CVF current reference with standby mode

Details CVF current reference with standby mode CVF current reference with standby mode

1996-09-17

2001-03-06

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific identifiable device, circuit, or system

With specific source of supply or bias voltage

C327S537000, C327S541000

Reexamination Certificate

active

06198339

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention pertains to a current reference circuit. In particular, this invention relates to an improved current reference circuit design, thereby providing exceptional tolerance and a standby mode which ensures circuit operation in the absence of an input frequency.
2. Background Art
Switched capacitor current reference circuits require an input frequency in order to produce an output current. This can be a problem in applications where the circuit must respond quickly at the moment the input starts switching or where the oscillator that provides the input frequency is itself powered by the current reference circuit.
Current reference circuits are widely used in microprocessors and ASICs to supply constant current for PLLs and other high speed circuits. Switched capacitor current references are recognized as having very good tolerances compared to alternative approaches since their output currents depend only on thin oxide capacitance (C
1
and C
2
), the input frequency, and a voltage reference. However, they suffer the disadvantage that they do not supply current until the input frequency begins switching, and they rise slowly due to the filtering elements required to remove ripple which is present in switched capacitor DC generating circuits. Prior art methods include use of DC reference circuits which nearly always have poorer tolerances, and use of expensive off-chip components. As technology progresses, power supply tolerance is becoming more demanding and the frequency must be very precise in many critical applications.
It is an object of the present invention to provide a current reference with excellent tolerance and no off chip components.
It is another object of the invention to provide an optional standby mode for a current reference circuit wherein the output is maintained in the absence of an input frequency.
SUMMARY OF THE INVENTION
A circuit suitable for chip implementation having a pair of switching capacitor current generators controlled by a frequency source and providing an output current to a common node. A high tolerance output circuit conducts the supplied output current from the common node to ground which is mirrored by a current path coupled to the supplied current path. The second current path includes a transistor coupled to the path of the supplied current and to a circuit output through which a reference current is provided in response to the mirror current flowing through the transistor. Another transistor coupled to a supply potential maintains a gate voltage of a transistor controlling the flow of the supplied output current into the output circuit. A filter comprising a series transistor and capacitor is connected between the supplied output current path and ground. A second large capacitor also filters and smooths the supplied output current. The supplied output current is mirrored a second time through a current path coupled to the supplied current path and which includes a transistor for providing a second reference current proportional to the current generated by the second mirror.
A back-up current generator may be included which provides standby current should the supplied output current disappear such as when an input frequency fails, for example. The back-up generator includes a transistor coupled to the common node for delivering the back-up current to the supplied current path when a voltage on the common node falls to about zero. The transistor turns off when the voltage on the common node rises to a normal operating level. The back-up generator may also include a pull-down device coupled to the common node for pulling the voltage on the common node to about zero when the switching capacitor current generators discontinue supplying the output current to the common node.
Other features and advantages of this invention will become apparent from the following detailed description of the presently preferred embodiment of the invention, taken in conjunction with the accompanying drawings.


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