Semiconductor device with termination resistance adjusting circuit

A semiconductor device includes a transmitter, a termination resistance adjusting section, a transmitter control section and a control signal generating section. The transmitter has two output terminals and operates based on a control current. The termination resistance adjusting section is connected with the output terminals of the transmitter and applies a termination resistance adjusted in response to a control signal to each of the output terminals of the transmitter. The transmitter control section supplies the control current to the transmitter in response the control signal. The control signal generating section compares a first voltage corresponding to an external resistance and a second voltage corresponding to an internal resistance whose precision is lower than that of the external resistance, and outputs the control signal to the termination resistance adjusting section and the transmitter control section based on the comparing result.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, and more particularly relates to a semiconductor device with a termination resistance adjusting circuit for adjusting a termination resistor.

2. Description of the Related Art

A technique is conventionally known in which a termination resistor is built in a semiconductor device in order to attain cost-down of a transmission system and reduction of a substrate mounting area. When the termination resistor built in the semiconductor device is not adjusted, there may be a high possibility that the resistance of the termination resistor has a wide distribution due to manufacturing variation of the termination resistor. If the resistance of the termination resistor is out of a desired range, signal reflection is caused to deteriorate transmission quality, which results in the decrease in a production yield. Also, the resistance of the termination resistor is a factor to determine an output signal voltage of a transmitting circuit in the transmission system. Therefore, stabilization of both the resistance of the termination resistor and the output signal voltage is required.

Conventional techniques are known in Japanese Laid Open Patent Application (JP-P2003-298395A and JP-P2004-32721A: first and second conventional examples), in which a resistance adjusting circuit is built in a semiconductor device to operate a circuit adequately even if the resistance of a termination resistor is out of a desired range.

FIG. 1is a block diagram showing the configuration of a termination resistance adjusting circuit100in the first conventional example. In the first conventional example, a stable reference current Iref is generated based on reference voltages VrefH and VrefL and an external resistance119and is applied to a replica resistor130. Voltages Va and Vb generated at that time, and the reference voltages VrefH and VrefL are compared by a control voltage generator120. Thus, the control voltage generator120recognizes the difference between the replica resistor130and the external resistance119from the comparison result and carries out an adjustment so that the resistance of the replica resistor130and that of the external resistance119are coincident with each other.

Also,FIG. 2is a block diagram showing the configuration of an impedance variable circuit200in the second conventional example. In the impedance variable circuit shown inFIG. 2, a synthetic resistance of resistors (201to209) is varied by controlling switches (SW1to SW9) and is used as a termination resistor.

FIG. 3is a circuit diagram showing a specific circuit configuration of a termination resistance adjusting circuit300, which is constituted from the above-mentioned termination resistance adjusting circuit100and the impedance variable circuit200. As shown inFIG. 3, the termination resistance adjusting circuit300is composed of a termination resistor generator101, a transmitting circuit102, a first reference current generator104, a second reference current generator105and a termination resistance controller106. In the termination resistance adjusting circuit300, the first reference current generator104generates a stable reference current Iref1based on a reference voltage Vref and an external resistance109, and applies the reference current Iref1to a replica resistor130of the termination resistance controller106. Also, the second reference current generator105generates a stable reference current Iref4based on a reference voltage Vref and an internal resistance108, and applies the reference current Iref4to a replica resistor131of the termination resistance controller106. The termination resistance controller106compares a voltage V1generated based on the current Iref1and the resistor130and a voltage V2generated based on the current Iref4and the resistor131, and recognizes the difference between the internal resistance108and the external resistance109from the comparing result. Then, the termination resistance controller106outputs a control signal Vcont to the termination resistor generator101based on the comparing result. The termination resistor generator101generates a resistance through separation and synthesis of resistors in response to the control signal Vcont, such that a precision of the internal resistance is coincident with that of the external resistance.

When the termination resistor is assumed to be a resistor value R150and the reference current is assumed to be a reference current Iref2, a transmitting circuit output voltage Vo is represented by:
Vo=R150*Iref2   (1)

In the conventional termination resistance adjusting circuit300shown inFIG. 3, the voltages V1and V2are compared. The voltage V2is generated when the current Iref4is applied to the resistor131. The current Iref4is generated based on a reference voltage Vref and the internal resistance108. The reference voltage Vref is supplied from a band gap power supply circuit or the like, in which an output voltage variation caused due to external factors such as a temperature variation, a power source voltage variation and the like is small.

The voltage V1is generated when the current Iref1is applied to the resistor130. The current Iref1is generated based on the reference voltage Vref and the external resistance109which is more stable than the internal resistance in an absolute precision. The reference voltage Vref is supplied from the above-mentioned band gap power supply circuit or the like.

Here, when the internal resistances108and131are assumed to be R108and R131, respectively, and the external resistance109and the internal resistance130are similarly assumed to be R109and R130, respectively, the voltages V1and V2are represented by the following equations.
V2=(Vref/R108)*R131   (2)
V1=(Vref/R109)*R130   (3)
In this case, since the internal resistances131and108have the same structure, the relative precision is insured. Therefore, the item of “R131/R108” in the above equation (2) has a constant value. Thus, the voltage V2is the stable voltage similar to the reference voltage Vref.

Also, since the external resistance109has an extremely high precision as compared with the internal resistance, the item of “Vref/R109” in the above equation (3) can be regarded as a constant value. Thus, the voltage V1is a value proportional to the internal resistance130.FIG. 4shows the above relation. When the voltages V1and V2are compared, if the voltage V1is determined to be excessively higher than the voltage V2(namely, the internal resistance is excessively high), the control signal Vcont is outputted to adjust the resistance R150to a low value. Consequently, the precision of the termination resistor150after the adjustment is similar to that of the external resistance109.

However, the actual adjustment is carried out in a step manner of a definite range. Thus, even in the ideally adjusted state, the resistances before and after the adjustment are discontinuous, which brings about an error depending on the adjustment resolution of the termination resistor150. In particular, it could be understood that the maximum error (ERR) is generated in the vicinity of the adjustment.

Under the assumption that the termination resistor has been adjusted, a fixed current Iref2is generated based on the stable power voltage Vref and the external resistance109and applied to the transmitting circuit102. Thus, the transmitting circuit output voltage Vo is a function of the termination resistor150, as represented by the following equation (4).
Vo=Iref2*R150   (4)

FIG. 5is a diagram showing the waveform of an output signal outputted from the termination resistance adjusting circuit300. With reference to the waveform shown inFIG. 5, the voltage error +ERR, −ERR remains in the output signal waveform due to the adjustment error of the termination resistor150. The voltage error +ERR, −ERR sometimes causes poor measurement reproducibility or a large deviation of the output voltage of the transmitting circuit102.

In order to avoid these problems, a method is known in which the adjustment resolution is made higher. However, if such a method is employed, the higher precision of the termination resistance controller106is required, which leads to a larger circuit scale. Also, since the number of switching circuits in the termination resistor generator101is increased, a capacitive load becomes greater, which restricts a frequency band.

The termination resistance adjusting circuit is desired in which the adjustment error to the termination resistor has no influence on the output voltage, without the increase in the circuit scale and the limitation on the frequency band.

SUMMARY OF THE INVENTION

In an aspect of the present invention, a semiconductor device includes a transmitter, a termination resistance adjusting section, a transmitter control section and a control signal generating section. The transmitter has two output terminals and operates based on a control current. The termination resistance adjusting section is connected with the output terminals of the transmitter and applies a termination resistance adjusted in response to a control signal to each of the output terminals of the transmitter. The transmitter control section supplies the control current to the transmitter in response the control signal. The control signal generating section compares a first voltage corresponding to an external resistance and a second voltage corresponding to an internal resistance whose precision is lower than that of the external resistance, and outputs the control signal to the termination resistance adjusting section and the transmitter control section based on the comparing result.

Here, the control signal generating section may include a first reference current generating section which generates a first reference current corresponding to the external resistance, a second reference current generating section which generates a second reference current corresponding to the internal resistance, and a termination resistor control section. The termination resistor control section generates the first voltage and the second voltage based on the first reference current and the second reference current, respectively, and supplies the control signal to the termination resistance adjusting section and the transmitter control section by comparing the first and second voltages.

Also, the transmitter control section may include a reference resistance adjusting section configured to apply a reference resistance corresponding to the termination resistor in response to the control signal, and a control current generating section configured to generate and supply the control current corresponding to the reference resistance to the transmitter.

In this case, the reference resistance adjusting section may include a reference basic resistor, a reference adjustment resistor, and a reference switch circuit configured to connect the reference adjustment resistor with the reference basic resistor in parallel in response to the control signal. Also, the control current generating section may include an amplifier having a positive input connected with a first reference voltage, a first transistor having a gate connected with an output of the amplifier, and a source connected with the reference resistance adjusting section and a negative input of the amplifier, and a current mirror circuit connected with a drain of the first transistor, and to supply the control current as an output current to the transmitter.

Also, the termination resistance adjusting section may include a basic resistor provided for each of the output terminals of the transmitter, a adjustment resistor provided for each of the output terminals of the transmitter, and a switch circuit configured to connect the adjustment resistor with the basic resistor in parallel in response to the control signal.

Also, it is preferable that a ratio of a resistance of the reference basic resistor and a resistance of the reference adjustment resistor is equal to a ratio of a resistance of the basic resistor and a resistance of the adjustment resistor.

Also, the transmitter may include a differential transistor pair configured to drive the output terminals of the transmitter, and a constant current source connected with the differential transistor pair, and configured to supply the differential transistor pair with a constant current determined based on the control current. Instead, the transmitter may include a differential transistor pair configured to drive the output terminals of the transmitter. The control current is supplied as a constant current for the differential transistor pair.

In another aspect of the present invention, a method of adjusting a transmitter in a semiconductor device is achieved by generating a control signal based on an external resistance and an internal resistance built in the semiconductor device, wherein a precision of the external resistance higher than that of the internal resistance; by adjusting load resistances of differential transistor pair as termination resistors in response to the control signal; and by adjusting a constant current for the differential transistor pair in response to the control signal.

The generating a control signal may be achieved by generating a first reference current corresponding to the external resistance; by generating a second reference current corresponding to the internal resistance; by generating a first voltage and a second voltage based on the first reference current and the second reference current, respectively; and by comparing the first and second voltages to generate the control signal.

Also, the adjusting load resistances of differential transistor pair may be achieved by connecting an adjustment resistor with a basic resistor in parallel in response to the control signal.

Also, the adjusting a constant current may be achieved by adjusting a reference resistance corresponding to the termination resistors in response to the control signal; and by supplying the control current corresponding to the reference resistance to the differential transistor pair.

In this case, the adjusting a reference resistance may be achieved by connecting a reference adjustment resistor with a reference basic resistor in parallel in response to the control signal to generate the reference resistance.

Also, the supplying the control current may be achieved by controlling a current flowing through the reference resistance based on a reference voltage; and by supplying as the control current a current corresponding to the current flowing through the reference resistance to the differential transistor pair by a mirror circuit.

Also, a ratio of a resistance of the reference basic resistor and a resistance of the reference adjustment resistor is equal to a ratio of a resistance of the basic resistor and a resistance of the adjustment resistor.

In another aspect of the present invention, a semiconductor device includes a transmitter having a differential transistor pair, a control section configured to generate a control signal based on an external resistance and an internal resistance built in the semiconductor device, wherein a precision of the external resistance higher than that of the internal resistance; a termination resistance adjusting section configured to adjust load resistances of the differential transistor pair as termination resistors in response to the control signal; and a transmitter control section configured to control a constant current for the differential transistor pair in response to the control signal.

Here, the transmitter control section may include a reference resistance adjusting section configured to adjust a reference resistance corresponding to the termination resistors in response to the control signal; and a supplying section configured to supply the control current corresponding to the reference resistance to the differential transistor pair.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a semiconductor device with a termination resistance adjusting circuit of the present invention will be described in detail with reference to the attached drawings.

FIG. 6is a circuit diagram showing the configuration of the termination resistance adjusting circuit10in the embodiment of the present invention. As shown inFIG. 6, the termination resistance adjusting circuit10is provided with a termination resistor generator1, a reference current corrector3, a first reference current generator4, a second reference current generator5and a termination resistance controller6. The reference current corrector3includes a reference resistance generator7and a reference current generator8.

The first reference current generator4generates a reference current Iref1based on a reference voltage Vref and an external reference resistor19. The first reference current generator4supplies the generated reference current Iref1to the termination resistance controller6. Similarly, the second reference current generator5generates a current Iref4based on the reference voltage Vref and an internal reference resistor18. The second reference current generator5supplies the generated reference current Iref4to the termination resistance controller6, as well as to the first reference current generator4. The termination resistance controller6applies the reference current Iref1to an internal resistance20. Consequently, a voltage of a node N1becomes a voltage V1, and the voltage V1is applied to a control signal generating circuit61. Similarly, the termination resistance controller6applies the reference current Iref4to an internal resistance21. Consequently, a voltage of a node N2becomes a voltage V2, and the voltage V2is applied to the control signal generating circuit61. The control signal generating circuit61compares the voltages V1and V2. The termination resistance controller6outputs the comparison result as a control signal Vcont to the termination resistor generator1and the reference current corrector3. The termination resistor generator1receives the control signal Vcont and adjusts a termination resistor11and a termination resistor14. In the following explanation, since the configuration and operation of the termination resistor14are similar to those of the termination resistor11, the termination resistor11is mainly explained.

The reference resistance generator7receives the control signal Vcont from the termination resistance controller6, adjusts the resistance of a reference resistor71to a resistance R51. The reference current generator8generates a reference current Iref3, which is obtained by applying a reference voltage Vref2to the adjusted resistor value R51, and supplies the reference current Iref3to the transmitting circuit2. The reference resistance generator7includes a reference resistor71, and the reference resistor71is composed of a basic resistor73and an adjustment resistor72to be connected in parallel to the basic resistor73. As shown inFIG. 6, the adjustment resistor72is connected to a switch circuit S1. The switch circuit S1is operated in response to the control signal Vcont outputted from the termination resistance controller6. Preferably, the switch circuit S1, basic resistor73and adjustment resistor72, which constitute the reference resistance generator7, have the same structures as those constituting the termination resistor generator1, so as to be matched with the termination resistor generator1.

The reference current generator8includes an operational amplifier OP1, a transistor MP1, a transistor MN1and a transistor MN2. As shown inFIG. 6, a gate of the transistor MP1is connected to an output end of the operational amplifier OP1. A source of the transistor MP1is connected to the reference resistance generator7. Also, the source of the transistor MP1is connected to a negative feedback input terminal of the operational amplifier OP1. The transistor MN1and the transistor MN2constitute a current mirror circuit. A drain of the transistor MP1is connected to the gate and drain of the transistor MN1. In the reference current generator8, the output of this current mirror acts as an output terminal.

FIG. 7is a circuit diagram showing the configuration of the transmitting circuit2in the embodiment of the present invention. As shown inFIG. 7, the transmitting circuit2is composed of a differential transistor pair. The adjusted termination resistors14and11function as load resistances of the differential transistor pair. In this embodiment, a constant current for the differential transistor pair is supplied as the reference current Iref3from the current mirror circuit of the reference current generator8. Although not showing, a constant current source may be separately provided and may supply a constant current to the differential transistor pair based on the reference current Iref3. When the resistance of the termination resistor11is assumed to be a resistance R50and the reference current Iref3is used, a transmission output level Vo of the transmitting circuit2is represented by:
Vo=R50*Iref3
fromFIG. 9.

The first reference current generator4has a stabilized band gap power supply or the like, and generates the reference current Iref1by applying a voltage Vref from the stabilized power supply to the external internal resistance R19. Also, the second reference current generator5has a stabilized band gap power supply similar to that of the first reference current generator4, and generates the reference current Iref4by applying the voltage Vref from the stabilized band gap power supply to the internal resistance18which is expected to have the same value as the external resistance R19. Thus, the reference current Iref1and the reference current Iref4can be represented by the following equations:
Iref1=Vref/R19
Iref4=Vref/R18

The generated reference current Iref1is applied to the internal resistance20, and the reference current Iref4is applied to the internal resistance21. The termination resistance controller6compares the voltage V1generated by the internal resistance20and the voltage V2generated by the internal resistance21. The voltages V1and V2can be represented by the following equations:
V1=Iref1*R20=Vref/R19*R20  (5)
V2=Iref4*R21=Vref/R18*R21  (6)

The termination resistor generator1receives the control signal Vcont outputted from the termination resistance controller6and drives the switch circuit. Since the switch circuit is turned on, the basic resistor13and an adjustment resistor12are connected in parallel. When the resistance of the adjustment resistor12is assumed to be a resistance R12and when the resistance of the basic resistor13is assumed to be a resistance R13, its synthesized resistance R50is changed as follows:
R50=R13*R12/(R13+R12)  (7)

Here, the reference resistance generator7similarly receives the control signal Vcont outputted from the termination resistance controller6and drives the switch circuit. Since the switch circuit is turned on, the basic resistor73and the adjustment resistor72are connected in parallel. When the resistance of the basic resistor73is assumed to be a resistor value R73and when the resistance of the adjustment resistor72is assumed to be a resistor value R72, its synthesized resistor value R51is changed as follows:
R51=R73*R72/(R73+R72)  (8)

The reference current generator8applies the reference voltage Vref2to the reference resistor value R51provided by the reference resistance generator7and generates the reference current Iref3. The reference current generator8supplies the current Iref3to the transmitting circuit. If the reference resistor value R51is adjusted, the reference current Iref3is also corrected at the same time.FIG. 8is a diagram showing an operational of the termination resistance adjusting circuit10in this embodiment. As shown inFIG. 8, the operation waveform of the reference current Iref3has a relation between the reference resistor R51and the termination resistor R50, and it can be represented by the following equation:
Iref3=Vref2/R51
For this reason, the output signal voltage Vo of the transmitting circuit2is represented by a product of the reference current Iref3and the resistance R50of the termination resistor11, and it is represented by the following equation:
Vo=Iref3*R50=Vref2*R50/R51  (9)

At this time, the termination resistor generator1and the reference resistance generator7are adjusted at the same time. As a result, the ratio between the resistance R12of the adjustment resistor12and the resistance R13of the basic resistor13, and the resistance R73of the basic resistor73and the resistance R72of the adjustment resistor72is set as follows:
R13:R12=R73:R72
Since the respective resistances are set as mentioned above, the following relation is always established between the synthesized resistance R50expressed by the equation (7) and the synthesized resistance R51expressed by the equation (8):
R50∝R51
At this time, the item of “R50/R51” in the equation (9) becomes a constant value in the meaning. Thus, the output voltage can be represented as a function that does not contain a term depending on a resistance, as follows:
Vo∝Vref2

FIG. 9is a diagram showing the output waveform of the transmitting circuit when the termination resistance adjusting circuit in the embodiment of the present invention is employed. With reference toFIG. 9, the output signal voltage Vo is outputted as a constant value. This implies that the constant output signal voltage Vo is obtained irrespectively of the controlled state of the internal resistance R51.

As described above, according to the present invention, it is possible to design the termination resistance adjusting circuit in which the adjustment error due to the resistance of the termination resistor has no influence on the output voltage.