Abstract:
A sample-and-hold interface circuit of a pickup head uses a circuit switch to control a voltage divider. When the circuit switch receives a signal Break, a voltage divider does not consume extra power to decrease the driving power consumption of a pickup head. And a controller can sample an unattenuated input signal to increase the signal/noise ratio.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an interface circuit, and more particularly to a sample-and-hold circuit to work between chips or circuits with different working voltages to provide a sample-and-hold function and protect the chips or circuits with a lower working voltage when operating at a higher voltage. 
     2. Description of the Prior Art 
     When a signal travels from a chip or circuit with higher working voltage to a chip or circuit with lower working voltage, there must be an interface circuit between the two chips or circuits to attenuate the signal to the range that the chip or circuit with lower working voltage can tolerate. For example, a pickup head usually works at 5V, and the controller of a DVD player works at 3.3V. If the writing voltage (3.3V-5V) of a pickup head inputs to a sample-and-hold circuit, like a Switched-OP, of a controller directly for a long time, the oxide layer manufactured in 3.3V process at the input terminal of the Switched-OP will break down to destroy the controller eternally. The prior art is shown as  FIG. 1 . A voltage divider, comprising a first resistor  1  and a second resistor  2 , is used to attenuate an input signal of a sample-and-hold circuit  3  down to the level lower than 3.3V to prevent this problem. 
     But in the above situation, the pickup head has to provide extra current “I” to the voltage divider. The extra current produces extra power consumption and thermal noise. Besides, the bigger the resistors are, the longer the setup time of the input of the Switched-OP is. Then the practical sampling time is decreased. Furthermore, the writing voltage unnecessary to be sampled and the reading voltage necessary to be sampled are both attenuated. The attenuated reading voltage combing with the noise at the back end makes the signal/noise ratio lower. 
     SUMMARY OF THE INVENTION 
     Obviously, the prior art mentioned in the above cannot satisfy the requirement of low power operation. One of the purposes of the present invention is to control the voltage divider to be open-circuit on the character that only a writing voltage needs to be attenuated when the input is a reading voltage to decrease power consumption without providing extra current. 
     Another purpose of the present invention is to increase the signal/noise ratio when the input is a reading voltage without being attenuated by the voltage divider. 
     A sample-and-hold interface circuit of a pickup head includes a pair of resistors in serial (voltage divider), a Switched-OP, a PMOS transistor and a timing circuit. The voltage divider can attenuate an input voltage to protect the Switched-OP with a lower working voltage. The Switched-OP provides a sample-and-hold function. The PMOS transistor works as the switch of the voltage divider. When the PMOS transistor is on, the pair of the resistors in serial work as a voltage divider. When the PMOS transistor is off, the pair of the resistors in serial is open-circuit. The timing circuit is used to control the on and off of the PMOS, and the timing for the Switched-OP to sample and hold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrated several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a diagram of the prior art of a sample-and-hold interface circuit; 
         FIG. 2  is an embodiment of the present invention, a sample-and-hold interface circuit of a pickup head; 
         FIG. 3  is an embodiment of the timing circuit of the present invention; 
         FIG. 4  is an embodiment of the delay circuit of the present invention; and 
         FIG. 5  is the signal timing diagram of the timing circuit. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 2 , it is one of the embodiments of the invention. An input signal  20  is sent from a pickup head  9  with 5V working voltage to a DVD control chip with working voltage 3.3V The input signal  20  processed by an interface circuit  10  is sent to a sample-and-hold circuit  7  of a DVD control chip, like a Switched-OP, to ensure the safety of the DVD control chip. The interface circuit  10  in this embodiment includes a voltage divider  23 , a circuit switch  6 , and a timing circuit  8 . In the embodiment, the voltage divider  23  including a pair of resistors  4  and  5  is connected to the sample-and-hold circuit  7  and the circuit switch  6 . A control signal  21  is output from the point between the serial resistors  4  and  5  to the sample-and-hold circuit  7 . 
     The sample-and-hold circuit  7  works for sampling the control signal  21 . The circuit switch  6  is in serial with the voltage divider  23 , and connected to the timing circuit  8 . In the embodiment, the circuit switch  6  includes a PMOS transistor connected the resistor  5  with the source, a low voltage, like ground, with the drain and timing circuit  8  with the gate. The circuit switch  6  works to control the voltage divider  23  to determine the character of the control signal  21 , like the amplitude. The timing circuit  8  is connected to the circuit switch  6  and sample-and-hold circuit  7 . The timing circuit  8  controls the on and off of the circuit switch  6  with a switching signal (signal Break), and the sampling of the sample-and-hold circuit  7  with a signal Set after receiving an activating signal (signal Sample). 
     Before inputting a writing voltage (3.3V-5V), a signal Sample will control the timing circuit  8  to make both of the signal Break and signal Sample at 0. At this moment, the circuit switch  6  is on, and the input signal  20  within the range of the writing voltage (3.3V-5V) is input afterward. The input signal  20  becomes the control signal  21  after attenuated by the voltage divider  23 , and partial the input signal  20  is sent to circuit switch  6 . At this moment, the present invention works as the prior art. After the input writing voltage is attenuated by the voltage divider  23 , the input voltage of the sample-and-hold circuit  7  is not over 3.3V 
     When the pickup head is inputting a reading voltage (1.4V-2.8V), the signal Sample will control the timing circuit  8  to make both of the signal Break and signal Sample at  1 , then the circuit switch  6  becomes off. At this moment, the resistor  5  is open-circuit and the voltage divider  23  cannot divide the voltage. Then the input voltage of the sample-and-hold circuit  7  approaches to the reading voltage. That means the input signal  20  is approximately equal to the control signal  21  output from the voltage divider  23 . The pickup head  9  does not need to provide extra current to drive the resistor  5 , therefore the power consumption can be reduced. 
       FIG. 3  is one of the embodiments of the timing circuit  8 . It includes a delay path including six delay circuits,  11   a ,  11   b ,  11   c ,  11   d ,  11   e  and  11   f  in serial, two multiplexers,  12   a , and  12   b , and four NOR gates,  13   a ,  13   b ,  13   c  and  13   d . The signal Sample of the reading and writing enters the delay circuit of  11   a  of the delay path. Signal S is output from the middle of the delay path, like the point between the  11   c  and  11   d , and becomes signal Set after passing through two serial NOR gates,  13   a  and  13   b . Each of the NOR gate  13   a  and  13   b  is connected to the ground with an input, therefore, they work as two serial inverters. Logically, the signal Sample is equivalent to the signal S. 
     Signal Sa and Sb are output from multiplexer  12   a  and  12   b  respectively, and equivalent to signal S in logic, but their delay times are different. The delay times of the signal Sa and Sb can be selected by the control signals of the multiplexer  12   a  and  12   b . The signal Sa and Sb are processed by two NOR gates  13   c  and  13   d  in serial to be output as the signal Break. One of the input of the NOR gate  13   d  is signal AlwaysOn. When the signal AlwaysOn is 0, the NOR gate  13   d  works as an inverter, and then the signal Break is equivalent to Sa+Sb in logic. When the signal AlwaysOn is 1, the signal Break output from the NOR gate  13   d  is always 0. The voltage divider  23  attenuates the input signal  20  to protect the DVD control chip.  FIG. 4  shows the diagram of the delay circuit of the present invention, including two serial inverters,  14   a  and  14   b.    
       FIG. 5  is the timing diagram of the related timing of the timing circuit. When the writing voltage (3.3V-5V) output becomes the reading voltage (1.4V-2.8V) output, an internal controller (not shown in the drawings) makes the signal Sample turn 0 into 1, passing through the delay circuits in  FIG. 3  to generate the signal Sa, Sb and S. At this moment, the AlwaysOn is 0, therefore, the signal Break is like Sa+Sb logically. It shows in  FIG. 5  that the length of the signal Break is as the same as the length from the beginning of the signal Sa to the end of the signal Sb. Because the sample-and-hold circuit  7  is not expecting to sample an attenuated reading voltage value, the length of the signal Break is a little longer than the length of the signal Set to ensure that the length of the input as a reading voltage through switching the sample-and-hold circuit  7  by signal Set can be selected by the multiplexer  12   a  and  12   b  when the resistor  5  of the voltage divider  23  in  FIG. 2  is open-circuit. 
     While the described embodiment represents the preferred form of the present invention, it is to be understood that modifications will occur to those skilled in that art without departing from the spirit of the invention. The scope of the invention is therefore to be determined solely by the appended claims.