Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
2003-01-09
2004-11-02
Tran, Anh Q. (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S033000, C326S090000
Reexamination Certificate
active
06812740
ABSTRACT:
CLAIM TO PRIORITY
Applicant hereby claims priority under 35 U.S.C. §119 to Japanese Application No. 2002-001957, filed Jan. 9, 2002, titled Low-Voltage Drive Circuit and Method for the Same, the disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to circuits for electromagnetic-induction pointing devices, and more specifically, to a low-voltage drive circuit for driving a sensor coil in an electromagnetic-induction coordinate input device at a constant current.
BACKGROUND OF THE INVENTION
For discharging a current to a sensor coil using a single power supply in low-voltage drive circuits in conventional electromagnetic-induction coordinate pointing devices, a common potential of sensor coils must be shifted to a midpoint potential. A reference circuit with low impedance is required to avoid noise. Such reference circuits have a complex circuit design. Furthermore, such reference circuits do not efficiently. utilize electric power. The drive circuits also include a complementary constant-current circuit unit, which has a drive source circuit segment and a drive sink circuit segment. Each of the segments requires a bias voltage of approximately 1 V. If a single 3 V power supply is used, sufficient drive voltage of the sensor coils is not achieved.
A conventional low-voltage drive circuit for driving a sensor coil in an electromagnetic-induction coordinate input device at a constant current is best shown in FIG.
3
. An AC signal is input to an input terminal IN
5
. A PNP transistor Tr
20
and a differential amplifier
30
are each connected to a power supply Vcc with a resistor R
20
therebetween. The PNP transistor Tr
20
and the differential amplifier
30
function as a drive source circuit segment. Based on the input AC signal, the differential amplifier
30
and the PNP transistor Tr
20
output a constant-current AC signal to one end of a sensor coil
10
. Also, an AC signal is input to an input terminal IN
6
. An NPN transistor Tr
21
and a differential amplifier
32
are each connected to ground with a resistor R
21
therebetween. The NPN transistor Tr
21
and the differential amplifier
32
function as a drive sink circuit segment. Based on the input AC signal, the differential amplifier
32
and the NPN transistor Tr
21
output a constant-current AC signal to one end of the sensor coil
10
. The PNP transistor Tr
20
and the NPN transistor Tr
21
are complementarily connected to each other, and combine the constant-current AC signals corresponding to the signals input from the input terminals IN
5
and IN
6
into one signal, which is then output.
The other end of the sensor coil
10
is connected to a reference circuit unit
34
, so that a common potential of the sensor coil
10
is shifted to a midpoint potential. Also, the other end of the sensor coil
10
is grounded with a capacitor C
20
therebetween. The reference circuit unit
34
is needed for AC driving the sensor coil
10
. Thus, the size of the circuit is increased, and thus current consumption in the circuit increases.
When the sensor coil
10
is AC driven, the midpoint of the sensor coil
10
has half the voltage of the power supply Vcc by using the reference circuit
34
, thus restricting a drive voltage range.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a drive circuit that reduces the size of the overall drive circuit, thereby reducing current consumption by driving a sensor coil at a constant current without using a reference circuit. The drive circuit has a wide driving voltage range by biasing a drive source circuit segment and a drive sink circuit segment separately.
A low-voltage drive circuit for driving a sensor coil in a coordinate input device at a constant current comprises: a constant-current output drive circuit unit; a DC coupling capacitor provided between an output of the constant-current output drive circuit unit and the sensor coil; and a constant-current output bias circuit unit for controlling a DC bias voltage of the output of the constant-current output drive circuit unit to be a midpoint potential of an operating voltage range for the constant-current output drive circuit unit. The midpoint potential is a target value of control. The vicinity of the midpoint potential, as a result of control, is also regarded as the midpoint potential, wherein the vicinity range is determined by a conventional method well known in the art. (The same applies to the following embodiments of the present invention.) The constant-current output drive circuit unit preferably includes a drive source circuit segment and a drive sink circuit segment, which function as a complementary constant-current circuit unit.
In a low-voltage drive circuit for driving a sensor coil in a coordinate input device at a constant current according to a second embodiment of the present invention, a constant-current output drive source circuit segment and a constant-current output drive sink circuit segment function as a complementary constant-current circuit unit. An output of the drive source circuit segment is connected to the sensor coil with a first DC coupling capacitor therebetween. An output of the drive sink circuit segment is connected to the sensor coil with a second DC coupling capacitor therebetween. The output of the drive source circuit segment is connected to a first constant-current output bias circuit unit, and the output of the drive sink circuit segment is connected to a second constant-current output bias circuit unit. The first and second constant-current output bias circuit units control DC bias voltages of the outputs of the drive source circuit segment and drive sink circuit segment, respectively, to be midpoint potentials of the respective operating voltage ranges of the drive source circuit segment and the drive sink circuit segment.
The first constant-current output bias circuit unit mentioned above may include a differential amplifier, a power supply, an amplifier, a resistor, a capacitor, and an NPN transistor (a bipolar transistor), or an FET or the like which corresponds to the bipolar transistor. In the drive source circuit segment, for example, a differential amplifier and a PNP transistor (a bipolar transistor) or an FET or the like which corresponds to the bipolar transistor convert an input AC signal to a constant-current AC signal that drives the sensor coil at a constant current via the DC coupling capacitor.
The second constant-current output bias circuit unit may include a differential amplifier, a power supply, an amplifier, a resistor, a capacitor, and a PNP transistor (a bipolar transistor), or an FET or the like which corresponds to the bipolar transistor. In the drive sink circuit segment, a differential amplifier and an NPN transistor (a bipolar transistor), or an FET or the like which corresponds to the bipolar transistor, may convert an input AC signal to a constant-current AC signal that drives the sensor coil at a constant current via the DC coupling capacitor.
A low-voltage drive method for driving a sensor coil in a coordinate input device at a constant current comprises the following steps: providing a constant-current output drive circuit unit; connecting a DC coupling capacitor between an output of the constant-current output drive circuit unit and the sensor coil; and controlling a DC bias voltage of the output of the constant-current output drive circuit unit to be a midpoint potential of an operating voltage range for the constant-current output drive circuit unit by a constant-current output bias circuit unit.
A low-voltage drive method for driving a sensor coil in a coordinate input device at a constant current according to the second embodiment of the present invention includes the steps of providing a constant-current output drive source circuit segment; connecting a first DC coupling capacitor between an output of the drive source circuit segment and the sensor coil; controlling a DC bias voltage of the output of the drive source circuit segment to be a midpoint
Liniak, Berenato & White, LLC
Tran Anh Q.
Wacom Co. Ltd.
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