Abstract:
A current-sensing switching circuit includes a first terminal clamp for the current feed for a first unit, based on the state of a second unit the state of which is detected at a second terminal clamp. The first terminal clamp may be switched, based on the current that is picked off at the second terminal clamp.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to electronic circuits, for example, to an electronic switching circuit for switching the voltage at an output terminal in response to the current that is picked off at a second output terminal. 
   BACKGROUND INFORMATION 
   Many electronic systems are designed for continuous operation, so that they are always able to respond to input signals. In order to save energy, these systems often have a rest mode or sleep mode, power only being supplied to those circuits that are essential for detecting input signals and which allow the idle system circuits to be switched on quickly. Input signals are normally detected by an electronic switching circuit supplying current to these idle circuits. For instance, recently introduced keyless locking systems for automobiles are able to lock or unlock automobiles in response to a radio-frequency signal transmitted by a transmitter of an electronic key. The locking circuit includes a self-polling ultra-high frequency (UHF) receiver circuit and an anti-interference, low-noise amplifier (LNA). Since the locking system should be able to respond to UHF signals, the power consumption of the circuits is kept to a minimum so as to prevent the car battery from being drained. The self-polling UHF receiver is able to be switched between a sleep mode and a run mode. The LNA may be switched in tandem so that it is supplied with power only when the UHF receiver is energized. 
   Conventional systems have achieved this by using a voltage comparator to monitor the output voltage of the UHF receiver circuit. When the UHF receiver switches from sleep state to operating state, the voltage comparator detects the change in the output voltage and supplies current to the LNA. According to this method, the output voltage of the UHF receiver is not well regulated. As a result, the LNA may mistakenly be energized due to electric noise and temperature fluctuations. Moreover, the sensitivity of the system is reduced if the LNA is not switched on in tandem with the UHF receiver. 
   SUMMARY 
   It is an object of the present invention to provide a reliable circuit arrangement for the supply of current in response to changes in an electric signal, or to provide at least a useful alternative. 
   The present invention relates to a current-sensing switching circuit including a first terminal for the supply of current for a first unit, based on the state of a second unit the state of which is detected at a second terminal, and in which the first terminal is switched based on the current picked off at the second terminal. 
   The second unit may be a high-frequency receiver having a run mode and a sleep mode, which, when in operation, is connected to the second terminal, it being possible that the first unit may be, for example, an LNA. During sleep mode, the picked off current may be below a threshold level and above the threshold level during run mode. 
   Example embodiments of the present invention are described below with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an exemplary embodiment of a UHF receiver circuit. 
       FIG. 2  is a circuit diagram of an exemplary embodiment of a current-mirror circuit. 
       FIG. 3  is a circuit diagram of an exemplary embodiment of a current-sensing circuit of the UHF receiver circuit. 
       FIG. 4  is a circuit diagram of a circuit arrangement of an exemplary embodiment of a current-sensing switching circuit and a UHF modulator in run mode. 
       FIG. 5  is a circuit diagram of a circuit arrangement of an example embodiment of a current-sensing switching circuit and a UHF modulator in sleep mode. 
   

   DETAILED DESCRIPTION 
   A receiver circuit  2  of a door-locking system of a vehicle as shown in  FIG. 1  may include a Motorola Romeo-2 receiver chip  4  for the demodulation of received UHF signals, and an anti-interference, low-noise amplifier (LNA)  6  for the amplification of the demodulated signals. Demodulator  4  has two operating modes: a run mode in which demodulator  4  drains a minimum current of 4 mA from current feed  10  via a load terminal  15 , and a sleep mode in which demodulator  4  only drains ≈270 μA. To save energy, terminal clamp  13  energizes the LNA only when demodulator  4  is in run mode. A current-sensing switching circuit  8  accomplishes this by measuring the current which demodulator  4  drains at load terminal  15  and by supplying a voltage that is close to the supply voltage when demodulator  4  is in run mode, and supplying a voltage that is close to zero when demodulator  4  is in sleep mode. A tandem switch, therefore, may ensure that demodulator  4  and LNA  6  run and idle in tandem. 
   Current-sensing switching circuit  8  includes a current-sensing circuit  16  and a Vcc switching circuit  44 . Current-sensing circuit  16  may be supported by a conventional current-mirror circuit, for example, as a constant current source. As shown in  FIG. 2 , a conventional current-mirror circuit  12  includes two nominally identical pnp transistors  21  and  22  (Siemens BC8568) the emitters of which are connected to a common supply-voltage bus bar  14 , and the bases of which are directly connected to one another. The collector of first transistor  21  is short-circuited with the base and grounded by a resistor  23 . The collector of second transistor  22  provides the output of the electric circuit and is connected to an external load. The current flowing through first transistor  21  is given by I=(V supply −V BE )/R. Since the base emitter transition junctions of both transistors may be identically forward-biased, the same current may flow through both transistors, and the collector current of second transistor  22  thus provides a constant current source. 
   Current-sensing circuit  16  may be a modified mirror circuit, as shown in  FIG. 3 , including a current-sensing resistor  31  in the supply bus bar between the emitters of the two transistors  21  and  22 , a third resistor  32  between the supply bus bar and the emitter of second transistor  22 , and a fourth resistor  33  between the collector of second transistor  22  and ground. The presence of current-sensing resistor  31  and of third resistor  32  causes an asymmetry in the emitter potentials of the two transistors, which is a function of the current drained by load  4 . The relative current flowing through first transistor  21  and second transistor  22  may be determined by the relative values of current-sensing resistor  31  and third resistor  32  as well. The resulting output voltage via fourth resistor  33  bears the same relationship to the load current. This output voltage may be used in current-sensing switching circuit  8  to operate Vcc switching circuit  44 , as shown in  FIGS. 4 and 5 . As a result, the resistances are selected according to the switching characteristics of this circuit. To be more specific: the value of third resistor  32  may be selected such that, in the unloaded state (i.e., in sleep mode), the voltage via fourth resistor  33  is lower than the lower threshold voltage required to activate Vcc switching circuit  44 . The value of current-sensing resistor  31  may be selected such that, given a normal load (i.e., in run mode), the output voltage via fourth resistor  33  is greater than the upper threshold voltage level required to activate Vcc switching circuit  44 . For circuit  2 , these voltages are 200 mV or 2V via fourth resistor  33 . 
     FIGS. 4 and 5  illustrate exemplary embodiments of a receiving circuit, which supplies the current to external LNA  6  for the, for example, two Motorola Romeo-2 demodulators  4  used and that run in the run or sleep mode. It is possible to divide these circuits into three logical blocks: a load  4 , current-sensing circuit  16  and Vcc switching circuit  44 . As described above, the output voltage of current-sensing circuit  16  is proportional to the current drained by load block  4 . This output voltage may be used to operate Vcc switching circuit  44 , which provides a connected output at terminal clamp  13 . The output voltage of current-sensing circuit  54  is run via an npn-transistor inverter. Switching circuit  54  switches current feed  10  of terminal clamp  13  of external LNA  6  when demodulator  4  is in run mode. 
     FIG. 4  shows demodulator  4  with an effective load of 1.1 k Ωand a current drain of 4.3 mA in run mode. Output  13  is 4.992 V of a supply voltage of 5.000 V. In sleep mode, as shown in  FIG. 5 , demodulator  4  has a load of 18 k Ωand drains a current of less than 0.3 mA. The corresponding output voltage at terminal clamp  13 , at ≈152 nV, is low. From this it may be inferred that, when demodulator  4  switches from sleep mode to run mode, the output voltage changes from ≈152 nV to 4.992 V, which may also be expressed as a change from 0.0 to 5.0 V. If terminal clamp  13  is used to switch on external LNA  6  in  FIG. 1 , LNA  6  may be energized when demodulator  4  is in run mode. Demodulator  4  and LNA  6  are switched on in tandem. 
   The example current-sensing switching circuit  8  described above includes an operating current of a switching element that is a more reliable indicator of the working state than the output voltage since it is less sensitive to electrical noise or temperature fluctuations. The current-sensing switching circuit according to the present invention may be very stable across a large temperature range if a matched transistor pair is used that is produced on a single substrate. Moreover, the current-sensing switching circuit may be less costly than a voltage-sensing switching circuit.