Patent Application
20050122139
The present invention generally relates to current drive circuits and, more particularly, to a current drive circuit of a current mirror type. A current mirror circuit is often used to feed a desired current to a load. A current mirror circuit generally used has the following structure. The gates of first and second transistors are connected to each other and so are the sources of the transistors. The sources of the transistors are grounded and the gates are connected to the drain of the first transistor. A target load is connected to the drain of the second transistor.
A reference current is fed to the drain of the first transistor and a drive current proportional to the reference current is fed to the load connected to the drain of the second transistor. The ratio between the reference current and the drive current, i.e. the mirror ratio, is determined by the ratio between source-drain currents of the first and second transistors. The source-drain current IDS is proportional to the channel width W of a transistor and inversely proportional to the channel length L thereof. Generally, the source-drain current is determined by the ratio W/L.
The ratio between the reference current and the drive current is determined by the ratio W/L of the first and second transistors. However, such a definition is based on an assumption that source-drain voltages VDS of the transistors are identical. Strictly speaking, it is known that the source-drain current IDS of a transistor is proportional to (VGS−Vth)2*(W/L)*(1+λVDS), meaning that IDS is slightly affected by VDS. λ indicates a channel length modulation coefficient, VGS indicates a gate-source voltage and Vth indicates a threshold voltage. Accordingly, even when W/L is designed properly, an accurate drive current is not obtained when VDS of one of the transistors is different from an ideal value.
The present invention has been done in light of the aforementioned problem and its objective is to provide a current drive circuit less dependent on VDS than the related art.
The present invention provides a current drive circuit of a current mirror type in which gates and sources of a first transistor and a second transistor are connected to each other, the sources of the transistors are grounded, the gates of the transistors are connected to the drain of the first transistor, a reference current is fed to the drain of the first transistor, a target load is connected to the drain of the second transistor, and a drive current proportional to the reference current is fed to the load, comprising: an adjustment circuit which makes a drain potential of the first transistor and a drain potential of the second transistor to approach each other while maintaining a direct connection between the drain of the second transistor and the load.
According to this structure, VDS of the first transistor and that of the second transistor approach each other so that an accurate drive current is obtained. Since the drain of the second transistor and the load are maintained in direct connection with each other, the drive current can be fed generally more accurately and efficiently than when an extra transistor or the like is introduced between the second transistor and the load.
The adjustment circuit may comprise: an operational amplifier having two inputs thereof connected to the drain of the first transistor and the drain of the second transistor, respectively; and a third transistor provided in series between the drain of the first transistor and the gates of the first and second transistors, and wherein an output of the operational amplifier is connected to a gate of the third transistor.
In an alternative mode, the adjustment circuit may comprise: a third transistor connected in series between the drain of the first transistor and the gates of the first and second transistors; and a fourth transistor having a source thereof connected to a gate of the third transistor and a drain thereof grounded, and being fed a constant current, and wherein a gate of the fourth transistor is connected to the drain of the second transistor.
The current drive circuit may further comprise a circuit which invalidates the operation of the adjustment circuit. The circuit (hereinafter, referred to as an invalidating circuit) may operate when VDS of the first transistor and that of the second transistor are close to each other. This is because the adjustment circuit is unnecessary when VDS of the first transistor is close to that of the second transistor. One of the advantages obtained by invalidating the adjustment circuit is reduction in power consumption.
The invalidating circuit may function when the gate-source voltage VGS is high. Since the source-drain current IDS is proportional to (VGS−Vth)2*(W/L)*(l+λVDS), the term (VGS−Vth)2 predominantly affects IDS when VGS is high. Since VDS affects IDS only slightly in this state, the aforementioned approach is useful.
The present invention also provides a current drive circuit comprising: a first route feeding a reference current; a second route including a target load and feeding a drive current to the load; a first resistor provided in series in the first route; a second resistor provided in series in the second route; an operational amplifier having two inputs thereof connected to an end of the first resistor (hereinafter, referred to as the top end of the first resistor) and an end of the second resistor (hereinafter, referred to as the top end of the second resistor), respectively, wherein a transistor is provided in the second route and a gate of the transistor is connected to an output of the operational amplifier. The first route and the second route constitute a current mirror circuit. Since the operational amplifier operates to match the potential at the top end of the first resistor and that of the second resistor, an accurate mirror ratio is produced. By utilizing the resistors and the operational amplifier, the problem of VDS dependency is eliminated.
The current drive circuits according to the invention may be built in an integrated circuit device (hereinafter, simply referred to as an LSI), and a route for feeding the drive current to the load external to the LSI via a terminal of the integrated circuit device may be formed. Since a power supply voltage applied to the load is unknown, reduction in the level of VDS dependency according to the invention is effective in maintaining an accurate drive current.
The drain of the third transistor Q3 is connected to the output of the constant current circuit 20 and the gates of the transistors. The source of the third transistor Q3 is connected to the drain of the first transistor Q1 and the inverting input of an operational amplifier 10. The gate of the third transistor Q3 is connected to the output of the operational amplifier 10. The non-inverting input of the operational amplifier 10 is connected to the drain of the second transistor Q2, the input of a detector 12 and a terminal 22. The operational amplifier 10 and the third amplifier Q3 operate as an adjustment circuit. The output of the detector 12 is connected to the gate of a transistor Qs, an n-channel FET. The detector 12 and the transistor Qs constitute a shut circuit for the adjustment circuit, the shunt circuit operating as an invalidating circuit. When the input voltage is higher than a predetermined voltage, the detector 12 brings the output thereof low, turns the transistor Qs for invalidation on and causes the current of the constant-current circuit 20 to bypass. This invalidates the third transistor Q3, thus causing the current drive circuit 100 to be returned to a conventional current mirror circuit.
A light-emitting diode 16 is provided as a load outside the LSI and a power supply voltage VDD is applied to the anode of the diode. The cathode of the light-emitting diode 16 is connected to a terminal 22 of the LSI.
It will be assumed that the output voltage of the constant current circuit 20 is indicated by Va, the drain voltage of the first transistor Q1 by Vb, and the drain voltage of the second transistor Q2 by Vc. Va is a sufficiently high voltage. When a reference current I1 is fed to the constant-current circuit 20, the on-state of the third transistor Q3 is controlled as a result of an imaginary short circuit being established in the operational amplifier 10. Consequently, Vb and Vc are substantially equal. With this, VDS of the first transistor Q1 and that of the second transistor Q2 are substantially equal to each other, VDS dependency of the mirror ratio is eliminated. The drive current I2 of a target value is fed to the light-emitting diode 16 and a desired light emission state is produced.
When VDD is sufficiently high, the effect from VDS is negligible as mentioned before. In this case, the detector 12 is operated so as to turn Qs on. The current drive circuit 100 as a whole is then returned to a conventional current mirror circuit. As a result, power loss in the third transistor Q3 is reduced to zero.
Thus, VDS matching using the adjustment circuit according to the first embodiment is useful since VDD is different from application to application and unknown when the LSI is designed.
The following description concerns only a difference from the structure of
It is assumed that Va and Vd are sufficiently high. Given that the threshold voltage of the third transistor Q3 and that of the fourth transistor Q4 are designated as Vth3 and Vth4, respectively, the system becomes stable in a state in which
Vb=Vd−Vth3
Vc=Vd−Vth4
Since it is possible to ensure that Vth3 and Vth4 are substantially equal, it naturally results that Vb=Vc so that the same advantage as available in the first embodiment is also available according to the second embodiment.
Indicating the voltage at the top end of the first resistor R1 as Va and the voltage at the top end of the second resistor R2 as Vb, the degree of on-state of the first transistor Q1 is controlled such that Va=Vb according to the operation of the operational amplifier 10. Accordingly, the drive current I2 is given by
I2=I1*R1/R2 (1)
By building the resistors with a high precision, high-precision control is enabled. Voltage adjustment using the operational amplifier 10 according to the third embodiment is useful since the power supply voltage of the load outside the LSI is unknown. A consideration of the precision of the resistor values will be given in the next embodiment.
In this structure, when the drive current I2 is greater than desired, the second fuse F2 or the fourth fuse F4, or both, are blown by laser trimming. When the drive current I2 is smaller than desired, the first fuse F1 or the third fuse F3, or both, are blown. In this way, the drive current I2 is generated with a high precision.
Not only the provision of an arrangement in which resistors are adjustable but also the improvement of the level of resistance value pairing of the first resistor R1 and the second resistor R2 are important.
The second resistor R2 is indicated as four discrete areas constituting a single wiring in a zig-zag manner. By providing the first resistor R1 and the second resistor R2 as zig-zag wirings, the characteristics thereof are matched with each other. Therefore, displacement of the resistance value of the first resistor R1 and that of the second resistor R2 occur, if ever, in the same direction so that a satisfactory level of resistor pairing is ensured. Thus, the drive current I2 close to the target value is produced. For similar reasons, the third resistor R3 and the fourth resistor R4 are provided in proximity to each other, and the fifth resistor R5 and the sixth resistor R6 are provided in proximity to each other.
Described above is an explanation based on the embodiments. The embodiment of the present invention is only illustrative in nature and it will be obvious to those skilled in the art that various variations in constituting elements are possible within the scope of the present invention. For example, the MOSFET transistors in the embodiments may be bipolar transistors.
According to the current drive circuit of the present invention, a drive current accurately proportional to a reference current is generated.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2003-409662 | Dec 2003 | JP | national |
