JP2000312150A - Current cell type d/a converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extend tolerance level of reference voltage and to extend degree of flexibility of a manufacturing process used, in the case of manufacturing current cell type digital/analog D/A converters as a semiconductor integrated circuit, while ensuring the required characteristics as a D/A converter. SOLUTION: A plurality of transistors(TRs) T2A1-T2A4, whose sources and drains are respectively connected in parallel of this D/A converter, configure a switch TR that becomes conductive in the case of a current output off state and bypasses the current to be outputted as a current cell to the ground. A TR control circuit, consisting of AND logic gates G1-G4 or the like individually, controls a signal given to the gates of the TRs T2A1 to T2A4 to select a TR to become conductive among a plurality of the TRs T2A1 to T2A4 in the case of a current output off state, thereby setting an optimum on-resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a current cell type D / A.
(Digital to analog) converter, especially D
While ensuring the characteristics required for the A / A converter, the allowable range of the reference voltage for setting the magnitude of the current supplied from the current cell is expanded, and the degree of freedom of the manufacturing process used when manufacturing as a semiconductor integrated circuit is increased. The present invention relates to a current cell type D / A converter that can be achieved.

[0002]

2. Description of the Related Art FIG. 1 is a circuit diagram mainly showing an example of a current cell used in a conventional current cell type D / A converter.

In the current cell, a transistor T1
A, T2A, and T3A constitute an output unit.
Also, by the transistors T1B, T2B and T3B,
A current setting unit is configured. The output section and the current setting section have a symmetric configuration. In an actual current cell type D / A converter, 2 ⁿ -1 current cells (n is the number of bits) of FIG. 1 are connected in parallel to a load resistor RL. In the figure, one of them is extracted.

In such a circuit, a current having the same magnitude as the current flowing through the transistor T1B in the current setting section is also passed through the transistor T1A in the output section by using a current mirror circuit. The magnitude of the current can be set externally by the resistance value of the external resistor RD.

[0005] The transistors T2A and T3A operate as switch transistors. Transistor T
A voltage in either the H state (power supply VDD) or the L state (ground GND) is applied to the gate of 2A as the control signal CTL, and no intermediate voltage from the power supply VDD to the ground GND is applied. The control signal CTL is D /
This signal is in the H state or the L state according to the code of the digital signal to be A-converted. On the other hand, the gate of the transistor T3A is at the VREF voltage which is an intermediate potential.
That is, it can be said that the transistor T3A is turned on with a very high resistance.

First, when an H state is applied to the transistor T2A, the transistor T2A is turned off. Then, in this case, the current set by the current setting unit and all the current flowing through the transistor T1A is output to the outside from the current output IOUT via the transistor T3A, and is converted into a voltage by the load resistance RL.

On the other hand, when the L state is applied to the transistor T2A, the transistor T2A turns on. Then, in this case, the transistor T2A and the transistor T2
Due to the resistance difference of 3A, the current flowing through the transistor T1A set by the current setting section is all the current flowing through the transistor T2A.
Will not flow to the transistor T3A,
It does not flow from the current output IOUT to the outside.

Therefore, using 2 ⁿ -1 current cells,
The above-described control signals CTL of these current cells are individually
The digital input signal can be converted to an analog signal by setting the H state or the L state according to the digital signal to be / A converted and adding the currents supplied from these current cells. Thus, the current cell type D / A converter functions.

Here, a current cell type D / A converter having 2 ⁿ -1 cells through which the same current flows is described. However, a current cell type D / A converter composed of n current cells having a weight of each bit, or only a part thereof is 2 ^m −1 (m is a part of the number of bits) of the same current. The same applies to a current cell type D / A converter constituted by a current cell through which a current flows, and the present invention can be applied.

[0010]

SUMMARY OF THE INVENTION In the circuit of FIG.
When the control signal CTL is in the L state, if the difference between the on-resistance of the transistor T2A and the on-resistance of the transistor T3B is too large, the balance of the current mirror circuit is lost, and the magnitude of the current flowing through the transistor T1A becomes It fluctuates from what is set by the resistance value of the external resistor RD. This affects the current consumption of the current cell type D / A converter.

In order to reduce the fluctuation of the current caused by this phenomenon flowing through the transistor T1A, the reference voltage V
This can be dealt with by lowering REF, thereby reducing the on-resistance of the transistor T3B, or reducing the size of the transistor T2A and increasing its on-resistance.

However, as described above, since the on / off of the transistor T3A is controlled by the resistance difference from the transistor T2A, if the on-resistance of the transistor T2A becomes too large, the control signal CTL is in the L state. , The current output from the current output IOUT to the outside cannot be cut off. Therefore, the transistor T2A
Cannot be made too small.

For this reason, it is necessary to find the optimum value of the size of the transistor T2A.

Also, LSI (large scale integrated)
The lower the power supply voltage of the circuit, the narrower the allowable range of the above-mentioned optimum value. Further, in a custom LSI, a process that meets customer requirements may be adopted, or a process may be manufactured in a different factory. Therefore, it is considerably difficult to find a common optimum value for all the processes. Further, when the reference voltage VREF is made variable, it may be said that the optimum value does not exist.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and has an allowable reference voltage for setting a magnitude of a current supplied from a current cell, while securing necessary characteristics as a D / A converter. It is an object of the present invention to provide a current cell type D / A converter capable of expanding a range and a degree of freedom of a manufacturing process used when manufacturing as a semiconductor integrated circuit.

[0016]

According to the present invention, the supply of a current unique to each of a plurality of current cells is made to flow to an output current terminal in each current cell in accordance with a digital input signal.
Or a current cell type D / A converter that converts a digital input signal into an analog signal by adding these output currents while switching whether to flow the current to the ground.
In order to solve the above-described problem, the transistor is configured by a plurality of transistors in parallel at a source and a drain, and a gate signal is individually given so that the number of transistors to be turned on among the transistors can be controlled. is there.

Further, in the above current cell type D / A converter, a circuit for detecting a reference voltage value applied to a gate of a switch transistor which is turned on when a current flows to an output current terminal in each current cell is provided. And a function of automatically controlling individual gate signals of the plurality of divided transistors based on the detection result of the reference voltage value.

In the above case, if the value of the reference voltage and the number of switch transistors to be turned on are known in advance, the number of transistors to be turned on among the plurality of transistors is appropriately controlled according to, for example, a manufacturing process. it can.

Hereinafter, the operation of the present invention will be briefly described.

The above-mentioned optimization includes setting the size of the transistor T2A in FIG. The transistor T2A is a switch transistor that causes a current supplied from the transistor T1A to flow to the ground in the corresponding current cell.

By configuring the transistor T2A with a plurality of parallel transistors, the reference voltage VREF of various magnitudes and various manufacturing processes used for manufacturing a semiconductor integrated circuit can be appropriately adjusted.
The size of the switch transistor can be set to an appropriate size.

For example, when the voltage of the reference voltage VREF is low, the on-resistance of the transistor T3A in FIG. 1 decreases. Therefore, it is necessary to reduce the on-resistance of the transistor T2A to suppress the leakage current when the transistor T3A is off. Therefore, when the present invention is applied, a transistor having a large transistor size is selected from a plurality of parallel transistors so as to increase the size of the transistor T2A, or the transistor is turned on among the plurality of transistors. Increase the number of things.

On the other hand, when the reference voltage VREF is high, the on-resistance of the transistor T3A in FIG. 1 increases and the on-resistance of the transistor T3B also increases. Therefore, the transistor T2A must also have a large on-resistance to suppress the fluctuation in the current of the transistor T1A. Therefore, when the present invention is applied, the transistor control circuit selects a transistor having a small transistor size from among a plurality of parallel transistors so as to reduce the size of the transistor T2A, or
The number of transistors turned on among the plurality of transistors is reduced.

Although the above description is about the magnitude of the reference voltage VREF, the size and number of a plurality of parallel transistors to be turned on are also determined with respect to variations in the manufacturing process and other conditions used when manufacturing as a semiconductor integrated circuit. It is possible to cope with the problem by appropriately setting them appropriately.

As described above, in this embodiment, the D / A
To expand the allowable range of the reference voltage that sets the magnitude of the current output from the current cell while securing the characteristics required for the converter, and to increase the degree of freedom in the manufacturing process used when manufacturing as a semiconductor integrated circuit. Can be.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 2 is a circuit diagram mainly showing a current cell portion used in the current cell type D / A converter of the embodiment to which the present invention is applied.

In the present embodiment, the transistor T2A in the conventional example shown in FIG. 1 is constituted by a plurality of transistors T2A1 to T2A4 which are parallel in the source and the drain. Further, these transistors T2A1 to T2A4
Is configured by a 2-bit A / D converter 10 and four AND logic gates G1 to G4.
It is controlled by a circuit corresponding to the transistor control circuit of the present invention.

The operation of the present embodiment will be described below.

Normal MOS (metal oxide semiconduc
In a tor) transistor, there is an exponential relationship between gate voltage and on-resistance between source and drain. That is, in the case of a P-channel MOS transistor, when the gate voltage increases, the on-resistance increases to a greater degree. Alternatively, in the case of an N-channel MOS transistor, when the gate voltage increases, the on-resistance decreases to a greater degree.

This relationship is hereinafter referred to as an exponential relationship between the gate voltage and the on-resistance.

Further, the source current of the MOS transistor slightly varies depending on the voltage between the source and the drain even in the current saturation region. This is also called a channel modulation effect. When the source-drain voltage Vds increases, the source current increases.

The phenomenon caused by the characteristics of the two transistors will be described in detail. In the current cell, FIG.
The current flowing through the transistor T1B in the above is passed through the transistor T1A by using a current mirror circuit. However, considering the case where the current flowing in the transistor T1A flows to the ground through the transistor T2A, the difference in the on-resistance between the transistor T3B and the transistor T2A having different gate voltages depends on the exponential relationship between the gate voltage and the on-resistance. In addition, a current setting resistor RD is provided between the transistor T3B and the ground, and the node A shown in FIGS.
And a potential difference occurs between the node B Then, due to the channel modulation effect of the transistor, the transistor T1A
Of the current cell type D / A converter becomes large, so that the current consumption of the current cell type D / A converter as a whole increases.

Therefore, in order to suppress the fluctuation, the voltage difference between the node A and the node B may be reduced, and as a method thereof, it is conceivable to reduce the transistor T2A and increase the on-resistance. However, ON / OFF of the transistor T3A is controlled by a difference in ON resistance from the transistor T2A. For this reason, the transistor T2A must not be too small, but must be set to an optimal size.

For example, for each voltage of the reference voltage VREF, the resistance value of the on-resistance of the transistor T3A at that voltage and the optimum value of the on-resistance of the transistor T2A at that voltage are as shown in FIG. And

In this embodiment, if the transistor T
The on-resistance of 2A1 is 1.25 ohms, the on-resistance of transistor T2A2 is 2.5 ohms,
It is assumed that the on-resistance of the transistor T2A3 is 5 ohms and the on-resistance of the transistor T2A4 is 10 ohms.

The two-bit A / D converter 10 has four signals L1 in which one is in the H state and the other is in the L state in accordance with the magnitude of the reference voltage VREF.
To L4. Accordingly, when the control signal CTL goes to the H state, the AND logic gates G1 to G4 cause the transistors T2A1 to T2A4 corresponding to the four signals L1 to L4 to be in the H state individually as shown in FIG. It is turned on and off, and one of them is turned on.
On the other hand, when the control signal CTL goes to the L state, the AND logic gates G1 to G4 activate the transistors T2A1 to T2A.
4 are all off.

The input voltage is shown in the section of the reference voltage VREF on the left side of FIG. The transistors T2A1 to T2A4 are turned on and off as shown in order to make the resistance of the transistor T2A follow the optimum value as shown in each of the voltages. The transistor T2A1 shown in FIG.
For items T2A4 to T2A4, "ON" is shown when ON, while a blank column for this item indicates OFF.

For example, when the reference voltage VREF is 1 volt, the transistor T2A having an on-resistance of 10 ohms
By turning on only 4 and turning off the transistors T2A1 to T2A3, the resistance value of the transistor T2A is adjusted to 10 ohms suitable for the optimum value. When the reference voltage VREF is 0.9 volts, only the transistor T2A3 having an on-resistance of 5 ohms is turned on, and the transistors T2A1, T2A2, and T2A4 are turned off. Operated by Ohm.

As described above, when the current flowing through the transistor T1A flows to the ground in the current cell, the transistors to be turned on in the transistors T2A1 to T2A4 are changed according to the magnitude of the reference voltage VREF. In addition, these transistors T2
The on-resistance of the transistor turned on in A1 to T2A4 is shown in FIG.
Will match or follow the optimal value shown in.

As described above, in the present embodiment, the on-resistance of the transistor T2A can be automatically and optimally set according to the magnitude of the reference voltage VREF. Therefore, it is possible to increase the allowable range of the reference voltage and the degree of freedom of the manufacturing process used when manufacturing as a semiconductor integrated circuit, while securing the characteristics required for the D / A converter.

In the present embodiment, an example has been described in which the on-resistance of the transistor T2A is automatically set in accordance with the magnitude of the reference voltage VREF. This is applicable when the relationship with the resistance value of T2A is clear, and when the relationship is unknown, 2 in FIG.
The terminal corresponding to the output signal of the bit A / D converter may be controlled, for example, from the outside.

In the embodiment described above, when one of the transistors T2A1 to T2A4 is turned off when the current supplied from the current output IOUT to the outside is off, the voltage of the reference voltage VREF increases. The most appropriate transistor size is selected accordingly. However, the variable transistor to be turned on in the present invention is not limited to this. For example, the number of transistors to be turned on in parallel or the number of transistors to be turned on in series may be varied to vary the transistors to be turned on, thereby varying the combined resistance of the on-resistance of the transistors.

[0044]

According to the present invention, it is possible to increase the allowable range of the reference voltage and the degree of freedom of the manufacturing process used when manufacturing as a semiconductor integrated circuit while securing the characteristics required for a D / A converter. Can be planned.

[Brief description of the drawings]

FIG. 1 is a circuit diagram mainly showing an example of a current cell used in a conventional current cell type D / A converter.

FIG. 2 shows a current cell type D / according to an embodiment to which the present invention is applied.
Circuit diagram focusing on current cell part used for A converter

FIG. 3 is a diagram showing, as an example, an optimum value of an on-resistance of a transistor T2A at each voltage of a reference voltage VEREF.

[Explanation of symbols]

10 2-bit A / D converters T1A to T3A, T1B to T3B, T2A1 to T2A4
... Transistor RP ... Resistance RL ... Load resistance G1-G4 ... AND logic gate

Claims (2)

[Claims] 1. A method according to claim 1, wherein the supply of a current specific to each of a plurality of current cells is switched between flowing to an output current terminal or flowing to ground in each current cell in accordance with a digital input signal. In a current cell type D / A converter that converts a digital input signal into an analog signal by adding a switch transistor, a switch transistor that is turned on when the unique current flows to the ground is replaced by a plurality of transistors in parallel at a source and a drain. And a gate signal is individually supplied so that the number of transistors to be turned on among these transistors can be controlled. 2. The current cell type D / A converter according to claim 1, wherein a reference voltage value applied to the gate of the switch transistor, which is turned on when a current flows to an output current terminal in each current cell, A current cell type D / A having a function of automatically controlling individual gate signals of the plurality of divided transistors based on the detection result of the reference voltage value.
converter.