Abstract:
A circuit for generating a negative voltage supply signal is disclosed. The circuit has a first capacitive element coupled to receive a switched input signal which switches at least between a first state with a positive first voltage and a second state with a second voltage, different from the first voltage. A second capacitive element is coupled to provide the negative voltage supply signal. Control circuitry is coupled to the first capacitive element and the second capacitive element. The control circuitry is arranged, in the first state of the switched input signal, to enable charging of the first capacitive element by the switched input signal, and, in the second state of the switched input signal, to enable charging of the second capacitive element by the first capacitive element to generate the negative voltage supply signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a circuit for generating a negative voltage supply signal. In particular, the invention relates to an inexpensive provision of negative voltage supply for use in or with an associated power supply device and/or an associated portable electronic apparatus. 
       BACKGROUND 
       [0002]    Electrical power supply devices for portable electronic apparatuses are in common use. Mobile terminals, such as mobile telephones and personal digital assistants (PDAs) for mobile telecommunications systems like GSM, UMTS, D-AMPS, CDMA2000, FOMA or TD-SCDMA, are common examples of portable electronic apparatuses. A mobile terminal will therefore be used as a non-limiting example of a portable electronic apparatus in the remainder of this specification. 
         [0003]    A mobile terminal is normally driven by positive supply voltages. Recent requirements from e.g. audio and illumination developments have however made it preferable or even mandatory to use a negative voltage supply in addition to the normal positive voltage supply. As for most mass-produced products, it is important to keep the components involved as few and simple as possible, to save space and costs. In other words, a problem can be seen in how to provide such a negative voltage supply for a portable electronic apparatus like a mobile terminal, which conventionally only has a positive voltage supply. 
       SUMMARY 
       [0004]    Generally, the present invention is based on the understanding that beneficial use can be made of an switched positive-voltage input signal, which is available in the portable electronic apparatus for other reasons, to generate a negative voltage supply signal. 
         [0005]    A first aspect of the present invention is therefore a circuit for generating a negative voltage supply signal. The circuit comprises a first capacitive element coupled to receive a switched input signal which switches at least between a first state with a positive first voltage and a second state with a second voltage, that is different from the first voltage and typically is substantially zero. The circuit also comprises a second capacitive element coupled to provide said negative voltage supply signal, and control circuitry coupled to the first capacitive element and the second capacitive element. The control circuitry is arranged, in the first state of the switched input signal, to enable charging of the first capacitive element by the switched input signal, and, in the second state of the switched input signal, to enable charging of the second capacitive element by the first capacitive element to generate the negative voltage supply signal. 
         [0006]    This represents a simple and inexpensive solution to the aforementioned problem. Beneficial use is made of a switched input signal which, as will be explained in more detail later, may serve another purpose as well in the portable electronic apparatus and therefore may already be available. 
         [0007]    In one or more embodiments, the control circuitry comprises a first rectifying element, such as a diode, which is forward-biased in the first state of the switched input signal and reverse-biased in the second state of the switched input signal; and a second rectifying element, such as a diode, which is reverse-biased in the first state of the switched input signal and forward-biased in the second state of the switched input signal. 
         [0008]    In one or more embodiments, the first capacitive element is a capacitor having a first end coupled to receive the switched input signal and a second end coupled to a first end of the first rectifying element. The second capacitive element is also a capacitor having a first end coupled to a second end of the first rectifying element and a second end coupled to provide the negative voltage supply signal. Furthermore, the second rectifying element has a first end coupled to the second end of the second capacitive element and a second end coupled to the second end of the first capacitive element. 
         [0009]    A node between the first end of the second capacitive element and the second end of the first rectifying element may be grounded. 
         [0010]    A second aspect of the present invention is a power supply device for an electronic apparatus. The power supply device comprises a switched signal source to generate a switched input signal which switches at least between a first state with a positive first voltage and a second state with a second voltage, different from the first voltage. The power supply device also comprises a positive voltage supply generator to generate a positive voltage supply signal, and a circuit for generating a negative voltage supply signal according to the first aspect of the present invention. 
         [0011]    Advantageously, the switched signal source is included in the positive voltage supply generator. By using an already existing positive voltage supply generator as the switched signal source, an efficient implementation can be done for the negative voltage supply signal generating circuit. In one or more embodiments, the positive voltage supply generator is thus a switching regulator, such as a step-up regulator, which includes an inductor, an electronic switch having an on-state and an off-state, a capacitor, and a controller. The controller is configured to cause the electronic switch to alternate between its on-state, during which electric energy is integrated in the inductor, and its off-state, during which the capacitor is charged from electric energy integrated in the inductor and the positive voltage supply signal is provided at one end of the capacitor. Advantageously, a node between the inductor and the electronic switch in the switching positive voltage supply generator is used as the switched signal source for the negative voltage supply signal generating circuit. 
         [0012]    Alternatively, the switched signal source may instead be derived from another device. Therefore, in an alternative embodiment, the switched signal source is a clock terminal or other pulsed-signal terminal of a digital logic circuit, such as a microcontroller, central processing unit (CPU), digital signal processor (DSP), etc. 
         [0013]    A third aspect of the present invention is a portable electronic apparatus having at least one electric device configured to be supplied with a positive voltage supply signal and a negative voltage supply signal. The portable electronic apparatus also has a circuit for generating the negative voltage supply signal in accordance with the first aspect of the present invention. The portable electronic apparatus may advantageously be a mobile terminal such as a mobile telephone or a personal digital assistant (PDA) for a mobile telecommunications system like GSM, UMTS, D-AMPS, CDMA2000, FOMA or TD-SCDMA. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    Objects, features and advantages of embodiments of the invention will appear from the following detailed description, reference being made to the accompanying drawings. 
           [0015]      FIGS. 1 and 2  are schematic front and rear views, respectively, of a portable electronic apparatus according to an embodiment of the present invention, embodied as a mobile terminal. 
           [0016]      FIG. 3  illustrates one exemplifying situation where a negative voltage supply is required in a mobile terminal. 
           [0017]      FIG. 4  is a schematic block diagram which serves to illustrate an embodiment of the invention on a conceptual level, with a circuit for generating a negative voltage supply signal and a power supply device in a portable electronic apparatus. 
           [0018]      FIG. 5  is a circuit diagram of a circuit for generating a negative voltage supply signal in association with a step-up regulator, constituting a power supply device according to one embodiment of the invention. 
           [0019]      FIG. 6  is a schematic graph which demonstrates voltage vs time characteristics of a switched input signal used in the embodiment of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    An embodiment of the invention will be now described with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the particular embodiment illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like elements are represented by like reference numerals. 
         [0021]      FIGS. 1 and 2  show a portable electronic apparatus in the form of a mobile terminal  100 . The mobile terminal  100  has a housing that includes a front side  101   F  and a rear side  201   R . The front side  101   F  has a user interface that includes a speaker  102 , a microphone  105 , a display  103 , and a set of keys  104  which may include an ITU-T type keypad (i.e., an alpha-numerical keypad representing keys 0-9, * and #) as well as special keys like soft keys. A joystick  107  or similar navigational input device (e.g. scroll keys, touchpad, or navigation key) is also provided. A system connector  106  forms an accessory interface to external devices, such as battery charger or hands-free unit. 
         [0022]    The rear side  201   R  has a camera  208  and an illuminating light emitting diode (LED)  209 . A battery case  210  is also provided at the rear side  201   R . 
         [0023]    Other external components may be provided, such as power switch, battery, volume controls and external antenna, but are not indicated in  FIGS. 1 and 2  for the sake of brevity. 
         [0024]    In addition to the external components referred to above, the mobile terminal  100  has various internal components, as is readily realized by those skilled in the art. Such internal components may include one or more central processing units (CPU), digital signal processors (DSP), display controllers, memories, various software and firmware—including a real-time operating system, device drivers and application programs—and electronic radio circuitry for establishing and maintaining a wireless link to a base station in an available mobile telecommunications system. As is well known per se, the electronic radio circuitry will typically comprise analog and digital components constituting a radio receiver and transmitter. Such components may include band pass filters, amplifiers, mixers, local oscillators, low pass filters, AD/DA converters, etc. The radio interface typically also includes associated communication service software in the form of modules, protocol stacks and drivers. Typically, the mobile terminal  100  will also include one or more interfaces for short-range supplemental data communication, such as a Bluetooth interface, an IrDA (infrared) interface or a wireless LAN (WLAN) interface. 
         [0025]      FIG. 3  illustrates a situation where a negative voltage supply is required in the mobile terminal  100 . A speaker  350  is driven by an input audio signal  340  having some DC level, such as 1.5 V, as is illustrated schematically at  342 - 344 . Since a DC level may damage the speaker  350 , a blocking capacitor  346  is provided to remove the DC level, as is seen schematically at  348 . In the situation shown in  FIG. 3 , the speaker  350  is part of a portable hands-free (PHF) accessory kit, and therefore the audio signal crosses the system connector  306  in a pair of leads in the form of a feed line  352  and a return line  354 . 
         [0026]    The PHF accessory kit also serves as an FM antenna  370 , which is connected to the return line  354 . In addition, an active circuit  360 , also part of the PHF accessory kit, is connected to the return line  354 . The active circuit includes an amplifier  362  and a microphone  364  and needs to be supplied with a negative supply voltage, as seen at  366 . 
         [0027]    Various other situations may occur where a negative voltage supply is required in the mobile terminal  100 . For instance, certain illumination components may require such a negative voltage supply. 
         [0028]    Reference is now made to  FIG. 4  which schematically illustrates a portable electronic apparatus, for instance the mobile terminal  100 , having an electric device  430  which needs positive as well as negative supply voltages, as seen at  432  and  434 . When the portable electronic apparatus  400  is a mobile terminal like mobile terminal  100 , the electric device  430  may for instance be the active circuit  360  of  FIG. 3 . 
         [0029]    In  FIG. 4 , a power supply device  401  is coupled to supply the electric device  430  with positive and negative supply voltages V out+  and V out−  via output supply terminals  428  and  416 , respectively. The power supply device  401  includes a voltage regulator  420  having input terminals  424 ,  426  for receiving a positive input DC voltage V in  (for instance from a battery) and for electric ground, respectively. The voltage regulator  420  generates at its output supply terminal  428  a positive voltage supply signal with a more or less constant positive DC level V out+ . 
         [0030]    In addition, the power supply device  401  of  FIG. 4  includes a negative voltage generator  410  in the form of a circuit for generating a negative voltage supply signal V out−  at the output supply terminal  416 . The negative voltage generator  410  is grounded at input terminal  414 , like the voltage regulator  420 . Unlike the voltage regulator  420 , however, the negative voltage generator  410  does not receive the positive input DC voltage V in  at the other input terminal  412 . Instead, the negative voltage generator  410  receives at its input terminal  412  a switched input signal  413  from a switched signal source  422 . As is seen in  FIG. 4 , the switched signal source  422  may be included in the voltage generator  420 , or it may alternatively be derived from a different device, such as a clock terminal or other pulsed-signal terminal of a digital logic circuit like a processor. With reference to  FIGS. 5 and 6 , the particulars about the switched signal source  422  and the switched input signal  413  that it provides to the negative voltage generator  410  (the circuit for generating a negative voltage supply signal) will be described in more detail with reference to one advantageous embodiment, where indeed the switched signal source  422  is included in the voltage generator  420 . First, however, it should be noticed that, of course, if any of the positive and negative supply voltages V out+  and V out−  are too high for the electric device  430 , appropriate adaptor circuitry may be provided in the power supply device  401 , the electric device  430 , or between these devices, to adjust the supply voltages to a suitable level, for instance by voltage division. 
         [0031]      FIG. 5  illustrates a power supply device  501  according to aforesaid one advantageous embodiment, including a step-up regulator  520  which may correspond to voltage regulator  420  of  FIG. 4 , as well as a negative voltage generator  510  (circuit for generating a negative voltage supply signal) which may correspond to negative voltage generator  410  of  FIG. 4 . 
         [0032]    The step-up regulator  520  is configured to receive a positive input DC voltage V in  and raise it to a higher voltage level V out+  under control by a controller U 1 . Such higher voltage level may for instance be needed by one or more white LEDs in the portable electronic apparatus—for instance the illuminating LED  209  in the mobile terminal  100  of  FIG. 2 . As will soon be described, this embodiment makes beneficial use of the presence of the step-up regulator  520  so as to generate also a negative voltage supply signal V out−  by means of the negative voltage generator  510 . In alternative embodiments, the regulator  520  may for instance be replaced by a charge pump. 
         [0033]    The operation of the circuitry in  FIG. 5  is as follows. The controller U 1  is configured to control, via a terminal EXT, an electronic switch in the form of an NMOS transistor T 1  to alternate between an on-state (closed) and off-state (open). To this end, the controller U 1  receives the positive input DC voltage V in  at a terminal VDD as well as a voltage over a capacitor C 1 , as divided by voltage divider R 1  and R 2  and representing the desired positive supply voltage V out+ . 
         [0034]    When T 1  is turned on and saturates, a current starts to integrate in an inductor L 1 . When T 1  is then turned off, the voltage at node A ( 422 ) goes high, and a diode D 1  is forward-biased, thereby allowing the electric energy stored in the inductor L 1  to be fed as a current to the capacitor C 1 . As previously mentioned, the voltage over capacitor C 1  is sensed by the controller U 1  via resistors R 1  and R 2 , allowing U 1  to stabilize V out+  by controlling the switching of T 1  between on-state and off-state. During the off-state of T 1 , when the capacitor C 1  is charged by the positive voltage at node A, beneficial use is made of this positive voltage at node A as a switched input signal  413  to be received at terminal  412  of the negative voltage generator  510 . The off-state of T 1  is referred to as S 2  in the graph shown in  FIG. 6 , from which it can be seen how the switched input signal  413  starts at a positive voltage of n V and decays to a lower but still quite positive voltage at the end of off-state S 2 . 
         [0035]    During T 1 &#39;s off-state S 2 , a first capacitor C 2  in the negative voltage generator  510  is thus charged simultaneously with the capacitor C 1  by the switched input signal  413 , i.e. the positive voltage at node A is applied to one end of the first capacitor C 2 , a second of which is connected to ground via a forward-biased first diode D 2  (node E in  FIG. 5 ). A reverse-biased second diode D 3  prevents at this stage charging of a second capacitor C 3  in the negative voltage generator  510 . At the end of state S 2 , the voltage over the capacitor C 2  in the negative voltage generator  510  may be for instance about 10 V. 
         [0036]    Then, when T 1  is switched again to its on-state S 1 , the switched input signal  413  will drop to zero voltage (as seen in  FIG. 6 ), since node A will be connected to ground via the conducting T 1 . Thus, the diode D 1  in the step-up regulator  520  as well as the first diode D 2  in the negative voltage generator  510  will both be reverse-biased, and no current will flow through them. As a consequence, the voltage at node C in  FIG. 5 , i.e. between the first capacitor C 2  and the first and second diodes D 2 , D 3  in the negative voltage generator  510 , will be about −10 V. Thus, D 2  will now be reverse-biased whereas D 3  will be forward-biased. Current will be drawn from the second end of the first capacitor C 2  through the second diode D 3  and start to charge the second capacitor C 3 . Thus, a negative voltage V out−  will be built up over C 3 . In the disclosed embodiment, C 2  and C 3  are of equal size; therefore the negative voltage V out−  over C 3  will be −10 V−[voltage drop over D 3 ], e.g. −10 V−(−0.4 V)=−9.6 V when D 3  is a Schottky diode. 
         [0037]    Resistor R 3  is a small load to allow V out−  to act as aforementioned negative voltage supply signal at the negative supply terminal  416  of the negative voltage generator  510 . 
         [0038]    The negative voltage generator  510  is therefore, in summary, a circuit for generating a negative voltage supply signal from a switched input signal at node A by converting and transferring voltage from the first capacitor C 2  to the second capacitor C 3  via a control circuitry in the form of the first and second diodes D 2 , D 3 . 
         [0039]    In the disclosed embodiment, the component values in the circuitry of  FIG. 5  may be as follows: 
         [0040]    C 0 : 100 nF 
         [0041]    C 1 : 4.7 μF 
         [0042]    C 2 : 1 μF 
         [0043]    C 3 : 1 μF 
         [0044]    D 1 : Schottky diode 
         [0045]    D 2 : Schottky diode 
         [0046]    D 3 : Schottky diode 
         [0047]    L 1 : 4.7 μH 
         [0048]    R 1 : 100 kΩ 
         [0049]    R 2 : 10 kΩ 
         [0050]    R 3 : 100 kΩ 
         [0051]    The invention has been described above in detail with reference to an embodiment thereof. However, as is readily understood by those skilled in the art, other embodiments are equally possible within the scope of the present invention, as defined by the appended claims.