Patent Publication Number: US-6212226-B1

Title: Supplemental power for battery operated modems

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the supplemental use of a switching power supply to draw power from a telephone line to assist a battery with powering the modem of a portable computer device. More particularly, this invention relates to a low noise constant input impedance switching power supply for drawing power from the input telephone line. 
     2. Background of Related Art 
     Many portable computer devices utilize modems for communications purposes. However, battery size and weight is an important consideration in the design and convenience of such devices. A balance must be reached between the size and weight of the overall device, dictated largely by the choice of battery, and an acceptable length of operation between charges. 
     Unfortunately, although operating an acceptable length of time when running typical application programs, portable computer devices utilize a large amount of power when communicating over a telephone line via a modem. The battery source which powers both the portable computer device and its modem is typically sized for general computing applications, and runs out of power quickly when actively communicating over a telephone line through a modem. Portable computer devices such as personal digital assistants (PDAs), hand held PCs (HPC), PCMCIA modems, and portable data terminals are designed to operate up to several hours on a single battery charge, but operate only fractions of an hour on a single battery charge when communicating via modem. Thus, although portable computer devices operate a sufficient length of time for quick data transfers over a modem powered completely by a battery, they require that external AC power be applied to allow for longer uses of the modem. It is therefore desirable for battery powered computer devices including a modem to draw power, in addition to the inherent battery, from a second power source. 
     The DC power inherent in a telephone line provides a convenient source of power, but there are often limitations and restrictions which limit the ability of a modem to derive power from the telephone line. For instance, present regulation in the United States is such that significant current may only be drawn from the telephone line when the telephone or modem is off hook or active. In order to hold the telephone line in an off hook condition, current in the approximate range of 20 mA to 150 mA must be drawn. The maximum amount of current available from the telephone line is also limited. 
     Modems which are designed to be powered entirely from the telephone line are known, but these designs suffer from an extremely constrained power budget. Moreover, modems are also subject to government constraints, e.g., FCC Part 68 requirements for telephones in the U.S., and limitations on effects and noise which may be placed back on the telephone line, placing further restrictions on the use of power from the telephone line. 
     Other conventional modems have inefficient power conversion apparatus making inefficient use of power from the telephone line. 
     In general, switching power supplies have previously been considered unsuitable for deriving power from the telephone lines because of the substantial amount of electrical noise inherent in such technology. A substantial amount of filtering is conventionally required to remove the noise generated back on the input lines, but this filtering would itself require a significant amount of power and thus degrade the net amount of power available from the telephone line. Switching power supplies are therefore conventionally avoided in the design of modems for computer devices not only because of the need for filtering but because of the additional stringent government standards as to the allowable maximum amount of disturbance of a telephone line due to a modem connection. 
     SUMMARY OF THE INVENTION 
     The present invention provides a modem for a portable computer device which draws only a portion of its required power from the telephone line using a low-noise switching power supply, which also can assist in keeping the battery for the portable computer device charged. 
     Switching power supplies are relatively inexpensive, utilize few components, and are relatively efficient. However, a major drawback preventing their widespread use in powering portable computer devices from a telephone line heretofore has been that because of the switching regulation of the output, they exhibit a high level of electrical noise to the input. 
     When a switching power supply is used to draw power from a telephone line, the input current is conventionally chopped, causing undesirable noise on the input telephone line, and jeopardizing compliance of the modem with required standards such as FCC Part 68 requirements in the United States. 
     According to one embodiment of the present invention, a shunt impedance placed in parallel with the input to a PWM switching power supply is switched in and out of the circuit to maintain the impedance of the input constant to the extent that a constant current draw is maintained. Thus, the impedance of the modem as sensed by the telephone line is balanced and constant despite the chopping of the current on the telephone line due to the switching of the power supply. Accordingly, the net input impedance is constant over time and electrical noise on the telephone line due to the modem is minimized or eliminated. 
     According to another embodiment, a constant current regulator is placed in series with the PWM switching power supply to maintain a constant impedance to the telephone line. 
     It is therefore an object of the invention to provide a portable computer device which operates longer between charges by utilizing available power from the telephone line. 
     It is also an object to provide a modem which utilizes available power from the telephone line efficiently and without causing significant electrical noise to the telephone line. 
     It is an additional object to provide a battery powered modem which utilizes power from a telephone line to trickle-charge the battery power source of the battery powered modem. 
     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the drawings, in which: 
     FIG. 1 shows a block diagram of the telephone interface and power supply of a battery operated modem according to the present invention. 
     FIG. 2 shows a block diagram of a first embodiment of the present invention utilizing a shunt impedance in parallel with the input to a PWM switching power supply. 
     FIGS. 3A and 3B are circuit diagrams of exemplary embodiments of the shunt impedance shown in FIG.  2 . 
     FIG. 4 is a circuit diagram of an exemplary embodiment of the PWM switching power supply shown in FIG.  2 . 
     FIG. 5 shows a block diagram of a second embodiment of the present invention utilizing a constant current regulator in series with a PWM switching power supply. 
     FIG. 6 is an exemplary circuit diagram of the constant current regulator and PWM switching power supply shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     FIG. 1 is a block diagram of a battery powered modem according to the present invention. 
     A standard two-wire telephone line  110 ,  112  from a central telephone office is connected to a standard telephone line connector such as an RJ-11 or other common telephone connector (not shown) on the portable computer device. The signal from the telephone line  110 ,  112  includes both an ac signal component and a dc power component and is input to the tip and ring inputs of a telephone line interface  100  of the modem. 
     The telephone line interface  100  in the exemplary embodiment is a data access arrangement (DAA) available from LUCENT TECHNOLOGIES. The DAA  100  includes transformers which provide isolation and balance while minimizing signal distortion within communications networks. The DAA  100  provides a barrier preventing electrical contact between the modem and the central telephone equipment. Located close to the phone jack, the transformers in the DAA  100  function as an isolation barrier that withstands high voltages between telephone lines, the rest of the modem circuitry, and the user. The telephone line interface  100  may alternatively be any SLIC chip or telephone interface accepting a common two or four-wire telephone input for a central telephone office. 
     The telephone line interface  100  extracts both an information signal  118  and a direct current (dc) voltage from the telephone line  110 ,  112 . The dc voltage component on telephone line  110 ,  112  is nominally 48 volts dc from the central telephone office, and is extracted from the telephone line  110 ,  112  by the telephone line interface  100  and output on power lines  114 ,  116 . 
     Although under present U.S. regulations the dc voltage can be made available for use by a modem on a telephone line essentially only when the modem is in the off hook state, the present invention is equally applicable to drawing power from the telephone line in either the off hook or on hook state. Thus, although the exemplary embodiments of the present invention comply with present U.S. regulations in drawing power from the telephone line only when the modem is in the off hook state, the invention can also be implemented to draw power from the telephone line in both the off hook and on hook states of the modem. 
     A power regulator  102  such as a switching regulator or linear regulator converts the dc voltage output from the telephone line interface  100  into a lower voltage, higher current dc voltage. The lower voltage, higher current dc voltage can be added to the power output from a battery  104  to power the portable computer device. The output voltage of power regulator  102  may be 5 volts, 3.3 volts, 2.5 volts, or any other common power voltage useful for powering circuitry and/or line drivers of a modem. The lower voltage, higher current dc voltage can either be added to the output power of battery  104 , or it can be used to trickle-charge the battery  104 , or both. Moreover, the lower voltage, higher current can be used to power only a specific component of the modem circuit  108 , e.g., telephone line interface  100 . 
     In the exemplary embodiment, the battery  104  is charged by the power drawn from the telephone line only when the modem is off hook. When the modem is on hook, the battery  104  is charged entirely by an external charge source  106 . 
     The output of the battery  104 , the output of the power regulator  102 , and/or the excess power available from the external charge source  106  when plugged into the portable computer device combine and assist each other to provide the dc power to the modem circuit  108  and telephone line interface  100  necessary for operation. 
     The information signal  118  from the telephone line interface  100  is input to the modem circuit  108 , which in the exemplary embodiment comprises a digital signal processor (DSP). The information signal  118  may include typical telephone quality voice, modulated data, or other standard analog or digital signals conventionally transmitted over a telephone line. 
     FIG. 2 is a detailed block diagram of an exemplary embodiment of the present invention. The telephone line interface  100 , telephone line  110 ,  112 , power lines  114 ,  116 , external charge source  106 , battery  104 , modem circuitry  108 , and information signal  118  are as discussed above with respect to FIG.  1 . In this embodiment, the power regulator  102  comprises a pulse-width modulated (PWM) switching power supply with a shunt impedance which is switched in and out of the circuit. 
     Regulation of the output of the PWM switching power supply  202  is accomplished by a feedback loop comprising an error amplifier or comparator  210  which compares the voltage output from the PWM switching power supply  202  to a voltage reference  206 . The voltage reference  206  outputs a voltage at the desired regulated level, e.g., 5 volts, 3.3. volts, or 2.5 volts. 
     A pulse-width modulator  204  supplies a PWM switching power supply  202  with a PWM clock signal ‘A’ in which the duty cycle is varied in response to the changing output dc level as determined by error amplifier or comparator  210 . PWM clock signal ‘A’ is a square wave of variable duty cycle. If the output of the comparator  210  indicates that the voltage output from the PWM switching power supply  202  is low, the positive pulse of PWM clock signal ‘A’ is widened. Conversely, if the output voltage is high, the positive pulse of PWM clock signal ‘A’ is narrowed. Inverter  212  provides an inverted PWM clock signal ‘/A’ for controlling the switchable shunt impedance  200 . 
     Oscillator  208  provides an oscillation frequency for pulse-width modulator  204 . The frequency of oscillator  208  is chosen to correspond to the chosen values of components in the PWM switching power supply  202 , e.g., the value of the capacitor and inductor. Typically, the frequency of oscillator  208  is chosen to be greater than audible frequencies, i.e., approximately 20 KHz and greater. 
     The switchable shunt impedance  200  is placed in parallel with dc voltage power lines  114 ,  116  output from the telephone line interface  100 , and controllably switched in and out of the circuit by the inverted PWM dock signal ‘/A’. 
     FIGS. 3A and 3B are circuit diagrams of exemplary embodiments of the switchable shunt impedance  200 . FIG. 3A shows a resistance or impedance  302  which is switched in and out of the circuit by an NMOS field effect transistor (FET)  300  in correspondence with the positive pulses of the inverted PWM clock signal ‘/A’. The resistance or impedance  302  in the exemplary embodiment is a resistor having a resistance approximately equal to the series resistance of the PWM switching power supply  202  and its load. Alternatively, a PMOS FET  304  together with resistance or impedance  306  can form the switchable shunt impedance  200  as shown in an alternative embodiment shown in FIG.  3 B. The switchable shunt impedance  200  is switched into the circuit on negative pulses of PWM clock signal ‘A’ to shunt the PWM switching power supply  202  during the time when the PWM switching power supply  202  is not charging its output charge capacitor. 
     Although field effect transistors (FETs) were used in the exemplary embodiments, any suitable semiconductor switch can be used, e.g., to switch the shunt resistance or impedance  302 ,  306  into and out of the circuit. Moreover, the particular semiconductor switch or FET used in the switchable shunt impedance  200  need not be the same component used in the PWM switching power supply  202  or other elements of the portable modem device. 
     The switchable shunt impedance  200  provides a path for a shunt resistance which minimizes load fluctuation presented to the telephone line  110 ,  112  arising from the switching of the series-pass FET in the PWM switching power supply. Therefore, noise generated into the telephone line from current chopping by the PWM switching power supply  202  is minimized. The PWM switching power supply  202  and switchable shunt impedance  200  provide the required load to the telephone line  110 ,  112  when the modem is in the off hook state, and maintains a high impedance condition when the modem is in the on hook state, in accordance with present FCC Part 68 regulations in the United States. 
     FIG. 4 is a circuit diagram of an example PWM switching power supply  202 . The switching power supply has often been thought of as an on-off device that controls the average direct current passing through it with duty-cycle or pulse-width modulation. The PWM switching power supply  202  shown in FIG. 4 is a step-down circuit wherein the output voltage is always less than the input voltage. 
     The PWM switching power supply  202  comprises semiconductor switch  400 , inductor  404 , charge capacitor  406 , and shunt voltage regulator  402 . Reversing the inductor  404  with the diode  402  and inverting the polarity of the capacitor  406  would provide a polarity reversing circuit wherein the output voltage can be greater or less than the input voltage. 
     The semiconductor switch  400  is an NMOS FET driven by PWM clock signal ‘A’ in the exemplary embodiment. The shunt voltage regulator  402  used in the present exemplary embodiment is a free-wheeling diode. 
     In operation, pulse-width modulator  204  outputs a square wave PWM clock signal ‘A’, which switches semiconductor switch  400  closed (or on) and open (or off). When the semiconductor switch  400  is driven closed (i.e., FET  400  on) by a positive pulse of PWM clock signal ‘A’, the power output from the telephone line interface  100  on power lines  114 ,  116  charges the charge capacitor  406 . Conversely, when PWM clock signal ‘A’ switches semiconductor switch  400  open (i.e., FET  400  off), inductor  404  maintains a charge on charge capacitor  406 . 
     The inductor  404 , charge capacitor  406  and shunt voltage regulator  402  are conventional devices selected according to design parameters shown in Table I. Vout is in volts, C is in microfarads, L is in henries, and Ipk is the peak inductor current in amperes. Table I relates only to the exemplary embodiment of FIG.  4 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Design Parameter 
                 General Equation 
               
               
                   
                   
               
             
            
               
                   
                 V out   
                 
                   
                     
                       
                         
                           
                             
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                               L 
                               ) 
                             
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                               ( 
                               
                                 I 
                                 pk 
                               
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                             t 
                             off 
                           
                         
                         - 
                         
                           V 
                           D 
                         
                       
                     
                     
                     
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 I pk   
                 
                   
                     
                       
                         2 
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                           ( 
                           
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                 t off   
                 
                   
                     
                       
                         
                           
                             ( 
                             L 
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                           - 
                           
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                 f 
                 
                   
                     
                       
                         1 
                         
                           
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                             OFF 
                           
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                             ON 
                           
                         
                       
                     
                     
                     
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
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                               V 
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                             I 
                             pk 
                           
                         
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                           ( 
                           
                             t 
                             off 
                           
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                 C 
                 
                   
                     
                       
                         
                           
                             ( 
                             
                               I 
                               pk 
                             
                             ) 
                           
                            
                           
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                               1 
                               / 
                               f 
                             
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                             ( 
                             8 
                             ) 
                           
                            
                           
                             ( 
                             
                               V 
                               RIPPLE 
                             
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                 Efficiency 
                 
                   
                     
                       
                         
                           
                             
                               V 
                               IN 
                             
                             + 
                             
                               V 
                               D 
                             
                             - 
                             
                               V 
                               T 
                             
                           
                           
                             V 
                             IN 
                           
                         
                         × 
                         
                           
                             V 
                             out 
                           
                           
                             
                               V 
                               out 
                             
                             + 
                             
                               V 
                               D 
                             
                           
                         
                       
                     
                     
                     
                         
                     
                   
                 
               
               
                   
                   
               
            
           
         
       
     
     FIG. 5 shows a second exemplary embodiment of the present invention. The telephone line interface  100 , telephone lines  110 ,  112 , external charge source  106 , battery  104 , modem circuitry  108 , pulse-width modulator  204 , error amplifier  210 , voltage reference  206 , oscillator  208  and information signal  118  are as described above. The second exemplary embodiment utilizes a constant current regulator  502  together with a PWM switching power supply  500  rather than a switchable shunt impedance  200  as in the first embodiment shown in FIG.  2 . The constant current regulator  502  and PWM switching power supply  500  are shown in more detail in FIG.  6 . 
     A constant current regulator  502  is placed in series with a PWM switching power supply  500  to regulate the current drawn by the PWM switching power supply  500 . Constant current regulator  502 , e.g., a diode, lowers its impedance to maintain a constant current flow, and thus presents a nearly constant load to the telephone line dc source, thus generating very little noise back into the telephone line  110 ,  112 . The value of the current regulation is chosen to allow sufficient charge of charging capacitor  608  when the semiconductor switch  602  is off, and to provide suitable loading to the telephone line  110 ,  112  in conformance with telephone standards. 
     A shunt semiconductor switch  602  is placed in parallel with the input to the PWM switching power supply  500  after the current regulator  502 . The semiconductor switch  602  may be an NMOS FET as in the present exemplary embodiment. The semiconductor switch  602  is controlled by an inversion of the PWM clock signal ‘A’, i.e., by inverted PWM clock signal ‘/A’ output from pulse-width modulator  204 . 
     The remainder of the PWM switching power supply  500  comprises inductor  604 , voltage regulator  606 , and charge capacitor  608 . The voltage regulator  606  in the exemplary embodiment is a Zener diode. 
     During operation, when the shunt FET  602  is switched off by the negative pulses of PWM clock signal ‘A’, charge capacitor  608  is charged at the rate allowed by current regulator  502 . When the shunt FET  602  is switched on by the positive pulses of PWM clock signal ‘A’, a virtual short circuit occurs in parallel with the diode  606  and in parallel with the inductor  604 /charge capacitor  608  series connection, and constant current regulator  502  regulates and maintains a constant current draw to the telephone central office via the telephone line. 
     While the invention has been described with reference to the exemplary preferred embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention.