Semiconductor device for use with a thermal print head

Metal-oxide semiconductors are used for the logic control circuit and the drive circuit of a thermal print head to eliminate defects such as breakdowns and erroneous operations caused by heat generated in the thermal head and to reduce current consumption.

BACKGROUND OF THE INVENTION 
The present invention relates to a semiconductor device in which a drive 
circuit for driving a thermal print head and a logic control circuit for 
controlling the thermal head are each constituted by a P-channel MOS, an 
N-channel MOS, or a complementary MOS, or by a plurality of such devices. 
A thermal print head 1 consists, as shown in FIGS. 1a and 1b, of a heating 
resistor 3, a drive circuit 4, and a logic control circuit 5 mounted on a 
ceramic substrate 2. 
Since the heating resistor consumes a large current, a bipolar element has 
heretofore been used in the drive circuit 4 for driving the thermal print 
head, as shown in FIG. 3. As the temperature rises, the leakage current 
and the amplification factor of the bipolar element increase, so that the 
current driving the thermal head increases. Heat is generated by the 
increase in current, and hence a larger current flows. This phenomenon is 
usually known as thermal runaway. The heat generator in the thermal head 
generates heat. Therefore, when a bipolar element is mounted on the same 
substrate, conditions worsen and, if the substrate of the thermal head has 
a poor heat radiating efficiency, the bipolar element undergoes thermal 
breakdown. Thermal runaway and erroneous operations caused by heat present 
serious problems concerning the reliability of the thermal head. 
Thermal print heads can be divided into the linear type and the serial 
type. In recent years, thermal heads of the linear type have been put into 
practice, and attention has been given to the generation of heat and 
consumption of large currents by bipolar elements. In the linear type of 
thermal head, 3 to 16 heat generator dots are provided per millimeter, and 
drivers are provided for each of them. For A-4 size, 1700 dots are 
required if 8 dots are provided per millimeter in the standard manner. 
Each dot must be supplied with a current of 20 mA to 80 mA to generate 
heat, which requires a power source with a large capacity. It therefore 
becomes necessary to reduce the current flowing into the logic circuit to 
as small as possible to minimize the load on the power source. 
FIG. 3 shows the conventional setup of a thermal head employing a bipolar 
element. A bipolar logic control circuit 5a and a drive circuit 4a are 
supplied with a voltage of 4 to 6 volts from a power source 7, and a heat 
generator (not shown) is supplied with a high voltage of 15 to 30 volts. 
The logic control circuit 5a produces and output control signal for 
controlling the operation of the drive circuit 4a. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to use metal-oxide semiconductors 
for the logic control circuit and drive circuit of the thermal head to 
eliminate the defects such as breakdowns and erroneous operations caused 
by heat generated in the thermal print head, as well as to reduce the 
current consumption so that the power source can be constructed at a 
reduced cost.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
An embodiment of the invention is illustrated in FIG. 4. 
The drive circuit 4 is constructed as an open drain of an N-channel MOS to 
satisfy the maximum voltage and drive current requirements, and the logic 
control circuit 5 is made up of a complementary MOS to reduce the 
consumption of electric current by the system. The development of the 
latch-up phenomenon by a high voltage and large current is prevented by 
using an N-channel MOS 9 for the drive circuit 4 as shown in FIG. 5. This 
effect can also be obtained even when a P-channel MOS is used for the 
drive circuit 4 provided it is constructed with an open drain. The same 
effect can also be obtained when a complementary MOS is used, but this 
would depend upon the conditions. The silicon substrate for forming a 
complementary MOS integrated circuit will usually be N-type. When an 
N-channel MOS is used for the drive circuit 4, however, the silicon 
substrate must be P-type. 
The reason why the metal-oxide semiconductor does not undergo thermal 
runaway is described below. 
The drive circuit 4 is constructed by using an N-channel MOS as shown in 
FIG. 5. The electric current flowing into an N-channel MOS 9 is given by 
the following equation: 
##EQU1## 
where: I: current flowing in the N-channel MOS 9, 
W: channel width of MOS 9, 
L: channel length of MOS 9, 
.mu.: mobility, 
C: gate capacity, 
V.sub.G : gate voltage of MOS 9, 
V.sub.T : threshold voltage of MOS 9, 
V.sub.D : drain-source voltage of MOS 9. 
In equation (1), the parameters which change with temperature are the 
mobility .mu. and the threshold voltage V.sub.T. The mobility .mu. has a 
negative temperature coefficient and, hence, acts to reduce the current I 
as the temperature rises. The threshold voltage V.sub.T also has a 
negative temperature coefficient so that (V.sub.G -V.sub.T) increases as 
the temperatue rises, and this acts to increase the current I. In equation 
(1), the temperature coefficients of the mobility .mu. and of the 
difference (V.sub.G -V.sub.T) between the gate voltage and the threshold 
voltage cancel and, hence, the current is automatically prevented from 
increasing despite any increase in temperature. Therefore, if thought is 
given on how to radiate the heat produced by the heating unit, the effect 
on the reliability by the heat produced by the drive circuit and by the 
heating unit in the thermal print head can be neglected. Accordingly a 
radiator plate can be designed easily, the thermal print head can be 
manufactured at a reduced cost, and the reliability can be enhanced. 
The system of the semiconductor device of the present invention and its 
operation will now be described. 
As illustrated in FIG. 4, the semiconductor device of the present invention 
consists of the logic control circuit 5, the drive circuit 4, and a level 
shifter 6. FIG. 5 illustrates the drive circuit 4 in detail and the gate 
of the N-channel MOS 9 is connected to the output of the logic control 
circuit 5. 
The feature of the present invention resides in that the logic control 
circuit 5 is driven by a low voltage by power source 7 to reduce its 
electric current consumption, and the drive circuit 4 of the heating unit 
3 is driven by a high voltage via the level shifter 6 and power source 8. 
According to the present invention, the heating unit which has heretofore 
been driven by a bipolar element is now driven by a MOS. Therefore, a 
current equivalent to that for the bipolar element must be supplied 
through the MOS, and thus a voltage as high as 15 to 30 volts must be 
applied to the drive circuit 4 from the level shifter 6. However, the 
logic control circuit 5, which consists of a CMOS, would consume a large 
current if driven by a voltage of 15 to 30 volts, which would offset the 
effects of the CMOS. The operating voltage of 4 volts to 6 volts is 
sufficient to accomplish the speeds of less than 10 MHz required by the 
CMOS. For reasons of current consumption and stability in operation, 
therefore, a voltage of 4 volts to 6 volts is applied to the logic control 
circuit 5. 
Up until now, a bipolar element has been used in the logic control circuit 
because of the drive using a large current, which results in thermal 
runaway and eventually thermal breakdown. According to the present 
invention, on the other hand, a large current is obtained by a MOS. 
Namely, the drive circuit is operated at a high voltage to correct the 
defects of the MOS, and the occurrence of thermal breakdown is prevented 
by utilizing the saturation of the driving current according to the 
temperature characteristic of the MOS. Another feature of the invention is 
that the consumption of electric current is reduced by the use of a CMOS. 
That is, a high-speed operation comparable to that of a bipolar element is 
accomplished with a current which is less than one-thirtieth the current 
needed conventionally. The present invention exhibits the effects of small 
current consumption, high speed, and increased reliability and therefore 
has wide applicability, as the dot density increases and the thermal head 
operates at faster speeds in the future. 
Although the above description has dealt with the invention as applied to a 
thermal head, it should be noted that the invention is in no way limited 
to a thermal print head, but can also be applied to a plasma display, a 
fluorescent display tube, and any other semiconductor devices which 
require a high voltage and large current. Further, although the 
description has explained the embodiment in which the drive circuit was 
made up of an N-channel MOS, the same effects can also be obtained when 
the drive circuit consists of a P-channel MOS or a CMOS.