Electric power conversion systems – Current conversion – With voltage multiplication means
Reexamination Certificate
2000-09-01
2002-02-05
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With voltage multiplication means
C307S110000, C327S536000
Reexamination Certificate
active
06344984
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a voltage multiplier having an intermediate tap and, more particularly, to a voltage multiplier for use in a LCD driver IC for driving a LCD panel.
(b) Description of the Related Art
A LCD driver IC for driving a LCD panel generally includes a power supply circuit outputting relatively high source voltages ranging between 5 and 20 volts. On the other hand, most current ICs including logic IC and memory IC use a relatively low voltage ranging between 3 and 5 volts. Thus, the power supply circuit in the LCD driver IC includes a voltage multiplier for multiplying a source voltage of 3 to 5 volts up to two to five times the supplied source voltage.
The power supply circuit in the LCD device, especially used in a personal digital assistant, generates a plurality of intermediate voltages in addition to the highest operatioanl voltage.
FIG. 1
shows a typical power supply circuit including a conventional voltage multiplier.
The power supply circuit
30
includes a voltage multiplier block
31
for multiplying an input source voltage VDD by “n” to generate a multiplied voltage VLCD equal to n×VDD where “n” is an integer. The power supply circuit
30
also includes a voltage generator block
32
which receives the multiplied voltage VLCD to generate a plurality of voltage levels V
1
to V
5
including a highest voltage V
1
and a lowest voltage V
5
.
The voltage generator block
32
includes a voltage divider
33
implemented by resistors R
1
to R
5
, a voltage follower A
1
which operate on a source voltage of VLCD and receives a reference voltage Vref to generate the voltage level V
1
, and a plurality of voltage followers A
2
to A
5
each of which operates on a source voltage of VLCD and receives one of output voltages of the voltage divider
33
to output a corresponding one of the voltage levels V
2
to V
5
.
In the conventional power supply circuit
30
, power dissipation can be suppressed by employing a higher resistance for the voltage divider
33
, reducing penetrating current flowing through the output stages of the voltage followers A
1
to A
5
, and stopping the bias current for the voltage followers A
1
to A
5
during a standby mode of the LCD device.
Due to the employment of lower power supply voltage, which is now reduced down to 1.5 to 2.0 volts, in the latest logic LSI, the multiplication factor of the voltage multiplier
31
has increased up to 6 to 9 times. Thus, further reduction of the power dissipation is requested in the power supply circuit
30
along with the current driveability of the voltage multiplier
31
. The current driveability of the voltage multiplier
31
generally involves a larger chip area for the LCD driver, which is undesirable especially in a personal digital assistant.
Patent Publication JP-A-10-31200 describes a power supply circuit for a LCD driver such as shown in FIG.
2
. The power supply circuit
40
includes a first voltage generator
41
for generating higher voltage levels V
1
and V
2
, and a second voltage generator
42
for generating lower voltage levels V
3
and V
4
. The first voltage generator
41
operates on a source voltage between a positive voltage level V
0
of a power source
43
and a median voltage level Vx, whereas the second voltage generator
42
operates on a source voltage between the median voltage level Vx and the negative voltage level V
5
of the power source
43
.
The median voltage level Vx is generated from the positive voltage level V
0
, the negative voltage level V
5
and bipolar transistors Tp and Tn. If the total output current from the operational amplifiers OP
1
and OP
2
constituting the voltage followers in the first voltage generator
41
is equal to the total output current from the operational amplifiers OP
3
and OP
4
in the second voltage generator
42
, the median voltage Vx is maintained at the exact median potential between voltage levels V
0
and V
5
. In this case, the power dissipation is reduced down to a half the power dissipation by the power supply circuit of FIG.
1
.
In an ordinary LCD device, specified voltage levels are often used in the LCD panel at a time. For example, if the first voltage generator
41
operates for driving a LCD panel, electric charge generated by the driving is temporarily stored in a capacitor C
1
connected between the median voltage line Vx and the negative source line. On the other hand, if the second voltage generator
42
operates for driving, the driving current reduces the charge stored in the capacitor C
1
. The capacitor C
1
alleviates a voltage fluctuation on the median voltage line Vx. However, if the unbalance of the drive current continues, the median voltage line Vx may rise or fall from the median voltage by a threshold. The rise or fall of the median voltage line Vx above the threshold is cancelled by turn on of the bipolar transistor Tp or Tn.
The turn on of the transistor Tp or Tn results in flow of the current from the V
0
line to the Vx line or from the Vx line to the V
5
line, and causes a power dissipation. In addition, the function of the capacitor C
1
is limited if the multiplication factor of the voltage multiplier increases.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a voltage multiplier which is capable of reducing power dissipation in the case of providing an intermediate voltage level in addition to the multiplied voltage from the power supply circuit having the voltage multiplier.
The present invention provides a voltage multiplier including a plurality of capacitors, a switching assembly for iteratively switching between a first position for connecting the capacitors in parallel for charging the capacitors with a first voltage and a second position for connecting the capacitors in series to form a serial branch, the serial branch having a first end outputting a second voltage, at least one first switch having a terminal connected to an intermediate position of the serial branch, the first switch being turned on, when the switching assembly connects the capacitors in series, to output a third voltage which is higher than the first voltage and lower than the second voltage.
In accordance with the voltage multiplier of the present invention, since the power supplied on the third voltage is substantially equal to the corresponding power supplied on the first voltage, the power dissipation in a LCD panel, for example, driven by the voltage levels generated based on the third voltage is reduced compared to the LCD panel driven by the voltage levels generated based on the multiplied voltage generated by the conventional voltage multiplier.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.
REFERENCES:
patent: 5532916 (1996-07-01), Tamagawa
patent: 10031200 (1998-03-01), None
Laxton Gary L.
NEC Corporation
Wong Peter S.
Young & Thompson
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