Patent Application
20090079406
The present invention relates in at least some of its embodiments to voltage regulation.
On-die low dropout voltage regulators have been used to provide low-noise power supplies phase-locked loop (PLL) circuits. As shown in
While LDO regulators have proven useful in many applications, they have performance limitations and problems with reliability, which are especially noticeable when a relative high power supply is used to power the regulator. These effects are particularly severe in PLL applications when, for example, the PLL goes in and out of power saving mode and the regulator output experiences hundreds of millivolts of step-function type changes. Moreover, traditional LDO regulator designs require too many components, which reduce valuable die space and increase circuit complexity.
a and 4b are graphs showing an example of the performance that may be achieved by the voltage regulator of
The feedback path is located between the first output, Out—1, of the amplifier and the non-inverting input of the amplifier, and includes a first control transistor 40 and a voltage divider 50 formed from resistors 51 and 52. The first control transistor 40 (e.g., a voltage adjuster) is coupled between a voltage source 90 (e.g., supply voltage Vccsfr) and an output node A. One of the resistors, e.g., 51, may be a variable resistor. In this case, the ratio of the voltage divider may be set by adjusting resistor 51.
The feed-forward path leads from the second output, Out—2, of the amplifier into the gate of a second control transistor 60. The first and second control transistors 40 and 60 are shown as having opposite conductivities, but in alternative embodiments different transistor types may be used. The feedback path intersects the feed-forward path at output node A, which outputs the regulated supply voltage, Vccpll. According to one application, the regulated supply voltage may be used as the supply voltage for a phase-locked loop, as the suffix “ccpll” indicates. However, in other embodiments, the supply voltage may be input into different circuits, and moreover virtually any type of circuit that requires a regulated voltage or power supply.
In operation, the amplifier serves to maintain a substantially constant supply voltage in spite of process, temperature, or other variations that may otherwise cause this voltage to fluctuate. To maintain a constant supply voltage, the amplifier operates to compare a feedback voltage proportional to supply voltage Vccpll to a reference voltage, which, for example, may be a bandgap reference voltage. Under normal operating conditions, the feedback voltage, generated from Out—1 and input into the non-inverting terminal of amplifier 10, equals the reference voltage and as a result the amplifier outputs a control signal (e.g., an analog voltage) which controls the amount of voltage transferred through the source and drain of transistor 40 from the voltage source 90.
The supply voltage charges capacitor 80, which operates not only to store a voltage proportional to Vccpll but also to prevent rapid fluctuations in the supply voltage especially when the inputs into the amplifier are different. In this latter case, the capacitor may therefore be said to perform a smoothing operation which helps to maintain a constant supply voltage.
The second output, Out—2, is also indicative of the difference in the amplifier inputs. Because the amplifier inputs are the same, another control signal is supplied to the gate of transistor 60 from the second output, Out—2, of the amplifier. This control signal couples output node A to reference potential 100 through transistor 60, and consequently no adjustment is made to the supply voltage, Vccpll. In accordance with at least one embodiment, the control signal from Out—2 may be considered a first control signal and the control signal from Out—1 may be considered a second control signal.
When the feedback voltage into the amplifier does not equal the reference voltage, at least to within some predetermined tolerance, both amplifier outputs may assume a same voltage. That is, the second control signal from Out—1 controls transistor 40 along the feedback path and the first control signal from Out—2 controls transistor 60 along the feed-forward path. When transistor 60 turns on more or less, a signal path to reference potential 100 (e.g., ground) is established which corrects, or helps maintain, the supply voltage at a predetermined constant value.
Thus, for example, when the supply voltage surges, a difference or error signal is supplied to the feed-forward path. This signal controls transistor 60 to reduce the voltage at node A, which reduces the voltage stored in the capacitor. This voltage continues to decline until the supply voltage returns to the predetermined constant value. When this occurs, the feedback voltage into the amplifier will equal the reference voltage, at which time transistor 60 reaches an equilibrium state and the voltage Vccpll from source Vccsfr is changed back to the constant value through transistor 40. Likewise, the feedback path experiences similar actions to keep the voltage Vccpll constant.
Using this one-amplifier design, the voltage regulator demonstrates high supply-voltage tolerance, and also allows for supply rejection optimization while maintaining positive regulator resistance for a given efficiency target. These effects serve to avoid power grid oscillation which translates into improved performance and reliability of the circuit (e.g., PLL) receiving the regulated output. Through its one-amplifier design, the regulator is also less complex and uses less die space.
In the
In addition, supply-oriented feed-forward compensation capacitor C4 is used on the feed-forward path. This capacitor slows down the feed-forward path and therefore helps to reduce the transient voltage level in response to voltage disturbance within the loop. Moreover, rather than penalizing the supply rejection due to the slow down, the compensation capacitor actually improves the supply rejection because it provides a short cut for the supply disturbance to the feed-forward path.
A supply-oriented feed-forward compensation capacitor C3 is used on the feed-back path. The compensation provided by this capacitor allows optimization of the supply rejection for a given regulator efficiency target while maintaining positive the regulator resistance. The larger the C3 capacitance, the faster the feed-back path and therefore the better supply rejection. However, a faster feed-back path pushes the regulator resistance (or conductance) more towards the negative region. Using capacitor C3, a fine balance can be obtained by choosing how far away the regulator resistance stays from the negative region to accommodate PVT and random variation, as well as aging impact.
a and 4b show an example of one level of performance that may be achieved using the voltage regulator of
Any reference in this specification to an “embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments.
Furthermore, for ease of understanding, certain functional blocks may have been delineated as separate blocks; however, these separately delineated blocks should not necessarily be construed as being in the order in which they are discussed or otherwise presented herein. For example, some blocks may be able to be performed in an alternative ordering, simultaneously, etc.
Although the present invention has been described herein with reference to a number of illustrative embodiments, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings and the appended claims without departing from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.
