A technique is described for improving the sampled current accuracy in current mirror circuits such as are often used to monitor the current output to a load. A reference voltage is established to feedback the load voltage conditions at the output to the current reference circuitry, thereby greatly reducing the error in the reference current over that which is produced by prior art circuits. The technique is shown as applied to a current limiting circuit for driving an output load. An alternative embodiment is also disclosed.
An integrated circuit (1) includes a voltage-sensing circuit (2) and a current mirror (3) for sensing both a voltage (Vi) and a current (Ii) at one input terminal (T) of the integrated circuit. The voltage-sensing circuit (2) has an input which is connected to the input terminal (T) and an output which supplies a voltage level indication (Vu). The current mirror (3) has an input which is also connected to the input terminal (T), a reference terminal which is connected to a reference potential, and an output which supplies an output current (Iu). The input of the current mirror has a high impedance as long as the voltage level (Vi) is below a threshold voltage (Vth) of the current mirror (3). Within a voltage range determined by this threshold voltage (Vth), the voltage level indication (Vu) is a measure of the voltage level (Vi). The current mirror (3) becomes active and the output current (Iu) is a mirrored input current (Ii) if the voltage level (Vi) exceeds the threshold voltage (Vth). The threshold voltage may be determined by an input transistor (T1) of the current mirror (3). This input transistor (T1), which is arranged as a diode, starts conducting when the voltage level (Vi) reaches the threshold voltage (Vth) of this diode.
A circuit (10) and method for limiting output currents in a circuit having at least two output drivers (12,14) with outputs (16,18) on which first and second out-of-phase output currents are produced includes a current source (34) and a first current mirror (28). The first current mirror (28) has a first side (30) connected to sense a current in the output drivers and has a second side (32) connected to the current source (34). A magnitude of the current in the output drivers (12,14) produces a proportional voltage at a connection (52) between the second side (32) and the current source (34). First and second control transistors (40,42) are each connected to provide a control current to control respective output currents of the drivers (12,14), the first and second control transistors (40,42) being connected to receive an input signal (46) to pass the control current. A circuit (48) is provided for modifying the control current to the first and second control transistors (40,42) according to the proportional voltage.
A semiconductor integrated circuit is disclosed in which a power MOSFET supplies a squib of automobile air bag systems with load current. The power MOSFET Q.sub.1 provides squib Z.sub.L with the load current, and load current signal which outputs from shunt resistor R.sub.s is provided an operational amplifier consisting of transistors Q.sub.4 -Q.sub.10 with a negative feedback signal, so that the load current to be supplied to the squib Z.sub.L is restricted. The negative feedback operation is interrupted by load current function interruption signal which inputs to terminal T.sub.6. A circuit which consists of two current mirror circuits composed of transistors Q.sub.4 -Q.sub.10 and constant current source I.sub.4 supplies the operational amplifier with constant current to interrupt the feedback operation.
A circuit arrangement (16) for producing a D.C. current, includes a feedback loop for producing a reference current from an output terminal (27) with a negative temperature coefficient. The feedback loop includes a current-source stage (17, 18) which feeds a current mirror stage (19, 20, 21) in response to a measuring current supplied from a current bank (24, 25, 28, 29), which also produces the output reference current. The output of the current mirror stage (19, 20, 21) drives a working impedance formed by the main current path of a transistor 22, across which a control voltage is developed which is applied to a control input (23) of the current bank (24, 25, 28, 29). The circuit arrangement is advantageously combined with a reference current source (1) arranged as a bandgap circuit for producing a reference current on an output terminal (2) with a positive temperature coefficient, by enabling the user to select a current reference with either a positive or negative temperature coefficient. Further, the respective reference current outputs with positive and negative temperature coefficients, properly dimensioned, may be combined to provide an output current from a common output terminal (35) which is independent of temperature in a predefined temperature range.
A telecommunications device (10) includes an input terminal (12,13) for coupling to an input line of a telecommunications network. A detection arrangement (17, 18, 23) is coupled to the input terminal (12,13) for providing an output voltage in response to a voltage on the input line exceeding a threshold level. An output terminal (21) is coupled to receive the output voltage from the detection arrangement (17, 18). The detection arrangement (17, 18) includes a control arrangement (17) for setting the threshold level in response to a received control signal (19).
