Abstract:
Method and apparatus for preventing current overdraw by a wireless modem sinking current from a host power supply of a host device (e.g. laptop computer or personal digital assistant (PDA)). The methods and apparatuses are applicable, but not limited to Personal Computer Memory Card International Association (PCMCIA) wireless modems and Global System for Mobile Communications/General Packet Radio Service (GSM/GPRS) wireless networks.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to wireless communications technologies. More particularly, the present invention relates to controlling the current draw of a wireless modem from a host power supply. 
   BACKGROUND OF THE INVENTION 
   Integrated circuit cards (or “PC cards”) are often used in mobile communications and computing. In one particular application, the PC card comprises a wireless modem that plugs into a PCMCIA slot of a laptop computer or personal digital assistant (PDA). PCMCIA is an acronym for “Personal Computer Memory Card International Association”, which sets forth standards for PC cards. Typically, the host device (i.e. laptop or PDA) is coupled to a direct current to direct current (DC/DC) converter, which converts the host power supply voltage to a voltage source that powers the PC card. 
   For efficient use of the converted power by the wireless modem, the output impedance of the PC card must be well matched to the input impedance of the modem antenna. An impedance mismatch can occur if the antenna is not oriented for proper transmission, is broken or is in contact with an object that inhibits its ability to radiate radio frequency (RF) power. A consequence of the mismatch is the transmission of a weak RF signal and the possibility of the following two scenarios occurring. First, the power control loop of the power amplifier (PA) in the wireless modem will act to increase the PA output power to compensate PA output loss due to antenna mismatch. Second, when the base station of the wireless network receives the weak RF signal, the base station will send a request for the modem to increase its output power, not knowing that the weak signal is attributable to the antenna mismatch. Both of these scenarios may result in the PA of the wireless modem drawing excessive current from the host power supply, i.e., more current than the supply is designed to properly supply. This phenomenon is often referred to as “current overdraw.” Current overdraw is undesirable since it can damage the power supply, cause the host device to reset, and/or cause the modem to overheat. 
   A solution to preventing current overdraw would be to use a fixed hardware limiter to limit the power amplifier DC power rail. Unfortunately, this solution has two problems. First, conventional hardware current limiters only guarantee about a 20% threshold accuracy. This degree of uncertainty in accuracy may be too large to manage and prevent current overdraw by the power amplifier. Second, using a fixed hardware limit may result in the sending of a “false alarm” of a current overdraw condition to a host that may, in fact, have the ability to deliver a particular current draw demand (i.e. a “strong” power supply). Conversely, using a fixed hard limit may result in the sending of no warning at all of a current overdraw condition to a host that may not have the ability to deliver a particular current draw demand (i.e. a “weak” power supply). These problems would be compounded in multiple-time-slotted systems, in which current demands vary depending on the number of transmission slots allocated per transmission burst. 
   SUMMARY OF THE INVENTION 
   Generally, the methods and apparatuses of the present invention relate to wireless modems. More particularly, the methods and apparatuses of the present invention relate to controlling the current draw of a wireless modem from a host power supply. The methods and apparatuses are particularly applicable, but not limited to, mult-time slot GSM (Global System for Mobile Communications)/GPRS (General Packet Radio Service) networks. 
   According to one aspect of the invention, a current limiter for a wireless modem includes a current sensor configured to measure a current drawn by a wireless modem from a power supply of a host device, a current-to-voltage converter coupled to the current sensor and operable to convert the measured current drawn by the wireless modem to a voltage representative of the measured current draw, a reference voltage generator operable to generate a reference voltage defining a maximum current that the modem may draw from the host device power supply, and a voltage comparator coupled to both the current-to-voltage converter and the reference voltage generator operable to compare the voltage representative of the measured current draw to the reference voltage and generate a current overdraw signal when the compare the voltage representative of the measured current draw is greater than the reference voltage. 
   According to another aspect of the present invention, a method of limiting the current draw of a PC card wireless modem from a host power supply includes the steps of determining a current drawn by the modem from the host power supply, converting the measured current to a voltage representative of the measured current draw, comparing the voltage representative of the measured current draw to the reference voltage, and generating a current overdraw signal when the voltage representative of the measured current draw exceeds the reference voltage. 
   Other aspects of the invention are described and claimed below, and a further understanding of the nature and advantages of the inventions may be realized by reference to the remaining portions of the specification and the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of an adaptive current limiting system, according to an embodiment of the present invention; 
       FIG. 2  shows an exemplary adaptive current limiter, which may be used in system shown in  FIG. 1 , according to an embodiment of the present invention; and 
       FIG. 3  shows an exemplary current sensor circuit, which may be used in the adaptive current limiter in  FIG. 2 , according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , there is shown a block diagram of an adaptive current limiting system  10 , according to an embodiment of the present invention. System  10  comprises a PC card wireless modem  100 , which includes baseband and RF components, and an adaptive current limiter  110 . PC card wireless modem  100  includes a connector  102  for connecting PC card wireless modem  100  to a socket  104  of a host  106 . Host  106  may comprise a laptop computer, a handheld computer, a personal digital assistant (PDA), or other device from which information (e.g. data) is to be radiated by an antenna  108  of wireless modem  100 . As explained in more detail below, adaptive current limiter  110  monitors the instantaneous current of the power amplifier (PA) of wireless modem  100 , via a PA sense line  112 , and dynamically adjusts a current limit threshold of the PA, according to, for example, the host power supply capability, the number of transmission time (Tx) slots allocated per transmission frame, and the RF band of the transmit signal. Adaptive current limiter  110  may also be configured to send an over-current signal to wireless modem  100 , via an over-current line  116 , if for example, the current draw of the PA exceeds an over-current level. Adaptive current limiter  110  may also send an error signal to a user of the system, via host interface  102 , if for example, the antenna mismatch is detected by system  10 . It should be pointed out here that whereas system  10  shows the adaptive current limiter  110  and PC card wireless modem as comprising separate components, in an alternative embodiment the current limiter would comprise part of the wireless modem and, therefore, not be a separate component. 
   Referring now to  FIG. 2  there is shown an exemplary adaptive current limiter  20 , which may be used in system  10 , according to an embodiment of the present invention. Current limiter  20  is coupled to a DC/DC converter  200 , which converts a DC input voltage DCIN from a power supply of a host device to a DC output voltage DCOUT. DC/DC converters are known in the art and, therefore, will not be described in detail here. DCOUT provides power to a PC card wireless modem, including the power amplifier (PA)  201  of the wireless modem, as shown in  FIG. 2 . In a specific exemplary embodiment, the host device power supply supplies a DC input voltage of 5 volts, DC/DC converter  200  is a buck type converter, which steps the DC input voltage down to an output voltage DCOUT having a nominal value of about 3.5 volts, and DCOUT is used to power a PCMCIA-compatible PC card wireless modem. 
   Current limiter  20  comprises a current sensor circuit  202 , a voltage comparator  204 , a central processing unit (CPU)  206  (e.g. a microprocessor, ASIC, or programmable controller), a digital-to-analog converter (DAC)  208  and a current limiter shutdown circuit  210 . In addition to accepting the over-current signal to decouple the power source from PA  201  when the current source by PA  201  exceeds an over-current level, current limiter shutdown circuit  210  may provide secondary protection with a secondary, and possibly higher, fixed over-current threshold. A secondary fixed over-current threshold would be used, for example, if CPU  206  is for some reason unable to respond to the interrupt signal from comparator  204 . This could happen, for example, if the firmware on the CPU is out of control. A device, which can be configured to perform the primary and secondary protection functions is part no. MIC2545A/2549A Programmable Current Limit High-Side Switch, sold by Micrel, Inc. Those skilled in the art will readily understand that other circuits may be used to perform these functions. 
   Current sensor circuit  202  detects the instantaneous DC current on the DC power rail of PA  201  and converts it to a voltage VOUT. VOUT is coupled to a first input of voltage comparator  204  and is compared to a reference voltage VREF, which is coupled to a second input of comparator  204 . Reference voltage VREF is generated by DAC  208  and has a value that is adjustable by CPU  206 . Other reference voltage generators may be used and those skilled in the art would readily understand this. CPU  206  is programmed to provide a digital signal, which as explained above is converted to an analog voltage by DAC  208 , so that the VREF threshold is determined by one or more of the following: host power supply capability, the number of present Tx slots and the RF operational band. The output of voltage comparator  204  provides an interrupt signal to CPU  206 , when VOUT is greater than reference voltage VREF, indicating that the instantaneous DC current drawn by PA  201  has exceeded a predetermined limit. 
   To enhance the accuracy of the adaptive current limiter circuitry, a pre-calibration process may be performed during factory testing. An exemplary pre-calibration process directs CPU  206  to program PA into different constant current draw states. For each of these current draw states, the DC current drawn by DC/DC converter  200  and the current sensor  202  output voltage VOUT are measured and recorded. The measured DC currents are then normalized and the VOUTs averaged. The normalized DC currents and averaged VOUTs are then used to determine an offset of the current limiter under test. The offsets may be stored in a none-volatile memory and used as compensation offsets that can be added to the reference voltage setting of voltage comparator  204 . Whereas a voltage offset compensation table is described here, those skilled in the art will understand that other compensation tables may be developed and used. For example, a temperature compensation table may also be used to enhance the accuracy of the adaptive current limiter circuitry. 
     FIG. 3  shows an exemplary current sensor circuit  30 , which may be used for the current sensor  202  in the adaptive current limiter in  FIG. 2 , according to an embodiment of the present invention. A sample (I SENSE ) of the current drawn by PA  201  produces a sense voltage drop V SENSE  across a sense resistor R SENSE . A mirror current I M  representing the sensed voltage is mirrored from a first differential amplifier  300  to a second differential amplifier  302  via transistor  304  and current mirror  306 . Second amplifier  308  is configured so that it produces VOUT, the voltage that is compared to VREF in  FIG. 2 . Other types of current-to-voltage circuits may be used for current sensor  202 . Accordingly, those skilled in the art will readily understand that the current sensor circuit  30  in  FIG. 3  is but one of many possibilities. 
   In a specific exemplary embodiment of the present invention, the adaptive current limiter  10  in  FIG. 1  (or  20  in  FIG. 2 ) is used to monitor and limit the current draw of a power amplifier in a PCMCIA wireless modem that is designed to conform to the multi-time slot GSM/GPRS communications protocol. In this particular embodiment, CPU  206  is programmed to instruct DAC  208  to provide a reference voltage VREF, which is dependent upon either a two-slot or four-slot transmission burst (i.e. class  10  or class  12  operation). Other parameters, such as for example, the current supplying ability of the host power supply or the frequency band of the signal transmitted by the wireless modem may be used to define the reference voltage VREF. According to this exemplary embodiment, the current draw threshold of adaptive current limiter  10  is adaptive to the one or more of the following conditions: (i) the type of host  106  that is being used, (ii) the number of TX time slots being used, and (iii) the RF frequency band of which the modem  100  is operating. The latter two conditions may change over time. Accordingly, a controller in the adaptive current limiter  110  (e.g. CPU  206  in  FIG. 2 ) may be configured to adjust the current limiter threshold voltage VREF dynamically based on changes in these conditions in real time. 
   CPU  206  may be programmed to respond to an interrupt signal indicative of a current overdraw condition in various ways. For example, in response to the interrupt signal, CPU  206  may be programmed so that an error signal ERR is sent to a user, the error signal prompting the user to adjust the antenna of the PC card wireless modem. CPU  206  may also be programmed to provide a current draw reduction signal to an input of PA  201  and/or may be programmed to produce a signal to reduce the number of Tx time slots used per transmission burst, in response to the interrupt signal. This response can protect the wireless modem from overheating the host power supply from being damaged or from resetting. Finally, CPU  206  may also be programmed to send a signal to the user that the antenna may have been damaged or broken, if the modem continues operating in a protected mode for an extended period of time. 
   In an alternative embodiment, if CPU  206  is capable of checking current sensor  202  output VOUT directly at the beginning of each Tx slot, DAC  208  and comparator  204  may be eliminated. According to this alternative embodiment VOUT would be coupled directly to CPU  206  and ADC and CPU firmware could periodically poll the PA current on, for example, every TX time slot. 
   Whereas the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.