Switched-capacitor regulators

[0001] This application claims priority from U.S. Provisional Application No. 60/645,966, filed Jan. 24, 2005, entitled AN AREA-EFFICIENT AND HIGH-ACCURACY SWITCHED-CAPACITOR REGULATOR. This Provisional Application is hereby incorporated herein by reference in its entirety.

[0002] The invention relates to switched-capacitor regulators, and in particular to switched-capacitor regulators with pseudo-continuous output regulation using a three-stage switchable operational amplifier and time-multiplexed pole-splitting compensation techniques. The invention is useful for decreasing chip area and improving the accuracy of output voltage.

[0003] Switched-capacitor regulators are needed in many portable electronic devices to provide supply voltages for different systems. Possible devices include personal digital assistants, notebook computers and mobile phones.

[0004] The importance of switched-capacitor regulators is demonstrated by their capability to provide dc-to-dc conversions without the use of inductors. Less conducted electromagnetic interference and cross couplings are then generated to other systems, since no magnetic components are used. Existing control methods adopted in switched-capacitor regulators require extra power transistors or auxiliary circuits to regulate the output voltage. These increase the chip area of the thereby increasing the manufacturing cost.

[0005] The accuracy of the output voltage is critical to switched-capacitor regulators. The accuracy is determined by line and load regulations and the load-transient recovery time of regulators, which in turn is dependent on the implementation of the controller. Existing implementations employ single-stage linear amplifier in the controller, which can affect the accuracy of the regulators.

[0006] A number of methods have been employed to maintain the output voltage of a switched-capacitor regulator. Both U.S. Pat. No. 6,445,623 and U.S. Pat. No. 6,411,531 disclose two single-ended switched-capacitor regulators and are illustrated in FIGS. 1 and 2 respectively. Additional circuitry (either a current source or a variable resistor connected between the input supply voltage and switches S1, S2, S3 and S4) is required to maintain the output voltage. However, the additional circuitry is implemented by power transistors; thereby increasing the chip area and manufacturing cost.

[0007] Similarly, U.S. Pat. No. 6,411,531 also describes a single-ended switched-capacitor regulator as shown in FIG. 3, which uses an extra current source connected between the power switches and ground for output-voltage regulation. Therefore, the area efficiency of the switched-capacitor regulator is not ideal

[0008] FIG. 4 shows a cross-coupled switched-capacitor regulator from U.S. Pat. No. 6,618,296. The cross-coupled design is similar to the configuration in U.S. Pat. No. 6,445,623, used to achieve dual phase operation. Extra power transistors are still required for regulating the output voltage, and this lowers the area efficiency of the regulator.

[0009] FIG. 5 shows a cross-coupled switched-capacitor regulator which maintains the output voltage without using extra power transistor, as described in W. Chen, W. H. Ki, P. K. T. Mok and M. Chan, “Switched-capacitor power converters with integrated low-dropout regulators”, IEEE International Symposium on Circuits and Systems, Sydney, Australia, Vol. III, pp. 293-296, 2001. Power transistors Ml4 and Mr4 are alternately turned off during the capacitor charging phase and regulate the output voltage of the control loop during the capacitor discharging phase. Therefore, Ml4 and Mr4 function as switching low-dropout regulators (SLDR) and the control scheme is known as an SLDR mechanism. By adopting SLDR control, an analog buffer should be used in order to enable Ml4 and Mr4 to operate in the saturation region during capacitor discharge phase. However, extra auxiliary voltage doubler are required to generate a high-voltage clock signal swinging up to 2V_DD so as to drive extra switches connected to the output of the analog buffer. Extra auxiliary voltage doublers increase both the area and total static current dissipation in the controller. In addition, since the gate-voltage swing of Ml4 and Mr4 is 2V_DD and increases with the input supply voltage, the SLDR control causes switching noise problems in the switched capacitor regulator. Moreover, gain stage and analog buffer in the controller cascaded with power transistors Ml4 and Mr4 results in a two-stage switchable opamp. Two-stage switchable opamps may not provide sufficient loop gain magnitude, and hence the accuracy of the switched capacitor regulator is not optimized.

[0010] There is a need for a control method for a switched-capacitor regulator that is able to efficiently and accurately regulate the supply voltage of electronic devices without requiring additional power transistors or auxiliary circuits.

[0011] Usually an additional power transistor is required to regulate the output voltage of a switched-capacitor regulator. By means of the present invention, a switched-capacitor regulator operates in pseudo-continuous regulator mode using three-stage switchable opamps with time-multiplexed pole-splitting compensation technique. The regulator controls the output voltage continuously during every capacitor-discharging phase without adding extra power transistor nor requiring high-voltage clock signals; thereby enhancing area efficiency. In addition, the use of three-stage switchable opamps increases loop-gain magnitude and improves line and load regulations by alternate switching of gain stage in different clock phases. The time-multiplexed pole-splitting compensation technique in the switchable opamp not only ensures stability but also reduces load-transient recovery time.

[0012] The switched-capacitor regulator according to the present invention decreases chip area and results in a comparatively low output ripple at low switching frequencies; which can improve the accuracy of the resulting output voltage.

[0013] The switched-capacitor regulator according to the invention is of particular advantage if applied in an electronic appliance, such as, but not limited to, a portable electronic device.

[0014] The present invention provides, in a first aspect, a switched capacitor regulator for regulating output voltage. The switched capacitor regulator comprises a supply input terminal capable of receiving a supply voltage, and two or more flying capacitors, with a regulation transistor located between each flying capacitor and the supply input terminal. The switched capacitor regulator further comprises a voltage control circuit, the activity of the regulation transistors being controlled by the voltage control circuit. The regulation transistors function as variable resistors, and an output of the two or more flying capacitors is used to adjust resistance of the regulation transistors.

[0015] The present invention provides, in a second aspect, a switched capacitor regulator for regulating output voltage. The switched capacitor regulator comprises a supply input terminal capable of receiving a supply voltage, and two or more flying capacitors, with a regulation transistor located between each flying capacitor and the supply input terminal. The switched capacitor regulator further comprises a voltage control circuit comprising a feedback resistance area comprising one or more feedback resistors located between the output of the flying capacitors and a ground terminal, a first gain stage connected to the feedback resistance area, and two or more second switchable gain stages, each connected to a regulation transistor and the first gain stage. The activity of the regulation transistors is controlled by the voltage control circuit, the regulation transistors functioning as variable resistors, and an output of the two or more flying capacitors is used to adjust resistance of the regulation transistors.

[0016] The present invention provides, in a third aspect, a switched-capacitor regulator configured for providing a regulated output voltage. The switched-capacitor regulator comprises a supply input terminal for receiving a supply voltage, and a voltage control circuit configured for regulation of the output voltage. The switched-capacitor regulator further comprises two flying capacitors, each of the flying capacitors charging and discharging alternately in every half-clock period, two regulation transistors, each of the regulation transistors coupled between the supply input terminal and one terminal of a flying capacitor, and controlled by the voltage control circuit. The switched-capacitor regulator further comprises an output load circuit comprising a load capacitor and a load device, the output load circuit configured for maintaining a voltage charge on the load device, and two feedback resistors connected in series, coupled between the output of the switched-capacitor regulator and ground terminal. The discharging phase occurs when one flying capacitor is coupled between regulation transistor and the output load circuit, and the charging phase occurs when the flying capacitor is disconnected from the output load circuit and is charged by the supply input terminal. The regulation transistors function as variable resistors, and an output of the two flying capacitors is used to adjust resistance of the regulation transistors.

[0017] The invention may also broadly be said to consist in any alternative combination of features as described or shown in the accompanying drawings. Known equivalents of these features not expressly set out are nevertheless deemed to be included.

[0031] The present invention is based on the concept of pseudo-continuous control switched-capacitor regulator. FIG. 6 shows an embodiment of the structure of the regulated cross-coupled voltage doubler with pseudo-continuous control. In the cross-coupled power stage, there are eight power transistors Ml1-Ml4 and Mr1-Mr4, and two flying capacitors C_f1 and C_f2 for voltage conversion. The additional circuitries Mal, Mar, Ral and Rar are used to reduce both the shoot-through current and switching noise of the converter, while the increase in area due to the additional circuitries is insignificant, as disclosed in H. Lee and P. K. T. Mok, “Switching Noise Reduction Techniques for Switched-Capacitor Voltage Doubler,” Proceedings of the IEEE 2003 Custom Integrated Circuits Conference, pp. 693-696, 2003. With pseudo-continuous control, both Ml2 and Mr2 operate as regulation transistors (also referred to as regulation switches), while other power transistors function as switches, therefore no additional power transistors are needed to continuously regulate the output. The converter according to the present invention is thus more area efficient than previous models. In preferred embodiments, the regulation switches are bipolar transistors or MOSFET transistors. The regulation switches may also function as variable resistors or gain stages. The principle of operation can best be described with reference to the timing diagram in FIG. 7. The control mechanism operates in non-overlapping clock phases Φ₁ and Φ₂ alternately. When Φ₂=V_DD, both power transistors Mr2, Mr4 are off and Mr1, Mr3 are on, so the voltage across C_f2 is increased to ˜V_DD in the charging phase. At the same time, Ml2 and Ml4 are on and C_f1 is in the discharging phase. Since both switches S_L1 and S_L2 are off, an appropriate dropout voltage (V_DO) is regulated across Ml2 by gain stages A₁ and A_L2 in the controller. The dropout voltage stacks on V_DD that is stored in C_f1 in the previous half-clock period to provide the desired regulated output voltage V_oreg, where V_oreg≈V_DO+V_DD=(1+R_f1/R_f2)V_ref. In the next half-clock cycle of Φ₁=V_DD, the operating phases C_f1 and C_f2 are swapped, so V_oreg is maintained by regulating Mr2 using gain stages A₁ and A_R2. The above actions repeat in every clock period, so continuous-output regulation is achieved as illustrated by the waveform of V_o. Since both power transistors Ml2 and Mr2 are regulated alternately corresponding to the discharging phase of C_f1 and C_f2, respectively, the proposed mechanism operates at pseudo-continuous mode. This pseudo-continuous control can also be used in other types of switched-capacitor dc-dc converters with different cross-coupled power stages.

[0032] V_o(1) and V_o(2) in FIG. 8 illustrate the output ripple in different load currents. Initially, the ripple increases with the load current. Due to continuous-output regulation, when the ripple amplitude reaches the limit defined by the load regulation, it stays constant with respect to the load-current change. Both upper and lower limits of the output are maintained at particular levels by varying V_DO. Low ripple can thus be attained, provided that the converter has a large low-frequency loop-gain magnitude. Moreover, the load-transient response of the a pseudo-continuous control converter is independent of a wide range of switching frequencies. The converter can achieve a fast load-transient response by widening the loop-gain bandwidth and high light-load power efficiency by using low switching frequency simultaneously.

[0033] To implement the pseudo-continuous control, gain stages in the controller cascaded with power transistors Ml2 and Mr2 result in a three-stage switchable opamp. The three-stage switchable opamp can increase the low-frequency loop-gain magnitude and improve the load regulation in low-voltage condition. Both switchable second gain stages regulate the output or turn off alternately in every half-clock period to save the static current dissipation in the controller.

[0034] The stability and loop-gain bandwidth in low-power condition are determined by the frequency compensation of the three-stage switchable opamp. One implementation of the pseudo-continuous control switched-capacitor regulator of the embodiment of FIG. 6 is shown in FIG. 9. Time-multiplexed enhanced active-feedback frequency compensation is employed in the pseudo-continuous control converter. The enhanced active-feedback frequency compensation consists of a damping-factor-control block and time-multiplexed active-capacitive feedback networks. Two compensation capacitors C_al and C_ar, with the positive gain stage A_a, establish an active-capacitive feedback network in every alternate half-clock period. The damping-factor-control block is located at the output of the first gain stage in order to eliminate the use of Miller capacitor. The enhanced active-feedback frequency compensation can increase the loop-gain bandwidth in low-power condition; thereby improving the load-transient response of the converter in low switching frequencies. The time-multiplexed dynamic feedforward stage generates extra dynamic current to the output of either second gain stages A_L2 or A_R2, according to clock signals Φ₁ and Φ₂ in order to realize a push-pull effect at the gates of power transistors Ml2 and Mr2. The push-pull effect relaxes the slew-rate limitation of driving Ml2 and Mr2 in low-power condition. As both Ml2 and Mr2 can be turned on and off much faster during switching, the load-transient response is further improved.

[0035] In preferred embodiments, the DC voltage gains of the first gain stage and the second switchable gain stages are greater than 1.

[0036] FIGS. 10(a) and 10(b) illustrate two alternative methods of implementing three-stage switchable opamps with time-multiplexed pole-splitting compensation network using serial complementary switches after second gain stages A_L2 and A_R2.

[0037] The embodiment of FIG. 6 has been implemented by way of an example in a 0.6 μm CMOS n-well process. The maximum output voltage is regulated at (2V_in−0.3)V. FIG. 11 demonstrates that the output ripple is maintained at 20 mV when the load current increases from 50 mA to 150 mA. This justifies the continuous-output regulation provided by the proposed pseudo-continuous control. FIG. 12 shows that the same output load-transient recovery time of ˜25 μs is achieved with a load change of 100 mA in 1 μs for the switching frequency varying from 200 kHz to 500 kHz. Fast load-transient response is thus realized across a wide range of switching frequencies by using the described three-stage switchable opamp.

[0038] The above describes some preferred embodiments of the present invention and indicates several possible modifications but it will be appreciated by those skilled in the art that other modifications can be made without departing from the scope of the invention.