JPH04265012A - Power-on resetting circuit - Google Patents

Abstract

PURPOSE:To enhance the detection accuracy of a comparator, and also, to use the output pulse of the comparator as a reset pulse by expanding its pulse width. CONSTITUTION:In a circuit for charging a capacitor 4 by a charging time constant CR1, when a power supply voltage V is turned on, and obtaining an output pulse 10 from a comparator 7, when this charging voltage V1 reaches a reference voltage VS by a Zener diode 5, by setting the power supply voltage V to 12V from conventional 5V, the reference voltage VS becomes high, the pulse width of the output pulse 10 is prolonged, and it is used as a reset pulse. In such a manner, a monostable multivibrator used conventionally in order to expand the pulse width of the output pulse can be omitted, and also, the detection accuracy of the comparator can be enhanced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power-on reset circuit that generates a reset pulse for resetting internal circuits of a microprocessor or the like when the power is turned on.

0002

[Prior Art] FIG. 3 is a configuration diagram showing a conventional power-on reset circuit, in which 1 is a power source for obtaining a 5V power supply voltage V, and 2 is a first resistor to which the power supply voltage V is applied. A charging resistor having a first resistance value R1. 3 is a discharging resistor as a second resistor connected in series with resistor 2, and has a second resistance value R2. A capacitor 4 is connected in parallel with the resistor 3 and has a capacitance C. 5 is a Zener diode for creating a predetermined reference voltage VS from the power supply voltage V via a resistor 6; 7 is a voltage V at the connection point of resistors 2 and 3;
1 and the reference voltage VS, and when V1≧VS, it becomes “L”
A comparator 8 that outputs a (low level) signal is triggered by the rising edge of the output pulse 9 of the comparator 7 to generate a reset pulse 10 having a predetermined pulse width T.
It is a mono multivibrator that outputs .

FIG. 5 shows a part of the circuit elements in FIG. 3 as an IC circuit 1.
1, a capacitor 4, a resistor 12, and a capacitor 13 are externally connected to the IC circuit 11.

Next, the operation will be explained with reference to a timing chart shown in FIG. In FIG. 4, when a power supply voltage V of 5V is applied at time t1, the output of the comparator 7 becomes “H”.
'' (high level), the capacitor 4 is charged through the resistor 2, and the voltage V1 rises. After the time T1 determined by the charging time constant CR1, this voltage becomes the reference voltage VS.
When the output voltage reaches "L", the output of the comparator 7 becomes "L", and as a result, an output pulse 9 having a pulse width T1 is obtained from the comparator 7. By triggering the mono multivibrator 8 at the rising edge of this output pulse 9, the mono multivibrator 8 generates a predetermined pulse width T.
A reset pulse 10 having the following values is obtained.

Next, the time T from time t2 to time t3
2, when the power supply voltage V is momentarily interrupted, the capacitor 4 is discharged through the resistor 3 with a discharge time constant CR2, and the voltage V1 becomes zero. When the power supply voltage V returns to 5V, the capacitor 4 is charged again and the voltage V1 rises, and the same operation as described above is performed to obtain the reset pulse 10. Note that the reference voltage VS is VS≦V×R2/R1+R2 …………(1
) has been selected.

As described above, in the conventional power-on reset circuit, the reset pulse width T and instantaneous power failure response speed are basically determined by the time constants of CR1 and CR2. Further, the voltage dividing resistance ratio by the resistors 2 and 3 for obtaining the voltage V1 is limited by the accuracy of the Zener diode 5. Therefore, in order to generate the reset pulse 10 in response to a short-term power failure due to a surge or the like, it is necessary to reduce the discharge time constant CR2. If the output pulse 9 (pulse width T1) of the comparator 7 in FIG. 3 is used as a reset pulse, and if the reset pulse width is T1 and the instantaneous power failure response time is T2, then T1=CR1, T2=CR2. Here, assuming that the required reset pulse width is 1 ms and the instantaneous power failure response time is 200 μs, R1=5R2. Therefore, when the discharging time constant CR2 is made small as described above, the charging time constant CR1 is also made small, so that the pulse width T1 becomes small and does not have a sufficient pulse width as a reset pulse. For this reason, conventionally, a mono-multivibrator 8 is provided after the comparator 7, and the pulse width T of the output pulse 9 is
1 is expanded to the reset pulse width T.

[0007]

[Problem to be solved by the invention] Conventional power-on
Since the reset circuit is configured as described above, it is necessary to provide the mono-multivibrator 8 in order to obtain the reset pulse 10 having a sufficient pulse width. Additionally, the Zener diode 5 that determines the reference voltage VS is generally shown in Table 1.
As illustrated in , the higher the voltage, the better the accuracy.

[0008]

[Table 1]

[0009] Therefore, the resistance values of resistors 2 and 3 are set to R1, for example.
=5R2, the reference voltage VS is as described above (
From equation 1), VS=0.83V, which is less than 1V. For this reason, the detection accuracy in comparator 7 decreases,
There were problems such as variations in the pulse width T of the reset pulse 10. This invention was made in order to solve the above-mentioned problems, and provides a power-on reset circuit that has high detection accuracy of the comparator and can widen the pulse width of the output pulse of the comparator and use it as a reset pulse. The purpose is to obtain.

0010

[Means for Solving the Problems] The power system according to the present invention
The on-reset circuit is one in which the power supply voltage V is set to 12V or higher.

[0011]

[Operation] By using the power supply voltage V of 12V or higher, the power-on reset circuit of the present invention can increase the reference voltage VS, which improves the detection accuracy of the comparator and improves the output pulse of the power-on reset circuit. The pulse width also becomes longer and can be used as a reset pulse as is.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, parts corresponding to those in FIG. 3 are designated by the same reference numerals and their explanations will be omitted. In FIG. 1, 12V is used as the power supply voltage V, and the mono-multivibrator 8 of FIG. 3 is omitted, and the output pulse of the comparator 7 is used as the reset pulse 10.

Next, the operation will be explained. When the resistance values of resistors 2 and 3 are selected in the relationship R1=5R1, the power supply voltage V
By setting 12V to 12V, the reference voltage VS becomes the above (1
), it is possible to make VS=2V higher than before. Therefore, a highly accurate Zener diode 5 can be used, and therefore the detection accuracy of the comparator 7 can be improved.

In addition, by increasing the reference voltage VS, as shown in FIG. 2, the time it takes for the voltage V1 to reach VS becomes longer, and as a result, the pulse width T1 of the output pulse of the comparator 7 becomes longer. The reset pulse width T can be set as follows. In addition, FIG. 2 shows momentary power failure response time T2=CR
2 is the same as the conventional case shown in FIG.

In the above embodiment, the power supply voltage V is 12V.
However, it will be more effective if a higher power supply voltage V of 24V or 48V is used.

[0016]

As described above, according to the present invention, by setting the power supply voltage to 12 V or more, the reference voltage of the comparator can be increased. can be made higher. As a result, the output pulse of the comparator can be used as a reset pulse, and the conventional mono-multivibrator can be omitted. Furthermore, effects such as being able to obtain a reset pulse without variations in pulse width can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a power-on reset circuit according to an embodiment of the present invention.

FIG. 2 is a timing chart showing the operation of the circuit.

FIG. 3 is a configuration diagram of a conventional power-on reset circuit.

FIG. 4 is a timing chart showing the operation of the circuit.

FIG. 5 is a configuration diagram when the same circuit is integrated into an IC.

[Explanation of symbols]

1 Power supply 2 Resistance 3 Resistance 4 Capacitor 5 Zener diode 7 Comparator 10 Reset pulse

Claims (1)

[Claims] 1. First and second resistors having first and second resistance values, respectively, and connected in series;
a power source that supplies a power supply voltage of 12 V or more to one end of the resistor; and a reference voltage that is less than or equal to the product of the power supply voltage and the first resistance value divided by the sum of the first and second resistance values. A Zener diode that obtains the voltage, a capacitor connected in parallel with the second resistor, and a voltage at the connection point of the first and second resistors is compared with the reference voltage and a reset pulse having a predetermined pulse width is output. A power-on reset circuit with a comparator that