up resistor or transistor at its output. As the power supply rises from 0 Volts, the pull-up provides a bias current, which causes the bandgap reference circuit to favor and snap into its normal operating state.
Many bandgap reference circuits include an operational amplifier or “opamp” to generate the reference voltage. Once the power supply is at a high enough level for the opamp to function properly, the current required to pull the bandgap reference circuit out of a latched off state condition can be quite high, especially if the output stage of the opamp has a strong current sinking capability, as is typically the case.
In general, a pull-up should be able to overdrive the output of an opamp while power supply voltage is too low for the opamp to be functional. However, if a bandgap reference circuit shuts off after it was fully operational, due to noise or other conditions for instance, the pull-up might not be strong enough to overdrive the opamp output and the bandgap can remain latched off.
The above issues relating to a latched off state similarly apply to other types of multi-stable circuits, which may enter and remain in an undesired state.
Non-limiting and non-exhaustive examples of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Methods and apparatuses for implementing state control of multi-stable electronics circuit are disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.
Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.
To illustrate,
It should be appreciated that the apparatus 10 of
As shown in the example depicted in
In the normal operating state of the example bandgap reference circuit 14 shown in
In one example, feed resistors 25 and 27 normally have the same resistance, which results in the currents through the emitter/collector conductive paths of the transistors 22 and 24 being equal. The different effective emitter sizes of the transistors 22 and 24, however, denoted in
In the illustrated example, the output of bandgap reference circuit 14 is the output of amplifier 20. In one example, the output of the amplifier 20 is generally 1.2V to 1.3V when the bandgap reference circuit 14 is in its normal operating state. With the arrangement shown in
When powered on, the bandgap reference circuit 14 becomes a bi-stable circuit with one of two stable operating points appearing at the output of amplifier 20. The desired operating state is with an output voltage of 1.2V in some examples, although different output levels may be desired in other examples. In the undesired state, the output of amplifier 20 is latched at 0V.
During startup, the output of the amplifier 20 initially starts at 0V in the depicted example. The polarity of the input offset of the amplifier 20 can make the bandgap reference circuit 14 favor its off state, and as noted above the circuit may remain in the off state after being powered up. Conventional startup arrangements using pull-up resistors or transistors connected to the output of the amplifier 20 overdrive the output of amplifier 20. Large currents can be required, especially after power supply voltage becomes large enough for the amplifier 20 to operate normally. As will be shown, examples in accordance with the teachings of the present invention provide a different approach.
In the latched off state, no current flows through the feed resistors 25 and 27 or the gain setting resistor 29. In the illustrated example, the state control circuit 12 detects the current flowing through one of the resistors, such as the resistor 25 in the depicted example, and generates a drive signal Idrive that is provided to an input of amplifier 20. Other sensing arrangements are also possible in accordance with the teachings of the present invention. For example, the voltages across the resistors 27 and 29, separately or in combination, are also indicative of the output level of the bandgap reference circuit and could be used to sense the output level. It would also be possible to provide separate elements to enable sensing of the output level of the bandgap reference circuit 14 in accordance with the teachings of the present invention.
As shown in the example depicted in
The state control circuit 12 thus detects the operating state of the bandgap reference circuit 14 (on or latched off) by effectively monitoring the current flowing through the transistor 22 that is connected to the positive input node of the amplifier 20. When the bandgap reference circuit is off, a current is injected into this node, pulling the output of the bandgap reference circuit 14 out of its latched off state.
In one example, the state control circuit 12 includes a differential input pair, which in the example shown in
In one example, state control circuit 12 determines whether the bandgap reference circuit 14 is on or off by sensing the current flowing through the resistor 25.
To illustrate, the voltage across the resistor 25, which is proportional to the current flowing through the resistor 25, is applied to the gate terminals of the differential input pair transistor Pm130 and transistor Pm232. When the bandgap reference circuit 14 is off, no current flows through the resistor 25, and the gate voltages of transistor Pm130 and transistor Pm232 are equal. Normally, this causes the drain currents of the differential pair transistor Pm130 and transistor Pm232 to be equal, except that transistor Pm130 and transistor Pm232 are different sizes. In one example, transistor Pm130 is twice as large as transistor Pm232 in the apparatus 10. As shown in
In the depicted example, the drive current Idrive raises the non-inverting input of amplifier 20, causing the bandgap reference circuit 14 to bootstrap itself into operation, resulting in the desired reference voltage at the output of amplifier 20. As the bandgap reference circuit 14 turns on, the voltage across the resistor 25 increases, which in turn decreases the gate to source voltage of transistor Pm130, gradually turning transistor Pm130 off. This results in substantially all of the Ibias current being carried by transistor Pm232 off into the dummy load transistor Nm136. In other examples, a drive signal may be provided to the inverting input of amplifier 20, or possibly to some other node at the input side of amplifier 20 in accordance with the teachings of the present invention.
Thus,
The transistors 30 and 32 shown in
The bandgap reference circuit 14 as illustrated in
As shown in the example depicted in
It is appreciated that the examples discussed above are described primarily in the context of an apparatus. Other examples in accordance with the teachings of the present invention are also contemplated.
It should be appreciated that the method 40 represents one illustrative example in accordance with the teachings of the present invention. Other examples for instance may include additional, fewer, or different operations performed in a similar or different order. The operations shown in
Although described above primarily in the context of an illustrative example application to bandgap reference circuits, examples in accordance with the teachings of the present invention may be used in conjunction with other types of circuits as well. For instance, many self-biased multi-stable circuits other than bandgap reference circuits include amplifiers and may have a latched off state or other undesired state. For example, in another example in accordance with the teachings of the present invention, an apparatus includes a self-biasing electronic circuit including an amplifier and having multiple stable operating states responsive to an output of the amplifier. A state control circuit can be coupled to the electronic circuit to sense an output of the electronic circuit and to provide, at an input of the amplifier, a drive signal responsive to the sensed output.
The drive signal causes the electronic circuit to avoid the undesired state. This may involve causing the electronic circuit to avoid entering the undesired state, causing the electronic circuit to transition out of the undesired state, and causing the electronic circuit to transition into one of the multiple states that is different from the undesired state.
It should be apparent from the foregoing that a bandgap reference circuit is one example of a self-biased multi-stable circuit in conjunction with which a state control circuit could be implemented in accordance with the teachings of the present invention.
In particular, the example circuit shown in
In one example, a state control circuit could be used substantially as described above in
The principles disclosed herein may also be applied to circuits having more than two stable states in accordance with the teachings of the present invention. A foldback limit circuit, for example, may be considered to have an off state, a normal operating state and a foldback state during which current is being limited. Foldback limit circuits often use amplifiers to provide the current limiting function. If the circuit output is folded back to zero, or even near zero, the circuit can latch off. This can potentially be prevented by providing a drive signal to an amplifier in the circuit to avoid the latched off state at startup or during operation in accordance with the teachings of the present invention. In this case, there are two desired operating states and an undesired latched off state.
Examples in accordance with the teachings of the present invention thus provide for state control of self-biased multi-stable electronic circuits. Indeed, overdriving an amplifier output can be difficult, especially where the output has a low impedance and high current sinking capability, as is often the case, or after initial startup. Providing a drive signal at an amplifier input in accordance with the teachings of the present invention enables a state control capability with much lower currents, due to the relatively high input impedance that is typical in amplifier designs. In one example, the current source 34 shown in
The above description of illustrated examples of the present invention, including what is described in the Abstract, are not intended to be exhaustive or to be limitation to the precise forms disclosed. For example, examples in accordance with the teachings of the present invention may be useful for avoiding an undesired state at startup. However, the disclosed techniques could also or instead be used to avoid such states during operation. A drive signal could be provided to an input of an amplifier to prevent the amplifier from latching off during normal operation, or to pull the amplifier out of the latched off state in the event that it latches off after startup.
In the case of a multi-stable circuit, it may be possible to provide different levels of drive signal depending on the alternate state into which operation of the amplifier or circuit is to be driven. For a circuit having two normal operating states and a latched off state for instance, different drive signal levels might be used to favor the two different normal operating states.
In addition, while specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present invention. Indeed, it is appreciated that the specific voltages, currents, times, etc., are provided for explanation purposes and that other values may also be employed in other embodiments and examples in accordance with the teachings of the present invention.
