Abstract:
A DC-DC converter controls a supply current (I IN ) provided to a rechargeable battery. The converter comprises an electrical input terminal that receives supply current (I IN ). An electrical output terminal is connected to the battery through a coil with a resistor in series therebetween. A controllable selector connects the input terminal to the output terminal during a first time interval in order to supply the battery and to connect the input terminal to a ground potential during a successive second time interval. Also, a feedback module generates a control signal for the selector from a resistor feedback signal, indicative of a variation of a battery charge current (I OUT ). The feedback module has an electronic block that receives the feedback signal. The electronic block processes the feedback signal to measure a variation of the supply current (I IN ) and provide the control signal to adjust the duration of the first time interval.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase application submitted under 35 U.S.C. §371 of Patent Cooperation Treaty application serial no. PCT/EP2010/054790, filed Apr. 13, 2010, and entitled DC-DC CONVERTER FOR THE CONTROL OF A BATTERY CHARGE CURRENT IN PORTABLE ELECTRONIC DEVICES, which application claims priority to Italy patent application serial no. MI2009A000790, filed May 11, 2009, and entitled DC-DC CONVERTER FOR THE CONTROL OF A BATTERY CHARGE CURRENT IN PORTABLE ELECTRONIC DEVICES. 
     Patent Cooperation Treaty application serial no. PCT/EP2010/054790, published as WO 2010/130514, and Italy patent application serial no. MI2009A000790, are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a DC-DC converter, particularly of the voltage reducer type (step-down converter or buck converter), for the control of a charge current provided to a battery in a portable device, and more particularly relative to the use of a USB (Universal Serial Bus) interface for the interconnection of the above-mentioned portable device, which is battery-supplied, to other devices 
     BACKGROUND 
     During the last years, battery-supplied portable electronic devices, such as mobile phones, digital cameras, digital video cameras, palm devices (PDA, Personal Digital Assistant) have had an increasing diffusion among users. 
     The success achieved by USB interfaces as connection means of personal computers (PCs) to other peripherals, such as, for example, printers, keyboards, pointing devices, memory card readers, and mass storage supports (pen drives), has induced consumer electronic devices manufacturers to provide also portable devices with such USB interfaces. 
     In fact, due to the growing number of multimedia functions and applications present in the portable devices which determine a high current consumption, it is necessary to frequently recharge the batteries of such devices. The USB connection ensures that such recharge occurs in a rapid manner, exploiting other portable devices. 
     However, a problem which is found in recharging a portable device battery by means of a USB connection is the difficulty in effectively controlling a supply current provided to the battery from the exterior through the USB connector, to prevent that such current exceeds maximum values allowed by the USB connection itself. 
     In order to obviate such drawback, a known solution employs a DC-DC converter having an input terminal connected to the USB connector, and provided with current detection devices associated to such input terminal. On the basis of the information provided by the above-mentioned detection devices, the converter is capable of controlling any variations from preset values of the external supply current provided to the portable device through the USB connector. Similar detection devices associated to an output terminal of the converter allow to control also variations of a charge current provided to the battery to be recharged. 
     However, such DC-DC converter of the known type comprises a number of discrete circuitry components which often make it too bulky and unsuitable for applications to portable devices in which, on the contrary, there is a tendency towards miniaturization and integration of the different components in order to reduce the implementation costs thereof. Furthermore, the functioning of such a DC-DC converter implies high power dissipation, which is unacceptable in many applications. 
     SUMMARY 
     Embodiments of the present invention provide an electronic DC-DC converter, particularly of the voltage reducer type (step-down converter), for the control of the supply current provided from the exterior to the battery through the USB connector in portable electronic devices, which is alternative to the known type converters and allows at least partially obviating the drawbacks set forth above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages of the above-mentioned electronic converter will result from the description reported herein below, of an exemplary embodiment, given by way of indicative, non-limiting example, with reference to the annexed figures, in which: 
         FIG. 1  schematically shows a battery-supplied portable electronic device; 
         FIG. 2  schematically shows an electronic DC-DC converter for the control of the battery charge current of the device in  FIG. 1 ; 
         FIG. 3  shows in detail a structure of a block of electronic components included in the electronic converter of  FIG. 2 ; 
         FIGS. 4A ,  4 B, and  4 C show by way of example, against time, waveforms of a charge current, an input current, and a duty-cycle signal relating to the electronic converter of  FIG. 2 , respectively, in a continuous conduction mode; 
         FIG. 4D  shows by way of example, against time, waveforms of voltage signals relative to the components unit of  FIG. 3  in a continuous conduction mode; 
         FIG. 4E  shows a detail of two successive periods of the waveform of  FIG. 4A ; 
         FIGS. 5A and 5B  show by way of example, against time, waveforms of a charge current and a duty-cycle signal relative to the electronic converter of  FIG. 2 , respectively, in a discontinuous conduction mode; 
         FIG. 5C  shows by way of example, against time, waveforms of voltage signals relative to the components unit of  FIG. 3  in a discontinuous conduction mode; 
         FIG. 5D  shows a detail of two successive periods of the waveform of  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows an electronic device  100 , preferably of the portable type, including a DC-DC converter  200  in accordance with an embodiment of the invention. The electronic device  100  illustrated in  FIG. 1  is, for example, a mobile phone, but in accordance with further embodiments of the invention, such device  100  can be a palm device (Personal Digital Assistant, or PDA), a portable MP3 file player, a digital camera, a digital video camera, a portable computer (for example, a notebook or a netbook). 
     The mobile phone  100  comprises a plurality of electronic components connected to a rechargeable battery  101  to receive a suitable supply current. This is schematically indicated in  FIG. 1  by means of dashed lines. Furthermore, as shown in the above-mentioned  FIG. 1 , the DC-DC converter  200  under discussion is electrically connected to the battery  101 . 
     In more detail, the mobile phone  100  comprises an antenna  10 , a transceiver unit  20  (Tx/Rx) connected to the antenna  10 , and an audio unit  30  (AV-CIRC) connected to the transceiver unit  20 . A loudspeaker  40  and a microphone  90  are connected to such audio unit  30  of the phone  100 . Furthermore, the mobile phone  100  is provided with a central processing unit (CPU)  60  for the control of various telephone and multimedia functions of the device, and especially for the control of the functioning of the transceiver unit  20  and the audio unit  30  on the basis of a control program stored in a system memory  80  (MEM), connected to the CPU  60 . Furthermore, the mobile phone  100  is provided with a display  70  provided with a screen  71  (for example, a liquid crystal display, DSPY) and a user interface  50 , such as an alphanumeric keyboard (K-B). 
     With reference to  FIG. 2 , an exemplary embodiment of an innovative electronic DC-DC converter for the control of the charge current of a battery in a portable device, such as, for example, the battery  101  of the above-mentioned mobile phone  100 , has been indicated with  200 . Such electronic converter  200  is, for example, a step-down converter. 
     The step-down DC-DC converter  200  is connected between the battery  101  to be recharged and a connector  201 , of the USB (Universal Serial Bus) type, which is connected to an external supply voltage source V in . Particularly, the converter  200  comprises an electrical input terminal  1  connected to the supply source V in  to receive a supply current I IN  from the exterior of the portable device  100 . For example, such external supply voltage V in  can be provided by an external computer, of the desktop or laptop type, or by any device which is connectable to the portable device  100  by a USB connection. 
     With reference to  FIG. 2 , the step-down DC-DC converter  200  further comprises an electrical output terminal  2  connected to the battery  101  to be recharged by means of a coil  203  and a sense resistor  204  mutually connected in series. It shall be noticed that the coil  203  and the sense resistor  204  are discrete circuitry components, that is, they are not integrated on a chip of semiconductor material. 
     In addition, the converter  200  comprises a controlled selector  202  including a first S 1  and a second S 2  switches, which are controlled by a circuitry control block DV. Such controlled selector operates so as to selectively connect/disconnect the input terminal  1  to the output terminal  2 , that is, to selectively connect/disconnect the battery  101  to be recharged to/from the external supply voltage V in . The first S 1  and the second S 2  switches are a PMOS transistor and a NMOS transistor, operating pull-push so as to not be both disabled at the same time, that is, open circuits. 
     In more detail, the controlled selector  202  is configured so that, both said first S 1  and second S 2  switches being active, that is, short circuits, the external supply voltage V in  is short-circuited toward a reference potential, for example, the ground potential GND, thus isolating the battery  101 . Vice versa, with only the first one S 1  of such switches being short-circuited, the battery  101  is connected to the external supply voltage V in  through the coil  203  and the sense resistor  204 . 
     The sense resistor  204  is employed to detect the value of a charge current I OUT  flowing within the coil  203 . Such charge current I OUT  is adapted to recharge the battery  101 . The sense resistor  204  includes a discrete resistor  204  of about 100 mΩ. 
     A voltage VR taken at the sense resistor  204  leads is proportional to an average value of the charge current I OUT , and represents a feedback voltage signal to be processed and sent, on a feedback branch of the DC-DC converter  200 , to drive the circuitry control block DV. 
     In particular, the feedback branch of the DC-DC converter  200  includes a feedback module  300  adapted to generate a control signal S from the feedback signal VR. Such control signal S is sent to the control block DV in order to control the first S 1  and the second S 2  switches. Preferably, the feedback module  300  includes, on the whole, integrated circuits. 
       FIG. 4A  shows, by way of example, a waveform of the current present in the coil  203  of the step-down DC-DC converter  200  against time (solid line curve) in a continuous conduction mode. Such coil  203  current coincides with the battery  101  charge current I OUT . The alternate trend of such charge current I OUT  depends on the connection/disconnection states of the coil  203  from the input voltage source V in  through the selector  202 . An average value I OUT/AV  of such charge current is represented by the dashed line of  FIG. 4A . 
       FIG. 4B  shows, by way of example, a waveform of the supply or input current I IN  (solid line curves) of the step-down converter  200  against time in a continuous conduction mode. In each period T of the signal, the waveform of the input current I IN  comprises a pulse which is determined by the concomitant closure (ON) of the first switch S 1  and by the opening (OFF) of the second switch S 2  during a first time interval t 1 . In a successive time interval T-t 1 , the first switch S 1  is open (OFF), while the second switch S 2  is closed (ON), so that the input current I IN  is null. An average value I IN/AV  of such input current I IN  is represented by the dashed line of  FIG. 4B . 
     The feedback electronic module  300  of the converter  200  includes a first  205  and a second  206  processing blocks which are adapted to receive and process the voltage signal VR taken at the sense resistor  204  heads. Such first  205  and second  206  blocks have their respective input terminals connected in parallel one to the other, and to the sense resistor  204  heads. The outputs of such blocks  205 ,  206  are connected to a logic block input  207  which is implemented, for example, by a digital port AND which is known to those skilled in the art. Particularly, such logic block  207  is adapted to receive a first PW 1  and a second PW 2  signals, respectively, from such first  205  and second  206  blocks, each of which is generated by processing the feedback voltage signal VR. Preferably, such first PW 1  and second PW 2  signals are pulse-width (PWM) modulated signals, and in phase one to the other. 
     It shall be noted that the logic block  207  operates so as to select one or the other of such first PW 1  and second PW 2  signals to be sent to the circuitry control block DV in order to control the opening/closure of the above-mentioned first S 1  and second S 2  switches. Advantageously, the logic block  207  operates so as to select the one of the two signals PW 1  and PW 2  having a respective duty cycle which is lesser than that of the other one. 
       FIG. 3  shows in detail the circuitry structure of the first block  205  on the whole. The same or similar members and components to those shown in the previous figures are indicated in  FIG. 3  with the same reference numerals. 
     In particular, such first block  205  comprises a voltage-voltage module converter  301  so configured as to receive the feedback voltage VR, which is indicative of the average charge current I OUT/AV , at the input terminals. Such first block is adapted to make available an output, on a respective first terminal A, a first voltage signal V A  with constant width which can be calculated based on the relationship:
 
 V   A   =G·I   OUT/AV   ·R   (1)
 
where G is the converter  301  gain, and R is the resistance value  204 .
 
     It shall be noted that the above-mentioned first terminal A of the converter  301  is connectable in series to a second input terminal B to a filter  302 , preferably a low pass filter, by means of a further controlled selector  303 . Such further selector  303  comprises a third S 3  and a fourth S 4  switches, which are controlled in order to selectively connect and disconnect the converter  301  to the/from the filter  302 . In particular, the selector  303  is a three-stage selector operating so that, when the third switch S 3  only is active (a short circuit), the converter  301  is directly connected to the filter  302 , that is, the voltage V A  on the first output terminal A is made available on the second input terminal B to the filter  302 . Instead, in the case where only the fourth switch S 4  is short-circuited, the filter  302  input terminal B is connected to the reference ground potential GND, and the converter  301  is disconnected from the filter  302 . Finally, in the case where both the switches S 3  and S 4  are disabled (open circuits), the terminals A and B continue to be floating. 
     Furthermore, the above-mentioned low pass filter  302  is connected in series with an integrator circuit  304  which employs a feedbacked operational amplifier  305  known to those skilled in the art. Particularly, an inverting terminal of the amplifier  305  is connected to a respective output terminal C of the filter  302 , by interposition of a first reactance R 1 . A further output terminal U of the integrator  304  is connected to said inverting terminal through a second reactance C 2 . Furthermore, the amplifier  305  is adapted to receive a reference voltage V ref  at a respective not-inverting terminal. 
     Advantageously, such reference voltage V ref  is made variable in order to adjust the value of the input current I IN  of the step-down converter  200 . 
     The integrator  304  output U is connected to an inverting input of an operational amplifier PWM  306 , the respective not-inverting input of which is connected to a sawtooth wave signal generator  307 . It shall be noted that the first pulse-width modulated signal PW 1  is made available at the amplifier PWM  306  output. 
     It shall be noted that the structure of the second block  206  is substantially similar to that of the first block  205 , even if it is free from of the selector  303 . In other words, the converter  301  and the low pass filter  302  are directly connected one to the other. 
     A functioning example of the step-down DC-DC converter  200  of the invention in a continuous conduction mode can be described with reference to  FIGS. 4A-4E . 
     Particularly, a constant voltage V A  related to the average charge current I OUT/AV  of the battery  101  on the basis of the relation (1) is present on the first output terminal A from the voltage-voltage converter  301 . Such constant voltage V A  is shown in  FIG. 4D  (dashed line). 
     The third switch S 3  is closure/opening controlled based on the duty-cycle signal D of  FIG. 4C , that is, such third switch S 3  is closed (ON STATE) during the time intervals when the duty-cycle signal D has a high value (1 logic) to connect the converter  301  output terminal A to the filter  302  input terminal B. Instead, the third switch S 3  remains open (OFF STATE) in the time intervals when the duty-cycle signal D has a low value (0 logic). 
     Vice versa, as regards the fourth switch S 4 , the latter is open (OFF STATE) in the time intervals when the duty-cycle signal D is high, while it is closed (ON STATE) in the time intervals when the duty-cycle signal D is low, thereby to connect the filter  302  terminal input B to the ground potential GND. 
     Accordingly, a voltage V B  which is applied to the filter  302  input terminal B is such that: V B =V A  during the ON STATE time intervals of the duty cycle D; V B =0 during the OFF STATE time intervals of the duty cycle D. Such voltage V B  has a pulsed trend, which is shown in  FIG. 4D  (solid line). 
     Advantageously, the waveform of the voltage V B  which is present on the second terminal B of the block  205  is indicative of the values taken by the input current I IN  of the DC-DC converter  200 . This is inferred by analyzing  FIG. 4E , which shows in detail two successive periods of the waveform of the battery  101  charge current I OUT . Particularly, in  FIG. 4E  a peak value of the above-mentioned current I OUT  is indicated with H, and I OUT/AV  represents the average charge current. Furthermore, indicating with T a complete period of closure/opening of the third S 3  and fourth S 4  switches, T/X being the duty cycle, and V ref  being the reference voltage, in the step-down DC-DC converter  200 , the input current I IN  and the charge current I OUT  of the battery  101  can be expressed as: 
                       I   IN     =       1   2     ·     1   X     ·   H       ⁢     
     ⁢       I   OUT     =       1   2     ·   H               (   2   )               
Furthermore, the following proportion is true:
 
     
       
         
           
             
               
                 V 
                 ref 
               
               : 
               
                 I 
                 IN 
               
             
             = 
             
               
                 V 
                 A 
               
               : 
               
                 I 
                 OUT 
               
             
           
         
       
       
         
           
             
               V 
               A 
             
             = 
             
               
                 
                   V 
                   ref 
                 
                 · 
                 
                   I 
                   OUT 
                 
               
               
                 I 
                 IN 
               
             
           
         
       
     
     Then, on the basis of (2), the voltage at the first terminal A can be expressed as,
 
 V   A   =V   ref   ·X  
 
While the voltage at the second terminal B
 
                       V   B     =         V   A     ·     1   X       +     0   ·     (     1   -     1   X       )           ⁢     
     ⁢       V   B     =       V   ref     ·   X   ·     1   X         ⁢     
     ⁢       V   B     =     V   ref               (   3   )               
From the latter of the previous equations (3), it is inferred that, in the continuous functioning mode, the feedbacked step-down DC-DC converter  200  operates so that, under stationary conditions, the voltage which is present at the filter  302  input terminal B takes the same value as the reference voltage V ref . Such reference voltage V ref  value is preset on the basis of the maximum value of the average input current I IN/AV  to the converter  200  that it is desired to be controlled.
 
     In operative terms, in the case where such average input current I IN/AV  exceeds the pre-established maximum value, this causes a resultant increase of the battery  101  average charge current I OUT/AV . Such current increase, detected by the sense resistor  204 , causes a resultant voltage V A  increase at the first terminal A and, by way of summary, a voltage V B  increase at the second terminal B. 
     If the voltage V B  exceeds the reference voltage V ref  value, the integrator circuit  304  generates an integrated signal at the integrator  304  output terminal U having a lower level than that that would be generated under stationary conditions on the basis of the equation between V B  and V ref . The comparison between the above-mentioned integrated signal and the sawtooth signal produced by the generator  307  generates the first signal PWM PW 1  having, in this case, a lower duty cycle than the one that such signal PW 1  would have under stationary conditions. 
     Then, such first signal PW 1  is compared by the logic block  207  to the second signal PWM PW 2  generated by the second block  206 . It shall be noted that the second pulse-width modulated signal PW 2  is indicative only of the average charge current I OUT/AV  of the battery  101 . Particularly, the second signal PW 2  duty cycle decreases/increases after an increase/decrease of the battery  101  charge current I OUT . 
     The logic block  207  selects and sends as the control signal S the one, between such first PW 1  and second PW 2  PWM signals, having a lower duty cycle. In any case, a feedback signal S is sent to the control block DV which is adapted to reduce the time intervals in which the first switch S 1  is closed and the second switch S 2  is open relative to the stationary conditions. In such a way, the average value of the input current I IN/AV  is reduced, and the input current I IN  is adjusted. 
     Similar considerations also apply when the voltage V B  results to be lower than the reference voltage V ref . In this case, the adjustment function performed by the first block  205  makes it so that the integrator circuit  304  generates an integrated signal at integrator  304  output terminal U, having a higher level than that generated under stationary conditions. The above-mentioned integrated signal, when compared to the sawtooth signal produced by the generator  307 , generates the first signal PW 1  PWM having, this time, a higher duty cycle than the one that would be generated under stationary conditions. On the basis of the comparison between the first PW 1  and the second PW 2  signals PWM, the logic block  207  sends the control signal S to the control block DV. 
     Therefore, on the basis of the adjustment of the reference voltage V ref  value, the DC-DC converter  200  of the invention allows controlling the average input current I IN/AV  value even when only a piece of information about the average value of the battery  101  charge current I OUT/AV  is available. 
     A functioning example of the step-down DC-DC converter  200  of the invention in the discontinuous conduction mode can be described with reference to  FIGS. 5A-5D . 
     In this case also, a constant voltage V A  connected to the battery  101  average charge current I OUT/AV  is present at the converter  301  output terminal A on the basis of the relationship (1). Such constant voltage V A  is shown in  FIG. 5C  (dashed line). 
     In the discontinuous mode, the third switch S 3  is closure/opening controlled on the basis of the duty-cycle signal D 1  shown in  FIG. 5B . Such third switch S 3  is closed (ON STATE) during the time intervals in which the signal D 1  is high (ON), in order to connect the converter  301  output terminal A to the filter  302  input terminal B. The third switch S 3  remains open (OFF STATE) both during a first time interval (OFF 1 ) and during a second time interval (OFF 2 ) in which the signal D 1  is low. 
     With reference to the fourth switch S 4 , the latter one is opened (OFF STATE) in the time intervals in which the signal D 1  is high. The fourth switch S 4  is closed (ON STATE) in the first time interval (OFF 1 ) in which the signal D 1  is low, in order to connect the filter  302  second input terminal B to the ground potential GND. Finally, such fourth switch S 4  returns to be open during the second interval (OFF 2 ) in which D 1  is low. 
     Accordingly, a voltage V B  which is applied to the filter  302  input terminal B is such that V B =V A  during the time intervals in which the duty cycle D 1  is high (ON STATE), V B =0 during the first time interval in which D 1  is low (OFF 1 ). Finally, V B =V ref  during the second time interval (OFF 2 ) in which D 1  is low. Such voltage V B  has a stepped trend, shown in  FIG. 5C  (solid line). 
       FIG. 5D  shows in detail two successive periods of the waveform of the battery  101  charge current I OUT . Particularly, in such  FIG. 5D , a peak value of said current I OUT  is indicated with H, and I OUT/AV  represents the average charge current. Furthermore, by indicating with T a complete closure/opening period of the third S 3  and of the fourth S 4  switches, with T/X the time interval ON STATE, with T/Y the first time interval OFF 1 , with T-T/Y the second time interval OFF 2 , and with V ref  the reference voltage, in the DC-DC converter  200 , the battery  101  input current I IN  and the charge current I OUT  can be expressed as: 
                       I   IN     =       1   2     ·     1   X     ·   H       ⁢     
     ⁢       I   OUT     =       1   2     ·     1   Y     ·   H               (   4   )               
Furthermore, the following proportion is true:
 
 V   ref   :I   IN   =V   A   :I   OUT  
 
Then, on the basis of (4), the voltage at the first terminal A can be expressed as,
 
               V   A     =       V   ref     ·     X   Y             
while the voltage at the second terminal B is
 
                       V   B     =         V   A     ·     1   X       +     0   ·     (       1   Y     -     1   X       )       +       V   ref     ·     (     1   -     1   Y       )           ⁢     
     ⁢       V   B     =         V   ref     ·     X   Y     ·     1   X       +       V   ref     ·     (     1   -     1   Y       )           ⁢     
     ⁢       V   B     =     V   ref               (   5   )               
Also in the discontinuous functioning mode, from the last one of the previous equations (5), it can be inferred that the feedbacked step-down DC-DC converter  200  operates so that, under stationary conditions, the voltage which is present at the low pass filter  302  input terminal B takes the same value of the reference voltage V ref .
 
     Since the second terminal B contains the information about the input current I IN , by fixing the reference voltage V ref , it is possible to adjust such input current I IN . 
     The exemplary step-down DC-DC converter  200  of the invention advantageously allows adjusting and reducing the input current I IN  by using only one sense resistor  204 . Such resistor  204  allows at the same time to accurately detect both input current I IN  variations and charge current I OUT  variations. 
     The DC-DC converter  200  has the advantage to require a minimal number of discrete circuitry components (particularly, only one sense resistor) that cannot be integrated. Therefore, such converter  200  has reduced overall dimensions compared to the known solutions, thereby resulting to be particularly adapted to be employed in portable electronic devices. 
     Furthermore, such minimal number of discrete circuitry components also implies a reduced power dissipation of the converter  200  compared to the devices of the known type. 
     To the above-described embodiments of the DC-DC converter, those of ordinary skill in the art, in order to meet contingent needs, will be able to make modifications, adaptations, and replacements of elements with functionally equivalent other ones, without departing from the scope of the following claims. Each of the characteristics described as belonging to a possible embodiment can be implemented independently from the other embodiments described.