High-efficiency, low noise, inductorless step-down DC/DC converter

[0002] The present invention relates to switching regulator circuits, and more particularly, to circuits and methods for maintaining high efficiency and low noise over broad current ranges in an inductorless, step-down, switching regulator circuit.

[0003] There are three important trends affecting the electronics industry: device miniaturization, declining supply voltages, and an increasing use of battery-power. These trends place great demands on power supply circuitry in miniature, battery powered, electronic devices, such as cellular telephones and personal digital assistants (PDA's). Miniaturization places limits on circuit size; a reduced supply voltage places stringent requirements on reducing supply ripple to provide adequate noise immunity for integrated circuits; and reliance on battery-power drives a need for high efficiency for prolonged battery life.

[0004] The purpose of a voltage regulator is to provide a predetermined and constant output voltage to a load from a poorly-specified or fluctuating input voltage source. Series regulators and switching regulators are two common types of voltage regulators. Low drop out (“LDO”) series regulators provide good regulation with very low noise, however, the current supply from the regulated output comes directly from the voltage source. Thus, the best efficiency possible from a LDO regulator is the ratio of the output voltage to the supply voltage which drops rapidly for supply voltages much larger than the output voltage.

[0005] Switching regulators are generally more efficient than series regulators. A switching regulator employs one or more switches (e.g., a power transistor) coupled either in series and/or parallel with the load. A control circuit turns the switches ON and OFF to transmit power to the output in discrete current pulses. An energy storage element, such as an inductor or capacitor, is used to convert the switched current pulses into a steady load current. Because inductors tend to be large components, switched capacitor converters are preferred in miniaturized devices.

[0006] In a conventional switched capacitor regulator, a capacitor is charged from the input voltage during a first part of a switch cycle and the charge is transferred to the output during a second part of the switch cycle. This cycle by cycle transfer of charge to the output produces ripple on the output. Furthermore, because charge is transferred during only a portion of a switch cycle, the regulator must be designed to supply much more charge per cycle than is typically required at the output. This can result in output ripple becoming unacceptably large under certain conditions. A couple hundred mV of ripple is not uncommon.

[0007] A feedback loop may be provided to regulate the output voltage by controlling operation of the switches to regulate the rate at which charge is transferred to the output. In a first type of switched capacitor regulator, the duration of a portion of the switching cycle is kept constant while the duration of the remaining portion of the switching cycle is changed. For example, the length of time during which charge is transferred to the output may be kept constant, while the length of time during which the capacitor is charged from the input is varied. This has the side effect of causing the frequency of a switching cycle to vary as a function of the output current. Because output ripple frequency is determined by the switching frequency, it too varies as a function of the output current making filtering the ripple more difficult.

[0008] In an alternative type of switched capacitor regulator, the duty cycle of a switching cycle is changed without changing the switching frequency. Constant frequency switched capacitor regulators generally provide lower output noise than variable frequency switching regulators by transferring only the amount of current necessary to keep the output in regulation on a cycle by cycle basis.

[0009] When a constant frequency switched regulator is supplying close to its rated output current, the efficiency of the overall circuit can be high. However, the efficiency is a function of output current and typically decreases at low output current due to the losses associated with operating the switching regulator. These losses include, among others, quiescent current losses in the control circuitry of the regulator, switch losses, switch driver current losses and the like.

[0010] It would therefore be desirable to provide DC/DC converter circuitry having high-efficiency.

[0011] It would also be desirable to provide DC/DC converter circuitry having low output voltage ripple.

[0012] It would also be desirable to provide DC/DC converter circuitry having an output voltage ripple with a substantially constant frequency.

[0013] In addition, it would be desirable to maintain high efficiency over broad current ranges, including low output currents, in a switching regulator circuit.

[0014] It is therefore an object of the present invention to provide DC/DC converter circuitry having high-efficiency.

[0015] It is also an object of the present invention to provide DC/DC converter circuitry having low output voltage ripple.

[0016] It is also an object of the present invention to provide DC/DC converter circuitry having an output voltage ripple with a substantially constant frequency.

[0017] In addition, it is an object of the present invention to maintain high efficiency over broad current ranges, including low output currents, in a switching regulator circuit.

[0018] These and other objects and advantages of the invention are provided by a switched capacitor converter having a number of switches and capacitors. The converter is operable in different modes to provide different step-down conversion ratios. Mode control circuitry selects the step-down conversion ratio to optimize efficiency as input voltage and load conditions vary.

[0019] The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

[0020] FIG. 1 is a simplified block diagram of a switching regulator circuit of the present invention;

[0021] FIG. 2 is a simplified schematic of the step-down charge pump and current control circuitry of FIG. 1; and

[0022] FIG. 3 is a simplified schematic of the switch/mode control circuitry of FIG. 1.

[0023] Referring first to FIG. 1, circuit 10 accepts an input voltage, V_IN, at terminal 12 and provides a regulated DC output voltage, V_OUT, at terminal 14 for driving load 16. For example, load 16 may be cellular telephone circuitry requiring a 1.5 volt regulated supply and V_INmay comprise a 3.6 volt battery. Circuit 10 includes step-down charge pump 18, mode control circuit 20, and current control circuitry 22. In addition, circuit 10 includes resistors 26aand 26b,voltage reference 27, and amplifiers 24 and 28 to provide feedback to mode control circuitry 20 and current control circuitry 22

[0024] To reduce ripple in the output voltage, an output capacitor 15 is often connected to output terminal 14. Typically, capacitor 15 has a large value to keep output ripple low. Resistor 17 represents the effective series resistance of capacitor 15. Although not shown in FIG. 1, additional resistors or capacitors may be used to further filter the output voltage and reduce ripple. As is known in the art, the values of these components must be considered when designing a feedback loop for the regulator circuit of FIG. 1.

[0025] Typically, a switched capacitor regulator is constructed using an integrated circuit controller with a few external components. The LT1054 Switched Capacitor Voltage Converter with Regulator, sold by Linear Technology Corporation, Milpitas, Calif., is such an integrated circuit controller for a switched capacitor converter and regulator. With the addition of external components, such as capacitors, resistors, diodes, or transistors, the LT1054 may be used to create many different types of voltage converters. For example, the data sheet for the LT1054, which is incorporated herein by this reference, includes sample circuits for voltage doublers, voltage inverters, voltage converters, and a digitally programmable voltage supply.

[0026] Referring now to FIG. 2, step-down charge pump 18 comprises a number of capacitors and switches coupled between input terminal 12 and output terminal 14. The switches are operated in a predetermined sequence to transfer charge from a voltage source at input terminal 12 to the capacitors and then from the capacitors to the output at output terminal 14. In accordance with the principles of the present invention, the switches are operated so as to control the amount of charge transferred from input terminal 12 to output terminal 14 and thereby regulate the output voltage.

[0027] Exemplary step-down charge pump 18 includes capacitors 30aand 30band switches S1-S7. Preferably, switches S1-S7 are MOSFET transistors although other types of switches may be used. Also shown in FIG. 2 are MOSFETs 40aand 40bconfigured as an inverter 41 and MOSFETs 42aand 42bconfigured as a current mirror 43.

[0028] In accordance with the principles of the present invention, step-down charge pump 18 may be operated in different modes, wherein a mode of operation is selected to improve the efficiency of the switched capacitor regulator.

[0029] In a first mode of operation, referred to herein as a “1-to-1”mode, switches S2-S7 are kept open, and switch S1 is closed. A clock signal, CLK, is applied to the gates of MOSFETs 40aand 40b. When the clock signal is HIGH, MOSFET 40ais turned OFF and MOSFET 40bis turned ON. Transconductance amplifier 45 sinks current from V_INat input terminal 12 through MOSFETs 42aand 40b. Because MOSFETS 42aand 42bare configured as current mirror 43, MOSFET 42bconducts a current approximately equal to the current through MOSFET 42amultiplied by the current mirror gain factor. This current is supplied via switch S1 and output terminal 14 to the output, including output capacitor 15 and load 16.

[0030] When the clock signal, CLK, is LOW, MOSFET 40aturns ON and MOSFET 40bturns OFF, causing MOSFETs 42aand 42bto turn OFF. As a result, no current flows through MOSFET 42aor 42b. Load current is supplied from output capacitor 15.

[0031] A voltage divider formed from resistors 46aand 46bprovides a voltage at node 32 proportional to the output voltage V_OUT. The output of transconductance amplifier 45 is a function of the difference between the voltage at node 32 and the reference voltage, V_REF. When the voltage at node 32 is less than V_REF, indicating that the output voltage is low, transconductance amplifier 45 sinks more current from MOSFETS 40band 42awhen clock signal CLK is LOW. Because of the current mirror configuration of MOSFETS 42aand 42b, MOSFET 42bprovides increased current to the output, thereby raising the output voltage, V_OUT. Conversely, when the voltage at node 32 is higher than V_REF, transconductance amplifier 45 sinks less current when the clock signal is LOW and MOSFET 42bprovides reduced current to the output, thereby lowering V_OUT.

[0032] When regulating, the voltage regulator of the present invention operates so that the average current supplied by current mirror 43 is equal to the average output current. In this mode of operation V_OUTis less than V_INby the voltage drops across switch S1 and MOSFET 42bdue to the output current. The regulator has an effective output impedance given by: $\begin{array}{cc}{R}_{\mathrm{out}}=\mathrm{GM}\times N\times \frac{R_{46b}}{R_{46a}+R_{46b}}\times \mathrm{DC}& \left(1\right)\end{array}$

[0033] where:

[0034] GM is the gain of transconductance amplifier 45,

[0035] N is the mirror gain factor for current mirror 43,

[0036] R46aand R46bare the values of resistors 46aand 46b, respectively, and

[0037] DC is the duty cycle of the clock signal.

[0038] The mode of circuit operation just described is referred to as 1-to-1 mode because the ratio of average input current to average output current is about 1 to 1 when the output voltage is being regulated. In a second mode of operation, step-down charge pump 18 operates in a “3-to-2” mode wherein the average input current is approximately two-thirds (⅔) the average output current. In the 3-to-2 mode, switches S1-S7 are controlled so that switches S1, S3, and S6 are ON and switches S2, S4, and S7 are OFF when the clock signal (CLK) is LOW. Conversely, switches S1, S3, and S6 are OFF and switches S2, S4, and S7 are ON when the clock signal is HIGH. Switch S5 remains OFF.

[0039] When CLK is HIGH, the current output of amplifier 45 is amplified by current mirror 43 which supplies current to V_OUT. Current mirror 43 also supplies current to capacitors 30aand 30bwhich are effectively connected in parallel between MOSFET 42band output terminal 14. When CLK goes LOW, capacitors 30aand 30bare connected in series between output terminal 14 and ground, and the charge stored on the capacitors is transferred to the output. Because current flows during both halves of a CLK cycle the efficiency of step-down converter 18 is improved.

[0040] In the 3-to-2 mode of operations, when the output voltage is being regulated, the average current through MOSFET 42bequals two-thirds (⅔) the average output current. Because the average input current is less than the average input current in 1-to-1 mode, the efficiency of the regulator is improved when the voltage regulator can be operated in the 3-to-2 mode. Losses in MOSFET 42band switches S1-S7, and the required load current, determine how low V_INmay be with respect to V_OUTwhile maintaining adequate voltage regulation in 3-to-2mode. However, when V_INis greater than about 1.5×V_OUTthe circuitry of FIG. 2 may be operated in 3-to-2 mode for improved efficiency.

[0041] The effective output impedance of switched capacitor step-down charge pump 18 in the 3-to-2 mode of operation is given by: $\begin{array}{cc}{R}_{\mathrm{out}}=\frac{3}{4}\times \mathrm{GM}\times N\times \frac{R_{46b}}{R_{46a}+R_{46b}}& \left(2\right)\end{array}$

[0042] To achieve a further improvement in efficiency, switched capacitor step-down charge pump 18 may also be operated in a “2-to-1” mode when V_INis greater than about twice V_OUT. In the 2-to-1 mode, switches S1 and S5 are ON and switch S2 is OFF when CLK is LOW, and switches S1 and S5 are OFF and switch S2 is ON when CLK is HIGH. Switches S3-S4, and S6-S7 are kept off. When CLK is LOW, current flows from V_INto V_OUTthrough MOSFET 42band switch S1. Current is also provided to charge capacitor 30a. When CLK goes high, the charge stored on capacitor 30ais transferred to V_OUTat output terminal 14. The effective output impedance of switched capacitor step-down charge pump 18 in the 2-to-1 mode of operation is given by: $\begin{array}{cc}{R}_{\mathrm{out}}=\mathrm{GM}\times N\times \frac{R_{46b}}{R_{46a}+R_{46b}}& \left(3\right)\end{array}$

[0043] A regulator according to the principles of the present invention is operable in a plurality of modes, e.g., 1-to-1 mode, 3-to-2 mode, or 2-to-1 mode, to provide improved operating efficiency over a wide range of input voltages. The optimum mode of operation is determined by the ratio of the input and output voltages, e.g., V_INto V_OUT, as well as, by switch losses and the actual output current. In one embodiment of a regulator in accordance with the principles of the present invention, a specific mode of operation may be preselected and fixed by a circuit designer by appropriately biasing configuration pins on a regulator controller integrated circuit. However, in a preferred embodiment of the present invention, the optimum mode of operation is automatically determined by switch/mode control circuitry 20 of FIG. 1. FIG. 3 is a schematic representation of an illustrative implementation of switch/mode control circuit 20.

[0044] Switch/mode control circuitry 20 includes resistors 50a-50cand comparators 52aand 52b. Resistors 50a-50cform a voltage divider network that provides voltage signals for determining the most efficient mode of operation of regulator circuit 10. The voltage signals are provided to inputs of corresponding comparators 52aand 52b. The other input to comparators 52aand 52bis provided by reference voltage, V_REFin series with an offset voltage 51 (V_OFF) that is proportional to the output current load. Resistors 50a-care chosen so that one or both of comparators 52aand 52bwill be on at the appropriate voltages on input 12. Specifically, both comparators are off when $V_{\mathrm{IN}}<\left(V_{\mathrm{REF}}+V_{\mathrm{OFF}}\right)\times \left(1+\frac{R_{50a}}{R_{50b}+R_{50c}}\right)$

[0045] which corresponds to the conditions under which the regulator should be operated in 1-to-1 mode. When $V_{\mathrm{IN}}\ge \left(V_{\mathrm{REF}}+V_{\mathrm{OFF}}\right)\times \left(1+\frac{R_{50a}+R_{50b}}{R_{50c}}\right)$

[0046] both comparators are on; which corresponds to the conditions in which the regulator should be operated in 2-to-1 mode. For intermediate input voltages, comparator 52ais off and comparator 52bis on, indicating that a 3-to-2 mode of operation would be most efficient.

[0047] However, as described previously, the optimal mode of operation is dependent not only on the input and output voltages, but also on voltage drops caused by the on resistance of switches S1 through S7 and the average load current. This variation is taken into account by offset voltage 51 (V_OFF) which is a function of the current feedback signal. The function is set to optimize the mode switching point for any given load condition. As the load current increases, the current feedback signal and the offset voltage also increase. This raises the input voltage at which the operational mode of regulator 10 changes from 1-to-1 mode, to 3-to-2 mode, and then to 2-to-1 mode.

[0048] Exemplary switching logic circuitry 54 combines the mode signals provided by comparators 52aand 52bwith timing signals, CLKA and CLKB, to operate switches S1-S7 according to the proper mode of operation. For example, when comparators 52aand 52bare both off, switching logic circuitry 54 provides a signal turning switch S1 ON. This corresponds to the 1-to-1 mode of operation. Otherwise, if one or both of comparators 52aand 52bare OFF, switch S1 is driven by the CLKB signal. Switches S2-S7 are controlled by switching logic circuitry 54 in an analogous manner.

[0049] Non-overlap clock generator 55 produces the clock signals CLKA and CLKB, such that there is a “blanking”period from one signal going low to the other signal going high. A blanking period is desirable to ensure that operation of switches S1-S7 do not result in shorting the switching nodes together, such as the output voltage, V_OUT, to ground. There are many means of designing a non-overlap clock generator, such as non-overlap clock generator 55, to ensure that all switches are turned off prior to turning any switches on, as one skilled in the art would know. Preferably, the CLKA signal is also used as the CLK signal of FIG. 2.

[0050] Connected to the reset signal of the non-overlap clock generator 55, is comparator 56. When the output of comparator 56 is high, the non-overlap clock generator operates as described previously. When the output of comparator 56 is low, the clock generator is forced into a static state, where the signal CLKA is held low. If signal CLKA is held low, no switching of switches S1-S7 takes place, which stops the charge transferred from the input 12 to the output 14. By stopping the switching and shutting down portions of the regulator circuit, efficiency at low output currents can be significantly improved. Comparator 56 goes low when the current feedback offset voltage 51 drops below a threshold voltage 57, signaling a light output load, which disables the switching as described above. As the output load 16 discharges capacitor 15, the output will drop, and the current feedback voltage will rise eventually causing the output of comparator 56 to return high, enabling the clock circuit 55 and charge transfer. This is similar to the operation of a conventional regulator, but unlike a conventional regulator, the amount charge transferred when coming out of burst mode is set by the threshold of comparator 56, effectively limiting the amount of charge transferred, which in turn limits the size of the output ripple voltage. Thus, a means for improved efficiency at light loads is achieved without producing excessive output ripple.

[0051] Additionally there are many means for providing output short circuit or over current protection. One simple means is to limit the amount of current the transconductance amplifier 45 can sink. By limiting of the amount of current sunk by transconductance amplifier 45, the current out of mirror transistor 42bis limited which in turn limits the effective output current. There are many means by which this may be accomplished as one skilled in the art would know.

[0052] Thus, an inductorless DC/DC regulator circuit and a method for maintaining high efficiency over broad current ranges has been provided. One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.