Abstract:
An AMP output protective circuit for an LCD panel source driver is disclosed. To solve a problem that internal diodes of PMOS/NMOS of an AMP output circuit are turned on, embodiments are characterized in making input and output voltages of the AMP in a charge sharing interval equal to HVDD and short-circuiting outputs of PAMP and NAMP with VRST_GH and VRST_GL lines, respectively. Accordingly, since there is no increase of voltage (Vth) attributed to a body effect, a speed is not reduced. An additional body bias control circuit is unnecessary. Power consumption can be reduced. Also, an AMP circuit can be more safely protected by adding an output reset function and an AMP protecting circuit.

Description:
The present application claims priority under 35 U.S.C. 119 to Korean Patent Application No. 10-2008-0134179 (filed on Dec. 26, 2008), which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     An LCD panel source driver converts a group of externally inputted digital signals to an analog signal at a corresponding level, amplifies the converted analog signal by a prescribed amplification gain, and then provides the amplified analog signal as a panel source signal for driving a display panel. 
       FIG. 1  is a diagram of an AMP output stage circuit for an LCD panel source driver according to a related art. Referring to  FIG. 1 , if an output of PAMP or NAMP in an AMP output stage circuit becomes greater or smaller than HVDD, it may cause a problem that a diode  102 / 104  within PMOS/NMOS loaded in an output circuit within an amplifier can be turned on. Particularly, this problem may be caused if the circuit is reactivated for a whole charge sharing interval  200 , as shown in  FIG. 2 , after completion of the charge sharing. 
     As mentioned in the foregoing description, in the AMP output stage circuit according to the related art, since a source voltage of the PMOS/NMOS is higher than a body voltage, a threshold voltage Vth attributed to a body effect is raised. In this case, a body bias control circuit is added, which increases a die area. Moreover, if an attempt is made to protect the output of the AMP using a protective diode only, the size of the diode needs to increase. Therefore, a corresponding die area must be increased as well. 
     SUMMARY 
     Embodiments relate to an amplifier output circuit, and more particularly, to an amplifier output protective circuit for an LCD panel source driver. Although embodiments are suitable for a wide scope of applications, they are particularly suitable for performing a switching control for amplifier protection in an AMP (amplifier) output circuit for an LCD panel source driver that uses ½ VDD. Accordingly, embodiments relate to an AMP output protective circuit, which eliminates the need for an additional body bias control circuit, reduces power consumption, and allows the AMP circuit to be more safely protected. 
     Embodiments relate to an AMP output protective circuit for an LCD panel source driver which may include a first amplifier biased by first and second power source voltages, a second amplifier biased by the second power source voltage and a third power source voltage, an input enable switching unit having switches connected between a first input line and an input terminal of the first amplifier and between a second input line and an input terminal of the second amplifiers, an input HVDD switching unit having switches connected between the input terminal of the first amplifier and a second power line for supplying the second power source voltage and between the input terminal of the second amplifier and the second power line, and an output HVDD switching unit having switched connected between an output terminal of the first amplifier and the second power line and between the second power line and the output terminal of the second amplifier. 
     The AMP output protective circuit may further include a first diode forward connected from the second power line toward the output terminal of the first amplifier and a second diode forward connected from the output terminal of the second amplifier toward the second power line. The AMP output protective circuit may further include a first output pad, a second output pad, and an output selection switching unit including switches switched to enable output stages of the first and second amplifiers to be selectively connected to the first and second output pads, respectively. In addition, the AMP output protective circuit may further include a first resistor connected between the first output pad and the output selection switching unit and a second resistor connected between the second output pad and the output selection switching unit. 
     The AMP output protective circuit may further include a first reset switch connected between the one end of the first resistor and a first reset line for supplying a first reset voltage and a second reset switch connected between the one end of the second resistor and a second reset line for supplying a second reset voltage. 
     Embodiments relate to a method of driving an AMP output protective circuit for an LCD panel source driver which may include: biasing a first amplifier by first and second power source voltages; biasing a second amplifier by the second power source voltage and a third power source voltage; connecting switches between a first input line and an input terminal of the first amplifier and between a second input line and an input terminal of the second amplifiers in an input enable switching unit; connecting switches between the input terminal of the first amplifier and a second power line for supplying the second power source voltage and between the input terminal of the second amplifier and the second power line in an input HVDD switching unit; and connecting switches between an output terminal of the first amplifier and the second power line and between the second power line and the output terminal of the second amplifier in an output HVDD switching unit. 
     Accordingly, since there is no increase of a voltage Vth attributed to a body effect, circuit speed is not reduced. Since an additional body bias control signal is unnecessary, die area and power consumption can be reduced. Moreover, by adding an output reset function and an AMP protective circuit, embodiments are able to protect the AMP circuit more safely. 
    
    
     
       DRAWINGS 
         FIG. 1  is a diagram of an AMP output stage circuit for an LCD panel source driver according to a related art. 
         FIG. 2  is a diagram of a general charge sharing interval. 
       Example  FIG. 3  is a diagram for a structure of an AMP output protective circuit for an LCD panel source driver according to embodiments. 
       Example  FIG. 4  is a diagram for a structure of an AMP output protective circuit for an LCD panel source driver according to embodiments. 
     
    
    
     DESCRIPTION 
     Embodiments provide an AMP output protective circuit for an LCD panel source driver, by which an input/output voltage of an amplifier can become HVDD by opening an input enable switch during a charge sharing interval and closing an input HVDD switch and an output HVDD switch. Embodiments provide an AMP output protective circuit for an LCD panel source driver, by which PAMP/NAMP output is short-circuited with a VRST_GH/VRST_GL line by opening an output HVDD switch, closing an output selection switch and closing an output reset switch, while input and output voltages of an amplifier in an AMP output circuit for an LCD panel source driver are made to become HVDD. In addition, embodiments provide an AMP output protective circuit for an LCD panel source driver, by which an AMP circuit can be safely protected by adding an AMP protective diode to an AMP circuit for an LCD panel source driver. 
     Example  FIG. 3  is a diagram for a structure of an AMP output protective circuit for an LCD panel source driver according to embodiments. Referring to example  FIG. 3 , an AMP output protective circuit for an LCD panel source driver may include a first amplifier PAMP, a second amplifier NAMP, an input enable switching unit  302 , an input HVDD switching unit  304 , an AMP output protecting unit  306 , an output HVDD switching unit  308 , an output selection switching unit  310 , an output reset switching unit  312 , a first output resistor R 1 , a second output resistor R 2 , a first output pad P 1 , a second output pad P 2 , and first to fourth output diodes D 1  to D 4 . 
     The first amplifier PAMP may be biased by a first power source voltage VDD and a second power source voltage HVDD. The second amplifier NAMP may be biased by the second power source voltage HVDD and a third power source voltage VSS. 
     Each of the first and second amplifiers PAMP and NAMP can include a buffer type amplifier having a gain set to 1 . The PAMP, NAMP and switching units ( 302 ,  304 ,  306 ,  308 ,  310 ) can use NMOS or PMOS operating at a half VDD (=½*VDD). Moreover, a charge sharing switch (not shown in the drawing) can include NMOS and PMOS operating at full VDD (=VDD). 
     The input enable switching unit  302  may include a first switch  322  connected between a first input line INP and an input terminal (e.g., a positive terminal) of the first amplifier PAMP and a second input line INN and an input terminal (e.g., a positive terminal) of the second amplifier NAMP. The input HVDD switching unit  304  may include a third switch  332  connected between the input terminal (e.g., a positive terminal) of the first amplifier PAMP and a second power line for supplying the second power source voltage HVDD and a fourth switch  334  connected between the input terminal (e.g., the positive terminal) of the second amplifier NAMP and the second power line. 
     The AMP output protecting unit  306  may include a first diode  342  forward connected from the second power line toward the output terminal of the first amplifier PAMP and a second diode  344  forward connected from the output terminal of the second amplifier NAMP toward the second power line. The output HVDD switching unit  308  may include a fifth switch  352  connected between the output terminal of the first amplifier PAMP and the second power line and a sixth switch  354  connected between the second power line and the output terminal of the second amplifier NAMP. 
     The output selection switching unit  310  may include first to fourth output switched SW 1  to SW 4 . The first output switch SW 1  may be connected between the output terminal of the first amplifier PAMP and one end of the first resistor R 1 . The second output switch SW 2  may be connected between the output terminal of the first amplifier PAMP and one end of the second resistor R 2 . The third output switch SW 3  may be connected between the output terminal of the second amplifier NAMP and one end of the first resistor R 1 . The fourth output switch SW 4  may be connected between the output terminal of the second amplifier NAMP and one end of the second resistor R 2 . 
     The other end of the first resistor R 1  may be connected to the first pad P 1 , while the other end of the second resistor R 2  may be connected to the second pad P 2 . The output reset switching unit  312  may include a first reset switch  362  and a second reset switch  364 . The first reset switch  362  may be connected between one end of the first resistor R 1  and a first reset line for supplying a first reset voltage VRST_GH. The second reset switch  364  may be connected between one end of the second resistor R 2  and a second reset line for supplying a second reset voltage VRST_GL. 
     The first output diode D 1  may be connected to the other end of the first resistor R 1  and a VDD power line for supplying a first power source voltage VDD. A forward direction of the first output diode D 1  starts from the other end of the first resistor R 1  toward the VDD power line. The second output diode D 2  may be connected to the other end of the second resistor R 2  and the VDD power line for supplying a first power source voltage VDD. A forward direction of the second output diode D 2  starts from the other end of the second resistor R 2  toward the VDD power line. 
     The third output diode D 3  may be connected to the other end of the first resistor R 1  and a VSS line for supplying a third power source voltage VSS. A forward direction of the third output diode D 3  starts from the VSS line toward the other end of the first resistor R 1 . The fourth output diode D 4  may be connected to the other end of the second resistor R 2  and the VSS power line. A forward direction of the fourth output diode D 4  starts from the VSS line toward the other end of the second resistor R 2 . 
     In the following description, in order to explain an operational process off an AMP output protective circuit, assume ‘first power source voltage (VDD)=16V’, ‘second power source voltage (HVDD)=8V’ and ‘third power source voltage (VSS)−0V’, 
     When a charge sharing switch is turned on, a reset voltage VRST can have a range between maximum ‘(16+8)/2=12V’ and minimum ‘(8+0)/2=4V’. Thereafter, if the charge sharing switch is turned off and the output selection switching unit  310  is turned on, the following two cases may occur. 1) If VRST=12V, since an NAMP output is greater than 8.7V (=8V+0.7V), it may cause a problem that an internal p-n diode of PMOS of an NAMP output stage circuit is turned on. 2) If VRST=4V, since a PAMP output is greater than 7.3V (=8 8 V−0.7V), it may cause a problem that an internal p-n diode of NMOS of a PAMP output stage circuit is turned on. 
     To solve the above two problems, both input and output voltages of each of the first and second amplifiers PAMP and NAMP may be made equal to the second power source voltage HVDD by opening the switches  322  and  324  of the input enable switching unit  302  connected to the first and second input lines INP and INN respectively in the charge sharing interval and selectively closing the switches  332  and  334  of the input HVDD switching unit  304  and the switches  352  and  354  of the output HVDD switching unit  308 . 
     After both of the input and output voltages of each of the first and second amplifiers PAMP and NAMP have been made equal to the second power source voltage HVDD, the switches  352  and  354  of the output HVDD switching unit  308  may be opened, the switches SW 1  to SW 4  of the output selection switching unit  310  may be closed, and the switches  362  and  364  of the output reset switching unit  312  may be closed. Therefore, the output stage of the first amplifier PAMP may be short-circuited with the first reset voltage line and the output stage of the second amplifier NAMP may be short-circuited with the second reset voltage line. 
     In this way, both of the input and output voltages of each of the first and second amplifiers PAMP and NAMP can be made equal to a preset voltage (e.g., the second power source voltage (HDD=8V)). Moreover, in order to protect the first and second amplifiers PAMP and NAMP more safely in a charge sharing interval, the AMP output protecting unit  306  may be used. The AMP output protecting unit  306  can be implemented with Schottky barrier diodes (hereinafter abbreviated SBD). Since a turn-on voltage of the Schottky barrier diode may be set to a turn-on voltage (0.3˜0.5 V) smaller than that (0.6˜0.88 V) of a diode within the PMOS or NMOS, the Schottky barrier diodes may be turned on before the diodes within the PMOS and NMOS are turned on. Therefore, these arrangements are able to protect the AMP circuit. 
     Example  FIG. 4  is a diagram for a structure of an AMP output protective circuit for an LCD panel source driver according to embodiments. Referring to example  FIG. 4 , an AMP output protective circuit for an LCD panel source driver according to embodiments may be almost identical to the former AMP output protective circuit for the LCD panel source driver, which is shown in example  FIG. 3 , except for the substitution of general diodes  410  and  412  for the Schottky barrier diodes  342  and  344 . 
     The circuit operation for the AMP protection may be identical to that shown in example  FIG. 3 . To protect the AMP in the charge sharing interval more safely, since the turn-on voltage of the diodes  410  and  412  of the AMP output protective circuit  406  is smaller than the turn-on voltage of the internal diodes of the PMOS and NMOS, the diodes  410  and  412  may be turned on before the internal diodes are turned on. Therefore, these arrangements are able to protect the AMP circuit. 
     It will be obvious and apparent to those skilled in the art that various modifications and variations can be made in the embodiments disclosed. Thus, it is intended that the disclosed embodiments cover the obvious and apparent modifications and variations, provided that they are within the scope of the appended claims and their equivalents.