Charge pump with temporally-varying adiabaticity

Abstract

Operation of a charge pump is controlled to optimize power conversion efficiency by using an adiabatic mode with some operating characteristics and a non-adiabatic mode with other characteristics. The control is implemented by controlling a configurable circuit at the output of the charge pump.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application 109
where η is the efficiency, VO is the measured converter output voltage, VIN is the measured converter input voltage, and N is the charge pump conversion ratio.

The controller directly measures the effect of selecting adiabatic vs. non-adiabatic charging on converter efficiency by comparing the average value of the output voltage VO over a complete charge pump cycle.

Other controller logic uses combinations of the approaches described above. For instance, the controller can confirm that the assessment of charge pump operating mode and estimation of efficiency increase by changing the charge pump charging mode.

A traditional method for operating the charge pump 100 is at a fixed frequency in which the switching occurs independently of the load requirement (i.e., the switches in FIG. 1 operate on a fixed time period). Referring to FIG. 6, during one cycle of the switching of the charge pump 100, a current I1 discharges from the capacitor C1 and a current IP discharges other of the capacitors in the charge pump 100. For a particular intermediate current IX, the longer the cycle time T, the larger the drop in voltage provided by the capacitor C1. A consequence of this is that the switching frequency generally limits the maximum intermediate current IX because the switching frequency for a particular load determines the extent of voltage excursions, and in some cases current excursions (i.e., deviations, variation), at various points and between various points within the charge pump 100 and at its terminals. For a particular design of charge pump 100, or characteristics of load and/or source of the charge pump 100, there are operational limits on the excursions.

Referring to FIGS. 7A-C, the intermediate voltage VX of the charge pump 100 is shown in various current and timing examples. Referring to FIG. 7A, at a particular intermediate current IX, the intermediate voltage VX generally follows a saw-tooth pattern such that it increases rapidly at the start of each state, and then generally falls at a constant rate. Consequently, the rate of voltage drop depends on the output current IO. At a particular output current IO and switching time, a total ripple voltage δ results, and a margin over the output voltage VO is maintained, as illustrated in FIG. 7A. (Note that the graphs shown in FIGS. 7A-B do not necessarily show certain features, including certain transients at the state transition times, and related to the high frequency switching of the regulator 320; however these approximations are sufficient for the discussion below).

Referring to FIG. 7B, in the output current IO in the circuit in FIG. 4 increases, for instance by approximately a factor of two, the ripple of the intermediate voltage VX increases, and the minimum intermediate voltage VMIN decreases and therefore for a constant output voltage VO the margin (i.e. across inductor 316) in the regulator 320 decreases. However, if the voltage margin decreases below a threshold (greater than zero), the operation of the regulator 320 is impeded.

Referring to FIG. 7C, to provide the regulator 320 with a sufficient voltage margin voltage the switching frequency can be increases (and cycle time decreased), for example, to restore the margin shown in FIG. 7A. Generally, in this example, doubling the switching frequency compensates for the doubling of the output current IO. However more generally, such direct relationships between output current IO or other sensed signals and switching frequency are not necessary.

In general, a number of embodiments adapt the switching frequency of the charge pump 100 or determine the specific switching time instants based on measurements within the charge pump 100 and optionally in the low-voltage and/or high-voltage peripherals coupled to the terminals of the charge pump 100.

In a feedback arrangement shown in FIG. 4, the controller 350 adapts (e.g., in a closed loop or open loop arrangement) the switching frequency. For any current up to a maximum rated current with a fixed switching frequency, the charge pump 100 generally operates at a switching frequency lower than (i.e., switching times greater than) a particular minimum frequency determined by that maximum rated current. Therefore, when the current is below the maximum, capacitive losses may be reduced as compared to operating the charge pump 100 at the minimum switching frequency determined by the maximum rated current.

One approach to implementing this feedback operation is to monitor the intermediate voltage VX and adapt operation of the charge pump to maintain VMIN above a fixed minimum threshold. One way to adapt the operation of the charge pump 100 is to adapt a frequency for the switching of the charge pump 100 in a feedback configuration such that as the minimum intermediate voltage VMIN approaches the threshold, the switching frequency is increased, and as it rises above the threshold the switching frequency is reduced. One way to set the fixed minimum threshold voltage is as the maximum (e.g., rated) output voltage VO of the regulator 320, plus a minimum desired margin above that voltage. As introduced above, the minimum margin (greater than zero) is required to allow a sufficient voltage differential (VX−VO) to charge (i.e., increase its current and thereby store energy in) the inductor 326 at a reasonable rate. The minimum margin is also related to a guarantee on a maximum duty cycle of the regulator 320.

A second approach adapts to the desired output voltage VO of the regulator 320. For example, the regulator 320 may have a maximum output voltage VO rating equal to 3.3 volts. With a desired minimum margin of 0.7 volts, the switching of the charge pump 100 would be controlled to keep the intermediate voltage VX above 4.0 volts. However, if the converter is actually being operated with an output voltage VO of 1.2volts, then the switching frequency of the charge pump 100 can be reduced to the point that the intermediate voltage VX falls as low as 1.9 volts and still maintain the desired margin of 0.7 volts.

In a variant of the second approach, rather than monitoring the actual output voltage VO, an average of the voltage between the switches 312, 314 may be used as an estimate of the output voltage VO.

In yet another variant, the switching frequency of the charge pump 100 is adapted to maintain the intermediate voltage VX below a threshold value. For example, the threshold can be set such that the intermediate voltage VX lowers or rises a specific percentage below or above the average of the intermediate voltage VX (e.g. 10%). This threshold would track the intermediate voltage VX. Similarly, a ripple relative to an absolute ripple voltage (e.g. 100 mV) can be used to determine the switching frequency.

Note also that the voltage ripple on the output voltage VO depends (not necessarily linearly) on the voltage ripple on the intermediate voltage VX, and in some examples the switching frequency of the charge pump 100 is increased to reduced the ripple on the output voltage VO to a desired value.

Other examples measure variation in internal voltages in the charge pump 100, for example, measuring the ripple (e.g., absolute or relative to the maximum or average) across any of the capacitors C1 through C4. Such ripple values can be used instead of using the ripple on the intermediate voltage VX in controlling the switching frequency of the charge pump 100. Other internal voltages and/or currents can be used, for example, voltages across switches or other circuit elements (e.g., transistor switches), and the switching frequency can be adjusted to avoid exceeding rated voltages across the circuit elements.

In addition to the desired and/or actual output voltages or currents of the regulator 320 being provided as a control input to the controller 350, which adapts the switching frequency of the charge pump 100, other control inputs can also be used. One such alternative is to measure the duty cycle of the regulator 320. Note that variation in the intermediate voltage VX affects variation in current in the Buck converter's inductor 326. For example, the average of the intermediate voltage VX is generally reduced downward with reducing of the switching frequency of the charge pump 100. With the reduction of the average output voltage VO, the duty cycle of the regulator 320 generally increases to maintain the desired output voltage VO. Increasing the duty cycle generally increases the efficiency of a Buck converter. So reducing the switching frequency of the charge pump 100 can increase the efficiency of the regulator 320.

It should be understood that although the various signals used to control the switching frequency may be described above separately, the switch frequency can be controlled according to a combination of multiple of the signals (e.g., a linear combination, nonlinear combination using maximum and minimum functions, etc.). In some examples, an approximation of an efficiency of the charge pump is optimized.

The discussion above focuses on using the controller 350 to adjust the switching frequency of the charge pump 100 in relatively slow scale feedback arrangement. The various signals described above as inputs to the controller 350 can be used on an asynchronous operating mode in which the times at which the charge pump 100 switches between cycles is determined according to the measurements. As one example, during state one as illustrated in FIG. 6, the intermediate voltage VX falls, and when VX−VO reaches a threshold value (e.g., 0.7 volts), the switches in the charge pump 100 are switched together from state one to state two. Upon the transition to state two, the intermediate voltage VX rises and then again begins to fall, and when VX−VO again reaches the threshold value, the switches in the charge pump 100 are switched together from state two back to state one.

In some examples, a combination of asynchronous switching as well as limits or control on average switching frequency for the charge pump are used.

Unfortunately, as the intermediate current IX decreases the switching frequency of the charge pump 100 decreases as well. This can be problematic at low currents because the frequency could drop below 20 kHz, which is the audible limit for human hearing. Therefore, once the frequency has dropped below a certain limit, a switch 344 closes and introduces a compensation capacitor 342. This forces the converter into non-adiabatic operation allowing the frequency to be fixed to a lower bound (e.g. 20 kHz). Consequently, the compensation capacitor 342 is introduced when either the duty cycle is low or when the output current IO is low.

Note that the examples above concentrate on a compensation circuit that permits selectively switching a compensation capacitor of a certain fixed capacitance onto the output of the charge pump. More generally, a wide variety of compensation circuits can be controlled. One example is a variable capacitor, which can be implemented as a switched capacitor bank, for example, with power of two capacitances. The optimal choice of capacitance generally depends on the combination of operating conditions (e.g., average current, pulsed current duty cycle, etc.) and/or circuit configurations (e.g., type of regulators, sources, load, pump capacitors), with the determining of the desired capacitance being based on prior simulation or measurement or based on a mechanism that adjusts the capacitance, for instance, in a feedback arrangement. In addition, other forms of compensation circuits, for example, introducing inductance on the output path, networks of elements (e.g., capacitors, inductors).

Note that the description focuses on a specific example of a charge pump. Many other configurations of charge pumps, including Dickson pumps with additional stages or parallel phases, and other configurations of charge pumps (e.g., series-parallel), can be controlled according to the same approach. In addition, the peripherals at the high and/or low voltage terminals are not necessarily regulators, or necessarily maintain substantially constant current. Furthermore, the approaches described are applicable to configurations in which a high voltage supply provides energy to a low voltage load, or in which a low voltage supply provides energy to a high voltage load, or bidirectional configurations in which energy may flow in either direction between the high and the low voltage terminal of the charge pump. It should also be understood that the switching elements can be implemented in a variety of ways, including using Field Effect Transistors (FETs) or diodes, and the capacitors may be integrated into a monolithic device with the switch elements and/or may be external using discrete components. Similarly, at least some of the regulator circuit may in some examples be integrated with some or all of the charge pump in an integrated device.

Implementations of the approaches described above may be integrated into an integrated circuit that includes the switching transistors of the charge pump, either with discrete/off-chip capacitors or integrated capacitors. In other implementations, the controller that determines the switching frequency of the charge pump and/or the compensation circuit may be implemented in a different device than the charge pump. The controller can use application specific circuitry, a programmable processor/controller, or both. In the programmable case, the implementation may include software, stored in a tangible machined readable medium (e.g., ROM, etc.) that includes instructions for implementing the control procedures described above.

It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.

Claims

1. An apparatus comprising a switched-capacitor charge pump configured to provide voltage conversion between first and second terminals thereof, a compensation circuit coupled to a first terminal of said charge pump, said compensation circuit having a first configuration and a second configuration, wherein, in said first configuration, said first terminal of said charge pump couples to a capacitance, wherein, in said second configuration, said capacitance is decoupled from said first terminal of said charge pump, and a controller circuit coupled to said charge pump and said compensation circuit, said controller circuit comprising an output for configuring said compensation circuit, and a first sensor input for accepting a first sensor-signal that, at least in part, characterizes operation of a circuit selected from the group consisting of said charge pump and a peripheral circuit directly coupled to said charge pump, wherein said controller circuit is configured to configure said compensation circuit based at least in part on said first sensor-signal to promote efficiency of power conversion between a power source coupled to said charge pump and a load coupled to said charge pump via said compensation circuit.

2. The apparatus of claim 1, wherein said first sensor-signal characterizes a voltage at said peripheral circuit.

3. The apparatus of claim 1, wherein said charge pump comprises a capacitor, and wherein said first sensor-signal characterizes a voltage across said capacitor.

4. The apparatus of claim 1, wherein at least some current that passes from said charge pump to said compensation circuit continues through to an inductor in said peripheral circuit.

5. The apparatus of claim 1, wherein said controller circuit comprises a second sensor input for accepting a second sensor-signal that, at least in part, characterizes operation of said circuit.

6. The apparatus of claim 1, wherein said controller circuit is configured to determine an operating mode at least in part based on said first sensor-signal, and to determine said configuration of said compensation circuit according to said determined mode.

7. The apparatus of claim 1, wherein said first sensor-signal characterizes a voltage at said first terminal.

8. The apparatus of claim 1, wherein said controller circuit is configured to couple said first terminal to said capacitance at times that optimize power-conversion efficiency.

9. The apparatus of claim 1, wherein said first sensor-signal characterizes said switching frequency of said charge pump.

10. The apparatus of claim 1, wherein said first sensor-signal characterizes a voltage at said second terminal.

11. The apparatus of claim 1, wherein said charge pump comprises a series-parallel charge pump.

12. The apparatus of claim 1, wherein said first sensor-signal characterizes a duty cycle of a pulsed current passing to or from said charge pump.

13. The apparatus of claim 1, wherein said first sensor-signal characterizes an average of a current passing to said charge pump.

14. The apparatus of claim 1, wherein said peripheral circuit comprises an inductor that is coupled to said compensation circuit.

15. The apparatus of claim 1, wherein said peripheral circuit is coupled between said compensation circuit and said first terminal and wherein said peripheral circuit comprises an inductor, switches, and an output capacitor, wherein control of said switches of said peripheral circuit adjusts a voltage across said output capacitor.

16. The apparatus of claim 1, wherein said peripheral circuit, which is coupled to said compensation circuit, comprises a switch that alternates between being open and being closed, wherein adjustment of a fraction of time during which said switch is closed.

17. The apparatus of claim 1, wherein said peripheral circuit is coupled between said compensation circuit and one of said first and second terminals and wherein said peripheral circuit comprises an inductor that is selectively connected to and disconnected from said compensation circuit.

18. The apparatus of claim 1, further comprising a regulator, wherein said regulator is coupled to said compensation circuit.

19. The apparatus of claim 1, wherein said compensation circuit is coupled to a regulator that achieves a selected output voltage by adjustment of a duty cycle thereof.

20. The apparatus of claim 1, further comprising a regulator, wherein said regulator is coupled between said compensation circuit and said high-voltage terminal.

21. The apparatus of claim 1, further comprising a regulator, wherein said regulator is coupled between said compensation circuit and said low-voltage terminal.

22. A method comprising carrying out voltage conversion, wherein carrying out voltage conversion comprises receiving a sensor signal that characterizes, at least in part, operation of a circuit that is selected from the group consisting of a switched-capacitor charge pump that provides voltage conversion between first and second terminals thereof and a peripheral circuit, wherein said peripheral circuit is directly connected to said switched capacitor charge pump, and based at least in part on said sensor signal, causing a compensation circuit that is coupled to a first terminal of said charge pump to transition between coupling and decoupling a capacitance from said first terminal, and wherein causing said compensating circuit to transition comprises causing said compensation circuit to transition at times that promote efficiency of power conversion between a power source coupled to said charge pump and a load coupled to said charge pump via said compensation circuit.

23. An apparatus comprising a power converter, said power converter comprising first and second terminals, said power converter being configured to cause a second voltage to be maintained at said second terminal in response to presence of a first voltage presented at said first terminal, wherein said power converter further comprises a compensation circuit, a controller circuit, a switching network, and capacitors, wherein said switching network interconnects said capacitors, wherein, as a result of transitioning between first and second states thereof, said switching network causes said capacitors to transition between corresponding first and second arrangements, wherein as a result of a transition, electrical charge propagates between said capacitors, wherein said controller circuit is connected to receive, from at least one of a first circuit and a second circuit, information indicative of an extent to which said propagation of said electrical charge between said capacitors results in energy loss, wherein said controller circuit is configured to cause said compensation circuit to transition between a first configuration and a second configuration based on said information, said transition being one that reduces said extent and that causes a capacitance of said compensation circuit to be switched into or out of communication with said first circuit, wherein said first circuit is a circuit that is formed by said switching network and said capacitors, and wherein said second circuit is a circuit that is directly connected to a circuit that is formed by said capacitors and said switching network.

24. The apparatus of claim 23, wherein said second circuit comprises an inductor.