JPH0365807A - Semiconductor filter circuit - Google Patents

Abstract

PURPOSE:To increase a margin against surge or noise or the like by connecting a pullup and pull-down resistor to an input terminal of an IC filter circuit and lowering one resistance when a signal input switch is closed and increasing when opened. CONSTITUTION:The circuit is provided with a signal input terminal 21a, clamp diodes 22, 23 connected between a power supply and the input terminal 21a and between the signal input terminal 21a and ground respectively, a pullup resistor 24 connected between the signal input terminal 21a and the power supply, a pull-down resistor 25 connected between the signal input terminal 21a and ground, a capacitor 26 connected between the signal input terminal 21a and ground or between the signal input terminal 21a and the power supply, and a buffer or a comparator 27 connected between the signal input terminal 21a and a signal output terminal 21b. Then either of the pullup and pull-down resistors 24, 25 is controlled so that the resistance is a 1st resistor when a signal input switch 29 is turned on and a 2nd resistance larger than the 1st resistance when the switch 29 is turned off. Thus, noise margin is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a semiconductor filter circuit for removing noise entering a signal input system of an electronic circuit.

B. Conventional technology For example, filter circuits for automotive electronic circuits have two functions: (1) absorb the energy of hundreds of surges generated by various inductive loads and prevent damage to the electronic circuit; Functions are required to prevent electronic circuits from malfunctioning by removing radio wave interference noise caused by strong electromagnetic waves from sources and high-frequency noise pulses caused by the above-mentioned surges.

Figure 3 is “Electrical Science Series CMO8 Applied Technical Report” 19
80, Telegram Publishing Co., Ltd., pages 47 and 48, is a circuit diagram showing an example of a conventional filter circuit having such a function.

In FIG. 3, a portion surrounded by a broken line is a filter circuit 1, which is composed of a protective resistor 2, a capacitor 3, and diodes 4 and 5 for voltage clamping. The connection point between the pull-up resistor 6 and the signal input switch 7 is connected to the input terminal A of the filter circuit 1 via the wiring connector 9, and the potential change at the input terminal A point due to opening and closing of the signal input switch 7 is converted into a digital signal. The signal is applied to the input terminal B of the electronic circuit 8 via the filter circuit 1 as a signal.

The operation of this circuit is as follows.

Noises caused by surges, radio wave interference, switch chatter, etc. that enter the wiring from the signal input switch 7 to the input end point A on the filter circuit are filtered and removed by a time constant determined by the protective resistor 2 and the capacitor 3. Ru. In addition, when an overvoltage input such as a surge is applied, the current caused by the overvoltage is released through the path between the diode 4 or 5 and the protective resistor 2, thereby preventing overvoltage and overcurrent damage to the electronic circuit 8. There is.

Also, as a peripheral function for vehicle filter circuits that are exposed to harsh environments such as vibration, temperature, and humidity.

(3) As shown in FIG. 3, it is necessary to reliably prevent malfunctions due to poor continuity of the wiring connector 9 connecting the filter circuit and the signal input switch 7.

This is caused by oxidation of the contacts, so the contact current (usually 1 mA to
2mA or more) should flow.

(4) Such filter circuits are extremely important in ensuring the reliability of electronic circuits, and in order to obtain even higher reliability, and to reduce assembly costs by reducing the number of parts, It is desirable to reduce the number of component parts.

As a solution to the above-mentioned (4), it is possible to integrate the filter circuit into an integrated circuit (hereinafter referred to as IC). From the perspective of cost reduction, it is desirable that the IC be a monolithic IC rather than a hybrid IC, and that the IC manufacturing process should not be a special process but the most common CMO8 standard process.

When both the filter circuit in Fig. 3 and the pull-up resistor 6, which is a peripheral circuit, are made into a CMO8IC, the protective resistor 2 is
When built-in C, protective resistor 2 installed at the input section of the IC
There is a risk that an overvoltage of several hundred volts will be applied directly to the IC, causing damage to the IC. Therefore, from the viewpoint of IC protection, a circuit as shown in FIG. 4 is created in which a protective resistor 2 is externally attached.

In FIG. 4, the part surrounded by the - dotted chain line is the CMOS fill 9 times 110 formed into an IC.

The protective resistor 2 is externally connected between the input terminal A of the CMOS filter circuit 10 and the wiring connector 9, and the pull-up resistor 6 is provided inside the CMOS filter circuit 10 in the same way as the clamping diode 4.5 and the capacitor 3. It will be done. Furthermore, it is provided with a CMOS buffer or comparator 11 for shaping the signal waveform.

The filter time constant is determined by an external protective resistor 2 (resistance value R) and a capacitor 3 (capacitance C) built into the filter circuit. In this case, it is difficult to incorporate a large-capacity capacitor of several tens of PF or more into an IC.

Depending on the case, a sampling circuit 1 that performs sampling and double verification, for example, as a filter using a digital circuit.
2 may be provided at the subsequent stage of the buffer circuit 11.

However, the problems that the C0 invention attempts to solve are 1. In the IC filter circuit as described above,
In order to satisfy the required functions (1) to (4) above, the input threshold voltage of the CMOS buffer or comparator 11 must satisfy the following conditions.

In order to prevent latch-up and destruction of the diodes due to the current 1 flowing through the diodes 4 and 5 when a surge is applied, for example, assuming that the surge voltage is ±300V,
It is necessary that ID≦100 mA. Therefore, the conditions for the resistance value R of the external protective resistor 2 are as follows: 300V/R≦100mA, and Ω≦R...■.

In addition, the connector contact current IC for reliably breaking the oxide film on the contacts of the wiring connector 9 is set to IC≧1mA, for example,
If the power supply voltage VDD=5V, the resistance value R of the protection resistor 2
The condition for the sum of and the resistance value r of the pull-up resistor 6 is 5V/(R+r)≧1mA.

Ω for R+r≦5...■ becomes.

Here, since the protective resistor 2 is a discrete element,
Variations in the resistance value R and changes with temperature are small and almost constant. On the other hand, since the pull-up resistor 6 is an element built into the IC, its resistance value r varies greatly and changes with temperature.

Therefore, if the resistance values R and r are set so that even the worst value satisfies the above condition (2), the following results.

R=3kΩ, Ω(rtyp) for r=1, Ω(raax) for L5, 0.
6 kQ (ruin) Therefore, signal input switch 7
The maximum value of the input voltage of the filter circuit 10 when Vo is on is, and the minimum value of the contact current of the connector 9 is Vo
o 5V. Therefore, the input threshold voltage VTH for Hi/Low determination of the CMOS buffer or comparator 1 is as follows: 4.17<VT)l<5V, which is quite close to the power supply voltage VDD.

Furthermore, it is practically undesirable to configure the circuit indicated by reference numeral 11 with a simple buffer circuit such as an inverter, and a comparator must be used.

When using such a comparator 11, if its input threshold voltage VT)l is close to the power supply voltage vDD, when surges, noise, etc. are superimposed on the input signal, the DC regeneration phenomenon by the clamp diode 4 will cause the voltage of the input signal to decrease. A shift occurs and the comparator 11 malfunctions.

Next, the state of the input waveform due to DC regeneration will be explained with reference to FIGS. 5 and 6.

FIG. 5 is a normal input waveform diagram when no surge or the like is applied. VA shown in FIG. 5(a) is the voltage waveform at the front stage of the protective resistor 2 shown in FIG. 4, and is Ov when the signal input switch 7 is on and 5V (VaD) when it is off. The input voltage waveform vrN of the filter circuit 0 shown in FIG. 4 is a voltage determined by the voltage division between the protective resistor 2 and the pull-up resistor 6 when the signal input switch 7 is on, as shown in FIG. 5(b). (<Voo), and when off it is 5v (
Voo). Further, the input threshold voltage VTH of the comparator 11 at this time is set between the voltages when the signal input switch 7 is on and off.

FIG. 6 is an input waveform diagram when radio interference noise is added. In this case, the voltage VA at the front stage of the protection resistor is
), the noise component 13 is superimposed on the voltage oV when the signal switch 7 is turned off and the voltage 5V when it is turned on. On the other hand, the input voltage VIN of the filter circuit 10 is determined by the time constant determined by the protective resistor 2 and capacitor 3 based on the original signal when the signal input switch 7 is turned on and off, as shown in FIG. The noise 14 filtered by is superimposed.

In this case 1. As is clear from FIG. 6(b), the DC regeneration of the power supply side clamp diode 4 causes asymmetry in the waveform at Hi. Therefore, the average level of the signal when it is Hi is lower than when there is no noise. the result,
The average level of the signal is the input threshold voltage V of the comparator 11
Lower than TH.

Even though the signal is Hi, it will malfunction as Lo.

In other words, in conventional filter circuits, the pull-up resistor has a large variation of ±40 to 50%, so it is easily affected by noise that forces the Hi/Lo judgment input threshold voltage to approach the power supply voltage, and the input signal However, there was a problem with the system malfunctioning and preventing accurate signal transmission. In addition, the Hi of the input signal,
There is a problem that the input voltage swing width associated with Lo is small and the margin against noise is also small.

An object of the present invention is to provide a semiconductor filter circuit that can increase the swing width of the input voltage associated with Hx g TJ o of the input signal, thereby increasing the margin against noise.

01 Means for Solving the Problems The present invention will be explained in conjunction with the figures showing an embodiment. In the present invention, the signal input terminal 21a is
1 to the signal input switch 29, and the signal output terminal 21b is connected to the electronic circuit 32 which receives digital signals accompanying the on/off of the signal input switch 29, and prevents noise etc. from entering the electronic circuit 32. applied to an integrated filter circuit 21 for filtering. The above purpose is the following 4i! It can be achieved with one precept.

Between the signal input terminal 21a and the power supply and the signal input terminal 21a
clamp diodes 22 connected between and ground, respectively.
．． 23, and a signal input terminal 21. a pull-up resistor 24 connected between the signal input terminal 21 a and the power supply, a pull-down resistor 25 connected between the signal input terminal 21 a and the ground, and the signal input terminal 21
A capacitor 26 connected between a and ground or between the signal input terminal 21a and the power supply, a buffer or comparator 27 connected between the signal input terminal 21a and the signal output terminal 21b, and pull-up and pull-down resistors 24.25
resistance value control means for controlling one of the resistance values so that the resistance value thereof becomes a first resistance value when the signal input switch 29 is on, and a second resistance value that is larger than the first resistance value when the signal input switch 29 is off. .

E6 action For example, a case where the signal input switch 29 is connected to the ground side will be explained. In this case, the pull-down resistor is configured as a variable resistor. When the signal input switch 29 is turned on and the input voltage VIN applied to the input terminal 21a is Lo, the resistance value of the variable pull-down resistor becomes a certain finite predetermined value. Further, when the signal input switch 29 is turned off and the input voltage VIN applied to the input terminal 21a is Hi equivalent to the power supply voltage VDD, the resistance value of the variable pull-down resistor becomes a predetermined value higher than when it is turned on. As a result, the input voltage VIN when the signal input switch 29 is turned on becomes a voltage divided by the combined resistance value of the protection resistor 31 and pull-up resistor 24 connected in parallel and the resistance value of the variable pull-down resistor 25, and is pulled down. It can be made lower than conventional circuits without resistance. Furthermore, since the resistance value of the variable pull-down resistor 25 is high when the signal input switch 29 is off, the input voltage VIN can be held at the power supply voltage VOO. Therefore, the input threshold voltage of the comparator 27 can be separated from the power supply voltage vDD, and even if noise is superimposed on the input signal, erroneous determination of Hi or Lo by the comparator 27 can be prevented. Moreover, since the swing width of the input terminal voltage when the input signal is Hi or Lo is increased, the noise margin can be increased.

In the above-mentioned sections and section E, which describe the present invention in detail, figures of embodiments are used to make the present invention easier to understand, but the present invention is not limited to the embodiments.

F. Embodiments Below, an embodiment of the present invention will be described in detail based on the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a semiconductor filter circuit according to the present invention.

In the figure, the part surrounded by a broken line is an IC-based semiconductor filter circuit 21, and this filter circuit 21 includes a voltage clamping diode 22.2 for overvoltage input protection.
3, a pull-up resistor 24 whose gate is grounded from a P-channel MOS transistor, and a variable pull-down resistor 25.

It includes a capacitor 26 for noise removal, a CMOS buffer or comparator 27, and a sampling circuit 28 added as necessary.

The variable pull-down resistor 25 includes a pull-down N-channel MOS transistor 25a, a P-channel MOS transistor 25b, and an N-channel MOS transistor 25e that make the degree of conductivity (resistance) of the MOS transistor 25a variable according to the input voltage Vrs.

In the filter circuit 21 having the above configuration, the clamping diode 22 on the power supply side is connected to the input terminal 21a and the power supply voltage Vpp.
A clamping diode 23 on the ground side is connected between the input terminal 21a and the ground. Further, the pull-up resistor 24 is connected between the input terminal 21a and the power supply voltage VDD, and the variable pull-down resistor 25 is connected between the input terminal 21a and the power supply voltage VDD.
Connected between a and ground. Capacitor 26 is connected between input terminal 21a and ground. A buffer circuit or comparator 27 and a sampling circuit 28 added as necessary are connected to an input terminal 21a and an output terminal 21b.
connected in series between.

Further, one end of an external protective resistor 31 is connected to the input terminal of the filter circuit 21, and the other end of the protective resistor 31 is connected to the signal input switch 29 via a wiring connector 30. An electronic circuit 32 is connected to the output terminal 21b of the filter circuit 2].

Next, the operation will be explained.

The P-channel MOS transistor constituting the pull-up resistor 24 is always conductive and connects the input terminal 21a to approximately the power supply voltage V.
It is pulled up to DD. When the signal input switch 29 is turned on and off, the input voltage VIN of the filter circuit wt21
changes, and the resistance value of the variable pull-down resistor 25 also changes in accordance with the change in the input voltage VIN.

First, when the signal input switch 29 is off, the input voltage V
IN becomes VIN:VDD, and accordingly, the P-channel MOS transistor 25 forming the pull-down resistor 25
b turns off. Therefore, N-channel MOS transistor 25c is not turned on and almost no drain current flows through pull-down N-channel MOS transistor 25a. That is, the variable pull-down resistor 25 becomes a high impedance state.

The input signal voltage of the comparator 27 is approximately the power supply voltage vDD
becomes.

Next, when the signal input switch 29 is turned on, contrary to the above, the P-channel MOS transistor 25b constituting the pull-down resistor 25 is turned on, so the N-channel MOS transistor 25c and the pull-down N-channel MoS transistor 25a become conductive, and this MOS transistor 25
A drain current flows at a. Therefore, the variable pull-down resistor 25 has a certain finite resistance value rpd. That is, when the signal input switch 29 is on, the protection resistor 3↓ and the pull-down resistor 25a are connected in parallel, and the potential of the input terminal 21a can be lower than that of the conventional circuit without the pull-down resistor 25a.

The input voltage VIN of the filter circuit 21 at this time is expressed by the following equation.

rpd・R Therefore, the resistance value R of the external protective resistor 31 is 300
From V/R≦100mA, R≧3 and Ω...■.

Further, if the contact current rc for reliably breaking the oxide film on the contact of the wiring connector 31 is IC≧1mA, and the power supply voltage Voo is 5V, then the resistance value R of the protective resistor 31 and the resistance value r of the pull-up resistor 24 The conditions between the resistance value rpd of the variable pull-down resistor 25 and the IC are: rpd-R rpd+R In addition, in the filter circuit 21, in order to satisfy the above conditions ① and ②, the following conditions must be satisfied. be.

That is, when preventing latch-up and destruction by the current ID flowing through the diodes 4 and 5 when a surge is applied, for example, assuming that the surge voltage is ±300V, the
The need for 100 mA is rpd+R.

Here, in this embodiment, the resistance value r of the pull-up resistor 24 and the resistance (firpd) of the variable pull-down resistor 25 are both MO8 resistors integrated into an IC in the filter circuit 21, and their resistance values are determined by the P-channel MOS transistor It is determined by the drain currents of the pull-up resistors 24 and 25b.Therefore, temperature changes and variations in the resistance values of the pull-up resistor 24 and the pull-down resistor 25a are almost the same.

Therefore, even the worst value including variations, etc. is the above ■.

If R, r, and rpd are set so as to satisfy the formula (2), the following will be obtained, for example.

R = 3, Ω (constant) r = 600 Ω (rtyp), 900 Ω (rsax
), 360Ω(r+ain)rpd=3 and Ω(rp
d(typ)), 4.5Ω(rpd(max)), 1
．． 8 is Ω(r pd (win)). Therefore, when the signal input switch 29 is on, the maximum value of the input voltage VIN of the filter circuit 21 and the minimum value of the contact current Ic of the wiring connector 30 are as follows.

As is clear from the above, even though the surge current ID and connector contact current Ic are the same as in the conventional case, the input voltage VIN when the signal input switch 29 is turned on cannot be set much farther from the power supply voltage VDD than in the conventional case. Accordingly, the level of the input threshold voltage VTH of the comparator 27 can be considerably lowered. Therefore, even if radio interference noise or the like is superimposed on the input signal when the signal input switch 29 is turned on or off, the problem that the average level of the input signal becomes higher than the input threshold voltage VTR as in the conventional case is eliminated, and the comparator 27 In addition to preventing malfunctions of the filter circuit, the swing of the input voltage of the filter circuit when the input signal is high or low can be increased, and the margin against surges and noise can be increased.

FIG. 2 is a circuit diagram showing another embodiment of the filter circuit according to the present invention.

In the embodiment shown in FIG. 2, a signal input switch 29 is connected to the power supply side, and accordingly, a capacitor 26 is connected between the input terminal 21a and the power supply voltage VDD, and a pull-up resistor 24 and a pull-down resistor 25 are connected. It is modified as follows.

That is, the pull-up resistor 24A is a variable pull-up resistor composed of a pull-up P-channel MO3h transistor 24a, an N-channel MOS transistor 24b, and a P-channel MOS transistor 24c, and the pull-up down resistor 25A is composed of an N-channel MOS transistor. and its gate is connected to the power supply voltage Voo.

In this embodiment, the pull-down resistor 25 is always conductive. When the signal input switch 29 is off, the pull-down resistor 25A is conductive, so the input voltage VIN of the filter circuit 21 is close to zero, and the N-channel MO
The S transistor 24b is turned off, and the variable pull-up resistor 2
4A becomes high impedance. Therefore, the input voltage VIN is maintained at approximately OV.

When the signal input switch 29 is on, the input voltage VIN shows a certain value, which turns on the N-channel MOS transistor 24b that constitutes the variable pull-up resistor 24A. Transistor 24a turns on and a certain drain current flows through it. Therefore, the variable pull-up resistor 24A has a certain finite resistance value rpd. Therefore, when the signal input switch 29 is on, the P-channel MOS transistor 24a and the protection resistor 31 are connected in parallel, so the potential of the input terminal 21a is V.
IN can be made higher than in the case without the pull-up resistor 24a. Therefore, in this embodiment as well, the same effects as in the above embodiment can be obtained.

In addition, in the above, the pull-up and pull-down resistors are M
Although it is constructed using an OS transistor, it is not limited to a MOS type, and may also be constructed from a bipolar type.

G1 Effects of the Invention As described above, according to the present invention, pull-up and pull-down resistors are connected to the input terminal of the IC-based filter circuit, and the value of one of the resistors is set to be low when the signal input switch is on and high when it is off. As a result, the input threshold voltage of a comparator or the like can be set to a value that is preliminarily separated from the voltage when the signal input switch is turned on, and accordingly, it is possible to prevent the filter circuit from malfunctioning in response to the input signal. Also, Hl of the input signal.

The swing width of the input voltage to the filter circuit in Lo mode can be increased, and the margin against surges, noise, etc. can be increased.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a semiconductor filter circuit according to the present invention. FIG. 2 is a circuit diagram showing another embodiment of the semiconductor filter circuit according to the present invention. FIG. 3 is a block diagram of a conventional filter circuit with a discrete configuration. FIG. 4 is a block diagram of a conventional IC filter circuit. FIGS. 5 and 6 are waveform diagrams illustrating a conventional example. 21: IC filter circuit 22.23: Clamping diode 24.24A nibble-up resistor 25.25A nibble-down resistor 26: Capacitor 27: Buffer circuit or comparator 28: Sampling circuit 29: Signal input switch 30: Wiring connector 31: Protection resistor Patent user Nissan Motor Co., Ltd.

Claims (1)

[Claims] A signal input terminal is connected to a signal input switch via an external protective resistor, and a signal output terminal is connected to an electronic circuit that receives a digital signal associated with turning on and off the signal input switch. and an integrated filter circuit that removes noise etc. that invades the electronic circuit, comprising: a clamp diode connected between the signal input terminal and the power supply and between the signal input terminal and the ground, respectively; and a clamp diode connected between the signal input terminal and the power supply. a pull-up resistor connected between the signal input terminal and the ground; a pull-down resistor connected between the signal input terminal and the ground; a capacitor connected between the signal input terminal and the ground or between the signal input terminal and the power supply; and the signal input terminal. a buffer or a comparator connected between the signal input switch and the signal output terminal; and one of the pull-up and pull-down resistors, the resistance value of which is a first value when the signal input switch is on.
1. A semiconductor filter circuit comprising: a resistance value control means for controlling the resistance value to be a resistance value of 1, and a second resistance value larger than the first resistance value when turned off.