Audio amplifier circuit and electronic apparatus including the same

An electronic audio amplifier circuit, operable in two modes and a battery powered portable audio apparatus incorporating the circuit; and associated apparatuses and methods. When in the first mode the audio apparatus is designed for the direct driving of headphones or a speaker. When in the second mode the audio apparatus is designed to drive a line input of an external amplifying apparatus with a signal that having a higher voltage amplitude for a given signal content than when driving headphone or a speaker. The circuit has common output stage circuitry for use in both modes, and a dual mode power supply circuit, ideally a charge pump circuit, for supplying the output stage in the first mode with a lower supply voltage than in the second mode.

The invention relates to audio amplifier circuits and electronic apparatus including such circuits. The invention has particular application in portable and battery powered audio apparatus having outputs for headphone or speaker and line level signals.

Today portable electronic audio apparatus such as MP3 players, radios and telephones with such functions built in are very popular. Generally they are usable in two modes, one mode when driving directly headphone or miniature loudspeakers and another mode when docked or connected by wire to drive the a ‘line’ input of higher power amplifying apparatus. In order for such devices to be miniaturised and exhibit high quality output at reasonable cost, the headphone driving is preferably done from a dual or split rail supply, generated on board the driver chip. This avoids the need for large DC blocking capacitors.

Dual rail supplies can be generated quite readily on board the amplifier chip by use of inverting charge pumps and/or DC-DC converters. Charge pumps are particularly advantageous because the size and cost of capacitors used is much less than that of the inductors and capacitors used in other types of DC-DC converter. However, the span of a dual rail supply generated by known charge pumps is generally twice the input voltage, meaning that the supply voltage to the output stage is generally much greater than the voltage amplitude of the output signal required to drive the low-impedance headphone or speaker. Particularly when using a high-fidelity linear output stage, the ‘headroom’ between the output level and the supply voltage translates directly into heat dissipation and wastage of battery power within the output stage.

When driving a line input, a higher load impedance is found, and optimum quality demands a relatively high voltage level in the output audio signal. As a result of these competing requirements, premium products in this market have adopted the practice of providing separate output stages, one used for line out situations via the docking connector and the other used for driving the headphone or similar load.

The invention provides a battery powered portable audio apparatus including an electronic audio amplifier circuit operable in two modes:a first mode for the direct driving of an audio output transducer anda second mode to drive a line input of an external amplifying apparatus, an output signal to be provided to the line input in the second mode having a higher voltage amplitude for a given signal content than an output signal driving the audio output transducer;wherein the circuit comprises common output stage circuitry for use in both modes, and a dual mode power supply circuit for supplying the output stage in the first mode with a lower supply voltage than in the second mode.

The amplifier circuit or electronic apparatus within which it is used may have a common output terminal for connection to the different loads. Alternatively or in addition, a dedicated output terminal may be provided for line level signals, as in the case of a docking terminal.

In a preferred embodiment, the dual mode power supply circuitry includes a dual mode charge pump operable in (at least) the first mode to divide an input supply voltage to provide a dual polarity supply to the output stage, the dual polarity supply spanning in total only the same or less than the input supply voltage, and in the second mode to provide a dual polarity supply to the output stage which spans greater than (for example twice) the input supply voltage.

The circuit may include means for limiting current at the signal output when operating in the second mode, so as to avoid over-driving any low impedance load (such as headphones) connected when a line input is expected.

The controller may be adapted to detect automatically the type of load connected and to select the first or second mode of operation automatically.

The detection may be made implicitly by reference to a volume setting input of the amplifier, for example when the first mode being selected when the volume is at a maximum.

The invention further provides audio apparatus including an amplifier circuit according to the invention as set forth above.

The audio apparatus may be in portable form.

The invention further provides communications apparatus (such as a portable phone) incorporating audio apparatus as set forth above.

The invention further provides headphone driving apparatus incorporating audio apparatus according to the preceding paragraphs, and a headphone jack connected to the output stage at least in the first mode. The headphone apparatus may include a separate output connector for outputting line level signals in the second mode.

The amplifier circuit may be operable to drive line level signals through the headphone jack in the second mode.

The invention further provides for a method of, on occasion, direct driving an audio output transducer having a relatively low impedance with signals in a first range of voltages, while, on separate occasions, driving a higher impedance line input of an external amplifying apparatus with signals in a higher range of amplitudes for a given signal content, using common output stage circuitry, the method comprising using a dual mode power supply to supply the common output stage circuitry in first and second modes, such that a lower supply voltage is supplied in the first mode, when the audio output transducer is being driven, than in the second mode when the higher impedance line input of an external amplifying apparatus is being driven.

The invention further provides for an electronic audio amplifier circuit operable in two modes:a first mode for the direct driving of an audio output transducer anda second mode to drive a line input of an external amplifying apparatus, an output signal to be provided to the line input in the second mode having a higher voltage amplitude for a given signal content than an output signal driving the audio output transducer;wherein the circuit comprises:common output stage circuitry for use in both modes, and a dual mode power supply circuit for supplying the output stage in the first mode with a lower supply voltage than in the second mode; andmeans for limiting the circuit's output current when operating in the second mode, such that maximum power output in the second mode is less than that in the first mode, in spite of the higher maximum voltage amplitude.

These and other features of the audio apparatus or the amplifier circuit, charge pump circuits and their applications in different electronic apparatus will be understood from a consideration of the detailed description of embodiments which follows. Other features are as described in the appended claims.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1ashows a portable electronic audio apparatus1in a first mode of operation, connected to drive a pair of headphones2. Apparatus1in the illustration is an MP3 player, but the same functions may be integrated into phones and multimedia players, laptop computers, PDAs and the like. Headphones2are connected to the apparatus via a lead3plugged into output jack4. The body of the apparatus may also include one or more miniature loudspeakers (not shown), which can be driven as an alternative audio output transducer, but are in principle equivalent to the headphones for the purpose of this description. As is well known, small size and weight, together with maximum battery life, are key attributes of premium products in this market. Manufacturing cost is an important factor across the market.

FIG. 1bshows the same apparatus1in a second mode of operation, where no headphones are connected. Instead, the apparatus is connected via a separate connector to a docking station5, which in turn drives larger loudspeakers6L,6R. Docking station5incorporates a higher powered amplifier than the portable device itself, and a separate power supply, usually mains-powered.FIG. 1cshows the same apparatus in a variation of the second mode, in which an external amplifier7and loudspeakers8L,8R are connected via a line lead9plugged into the headphone jack4in place of the headphones.

The apparatus1is required to drive very different loads in these modes. A headphone or loudspeaker will typically have an impedance of 32 ohms or less. Into this load, an output amplitude of, say 100 mV RMS will provide a moderate volume, (say −12 dB) from full scale, corresponding to a power 30 mW. When driving the line input of a larger amplifier in the second mode, the load impedance is typically 10 kilohms or more, and a full scale signal amplitude (0 dB) of 2V RMS is appropriate for maximum quality. If the output stage driving the socket in Mode1is capable of providing a 2V RMS signal, its supply voltage must be greater than 2V. When driving a headphone load in Mode1from the same output stage, most of the output stage power consumption is dissipated in the form of heat, as the supply voltage is dropped to the 100 mV level in the transistors of the output stage. If, to increase battery life, the designer opts not to provide the full 2V RMS for line output situations, a poorer signal is the result.

As a result of these competing requirements, premium products in this market have adopted the practice of providing separate output stages, one used for driving headphone/speaker loads2, and another for line out situations via the docking connector4. Each output stage can be driven from a power supply appropriate to the voltage range, maintaining power efficiency and quality in each application. Needless to say, the requirement for separate output stages and separate power supplies for them increases the size and cost of the apparatus undesirably.

FIG. 2shows in block schematic form novel output circuitry for an audio apparatus of the type shown inFIGS. 1a-1c. Here, output stage supply voltages Vout+ and Vout− are generated by a dual mode charge pump (DMCP) circuit10. DMCP10is supplied from a single rail supply voltage VDD, which may be a battery voltage or some intermediate supply generated on board the apparatus. It has different modes of operation which can provide different supply voltages, enabling a common output stage to be used in both headphone and line output modes of use. Novel examples of how DMCP10can be implemented in a very compact and cost-effective way are provided later with reference toFIG. 4onward. In the description that follows, labels VDD, Vout+ etc. are used to refer to either the respective terminals or the voltage at that terminal, according to context.

Referring toFIG. 2, two channels of audio signal processing and output are depicted, with suffices ‘L’ and ‘R’ representing left and right channels of stereo audio. The supply voltage VDD is illustrated as supplying processing circuitry20in each channel. The input signal S1maybe an analogue signal or a digital signal. In the case where S1is an analogue signal then the processing circuitry20will be purely analogue type circuitry such as op-amps, multiplexers, gain blocks etc. In the case where S1is a digital signal and the output stage is analogue, then the processing circuitry20may be a mixture of digital and analogue circuitry where signal S1is fed, either directly or through some digital signal processing, into a DAC (not illustrated) and the output of the DAC is then fed into the analogue circuitry as mentioned above. The processing circuitry may include a volume function, controlled by a volume control22as illustrated. Voltage supplies and control circuits are shown as being common to both channels, although they could be duplicated also if desired (for example to reduce cross-talk).

The processing circuitry20outputs a processed signal S2that in this particular embodiment is an analogue signal that is passed into a level shifter30that may be implemented by a simple DC-blocking capacitor for example. The level shifted signal S2′ is fed into the output amplifier40which outputs an amplified output signal S3to the headphone jack or docking connector. Two alternative loads are illustrated being driven by the amplifier, the headphone or speaker transducer50or a line input50′

The output amplifier40could in principle be a switching (Class D or PWM) amplifier, or a 1-bit digital (sigma-delta) type output stage, in which case the signals S1, S2may be digital in form right through to input to output, or may begin in analogue form and be converted to digital form in the processing circuit20. The present embodiment assumes a linear amplifier, however.

The input signal S1, if analogue, and analogue signals in the processing circuitry20, will normally be referenced midway between ground potential and VDD, whereas the level shifted signal S2′ is referenced about ground, as required by the output amplifier operating from the split rail supply Vout+, Vout−. Operating from a split-rail supply avoids the need for a DC-blocking capacitor between the output amplifier and the headphone jack. This is important in the trade-of between performance, compactness and cost in the product, because a large capacitor is needed if low frequencies are not to be impaired, and for the output to be tolerant of a wide range of headphone impedances.

In the example of the portable apparatus shown inFIGS. 1a-1c, two modes (or more) can be defined: a Mode1may be used to drive a low impedance load such as headphones3while Mode2may be used to drive a high impedance load such as a line input. In the case where a docking connector, separate from headphone jack4, is used in Mode2(FIG. 1b), the control circuit can decide directly which mode applies, from a general signal indicating docked status. As mentioned already, performance, size and cost are conflicting requirements when driving headphones at one time and line inputs at another. Rather than provide separate output stages for these modes, the present application proposes to use common output stage components in both modes, but switching to different power voltages. To enable this, the supplies Vout+ and Vout− are generated differently by the Dual Mode Charge Pump10in each mode, preferably using novel charge pump arrangements as described below.

For Mode1(headphone) operation, the output stage supply voltages Vout+ and Vout− are substantially +VDD/2 and −VDD/2. In this way, the wasteful voltage drop across the output stage transistors when driving the load at low voltage is greatly reduced, and battery life and cooling requirements relaxed substantially. For Mode2operation (line output), the DMCP can generate for example +VDD and −VDD supplies, permitting the output amplifier to drive a full scale signal of greater than 1V RMS.

The mode of operation of DMCP10is determined by control circuit24. Where the same jack4is used in Mode2as in Mode1, mode selection may be determined in several ways. As a first option, a switch or menu option may be available to the user to select explicitly between sound and line output modes. The user setting may alternatively be implicit in the act of turning the volume control22to full scale, on the assumption that headphones will not be used at the maximum level in practice. Alternatively it may be possible to sense the by automatically sensing the output impedance or output current supply or even jack socket versus docking station operation in the case of a portable audio device. These options are indicated by dashed lines in the drawing.

In the case of using the volume control to “Mode select”, setting the charge pump to Mode2could in principle over-drive the headphones, damaging them and/or damaging the hearing of the user. This risk is mitigated by the fact that, in practice, should the volume be set high under normal circumstances, the output supply voltages will collapse due to the fact that the load's power requirements are greater than that for which the DMCP10has been designed. Nevertheless, specific safeguards in the form of extra circuitry (not illustrated), can be put in place to monitor for such a situation so as to disable the DMCP10or another part(s) of the circuitry. Alternatively, the effective output resistance of the output amplifier40or its voltage supply can be increased under control of the mode selection signal, thereby to limit the current deliverable at the outputs. An additional 100 ohms resistance will not be significant when placed in series with 10 kilohms or more at a line input, while it will significantly limit the current that can be put through a 32 ohm load.

For cost and size reasons, it is important to be able to integrate the functions of an MP3 player, mobile phone or any other application into a small number of integrated circuits. Therefore it is advantageous to integrate the circuitry for supply voltage generation, in this case the DMCP10, together with the functional circuitry20,30,40etc. Generally speaking, the DMCP10includes capacitors which cannot realistically be integrated and have to be located off-chip, with consequences for chip-pin-count and overall circuit size. The DMCP examples to be described use novel charge pump circuits to minimise size and/or maximise performance, and also to divide the input voltage into half or even smaller fractions, reducing power dissipation in Mode1particularly.

Examples of Dual Mode Charge Pump construction and operation will now be described.

FIG. 3illustrates, for background purposes, a prior art inverting charge-pump (ICP) circuit100which generates a negative output voltage (Vout−) from a positive input voltage (+VDD). In ideal conditions Vout− will substantially equal −VDD thus resulting in a total voltage across the nodes N1-N2of 2×VDD. The circuit100comprises three capacitors, one flying capacitor Cf and two reservoir capacitors CR1, CR2, and a switch array110. Circuit100is controlled by a controller120which controls the switch array110thus causing circuit100to switch between two main states as explained below.

To generate the voltage Vout−, the controller operates the switch array110to repeat the following steps at a high frequency. Firstly, flying capacitor Cf is connected between the input voltage node N1and the common reference voltage node N3(as illustrated inFIG. 2b). Therefore the flying capacitor Cf charges up to voltage +VDD. Subsequently, flying capacitor Cf is connected in parallel with the negative reservoir capacitor CR2, that is its connected across the common reference voltage node N3and the output voltage node. Assuming capacitor CR2is initially charged to zero volts in this first cycle, capacitor CR2will share charge with capacitor Cf, to give an equal voltage across each capacitor. Since the positive plates of capacitors Cf and CR2are connected to the common reference voltage node N3(ground), node N2sees a voltage somewhat more positive than −VDD relative to node N3, depending on the respective sizes of Cf and CR2.

In each cycle, capacitor CR2will be further charged, eventually reaching a steady state after a plurality of 4-step cycles. By this time, capacitor CR2is already charged to (and therefore Vout−equals) substantially −VDD, and consequently Cf no longer adds any further significant charge so long as no current is drawn by the load. If a load is applied to Vout−, it will continuously discharge capacitor CR2. This charge is then replaced by charge from capacitor Cf, resulting in Vout− being somewhat more positive than −VDD. The average difference and voltage ripple will depend on the values of Cf, CR2, the switching frequency and the load characteristics. The frequency will be chosen to be far above the audio frequency band of the signals being processed, for example 100 kHz or more.

It should be noted that a closed-loop feedback control can be added to the charge pump, by which the output voltage Vout− can be regulated such that it is anywhere between approximately ground potential and −VDD. However, the charge pump itself is most efficient when the output voltage Vout− equals −VDD. In practice the target voltage will probably be set slightly above −VDD in order to reduce ripple.

The prior art charge pump circuits100can only generate output voltage −VDD, meaning that the rail-to-rail magnitude of the amplifier supply is greater than the input voltage (VDD−(−VDD)=2VDD). This can be disadvantageous in certain applications, as it may not allow the circuitry being supplied to run efficiently, for example when such an circuit is being used to power circuitry that amplifies a signal with a maximum amplitude much smaller than the amplifier circuitry's power supply +/−VDD.

FIG. 4aillustrates a novel dual mode charge-pump (DMCP) circuit400which comprises three capacitors—one flying capacitor Cf and two reservoir capacitors CR1, CR2—and a switch array410. DMCP400is a first example for a circuit suitable for use as the DMCP10in the apparatus ofFIGS. 1a-1cand2. Circuit400is controlled by a controller420which controls the switch array410thus causing circuit400to switch between various states to implement the different modes of operation, as explained below. Clock signals (not shown) are provided to the controller, which may be generated within DMCP400or shared with other circuitry on chip. The circuit400in operation uses flying capacitor Cf to transfer packets of charge from an input supply to the reservoir capacitors at high frequency, in such a way as to generate positive and negative output voltages (Vout+ & Vout−) from a positive input voltage (+VDD). The values of these output voltages depend on the mode selected. To aid explanation, various circuit nodes are labelled, including node N10connected to receive the input supply voltage VDD, node N11being a common (ground) node and nodes N12and N13forming the output terminals for Vout+ and Vout− respectively.

Connected to the outputs Vout+, Vout−, and N11(0V) is a load450. In reality this load450may be wholly or partly located on the same chip as the power supply, or alternatively it may be located off-chip. Example applications will be described with reference toFIGS. 23 to 25below.

As its name implies, DMCP400is operable in two main modes. All of these modes will be explained in more detail below. Naturally the principles of the dual mode circuit can be extended to multiple modes.

In the first main mode, referred to below as Mode1, the DMCP400operates such that, for an input voltage +VDD, the DMCP400generates outputs each of a magnitude which is a half of the input voltage VDD. In other words, the output voltages generated in this first mode are nominally of magnitude +VDD/2 and −VDD/2. When lightly loaded, these levels will, in reality, be +/−(VDD/2−Iload.Rload), where Iload equals the load current and Rload equals the load resistance. It should be noted that, in this case, the magnitude (VDD) of output voltage across nodes N12& N13is the same, or is substantially the same, as that of the input voltage (VDD) across nodes N10& N11, but shifted. This mode may therefore be referred to as a ‘level shifting’ mode. In the second main mode (Mode2) the DMCP400produces a dual rail output of +/−VDD.

This particular form of charge pump has significant advantages over known circuits, in particular because of the ability to generated a reduced, bipolar supply using only a single flying capacitor. Prior circuits for generating reduced output voltages requires additional flying capacitors. The flying capacitor and reservoir capacitors are often of a size that they need to be located off-chip, and so eliminating one capacitor and two IC pins is highly beneficial. The present invention not to be taken as being limited in its application to the particular form of DMCP illustrated here, however, and is potentially applicable in other multi-mode charge pump circuits whether they be known or, as yet, unknown.

FIG. 4bshows more internal detail of the DMCP100. Here it can be seen that the switch array410comprises six main switches S1-S6each controlled by corresponding control signal CS1-CS6from the switch control module420. The switches are arranged such that first switch S1is connected between the positive plate of the flying capacitor Cf and the input voltage source, the second switch S2is between the positive plate of the flying capacitor and first output node N12, the third switch S3is between the positive plate of the flying capacitor and common terminal N11, the fourth switch S4is between the negative plate of the flying capacitor and first output node N12, the fifth switch S5is between the negative plate of the flying capacitor and common terminal N11and the sixth switch S6is between the negative plate of the flying capacitor and second output node N13. Optionally, there may be provided a seventh switch S7(shown dotted), connected between the input voltage source (node N10) and first output node N12. These switches are the ones appropriate to the modes to be described. The provision of further switches to enable other modes of operation is of course not excluded.

It should be noted that the switches can be implemented in a number of different ways (for example, MOS transistor switches or MOS transmission gate switches) depending upon, for example, an integrated circuit's process technology or the input and output voltage requirements. The selection of appropriate implementations is well within the capability of the skilled reader.

Also shown in greater detail is the control module420which, at least notionally, comprises mode select circuit430for deciding which of two control functions420a,420bto use, thus determining which mode the DMCP operates in. The mode select circuit430and the controllers420a, etc. are notional blocks in that they represent different behaviours of the control module in implementing different operating modes of DMCP400. They can be implemented by separate circuits as just described. In practice, they are just as likely to be implemented by a single circuit block or sequencer with hardwired logic and/or sequencer code determining which behaviour is implemented at a given time. As also described below, where a given mode can be implemented in a range of variants, the designer may select variants which simplify the generation of the control signals, when all the different modes are considered together.

In a main operational embodiment of Mode1, there are three basic states of operation, repeated in high-frequency cycles of three phases, which may be referred to as P1, P2, P3. When DMCP400is operating in Mode1, switch S7, where present, is always open and is therefore not shown when describing this mode.

FIGS. 5aand5bshow the switch array410operating in a first state, “State1”. Referring toFIG. 5a, switches S1and S4are closed such that capacitors Cf and CR1are connected in series with each other and in parallel with the input voltage +VDD. Therefore, capacitors Cf and CR1share the input voltage +VDD that is applied across them.FIG. 5bshows an equivalent circuit for the state1operation with voltage +VDD effectively applied across nodes N10& N11.

It is preferable for applications that require symmetrical, but opposite polarity, output voltages, that the values of capacitors Cf and CR1are equal such that each capacitor Cf, CR1changes voltage by an equal increment when connected in series across a voltage source. If both capacitors are initially discharged, or indeed previously charged to any equal voltages, they will end up each with a voltage equal to half the applied voltage source, in this case one half of the input voltage VDD.

FIGS. 6aand6bshow the switch array410operating in a second state, “State2”. Referring toFIG. 6a, switches S3and S6are closed such that capacitors Cf and CR2are connected in parallel with each other and between nodes N11and N13. Therefore, the voltage across capacitor Cf equalises with that across capacitor CR2.FIG. 6bshows an equivalent circuit for this State2condition.

It should be noted that the value of reservoir capacitor CR2does not necessarily need to be the same as that of flying capacitor Cf. If capacitor CR2is much larger than capacitor Cf, it will require more cycles to charge up to or close to VDD/2. The value of reservoir capacitor CR2should be chosen depending upon expected load conditions and required operating frequency and output ripple tolerance.

Over a plurality of cycles alternating only States1and2, the voltages across the capacitors Cf and CR2would, under ideal conditions, converge to a voltage +/−VDD/2. However, the presence of a significant load on the charge pump's output terminals will result in a respective voltage droop in Vout+, Vout− away from +/−VDD. If the load is symmetric, and there is equal current magnitude on both Vout+ and Vout−, then the symmetry of the system will result in both outputs drooping by the same amount.

However, if, for example, there is a significant load on Vout+ but no load or a light load on Vout−, then the voltage across capacitor CR1will reduce. This will result in a larger voltage across capacitor Cf at the end of State1which will then be applied to capacitor CR2in State2. If only States1and2were used, the flying capacitor Cf would then be connected in series with capacitor CR1in State1but still having a larger voltage across it, even initially. Therefore, voltages Vout+ and Vout− will both tend to droop negatively, that is to say that the common mode is not controlled.

To avoid this effect, a third state, State3, is introduced, and States1to3are repeated in Phases1to3over successive cycles.FIGS. 7aand7bshow the switch array410operating in this State3operation. Referring toFIG. 7a, in State3, switches S2and S5are closed such that capacitors Cf and CR1are connected in parallel with each other and between nodes N11and N12. Therefore, both capacitors Cf and CR1become charged up to an equal voltage, despite any difference between of their previous voltages. In steady state (after many cycles) this becomes approximately VDD/2.FIG. 7bshows an equivalent circuit for this State3condition.

The circuit, therefore ends State3with equalised voltages, after which it returns to State1. Consequently the circuit will, in principle, enter Phase1of the next cycle in State1with Vout+=+VDD/2, depending upon load conditions and switching sequence.

In States2and3, the voltages across the various capacitors that are connected in parallel may not actually, in practice, completely equalise in a single sequence, particularly if the switching frequency is high, relative to the DMCP's R-C time constant. Rather, in each sequence of states a contribution of charge will be passed from capacitor to capacitor. This contribution will bring each output voltage to the desired level under zero, or low, load conditions. Under higher load conditions, the output reservoir capacitors CR1, CR2will typically achieve a lower voltage (with some ripple). The size of each of the capacitors needs simply to be designed such that the reduction of common mode drift is within acceptable bands, for all expected load conditions, Alternatively, or in addition, larger switches, with less on-resistance, could be employed.

FIG. 8illustrates the non-overlapping control signals (CS1-CS6) for controlling the switches (S1-S6) during the three states (1,2and3) of the main operational embodiment of Mode1. As discussed above, this represents only one example out of many possibilities for the controlling sequence.

It should be appreciated that the open-loop sequencing of the above three states does not necessarily need to be observed. For example the state sequences could be:1,2,3,1,2,3. . . (as described above); or1,3,2,1,3,2. . . ; or1,2,1,3,1,2,1,3. It should also be apparent that it is not necessary that the third state be used as often as the other two states, for instance a sequence of1,2,1,2,1,2,3,1. . . can be envisaged. It may even be envisaged to dispense with the third state altogether, albeit only in the case of well-balanced loads, or with alternative schemes for common-mode stabilisation.

Other switching and sequencing scenarios exist. For example, in one alternative operational Mode1embodiment: State1could be replaced by a fourth state, “State4” whereby switches S1and S5are closed (all other switches are open). In this state capacitor Cf charges up to input voltage +VDD. A fifth state, “State5” would then operate with switches S2and S6closed (all other switches open) such that flying capacitor Cf is connected across reservoir capacitors CR1and CR2in series (which, in this scenario, may be equal in capacitance). This particular example of an alternative switching and sequencing scenario has the drawback that there is no common-mode control and therefore would suffer from common-mode drift. However, this common-mode drift can be “reset” by altering the switching sequence at appropriate intervals during the “normal” switching and sequencing cycle. These alterations can be predetermined, or initiated in response to observed conditions.

It should be noted that the sizes of capacitors Cf, CR1, CR2, can be selected to meet the required ripple tolerances (versus size/cost) and consequently the clock phase duration for each state need not necessarily be of ratio 1:1:1.

While the above describes an embodiment wherein Mode1generates outputs of +/−VDD/2, it will be understood by the skilled person that the above teaching could be used to obtain outputs of any fraction of VDD by increasing the number of flying capacitors Cf and altering the switch network accordingly. The relationship between output and input in this case is Vout+/−=+/−VDD/(n+1) where n equals the number of flying capacitors Cf. It will also be appreciated that circuits with more than one flying capacitor as described will still be capable of generating outputs of +/−VDD/2 as well as outputs for every intermediate integer denominator between +/−VDD/2 and +/−VDD/(n+1) depending on its control. For example, a circuit with two flying capacitors can generate outputs of VDD/3 and VDD/2, one with three flying capacitors can generate outputs of VDD/4, VDD/3 and VDD/2 and so on.

As mentioned above, the DMCP is also operable in a second mode, Mode2, where it produces a dual rail output of +/−VDD (+VDD again being the input source voltage level at node N10). In Mode2, switch S4is always open.

Furthermore, in Mode2, the circuit is operable in four sub-Modes, referred to as Modes2a,2b,2cand2d. Optional switch S7is only used in Modes2cand2d. Consequently, if switch S7is not included, Mode2is only operable in sub-Modes2aand2b.

In Mode2athe DMCP has two basic states of operation.FIG. 9ashows the circuit operating in the first of these states, “State6”. In this state, switches S1, S2and S5are closed (S3, S4and S6are open). This results in capacitors Cf and CR1being connected in parallel across the input voltage +VDD, between nodes N10& N11. Therefore, capacitors Cf and CR1each store the input voltage +VDD.FIG. 9bshows an equivalent circuit for the State6operation.

FIG. 10ashows the circuit operating in the second of these states, “State2”, which is, in fact, the same state as state2in Mode1, whereby switches S3and S6are closed (S1, S2, S4and S5are open). Therefore capacitors Cf and CR2are connected in parallel between common node N11and second output node N13. Therefore, capacitors Cf and CR2share their charge and Node13exhibits a voltage of −VDD after a number of state sequences.FIG. 10bshows an equivalent circuit for this State2of operation.

FIG. 11illustrates the non-overlapping control signals (CS1-CS3& CS5-CS6) for controlling the switches (S1-S3and S5-S6) during the two alternating states of Mode2(a). The sequence of states in this mode is therefore 6, 2, 6, 2, 6, . . . etc.

FIG. 12ashows an additional state, “State7”, which can be introduced into this Mode2(a) sequence to create a slightly different implementation, referred to now as Mode2(b). In State7, switches S1and S5are closed (S2, S3, S4and S6are open). This state7connects the flying capacitor Cf across the input voltage +VDD. This state can be followed by states6then2and then back to7etc.FIG. 12bshows an equivalent circuit for this State7operation.

FIG. 13illustrates the non-overlapping control signals (CS1-CS3& CS5-CS7) for controlling the switches (S1-S3and S5-S7) to generate a repeating sequence of the three states7,6,2,7,6,2, etc. . . . that defines Mode2(b). Again, this represents only one example out of many possibilities for the controlling sequence. The inclusion of State7before State6is intended to isolate CR1from the influence of CR2, and hence combat cross-regulation. On the other hand, the inclusion of State7reduces the time available for charge transfer in the main States2and6, so that regulation as a whole may be improved if State7is simply omitted (Mode2(a)). These are design choices.

Whichever pattern is chosen, one of the states may be used less frequently than the others (as was described above in relation to Mode1). For instance, if the loads on the two output nodes N12, N13are unbalanced (either permanently or according to signal conditions), one of the States6and2could be included less frequently than the other, as capacitor CR1may need to be charged less frequently than capacitor CR2or vice versa.

Modes2(c) and (d) are further alternative modes of operation to generate +/−VDD, which are possible when the DMCP is provided with switch S7. This switch may used to replace the combined functionality of switches S1and S2for generating the positive output voltage at node N12in applications where the high-side load, i.e. the load connected between nodes N12and N11, does not require a lot of current. This may be where the load has a high input resistance as with a “Line Output” for a mixer for example. In such a case the size and the drive requirements of switch S7can be reduced and modified compared to those of switches S1and S2. Indeed, switch S7can be constantly switched on during operation in Mode2(c) which has advantages in that there is less power required to drive the switches and switch S7would not, in the case of a MOS switch implementation, inject any charge into either nodes N10or N12due to its parasitic gate-drain and gate-source capacitances. It should also be noted that switch S1is still required to operate so as to generate the negative output voltage −VDD. Still further, it should be noted that switch S2may be operated on an infrequent basis so as to also connect the flying capacitor Cf and high-side reservoir capacitor CR1in parallel.

FIG. 14illustrates the non-overlapping control signals (CS1-CS3& CS5-CS7) for controlling the switches (S1-S3and S5-S7) during the two alternating states of Mode2(c). Summarising Mode2(c), therefore, switch S7is permanently (or near permanently) closed. A modified State6is used to charge the flying capacitor Cf and capacitor CR1in parallel, this now being achieved by having switches S1, S5and S7closed only. A modified State2is then used to transfer this charge to capacitor CR2via switch S3, S6as before, but this time with capacitor CR1still having voltage VDD across it due to S7being closed.

FIG. 15illustrates non-overlapping control signals (CS1-CS3& CS5-CS7) for controlling the switches (S1-S3and S5-S7) during three states in a variation of Mode2(c) referred to as Mode2(d). The difference relative to Mode2(c) is similar to the difference between Modes2(a) and2(b), in that an extra phase is inserted with the switches in State7, wherein switches S1and S5are closed (S2, S3, S4and S6are open; S7can remain closed throughout). Note that Mode2(d) follows a sequence 7, 2, 6, 7, 2, 6 . . . rather than 7, 6, 2. There is not necessarily any great difference in the effect of these modes, but the freedom to vary the sequence can simplify the control logic, as will be seen in the discussion below.

Table 1 illustrates the switch (S1-S7) states for the seven states described above, with a “0” representing an open switch and a “1” representing a closed switch. Note that the switch network and controller do not need to implement all states1to7, if only a subset of the described modes will be used in a particular implementation.

Again, these four example sequences and seven or eight different states of the switch network are not the only possibilities for the controlling sequence. Again, a number of different sequence implementations are possible and some of these states may be used less frequently than others, depending on load.

FIG. 16illustrates a similar DMCP900circuit as illustrated inFIG. 4except that the DMCP900also includes two comparators910a,910bfor regulating the two output voltages.

It should be noted that DMCP900represents a closed-loop DMCP. Each of the comparators910a,910bcompares their respective charge pump output voltages (Vout+, Vout−) with a respective threshold voltage (Vmin+, Vmin−) and outputs a respective charge signal CHCR1and CHCR2. These charge signals CHCR1, CHCR2are fed into the switch control module1420to control the switch array1410causing the DMCP to operate charging either the relevant reservoir capacitor. If either output voltage droops past its respective threshold, the charge pump is enabled; otherwise the charge pump is temporarily stopped. This reduces the power consumed in switching the switches, especially in conditions of light load.

This scheme allows output voltages up to +/−VDD/2. It should be further noted that in this configuration, the DMCP900may be used to generate higher voltages, but with a drop in efficiency. In this case, the reference voltages (Vmin+/Vmin−) can be adjusted to adjust the output voltages accordingly. The flying capacitor Cf is charged up to +VDD (via switches S1and S5) and then connected in parallel across either reservoir capacitor CR1(via switches S2, S5) or CR2(via switches S3, S6) to raise their voltages to the levels set by the reference voltages. Such an operation increases the ripple voltages on the reservoir capacitors CR1, CR2but it also reduces switching losses. However, by scaling the reservoir capacitors CR1, CR2relative to the charging capacitor Cf, the ripple voltages can be reduced.

FIG. 17ais a block diagram of a second main embodiment of the Dual Mode Charge-Pump1400. As with the previous embodiment there are two reservoir capacitors CR1and CR2, a switch array1410controlled by a switch control module1420(which may be software or hardware implemented) However, there are now two flying capacitors Cf1and Cf2. DMCP1400again operates to produce outputs of +/−VDD/2 in a first mode and +/−VDD in a second mode. While this embodiment uses an extra flying capacitor, it has the advantage over the DMCP400with a single flying capacitor in that the output voltages Vout+/−now have improved cross-regulation characteristics.

FIG. 17bshows a more detailed version of the circuit1400and, in particular, detail of the switch array1410is shown. The switch array1410comprises eight switches S1-S8each controlled by corresponding control signal CS1-CS8from the switch control module1420. The switches are arranged such that first switch S1is connected between the positive plate of the first flying capacitor Cf1and the input voltage source, the second switch S2between the positive plate of the first flying capacitor Cf1and first output node N12, the third switch S3between the positive plate of the flying capacitor and the positive plate of the second flying capacitor Cf2, the fourth switch S4between the negative plate of the first flying capacitor Cf1and common terminal N11, the fifth switch S5between the negative plate of the first flying capacitor Cf1and the positive plate of the second flying capacitor Cf2, the sixth switch S6between the negative plate of the first flying capacitor Cf1and the negative plate of the second flying capacitor Cf2, the seventh switch between the negative plate of the second flying capacitor Cf2and common terminal N11and an eighth switch between the negative plate of the second flying capacitor Cf2and second output terminal N13. It should be noted that the switches can be implemented in a number of different ways (for example, MOS transistor switches or MOS transmission gate switches) depending upon, for example, an integrated circuit's process technology or the input and output voltage requirements. Also shown in greater detail is the control module1420which comprises a mode select circuit1430for deciding which controller1420a,1420bor control program to use, thus determining which mode the DMCP operates in. Alternatively, the mode select circuit1430and the controllers1420a,1420bcan be implemented in a single circuit block (not illustrated).

The DMCP1400, in one operational embodiment of its first mode, has three basic states of operation as shown below.

FIGS. 18aand18bshow the switch array1410operating in a first state, “state1”. Referring toFIG. 18a, switches S1, S5and S7are closed such that capacitors Cf1and Cf2are connected in series with each other and in parallel with the input voltage +VDD (N10& N11). Therefore, capacitors Cf1and Cf2share the input voltage +VDD that is applied across them.FIG. 18bshows an equivalent circuit for this state1operation with voltage +VDD effectively applied across nodes N10& N11.

It is preferable, for applications that require symmetrical, but opposite polarity, output voltages, that the values of capacitors Cf1and Cf2are of equal such that each capacitor changes voltage by an equal increment when connected in series across a voltage source. If both capacitors are initially discharged, or indeed previously charged to any equal voltages, they will end up each with a voltage equal to half the applied voltage source, in this case one half of the input voltage VDD.

FIGS. 19aand19bshow the switch array1410operating in a second state, “state2” Referring toFIG. 19a, switches S2, S4, S5and S8are closed such that capacitors Cf1and CR1and Cf2and CR2are respectively connected in parallel with each other. Therefore, the voltage across capacitor Cf1equalises with that across capacitor CR1such that the voltages across capacitors Cf1, CR1equalise. Over a plurality of state sequences, the voltages across capacitors Cf1, CR1will converge to a voltage VDD/2. Similarly, the voltages across capacitors Cf2and CR2will also equalise and eventually converge to VDD/2.FIG. 19bshows equivalent circuits for this state2operation.

It should be noted that the value of reservoir capacitors CR1and CR2do not necessarily need to be the same as that of flying capacitors Cf1and Cf2. If capacitor CR1and/or CR2is much larger than capacitor Cf1and/or Cf2, they will require more state sequences to charge up to, or close to, VDD/2. The value of reservoir capacitors CR1, CR2should be chosen depending upon expected load conditions and required operating frequency and output ripple tolerance.

As with all the charge pumps100,400,900described above, the presence of a significant load on the charge pump output terminals will result in a voltage droop in Vout+, Vout− away from +/−VDD/2. If the load is symmetric, that is there is equal current magnitude on both Vout+ and Vout−, then the symmetry of the system will result in both outputs drooping by the same amount.

However, if for example there is a significant load on Vout+ but no load or a light load on Vout−, then the voltage across capacitor CR1will reduce, while that across CR2will remain the same, or substantially the same. This will result in a reduction in the voltage across Cf1during state2. As a result of this there will be a larger voltage across capacitor Cf2at the end of state1, which will then be applied to CR2in state2, while at the same time, capacitor Cf1will again be connected in series with capacitor CR1, but still having a smaller voltage across it, even initially. Therefore, the output voltages Vout+ and Vout− will both tend to droop negatively, that is to say, the common mode is not controlled.

To avoid this effect, a third state of operation is introduced.

FIGS. 20aand20bshow the switch array1410operating in this third state, “state3”. Referring toFIG. 20a, switches S3and S6are closed such that the two flying capacitors Cf1and Cf2are connected in parallel with each other. Both capacitors Cf1and Cf2become charged up to an equal voltage, despite any difference between of their previous voltages. In steady state this becomes approximately VDD/2.FIG. 20bshows an equivalent circuit for the state3operation.

As mentioned in the previous embodiment, in states2and3, the voltages across the various capacitors that are connected in parallel may not actually completely equalise in practice, particularly if the switching frequency is high relative to the DMCP's R-C time constant. Therefore, the same considerations as in the previous embodiment must be taken into account when considering capacitor sizes so that any reduction in the output voltage remains within acceptable bounds.

It should be appreciated that the open-loop sequencing of the above three states does not necessarily need to be observed. For example the state sequences could be:1,2,3,1,2,3. . . (as described above); or1,3,2,1,3,2. . . ; or1,2,1,3,1,2,1,3. It should also be apparent that it is not necessary that state3be used as often as the other two states,1and2, for instance a sequence of1,2,1,2,1,2,3,1. . . can be envisaged. It may even be envisaged to dispense with state3altogether albeit only in the case of well-balanced loads, or with alternative schemes for common-mode stabilisation.

Other switching and sequencing scenarios exist. For example, in one alternative operational embodiment: State1could be replaced by another state, “state4” whereby switches S1and S4are closed (all other switches are open) or a fifth state, “state5” where S1, S3and S7are closed. In these states either capacitor Cf1or Cf2charges up to input voltage +VDD. A sixth state, “state6”, with S2and S8closed (all other switches open) or a seventh state, “state7”, with switches, or S2, S3or S8closed would then operate such that the charged flying capacitor Cf1or Cf2is connected across reservoir capacitors CR1and CR2(which, in this scenario, may be equal in capacitance). It should be noted that this particular example of an alternative switching and sequencing scenario has the drawback that there is no common mode control and therefore such a switching and sequencing scenario would suffer from common mode drift. However, this common mode drift can be “reset” by altering the switching sequence at appropriate intervals during the “normal” switching and sequencing cycle. These alterations can be predetermined, or initiated in response to observed conditions.

FIG. 21illustrates the non-overlapping control signals (CS1-CS8) for controlling the switches (S1-S8) during the three states (1,2and3) of the main operational Mode1embodiment of this second main embodiment of the DMCP. As discussed above, this represents only one example out of many possibilities for the controlling sequence.

As before, this second main embodiment of the DMCP is operable in a second mode to obtain output signals at levels +/−VDD. When operating in Mode2this DMCP1400has two basic states of operation. In both cases switches S2and S4are permanently closed.

FIG. 22ashows the first of these states “state8”, in which, switches S1, S3and S7are closed, as well as the permanently closed S2and S4. This results in capacitors Cf1, Cf2and CR1being connected in parallel across the input voltage +VDD, between nodes N10& N11(Cf1and CR1are permanently connected in parallel in this mode). Therefore, the three capacitors Cf1, Cf2, CR1are allowed to charge up to +VDD.FIG. 19bshows an equivalent circuit for this state8operation.

FIG. 23ashows a circuit diagram for the second of these states, “state2”, which is also the second state of mode1operation. It can be seen that switches S2, S4, S5and S8are closed.FIG. 20bshows an equivalent circuit for this state2operation. This state2is described in detail above. However in this case each flying capacitor Cf1, Cf2is charged up to +VDD after state8, and therefore when the voltages across capacitors CR1and CR2equalise with their respective flying capacitor Cf1, Cf2, outputs Vout and Vout− will sit at VDD and VDD− respectively.

FIG. 24illustrates the non-overlapping control signals (CS1-CS8) for controlling the switches (S1-S8) during Mode2of this second main embodiment of the DMCP1400. Again, this represents only one example out of many possibilities for the controlling sequence.

Table 2 illustrates the switch (S1-S8) states for the eight states that this second main embodiment of the DMCP1400can operate in, with a “0” representing an open switch and a “1” representing a closed switch. States1,2and3are used in the main operational embodiment of this DMCP1410in Mode1, while the states4,5,6and7are used in an alternative operational embodiment of same basic mode. States2and8are used Mode2of this DMCP1410. It follows that the switch network and controller do not need to implement all states1to8, if only a subset of the described modes will be used in a particular implementation.

FIG. 25illustrates a closed loop equivalent1900of this second main embodiment of the DMCP1400circuit, similar to DMCP900. Again it is largely similar to the open loop DMCP1400but further includes two comparators1910a,1910bfor regulating the two output voltages.

Each of the comparators1910a,1910bcompares their respective charge pump output voltages (Vout+, Vout−) with a threshold voltage (Vmin+, Vmin−) and each respective comparator1910a,1910boutputs a respective charge signal CHCR1, CHCR2. These charge signals CHCR1, CHCR2are fed into the switch control module1420to control the switch array1410causing the DMCP to operate charging either the relevant reservoir capacitor. If either output voltage droops past its respective threshold, the charge pump is enabled; otherwise the charge pump is temporarily stopped. This reduces the power consumed in switching the switches, especially in conditions of light load. It is apparent that, as both reservoir capacitors CR1, CR2are charged in a single state (state2), that there need only be a single charge signal CHCR which causes the DMCP to charge both reservoir capacitors CR1, CR2.

It should be further noted that in thisFIG. 22configuration, the charge pump1400may be used to generate any required voltages, but with a drop in efficiency. In this case, the reference voltages (Vmin+/Vmin−) can be adjusted to adjust the output voltages accordingly. The flying capacitors Cf1, Cf2are charged up to +VDD and then each is connected in parallel across one of the reservoir capacitors CR1or CR2to raise their voltages to the levels set by the reference voltages. Such an operation increases the ripple voltages on the reservoir capacitors CR1, CR2but it also reduces switching losses. However, by scaling the reservoir capacitors CR1, CR2relative to the flying capacitors Cf1, Cf2, the ripple voltages can be reduced.

FIG. 26illustrates a further embodiment of any of the novel Dual Mode Charge Pumps400,9001400,1900described above, wherein one of a number of different input voltage values may be selected as an input voltage to the DMCP400,9001400,1900. It shows an input selector1000having a number of different voltage inputs (+Vin1to +Vin N), the actual input chosen being determined by control input Ic. The chosen voltage level then serves as the input voltage VDD for the Dual Mode Charge Pump400,900,1400,1900.

From the above description, referring also toFIGS. 2aand2b, it will be understood how these DMCP circuits can be applied in portable audio and other apparatus to provide adaptability to different modes of use, without the same sacrifices of quality/cost/battery life and so forth that are problematic in the state of the art.

Many other modifications are possible in the control scheme, the form of the controller and even specifics of the switch network. The skilled reader will appreciate that the above and other modifications and additions are possible to these circuits, without departing from the spirit and scope of the invention as defined in the appended claims. Accordingly, the above described embodiments are presented to illustrate rather than limit the scope of the invention. For interpreting this specification and claims, the reader should note that the word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, the singular article “a” or “an” does not exclude a plurality, and a single element may fulfil the functions of several elements recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Where a claim recites that elements are “connected” or are “for connecting”, this is not to be interpreted as requiring direct connection to the exclusion of any other element, but rather connection sufficient to enable those elements to function as described. The skilled reader will appreciate that a good, practical design might include many auxiliary components not mentioned here, performing, for example, start-up and shutdown functions, sensing functions, fault protection or the like, some of which have been mentioned already, and none of which detract from the basic functions characteristic of the invention in its various embodiments described above in the claims.