Adjustable current limiting/sensing circuitry and method

  
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A circuit and method are provided for accurate and adjustable current limiting/sensing in a power IC (100). In particular, a current limiting/sensing circuit (110) and method of use are provided that substitutes a transistor (MD2) in place of a resistor. Consequently, all of the components (MD1, MD2, MDout) in the IC (100) may be identical transis
Adjustable current limiting/sensing circuitry and method - schematic

tors, which may be fabricated by one process and integrated in one power structure. Therefore, process variations from device to device and errors due to thermal gradients between components may be minimized, thereby reducing the complexity and fabrication costs of the power ICs. Additionally, a user may readily adjust the trip/sensing point of the current limiting circuit (110) without having to physically alter individual components in the IC (100). a first input circuit connected to said output circuit and operable to generate a sense current, said first input circuit responsive to said first input signal; a second input transistor connected to said first input circuit and operable to generate a sensing voltage proportional to said sense current, said second input transistor responsive to a second input signal; and a third input transistor having an emitter of the third input transistor directly connected to a source/drain of said second input transistor, a collector of the third input transistor directly connected to an input of said first input circuit, and an input of said output circuit, said third input transistor being operable to switch responsive to said sensing voltage applied to an input of the third input transistor and thereby vary the variable output current of said output circuit responsive to said second input signal. This application is a Continuation, of application Ser. No. 08/655, 658, filed Jun. 3, 1996 now abandoned, and is a Continuation of Ser. No. 08/447, 284, filed May 22, 1995 now abandoned, and is Continuation of Ser. No. 08/145, 285 filed Oct. 29, 1995 now abandoned. In designing semiconductor power integrated circuits (ICs), it is often prudent to include limiting/sensing circuitry to monitor and, if necessary, control the IC`s output current or current flowing through the circuit. For example, current limiting/sensing circuitry may be provided to monitor and control the output current of high or low power IC switches or amplifiers. Also, current limiting/sensing circuitry may be used in an IC for fault protection, such as, for example, in detecting internal or external short circuits. In such an application, whenever an excessive amount of current is detected flowing either in the IC or through its output structure, the current limiting/sensing circuit may be designed to trip ” and turn the IC, or if desired, any associated circuitry off ”. Using a typical design for a current limiting/sensing circuit in an IC, a portion of the output current to be monitored is passed through a resistor. A voltage drop is developed across the resistor, which is proportional to the output current. Consequently, as the IC`s output current is increased, the current through the resistor is increased, which increases the voltage drop across the resistor. Conversely, as the output current is decreased, the voltage drop across the resistor is accordingly decreased. Therefore, by monitoring the voltage drop across the resistor, the IC`s output current may be monitored and thereby controlled, if so desired. FIG. 1 shows a typical semiconductor power IC design that includes a current sensing circuit having a FET arranged in combination with a diffusion or poly-silicon resistor to form a voltage divider. With an appropriate signal applied to the GATE IN input lead, IC 10 will conduct or be turned on ”. Current I1 passes through sensing transistor MD1 and resistor R1, while current I2 passes through output transistor MDout. Current I1 is proportional to current I2, as defined by the width/length (w/l) ratio of MD1 to



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