Digital-to-analog converter with short integration time constant

1. Field of the Invention

The present invention relates to a digital-to-analog (D/A) converter, which is applicable to a liquid crystal display drive circuit or the like that generates a voltage in response to a digital signal to drive a liquid crystal display device.

2. Description of the Background Art

Reference is first made to FIG. 2 , which is a schematic block diagram showing a conventional digital-to-analog converter disclosed by U.S. Pat. No. 6,556,162 to Brownlow et al. The digital-to-analog converter is a two-stage conversion system that comprises a first-stage converter section 1 and a second-stage converter section 2 . The first-stage converter section 1 selects, according to the m most significant bits (MSBs) in the digital signal DI of k (=m+n) bits, a pair of adjacent voltages from the voltages of 0 to 2 m . The first-stage converter section 1 then outputs the selected voltages as an upper voltage limit VH and a lower voltage limit VL. The second-stage converter section 2 divides the voltage between the upper voltage limit VH and the lower voltage limit VL into 2 n segment voltages which are equal to each other. The second-stage converter section 2 then selects one of the segment voltages according to the n least significant bits (LSBs) in the digital signal DI.

The second-stage converter section 2 comprises: a resistive voltage divider 3 which divides the voltage between the upper limit voltage VH and the lower limit voltage VL into the 2 n equal segment voltages; a switch group 4 which selects one of the 2 n segment voltages to output the selected one; and a decoder (DEC) 2 a which decodes the n bits to turn on a corresponding switch of the switch group 4 .

To the output of the second-stage converter section 2 , i.e., to the output of the switch group 4 , a capacitive load CL such as a liquid crystal display device is connected via an output buffer including a voltage-follower-connected operational amplifier (OP) 5 .

In the digital-to-analog converter, the first-stage converter section 1 selects, in response to the MSB bit in the digital signal DI, a pair of adjacent voltages VH and VL from the voltages of 0 to 2 m provided by the low-impedance reference voltage source. The second-stage converter section 2 selects a voltage specified by the LSB bit in the digital signal DI from the 2 n equal segment voltages divided from the voltage between the voltage limits VH and VL.

In the above-described digital-to-analog converter, however, the voltages of 0 to 2 m that are provided to the first-stage converter section 1 a are generated generally by a resistive voltage divider having lower resistance. Between the voltages VH and VL outputted from the first-stage converter section 1 connected in parallel is the resistive voltage divider 3 of the second-stage converter section 2 . Smaller resistance of the resistive voltage divider 3 may therefore vary the voltages VH and VL, providing inaccurate analog voltages. The liquid crystal display device or the like will require as many digital-to-analog converters as the number of display electrodes to be connected in parallel with the resistive voltage divider having lower resistance. A plurality of digital-to-analog converters operating at the same time may cause the reference voltage to vary more significantly.

Larger resistance of the resistive voltage divider 3 may increase the time constant of the integrator circuit. The time constant comes from the combination of that resistance with the input capacitance of the operational amplifier 5 . A larger time constant may provide a lower response speed, which causes the display device not to follow the fast motion of a displayed image.

It is an object of the present invention to provide a two-stage conversion type of digital-to-analog converter that has high conversion accuracy and a high response speed.

In accordance with the present invention, a digital-to-analog converter for converting a digital signal including a plurality of bits into a corresponding analog voltage, comprises: a first divider for dividing a voltage between a lower reference voltage and an upper reference voltage into a plurality of stepwise voltages; a first selector for selecting, according to at least one most significant bit in the digital signal, a pair of lower limit voltage and upper limit voltage adjacent to each other among the plurality of stepwise voltages; a second divider having a resistor for dividing the voltage between the lower limit voltage and the upper limit voltage into a plurality of divided voltages; a second selector for selecting, according to at least one least significant bit in the digital signal, a voltage associated with the at least one least significant voltage among the plurality of divided voltages as an output voltage; and a switch provided in parallel with the resistor in the second divider for short-circuiting opposite ends of the resistor for a predetermined period of time.

In accordance with the present invention, when the digital signal changes, a switch short-circuits the resistor in the second divider in response to a load signal that is provided for the predetermined period of time. The second divider thus has low total resistance, which may reduce the time constant of the integration circuit. The time constant comes from the combination of the total resistance and the input capacitance of the second selector, the buffer amplifier or the load connected to the second divider. The output voltage may thus quickly change to follow the digital signal change. When the load signal stops, the switch is opened and the second divider resistance returns to its normal value, allowing the output voltage to quickly change to the normal voltage. The present invention may thus provide a digital-to-analog converter that has high conversion accuracy and a high response speed.

The above-described switches may be replaced with switches that reduce the resistance of the respective resistors of the second divider to a predetermined value. The switches may be controlled in response to a load signal that is provided for a predetermined period of time when the digital signal changes.

The objects and features of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic block diagram showing a preferred embodiment of the digital-to-analog converter according to the present invention;

FIG. 2 is a schematic block diagram showing a conventional digital-to-analog converter;

FIG. 3 shows signal waveforms appearing in operation of the digital-to-analog converter shown in FIG. 1 ; and

FIG. 4 is a schematic block diagram of the digital-to-analog converter of an alternative embodiment according to the present invention.

FIG. 1 is a schematic block diagram of the digital-to-analog converter of an embodiment-according to the present invention. The digital-to-analog converter of the embodiment is also the type of two-stage converter similar to what is shown in FIG. 2 . The illustrative digital-to-analog (D/A) converter is adapted to generate an output voltage OUT to drive a liquid crystal display device in response to the digital signal DI represented by a plurality (k) of bits, where k is a natural number equal to m+2, m also being a natural number.

The digital-to-analog converter comprises a first-stage resistive voltage divider 10 that is adapted to divide a drive voltage ranging from a lower reference voltage VRL, for example, 8 volts (V), to an upper reference voltage VRH, for example, 16 V, into 2 m +1 different stepwise voltages. The resistive voltage divider 10 comprises 2 m component resistors connected in series, each resistor having its resistance r equal to about 100 [ohm], for example. The resistive voltage divider 10 generates 2 m +1 different stepwise voltages across its opposite terminals and the connection points or nodes in between. The resistive voltage divider 10 outputs the 2 m +1 different stepwise voltages to the first-stage selector (SEL) 20 . The embodiment is only for illustrative purposes and are not intended to limit the scope of the invention. Signals are designated with reference numerals of connections on which they appear.

The selector 20 functions as selecting in pair adjacent ones of the 2 m +1 different stepwise voltages according to the m most significant bits (MSBs) in the digital signal DI. The selector 20 then outputs the selected pair of voltages as an upper voltage limit VH and a lower voltage limit VL.

The selector 20 outputs the lower limit voltage VL to one end of a component resistor 31 , included in the second-stage resistive voltage divider 40 . The component resistor 31 1 has its other end connected to a node N 1 . The node N 1 connects via a switch 32 to one end of the resistor 31 2 . The resistor 31 2 has its other end connected to another node N 2 . The node N 2 connects via a resistor 31 3 to still another node N 3 . To the node N 3 is connected one end of the resistor 31 4 . To the other end of the resistor 31 4 is provided the upper limit voltage VH outputted from the selector 20 . each of these resistors 31 1 to 31 4 has its resistance R (for example, 200 k[ohm]) much greater than the resistors included in the resistive voltage divider 10 .

Between the nodes N 1 and N 2 , a switch 33 1 is connected. Between the nodes N 2 and N 3 , a switch 33 2 is connected. Further, across the resistor 31 4 , a switch 33 3 is connected. These switches 33 1 to 33 3 are in the conductive or ON state thereof when a load signal LD is at “H” level, thus short-circuiting the resistors between the nodes. The load signal LD has its waveform becoming its high “H” level only for a predetermined period of time when the digital signal DI changes or transfers its logical state. To the nodes N 1 to N 3 , the second-stage selector 40 is connected.

The second-stage selector 40 functions as selecting, according to the two least significant bits (LSBs) in the digital signal DI, the divided or segment voltage of one of the nodes corresponding thereto. The selector 40 comprises a decoder (DEC) 41 that decodes the two least significant bits in the digital signal DI, and switches 42 1 to 42 4 that are ON and OFF controlled with the signal decoded by the decoder 41 .

The decoder 41 is operative in response to the two least significant bits in the digital signal DI to raise one or ones of its output signals s 0 , s 1 , s 2 , and s 3 which corresponds or correspond to the values 0, 1, 2, and 3 represented by the two least significant bits to the high “H” level thereof. The remaining signals not corresponding to the values represented by the two least significant bits are tendered to the low “L” level thereof. The-decoder 41 also outputs a signal/s 0 to the switch 32 at the same time, the signal/s 0 being obtained by inverting the logical level of the signal s 0 . The switch 32 takes its ON state when the signal/s 0 is at its “H” level.

The switch 42 1 is connected between the node N 1 and the node N 4 on the output side, and is controlled with the signal s 0 . The switch 42 2 is connected between the nodes N 1 and N 4 , and is controlled with the signal s 1 . Further, the switch 42 3 is connected between the nodes N 2 and N 4 , and is controlled with the signal s 2 . Finally, the switch 42 4 is connected between the nodes N 3 and N 4 , and is controlled with the signal s 3 . These switches 42 1 to 42 4 take the ON state thereof when the signals s 0 to s 3 become “H” level, respectively.

To the node N 4 is connected an output buffer including a voltage-follower-connected operational amplifier 50 . The operational amplifier 50 is adapted for producing a signal AO via a switch 60 to a capacitive load CL, such as a liquid crystal display device, in the form of output voltage OUT. The switch 60 is rendered to its ON state when the inverse load signal/LD obtained by inverting the load signal LD is at its “H” level.

Although FIG. 1 shows the first-stage resistive voltage divider 10 having its output interconnected to only one digital-to-analog converter including the selectors 20 and 40 , the digital-to-analog converter may be applied in practice to a liquid crystal display drive circuit in such a fashion that the resistive voltage divider 10 has its output interconnected to the same digital-to-analog converters provided in parallel as many as the number of display electrodes of the liquid crystal display device.

Now reference will also be made to FIG. 3 showing signal waveforms appearing in operation of the digital-to-analog converter shown in FIG. 1 . In operation, for example, at time t 0 when the operation is stable, the selector 20 selects, according to the m most significant bits in the digital signal DI, stepwise voltages 9.0 V and 9.1 V as the lower limit voltage VL and the upper limit voltage VH, respectively, and the two least significant bits in the digital signal DI have a value of binary “0”. The load signal LD is at its “L” level, all of the switches 33 1 to 33 3 are in the OFF state thereof, and the switch 60 is in its ON state.

Because the two least significant bits in the digital signal DI have a value of binary “0”, the signals s 0 and /s 0 outputted from the decoder 41 are of the “H” and “L” levels thereof, respectively. The switch 32 thus becomes its OFF state, thus providing the lower limit voltage VL to the node N 4 through the resistor 31 1 , node N 1 , and switch 42 1 . The node N 4 thus has the resultant voltage VN 4 equal to 9.0 V. The operational amplifier 50 then outputs the lower limit voltage VL (9.0 V) as the output signal AO. The output signal AO is provided through the switch 60 to the liquid crystal display device CL as the output voltage OUT.

Now, at time t 1 , the digital signal DI changes its state such that the selector 20 selects, according to the m most significant bits in the digital signal DI, stepwise voltages 11.5 V and 11.6 V as the lower limit voltage VL and upper limit voltage VH, respectively, and such that the two least significant bits have a value of binary “1”. When the digital signal DI changes, the load signal LD becomes its “H” level for the predetermined period of time.

Because the two least significant bits in the digital signal DI have a value of binary “1”, the signals /s 0 and s 1 become the “H” level thereof, and the signals s 0 , s 2 , and s 3 become the “L” level thereof. The switches 32 and 42 2 thus become the ON state thereof. Because the load signal LD is at its “H” level, the switches 33 1 to 33 3 become the ON state thereof. The switches 33 1 to 33 3 , therefore, short-circuit the resistors 31 2 to 31 4 , causing the divided or segment voltages on the nodes N 1 to N 3 to be-the upper limit voltage VH (11.6 V). The voltage of the node N 1 is provided via the switch 42 2 to the node N 4 . Because the upper limit voltage VH is applied to the node N 4 via the switches 33 1 to 33 3 in the ON state thereof, the voltage VN 4 of the node N 4 increases rapidly to 11.6 V of the upper limit voltage VH regardless of the input capacitance of the operational amplifier 50 or the like. The operational amplifier 50 outputs the voltage VN 4 as the output signal AO. The output voltage OUT is not, however, outputted because the switch 60 at the output is in its OFF state.

At time t 2 , the digital signal DI remains its current state, and the load signal LD returns to its “L” level. The switches 33 1 to 33 3 thus become the OFF state thereof, allowing the four resistors 31 1 to 31 4 connected in series to connect the voltages VL and VH. The node N 1 thus changes its voltage from 11.6 V to 11.525 V of the normal voltage. The voltage VN 4 of the node N 4 also changes its voltage from 11.6 V to 11.525 V. Because the input capacitance of the operational amplifier 50 or the like has already been charged to 11.6 V, the voltage VN 4 of the node N 4 may quickly change to 11.525 V. The operational amplifier 50 outputs the voltage VN 4 as the signal AO. The signal AO is then provided via the output-side switch 60 in its ON state to the liquid crystal display CL as the output voltage OUT. Subsequently, the same operation will repeat every time the digital signal DI changes periodically.

As described above, the digital-to-analog converter in this illustrative embodiment includes the switches 33 1 to 33 3 that short-circuit, when the digital signal DI changes its logical state, the resistors 31 2 to 31 4 in the second-stage resistive voltage dividers in order to rapidly increase the voltages of the nodes N 1 to N 3 to the upper limit voltage VH, and in order to charge the node N 4 on the input of the operational amplifier 50 to the upper limit voltage VH. This may have an advantage that even when the digital signal DI largely changes its state, the digital-to-analog converter may output, correspondingly to the signal DI, the output signal AO having higher conversion accuracy and a higher response speed.

As described above, the switch 32 resides in series with the second-stage resistive voltage dividers 31 1 to 31 4 . When the two least significant bits in the digital signal DI have a value of binary “0”, the switch 32 isolates the resistive voltage dividers from the output of the selector 20 , providing the lower limit voltage VL to the node N 4 . The second-stage resistive voltage dividers are thus not connected in parallel when the two least significant bits have a value of binary “0”, thereby further reducing the error.

The present invention is not limited to the above-described illustrative embodiment, but various modifications may be made thereon. For example, in the illustrative embodiment, the second-stage selector 40 references the two least significant bits in the digital signal DI to select the final voltage. Alternatively, the second-stage selector 40 may use three or four least significant bits rather than two bits to select the target voltage.

In the illustrative embodiment, when the two least significant bits in the digital signal DI have a value of binary “0”, the switch 32 isolates the second-stage resistive voltage divider from the output of the selector 20 , thereby providing the node N 4 with the lower limit voltage VL. Alternatively, there may be provided a second-stage resistive voltage divider that is always connected and a selector that selects the output of the divider.

In the illustrative embodiment, when the load signal LD is at its “H” level, the upper limit voltage VH is outputted to the node N 4 . Alternatively, the switches 32 and 33 may be differently arranged to output the lower limit voltage VL. In addition, the switch 60 on the output side may be omitted.

Well, FIG. 4 will be referred which schematically shows in a block diagram the digital-to-analog converter of an alternative embodiment according to the present invention. Like elements are designated with the same reference symbols or numerals.

The digital-to-analog converter depicted in FIG. 4 may be the same as the digital-to-analog converter shown in FIG. 1 except that the resistors 31 1 to 31 4 in the second-stage resistive voltage divider and the switches 33 1 to 33 3 that short-circuit the resistors 31 2 to 31 4 shown in FIG. 1 are replaced with a resistive voltage divider and a switch group that are slightly different in configuration. Specifically, the selector 20 is interconnected to output the voltage VL to a node N 0 . between the nodes N 0 and N 1 , a resistor 35 1 is connected, and between the nodes N 1 and N 2 a resistor 35 2 is connected. Further, between the nodes N 2 and N 3 connected is a resistor 35 3 . The selector 20 is further interconnected to output the voltage VH via a resistor 35 4 to the node N 3 . The resistors 35 1 to 35 4 have the same resistance R, equal to, e.g., 200 k[ohm], much greater than the resistance of the resistors included in the resistive voltage divider 10 .

The resistors 35 1 to 35 4 each include taps therein. The switches 36 1 to 36 4 are interconnected to short-circuit the respective taps. The switches 36 1 to 36 4 are turned to the ON or conductive state thereof whenever the load signal LD is at its “H” level. When the switches 36 1 to 36 4 are in the ON state thereof, the resistors 35 1 to 35 4 have a reduced resistance, for example, about half as large as the normal resistance.

The nodes N 0 , N 1 , N 2 , and N 3 connect to the selector 40 that selects the input according to the two least significant bits in the digital signal DI. The selector 40 selects the voltage VN 4 and outputs it to the operational amplifier 50 . Other configurations may be the same as those shown in and described with reference to FIG. 1 .

When the digital signal DI changes periodically, the digital-to-analog converter shown in FIG. 4 in turn changes the load signal LD to its “H” level for a predetermined period of time. The switches 36 1 to 36 4 thus become the ON state thereof, and half the normal resistance value is provided for each of the resistors 35 1 to 35 4 in the second-stage resistive voltage divider that is connected in parallel with the output of the selector 20 . The voltage VN 4 provided to the operational amplifier 50 , therefore, changes relatively quickly to its final voltage. At this point, however, a small resistance is provided for the second-stage resistive voltage divider that is connected in parallel with the first-stage resistive voltage divider 10 , thus causing errors in the pair of voltages VL and VH outputted from the selector 20 .

When the load signal LD returns to its “L” level, the switches 36 1 to 36 4 become the OFF state thereof, and the normal resistance is provided for each of the resistors 35 1 to 35 4 in the second-stage resistive voltage divider that is connected in parallel with the output of the selector 20 . The pair of voltages VL and VH outputted from the selector 20 is thus adjusted to voltages with smaller errors. Accordingly, the voltage VN 4 outputted from the selector 40 is adjusted to the correct value. The adjusted voltage that is significantly small may quickly complete the adjustment operation.

As described above, the digital-to-analog converter in the alternative embodiment includes the switches 36 1 to 36 4 that are adapted to reduce, when conductive, the resistance of each of the resistors 35 1 to 35 4 in the second-stage resistive voltage divider when the digital signal DI changes its logical state. The input of the operational amplifier 50 may thus quickly be rendered to its target voltage when the digital signal DI changes. The resistors 35 1 to 35 4 then return to the normal resistance thereof, quickly completing the adjustment operation. The alternative embodiment may thus have the same advantage as the illustrative embodiment shown in and described with reference to FIG. 1 .

The alternative embodiment may be more advantageous than the illustrative embodiment shown in FIG. 1 when the pair of voltages VL and VH selected by the selector 20 has a larger voltage difference, for example, 0.5 V or more, therebetween, because the input of the operational amplifier 50 may become the target voltage when the load signal LD is at its “H” level. In the illustrative embodiment shown in FIG. 1 , the input of the operational amplifier 50 becomes the voltage VH when the load signal LD is at its “H” level, and the input changes, after the load signal LD returns to its “L” level, to the target voltage specified by the two least significant bits.

The present invention is not limited to the above-described alternative embodiment, but various modifications may be made thereto. For example, as described earlier, the second-stage selector 40 may use three or four least significant bits in the digital signal DI to select the final target voltage.

The values of the voltage and resistance in the above-described embodiments are shown by way of example, and may be set to any values according to the circuit condition to which the invention is applied.

The entire disclosure of Japanese patent application No. 2005-198610 filed on Jul. 7, 2005, including the specification, claims, accompanying drawings and abstract of the disclosure is incorporated herein by reference in its entirety.

While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.