Integrated circuit for timepiece

The integrated circuit, including MOS transistors, for driving the step motor of an analog display is substantially reduced in size by boosting the transistor gate voltage above the battery voltage applied to the source-drain terminals.

BACKGROUND OF THE INVENTION 
This invention relates generally to an integrated circuit for a timepiece 
of the type used for driving the step motor of an analog display, and more 
particularly, to a circuit where the size of the transistors is 
substantially reduced. In a conventional electronic analog timepiece, 
since there is only one supply voltage, generally a battery, the 
transistors for driving a step motor are driven by the same voltage as is 
used for integrated timekeeping circuits. Also, in hybrid watches having 
both an analog display and a digital display, the step motor driving 
transistors again are driven by the same battery voltage as in the 
simpler, purely analog timepiece. For driving the step motor, a high level 
of current, several hundred microamperes, flows, although the peak current 
continues for only a short time period. To accommodate this high current 
requirement, the CMOS integrated circuit structure, that is, in particular 
the driving transistors, requires a large surface area. Accordingly, the 
cost of the integrated circuit chip is greatly influenced by the area of 
the driving transistors. It is desirable that the size of the integrated 
circuit chip be small as possible so as to decrease the cost of the chip 
and also raise its reliability. When the area for the step motor driving 
transistors is made smaller, the entire area of the integrated circuit 
chip for the timepiece can be made smaller. The same object of smaller 
size is equally applicable in the hybrid watch having both analog and 
digital displays. 
What is needed is an integrated circuit for a timepiece which has a small 
area and yet provides the high current carrying capability required for 
operation of the step motor in the analog mode of operation. 
SUMMARY OF THE INVENTION 
Generally, in accordance with the invention, an integrated circuit for an 
analog timepiece, which is small and reliable, is provided. The integrated 
circuit, including MOS transistors for driving the step motor of an analog 
display is substantially reduced in size by boosting the transistor gate 
voltage above the battery voltage applied to the sourcedrain terminals. In 
a hybrid timepiece having both analog and digital displays, the digital 
timekeeping circuits also operate at a voltage level elevated above the 
battery supply voltage. Elevation of the gate potentials allows for a 
reduction in transistor area while still providing the current flow 
required to drive the step motor for the analog display. 
Accordingly, it is an object of this invention to provide an improved 
integrated circuit for a timepiece having small size and suited for 
driving a step motor for an analog display. 
Another object of this invention is to provide an improved integrated 
circuit for a timepiece wherein transistor gates are driven with a higher 
potential than is provided for the source-drain, and transistor size is 
reduced. 
A further object of this invention is to provide an improved integrated 
circuit for a timepiece having both digital and analog displays wherein 
voltage is elevated for digital timekeeping functions and for the 
transistor gates used in driving the step motor. 
Yet another object of this invention is to provide an improved integrated 
circuit for an analog timepiece wherein a MOS transistor drives a step 
motor and the transistor source and substrate potentials are the same as 
that of the power supply and the gate is driven by a voltage generated 
within the integrated circuit which is a greater voltage than the power 
supply. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification. 
The invention accordingly comprises the features of construction, 
combination of elements, and arrangement of parts which will be 
exemplified in the construction hereinafter set forth, and the scope of 
the invention will be indicated in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention relates to a circuit for controlling the MOS transistors in 
an integrated circuit for a timepiece, the MOS transistors driving the 
indicating hands of a timepiece by means of a step motor. In the prior 
art, in the so-called analog display timepiece having a second, minute and 
hour hand, an oscillator circuit, divider circuit and MOS transistors for 
driving a step motor, hereinafter also referred to as the driving 
transistors, are all operated by an external supply voltage, which in a 
small timepiece is a battery. A functional block diagram of a conventional 
analog timepiece is shown in FIG. 1. The timepiece includes an oscillator 
circuit 1 outputting a high frequency standard signal, a divider circuit 2 
receiving the output of the oscillator 1 and dividing down the standard 
frequency signal to a lower frequency signal for timekeeping. Also 
included are a circuit 3 for controlling the output of the divider 2 to 
provide the proper wave shape to a driver circuit 4. The output of the 
driver circuit 4 is fed to a step motor and analog display in the 
conventional manner. Within the oscillator circuit 1, a quartz crystal 
vibrator is connected to an integrated circuit (IC) so as to provide an 
oscillation having a stable frequency, generally, 32,768 Hz, for input to 
and division within the divider circuit 2. The control circuit 3, as 
stated, performs the function of determining the pulse width for driving 
the step motor which in turn drives the indicating hands of the analog 
display. The driving circuit 4 provides the electrical energy which is 
necessary to operate the step motor using the signals of the pulse width 
determined in the output control circuitry. Generally speaking, a high 
level of energy is delivered over a short period of time in pulses to 
operate the step motor. 
FIG. 2 is a detailed schematic of the driving circuit 4. P-channel 
transistors 6,7 are connected in series with N-channel transistors 8, 9 
respectively across the external power source or battery (not shown). A 
resistor 10 represents the load of the step motor and it is connected 
between the P- and N-channel resistors in each branch of the circuit. The 
gates of the channel transistors in one branch, namely, the channel 
transistors 6, 8, are driven by a common gate signal A. The transistors in 
the other branch, namely, P-channel transistor 7 and N-channel transistor 
9 are driven by another common gate signal B. Because the commonly driven 
transistors are of different types, it is apparent that only one 
transistor in each branch is capable of conducting when a driving signal 
pulse is applied to the gate. Pulse signals for controlling the gates are 
shown in FIG. 3 and identified as A and B to correspond with the gates of 
FIG. 2 where the signals are applied. 
When both signals A, B are high, for example, at time t1 of FIG. 3, both 
N-channel transistors 8, 9 are electrically conductive whereby both ends 
of the load resistor 10 of the step motor are electrically connected with 
the negative terminal of the power supply. There is no voltage 
differential across the resistor 10 and no current flows in the step 
motor. When a voltage pulse of narrow width, generally, 3 to 10 
milliseconds, is applied as shown for signal A at time t2 of FIG. 3, the 
P-channel transistor 6 and the N-channel transistor 9 are electrically 
conductive and the P-channel transistor 7 and N-channel 8 are turned off. 
During the time of the pulse, the load resistor 10 is connected in series 
with the transistors 6, 9 across the power supply terminals and a current 
flows from left to right (FIG. 2) through the step motor whereby the 
indication hands are advanced. Then, in time a similar pulse B, occurring 
at time t3 of FIG. 3, causes the P-channel transistor 7 and the N-channel 
transistor 8 to be electrically conductive while the transistors 6, 9 are 
turned off. This condition places the load resistor 10 in series with 
transistors 7, 8 across the power supply and a current flows through the 
step motor in the reverse direction. This is a brief description of an 
integrated circuit function for an analog timepiece. 
When designing such an integrated circuit, the area of the driving circuit 
4 presents a problem. The operation voltage is an external supply voltage 
which is generally a battery. The supply voltage drops at the time when 
the motor is driven because of the internal resistance of the battery. 
Thus, a large amplifying coefficient is neccessary for the driving 
transistors in order to provide the power required by the motor. Thus, the 
area of the driving circuit 4 becomes approximately one millimeter square, 
which amounts to 20 percent of the integrated circuit chip area in an 
ordinary integrated circuit for an analog timepiece. Thus, space, 
primarily area, is inefficiently used. Further, when driving a larged 
sized step motor, or when the other circuit portions are small in area, 
there have been instances where the driving transistors for the step motor 
occupy 50 percent of the integrated circuit chip area. Therefore, it is 
readily understandable that in view of the cost and reliability in 
manufacture of transistors, it is highly advantageous and profitable to 
make the area for the motor driving transistors as small as possible 
consistent with the load requirements. 
In recent years, hybrid watches have appeared on the market. A hybrid watch 
includes the functions both of an analog watch and of a digital watch and 
has both types of display. A functional block diagram of a hybrid watch is 
shown in FIG. 4. The timepiece includes an oscillator circuit 1 for 
producing a high frequency standard signal, a divider circuit 2 for 
dividing down said high frequency signal to a lower frequency signal 
suitable for timekeeping, an output controlling circuit 3, and a circuit 4 
for driving a step motor and analog display. These are the same components 
as in the ordinary analog watch of FIG. 1. Additional circuit blocks are 
necessary for performance of the digital functions, including a second 
counter 12, minute counter 13, and hour counter 14. Decoders 15, 16, 17 
transduce the contents of the second, minute and hour counters 12, 13, 14 
respectively into necessary timekeeping data. Driving circuits 18, 19, 20 
for driving a liquid crystal display operate in accordance with the 
outputs of the respective decoders. A booster circuit 11 boosts the 
external supply, or battery voltage, to double or three times the normal 
source level. Voltage boosters to accomplish such a doubling or tripling 
in voltage are well known and need no further description here. The 
voltage output of a single silver oxide battery is not sufficient for 
driving and display of many kinds of liquid crystals. Therefore, the 
liquid crystal display is driven after boosting the voltage by means of a 
booster circuit 11. 
A driver circuit 21 connects to the common side of the liquid crystal 
electrodes in a known manner and an interface circuit 22, namely, a 
transducer circuit for raising the signal level generated by the battery 
voltage to the same level as the boosted voltage from the booster 11. This 
interface circuit 22 may be inserted anywhere so long as the liquid 
crystal drivers 18, 19, 20 are driven by a boosted voltage. However, 
generally speaking, the interface circuit 22 is inserted between the 
second counter 12 and the divider 2. A broken line 23 shows the circuit 
portions which are driven by a boosted voltage. In the timepiece of FIG. 
4, the driving transistors in the driver 4 are driven by the external 
supply voltage of the battery in the same manner as shown in FIGS. 1 and 
2. Thus, in the integrated circuit for a hybrid watch as in FIG. 4, the 
area for the step motor driving transistors will be substantially the same 
as the area for the same transistors of the integrated circuit for a watch 
which is solely analog as in FIG. 1. 
In a timepiece in accordance with this invention, it is an object that the 
area of the driving transistors is made small so as to achieve beneficial 
results with regard to cost and reliability of the integrated circuit. A 
functional block diagram of a hybrid timepiece in accordance with this 
invention is shown in FIG. 5. The oscillator 1, divider 2, output control 
3, voltage booster 11, counters 12, 13, 14, decoders 15, 16, 17, drivers 
18, 19, 20, interface circuit 22, motor and analog and digital displays 
are the same as those shown in a conventional hybrid watch of FIG. 4. 
In FIG. 5, an interface circuit 24 is located between the output control 
circuit 3 and the driving circuit 4, and more particularly, the circuit 24 
is between the signals from the output control circuit 3 and the gates of 
the step motor driving transistors as more clearly shown in FIG. 6. The 
interface circuits 24 boost the voltage of the signals A, B (FIG. 3) so 
that the magnitude of change in voltage when the pulses occur is changed 
from the level of the external supply or battery to that of a boosted 
voltage, for example, doubled or tripled. As in FIG. 2, the source-drain 
current through the transistors 6, 7, 8, 9 is drawn directly from the 
external supply voltage or battery without boosting. Use of the battery 
voltage directly is continued because a boosted voltage cannot provide a 
sufficiently large current output for driving a step motor although the 
boosted voltage can control the gates. A broken line 25 shows the circuit 
portions which are driven by a boosted voltage. 
As stated, FIG. 6 is a circuit diagram similar to FIG. 2 and including 
interface circuits 24 connected to the gates of the driving transistors. 
The voltage level of the gate controlling signal A, B (FIG. 3) in 
accordance with this invention, are made the same as the internal boosted 
voltage from the booster 11. By raising the effective gate voltage on the 
driving transistors 6, 7, 8, 9, the area of the driving transistors can be 
decreased. Assume the external supply voltage or battery voltage (VGS) of 
1.58 volts drops to 1.3 volts at the time of driving the step motor 
because of the internal resistance of the external power source. Further, 
assume that a current which flows in the load resistor 10 is 500 
microamperes; ON potential (VDS) of the P-channel transistor 6 in FIG. 2 
is 0.1 volts; the threshold voltage (Vth) is 0.75 volts and the amplifying 
rate of the transistor is .beta.. Then, an approxiate equation for current 
in a conventional circuit is as follows: 
EQU I=.beta.{(VGS-Vth).times.VDS-1/2VDS.sup.2 }500 .times.10.sup.-6 
=.beta.{(1.3-0.75).times.0.1-1/20.1.sup.2 }.beta..div.0.92.times.10.sup.-2 
(1) 
I: current 
.beta.: amplifying rate 
VGS: voltage between gate and substrate 
Vth: threshold voltage 
VDS: voltage between drain and substrate 
When as a result of the interface circuits 24, the voltage is doubled from 
1.3 to 2.6 and this value is reduced to 2.5 volts because of losses and 
inefficiencies in the booster, the following equation is obtained: 
EQU 500.times.10.sup.-6 =.beta.{(2.5-0.75).times.0.1-1/20.1.sup.2 
}.beta..div.0.29.times.10.sup.-2 (2) 
The equations 1, 2, indicate that the necessary amplifying rate .beta. of 
the transistors in a circuit in accordance with this invention is 
approximately 23 percent of the amplifying rate required in the 
conventional circuit of FIG. 2 without voltage boosting in order to 
produce the same assumed value of driving current, that is, in this 
example 500 microamperes. When the external power source or battery and 
the internal boosted power source 11 have their plus sides in common, this 
arrangement is effective on the P-channel transistor. When the internally 
boosted power source and the external power source are connected in common 
on the negative side in view of the integrated circuit manufacturing and 
circuit arrangement, the same beneficial effect results in N-channel 
transistors. Even considering the area of the interface circuits 24, the 
area of a driving transistor and the associated interface circuit is about 
one-third of the area for the conventional transistor. Circuit arrangement 
and the area of the integrated circuit chip becomes smaller. 
Additionally, another result can be anticipated when the external power 
supply or battery has an internal impedance. Assuming again that the 
external supply or battery voltage is 1.58 volts just before the time when 
it drives the step motor. Also assume that the voltage drops to 1.30 volts 
at the time when the step motor is driven. In such a case, the voltage can 
be boosted before the motor is driven, giving a result, for example, with 
a doubler, of 1.58.times.2=3.16 volts. Since the time for driving the step 
motor is short and a suitable capacitor is inserted in parallel with the 
boosted voltage, it is possible to maintain the voltage level of the 
boosted power source when the external supply voltage or battery voltage 
drops for only a short period of time as is the case here. Therefore, the 
value of .beta. can be reduced by 15 percent from the value in equation 2, 
and the area of the integrated circuit can also be decreased by 
approximately another 15 percent. 
The circuit construction in accordance with this invention is not limited 
only to the construction wherein the interface circuit 24 is positioned 
between the output control circuit 3 and the driving transistors in the 
driver 4. The same beneficial effect of reducing the area for the 
transistors can be obtained by positioning interface circuitry for voltage 
boosting between the divider stages 2 and the output control circuit 3. A 
circuit construction in accordance with this invention is applicable not 
only to a transistor for driving a step motor but also in other 
applications. When the potentials of the source of a driving transistor 
and of the substrate are dependent upon an external supply voltage, and 
when the controlling gate of the driving transistor is driven by a voltage 
increased above the value of the external supply voltage by means of an 
internal booster circuit, the area of the driving transistor can be 
decreased. 
For an integrated circuit in which a great capacity is required for a 
driving transistor and in which the area of the driving transistor 
occupies a substantial part of the chip area, some beneficial effect is 
obtained by boosting the voltage only for controlling the gate of the 
driving transistor even when there is no necessity of other internal 
voltage boosting. This is so whether such an integrated circuit is used 
for a hybrid watch or for a purely analog watch. In such a construction, 
with a booster circuit capable only of increasing the voltage of the gate 
of a driving transistor, the area of the booster circuit is also small. An 
externally attached capacitor (not shown) for boosting voltage, which is 
necessary for an integrated circuit in a hybrid watch can be incorporated 
in a small area within the integrated circuits. 
In a timepiece in accordance with this invention, a lithium battery having 
a voltage of more than three volts has been used with satisfactory 
results. However this invention is also suited to the situations where the 
voltage is three volts or less. 
FIGS 7 and 8 show alternative circuits for the driver 4 of FIGS. 4 and 5 
respectively. In these circuits the load resistor 54 of the step motor is 
located in series with a P-channel MOS transistor 52 and an N-channel MOS 
transistor 53. The series circuit is connected across the external power 
supply or battery as described above. When a high signal is applied to the 
input of an invertor 51, and to the gate of the N-channel transistor 53, 
both transistors 52, 53 are conductive and current flows through the motor 
resistor 54 to drive the analog display. In FIGS. 7 and 8 only a single 
input to the invertor is necessary to drive the motor whereas in the 
previously described circuits of FIGS. 2 and 6 two signals were required. 
The area required for the transistors on the integrated circuit is reduced 
in the circuit of FIG. 8 in accordance with this invention where the 
signal to the inverter 51 first passes through an interface circuit 55 
wherein the voltage level is boosted above the voltage level of the 
external power source. As explained in relationship to equations 1 and 2 
above, boosting the voltage applied to the gates of the transistors 
reduces the amplification rate which is needed to provide a given quantity 
of current for driving the step motor. Accordingly, the area of the 
integrated circuit devoted to the transistor is reduced, and size, cost 
and reliability are improved. 
It can thus be seen that the objects as forth above, among those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in the above construction without 
departing from the spirit and scope of the invention, it is intended that 
all matter contained in the above description as shown in the accompanying 
drawings shall be interpreted as illustrative and not in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention herein described 
and all statements of the scope of the invention which as a matter of 
language, might be said to fall therebetween.