1. Field of the Invention
The present invention is related to power supply regulators for radio frequency applications and more particularly to a shunt regulator for high frequency applications.
2. Background Description
Voltage regulators are well known. An ideal voltage regulator provides a constant voltage regardless of load. Thus, the voltage regulator provides the same voltage under no load (at no current) as it does fully loaded. Current used by circuits in the complementary insulated gate field effect transistor (FET) technology, commonly known as CMOS, primarily, is switching current with negligible static (or DC) current flow.
CMOS circuit current flow usually occurs only during switching, primarily, either to charge or discharge the circuit""s load (capacitance). Thus, digital circuits that are synchronized by a common clock signal may exhibit sporadic episodes of very high switching current, e.g., from a counter switching from FFFF16 to OOOO16. By contrast, typical radio frequency circuits, such as may be used in a radio telephone or cellular phone, exhibit relatively uniform switching and, therefore, have relatively uniform current. Variations between no load and full load current, such as may occur with digital circuits, can cause large switching noise that must be filtered to prevent errors in CMOS analog circuits on the same integrated circuit substrate or chip. Shunt regulators may be used to reduce switching and other current related noise.
A typical shunt regulator includes an alternate current path for regulator current, the regulator supplying constant current during no load conditions as well as during full load. The parallel current path maintains an effective load such that even as the load varies, the regulator supplies constant (full load) current with excess current being shunted through the parallel path. Many state of the art shunt regulators are designed to maintain effective constant current. That unused portion of the full load current in excess of the load current is shunted through a shunt device in the shunt regulator and so, wasted.
High frequency circuits, and especially radio frequency (RF) circuits, are very sensitive to noise. Switching noise from digital circuits can easily couple into radio frequency circuits thereby, degrading circuit performance. As higher degrees of integration are being achieved, larger numbers of complex high frequency circuits are being integrated onto a single integrated circuit chip. Further, as digital circuit performance improves, digital functions are also being combined with RF functions onto monolithic integrated circuit chips integrating more and more digital and RF circuits onto the same chip and resulting in an increased number of local potential noise sources on a given chip. To mitigate this problem on-chip voltage regulators may be used to provide separate isolated voltage supplies for digital and for RF circuits, thereby, isolating digital switching currents from input power supplies and from other on-chip RF circuits. This approach significantly reduces the digital switching noise that might otherwise couple into the RF circuits. Typically, shunt regulators are used for this type of isolation.
A typical prior art shunt regulator is described in U.S. Pat. No. 4,366,432 entitled xe2x80x9cHighly Stable Constant-Voltage Power Source Devicexe2x80x9d to Noro. Noro describes a shunt regulator that is biased to deliver a constant current to a parallel combination of a load and shunt device. This constant current value must be set high enough to supply the maximum load current and any lesser variations thereof with the excess passing through the shunt device. Consequently, Noro""s shunt regulator constantly provides the maximum current and, when there is no load current, i.e., it is unloaded, all of the supply current is shunted through the shunt device. Accordingly, when the load is less than full, Noro wastes some of the power supplied.
U.S. Pat. No. 5,260,644 entitled xe2x80x9cSelf-Adjusting Shunt Regulator and Methodxe2x80x9d to Curtis describes a shunt regulator which attempts to isolate power supply and load, automatically adjusting supply current to compensate for load variations. Unfortunately, the Curtis shunt regulator also consume and excess power when it is not fully loaded. The Curtis shunt regulator varies shunt current to compensate for changes in load currents such that fluctuation of total supply current are only a small fraction of fluctuations of load current.
These prior art shunt regulators are used to provide isolation between a chip power supply and a digital circuit portion of the chip load. These shunt regulators can tolerate rapidly varying load current. Unfortunately, these typical prior art shunt regulators suffer from excessive power consumption, while only providing limited isolation between digital and RF circuits that are integrated onto the same integrated circuit chip.
Thus, there is a need for improved shunt regulators that more effectively isolate digital circuits from RF circuits integrated onto the same chip, but with a minimum increase in load current.