Patent Document

INCORPORATION BY REFERENCE 
     The disclosure of U.S. Pat. No. 7,084,697 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a booster circuit for generating a higher voltage than a power supply voltage, and particularly to a charge pump style booster circuit. 
     2. Description of Related Art 
     A display unit of a portable information device such as cellular phone and Personal Digital Assistant (PDA) is configured to operate at a low power supply voltage to save power consumption. On the other hand, a display panel for displaying processed information sometimes requires a higher voltage than a power supply voltage. Generally a circuit for driving a display panel is provided with a booster circuit for boosting a power supply to generate a necessary driving voltage. 
     An example of booster circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2005-45934. The booster circuit is a charge pump style as shown in  FIG. 3  that includes P-channel MOS transistors M 1 , M 3  to M 8 , an N-channel MOS transistor M 2 , capacitors C 1  and C 2 . The booster circuit is configured as shown in  FIG. 3  and boosts a power supply voltage VDD by twice in response to a clock signal CLK to generate the voltage as Vout. 
     The circuit in  FIG. 3  includes a P-channel MOS transistor M 9  between a power supply line VDD and the capacitor C 2  (i.e. Vout line) in order to speed up start-up at power-on. 
     A booster circuit needs to be operated to display information, however it is not necessary to display information at any time. Operating a booster circuit at all the time only consumes unnecessary power. Thus a boosting operation of a booster circuit is suspended while there is no information to be displayed. The circuit shown in  FIG. 3  fixes a clock CLK to high-level, turns on the transistors M 1  and M 2 , turns off the transistor M 3  and M 4  to charge the capacitor C 1  while a boosting operation is suspended. Further, the circuit fixes a control signal CNT supplied to agate of the transistor M 9  to low-level and electrically connects the VDD line and the Vout line, so that the capacitor C 2  is charged to a level of VDD. Charging the capacitor C 2  speeds up start-up when resuming a boosting operation. 
     However in recent years, there are increasing requests from clients to generate a ground level potential as Vout while a boosting operation is suspended, so as to ensure that an operation of a circuit receiving Vout is stopped while a boosting operation is suspended. Setting Vout to a ground level while a boosting operation is suspended causes a signal necessary to operate a booster circuit such as a clock signal CLK to be the ground level, thereby not ensuring to stop a boosting operation. This is because that the signal such as the clock signal CLK requires twice the VDD level during a boosting operation and is generated by a circuit operated on Vout voltage. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a booster circuit for boosting and outputting a voltage between a power supply potential line and a reference potential line using a capacitor connected between a boosted voltage output node and the reference potential line that includes a first switch for separating the capacitor from the boosted voltage output node while a boosting operation is suspended, a second switch connected in, parallel to the capacitor and being conductive while the boosting operation is suspended, and an electric path between the power supply potential line and the boosted voltage output node while the boosting operation is suspended. 
     With the booster circuit of the present invention, the capacitor is discharged to be a reference potential by the second switch while the boosting operation is suspended. The boosted voltage output node is electrically separated from the capacitor and also supplied with a voltage from the power supply potential line. This ensures that the boosting operation is suspended. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram according to a first embodiment of the present invention; 
         FIG. 2  is a circuit diagram according to a second embodiment of the present invention; 
         FIG. 3  is a circuit diagram according to a conventional technique; and 
         FIG. 4  is a cross-sectional diagram showing substantial part of a semiconductor integrated circuit for explaining a latch up in the circuit of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
     First Embodiment 
     An embodiment of the present invention is described hereinafter in detail with reference to the drawings.  FIG. 1  is a view showing a booster circuit according to a first embodiment of the present invention. The booster circuit is included with other functional blocks in a semiconductor integrated circuit (hereinafter referred to as an IC chip). Capacitors C 1  and C 2  are connected as external components to the IC chip  100 . Thus the chip  100  includes connection terminals (connection pins)  103  and  104  for the capacitor C 1 , and a connection terminal  105  for the capacitor C 2 . A charge pump circuit  150  comprising the booster circuit includes transistors M 1  to M 8  as shown in  FIG. 3 . 
     The IC chip  100  further includes a VDD terminal  101  as a power supply potential and a ground (GND) terminal  102  as a reference potential. A power supply potential line from the VDD terminal  101  is connected to the charge pump circuit  150 , a control circuit  200 , and an output end of boosted voltage, which is a Vout 1 , via a P-channel MOS transistor M 12 . The node Vout 1  is connected to a level shift circuit  250  (a first boosted voltage application circuit). Further, the node Vout 1  is connected to a capacitor C 2  connection terminal  105  via a P-channel MOS transistor M 10 . The terminal  105  (output node Vout 2 ) is connected to the boosting voltage circuit  300 , (a second boosted voltage application circuit). Further, the node Vout 2  is connected to a GND terminal  102  via an N-channel transistor M 11 . The transistor M 11  is connected in parallel to the capacitor C 2 . 
     The control circuit  200  generates a clock signal CLK necessary for a boosting operation and also a control signal CNT for activating and deactivating a boosting operation. These signals CLK and CNT are level shifted by the level shift circuit  250  to boosting clock signals ICL and ICLB having complementary phases, and to boosting operation control signals ICN and ICNB having complementary phases. The signal ICN is supplied to a gate of the transistor M 12 , and the signal ICNB is supplied to gates of the transistors M 10  and M 11 . 
     When the IC chip power is turned on, the boosting voltage output node Vout 1  and the capacitor C 2  are charged along with an increase in a potential of the terminal VDD due to the power-on via the transistors M 10  and M 12  because the circuit nodes in the IC chip  100  has no charge at all. On the other hand the control circuit  200  is activated by the power supply voltage VDD. Thus if the IC chip  100  requires a boosted voltage, the control circuit  200  starts generating a clock signal CLK together with making a control signal CNT be a level activating a boosting operation (for example high-level). This fixes a signal ICNB to be low-level, while a signal ICN to be high level along with an increase in the power supply voltage VDD. At this time, a back gate of the transistor M 12  is connected to M 10  side, creating parasitic effect. The parasitic effect continues to charge the node Vout 1  and the capacitor C 2 . 
     With an increase in a power supply voltage level supplied to the level shift circuit, the clock signals ICL and ICLB are raised to a level necessary to operate the charge pump circuit  150 . Then the charge pump circuit starts a voltage boosting operation using the capacitors C 1  and C 2 . 
     The charge pump circuit  150  having a configuration shown in  FIG. 3  boosts the nodes Vout 1  and Vout 2  to a level twice as high as the power supply voltage VDD (which is 2×VDD). 
     This voltage is supplied from the node Vout 2  to the boosted voltage application circuit  300 , for a display panel requiring a level higher than the power supply voltage VDD to be operated, for example. 
     If a display panel does not need to be operated, the control circuit  200  changes the control signal CNT to a level deactivating a boosting operation (for example low-level). Further, generation of a clock signal CLK is stopped. However if the clock signal CLK is used in other circuits not shown, the clock signal CLK is continued to be generated. The signal ICN is inverted to low-level and the signal ICNB is inverted to high-level. Further, the level shift circuit  250  is configured in a way that the boosting clock signal ICL and ICLB are fixed to high and low levels respectively by the control signal CNT using an NAND gate, for example. 
     A high-level signal ICNB turns off the transistor M 10  and turns on the transistor M 11 . Consequently the capacitor C 2  is discharged and a potential of the output node Vout 2  is pulled down to ground level, which is low-level. On the other hand a low-level signal ICN turns on the transistor M 12 , thereby enabling the output node Vout 1  to stay at a VDD level even while a boosting operation is suspended. 
     While a potential of the terminal  105  is pulled down to low level while a boosting operation is suspended, the power supply voltage VDD is supplied to the level shift circuit  250 , a first boosted voltage application circuit. This accordingly satisfies the requests from clients and also a voltage necessary to activate a boosting operation is supplied to the level shift circuit  250 , a first boosted voltage application circuit. Therefore, the level shift circuit  250  needs not to be changed to ensure fixing a voltage in the charge pump circuit  150  to a state the boosting operation is suspended by a signal from the level shift circuit  250 . 
     Second Embodiment 
       FIG. 2  is a view showing a second embodiment of the present invention. In  FIG. 2 , components identical to those in  FIG. 1  are denoted by reference numerals identical to those therein with explanation omitted. In this embodiment, a resistance  350  is provided instead of the transistor M 12  in  FIG. 1 . 
     There are possibilities that following issue could arise from using the transistor M 12  in  FIG. 1 . The issue is explained hereinafter in detail. In the circuit of  FIG. 1 , the terminal  105  becomes low-level while a boosting operation is suspended. Each time a boosting operation is activated, the power supply voltage VDD is charged to the capacitor C 2  having 0V through the transistor M 12 . 
     If the IC chip is formed on a P type semiconductor substrate, the transistor M 12  includes PNP parasitic transistor for the P type semiconductor substrate. If the parasitic transistor is turned on, an NPN parasitic transistor included in the same semiconductor substrate is turned on, resulting both parasitic transistors in a thyristor operation. 
       FIG. 4  is a cross-sectional structure of the transistors M 12  and M 2  formed on a P type semiconductor substrate. In  FIG. 4 ,  11  indicates a P type semiconductor substrate having an N well  12  formed therein. Drains and sources of P type regions  13  and  14 , and a back gate contact of an N type region  15  are formed in the N well  12  to configure the transistor M 12 . Further, drains and sources of N type regions  16  and  17 , and a back gate contact of a P type region  18  are formed in the P type semiconductor substrate  11  to form the transistor M 2 . The P type region  13  of the transistor M 12  is connected to a VDD terminal  101 , and the P type region  14  and the N type region  15  are connected to the capacitor C 2 . The N type region  17  and the P type region  18  of the transistor M 2  are connected to the GND terminal  102 . 
     Each time a boosting operation is activated, the power supply voltage VDD is charged to the capacitor C 2  having 0V, and a forward current flows from the P type region  13  of the transistor M 12  to the N well  12 . This causes a parasitic PNP transistor Q 1  comprised of the P type region  13 , the N well  12 , and the P type semiconductor substrate  11  to be turned on, and a potential of the P type semiconductor substrate  11  to rise towards VDD. The rise in the potential could turn on a parasitic NPN transistor Q 2  formed by the N well  12 , P type semiconductor substrate  11 , and N region  17 . If the parasitic transistors Q 1  and Q 2  are turned on, a thyristor operation by the parasitic transistors Q 1  and Q 2  could generate a latch-up, causing a large current to flow between the VDD terminal  101  and the GND terminal  102 . Therefore, a boosting operation may not properly be activated in the booster circuit. 
     To properly activate the boosting operation in the booster circuit, a Schottky diode with a smaller Vf than a PN junction forward voltage Vf by the P type region  13  and the N well  12  must be connected between the VDD terminal  101  and the terminal  105 , so that the parasitic transistor Q 1  will not be turned on. Connecting a Schottky diode introduces other problems such as an increase in the number of external components and the size of an area where components are mounted. 
     On the other hand the resistance  350  is used in the circuit of  FIG. 2 , thereby capable of preventing a latch-up when activating the boosting operation even without a Schottky diode connected between the VDD terminal  101  and the terminal  105 . By using the resistance  350 , VDD potential difference is generated in both ends of the resistance  350  when a boosting operation is activated, thereby generating a current in the resistance  350 . However by specifying a resistance value of the resistance  350  to an appropriate range, a level of the current can be acceptable in comparison to an operating current in the booster circuit including load. 
     In the first and the second embodiment, only the level shift circuit  250  is provided as a first booster voltage application circuit to be connected to the output node Vout 1 . However other circuit activated at a VDD level while a boosting operation is deactivated may be connected if necessary. 
     It is apparent that the present invention is not limited to the above embodiment and it may be modified and changed without departing from the scope and spirit of the invention.

Technology Category: h