(1) Field of the Invention
This invention relates generally to current mirrors and relates more particularly to the measurement of the load current of current mirrors.
(2) Description of the Prior Art
Current mirrors are the most widely use analog circuit. Most of the transistors in an analog integrated circuit are parts of current mirrors. A current mirror may be thought of as an adjustable current regulator, the current limit being easily set by a single resistance.
Current mirrors are used as current sources. An ideal current source has infinite output impedance. That is, the output current does not change, even for large swings in output voltage. This means that actual current mirrors have very high impedance.
A challenge to the designers of electronic circuits is to define a low-cost solution to measure a load current, especially in integrated circuits, without impacting the load current. Current mirrors can offer a solution to this task. This is especially important for diagnostic and safety circuits, especially for short circuit detection. There are various patents in regard of this area.
U.S. Pat. No. 6,531,885 (to Manheuve et al.) describes an apparatus and method for testing supply connections of an electronic device by using a current mirror configuration through using a particular connection of the branches of the current mirror to the supply line. Such connection results in unbalanced operation of the current mirror but depending whether the supply connection under test is proper or not, the unbalance is essentially different, resulting in a high sensitivity of the test device.
U.S. Pat. No. 6,392,392 (to Nakahara) discloses an over-current detecting circuit (OCD Circuit) for comparing a voltage drop by an ON-state resistance of an output transistor with a reference voltage, to detect an over-current state of the output transistor. This OCD Circuit is provided with a first power source and an RVG Circuit, which outputs a first reference voltage on the basis of a voltage supplied from the first power source. The OCD Circuit is also provided with a constant-current source which generates a constant current having a second temperature characteristic on the basis of the first reference voltage, and a current mirror circuit which inputs the constant current. Moreover, the OCD Circuit is provided with a current-voltage converting circuit, which converts an output current from the current mirror circuit, to a voltage and outputs a reference voltage, which has a temperature characteristic in proportion to the second temperature characteristic. A first temperature characteristic of the output transistor is compensated by the temperature characteristic of the reference voltage.
U.S. Pat. No. 5,159,516 (to Fujihira) describes an improved over-current-detection circuit for detecting over-current condition of a main current flowing through a semiconductor power device using a constant-current device to provide a proportionally enhanced potential difference representative of increases in such current. The proportionally enhanced potential difference increases the accuracy of measuring the current flowing between first and second main-current terminals of the semiconductor power device, thereby providing more accurate over-current detection, without requiring an increase in the accuracy of a voltage comparator. The power semiconductor device is coupled to a current-mirror element having a shunt-current terminal. The over-current-detection circuit incorporates a constant-current device connected between the second main-current terminal and the shunt-current terminal. The constant-current device maintains the shunt current at a substantially constant level, after the shunt current rises to a predetermined level. The over-current-detection circuit also incorporates determining means, which is coupled across the constant-current device and includes a voltage comparator. The voltage comparator provides an over-current-detection signal when the potential difference across the constant-current device exceeds a predetermined voltage. The over-current-detection signal is used to control the main current of the semiconductor power device. The invention provides an over-current-detection circuit with superior over-current-detection accuracy using conventional power-IC manufacturing technology.