Voltage and current regulators with switched output capacitors for multiple regulation states

A device and method of providing any one of a plurality of desired levels of a regulated signal output to a load is described, wherein each desired level is a function of a corresponding reference signal. The device is configured and the method is designed to (1) store each desired level of the regulated signal output on a switchable storage device; and (2) selectively switch the correct storage device to the output when switching from one regulated state to another so as to establish the desired level of regulated signal output.

FIELD OF THE DISCLOSURE

The disclosure relates generally to voltage and current regulators, and more specifically to regulators using switchable output capacitors for improving the output voltage response time of regulators when switching from one regulation state to another.

BACKGROUND OF THE DISCLOSURE

In prior art applications, such as generally shown inFIG. 1, the typical current or voltage regulator10includes regulator control circuit12and a control loop or feedback network14for regulating the output22provided to the load16. The voltage output of the regulator10is usually set by a reference signal (current or voltage) SREFindicated at18, while the output of the regulator10is typically bypassed with a single large capacitor20. When the desired output voltage VOUTis required to change by a significant amount, the large output capacitor20must be charged or discharged to achieve the new regulation voltage VOUT. This causes the transition time between regulated states to be excessively long and impractical for applications where the transition times must be less than several micro seconds. The large output capacitor20thus directly limits the step-response of the regulator's control loop.

More specifically, in order to change the regulation state of the regulator, the reference signal SREFis changed at the input18. When the reference signal SREFis changed, the slew-rate of the output Voutat22is limited to the current sinking or sourcing capabilities of the regulator12, the impedance of the load16, the size of the output capacitor20, and the bandwidth of the regulator's control loop14. For a stable control loop, the rise-time or decay time of the output may be limited from tens to hundreds of microseconds. This may be acceptable for systems where a single regulation state is desired, but can be unacceptable where the regulator is designed to operate in any one of a plurality of regulation states. It is desirable to provide a solution to allow a very fast response time to change from one regulation state to another without redesigning the control-loop, changing the bandwidth of the control-loop, or reducing the size of the output capacitor.

DETAILED DESCRIPTION OF THE DISCLOSURE

The following describes a system for and method of improving the response time of the output of a regulator when switching from one regulated state to another. Regulators which include control-loops have a finite bandwidth when responding to changes in regulated states. The system and method described herein has the effect of increasing the bandwidth without affecting the stability of the system or the output ripple at the output of the regulator where the load is connected.

In one embodiment the system includes a plurality of output bypass capacitors that are each charged to a voltage corresponding to the desired voltage output for a corresponding one of the desired regulated states. The capacitors are controlled so that they can be individually switched to bypass the output so as to immediately bring the voltage of the output to the desired level corresponding to its new regulation state. By switching each of the load capacitors, the voltage and current in the load may be changed as rapidly as the switches change states. Since the output capacitors are each very large, each of the capacitors provide the energy to the load until the regulator's control loop takes over and provides energy to the load while at the same time refreshing the capacitor providing the initial output voltage. At least two capacitors, corresponding to at least two regulated states, are required, although there is no limitation on the number of output capacitors or states that may be regulated. By switching the appropriate output capacitor, transition times between two regulated states can be reduced two orders of magnitude to several microseconds.

FIG. 2illustrates one embodiment of a regulator30comprising a regulator control circuit32, feedback network34, and a plurality of switchable output bypass capacitors C1, C2. . . Cn. The capacitors are connected in parallel with each other and with load38. Each capacitor is also connected to system ground through a respective switch40a,40b. . .40n. In addition to any other inputs (not shown) required to operate regulator30, the regulator also includes a plurality of inputs constructed to receive signal inputs respectively representing a plurality of reference voltages (in the case of a voltage regulator) VREF1, VREF2. . . VREFn. A plurality of inputs are also provided for receiving control inputs S1, S2. . . Snfor respectively controlling the switches40. In this embodiment the voltage across capacitor C1is determined by the reference voltage VREF1, the voltage across capacitor C2is determined by the reference voltage VREF2, and so on for all the reference voltages and capacitors. The individual switches40are controlled by the control inputs, with control input S1controlling switch40a, control input S2controlling switch40b, and so on for all of the control inputs and switches.

In operation, each of the capacitors of the embodiment ofFIG. 2is precharged to provide a desired voltage VOUTat the output44to be applied to the load38by closing the corresponding switch and applying the appropriate signals at the inputs S and VREF. Once each capacitor C is precharged, the corresponding switch40is opened and the charge remains stored on the capacitor.

Once all of the capacitors are charged, the regulated state is controlled by the control inputs to the regulator. The voltage across C1is determined by the voltage at VREF1, the voltage across C2is determined by the voltage at VREF2, and so on forth for all references and output capacitors. The application of a control input S determines the regulation state, and in particular the reference voltage VREFto be used. Accordingly, in this embodiment the corresponding output capacitor C is switched onto the output terminal44, with the remaining switches remaining open so as to provide the correct VOUTfor the selected regulation state. With each capacitor being sized so as to be capable of being charged to a predetermined voltage as a function of the desired level of the regulated signal output, controlling the switches allows for selectively connecting at least one of the capacitors to the load depending on and as a function of the desired level of the regulated signal output so that when the reference signal is changed, at least one select capacitor is concurrently connected to the load so as to concurrently provide the desired level of the regulated signal output to the load.

While theFIG. 2embodiment is shown with a switch40connected between a corresponding capacitor C and system ground, the regulator will work equally as well if each switch40and capacitor are exchanged so that the capacitor is connected between the corresponding switch and system ground, as shown as the embodiment illustrated inFIG. 3.

Further details of one embodiment of the regulator are shown inFIG. 4. The regulator is shown as an exemplary voltage regulator50. The S inputs are applied to the control logic52, while the VREFinputs are connected to switches54. Switches54are each controlled by the control logic52. When each switch54is closed the corresponding VREFinput is connected to the non-inverting input of the error amplifier56. The output of the error amp56is applied to the power stage58. The latter in turn is connected to the output44, and to the voltage divider60. The voltage divider60is connected to system ground, while the tap of the voltage divider is connected to the inverting input of the error amplifier56. Control logic52includes logic for selectively closing a set of switches comprising a switch54and the corresponding switch40so that the desired VREFis connected to the non-inverting input of the error amplifier56. When one set of switches40and54is closed, a desired value of VREFis connected to the input of the error amplifier, and a desired capacitor C is connected between the regulator output44and system ground. Precharged capacitor C will immediately set the output voltage to the precharged voltage level, while the regulator slews to the level through its normal feedback process. In this way the output is brought to the desired level much more quickly than otherwise allowed by various factors including limitations due to the current sinking or sourcing capabilities of the regulator, the impedance of the load, the size of a single output capacitor, and the bandwidth of the regulator's control loop.

In another embodiment, the regulator shown inFIG. 5is an example of a current regulator. As illustrated, current regulator70includes the control inputs S each controlling a respective set of switches54and40. In this instance, the desired reference inputs are currents IREF1, IREF2. . . IREFn. When the appropriate switch54is closed the corresponding IREFis applied to the non-inverting input of the error amplifier72. The input of the error amplifier72has its non-inverting input connected through resistor74, which in turn is connected the node forming the output44of the regulator. The output of the error amplifier72is connected to the input of the power stage76, which in turn has its output connected to the inverting input of the amplifier72. A resistor78is connected between the inverting input of error amplifier72and the resistor74. In operation, each set of switches is closed to allow a corresponding IREFto flow into the current regulator control circuit, and charge the corresponding capacitor C at the output of the control circuit. When the switches40are open, the corresponding capacitors will hold the appropriate charge corresponding to the respective references currents IREF. The output voltage across each capacitor is determined by the corresponding regulated current flowing through the load38. When a particular regulation state is desired, the appropriate control switch S is applied to close the corresponding set of switches54and40connecting the desired IREFto the input of amplifier72. As the amplifier slews to the reference value at it non-inverting input, the desired value of the output voltage is applied from the precharged capacitor C that is connected through the appropriate switch40to the output44.

In yet another embodiment, the regulator shown inFIG. 6is an example of a voltage regulator employing a plurality of error amplifiers EA. As illustrated, the regulator80includes the control logic82for controlling the operation of each set of switches84and86in response to the control inputs S. In this illustrated embodiment, an error amplifier EA is provided for each regulation state. According to this embodiment, each error amplifier EA1, EA2. . . EAn has its input connected to receive one of the reference voltages, and a separate switch84for selectively connecting the output of the amplifier to the input of the power stage88. The output of stage88is connected to resistor divider90, the tap of which is connected to the inverting input of each error amplifier EA. Thus, when the regulator80needs to be set for a particular regulated state, the appropriate control input S will close the correct switch84and switch90corresponding to the desired regulated state. This will connect the correct error amplifier EA with the power stage88, and the correct capacitor C to system ground, so as to provide the corresponding regulated voltage (stored on the correct capacitor) to the output94and load92while the error amplifier EA slews to its regulated output value determined as a function of the input VREF.

The major advantage of providing the multiple capacitors, so as to store each precharged output voltage at a predetermined desired level for each regulated state, is illustrated by the comparator experimental results between a regulator employing a plurality of switched capacitors and the prior art approach.FIG. 7illustrates the response of changing from one regulated state to another using the prior art regulator similar to that shown inFIG. 1. As shown when the reference voltage100is changed at time t1, so as change from level A to level B, the output of the regulator slews from level VOUT1to level VOUT2. However, the output does not change as quickly as the change in the application of the reference voltage. Instead it takes time as indicated at102to slew from one output value to the next. As shown, while the reference voltages are switched very quickly from one reference value to the next, it takes the output voltage significant time to respond. In the example shown the reference voltage switches from one value to the next almost instantaneously, while it takes more than 200 microseconds for the output voltage to settle at its new value for the new regulated state.

FIG. 8illustrates the response of a regulator employing a plurality of switched capacitors. As can be seen, when the control signal at level110for one regulated state is changed to another control signal112for a new desired regulated state the transition still occurs relatively quickly relative to the output response. However, in this instance the output voltage is change as illustrated at114almost 100 times faster than the response shown as the output response inFIG. 7because of the value stored on the corresponding capacitor for the new regulation state is immediately applied to the output of the regulator in response to the change in control signals.

The comparative differences between the results illustrated inFIGS. 7 and 8are more clearly show inFIG. 9, where both results are plotted on the same graph. The control and VREFs are superimposed at120for simplification purposes, while the output response of the regulator of the prior art type is shown at122, and the output response of a regulator using multiple switched capacitors is shown at124.

It should be appreciated that while the storage devices are described as capacitors, other types of storage devices can be used, such as inductors. Further, more than one capacitor can be used to establish a regulated state by switching more than one capacitor to the output when switching to a new regulated state.

An example of an application of the regulator with a plurality of switched capacitors is a control regulator that can be used to provide any one for a plurality of regulated operating states of an LED where a plurality of different regulated states are possible. For example, such an arrangement might require three regulated states including zero current, a low level current (0 to 4 A) and high current (4 to 30 A). However, it should be appreciated that the plural switched capacitor arrangement can applied to any regulation scheme where two or more states are desired with a rapid transition time between the states is required.

While there has been illustrated and described particular embodiments of the present disclosure, it will be appreciated that numerous changes and modifications will occur to those skilled in the art. Accordingly, it is intended that the appended claims cover all those changes and modifications which fall within the spirit and scope of the present disclosure.