Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Slope control of leading or trailing edge of rectangular or...
Reexamination Certificate
1999-09-02
2001-08-28
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Slope control of leading or trailing edge of rectangular or...
C327S108000, C327S437000, C330S288000, C323S316000
Reexamination Certificate
active
06281730
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to driver circuits, and in particular, to a digital driver circuit that provides a controlled slew rate at the output of the circuit.
2. Description of the Related Art
Driver circuits are used for communicating digital information between different portions of electronic systems. For example, in mixed signal systems that have both analog and digital circuits, information in the form of analog signals may be received by one section of an electronic system and converted into digital information. Once converted into digital form, the information may be communicated across the system to a microprocessor, microcontroller, or digital signal processor. Driving digital data between the different portions of an electronic system presents many design challenges. First, digital signal lines between devices in an electronic system usually have large capacitances and parasitic inductances associated with them. Large CMOS devices utilizing large current values are typically used to drive the large capacitive loads at high speeds. However, at high switching frequencies, the large currents through the parasitic inductances cause large voltage spikes that can exceed noise margin requirements. This switching noise becomes even more of a problem when the noise couples into the analog portions of a mixed signal integrated circuit.
Another challenge associated with designing digital drivers is to maintain a constant output current across variations in process, temperature, and supply voltage. The output current of a typical CMOS driver will vary with process, temperature, and supply voltage. In order to guarantee the desired operation at nominal values, typical drivers are designed to output a nominal current which is greater than the minimum required under normal conditions. However, in a fast process, this current can become several times more than the minimum required. This large current can result in high voltage slew rates which can inject noise into the analog portions of the mixed signal system.
Accordingly, it would be desirable to have a digital driver circuit with a controlled current output, or slew rate, to reduce the switching noise in the mixed signal system. Additionally, it would be desirable to have a digital driver that maintains a constant current output across process, temperature, and supply variations.
SUMMARY OF THE INVENTION
A driver circuit in accordance with the present invention combines current controlled current source and sink circuits, which are independent of process, temperature, and supply voltage, and voltage controlled current source and sink circuits to control the slew rate at the output of the driver circuit and thereby reduce switching noise. The driver includes an output transistor coupled to an output node, a current source, a current mirror transistor having a control node connected to the control node of the output transistor and a conduction path coupled to the current source, and a voltage controlled switch coupled between the conduction path of the current mirror transistor and the control node of the output transistor. The voltage controlled switch is coupled to the output node and is open when the output node is within a first voltage range, and is closed when the output node is within a second voltage range.
In accordance with another embodiment of the present invention, a first input transistor has a control node coupled to receive a circuit input and a conduction path coupled in series between the current source and the control node of the output transistor. In accordance with another embodiment of the present invention, a second input transistor of complementary device type to the first input transistor has a control node coupled to receive the circuit input and a conduction path coupled both the conduction path of the first input transistor and to the control node of the output transistor.
In accordance with another embodiment of the present invention, a controlled slew rate driver includes timing and control logic for receiving input signals, a high side driver stage for charging a capacitive output node, and a low side driver stage for discharging a capacitive output node. The high side driver and the low side driver are substantially similar and are comprised of complementary device types.
In accordance with another embodiment of the present invention, selectable current source is coupled to the current source in the driver circuit for digitally controlling the output slew rate of the controlled slew rate driver. The selectable current source includes a current mirror array in which current gate transistors are selectively activated to increase the amount of current supplied at the output of the selectable current source. In one embodiment the current gate transistors are controlled by digital outputs of a digital logic circuit.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.
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National Semiconductor Corporation
Stallman & Pollock LLP
Wells Kenneth B.
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