Four-Level Voltage Supply for Liquid Crystal Display

The present invention relates to a power supply circuit for providing a liquid crystal display with desired voltage levels. More particularly, the present invention relates to the power supply circuit of the above described type which can simplify circuit configuration and then facilitate fabrication of the power supply circuit.

The inventors have proposed an earlier power supply circuit suitable for supplying a liquid crystal energizing circuit with desired voltage levels or potentials as shown and described in copending application, FOUR-LEVEL VOLTAGE SUPPLY FOR LIQUID CRYSTAL DISPLAY, Ser. No. 685,261, now U.S. Pat. No. 4,050,064, filed May 11, 1976 by Shintaro Hashimoto and Yuuichi Sato and assigned to the same assignee as the present invention, the disclosure of which is incorporated herein by reference. The earlier circuit as shown in FIG. 1, includes an input terminal In.sub.1 connected to a reference potential or OV, another input terminal In.sub.2 connected to a constant voltage source VC and output terminals a-e for providing the liquid crystal display energizing circuit with desired potentials. The input terminal In.sub.1 is connected directly to the output terminal a and the second input terminal In.sub.2 is connected directly to the output terminal e. Resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4 of the substantially same resistance value are serially connected between the input terminals In.sub.1 and In.sub.2 and the output terminals b, c and d are coupled with the respective middle points of the series circuit of R.sub.1, R.sub.2, R.sub.3 and R.sub.4.

There is also provided a complementary MOS circuit which comprises a P channel MOS transistor Tr.sub.8 connected in parallel with the resistor R.sub.1 and an N channel MOS transistor Tr.sub.9 connected in parallel with the resistor R.sub.4. The transistors Tr.sub.8 and Tr.sub.9 are switchable between ON and OFF states in response to control signals A.

When the control signal A is OV, the transistor Tr.sub.9 is ON to establish a short circuit between In.sub.2 and d such that voltage between In.sub.1 and In.sub.2 is divided through the use of the resistors R.sub.1, R.sub.2 and R.sub.3 to produce outputs VA, VB and VC via terminals b-d. It is concluded that a = OV, b = VA, c = VB and d = e = VC. For the DSM type of liquid crystal displays with an ignition voltage of 18V, VC = -18V, VB = -12V and VA = -6V.

Conversely, when the control signal A is VC (-18V), the transistor Tr.sub.8 is ON to establish a short circuit between In.sub.1 and b such that voltage between In.sub.1 and In.sub.2 is divided by R.sub.2, R.sub.3 and R.sub.4 thereby to produce outputs VA and VB via C and D at terminals c, d, and e. As a consequence, a = b = OV, c = VA, d = VB and e = VC.

Nevertheless, although the above described power supply circuit including as the switching means the complementary MOS circuit as shown in FIG. 1 is advantageous from the viewpoint of circuit technique since the complementary MOS circuit provides output voltage levels approximately equal to the input voltage levels, it is still difficult to fabricate the complementary MOS circuit configuration at a low cost.

Accordingly, it is an object of the present invention to provide an improvement in the earlier proposed liquid crystal display driving circuit which does not include a complementary MOS transistor circuit configuration as a switching means. In accordance with the concept of the present invention, the power supply circuit for the liquid crystal display energizing circuit is adapted in such a way as to constitute the switching means by only P channel MOS transistors (or N channel MOS transistors).

A better understanding of the present invention may be had from a consideration of the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a circuit diagram of a power supply circuit using complementary MOS transistors;

FIG. 2 is a circuit diagram of a power supply circuit constructed in accordance with one preferred form of the present invention;

FIG. 3 is an explanatory diagram for the purposes of illustrating the operation of the circuit of FIG. 2;

FIG. 4 is a circuit diagram of another preferred form of the present invention;

FIG. 5 is a circuit diagram of still another preferred form of the present invention;

FIG. 6 is a circuit diagram of the portion A of the circuit of FIG. 5;

FIG. 7 is a timing diagram of waveforms of signals which occur within the circuit of FIG. 6;

FIG. 8 is a block diagram of a peripheral configuration of the power supply circuit of the present invention;

FIG. 9 is a circuit diagram of a circuit providing segment signals for a liquid crystal display; and

FIG. 10 is a timing diagram of signals which occur within the circuit of FIG. 9.

Referring now to FIG. 2, there is illustrated a power supply circuit constructed in accordance with the present invention which includes the same parts as shown in FIG. 1, that is, the input terminals In.sub.1, In.sub.2, the output terminals a-e and the resistors R.sub.1, R.sub.2, R.sub.3, R.sub.4. While the P channel MOS transistor Tr.sub.8 is connected in parallel with the resistor R.sub.1 in the similar manner, a P channel MOS transistor Tr.sub.9 ' (not N channel MOS transistor as in FIG. 1) is connected in parallel with the resistor R.sub.4. The control signal A is applied to the gate of the transistor Tr.sub.8 and the inverted signal via an inverter is applied to the gate of the transistor Tr.sub.9 '. It will be noted that the above inverter comprises P channel MOS transistors Tr.sub.10, Tr.sub.11. An additional resistor R.sub.5 is provided in series with the resistor R.sub.4 and the P channel transistor Tr.sub.9 ' .

In the case where as in FIG. 2 the switching means are constituted by the P channel MOS transistors Tr.sub.8 and Tr.sub.9 ', when the control signal A is OV, the transistor Tr.sub.9 ' is ON to short between In.sub.2 and d. When the control signal A is VC', the transistor Tr.sub.8 is ON to short between In.sub.1 and b.

As viewed from FIG. 3, when the gate is -V and the source is -V for the P channel MOS transistors, V.sub.OUT will be reduced by the threshold voltage V.sub.t and thus assume - (V - V.sub.t). This implies that -V cannot be outputted as V.sub.OUT. It will be understood that with the complementary MOS transistor configuration set forth in the foregoing paragraphs with respect to FIG. 1, -V can be completely outputted as V.sub.OUT.

To this end, in FIG. 2 the source of the transistor Tr.sub.9 ' is connected to the point f (the junction with the output terminal e). If the voltage fV at that point is for example -6V, the gate voltage of the transistor Tr.sub.9 ' should be negative with respect to the source voltage namely -6V by at least the threshold voltage (generally, -2 to -3V) in order to derive -6V via the drain of the transistor Tr.sub.9 ' or d. The OFF output voltage of the inverter of Tr.sub.10 and Tr.sub.11 should be therefore decreased to such extent. The OFF output voltage of the inverter is determinative upon the constant voltage source VC' at In.sub.2 and therefore is VC'-V.sub.t. VC' is preliminarily established to be at least fV + 2V.sub.t and thus -6V of fV is outputted via d.

The resistor R.sub.5 provided between In.sub.2 and f is selected to meet the predetermined interrelationship between VC' of In.sub.2 and fV of f. In other words, when Tr.sub.8 is ON and Tr.sub.9 is OFF the potential fV at the point f is written as follows: ##STR1## And when Tr.sub.8 is OFF and Tr.sub.8 ' is ON the potential fV is as follows: ##STR2##

It will be obvious from the above equations that if R.sub.1 = R.sub.4 is satisfied, fV becomes fixed. Proper choice of the resistor R.sub.5 enables establishment of desired interrelationship between VC' and fV. A Zener diode may be employed instead of the resistor R.sub.5 to set fV of the point f.

In FIG. 2, VC' of In.sub.2 is, in fact, selected at -16V and the output of Tr.sub.10 is about -12 to -13V. Accordingly, since sufficiently low voltage is applied to the gate of Tr.sub.10, -6V of f can be derived from d without any variations therein.

With such an arrangement, when the control signal A is OV, Tr.sub.9 ' is ON to establish a short circuit between In.sub.2 and d such that voltage division through the resistors R.sub.1, R.sub.2 and R.sub.3 results in d = e = -6V, c = -4V, b = -2V and a = OV. The liquid crystal display of the FEM type is ignited upon application of 6V. Conversely, when the control signal A is VC', Tr.sub.8 is ON to short the circuit between In.sub.1 and b such that a = b = OV, c = -2V, d = -4V and e = -6V through the use of the voltage divider of R.sub.2, R.sub.3 and R.sub.4.

FIG. 4 is a modification in the circuit of FIG. 2 wherein the inverter circuit is implemented with a MOS device of the ED (enhancement/depletion) type. Tr.sub.10 is of the enhancement type while Tr.sub.11 is of the depletion type. Since the gate voltage of Tr.sub.9 ' is in proximity to VC' of In.sub.2, it is not necessary that VC' be negative to such extent as discussed above.

FIG. 5 shows still another preferred form of the invention which resembles that of FIG. 2 with exception of the transistor Tr.sub.9 ' and the inverter circuit. The portion including the transistor Tr.sub.9 ' and the inverter circuit is denoted as circuit A'.

The circuit A' is of the circuit configuration as shown in FIG. 6 wherein P channel MOS transistors Tr.sub.9 ' and Tr.sub.9 " are connected in parallel with the sources connected to the point f (the junction with the output terminal e) and the drains connected to the junction with the output terminal d.

The circuit A' comprises a pair of bootstrap circuits X and Y coupled to output fV of the point f via the output terminal d and 2/3fV via the output terminal d.

On the first bootstrap circuit X there is provided an inverter circuit of P channel MOS transistors Tr.sub.12 and Tr.sub.13 at a one terminal of a capacitor C.sub.1, the one terminal of the capacitor C.sub.1 being biased through a MOS resistor of Tr.sub.12 and the other terminal thereof being switchable through a MOS circuit of the ED type consisting of transistors Tr.sub.14 and Tr.sub.15. Repetition signals a.sub.1 of relatively high frequency are supplied for Tr.sub.15.

To this end, the output voltage of Tr.sub.15 and in other words the other terminal of the capacitor C.sub.1 assumes OV and VC' (for example -6V). When OV, the capacitor C.sub.1 is charged to about .vertline.VC'.vertline.. At a moment that the output of Tr.sub.15 is changed from OV to VC', the potential of A.sub.1 (the one terminal of the capacitor C.sub.1) will be changed from VC' to 2VC'. If the output of A.sub.1 is applied to the gate of Tr.sub.9 ' to effectively utilize such variation to 2VC', VC' of the point f can be outputted via the output terminal d because of the gate held at 2VC'.

The potential of 2VC' is not of a permanent character and therefore varies in accordance with developments of the discharge procedure. Therefore, Tr.sub.9 ' is rendered ON for only a short period of time. For this reason VC' or 2/3VC' cannot be continuously outputted via the output terminal d. The P channel MOS transistor Tr.sub.9 " of the second bootstrap circuit Y, therefore, is connected in parallel with Tr.sub.9 '. The output of A.sub.2 applied to the gate of Tr.sub.9 " permits VC' of the point f to be outputted via the output terminal d without variations and therefore VC' or 2/3VC' to be permanently outputted via the output terminal d in cooperation with the first bootstrap circuit X.

The output of A.sub.2 set forth above is obtainable from the second bootstrap circuit Y in the same manner as that of the circuit X. The repetition signals a.sub.2 are applied to Tr.sub.19 within the MOS circuit device of the ED type.

FIG. 7 is a timing diagram showing waveforms of signals within the circuit of FIG. 6. The signals a.sub.1 and a.sub.2 are of the waveforms designated a and b such that periods of time for remaining OFF state of Tr.sub.15 and Tr.sub.19 are different but somewhat overlapped with each other. c designates the waveform of the control signal A, d designates that of the output of A.sub.1, e designates that of the output of A.sub.2 and f designates that of the output of the output terminal d.

When the control signal A is OV, Tr.sub.12 and Tr.sub.16 are ON. When the a.sub.1 signal is OV, the output of Tr.sub.15 is OFF and hence VC'. The potential of A.sub.1 falls from VC' to 2VC' to make Tr.sub.9 ' ON. If the a.sub.2 signal is OV, Tr.sub.19 is OFF and its output is VC'. As a result, the potential of A.sub.2 falls from VC' to 2VC' thereby to render Tr.sub.9 " ON. As ON period for Tr.sub.9 ' is placed to overlap with that for Tr.sub.9 ", fV = VC' of the point f is successively developed at the output terminal d as suggested by f.

On the other hand, when the control signal A is VC', Tr.sub.13 and Tr.sub.17 are ON to keep the potentials of A.sub.1 and A.sub.2 at OV and in addition to render Tr.sub.9 ' and Tr.sub.9 " OFF. This is the correspondence to that the circuit A' of FIG. 5 is rendered OFF.

In summary, when the control signal A is OV, Tr.sub.9 ' and Tr.sub.9 " are ON to establish shorted circuit between In.sub.2 and the output terminal d, followed by that voltage division is effected by the resistors R.sub.1, R.sub.2 and R.sub.3 and consequently d = e = -6V, c = -4V, b = -2V and a = OV. Conversely, when the control signal A is VC', Tr.sub.8 is ON and circuit between In.sub.1 and b is shunted such that voltage division by the resistors R.sub.1, R.sub.2 and R.sub.3 results in a = b = OV, c = -2V, d = 4V and e = -6V. Provision of the constant voltage source of more negative voltage as shown in FIG. 2 is not needed due to a parallel combination of the two bootstrap circuits X and Y as shown in FIG. 6. Therefore, a constant voltage source of -6V can be employed (in FIG. 2 -16V) and Tr.sub.8 and Tr.sub.9 ' as switching means can be implemented with a P channel MOS transistor.

As depicted by f showing the waveform of the output at the output terminal d in FIG. 7, under the conditions the control signal A is OV, the signal a.sub.1 is VC' and the signal a.sub.2 is OV at the time t.sub.0, Tr.sub.8, Tr.sub.9 ' and Tr.sub.9 " are to be OFF (as A.sub.2 is not 2VC'). Thus, fV of the point f has to be outputted via the output terminal d since Tr.sub.8 and the circuit A' are both placed in OFF state in FIG. 5. However, there is created the possibility of providing undesired potentials. By sufficiently increasing frequency of the signals a.sub.1 and a.sub.2 as compared with frequency of the control signal A, the possibility can be avoided or ignored.

Although in the circuit of FIG. 6 Tr.sub.14 and Tr.sub.15 are of the ED type MOS configuration, they may be in the same form as the P channel MOS transistor Tr.sub.12.

FIG. 8 is a schematic of a peripheral circuit arrangement which utilizes the potential distributing circuits stated above with respect to FIGS. 2, 4 and 5. By way of a one chip MOS/LSI calculator, an LSI chip internally contains the potential distributing circuit in addition to a conventional arithmetic control circuit. The LSI chip is supplied with VC' from the constant voltage source. And, as obvious in the art of calculators, the LSI chip receives key signals from a keyboard and provides display signals for a liquid crystal display. The constant voltage source VC' is coupled as V.sub.DD to enable terminals of the LSI chip to energize P channel MOS transistors within the control circuit.

For example, in the case where VC' of -16V as shown in FIG. 2 is employed, P channel MOS transistors within the control circuit are of the high threshold type powered with -16V, thereby reducing the number of power or enable terminals of the LSI chip as small as possible. In many cases, the LSI chip needs V.sub.GG power source somewhat smaller than V.sub.DD for establishment of clock pulse levels but VC' may be the correspondence to V.sub.GG.

FIG. 9 shows an example of a circuit arrangement adapted for providing segment signals SI in response to the potentials from the potential distributing circuit. This comprises a couple of bootstrap circuits M and N and resembles essentially the circuit arrangement shown in FIG. 6 wherein the P channel MOS transistors Tr.sub.1 and Tr.sub.2 are connected in parallel. The commonly connected sources of Tr.sub.1 and Tr.sub.2 are led to the output terminal of the segment signal SI and the drain of a P channel MOS transistor Tr.sub.3. The drains of Tr.sub.1 and Tr.sub.2 are connected to the output terminal d of the potential distributing circuit. Tr.sub.3 has its source connected to the output terminal b of the potential distributing circuit and its gate receiving segment selection signals s. The circuit arrangement including a capacitor C.sub.1, Tr.sub.12 to Tr.sub.15 within the bootstrap circuit M and a capacitor C.sub.2, Tr.sub.16 to Tr.sub.18 within the bootstrap circuit N, operates in the same mode as that of FIG. 6.

A time diagram of the waveforms of signals in operation of the circuit of FIG. 9 is shown in FIG. 10. a designates the signal a.sub.1 applied to the gate of Tr.sub.15, b designates the signal a.sub.2 applied to the gate of Tr.sub.19, c designates coincidence between the control signal A and the segment selection signal s which is derived via a decoder from a register, d designates the waveform of the output of A.sub.1, e designates the waveform of the output of A.sub.2 and f designates the waveform of the segment signal SI. The segment SI assumes either one of the potentials supplied from the output terminals b and d in accordance with the potential of the segment selection signal s.

Although there has been described above a specific arrangement of the liquid crystal power supply circuit in accordance with the invention for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that invention is not limited thereto. Accordingly, any modifications, variations or equivalent arrangements which may occur to those skilled in the art should be considered to be within the scope of the invention.