Patent Grant
8493040
Voltage regulators are often used in electronic devices to generate a stable output voltage from a varying power supply. The current load of a device may change dynamically during operation. This change may cause fluctuations in the output voltage, which may adversely affect operation of the device. A voltage regulator adjusts supplied power according to changes in the load in order to maintain a stable voltage.
Some voltage regulators are configured to exhibit a low dropout voltage. In these contexts, the term dropout voltage is generally used to refer to the minimum difference between the input unregulated voltage to the LDO regulator (such as a battery or power bus) and the regulated voltage output from the LDO regulator at max output current conditions. Linear regulators maintain the regulated output voltage while an unregulated voltage supply remains above the dropout voltage. LDO regulators exhibit a relatively small dropout voltage that helps extend the life of the battery because the LDO regulator can continue to provide a regulated voltage until the battery is discharged to a value that is within a relatively close range (e.g., 100-500 millivolts) of the regulated voltage.
In one embodiment, a regulator circuit having a low drop out voltage is provided. The regulator circuit includes a control circuit configured and arranged to adjust an oscillation frequency of a variable frequency oscillator in response to a feedback signal indicating the regulated output voltage. A charge pump is coupled to an output of the variable frequency oscillator and is configured to charge one or more energy storage elements in response to the output of the variable frequency oscillator. The regulator circuit includes a plurality of output stages, each output stage having an input driven by the output of the charge pump and being configured to drive the regulated output voltage. Each output stage is selectably enabled or disabled in response to a respective enable signal provided to the output regulator by an enable control circuit.
In another embodiment, a circuit for producing a regulated voltage is provided. The circuit includes a charge pump having a control input coupled to an output of a fixed frequency oscillator. The charge pump is configured and arranged to charge and discharge a plurality of energy storage elements, in response to the output of the variable frequency oscillator, to produce a voltage signal at a rate controlled by the fixed frequency oscillator. The regulator includes a plurality of selectably enabled output stages driven by the voltage signal and configured to provide a respective regulated output voltage, in response to a respective enable signal provided to the output stages.
In yet another embodiment, a regulator circuit is provided. The regulator circuit includes a charge pump having a control input coupled to an output of an oscillator. The charge pump is configured and arranged to charge one or more energy storage elements, in response to the output of the oscillator, to produce voltage signal at a rate controlled by the oscillator. The regulator circuit also includes a plurality of output stages. Each output stage has an input driven by the voltage signal and configured to provide one or more respective regulated output voltages in response to respective enable signals provided to the output stages. A control circuit is configured and arranged to adjust the voltage signal, via the charge pump, in response to one or more of the one or more respective regulated output voltages.
The above discussion is not intended to describe each embodiment or every implementation. The figures and following description also exemplify various embodiments.
Various example embodiments may be more completely understood in consideration of the following detailed description in connection with the accompanying drawings, in which:
While the disclosure is amenable to various modifications and alternative forms, some various implementations thereof have been shown by way of example in the drawings and are described in detail below. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments shown and/or described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure.
The disclosed embodiments are believed to be applicable to a variety of different types of processes, devices, and arrangements for use with various regulator circuits. While the embodiments are not necessarily so limited, various aspects of the disclosure may be appreciated through a discussion of examples using this context.
One or more embodiments provide a power efficient voltage regulator with low drop-out voltage. The regulator includes a plurality of output stages, which are driven by an output of a charge pump. Use of the charge pump allows to drive the inputs of the output stages above the source voltage Vdd. In this manner, a low dropout voltage is achieved. The charge pump generates the voltage used to drive the output stages in response to an output of a variable frequency oscillator. The frequency of the variable frequency oscillator adjusted in response to a feedback control signal. The feedback control signal is generated by comparing an output voltage to a reference voltage. As the feedback voltage increases and approach the reference voltage, the feedback control signal reduces the frequency of the variable frequency oscillator and the current drawn by the charge pump is reduced proportionally. In this manner, when less load is placed on the regulator, less power is required to operate the regulator.
A feedback circuit may be configured to control the variable frequency oscillator in a number of different way. In one or more embodiments the variable frequency oscillator is implemented with a voltage controlled oscillator and is controlled by a voltage difference amplifier configured to compare an output voltage with a reference voltage. In one or more other embodiments, the variable frequency oscillator is implemented with a current controlled oscillator and is controlled by a transconductance amplifier configured to compare the output voltage with the reference voltage. In yet one or more other embodiments, the variable frequency oscillator is implemented with a voltage controlled oscillator that is controlled by a differential signal generated from the difference between the output voltage and the reference voltage.
In one or more embodiments, a feedback control voltage is generated using a replica output stage rather than from an actual output of the regulator. As a result, an increase in load current on one of the output stages will not cause the regulator to increase voltage signal output from the charge pump. This allows several output stages to be driven from the same voltage without transferring noise between output stages and may be useful in a number of applications that are sensitive to power supply noise.
In one or more embodiments, a voltage regulator includes a plurality of output stages that may be selectably enabled or disabled. The output stages may provide power for different sections of a power grid or for separate circuits. Different ones of the output stages may be biased differently to produce different output voltages. Through biasing and/or selective enabling, different output stages may be programmably adjusted to produce different regulated output voltages.
The minimum supply voltage of the transistor 114 is equivalent to VDD>Vo+VGS+VDS+RonIout where Vo is the required output voltage, VGS is the Gate-to-Source voltage of the output transistor (equal to VGS=Vtn+Veff), VDS is the saturation voltage of transistor 114, Iout is the load current and Ron is the “on resistance” of switch 116. Use of the charge pump to drive the gate of the NMOS transistor allows each of the output stages 112 and 113 to operate using lower supply voltage. In this manner, a low dropout voltage is achieved.
Various NMOS transistors as discussed herein may exhibit a low output impedance and provide built-in feedback during operation. If the output load current demand rapidly increases, the accompanied drop in output voltage will automatically increase the Gate-to-Source bias of the output transistor. Due to this increase in voltage, the transistor can automatically source more current to the load. The feedback loop does thus not limit the response time and the output voltage does not collapse. P-MOS type regulators, which may also be implemented with various embodiments, rely on the feedback loop to increase the Gate-to-Source voltage of the output transistor and require additional output capacitance to handle to load current step to compensate for the loop response time. Faster control loops employ smaller decoupling capacitors at the expense of greater current consumption. For ease of illustration, the examples and embodiments are illustrated and described herein using NMOS transistors to drive a regulated output voltage of each output stage 112. However, various embodiments including those described herein are not so limited and may alternatively be configured to use PMOS transistors.
Each output stage is selectably enabled or disabled by a respective PMOS transistor 116 in response to a respective enable signal (En_1, En_n) provided to the output regulator by an enable control circuit (not shown) and may be used to individually enable or disable power to the connected loads. Output stages may provide power for different sections of a power grid or for separate circuits. The enabling/disabling mechanism may be used to conserve power by eliminating leakage currents in unused area of a chip or provide more efficient load balancing across a power network. For example, in some applications, the enable control may be configured to enable and disable one of the plurality of output stages 112 and 113 in response to large increases or decreases in load current demands.
The regulator circuit 100 includes a control circuit configured and arranged to adjust an oscillation frequency of a variable frequency oscillator 102 in response to a feedback signal indicating the regulated output voltage. A charge pump 104 is coupled to an output of the current controlled oscillator 102 and is configured to charge one or more energy storage elements of the charge pump in response to the output of the variable frequency oscillator 102.
The control circuit includes a replica output stage 110 that is configured similar to the plurality of output stages 112 and 113, a difference amplifier 106, and a reference voltage generator 108. The replica output stage 110 derives a feedback voltage Vfb that is proportional to the output voltage Vout. In this example, the replica output stage 110 is implemented to be the same as one of the output stages 112 and 113, except that the gate of the enable PMOS transistor 116 is connected to ground. This causes the replica output stage 110 to always be enabled.
The difference amplifier 106 compares the output of the replica output stage Vfb to a reference voltage Vref generated by the reference voltage and current generator 108 to produce an error signal that is used to control the frequency of the variable frequency oscillator. In this embodiment, the variable frequency oscillator 102 is implemented using a current controlled oscillator and the difference amplifier 106 is implemented with a transconductance amplifier 106. During startup, the output voltage Vout and the feedback voltage Vfb are low and the transconductance amplifier 106 supplies (about) maximum input current to the current-controlled oscillator 102. As the output voltage Vout and the feedback voltage Vfb increase and approach a reference voltage Vref, the output current of the transconductance amplifier 106 diminishes. As a result, the frequency of the current controlled oscillator 102 reduces and the current drawn by the oscillator 102 and charge pump 104 reduce proportionally.
Because feedback of the control circuit is provided using a replica feedback stage 110, an increase in load current on one of the output stages 112 and 113 will not cause the regulator to increase voltage signal output from the charge pump. The net effect is reduced load regulation as well as reduced peak-to-peak noise of the output voltage. This allows several output stages to be driven from the same voltage without transferring noise between output stages. This is useful to isolate noisy and power hungry loads from those that may be particularly sensitive to power supply noise.
In one or more embodiments, the charge pump includes a small leakage circuit that ensures stability and a minimum frequency of operation regardless of the frequency of the variable frequency oscillator. In some other embodiments, the leakage circuit may be implemented external to the charge pump.
In this embodiment, the regulator 200 implements replica feedback stage 210 to provide a leakage path at the output of the charge pump by coupling the gate and drain of NMOS transistor 220 to implement a diode. As a result, no leakage internal to the charge pump is needed, resulting in less current consumption. Assuming equal leakage currents between the regulators of
In the regulator circuits illustrated in
In some applications, use of voltage signaling to implement feedback control may increase susceptibility to noise and require more attention during the design and layout phase. For such applications, the regulator may be implemented to control the variable frequency oscillator using differential signaling, which may improve noise immunity.
In some embodiments, different output stages of the regulators shown in
In one or more embodiments, a regulator circuit is provided that does not include a difference amplifier to provide feedback to the oscillator.
The regulator circuit includes a plurality of output stages 512 and 513 that are driven by a voltage signal vhv produced by the charge pump 504. Each output stage has an input driven by a voltage signal output from the charge pump 504. Each output stage (e.g. 513) drives one or more regulated output voltages (vout1−voutn) using a transistor 518 arranged in a source follower configuration with respective current source 522. Current source 522 biases the transistor 518 according to a reference current iref2.
The charge pump 504 charges one or more energy storage elements to produce a voltage signal at a rate controlled by the fixed frequency oscillator. A control circuit 506 is configured to limit voltage provided to a power supply pin of the charge pump 504 in response to the regulated output voltage of one or more of the plurality of output regulators. The control circuit 506 is a replica of the transistor 118 of the one of the output stages. This ensures sufficient voltage vhv is output from the charge pump 504 without exceeding technology limits.
Transistor 516 of each output stage (e.g. 513) operates with transistor 510 to mirror reference current iref provided by reference current/voltage generator 508. The mirrored current iref biases a replica transistor 520, which provides a gate voltage to NMOS transistor 518. By controlling the source of the replica transistor devices via a voltage reference/voltage clamp circuit, the output voltage is changed accordingly. Thus, independently programmable output voltages can be generated.
A startup circuit provides the initial supply voltage to the charge pump. The startup circuit may be implemented, for example, by a comparator that turns on a weak switch driving a clamp circuit. The comparator monitors an internal voltage and compares it to one of the reference voltages, to decide when the internal replica bias is sufficient. Until then, it enables a weak switch to supply. In order to prevent voltage overshoot, a weak switch is used that drives a clamp circuit. The clamp only draws current if the supply exceeds a maximum level. After the comparator turns off the switch, it will not consume any current.
Each output stage may be selectably enabled or disabled by PMOS transistor 514 in response to a respective enable signal (En_1, En_n) provided to the output regulator by an enable control circuit, and may be used to power one or more loads. Output stages may provide power for different sections of a power grid or for separate circuits as described with reference to
The embodiments are thought to be applicable to a variety of applications which require one or more regulated voltages such as a personal electronic device, a hand-held device, a computer device, etc. Based upon the above discussion and illustrations, those skilled in the art will readily recognize that various modifications and changes may be made without strictly following the exemplary embodiments and applications illustrated and described herein. Furthermore, various features of the different embodiments may be implemented in various combinations. Such modifications do not depart from the true spirit and scope of the present disclosure, including that set forth in the following claims.
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