Abstract:
The present invention discloses a voltage detection type overcurrent protection device, which applies to the output stage of a CMOS Class-D audio amplifier. Generally, a Class-D audio amplifier is used to drive a high-load loudspeaker; therefore, it needs a high-current driver. When there is a short circuit in the load, the high current will burn out the driver stage. The present invention detects the output voltage to indirectly monitor whether the output current is too large. Once an overcurrent is detected, the output-stage transistor is turned off to stop high current lest the circuit be burned out.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an overcurrent protection device for a Class-D amplifier, particularly to a voltage detection type overcurrent protection device for a Class-D amplifier, which applies to a two-state output (1 or 0) high-power conversion efficiency Class-D amplifier or a general PWM system. 
   2. Description of the Related Art 
   Refer to  FIG. 1 . Generally, a Class-D amplifier adopts a current sensor to detect current, wherein a current mirror  12  copies an output current to another circuit by a very small proportion, and detects whether the copied current exceeds a threshold. Such a design usually at least needs an operational amplifier  16  and a current comparator  14 . Therefore, when realized with an integrated circuit, the conventional current detection circuit is more complicated, less power-efficient and less space-efficient. 
   Refer to  FIG. 2  for a conventional CMOS (Complementary Metal Oxide Semiconductor) Class-D amplifier. An audio signal is amplified by an audio amplifier  18  and then output to a comparator  20 . The comparator  20  compares the amplified audio signal with a signal of a triangle generator  22  and outputs the result to a clock logic module  24 . CMOS transistors  26  succeed to the clock logic module  24  and function as the output stage. The output terminals of the CMOS transistors  26  are connected to a filter circuit containing inductors  28  and capacitors  30 , and the filter circuit is then connected to a loudspeaker  32 . 
   A Taiwan patent No. 560125 disclosed a “Current Detection and Overcurrent Protection Circuit for Transistors of PWM Amplifier”. The Claim  4  of the prior-art patent disclosed an overcurrent protection circuit for a switching circuit, which uses at least one switching transistor P 1  to provide a given load current IL for a load and which comprises: a sampling/holding capacitor C 2 , a switch  31 , an overcurrent detection device  50 , and a controller  60 . The sampling/holding capacitor C 2  is used to temporarily hold a terminal voltage VP 1  of the turned on switching transistor P 1 . The switch  31  is arranged in between the switching transistor P 1  and the sampling/holding capacitor C 2 , and is turned on synchronously with the switching transistor P 1 . The overcurrent detection device  50  is used to determine whether a detected voltage VS 1 , which is corresponding to a voltage charging the sampling/holding capacitor C 2 , exceeds a predetermined reference voltage VREF. When the detected voltage VS 1  is over the reference voltage VREF, the controller compulsorily turns off the switching transistor P 1 . From the above description, it is known that the prior-art overcurrent protection circuit uses the sampling/holding capacitor to detect overcurrent. 
   Contrasting to the prior art, the present invention proposes a voltage detection type overcurrent protection device to detect overcurrent. 
   SUMMARY OF THE INVENTION 
   The primary objective of the present invention is to provide a voltage detection type overcurrent protection device for a Class-D amplifier, which is used to perform overcurrent protection for a rear-stage high-current driver. 
   Another objective of the present invention is to provide a voltage detection type overcurrent protection device for a Class-D amplifier, which can be realized with an integrated circuit more simply and space-efficiently and will operate more power-efficiently. 
   The present invention proposes a voltage detection type overcurrent protection device for a Class-D amplifier, which is used to perform overcurrent protection for a rear-stage high-current driver. A driver stage usually comprises large-area PMOS and NMOS so that it can provide sufficient current for a high-load loudspeaker. As a Class-D amplifier outputs digital PWM signal of 0 and 1, the PMOS (P-channel MOS) transistors and NMOS (N-channel MOS) transistors in the output stage thereof output on-off or off-on signals. 
   Below, the embodiments of the present invention are described in detail in cooperation with the attached drawings to make easily understood the objectives, technical contents, characteristics and accomplishments of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing a prior-art circuit; 
       FIG. 2  is a block diagram of conventional class D amplifier; 
       FIG. 3  is a diagram schematically showing a circuit according to the present invention; 
       FIG. 4  is a diagram schematically showing overcurrents in a circuit according to the present invention; 
       FIG. 5  is a diagram schematically showing that a first voltage comparator detects an overcurrent according to the present invention; and 
       FIG. 6  is a diagram schematically showing that a second voltage comparator detects an overcurrent according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Refer to  FIG. 3  for an output stage of a Class-D amplifier. Firstly, an audio signal is converted into a PWM signal (the output of a Class-D amplifier). Next, a clock logic module  42  converts the PWM signal into non-overlap clock signals, which control the output-stage CMOS transistors  44  and  46  to function as switches and drive a load, such as a loudspeaker. The CMOS transistors include a PMOS transistor  44  and a NMOS transistor  46 , and the gates of the PMOS transistor  44  and NMOS transistor  46  are connected to the clock logic module  42 . In  FIG. 3 , VDD denotes the voltage of the power source. The output terminals of the CMOS transistors are respectively connected to a first voltage comparator  48  and a second voltage comparator  50 . 
   A reference voltage circuit  52  is connected to the first voltage comparator  48  and the second voltage comparator  50 . According the preset values of the short-circuit current, the reference voltage circuit  52  provides two different reference voltages for the first voltage comparator  48  and the second voltage comparator  50 . The input terminals of a digital voltage debounce device  54  are respectively connected to the output terminals of the first voltage comparator  48  and the second voltage comparator  50 . The input terminals of an overcurrent protection module  56  are connected to the digital voltage debounce device  54 , and the output terminal of the overcurrent protection module  56  is connected to the clock logic module  42 . 
   When there is an overcurrent occurring, it means that the output has a very small resistance with respect to the power source or the ground, and that the output voltage is greatly dragged upward or downward. At this time, the output voltage is no more the ideal VDD or 0 but between VDD and 0. Below is derived the relationship between the output current and the output voltage. 
   Refer to  FIG. 4 . Suppose VDD denotes the voltage of the power source, R ON     —     p  the turn-on resistance of the PMOS transistor, R ON     —     N  the turn-on resistance of the NMOS transistor, R LOAD  the resistance of the external load, R SC  the short-circuit resistance, V OUT  the output voltage, I OUT  the output current, and I IN  the input current. 
   There are two cases to be discussed below. 
   Case I 
   In case I, the PMOS transistor  44  is turned on, and the NMOS transistor  46  is turned off, and I OUT =VDD/(R LOAD +R ON     —     P ). 
   In a normal state, the short-circuit resistance R SC  does not exist, and R LOAD &gt;&gt;R ON     —     P , and V OUT =VDD×R LOAD /(R LOAD +R ON     —     P )≈VDD. 
   When there is an overcurrent occurring, R SC ≈0, and the parallel resistance of R SC  and R LOAD  is R EQ =(R SC //R LOAD ). Thus, R EQ ≈0, and V OUT =VDD×R EQ /(R EQ +R ON     —     P )&lt;VDD. In other words, the smaller the short-circuit resistance R SC , the smaller the output voltage V OUT . 
   Case II 
   In case II, the PMOS transistor  44  is turned off, and the NMOS transistor  46  is turned on. 
   In a normal state, the short-circuit resistance R SC  does not exist, and V OUT =0. 
   When there is an overcurrent occurring, R SC ≈0, and the parallel resistance of R ON     —     N  and R LOAD  is R N =R ON     —     N //R LOAD . Thus, I IN =VDD/(R SC +R N ), and V OUT =VDD×R N /(R N +R SC )&gt;0. In other words, the smaller the short-circuit resistance R SC , the greater the output voltage V OUT . 
   The short-circuit current can be calculated from the output voltage. Thus, the reference voltages of the first voltage comparator  48  and the second voltage comparator  50  can be calculated from the predetermined critical short-circuit currents. When the output of the first voltage comparator  48  or the second voltage comparator  50  has been 1 for a long time (&gt;200 ns), the digital voltage debounce device  54  also outputs a signal of 1 to warn that there is an overcurrent. After receiving the warning, the overcurrent protection module  56  turns off the output-stage transistors  44  and  46  and stops high current lest the circuit be burned out. 
   Refer to  FIG. 5  for the overcurrent protection of the PMOS transistor  44 . The clock logic module  42  can control the power amplifier to output 1 or 0. When CMOS driver output stage is expected to output 1, the first voltage comparator  48  begins to operate, and the clock logic module  42  sends an enable signal to the first voltage comparator  48 . When CMOS driver output stage is expected to output 0, the first voltage comparator  48  will not operate. The first voltage comparator  48  can check whether the voltage is pulled down to below the reference voltage. If the voltage is below the reference voltage, the first voltage comparator  48  will output 1 to indicate an overcurrent. 
   Refer to  FIG. 6  for the overcurrent protection of the NMOS transistor  46 . When CMOS driver output stage is expected to output 0, the second voltage comparator  50  begins to operate, and the clock logic module  42  sends an enable signal to the second voltage comparator  50 . When CMOS driver output stage is expected to output 1, the second voltage comparator  50  will not operate. The second voltage comparator  50  can check whether the voltage is pulled upward to over the reference voltage. If the voltage is over the reference voltage, the second voltage comparator  50  will output 1 to indicate an overcurrent. 
   When the output of the power amplifier shifts from 1 to 0 or from 0 to 1, the power source is likely to have voltage bounce, which may result in instantaneous instability of the power source and may cause the comparator to malfunction. The digital voltage debounce device  54  allows only a longer interval of logic “1” signal to pass but eliminates an instantaneous logical “1” signal lest malfunction occur, wherein the instantaneous logical “1” signal is referred to a logical “1” signal having a length of from 100 to 300 ns. After passing through the digital voltage debounce device  5 , the long interval of logic “1” signal, which is originally output by the voltage comparator, is still a “1”-level signal. At this time, the overcurrent protection module  56  turns off the output-stage transistors  44  and  46  to stop high current. 
   In conclusion, the present invention proposes a voltage detection type overcurrent protection device to protect the high-current rear-stage driver from overcurrent. The present invention outputs the same signal as a PWM system does and thus may function as a protection device of a general PWM system. The present invention can be realized with an integrated circuit more simply and space-efficiently and will operate more power-efficiently. 
   The preferred embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Therefore, any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.