JPH096450A - Constant current circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] To reduce the dependence of the constant current output Io of the constant current circuit composed of MOS transistors on the power supply voltage Vd,
In addition, constant current output even when the power supply voltage V of the load 1 is high
Prevent the destruction of the Io transistor 12. [Structure] Current mirror circuit 10 and its driven transistor 12
, A control transistor 20 connected in series with the resistor 30, a resistor 30 connected in series therewith, and a setting transistor 40 whose gate is controlled by the voltage drop of the resistor connected in series with the reference transistor 11 of the current mirror circuit 10, The gate of the control transistor 20 is the reference transistor of the current mirror circuit 10.
The driven current If of the current mirror circuit 10 is taken out as a constant current output Io while being controlled by the potential at the connection point between 11 and the setting transistor 40. Further, a DMOS structure is used for at least the driven transistor 12 of the current mirror circuit 10 to prevent the load 1 from being destroyed by the power supply voltage V.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant current circuit using a so-called current mirror circuit, which is suitable for supplying a constant current to a load which is incorporated in an integrated circuit device and receives a predetermined power supply voltage. Regarding what is composed.

[0002]

2. Description of the Related Art In a constant current circuit composed of the above-mentioned MOS transistors, a constant current, which is the saturation current, is generated as a reference current on the side of the reference transistor using a current mirror circuit, and the current on the side of the driven transistor. If the driven current proportional to the reference current is taken out as the constant current output, it is advantageous in that the constant current output can be easily supplied to the loads receiving various power source voltages at various current values. FIG. 2 shows a basic circuit configuration example of such a constant current circuit.

The reference transistor 11 of the current mirror circuit 10 shown in FIG. 2 receives a power supply voltage Vd of, for example, 5V for an integrated circuit through a current setting resistor 11a, and its gate and drain are short-circuited to form a gate / source. In the meantime, since it operates in the saturation mode by receiving a voltage higher than the operation threshold value of the gate, the reference current Ir, which is a constant current set by the resistor 11a, flows through the reference transistor 11. As is well known to the driven transistor 12 of the current mirror circuit 10 whose gate is commonly connected to this, a current If which is exactly proportional to the above-mentioned reference current Ir.
Is driven, the driven current If is supplied as a constant current output Io to the load 1 receiving another power supply voltage V. When the so-called mirror coefficient of the current mirror circuit 10 is M, this constant current output is represented by Io = If = MIr.

[0004]

The conventional constant current circuit as shown in FIG. 2 has an advantage that the value of the constant current output Io supplied to the load 1 can be set arbitrarily and accurately even when the load 1 receives various power supply voltages V. However, when the power supply voltage Vd fluctuates for some reason, there is a problem that the current value of the constant current output Io is very sensitive to the change and tends to change, and the power supply voltage V of the load 1 is a current mirror circuit. When the voltage is higher than the power supply voltage Vd on the side of the reference transistor 11 of 10, the source-drain breakdown voltage cannot be withstood immediately after the power is turned on or when noise is mixed in the power supply voltage V, and the driven transistor 12 may be destroyed.

The current value of the constant current output Io is easily affected by the fluctuation of the power supply voltage Vd because the reference current Ir of the current mirror circuit 10 is
Is a functional relationship with the power supply voltage Vd. That is,
Since the reference transistor 11 of the current mirror circuit 10 operates in a saturated state in which the gate is short-circuited with the drain, the reference current Ir, which is the saturation current, has a relationship with the power supply voltage Vd represented by the following equation.

Ir = k (Vd-IrRr-Vt) ² where k is a unique constant related to the structure of the reference transistor 11, Rr is the resistance value of the resistor 11a, and Vt is the reference transistor.
11 is the operating threshold of the gate. This formula is the reference current Ir
Solving this is a quadratic equation for

[0007]

The Equation 1] Ir = (2kRrDv + 1-D 1/2) / 2kRr 2 (1). However, Dv = Vd−Vt and D = 4kRrDv + 1, both of which are functions of the power supply voltage Vd. In this way, since the reference current Ir depends on the power supply voltage Vd, it and Io =
The constant current output Io, which is in the relationship of MIr, also has voltage dependency, and when the mirror coefficient M is set large, the change in the current value becomes particularly large.

In order to solve such a problem, the present invention has its main purpose to reduce the dependency of the constant current circuit on the power supply voltage, and to prevent the destruction of the driven transistor of the current mirror circuit. It has a subordinate purpose.

[0009]

In the constant current circuit of the present invention, a current mirror circuit including a reference transistor and a driven transistor, a control transistor connected in series with the driven transistor of the current mirror circuit, and a control transistor A resistor connected in series and a setting transistor connected in series to the reference transistor of the current mirror circuit and having its gate controlled by the voltage drop of the resistor are used, and the gate of the control transistor is set to the setting transistor and the reference of the current mirror circuit. The above object is achieved by extracting the driven current of the current mirror circuit as a constant current output while controlling it by the potential at the interconnection point with the transistor. In such a circuit of the present invention, the value of the constant current output is set by the ratio of the operating threshold value of the gate of the setting transistor to the value of the resistor connected in series to the control transistor. The driven current can be set to a predetermined multiple of the reference current.

In the present invention, the reference transistor and the driven transistor of the current mirror circuit can be operated with different power supply voltages, and when the latter power supply voltage is higher, at least the driven transistor of the current mirror circuit is destroyed. It is advantageous to use DMOS transistors for protection. Further, this current mirror circuit can be constructed by an n-channel transistor to take out a sink current by the driven transistor as a constant current output, or can be constructed by a p-channel transistor to take out a source current by the driven transistor as a constant current output.

[0011]

The present invention makes use of the proportionality of the driven current of the current mirror circuit to the reference current, and makes the voltage dependence almost dependent on the resistance value of the series resistance of the control transistor and the operating threshold value of the gate of the setting transistor. In addition to setting a constant current output that does not have a constant current output, a feedback control system that includes a control transistor and a setting transistor controls the current value of the constant current output to be always constant according to this setting value. By using a transistor having a DMOS structure as at least the driven transistor of the circuit, the driven transistor is prevented from being destroyed when operated at a power supply voltage higher than that of the reference transistor side.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. The figure (a) shows an embodiment of the constant current circuit of the present invention which extracts the sink current by the driven transistor of the current mirror circuit as the constant current output, and the figure (b) extracts the source current due to it. 2 are assigned the same reference numerals as those corresponding to FIG.

In the embodiment shown in FIG. 1A, the constant current circuit is composed of an n-channel type MOS transistor, and the current mirror circuit 10 shown on the upper part of the constant current circuit has its reference transistor as in the conventional example shown in FIG. 11 receives a low-voltage power supply voltage Vd of, for example, 5V via the current setting resistor 11a, and its driven transistor 12 receives a different power supply voltage V of, for example, several to several tens of volts, and a driven current as a constant current output Io to the load 1. Although If is supplied in the form of a sink current, in the constant current circuit of the present invention, the driven transistor 12 is provided between it and the ground potential point E unlike the conventional one.
The control transistor 20 and the resistor 30 are connected in series on the side, and the setting transistor 40 is connected in series on the side of the reference transistor 11 as shown in the figure.

Since the power supply voltage V on the driven transistor 12 side of the current mirror circuit 10 is high as described above, in this embodiment, the driven transistor 12 is a so-called DMOS transistor having a withstand voltage of about 100 V, for example.
Prevents damage immediately after charging. In order to keep the proportionality between the reference current and the driven current of the current mirror circuit 10 as accurate as possible, it is preferable to use a transistor having the same DMOS structure as that of the driven side for the reference transistor 11 as well. Also in the present invention, it can be appropriately set as required. Especially constant current output Io
In order to reduce the voltage dependence of the above-mentioned, as will be described later, it is desirable to set the reference current Ir flowing through the reference transistor 11 of the current mirror circuit 10 sufficiently small.

In the circuit of the present invention, the gate of the control transistor 20 connected in series to the driven transistor 12 is given the potential at the interconnection point of the reference transistor 11 and the setting transistor 40 as shown in the figure, and the gate of the setting transistor 40 is applied. Is controlled by the voltage drop due to the driven current If of the resistor 30 connected in series to the control transistor 20. The setting transistor 40 preferably has a structure in which the ratio W / L of the channel width W to the channel length L is large, and the reference current Ir flowing through the setting transistor 40 is preferably set small as described above. As a result, the resistance that is the potential applied to the gate of the setting transistor 40
The voltage drop of 30 becomes almost equal to the operation threshold value Vt. Therefore, when the resistance value of the resistor 30 is R, Vt = RIf and the driven current If is the constant current output Io, so the following equation holds.

[0016]

## EQU2 ## Io = Vt / R (2) Thus, the current value of the constant current output Io is set by the ratio of the resistance value R of the resistor 30 and the gate threshold Vt of the setting transistor 40. As can be seen by comparing with the above equation (1), in the circuit of the present invention, unlike the conventional case, the influence of the fluctuation of the power supply voltage Vd on the current value of the constant current output Io can be eliminated in principle. Further, the control transistor 20 serves to keep the constant current output Io set in this way constant. That is, if the constant current output Io increases even slightly, for example, from the set value, the voltage drop due to the resistor 30 increases, so the potential of the gate of the setting transistor 30 slightly increases and the reference current Ir is increased.
However, since the potential at the interconnection point between the reference transistor 11 and the setting transistor 40 decreases accordingly,
The control transistor 20 receiving this at its gate outputs a constant current
It works by decreasing Io and keeping it at the set value.

In the embodiment shown in FIG. 1B, the constant current circuit is p
It is composed of a channel type MOS transistor. The reference transistor 13 of the current mirror circuit 10 shown in the lower part of the figure is grounded via a current setting resistor 13a, and the driven transistor 14 gives a driven current If to the grounded load 1 as a constant current output Io in the form of a source current. . The driven transistor 14 passes through the control transistor 21 and the resistor 30, and the reference transistor 13 passes through the setting transistor 41.
Receive. Setting transistor 41 due to voltage drop across resistor 30
Of the control transistor 21 and the potential of the interconnection point between the reference transistor 13 and the setting transistor 41.
The point of giving to the gate is the same as that of the previous embodiment of FIG. 1 (a).

Although this embodiment is not suitable for supplying the constant current output Io to the load 1 that receives a power supply voltage different from the power supply voltage Vd, the current value of the constant current output Io is a setting transistor for the resistance value of the resistor 30. The point is set by the ratio of the gate threshold value of 41, and the current value of the constant current output Io is controlled to be constant according to this set value by the control transistor 21, which is exactly the same as the previous embodiment. Therefore, even in the embodiment of FIG. 1 (b), the set value of the constant current output Io is hardly affected by the fluctuation of the power supply voltage Vd, and further, the current value is always accurately and exactly controlled according to the set value. The same effect as the embodiment of FIG. 1 (a) can be obtained.

[0019]

As described above, according to the present invention, as a constant current circuit composed of MOS transistors, a current mirror circuit, a control transistor connected in series to its driven transistor and its series resistance, and a reference transistor of the current mirror circuit. A setting transistor connected in series with the control gate whose gate is controlled by the voltage drop of a resistor is used, and the gate of the control transistor is controlled by the potential at the interconnection point between the setting transistor and the driven transistor of the current mirror circuit, while the current mirror circuit is driven. Since the current is output as a constant current output, (a) the value of the constant current output is set by the ratio of the gate threshold value of the setting transistor to the resistance value of the series resistance of the control transistor, and the current value Make sure that it is hardly affected by fluctuations,
(b) With the feedback system including the control transistor and the setting transistor, the current value of the constant current output can be accurately kept constant as set.

The embodiment in which the reference transistor and the driven transistor of the current mirror circuit are operated by different power supply voltages is advantageous in expanding the application range and application of the constant current circuit according to the present invention, and the current mirror circuit. The embodiment in which the DMOS transistor is used as at least the driven transistor has the effect of preventing breakdown when the load receives a high voltage and improving the operation reliability of the constant current circuit.

The embodiment in which the driven current flowing in the driven transistor of the current mirror circuit is set to a predetermined multiple of the reference current flowing in the reference transistor is advantageous in expanding the settable range of the constant current output, and the constant value output is fixed. There is an effect that the reference current of the current mirror circuit is set to be smaller than the current output and the constant current output value can be accurately set by the ratio of the gate threshold value of the setting transistor to the resistance value of the series resistance of the control transistor.

The current mirror circuit is an n-channel MOS.
The embodiment configured by a transistor and extracting the sink current by the driven transistor as a constant current output is suitable for use in supplying the constant current output to a load that receives a power supply voltage different from the power supply voltage on the reference side of the current mirror circuit. The embodiment in which the current mirror circuit is configured by the p-channel transistor and the constant current output is taken out from the driven transistor is not suitable for this application, but has an effect that the constant current output can be set large as the source current.

[Brief description of drawings]

FIG. 1 shows an embodiment of a constant current circuit according to the present invention.
(a) is a circuit diagram showing an embodiment of extracting a sink current by a driven transistor of a current mirror circuit as a constant current output,
FIG. 3B is a circuit diagram showing an embodiment in which the source current of the driven transistor is taken out as a constant current output.

FIG. 2 is a circuit diagram showing a typical configuration of a conventional constant current circuit.

[Explanation of symbols]

1 load 10 current mirror circuit 11 n-channel reference transistor of current mirror circuit 12 n-channel driven transistor of current mirror circuit 13 p-channel reference transistor of current mirror circuit 14 p-channel driven transistor of current mirror circuit 20 n-channel type control transistor 21 p-channel type control transistor 30 Resistance connected in series with the control transistor 40 n-channel type setting transistor 41 p-channel type setting transistor If Current mirror circuit driven current Io Constant current output Ir current Reference current of mirror circuit Vd Current supply voltage of reference side of mirror circuit V Power supply voltage for load

Claims (7)

[Claims] 1. A constant current circuit comprising a MOS transistor, comprising a current mirror circuit, a control transistor connected in series with a driven transistor of the current mirror circuit, and a resistor connected in series with the control transistor. And a setting transistor which is connected in series to the reference transistor of the current mirror circuit and whose gate is controlled by the voltage drop of the resistor. The gate of the control transistor is connected to the interconnection point of the setting transistor and the reference transistor of the current mirror circuit. The constant current circuit is characterized in that the driven current of the current mirror circuit is taken out as a constant current output while being controlled by the potential of. 2. The constant current circuit according to claim 1, wherein the reference transistor and the driven transistor of the current mirror circuit are operated by different power supply voltages. 3. A constant current circuit according to claim 1, wherein a transistor having a DMOS structure is used as a driven transistor of the current mirror circuit. 4. The circuit according to claim 1, wherein the value of the constant current output is set by the ratio of the operating threshold value of the gate of the setting transistor to the resistance value of the resistor connected in series to the control transistor. Constant current circuit characterized by. 5. The circuit according to claim 1, wherein the driven current flowing through the driven transistor of the current mirror circuit is set to a predetermined multiple of the reference current flowing through the reference transistor and is taken out as a constant current output. Constant current circuit. 6. The constant current circuit according to claim 1, wherein the current mirror circuit is composed of an n-channel transistor and the sink current of the driven transistor is taken out as a constant current output. 7. A constant current circuit according to claim 1, wherein the current mirror circuit is composed of a p-channel transistor and the source current of the driven transistor is taken out as a constant current output.