Method of controlling a switched-mode power supply having a single inductive element and several outputs, and corresponding power supply, in particular for a cellular mobile telephone

The invention relates in general to switched-mode power supplies, and more particularly to switched-mode power supplies including a single inductive element and several regulated outputs.

The invention applies advantageously, but not limitingly, to battery-powered portable systems, in particular but not exclusively the remote terminals of wireless communication systems, for example cellular mobile telephones. Integrated systems for energy management in portable applications (battery operation) increasingly require different output voltages that are to be regulated, on the basis of an ever lower battery voltage.

A traditional approach includes creating a voltage regulated on the basis of the supply voltage with a linear regulator. This regulator uses an external capacitor to achieve its function. And use is made of as many linear regulators as outputs to be regulated. A drawback of this type of regulator is its low efficiency.

To alleviate this problem of low efficiency, use is made of switched-mode power supplies, also called “DC-DC converters”. The efficiencies are then close to SS to 90%, while under the same conditions, a linear regulator would have an efficiency of only 50%. However, a switched-mode power supply uses an inductive element in addition to a capacitor.

To regulate several outputs, it is conceivable to use several switched-mode power supplies. But this then presupposes the implementation of as many inductive elements, thereby increasing the cost and the surface area of the integrated circuit. Hence, the use has been envisaged of switched-mode power supplies capable of regulating several distinct outputs with a single inductive element. However, it is not always possible to retain a sufficient dynamic range between the various output voltages.

Additionally, contemporary switched-mode power supply systems do not make it possible to create, at output, regulated voltages with different signs. One approach for obtaining a positive voltage and a negative voltage includes using at least two linear regulators, with their aforesaid drawbacks, and a large number of external components.

According to a mode of implementation, it is proposed to regulate various independent outputs in terms of energy levels, using a single inductive element and exhibiting a larger dynamic range, or difference of voltages between two regulated outputs.

According to a mode of implementation, it is possible to generate a negative voltage without calling upon technologies making it possible to manage high voltages.

According to an aspect, there is proposed a switched-mode power supply, comprising a switching cell comprising a single inductive element with two connections nodes or points and several individually selectable outputs, in which the two connections of the inductive element are joined respectively to at least two of the individually selectable outputs.

It is thus possible to generate and regulate at least two different voltages, one positive and one negative. This makes it possible to regain on the one hand a correct dynamic range on the basis of a low battery voltage. Moreover, for certain applications such as audio amplifiers, this also makes it possible to obtain symmetric supply voltages making it possible to reduce the filtering and the design problems related to a nonzero common mode. Finally, such a power supply may be realized on silicon and can generate a negative voltage without calling upon technologies making it possible to manage high voltages. The controller may also be able to simultaneously select at least two of the individually selectable outputs.

According to an embodiment, the switching cell includes a first controllable selection unit or means possessing an input connection linked to a first connection of the inductive element and one or more output connections respectively linked to the output or outputs joined to the first connection of the inductive element. A second controllable selection unit or means includes an input connection linked to the second connection of the inductive element and one or more output connections respectively linked to the output or outputs joined to the second connection of the inductive element, and a capacitor per output includes a connection linked to the corresponding connection of the corresponding selection means.

The switched-mode power supply may also comprise two breakers linked respectively between a supply voltage and one of the connections of the inductive element, and between ground and one of the connections of the inductive element.

According to an embodiment, the switching cell is controllable cyclically by a slaving loop. The slaving loop comprises determination means able, in the course of each conduction cycle, to determine a total energy corresponding to the sum of the elementary energies required respectively by all the outputs in the course of this cycle, and control means able, in the course of this cycle, to control the switching cell so as to inject, possibly several times, the total energy into the inductive element, to select, possibly repeatedly, the outputs requiring a nonzero elementary energy and to restore on each selected output, the corresponding elementary energy.

This embodiment makes it possible to obtain conduction of continuous type, that is to say continuity of the current in the inductive element from one cycle to another. The controller or control means may also be able, in the course of each conduction cycle, to select successively and in an identical predetermined order for all the cycles, the outputs requiring a nonzero elementary energy. One of the outputs joined to the first connection of the inductive element and one of the outputs joined to the second connection of the inductive element can exhibit voltages of opposite signs and of equal absolute values.

According to another aspect, there is proposed a terminal of a wireless communication system, incorporating a switched-mode power supply, such as defined above. This terminal may define a cellular mobile telephone. When the terminal comprises a battery and an audio amplifier, the switched-mode power supply is powered by the battery of the terminal and the voltage for regulating the audio amplifier can be the voltage difference available between two outputs joined respectively to the two connections of the inductive element of the switched-mode power supply.

According to another aspect, there is proposed a method of controlling a switched-mode power supply, comprising a switching cell comprising a single inductive element with two connections and several individually selectable outputs, in which, the two connections of the inductive element being respectively joined to at least two of the individually selectable outputs, the control of the switching cell comprises the selection of at least two outputs respectively joined to the two connections of the inductive element. In particular, the control of the switching cell may comprise the simultaneous selection of at least two outputs respectively joined to the two connections of the inductive element.

According to a mode of implementation, the switching cell is controllable cyclically and, in the course of each conduction cycle, a total energy corresponding to the sum of the elementary energies required respectively by all the outputs in the course of this cycle is injected, possibly several times, into the inductive element; the outputs requiring a nonzero elementary energy are selected, possibly repeatedly and the corresponding elementary energy is restored on each selected output.

In particular, in the course of each conduction cycle, it is possible to select successively and in an identical predetermined order for all the cycles, the outputs requiring a nonzero elementary energy. It is possible to select two outputs joined respectively to the two connections of the inductive element according to a command so as to obtain voltages of opposite signs and of equal absolute values.

In an advantageous application, the switched-mode power supply can be incorporated into a terminal of a wireless communication system, for example a cellular mobile telephone, comprising a battery and an audio amplifier. The switched-mode power supply is then for example powered by the battery of the terminal and the voltage of the audio amplifier is then for example regulated with the voltage difference available between two outputs joined respectively to the two connections of the inductive element of the switched-mode power supply.

Other advantages and characteristics of the invention will be apparent on examining the detailed description of nonlimiting embodiments and modes of implementation and the appended drawings, in which:

FIG. 1 is a schematic block diagram illustrating a cellular mobile telephone incorporating a switched-mode power supply;

FIG. 2 is a schematic diagram illustrating in greater detail, a switching cell for a switched-mode power supply;

FIG. 3 is a schematic diagram illustrating in greater detail, an embodiment of a switched-mode power supply comprising two outputs;

FIG. 4 is a schematic diagram illustrating a mode of implementation of the method of controlling a switched-mode power supply.

In FIG. 1 , the reference TP globally designates a remote terminal of a wireless communication system, for example a cellular mobile telephone. This mobile telephone comprises an audio amplifier AA. The audio amplifier AA is supplied by two symmetric regulated voltages +VS and −VS, delivered by two outputs of a switched-mode power supply ALM. Additionally, the switched-mode power supply ALM is supplied by a supply voltage VE delivered by a battery. By way of example, this voltage VE can vary, as a function of the state of the battery, between 2.5 volts and 5.5 volts.

Reference will now be made more particularly to FIGS. 2 to 4 , for a particular embodiment of a switched-mode power supply ALM. In the embodiment described, the switched-mode power supply is a power supply of the voltage step-down type, in the sense that the regulated voltage delivered on each of the outputs of the power supply is equal to the product of the supply voltage VE times a coefficient α less than 1. This coefficient α is the duty ratio of the switched-mode power supply. That said, the invention is not limited to the use of a switched-mode power supply of the “voltage step-down” type, but could also apply to a power supply of the “voltage step-up” type.

The switched-mode power supply comprises a switching cell CCM formed of two breakers IT 1 and IT 2 , respectively including in this example a P-channel field-effect transistor (PMOS transistor) supplied by the supply voltage VE, and by an N-channel insulated-gate field-effect transistor (NMOS transistor) connected to ground. The switching cell CCM here also comprises a single inductive element L linked between a first connection BC 1 common with the breaker IT 1 , and a second connection BC 2 common with the breaker IT 2 .

The switching cell CCM also comprises several outputs OUT 11 -OUT 1 j and OUT 21 -OUT 2 i , which also form the outputs of the switched-mode power supply. Each output OUT 1 j , OUT 2 i is associated with a capacitor C 1 j , C 2 i linked between this output and ground. The capacitor C 1 j , C 2 i , associated with each output OUT 1 j , OUT 2 i , thus plays the role of energy reservoir so as to continue to supply the unselected output and maintain the voltage on the output, until the latter is again selected.

Additionally, for each output, a load CH 1 j , CH 2 i supplied by the regulated voltage VS 1 j , VS 2 i delivered by the corresponding output has been represented in FIG. 2 . Additionally, a current IS 1 j , IS 2 i flows in this load CH 1 j , CH 2 i . Thus, in the example of the application of the cellular mobile telephone mentioned above, the loads CH 11 and CH 21 may be for example the audio amplifier AA, and the regulated voltages VS 11 and VS 21 may be respectively the voltages −VS and +VS of the power supply of the audio amplifier AA.

Finally, each output OUT 1 j is linked to the connection BC 1 by way of an elementary breaker, here formed of a selection transistor MOS 1 j , which can be an N-channel or P-channel transistor. Likewise, each output OUT 2 i is linked to the connection BC 2 by way of an elementary breaker, here formed of a selection transistor MOS 2 i , which can also be an N-channel or P-channel transistor. The set of the elementary breakers MOS 11 -MOS 1 j defines a first selection means MSEL 1 , and the set of the elementary breakers MOS 21 -MOS 2 i defines a second selection means MSEL 2 . The first and second selection means MSEL 1 , MSEL 2 make it possible to individually select the outputs of the switched-mode power supply.

A particular embodiment is now described in which the switched-mode power supply is controlled by a slaving loop in such a way as to store a defined energy in the inductive element and then to restore this energy at the various outputs. However, the invention is not limited to this particular embodiment but also comprises any type of switched-mode power supply. In particular, the switched-mode power supply can be devoid, for example, of a slaving loop and systematically restore the energy at an output after or during the storage of this energy in the inductive element.

The breaker IT 1 is successively and alternately closed and open, so as to allow storage of energy in the inductive element, followed by restoration of this energy. The duration for which the breaker is open and closed in the course of each conduction cycle depends on the duty ratio α. The energy stored in the inductive element corresponds to the total energy required by each of the outputs OUT 1 j , OUT 2 i of the power supply. In particular, the switched-mode power supply ALM also comprises a slaving loop making it possible to drive the switching cell to obtain the output voltage desired on each of the outputs.

In the example illustrated in FIG. 3 , a switching cell with two outputs has been represented. Each of the desired output voltages VS 21 and VS 11 is a regulated voltage whose value is fixed by the value of an external reference voltage VREF 1 and VREF 2 , respectively. It is these reference voltages VREF 1 and VREF 2 which have opposite signs, for example, in the cellular mobile telephone application mentioned above.

The slaving loop BAS comprises a first elementary error amplifier or amplification means AMP 1 , whose positive input receives the voltage VREF 1 , and whose negative input is linked to the first output connection OUT 11 . The difference between the external reference voltage VREF 1 and the output voltage VS 11 , is consequently amplified in the error amplifier AMP 1 , and the corresponding error voltage VERROR 1 is an image of the elementary energy required on the output OUT 11 .

The slaving loop BAS likewise comprises a second elementary error amplifier or amplification means AMP 2 receiving, on the one hand, the reference voltage VREF 2 and, on the other hand, the voltage VS 21 . This amplifier AMP 2 delivers an error voltage VERROR 2 , which is also representative of the elementary energy required on the output OUT 21 .

The slaving loop BAS comprises, additionally, a summation means SUM 0 , whose two inputs are respectively joined to the two outputs of the elementary error amplifiers AMP 1 and AMP 2 . The output of the amplification means SUM 0 consequently delivers a voltage VERROR 0 which is representative of the total energy corresponding to the sum of the elementary energies required respectively by all the outputs in the course of each conduction cycle.

The slaving loop BAS also comprises a control pre-stage formed here of several comparators CMP 0 , CMP 1 and CMP 2 . More precisely, the comparator CMP 0 , dubbed here main comparator, receives on its negative input the error voltage VERROR 0 , and on its positive input a voltage VRAMP corresponding to a ramp signal SRP generated in a conventional manner by a ramp generator GR of structure known per se. The output of the main comparator CMP 0 consequently delivers a main pulsed control signal PWM 0 .

Additionally, the comparator CMP 1 is a comparator termed “elementary”, which is associated with the output OUT 11 . This comparator CMP 1 receives on its negative input the voltage VERROR 1 and on its positive input the voltage VRAMP. It delivers an elementary pulsed control signal PWM 1 . Likewise, the comparator CMP 2 is a comparator termed “elementary”, associated with the output OUT 21 , which delivers, after having received on its two inputs the error voltage VERROR 2 and the voltage VRAMP, an elementary pulsed control signal PWM 2 . The signals PWM 0 , PWM 1 and PWM 2 are consequently pulsed signals the width of whose pulses is modulated by the level of the error voltage.

The slaving loop BAS control means additionally comprise a digital control stage ETNC, receiving the signals PWM 0 , PWM 1 , PWM 2 , as well as a clock signal CLK, which is additionally used to reinitialize the ramp generator at the start of each conduction cycle (this operation is performed on the rising edge of the clock CLK, for example). On the basis of the signals PWM 0 , PWM 1 and PWM 2 , the control stage ETNC will formulate control signals intended to control, on the one hand, the breakers IT 1 and IT 2 of the switching cell CCM, and, on the other hand, the selection means, that is to say the transistors MOS 11 and MOS 21 .

In a general manner, as illustrated in FIG. 4 , there exist, for the switched-mode power supply with two outputs, four different possible configurations for the breakers IT 1 and IT 2 , as well as for the transistors MOS 11 and MOS 21 of the selection means. In these figures, the reference IL designates the current flowing in the inductive element L. In configuration No. 1 , the breakers IT 1 (PMOS transistor) and IT 2 (NMOS transistor) are conducting. They make it possible to inject energy into the inductive element L. In configuration No. 2 , the breaker IT 1 and the transistor MOS 21 are conducting. The output OUT 21 is consequently selected. The outputs OUT 11 and OUT 21 can also be selected simultaneously with configuration No. 3 , in which it is the transistors MOS 11 and MOS 21 that are conducting. Finally, the output OUT 11 can be selected with configuration No. 4 , in which the breaker IT 2 and the transistor MOS 11 are conducting.

By considering the duty ratios E 1 , E 2 , E 3 , E 4 of respectively configuration 1 , configuration 2 , configuration 3 , configuration 4 which are equal to the ratio of the time, during a cycle, in which the switching cell is in a given configuration to the time of a cycle, it is possible to deduce therefrom the mean voltages VS 11 and VS 21 . Thus, by considering that the output loads CH 11 and CH 21 are identical and equal to CH and that the resistance of the inductive element is RL, this then gives:

Therefore, by choosing duty ratios such that: E 1 =E 4 , this actually gives: VS 21 =−VS 11 .

In a general manner, according to the particular embodiment described above, the various outputs which require energy will be polled and selected successively in a predetermined order which is the same for all the cycles. This makes it possible to ensure continuity of current in the inductive element L between the end of one conduction cycle and the start of the following conduction cycle.

However, it is also possible, in the course of one and the same conduction cycle, to select each of the configurations 1 , 2 , 3 or 4 several times according to the energy to be restored on each of the outputs OUT 11 , OUT 21 . Likewise, it is also possible, in the course of one and the same cycle, not to select one or more of the configurations 1 , 2 , 3 , 4 . The embodiment described thus makes it possible to retain a current in the inductive element L which is relatively low, even when the switched-mode power supply is used under voltage step-up.

Additionally, the embodiment may comprise only three external components to produce two symmetric voltages: an inductor and two capacitors. The transistors MOS 1 j and MOS 2 i of the selection means MSEL 1 , MSEL 2 of the switching cell CCM can be N-channel or P-channel transistors. Their choice will depend on the application and the regulated value of the output voltages.

The invention is not limited to the embodiments which have just been described but embraces all variants thereof. Thus the elementary breakers of the selection means MSEL 1 , MSEL 2 may be formed of any type of switch that can be produced as an integrated circuit, for example bipolar transistors or IGBT transistors. The same holds for the breakers IT 1 and IT 2 .