JPH0487412A - Ripple filter circuit - Google Patents

Abstract

PURPOSE:To prevent the influence of a fluctuation from appearing in a ripple filter by providing a comparator for comparing the output voltage of a ripple filter circuit and a power supply voltage, and controlling a third transistor in the cut-off direction by the output signal of the comparator circuit at the time when the power supply voltage is dropped. CONSTITUTION:When a power supply voltage VCC is dropped, the base of a transistor Tr8 is also dropped in a certain ratio (R6/(R6+R7)). However, since a comparator part A2 is added, as for the base potentials of transistors Tr7, Tr8, when the voltage VCC of a current line is changed downward suddenly, the base potential of Tr7 becomes higher than the base potential of Tr8. Therefore, Tr7 is turned on, and also, Tr6 is turned on. Thus, since Tr3 is in the turn-off direction, it becomes a high impedance, and it can be prevented that a ripple is mixed in from the power source line VCC. Accordingly, even if the power supply voltage of the power source line is fluctuated suddenly, the fluctuation of the power source line does not appear in the ripple filter output.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a ripple filter circuit suitable for integration into an IC (integrated circuit), and particularly to a ripple filter circuit in which the ripple filter rejection rate does not decrease even when the power supply voltage decreases. .

[Prior Art] Conventionally, in order to remove power supply ripple in a power supply line, an integrating circuit made of a resistor and a capacitor as shown in FIG. 3 was generally inserted to remove the ripple voltage.

In FIG. 3, the ripple signal Vr input to the terminal 6
is attenuated as shown by the following equation, and Vo is output to the output terminal 7.

Vo-V r ·(1/W-C1) / (R8+1/W-CI) ... (5) W-2 · π · f (f: frequency of ripple signal) [Problem to be solved by the invention] In the conventional ripple filter circuit described above, the ripple removal rate cannot be sufficiently increased unless the value of the resistor or capacitor is increased.

For this reason, the circuit shown in FIG. 3 is not suitable for integration into an IC, and has large power loss, making it unsuitable for equipment that utilizes a low power supply voltage.

The present invention has been made in view of the above-mentioned problems, and its purpose is to provide a ripple filter circuit that is suitable for integration into an IC and whose output is unlikely to fluctuate even when the power supply voltage drops momentarily. .

[Means for Solving the Problems] In order to achieve the above object, the present invention improves a ripple filter circuit.

In other words, the first and second transistors are differentially connected;
A ripple filter circuit having a third transistor having a base connected to the collector of the first transistor, an emitter connected to a power supply, and a collector connected to the collector of the first transistor, wherein fourth and fifth transistors are differentially connected. A comparison circuit is provided to compare the output voltage of the ripple filter circuit with a power supply voltage, and the third transistor is controlled in a cut-off direction by the output signal of the comparison circuit when the power supply voltage decreases.

[Function] In the present invention, by controlling the third transistor from the fourth transistor of the comparator circuit in the direction of cutting off (to high impedance) when the power supply voltage drops, ripples from the superimposed line VCC can be prevented. This prevents the effects of fluctuations from appearing on the ripple filter output.

[Example] Preferred embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a circuit diagram of a ripple filter circuit according to the present invention.

Circuit Configuration Description The ripple filter circuit 1 mainly includes a ripple filter section A1 and a comparator section A2.

The ripple filter section A1 includes a second transistor Tri and a first transistor Tr2.

Further, the comparator section A2 includes a fourth transistor Tr7 and a fifth transistor Tr3.

Circuit Connection Description The ripple filter circuit 1 integrated on the semiconductor substrate includes a power input terminal 10, a ground terminal 20, an electrolytic capacitor connection terminal 4.5, and a ripple filter output terminal 3. Placed.

In the ripple filter circuit 1, a resistor R6, a resistor R7, and a collector-emitter path of a transistor T11 are connected in series between the positive power supply VCC and ground.

Further, a resistor R1 and a capacitor C1 are connected in series between the power supply vCC and the ground.

The collector of the transistor Tri of the ripple filter section A1 is connected to the power supply VCC via the resistors R4 and R2, and the base of the transistor Tri is connected via the resistor R3 to the connection point of the resistors R2 and R4, and further via the terminal 4. It is connected to an electrolytic capacitor C2, and the emitter of the transistor Tri is connected to the emitter of the transistor Tr2 and to the collector of the transistor Tr5. The base of the transistor Tr2 is connected to the collector, and also to the resistor RIO and the electrolytic capacitor C3 via the terminal 3, and further connected to the collector of the transistor Tr3 and the base of the transistor Tr7.

The base of the transistor Tr3 is connected to the collector of the transistor Tri and the collector of the transistor 6, and the emitter of the transistor Tr3 is connected to the power supply VCC.

The emitter of the transistor Tr6 is connected to the power supply VCC, and the base is connected to the collector of the transistor Tr7 and the resistor R5.
It is connected to the power supply VCC via.

The emitter of the transistor Tr7 of the comparator A2 is connected to the emitter of the transistor Tr8 and the collector of the transistor TrlO, the collector of the transistor Tr8 is connected to the power supply VCC, and the base is connected to the resistor R6 and the resistor R7.
connected to the connection point.

Circuit operation description If comparator section A2 does not exist, power supply voltage VCC
When the voltage suddenly decreases, the voltage between the collector and emitter of the control transistor Tr3 becomes saturated, and fluctuations in the power supply voltage appear directly at its collector.

Therefore, the output terminal 3 would be affected by power fluctuations in the power supply line, but since the comparator section A2 is present, such fluctuations in the output terminal 3 can be prevented.

In other words, when the power supply voltage VCC falls, the transistor Tr
The base of 8 also falls at a certain ratio (R6/(R7 of R6)). That is, the base voltage VB(
Tr8) is VB(Tr8) -VCC-(R6XVBE (Tr9))/(R in R6
7) ... (1) (VBE (
Tr9): base-emitter voltage of transistor Tr9) From equation (1), the base voltage VB (Tr8) of transistor Tr8 changes as VCC changes.

Next, the output potential VO(
3) can be determined as follows.

VO (3) VB (Tri) -VCC- (VBE (Tr 3)X (R2/R
R4) +IB (Tr 1) XR3)-VCC
-VBE (Tr3)x (R4 in R2/R2)
... (2) (V
B (Tr 1), : Base voltage of transistor Tri) However, 1B (Tri)xR3<VBE (Tr3)X (R
2/R2 in R4) ... (2) (IB (
Tri): Base current of transistor Tri) Note that the ripple removal rate is expressed by the following equation.

Rr-20log (1/ (c2+c3))...
(3) Here, assuming that there is no comparator section A2, if the power supply voltage VCC suddenly drops, the potential cannot change suddenly and remains constant because the electrolytic capacitor C2 is attached to the electrolytic capacitor connecting terminal 4. Therefore, transistors Tri and T
r2 maintains the state before the power supply voltage drops, transistor Tr3 is saturated, and as a result, the output potential VO(3)
decreases as the power supply voltage decreases.

Therefore, power supply fluctuations appear as they are at the output terminal 3 of the ripple filter circuit, and the ripple removal rate Rr deteriorates.

However, since the comparator section A2 is added, the above does not occur. In other words, transistors Tr7, T
When the voltage VCC of the power supply line suddenly changes downward, the base potential of ri3 becomes higher than the base potential of transistor Tr7, so that
Transistor Tr7 is turned on, and further transistor Tr7 is turned on.
5 is also turned on.

As a result, the transistor Tr3 is in the OFF direction, resulting in high impedance, and it is possible to prevent ripples from being mixed in from the power supply line VCC.

Next, the operation of the ripple filter circuit in response to changes in the power supply conditions of the power supply line vCC will be described.

(1) When the power supply is constant (when VCC does not change): The output is determined from the above equation (2). Comparator section A2
The transistor Tr7 is turned off, and the transistor Trg is turned on. This is determined from the above equation (1).

(2) When the power supply fluctuation is upward: Although the collector-emitter voltage of the control transistor (3) increases, the collector-emitter impedance of the transistor (3) remains high, and the transistor Tr3 is turned off.

Therefore, mixing of ripples is prevented.

(3) When the power supply fluctuation is downward: The collector-emitter voltage VCE (T-r3) of the control transistor Tr3 is expressed as follows based on the above equation (2).

VCE (Tr3) -VCC-VO(3) -VC<;-(VCC-VBE (Tr3)x (R2
/ (R4 in R2)) -VBE (Tr3)X (R2/ (R2+R4)
))... (4) From the above equation (4), the power supply fluctuation value of VCC is
When the collector-emitter voltage of r3 becomes less than VCE,
Since the transistor Tr6 is turned on and drives the transistor Tr3 in the cutoff direction, the output terminal 3 is not affected by power fluctuations.

Therefore, mixing of ripples is prevented.

Next, a second embodiment will be explained using FIG. 2.

In the second embodiment as well, the ripple filter circuit is mainly composed of a ripple filter section A1 and a comparator section A2, as in the first embodiment.

The ripple filter circuit has a + power input terminal 1, a - power input terminal 2, electrolytic capacitor connection terminals 4 and 5, and a ripple filter output terminal 3.

The difference from the first embodiment is that a transistor Tr12 and a transistor Tr13 are added. In other words,
The collector of the transistor Tr12 is the transistor Tri
Electrolytic capacitor C via the base of resistor R3 and terminal 4
1, its emitter is connected to one power source VEE, its base is connected to the base and collector of transistor Tr13, its emitter is connected to one power source VEE, and its collector is connected to the collector of transistor Tr6. In the case of FIG. 2, when the power supply voltage drops, transistor Tr6. Tr
13. Tr12 turns on and lowers the base voltage of transistor Trl. As a result, the collector current of the transistor Tri decreases, and if the transistor Tr3 is controlled in the cut-off direction, even if a sudden power fluctuation occurs on the power supply line, the output terminal 3
Ripple does not appear in .

[Effects of the Invention] As described above, the present invention has a comparator section, and receives a control signal from the transistor of the comparator section in the direction of turning off (to high impedance) the transistor of the ripple filter section when the power supply voltage momentarily fluctuates. This has the effect that even if the power supply voltage of the power supply line fluctuates rapidly, this fluctuation in the power supply line can be prevented from appearing in the output of the ripple filter.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram explaining a first embodiment according to the present invention, FIG. 2 is a circuit diagram explaining a second embodiment according to the present invention, FIG. 3 is a conventional circuit diagram. 1... Ripple filter circuit A1... Ripple filter section A2... Comparator section 01~C3... Capacitor R1-R8...Resistance Tri ~Tr13- transistor Figure 3

Claims (1)

[Claims] First and second transistors that are differentially connected; and a third transistor whose base is connected to the collector of the first transistor, whose emitter is connected to a power supply, and whose collector is connected to the collector of the first transistor. A ripple filter circuit comprising: a comparator circuit that includes differentially connected fourth and fifth transistors and compares the output voltage of the ripple filter circuit with a power supply voltage; and when the power supply voltage decreases, the output of the comparator circuit A ripple filter circuit characterized in that the third transistor is controlled in a cutoff direction by a signal.