JPH0698782B2 - Thermal head - Google Patents

Description

The present invention relates to a direct drive type thermal head for gradation printing.

[Outline of Invention]

The present invention, in the direct drive type thermal head for gradation printing, improves the voltage variation between the driver IC chips of the thermal head by improving the variation of the voltage of the driver IC element when the current is applied by using a variable resistance element for adjustment. It is possible to prevent the occurrence of density unevenness of the print based on the variation of.

[Conventional technology and its problems]

Conventionally, the circuit configuration of a direct drive type thermal head in which a switching element (for example, a bipolar transistor (Tr) or a field effect transistor (FET)) that connects and disconnects a current to each heating resistor is connected is equivalent to, for example, FIG. As shown in the circuit, in recent years the switching element Tr
_The monolithic IC chips (called driver IC chips) 10 and 11, which have _1-32 and a gate circuit, a latch circuit, a shift register path, etc. in one chip, have come to be used.
In FIG. 5, the driver IC chips 10 and 11 are switch elements T.
r _1, Tr _2, ......... are 32-bit integrated. Each one
The equivalent circuit of only bits is as shown in FIG. When the applied voltage is constant voltage V ₁ , the heating resistance R in Fig. 6
_{Between H1} , energizing resistance R _ON1 , wiring resistance R _P1 and heating voltage V _H1 Have a relationship.

The applied power W (Watt / dot) per print dot is The heat generation energy E (joule / dot) is E = W · Ton (2) (where Ton is energization time). Therefore, in the gradation printing thermal head, in order to change the heat generation energy E for each gradation, control is performed by the energization time of the switch elements Tr ₁ , Tr ₂ , .... Therefore, unless the applied power W is constant, proper gradation printing cannot be obtained.

When it is desired to print the entire screen in a halftone color using a line type thermal head, if the variation ΔR of R and the variation ΔV of the applied voltage V in Expression 1 are not as small as possible, the color in the sub-scanning direction of the screen is obtained. It causes unevenness.
ΔV is determined by the regulation of the power supply, etc., and can be set in advance with high accuracy. On the other hand, ΔR is the variation ΔR of the heating resistance R _H of the heating resistor, the wiring resistance R _P, and the energization resistance R _{ON.
It is} the sum of _H , ΔR _P and ΔR _ON . In terms of manufacturing technology, ΔR _H has a limit of ± 5% for thin films and ± 10% for thick films, and ΔR _P can be solved by considering the pattern beforehand, so driver ICs with small ΔR _ON Required. But this has its limits.

In the circuit configuration of the conventional example shown in FIG. 5, generally, in the equivalent circuit in the driver IC chips 1, 2, ..., As shown in FIG. _{1 ~Tr 8, Tr 9 ~Tr 16} , by providing four ground pads G ₁ ~G ₄ are summarized to the ground of one by one ........., wiring resistance on the chip, the voltage drop due to, for example, Al It is preventing. Ideally, a ground pad should be provided for each switch element, but in that case, the number of wires increases, which is not preferable because the cost for mounting and the unit price of the chip increase. Therefore, the number of ground pads is minimized. Therefore, when the switching element is Tr, the collector saturation voltage variation ΔVc _E (s
at) is not a solution. That is, variations among driver IC chips are likely to occur.

[Means for Solving the Problems] In view of the above problems, the present inventors have improved the variation in the voltage during energization of the driver elements in the direct drive type thermal head, and have a driver IC chip of the thermal head for gradation printing. As a result of intensive research to prevent the occurrence of density unevenness due to the difference in applied power between the two, the present invention has been completed.

That is, the present invention, on the insulating substrate, a plurality of recording heating resistor, a driver IC for controlling the current to the resistor,
A direct drive type thermal head integrally provided with a variable adjusting resistance element for buffering variation in resistance when the switch element in the driver IC is energized, wherein the switch element is divided into a plurality of groups. , Each of the groups is led to one of the adjusting resistance elements, respectively.

The adjusting resistance element is preferably a resistor formed by thick film printing, or may be a thin film resistor formed by sputtering.

Further, it is preferable that the adjusting resistor is made of the same material as the recording heating resistor.

Further, it is preferable that the adjusting resistance element is composed of a plurality of IC bonding wires connected in parallel.

〔Example〕

 The present invention will now be described with reference to the drawings.

As shown in FIG. 1, a variable variable resistance element Ra for adjustment is inserted in series for each driver IC chip. At this time, at the constant voltage V ₂ , the heating voltage V _H2 is Therefore, by inserting the resistance element Ra, the variation in R _ON2 is absorbed and V _H2 becomes constant. Resistance element Ra ₁ , R
a ₂ , Ra ₃ ……………… for driver IC chip units 1, 2,
Fig. 2 shows an example of an equivalent circuit when it is placed in 3 ………….

The adjustment resistance element is also used for each driver IC chip 1, 2,
..., switch elements Tr ₁ , Tr ₂ ... are divided into several groups, for example, as shown in FIG. 3, switch elements Tr _{1 to} Tr ₃₂ of chip 1 are divided into 4 groups of 8 groups each, Each of the ground terminals G _{1 to} G ₄ of each group has a resistance element Ra ₁₁ for adjustment,
Ra ₁₂ , Ra ₁₃ and Ra ₁₄ can be used by being inserted.

The variable resistance element for adjustment used in the present invention can be formed by various methods, and is preferably shown in FIG.
It is formed by the method shown in FIG. That is, in FIG. 4A, the notch 5 is formed in the resistor 4 formed by thick film printing and trimmed, and the resistance is adjusted by the size of the notch. This resistor is preferably made of the same material as the recording heating resistor, and by doing so, both can be manufactured simultaneously in the thermal head manufacturing process.

In addition, as shown in FIG. 4 (b), the adjustment resistor is Au,
It can be formed by adjusting the number and length of the wires 6 such as Al.

Furthermore, as shown in FIG. 4 (c), the resistor 7 and the wire 8 can be used in combination to form an adjusting resistor, and the wire 8 can be cut one by one for adjustment. You can

〔The invention's effect〕

Since the thermal head of the present invention is provided with a variable resistance element for adjustment for each driver IC chip so as to improve the variation in resistance during energization, it has been conventionally performed before mounting the driver IC chip on a mount. In addition, the step of measuring the resistance during energization and sorting the chips becomes unnecessary, and the problem of accuracy due to the conventional measurement error of the resistance during energization in the chip state is solved.

The present invention solves the drawback that if the chip is mounted and mounted without measuring the resistance during energization, it is generally difficult to replace the chip after assembly and the yield is deteriorated.

Further, when printing halftone over the entire surface, density unevenness in the sub-scanning direction is eliminated, and the image quality with gradation can be improved.

In addition, the potential of the base of the switch element or the ground of the gate rises, and malfunctions due to the influence of noise can be prevented by the thermal head of the present invention.

[Brief description of drawings]

FIG. 1 is an equivalent circuit of only one bit in the embodiment of the present invention, FIG. 2 is an equivalent circuit configuration diagram of the embodiment of the present invention, FIG. 3 is an equivalent circuit of another embodiment, and FIG. 4 (a). (C) is a plan view for explaining the method of forming the adjusting resistance element, FIG. 5 is an equivalent circuit of the conventional example, FIG. 6 is an equivalent circuit of only 1 bit in FIG. 5, and FIG. 7 is another conventional circuit. It is an equivalent circuit block diagram of an example driver IC chip. In the reference numerals used in the drawings, Ra, Ra ₁ , Ra ₂ , ..., Ra _{11 to} Ra ₁₄ ... adjusting resistance elements G _{1 to} G ₄ ... ground pads ₁ , ₂ , ₃ , ₁₀ , ₁₁ ...... Driver IC chip 4 …… A resistor.

Claims (4)

[Claims] 1. A plurality of heating resistors for recording, a driver IC for controlling a current to the resistors, and a switching element in the driver IC for absorbing a variation in resistance when energized on an insulating substrate. A direct drive type thermal head integrally provided with a variable adjusting resistance element, wherein the switch elements are divided into a plurality of groups, and each group is led to one adjusting resistance element. A thermal head characterized by that. 2. The thermal head according to claim 1, wherein the adjusting resistance element is a resistor formed by thick film printing. 3. The thermal head according to claim 1, wherein the adjusting resistor is made of the same material as the recording heating resistor. 4. A plurality of the adjusting resistance elements are connected in parallel.
The thermal head according to claim 1, comprising a wire for IC bonding.