Abstract:
A power management circuit for a system that has combined power supplies from an inductively coupled circuit and from a battery comprises voltage sensing circuitry for sensing the voltage of each of the power supplies. A switching arrangement selectively connects one of the power supplies with a user or with plural users. The switching arrangement is controlled by appropriate control circuitry in response to outputs from the voltage sensing circuitry. The power management makes the best use of energy received over the inductive interface to preserve battery lifetime.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to a power management circuit and, in particular, to a power management circuit for use with a system that has combined power supplies from an inductively coupled circuit and from a battery.  
         BACKGROUND OF THE INVENTION  
         [0002]    Transponders are typical systems that use inductive coupling for data transfer and also for energy transfer. The inductive interface includes an antenna coil and a capacitor forming a resonant circuit, and a rectifier with a storage capacitor. Since a transponder is supplied from inductively transmitted energy, it may be battery-less. An example of a battery-less transponder is a vehicle immobilizer. A vehicle immobilizer is a portable device that may include a physical key fitting into a lock, in addition to a transponder. While the basic function of such a device is that of a reliable key, other features may be added. For example, remote control functionality is among the features that respond to the ongoing demands for comfort. Remote control generally involves a RF transceiver, which needs to be powered from a battery. In spite of their increasing complexity, these systems are mass products and it is essential to keep the system costs as low as possible. A considerable reduction of the total system cost is achieved by combining plural functions in a single circuit and by sharing resources such as a controller, encryption circuits or memory, allowing to reduce the chip size. The shared circuit parts must be supplied regardless whether a supply is available from the inductive interface or from a battery. Since each power supply can have a voltage between zero and a maximum voltage up to e.g. 8 volt, an intelligent power management is required. The power management should make the best use of energy received over the inductive interface to preserve battery lifetime. In addition, energy received over the inductive interface in excess of that required for the basic functionality, should be made available to charge the battery (if a rechargeable battery is used). Also, conflicts between the power supplies must be avoided.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention provides an intelligent power management circuit that allows an optimum use of energy from two different power supplies, one of which has an inductive interface and the other of which has a battery. Specifically, the invention provides a power management circuit for a system that has combined power supplies from an inductively coupled circuit and from a battery. The power management circuit comprises voltage sensing circuitry for sensing the voltage of each of the power supplies. A switching arrangement selectively connects one of the power supplies with a user or with plural users. The switching arrangement is controlled by appropriate control circuitry in response to outputs from the voltage sensing circuitry.  
           [0004]    In the preferred embodiments of the invention, the power management circuit allows particular circuit parts, such as the immobilizer analog circuits, to be supplied from the inductively coupled circuit independent of the battery supply. Other digital and shared circuit parts may be supplied through voltage regulators near the minimum required voltage level to reduce current consumption. The digital and shared circuit parts may be alternately supplied from the inductive interface, also in case of an empty battery, or from the battery supply in response to the results of a voltage comparison. If both supplies are available, a preference is given to the supply from the inductive interface to preserve battery lifetime.  
           [0005]    In a further preferred embodiment, circuit parts that have a relatively high current consumption such as a RF transmitter may be supplied from the inductive interface. In that case, a voltage drop below the minimum required for operation of other essential circuit parts is avoided by monitoring the voltage level and inserting a voltage regulator.  
           [0006]    Where a rechargeable battery is used, the preferred embodiment of the invention permits excess energy received over the inductive interface to be used for the purpose of charging the battery. A separate controller may be used to control the charging process if the supply voltage is sufficient for operation of the controller. Below the voltage level required for operation of the controller, a voltage regulator in combination with a switch protects the power supply from being heavily loaded by an empty battery, thereby avoiding a voltage drop below the minimum level required for operation of essential circuit parts.  
           [0007]    Another advantage of an embodiment is the option to provide a backup supply from the inductive interface to permit battery replacement. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    Further advantages and features will appear from the following detailed description of preferred embodiments with reference to the appending drawings.  
         [0009]    [0009]FIG. 1 is a sketch illustrating a typical environment for a device that includes the inventive power management circuit;  
         [0010]    [0010]FIG. 2 is a block diagram of the power management circuit;  
         [0011]    [0011]FIG. 3 is a circuit diagram of a voltage sensing circuit;  
         [0012]    [0012]FIG. 4 is a circuit diagram of a combined threshold circuit and voltage regulator; and  
         [0013]    [0013]FIG. 5 is a flow chart illustrating a battery charge process.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    With reference to FIG. 1, the sketch schematically depicts a safety system behind a vehicle dashboard  10  with a key-lock cylinder  12  mounted therein. An air coil  14  surrounds the key-lock cylinder  12 . A portable device  16 , i.e. a physical key that fits into the key-lock cylinder  12 , is equipped with an electronic identification device  18 . A RF transceiver  20  is connected to the air coil  14 . The RF transceiver  20  is connected to a controller  22 , which is also connected to a vehicle immobilizer  24 .  
         [0015]    The electronic identification device  18  includes a transponder that co-operates with transceiver  20 . Specifically, the transponder within device  18  is inductively coupled with air coil  14  by means of an internal antenna coil  18 a. In operation, the RF transceiver  20 , under control of controller  22 , drives air coil  14  with RF interrogation signals. The interrogation signals convey both data and electromagnetic energy and are received by the internal antenna coil  18   a  within device  18 . Since the device  18  is inductively coupled with the safety system installed within the vehicle, it may be battery-less.  
         [0016]    So far, the system depicted in FIG. 1 is conventional; it may be found in modern vehicles.  
         [0017]    More sophisticated systems require the portable device  16  to support comfort functions such as remote control and remote keyless entry. These functions by necessity involve a RF transmitter that must be supplied from a battery. To preserve battery lifetime, an intelligent power management is required.  
         [0018]    When a rechargeable battery is used for the portable device  16 , the stationary safety system behind dashboard  10  may function as a charge station. It may receive battery condition indication signals from the portable device  16  and, in response, initiate a battery charge process.  
         [0019]    With reference now to FIG. 2, a power management circuit  30  for use in an electronic identification device  18  of FIG. 1 has an inductive interface formed by a parallel resonant circuit that includes an inductivity LR (materialized by antenna coil  18   a ) and a capacitor CR. A storage capacitor CL is connected between a ground terminal GND and one of the nodes, referred to as terminal VCL, of the parallel resonant circuit including LR and CR. The other node, referred to as terminal RF 1 , of the parallel resonant circuit including LR and CR is connected to the cathode of a rectifier diode D the anode of which is connected to the ground terminal GND.  
         [0020]    Diode D may be a substrate diode if the power management circuit  30  of FIG. 2 is implemented in a CMOS technology.  
         [0021]    Diode D and storage capacitor CL form a rectifier circuit. Provided that, in operation, the antenna coil  18   a  is inductively coupled with air coil  14  and the latter is driven with appropriate signals, terminal VCL constitutes a first power supply terminal referenced to ground terminal GND. In practical implementations, the voltage level of terminal VCL may vary between zero and a maximum of less than  8  volt. Accordingly, a first power supply is formed by an inductively coupled circuit.  
         [0022]    A second power supply is formed by a rechargeable battery BAT connected between ground terminal GND and terminal VBAT of circuit  30 . A buffer capacitor CBAT may be connected across battery BAT.  
         [0023]    The power management circuit  30  has at least three different supply outputs.  
         [0024]    A first supply output is directly taken from terminal VCL. In the specific embodiment, this is a supply for the vehicle immobilizer analog circuits.  
         [0025]    A second supply at a terminal referred to as VCCPCU is selectively taken either from terminal VCL, or from terminal VBAT. In both cases, a voltage regulator and a switch are inserted, as will be disclosed more in detail. In the particular embodiment disclosed, the second supply is a supply for shared digital circuits.  
         [0026]    Whether the supply at terminal VCCPCU is taken from terminal VCL or from terminal VBAT, is decided by a voltage sensing circuit  32  which has an input connected to terminal VCL and an input connected to terminal VBAT. The voltage sensing circuit  32  also has two control outputs, each for controlling one of two switches SW  1  and SW  2  in the supply paths from terminals VCL and VBAT, respectively. The supply path from terminal VCL includes a voltage regulator  34  in series with switch SW  1 . The supply path from terminal VBAT includes a voltage regulator  36  in series with switch SW  2 .  
         [0027]    A third supply output at a terminal VBATI is also taken from terminal VCL, but through a threshold circuit  38  and a voltage regulator  40 , as will be disclosed more in detail. In the particular embodiment considered here, the third supply is for a remote keyless entry controller.  
         [0028]    Although not shown in FIG. 2, a fourth supply may be taken directly from terminal VBAT, for a transmitter of a remote control system, for example.  
         [0029]    With reference to FIG. 3, the voltage sensing circuit  32  will be disclosed more in detail. The voltage sensing circuit in FIG. 3 has a first supply from terminal VCL and a second supply from terminal VBAT, both referenced to the common ground terminal GND. A resistor R 1  is connected in series with a Zener diode ZD 1  between VCL and GND. The common node between R 1  and ZD 1  is connected to the gate of a PMOS transistor MP 1 , which has its source connected to VCL and its drain connected to GND through a current source. The drain of MP 1  drives the gate of a NMOS transistor MN 1 , the source of which is connected to GND and the drain of which is connected to VBAT through a current source. The drain of MN 1  drives the gate of another NMOS transistor MN 2 , the source of which is connected to GND and the drain of which is connected to VCL through a current source.  
         [0030]    A first buffer B 1  is driven by the drain of MN 2  and supplied from VCL. A second buffer B 2  is driven by the drain of MN 1  and supplied from VBAT. Buffer B 1  has an output at a terminal YESVCL, and buffer B 2  has an output at terminal NOVCL. A latch circuit L incorporating a level-shifter has an enable input connected to terminal YESVCL and a reset input connected to terminal NOVCL. Output Q of latch circuit L operates switch SW  1 , and the inverted output nQ operates switch SW  2 .  
         [0031]    In operation, MP 1  remains blocked until the Zener voltage at ZD 1  is reached. The Zener voltage of ZD 1  is selected to correspond to the minimum required voltage level for proper operation of the logic circuitry intended to be supplied from VCL.  
         [0032]    With MP 1  blocked (OFF), MN 1  is also blocked, and MN 2  is conducting (ON). Consequently, terminal YESVCL is at a level near GND and terminal NOVCL is at level VBAT if VBAT is present. The battery is not loaded in this state. Output nQ of latch circuit L is high, switch SW  2  is closed; output Q is low and switch SW  1  is open.  
         [0033]    With MP 1  conducting (ON), MN 1  is also conducting, and MN 2  is blocked (OFF). Consequently, terminal YESVCL is at a level near VCL and terminal NOVCL is at level near GND. Output Q of latch circuit L is high, switch SW  1  is closed; output nQ is low and switch SW  2  is open. Here, the battery is only loaded with one unit current (typ. 100 nA). This is not permanent because this state exists only if VCL is derived from the RF field.  
         [0034]    The voltage sensing circuit as disclosed has several important aspects.  
         [0035]    First, it addresses the problem of a reliable existence check for both supplies (VCL and VBAT). A voltage sensing circuit cannot provide a reliable output when it has no supply for itself.  
         [0036]    Second, the alternative connection of either of the two supplies works without any voltage drop.  
         [0037]    Third, two high-active signals are available to indicate the existence and the non-existence of VCL (Note: a low-active signal is not distinguishable from a missing supply).  
         [0038]    Fourth: the signal indicating the non-existence of VCL (NOVCL) is driven from VBAT. Consequently, a weak VCL is indicated only with VBAT, independent of VCL. Thus, SW  2  is closed only with VBAT present.  
         [0039]    Fifth: The signal indicating the existence of VCL (YESVCL) is derived from VCL itself. This means that SW  1  is closed even without the existence of VBAT.  
         [0040]    Sixth: When both supplies (VCL and VBAT) exist, VCL is the dominant supply to preserve battery lifetime.  
         [0041]    Seventh: No battery consumption occurs in the standby mode, i.e. when VCL is non-existent. This ensures battery lifetime over years.  
         [0042]    Voltage regulators  34  and  36  (FIG. 2) ensure that the connected user at terminal VCCPCU is supplied at the minimum required voltage level for proper operation to reduce the total current consumption.  
         [0043]    The preferred embodiment of the power management circuit includes a battery charge feature and a battery backup feature for battery replacement.  
         [0044]    With reference to FIG. 4, threshold circuit  38 , also referred to as a pre-regulator in FIG. 2, includes a series connection of a resistor R 2 , a Zener diode ZD 2  and an NMOS transistor MN 3  between VCL and GND. The node between Resistor R 2  and Zener diode ZD 2  is connected to the gate of a pass transistor PCH 1 , the source of which is connected to VCL and the drain of which is connected to an input VVC_IN of voltage regulator  40 . An enable input is applied to the gate of transistor MN 3  and to voltage regulator  40  from the output of an OR gate (FIG. 2) that has inputs ENVBUP and ENCHRG.  
         [0045]    In operation, pass transistor PCH 1  remains blocked until VCL reaches the Zener voltage of diode ZD 2  and an enable signal is received. With a sufficient voltage level of VCL, voltage regulator  40  receives a supply at input VCL_IN and provides a regulated output V_Regulated if an enable input is received. It should be understood that the output V_Regulated is equivalent to supply output VBATI in FIG. 2. This prevents VCL from dropping below a sufficient voltage level, independent of loads at VBAT 1 .  
         [0046]    With reference to FIG. 2, a switch SW  3  is provided to connect supply terminal VBATI to terminal VBAT in response to a battery charge command (issued by a charge station, see FIG. 1). Switch SW  3  is controlled by an output of a gate circuit that receives input ENCHRG and inverted input ENVBUP, i.e. NENVBUP.  
         [0047]    Terminals VBAT and VBATI may be provided with buffer capacitors CBAT and CBATI, respectively, as seen in FIG. 2.  
         [0048]    Another feature apparent from FIG. 2 is a switch SW  5  that connects terminals VCL and VBAT with each other in response to a command VBATON.  
         [0049]    With reference now to FIG. 5, a battery charge process is started at step  100 . If an identification device  18  is detected in step  102 , the charge station (FIG. 1) switches its LF transmitter on in step  104  to drive air coil  14 . In step  106 , a battery charge command is sent. In step  108 , charge is enabled. In step  110 , the charge station sends charge information to the charge controller within device  18 . If the charge controller in step  112  determines that the charge information was supplied and received, a battery charge loop is entered at step  114 . In step  114 , the charge controller measures the battery voltage VBAT. As long as the battery voltage is below the rated maximum, as determined in step  116 , the charge process is continued with waiting step  118 . Otherwise, the charge loop is exited at step  120  with Charge disabled.  
         [0050]    If step  112  determined that no charge information was received, or if step  120  returned with Charge disabled, step  122  continues with a several minutes waiting period for the charge station. Thereafter, step  124 , the LF transmitter in the charge station is switched off. A “Disable Charge” command is then issued in step  126 , and the loop returns to step  102 .