Patent Publication Number: US-6903589-B2

Title: Output driver circuit with automatic slew rate control and slew rate control method using the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor device, and more particularly, the present invention relates to an output driver of a semiconductor device and to a method of controlling the slew rate thereof. 
   A claim of priority is made to Korean Patent Application No. 2003-1592 filed on Jan. 10, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   2. Description of the Related Art 
   In a semiconductor device, an output driver is used to output signals externally from the device. The slew rate of an output signal from the output driver is a significant factor for devices operating at a high-frequency. This is because the slew rate must generally be increased for a high-frequency operation as compared to a low-frequency operation. 
   The slew rate of the output signal is typically adjusted using one of two methods. In a first method, a signal output from an output driver is measured and the slew rate of the signal is adjusted using a predetermined selection signal according to the result of measurement. In a second method, the slew rate of an output signal is adjusted according to the operation of a circuit that detects a process voltage temperature (PVT) which exhibits some correlation to slew rate. 
   However, the first method is disadvantageous because it is generally inconvenient to measure the slew rate of the output signal using conventional measurement techniques. The second method is also disadvantageous because the slew rate is not adjusted based an actually measured slew rate, and inaccuracies may therefore be introduced. 
   SUMMARY OF THE INVENTION 
   The present invention provides an output driver in which the slew rate of an output signal is automatically detected and is adjusted based on the detection result. 
   The present invention also provides a method of adjusting the slew rate of an output signal output from such an output driver. 
   According to an aspect of the present invention, there is provided a circuit for outputting a signal having a target slew rate. The circuit includes an output driver, a slew rate detection circuit, a pulse width comparison circuit, and a slew rate control circuit. The output driver outputs an output signal and is responsive to a slew rate control signal to increase or decrease a slew rate of the output signal. The slew rate detection circuit compares the output signal from the output driver with a first reference voltage and generates a first comparison output signal which exhibits a voltage transition when the output signal reaches the first reference voltage, and compares the output signal with a second reference voltage and generates a second comparison output signal which exhibits a voltage transition when the output signal reaches the second reference voltage. The pulse width comparison circuit compares a first pulse width value with a second pulse width value, the first pulse width value corresponding to a time delay difference between the voltage transition of the first comparison output and the voltage transition of the second comparison output signal, and the second pulse width value corresponding to a target slew rate of the output signal from the output driver. The slew rate control circuit generates the slew rate control signal in response to a signal output from the pulse width comparison circuit. 
   According to another aspect of the present invention, there is provided a method of controlling the slew rate of an output signal output from an output driver. The output signal from the output driver is compared with a first reference voltage and a first comparison output signal is generated which exhibits a voltage transition when the output signal reaches the first reference voltage. The output signal is compared with a second reference voltage and a second comparison output signal is generated which exhibits a voltage transition when the output signal reaches the second reference voltage. A first pulse width value is then compared with a second pulse width value. The first pulse width value corresponds to a time delay difference between the voltage transition of the first comparison output signal and the voltage transition of the second comparison output signal, and the second pulse width value corresponds to a target slew rate of the output signal from the output driver. The slew rate of the output signal is decreased when the first pulse width value is smaller than the second pulse width value, and the slew rate of the output signal is increased when the first pulse width value is larger than the second pulse width value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings, in which: 
       FIG. 1  shows a diagram illustrating basic principles of a method of controlling the slew rate of an output signal output from an output driver, according to a preferred embodiment of the present invention; and 
       FIG. 2  is a block diagram illustrating the structure of an output driver circuit with automatic slew rate control, according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art. The same reference numerals represent the same elements throughout the drawings. 
     FIG. 1  shows a diagram illustrating basic principles of a method of controlling the slew rate of an output signal output from an output driver according to a preferred embodiment of the present invention. In  FIG. 1 , OUT 1  denotes a desired output signal having a target slew rate, OUT 2  denotes an output signal having a slew rate that is greater than that of the desired output signal OUT 1 , and OUT 3  denotes an output signal having a slew rate that is less than that of the desired output signal OUT 1 . 
   Hereinafter, a method of controlling the slew rate of a signal output from an output driver according to a preferred embodiment of the present invention will be described in detail with reference to FIG.  1 . In this embodiment, the slew rate of an output signal is detected using two reference voltages, i.e., a first reference voltage VREF 1  and a second reference voltage VREF 2 . Here, the level of the second reference voltage VREF 2  is higher than that of the first reference voltage VREF 1 . 
   In this method, first, a target slew rate of an output signal is set from an external source. For example, when an output driver is included in a semiconductor memory device such as synchronous dynamic random access memory (DRAM), the target slew rate is set from external the semiconductor memory device during a mode register set (MRS) operation in the semiconductor memory device. 
   The target slew rate may be externally supplied in the form of a digital value indicative of a time delay difference A between a target signal CP 1 (A) and a target signal CP 2 (A) of the desired output signal OUT 1  having the target slew rate. Here, the desired output signal OUT 1  having a target slew rate is assumed to be an ideal signal. The target slew rate can be changed by externally adjusting the digital value. 
   After setting the target slew rate of the desired output signal, the slew rate of the actual output signal is monitored as to whether is greater than or less than the target slew rate. 
   That is, assume first that the signal OUT 2  is actually output from the output driver, and accordingly, the actual slew rate of the output signal is greater than the target slew rate. The level of the actual output signal OUT 2  is compared with that of the first reference voltage VREF 1  so as to generate a comparison output signal CP 1 (B). As shown in  FIG. 1 , the signal CP 1 (B) exhibits a voltage transition when the signal OUT 2  reaches VREF 1 . Next, the level of the actual output signal OUT 2  is compared with that of the second reference voltage VREF 2  so as to generate a comparison output signal CP 2 (B). Again, the signal CP 2 (B) exhibits a voltage transition when the signal OUT 2  reaches VREF 2 . The time delay difference between the voltage transitions of signals CP 1 (B) and CP 2 (B) is denoted in  FIG. 1  by reference B. 
   Now assume that the signal OUT 3  is actually output from the output driver, and accordingly, the actual slew rate of the output driver is less than the target slew rate. The level of the actual output signal OUT 3  is compared with that of the first reference voltage VREF 1  so as to generate a comparison output signal CP 1 (C). As shown in  FIG. 1 , the signal CP 1 (C) exhibits a voltage transition when the signal OUT 3  reaches VREF 1 . Next, the level of the actual output signal OUT 3  is compared with that of the second reference voltage VREF 2  so as to generate a comparison output signal CP 2 (C). Again, the signal CP 2 (C) exhibits a voltage transition when the signal OUT 3  reaches VREF 2 . The time delay difference between the voltage transitions in signals CP 1 (C) and CP 2 (C) is denoted in  FIG. 1  by reference number C. 
   By comparing the time delay difference B or C with the target value A, a determination is made as to whether the actual slew rate of the output signal should be increased or decreased. 
   That is, if the time delay difference B is detected which is smaller than the time delay difference A, the slew rate of the actual output signal OUT 2  is reduced in a direction towards the target slew rate of the desired output signal OUT 1 . On the other hand, if the time delay difference C is detected which is greater than the time delay difference A, the slew rate of the actual output signal OUT 3  is increased in a direction towards the target slew rate of the desired output signal OUT  1 . In this manner, by repeatedly increasing or decreasing the slew rate of the output signal based on the detected time delay difference, the actual slew rate of the output signal is brought into conformity with the target slew rate. 
     FIG. 2  is a block diagram illustrating the structure of a circuit having an automatic slew rate control mechanism according to a preferred embodiment of the present invention. The circuit of  FIG. 2  operates according to the method explained above in connection with  FIG. 1   
   As shown, the circuit of  FIG. 2  includes an output driver  21 , a slew rate detection circuit  23 , a pulse width comparison circuit  25 , and a slew rate control circuit  27 . 
   The slew rate detection circuit  23  compares an output signal OUT output from the output driver  21  with the first reference voltage VREF 1  and outputs the result of the first comparison as a signal CP 1 . Also, the slew rate detection circuit  23  compares the output signal OUT with the second reference voltage VREF 2  and outputs the result of comparison as a signal CP 2 . As previously mentioned, the level of the second reference voltage VREF 2  is higher than that of the first reference voltage VREF 1 . 
   The pulse width comparison circuit  25  compares a first pulse width with a second pulse width. Here, the first pulse width corresponds to a time delay difference, i.e., a time delay difference B or C as shown in  FIG. 1 , between the rising edges (voltage transitions) of the first and second comparison output signals CP 1  and CP 2 . The second pulse width corresponds to a target slew rate of the output signal OUT output from the output driver  21 , i.e., a time delay difference A as shown in  FIG. 1 , between the first and second target comparison output signals CP 1  and CP 2 . 
   As described above, the target slew rate is a digital value that is set from an external source. Therefore, the target slew rate can be changed by adjusting a digital value from external the semiconductor memory device. If the output driver is included in a semiconductor memory device such as synchronous DRAM, the target slew rate may be set external the semiconductor memory device during a mode register set (MRS) operation in the semiconductor memory device. 
   In response to a signal output from the pulse width comparison circuit  25 , the slew rate control circuit  27  generates a control signal SCNT which is supplied to a control input of the output driver  21 . The output driver  21  is responsive to the control signal SCNT to either increase or decrease the actual slew rate of the output signal. 
   More specifically, when the pulse width comparison circuit  25  compares the first (measured) pulse width with the second (target) pulse width and determines that the first pulse width is smaller than the second pulse width, the slew rate control circuit  27  sets a value of the control signal SCNT which causes the output driver  21  to decrease the slew rate of the output signal OUT. In contrast, when the first pulse width is larger than the second pulse width, the slew control circuit  27  sets a value of the control signal SCNT which causes the output driver  21  to increase the slew rate of the output signal OUT. In this way, the slew rate of the output signal OUT can be adjusted to the target slew rate. 
   As described above, an output driver with automatic slew rate control according to the present invention, and a slew rate control method according to the present invention, are advantageous in that the slew rate of an output signal is automatically detected in a semiconductor memory chip including the output driver, and thus, the slew rate can be adjusted based on the detection result. 
   While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.