Semiconductor memory device having internal circuit screening function

A semiconductor memory device includes an internal voltage generation circuit controlling an internal voltage supplied to an internal circuit in accordance with a reference voltage, a reference voltage generation circuit generating the reference voltage, a plurality of signal terminals for transmitting and receiving a signal to and from an outside of the semiconductor memory device, and a reference voltage change indication circuit for indicating a change of the reference voltage on the basis of a binary input signal to each of the signal terminals with respect to the reference voltage generation circuit during a test.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device and particularly relates to an internal voltage generation circuit capable of adjusting internal voltage during a test.

2. Description of the Background Art

Generally, periods in which failures occur to a semiconductor memory device are roughly divided into three periods, which periods are also referred to as a an initial failure period, a chance failure period and a wear-out failure period in the order of time.

In the initial failure period, a defect at the time of the manufacture of a semiconductor memory device appears as a failure. The initial failure period is a period in which an initial failure occurs right after starting the use of the semiconductor. The rate of this initial failure sharply decreases with the passage of time. The initial failure period is followed by the chance failure period in which a low failure rate continuous for a certain period of time. With time, the life of the semiconductor memory device nears the useful life thereof and the semiconductor memory device enters the wear-out failure period in which the failure rate suddenly increases. If the operation reliability of the semiconductor memory device while being in use is considered, it is necessary to use the semiconductor memory device within the chance failure period. Namely, it is necessary to remove semiconductor memories to which initial failures occur before shipment. To this end, semiconductor memories are subjected to accelerated operation aging for a certain period of time and to screening for removing defects having initial failures.

To perform efficient screening, it is necessary to discover an initial failure in short time. Generally, a screening method for raising internal voltage which is used as operating power supply voltage in semiconductor memory device from voltage in normal operation, applying high field stress to the memory and thereby screening semiconductor memories is used.

FIG. 8 is a conceptual view of a conventional internal voltage generation circuit 20 which generates internal voltage applied to the internal circuit of a semiconductor memory device.

Referring to FIG. 8 , internal voltage generation circuit 20 includes reference voltage generation circuits 300 a to 300 c which generate reference voltages REF 1 to REF 3 , respectively, and internal voltage generation units 400 a to 400 c which receive corresponding to reference voltages REF 1 to REF 3 , and generate internal voltages V 1 to V 3 respectively.

FIG. 9 is a circuit block diagram of reference voltage generation circuit 300 a generating reference voltage REF 1 . Since reference voltage generation circuits 300 a to 300 c are equal in configuration, the configuration of reference voltage generation circuit 300 a will be typically explained herein.

Referring to FIG. 9 , reference voltage generation circuit 300 a includes a current mirror amplifier 310 , a starting circuit 320 which operates at startup, a constant current circuit 330 which generates a constant current, a tuning circuit 340 and a reference voltage setting circuit 350 .

Reference voltage setting circuit 350 sets the voltage level of an internal node to be described later. Current mirror amplifier 310 generates a reference voltage in accordance with the voltage level of this internal node. Tuning circuit 340 and constant current circuit 330 are used to adjust the voltage level of the internal node. Constant current circuit 330 supplies a constant current to reference voltage setting circuit 350 , and tuning circuit 340 adjusts a resistance element to be described later and tunes the voltage level of the internal node. Starting circuit 320 indicates the activation of constant current circuit 330 when the power of the semiconductor memory device is turned on.

Current mirror amplifier 310 includes P-channel MOS transistors 311 and 312 , and N-channel MOS transistors 313 to 315 . P-channel MOS transistor 311 and N-channel MOS transistor 313 are connected in series between a power supply voltage VCC and a node N 1 through a node N 2 and the gates of P-channel MOS transistors 311 and N-channel MOS transistor 313 are connected to node N 2 and an internal node N 6 , respectively. P-channel MOS transistor 312 and N-channel MOS transistor 314 are connected in series between power supply voltage VCC and node N 1 through a node N 0 and the gates of P-channel MOS transistors 312 and N-channel MOS transistor 314 are connected to node N 2 and node N 0 , respectively. Further, N-channel MOS transistor 315 is connected between node N 1 and a ground voltage GND and the gate thereof is connected to a node N 4 .

By such a current mirror structure, current mirror amplifier 310 sets reference voltage REF 1 generated at node N 0 at the voltage level of voltage Vn 6 of internal node N 6 connected to the gate of N-channel MOS transistor 313 .

Starting circuit 320 includes a P-channel MOS transistor 321 and an N-channel MOS transistor 322 .

P-channel MOS transistors 321 and N-channel MOS transistor 322 are connected between power supply voltage VCC and ground voltage GND through a node N 3 and the gates of P-channel MOS transistors 321 and N-channel MOS transistor 322 are connected to ground voltage GND and a node N 4 , respectively.

At startup, starting circuit 320 raises the voltage level of node N 3 in response to the rise of power supply voltage VCC. Following this, an N-channel MOS transistor 323 which is provided in constant current circuit 330 becomes conductive, nodes N 4 and N 5 are electrically connected to each other and constant current circuit 330 is activated. It is noted that starting circuit 320 turns N-channel MOS transistor 323 into a nonconductive state after the passage of a predetermined period. This is because the voltage level of node N 3 decreases if N-channel MOS transistor 322 is conductive.

P-channel MOS transistors 331 and N-channel MOS transistor 334 are connected in series between power supply voltage VCC and ground voltage GND through node N 5 and the gates of P-channel MOS transistors 331 and N-channel MOS transistor 334 are connected to nodes N 5 and N 4 , respectively. Resistance 332 , P-channel MOS transistors 333 and N-channel MOS transistor 335 are connected in series between power supply voltage VCC and ground voltage GND through node N 4 and the gates of P-channel MOS transistors 333 and N-channel MOS transistor 335 are connected to nodes N 5 and N 4 , respectively.

N-channel MOS transistor 323 is connected between nodes N 4 and N 5 and the gate thereof is connected to node N 3 . N-channel MOS transistors 334 and 335 constitute a current mirror circuit. If N-channel MOS transistors 334 and 335 have high channel resistances, the same current is carried to P-channel MOS transistors 331 and 333 by N-channel MOS transistors 334 and 335 which constitute a current mirror circuit.

Reference voltage setting circuit 350 includes P-channel MOS transistors 302 and 351 to 361 , and an inverter 362 .

P-channel MOS transistor 302 is connected between power supply voltage VCC and internal node N 6 and the gate thereof is connected to node N 5 . P-channel MOS transistors 351 to 357 are connected in series between internal node N 6 and ground voltage GND and the gates thereof are connected to ground voltage GND. P-channel MOS transistors 358 to 361 are provided as transistor switches so as to short-circuit P-channel MOS transistors 352 to 355 , respectively (which P-channel MOS transistors 358 to 361 will be also referred to as transistor switches hereinafter), and the gates thereof receive the input of tuning circuit 340 . The gate of P-channel MOS transistor 361 receives a signal input inverted from the output signal of tuning circuit 340 by inverter 362 .

P-channel MOS transistor 302 has the same size (same ratio of channel width to channel length) as that of P-channel MOS transistor 331 . A constant current Ict which is the same in magnitude as a current carried to P-channel MOS transistor 331 , is carried to this P-channel MOS transistor 302 .

The channel resistances of P-channel MOS transistors 351 to 357 causes voltage drop due to their resistance components. It is assumed herein that the channel resistances of P-channel MOS transistors 358 to 361 are sufficiently lower than those of P-channel MOS transistors 351 to 357 .

Therefore, if a combined channel resistance of P-channel MOS transistors 351 to 357 is assumed as Rc, a constant voltage Vn 6 generated at internal node N 6 is expressed by the following equation.

Accordingly, constant voltage Vn 6 can be adjusted by selectively setting transistor switches 358 to 361 and changing combined channel resistance Rc. As already described, the conductive states of P-channel MOS transistors 358 to 361 can be selectively set by tuning circuit 340 .

FIG. 10 is a circuit block diagram of tuning circuit 340 .

Referring to FIG. 10 , tuning circuit 340 includes tuning units 344 a to 344 d which are provided to correspond to P-channel MOS transistors 358 to 361 , respectively.

Since tuning units 344 a to 344 d are equal in configuration, tuning unit 344 a will be typically described herein.

Tuning unit 344 a includes a fuse element 343 a which serves as a program element, an N-channel MOS transistor 341 a , and an inverter 342 a . Fuse element 343 a and N-channel MOS transistor 341 a are connected in series between power supply voltage VCC and ground voltage GND through a connection node Nh, and the gate of N-channel MOS transistor 341 a is connected to node N 4 . In addition, inverter 342 a inverts the signal transmitted to connection node Nh and transmits the inverted signal to the gate of P-channel MOS transistor 358 .

Fuse element 343 a is blown in response to the incidence of a laser beam applied from the outside of the memory and the state of fuse element 343 a changes from a conductive state to a nonconductive state. As a result, tuning unit 344 a changes the state of P-channel MOS transistor 358 from a conductive state to a nonconductive state when the fuse is blown. The same thing is true for remaining tuning units 344 b to 344 d.

Referring back to FIG. 9 , a case where the channel resistance ratio of P-channel MOS transistors 352 to 355 is, for example, 1:2:4:8, will be considered.

In an initial state, transistor switches 358 to 360 are conductive and transistor switch 361 is nonconductive. Accordingly, P-channel MOS transistor 355 functions as a resistance element.

In this state, combined channel resistance Rc can be adjusted and constant voltage Vn 6 can be raised or lowered in accordance with the tuning of tuning circuit 340 based on a predetermined combination of tuning units. It is, therefore, possible to correct the deviation of a target level which has been set in a design phase by conducting tuning to thereby blow fuse elements after designing the memory.

For example, in tuning circuit 340 , if fuse element 343 a is blown, P-channel MOS transistor 358 becomes nonconductive and P-channel MOS transistor 352 functions as a resistance element. As a result, combined channel resistance Rc increases and constant current Vn 6 rises. Accordingly, the voltage level of the reference voltage in an initial phase is corrected to follow the target level of the reference voltage by tuning.

Reference voltage generation circuit 300 a also includes an N-channel MOS transistor 301 and a DQM terminal as an external terminal, both of which are used during a test.

N-channel MOS transistor 301 is connected between DQM terminal and node N 0 and the gate thereof receives a test mode signal TM which is activated to H level during a test. Namely, during a test, by inputting test mode signal TM, N-channel MOS transistor 301 can be turned into a conductive state and reference voltage REF 1 can be inputted into transistor 301 directly from the outside of the memory using DQM terminal.

By adopting such a configuration, it is possible to directly input the reference voltage from the outside during a test, so that the internal voltage can be set at arbitrary level and a screening test can be easily executed. Further, the setting of the internal voltage during the test can be facilitated.

Nevertheless, as shown in three types of internal voltages V 1 to V 3 in FIG. 8 , a semiconductor memory device is provided with a plurality of levels of internal voltages to correspond to various internal circuits, respectively. Therefore, it is necessary to provide many DQM terminals to input the reference voltage (REF 1 in FIG. 9 ) so as to conduct a screening test in the configuration shown in FIGS. 9 and 10 .

As already described, since it is necessary to fixedly input a constant voltage for a test into each DQM terminals as an external terminal, the DQM terminal cannot be used to input/output the other test signals. Because of the limited number of terminals, therefore, it is difficult to minutely adjust all the internal voltages during a test based on the configuration shown in FIG. 9 .

Furthermore, since such a screening test is intended to accelerate the defect of an internal circuit, it is considered to suffice that the internal voltage can be slightly raised or lowered from the reference voltage which is set.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor memory device capable of performing a screening test to internal circuits without directly inputting reference voltages from an outside of the semiconductor memory device and without increasing the number of external terminals during the test.

According to one aspect of the present invention, a semiconductor memory device includes: an internal voltage generation circuit; a reference voltage generation circuit; a plurality of signal terminals; and a reference voltage change indication circuit.

The internal voltage generation circuit controls an internal voltage supplied to an internal circuit in accordance with a reference voltage. The reference voltage generation circuit generates the reference voltage. The plurality of signal terminals transmit and receive a signal to and from an outside of the semiconductor memory device.

During a test, the reference voltage change indication circuit indicates a change of the reference voltage on the basis of a binary input signal to each of the signal terminals with respect to the reference voltage generation circuit.

Therefore, a main advantage of the present invention is to indicate a change of a reference voltage on the basis of a binary input signal to signal terminals during a test. Accordingly, it is possible to adjust an internal voltage without necessity for directly setting a level of a reference voltage with test dedicated external terminals and without increasing the number of the external terminals to efficiently perform a screening test.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A embodiment of the present invention will be described hereinafter in detail with reference to the drawings. It is noted that the same or corresponding constituent elements are denoted by the same reference symbols in the drawings and will not be repeatedly described.

Referring to FIG. 1 , a semiconductor memory device 1 executes random access in response to external control signals and address signals A 0 to An (where n is a natural number), and executes the input/output of data DQ. The control signals include a clock signal CLK, a write enable signal WE which is a write permission signal, a column address strobe signal CAS for reading an address in a column direction at appropriate timing, a row address strobe signal RAS for reading an address in a row direction at appropriate timing, and a chip select signal CS for selecting a chip.

Semiconductor memory device 1 includes a control circuit 100 which controls overall semiconductor memory device 1 in response to the control signals and the like, a memory array 6 which includes a plurality of memory cells arranged in a matrix, a control terminal 7 which receives the input of the control signals, an address terminal 8 which receives the input of address signals A 0 to An, and a data terminal 9 which is the input/output terminal of data DQ.

Semiconductor memory device 1 also includes a row/column address buffer 2 , a row select circuit 4 and a column select circuit 5 .

Row/column address buffer 2 receives address signals A 0 to An and generate a row address RA and column address CA. Row select circuit 4 executes the selection of a row in memory array 6 in response to row address RA. Column select circuit 5 executes the selection of a column in memory array 6 in response to column address CA.

Semiconductor memory device 1 further includes a data input/output circuit 3 and an internal voltage generation circuit 1000 .

Referring to FIG. 2 , control circuit 100 outputs control signal and test mode signal TM based on input according to a predetermined combination of the control signals (clock signal CLK, write enable signal WE, column address strobe signal CAS, row address strobe signal RAS and chip select signal CS) and address signal A 0 .

Internal voltage generation circuit 1000 includes voltage level change indication circuits 210 a to 210 c which indicates the rise or fall of reference voltages REF 1 to REF 3 , respectively, based on address signals A 1 to A 15 and control signal o, reference voltage generation circuits 300 a to 300 c which generate reference voltages REF 1 to REF 3 , respectively, and internal voltage generation units 400 a to 400 c which generate internal voltages V 1 to V 3 in accordance with reference voltage REF 1 to REF 3 , respectively.

Referring to FIG. 3 , control circuit 100 generates control signal and test mode signal TM for indicating the internal operation of internal voltage generation circuit 1000 during a test in accordance with a predetermined combination of the control signals.

Control circuit 100 includes NAND circuits 101 and 105 , NOR circuits 102 and 104 , inverters 103 and 107 to 109 , and a transfer gate 106 . NAND circuit 101 receives the input of write enable signal WE, row address strobe signal RAS and column address strobe signal CAS, and outputs a NAND logic operation result for the both signals to NOR circuit 102 . NAND circuit 105 receives the input of chip select signal CS and clock signal CLK, and outputs a NAND logic operation result for the both signals to NOR circuit 102 .

Transfer gate 106 transmits address signal A 0 to a node NN 1 in response to the output signal of NOR circuit 102 . Inverter 108 inverts the signal transmitted to a node NN 1 and transmits the inverted signal to node NN 2 . Inverter 107 inverts the signal transmitted to a node NN 2 and transmits the inverted signal to node NN 1 . Therefore, inverters 107 and 108 form a latch circuit. It is noted that inverter 107 is inferior to inverter 108 in driving capability.

Inverter 109 outputs, as test mode signal TM, the inverted signal of the signal transmitted to node NN 2 . In addition, NOR circuit 104 outputs a NOR logic operation result as control signal based on the inverted output signal of NOR circuit 102 through inverter 103 and the signal transmitted to node NN 2 .

For example, control signal and test mode signal TM are set to become H level when the control signals (WE, RAS, CAS, CS and CLK) and address signal A 0 are all at H level. Otherwise, control signal and test mode signal TM are both set at L level.

Since voltage level change indication circuits 210 a to 210 c are equal in configuration, voltage level change indication circuit 210 a will be typically described. Referring to FIG. 4 , voltage level change indication circuit 210 a generates an up indication signal UP indicating the rise of the voltage level of the reference voltage or a down indication signal DN indicating the fall of the voltage level of the reference voltage during a test based on address signals A 1 to A 5 and control signal .

Referring to FIG. 4 , voltage level change indication circuit 210 a includes NAND circuits 211 , 213 and 216 , NOR circuits 212 and 214 , and an inverter 215 .

NAND circuit 211 receives the input of address signals A 1 to A 3 , and outputs a NAND logic operation result to one of the input sides of NOR circuit 212 . NAND circuit 213 receives the input of address signals A 4 and A 5 and control signal , and outputs a NAND logic operation result to the input sides of both of NOR circuits 212 and 214 . NAND circuit 216 receives the input of the inverted signal of address signal A 1 inputted through inverter 215 and the input of address signals A 2 and A 3 , and outputs a NAND logic operation result to one of the input sides of NOR circuit 214 . NOR circuit 212 outputs a NOR logic operation result based on the input of NAND circuits 211 and 213 as up indication signal UP. NOR circuit 214 outputs a NOR logic operation result based on the input of NAND circuits 213 and 216 as down indication signal DN.

Voltage level change indication circuit 210 a sets one of up indication signal UP and down indication signal DN at H level based on a predetermined combination of address signals A 1 to A 5 . It is noted that control signal is an activation signal for activating voltage level change indication circuit 210 a . That is, when control signal is at L level, both up indication signal UP and down indication signal DN are at L level irrespectively of the combination of address signals A 1 to A 5 . Therefore, during operations other than a test, voltage level change indication circuit 210 a does not indicate the rise or fall of the voltage level of the reference voltage.

For example, when address signals A 1 to A 5 and control signal are all at H level, up indication signal UP is set at H level. In response to this, the reference voltage during a test rises. When only address signal A 1 is at L level and the other signals are all set at H level, down indication signal DN is set at H level. In response to this, the reference voltage falls during a test.

Since the same thing is true for remaining voltage level change indication circuits 210 b and 210 c , they will not be repeatedly described herein in detail. Voltage level change indication circuit 210 b generates up indication signal UP and down indication signal DN in accordance with a predetermined combination of address signals A 6 to A 10 . In addition, voltage level change indication circuit 210 c generates up indication signal UP and down indication signal DN in accordance with a predetermined combination of address signals A 11 to A 15 .

Referring to FIG. 5 , while reference voltage generation circuit 300 a will be typically described, reference voltage generation circuits 300 b and 300 c are equal in configuration to reference voltage generation circuit 300 a.

Referring to FIG. 5 , reference voltage generation circuit 300 a differs from reference voltage generation circuit 300 a in the conventional art shown in FIG. 9 in that a counter section 390 is further provided and that reference voltage setting circuit 350 is replaced by a reference voltage setting circuit 380 .

Reference voltage setting circuit 380 differs from reference voltage setting circuit 350 in that a test voltage setting circuit 370 which sets the level of the reference voltage during a test is further provided. Since the remaining constituent circuits are the same as those described with reference to FIG. 9 , they will not be repeatedly described herein in detail.

Test voltage setting circuit 370 is intended to raise or lower constant voltage Vn 6 in a normal state step by step.

Test voltage setting circuit 370 includes P-channel MOS transistors 371 to 378 and an inverter 379 .

P-channel MOS transistors 371 to 374 are connected in series between internal node N 6 and P-channel MOS transistor 352 and the respective gates of P-channel MOS transistors 371 to 374 are connected to ground voltage GND. Therefore, P-channel MOS transistors 371 to 374 function as resistance elements as in the case of P-channel MOS transistors 353 to 356 described above.

Further, P-channel MOS transistors 375 to 378 are provided to correspond to P-channel MOS-transistors 371 to 374 , and to function as transistor switches which short-circuit corresponding P-channel MOS transistors, respectively. The gate of each of P-channel MOS transistors 375 to 378 is controlled by input from counter section 390 . It is noted, however, the gate of P-channel MOS transistor 378 receives the input of the inverted signal of the output signal of counter section 390 through inverter 379 .

It is assumed that signals inputted from counter section 390 are all at L level in normal operation. Following this, it is set that only P-channel MOS transistor 374 functions as a resistance element in normal operation. If so setting, during a test, as in the case of the tuning operation of tuning circuit 340 described above, the combined channel resistance of P-channel MOS transistors 371 to 374 which function as resistance elements is adjusted by counter section 390 and constant voltage Vn 6 which is at the voltage level of internal node N 6 is set. According to the present invention, therefore, it is possible to further increase or decrease the voltage level of constant voltage Vn 6 employed in the normal operation, during a test.

For example, it is assumed that the channel resistance ratio of P-channel MOS transistors 371 to 374 is 1:2:4:8. It is also assumed that the combined channel resistance of the channel resistances of test voltage setting circuit 370 is Rd. In an initial state, it is assumed that P-channel MOS transistors 375 to 377 are conductive and that P-channel MOS transistor 378 is nonconductive. Only P-channel MOS transistor 374 functions as a resistance element.

Accordingly, if P-channel MOS transistor 378 is made conductive, the value of combined resistance Rd falls from the initial state. Conversely, if P-channel MOS transistor 375 is made nonconductive, the value of combined resistance Rd rises from the initial state. By allowing counter section 390 to selectively make P-channel MOS transistors 375 to 378 conductive or nonconductive, it is possible to increase or decrease constant voltage Vn 6 during a test step by step.

It has been described in connection with reference voltage generation circuit 300 a shown in FIG. 9 that tuning circuit 340 adjusts combined channel resistance Rc of P-channel MOS transistors 352 to 355 and thereby corrects the deviation of the target level of the reference voltage set in a design phase. According to the present invention, in an initial state, since P-channel MOS transistor 374 in test voltage setting circuit 370 functions as a resistance element in the normal operation, tuning circuit 340 adjusts combined channel resistance Rc to which the channel resistance of P-channel MOS transistor 374 is further added, and thereby executes tuning for correcting the deviation of the reference voltage set in a design phase.

Counter section 390 includes counters 500 a to 500 d.

Counter section 390 executes the tuning of test voltage setting circuit 370 in accordance with the input of up indication signal UP, down indication signal DN and test mode signal TM.

Each of counters 500 a to 500 d receives the input of up indication signal UP, down indication signal DN, test mode signal TM and a counter input signal CIN, and generates an output signal OUT and a counter output signal COUT.

In addition, output signals OUT of counters 500 a to 500 d are transmitted to the gates of P-channel MOS transistors 375 to 378 serving as transistor switches, respectively.

Further, counter output signal COUT of counter 500 a is inputted into next counter 500 b as a counter input signal. Likewise, counter output signals COUT of counters 500 b and 500 c are inputted into next counters as counter input signals CIN, respectively. It is noted that power supply voltage VCC, i.e., H level voltage is always inputted as counter input signal CIN of counter 500 a.

Since counters 500 a to 500 d are equal in configuration, counter 500 a will be typically described.

FIG. 6 is a circuit block diagram of counter 500 a.

Gate circuit 540 outputs one of the signals transmitted to nodes N 11 and N 12 to latch control circuit 510 in response to counter input signal CIN. Gate circuit 540 includes transfer gates 541 and 542 , and an inverter 543 .

The gates of transfer gates 541 and 542 receive counter input signal CIN and an inverted signal through inverter 543 and are complementarily turned on/off.

If counter input signal CIN is, for example, at H level, the signal transmitted to node N 12 is outputted to latch control circuit 510 . If counter input signal CIN is at L level, the signal transmitted to node N 11 is outputted to latch control circuit 510 .

NOR circuit 501 receives the input of up indication signal UP and down indication signal DN, outputs a NOR logic operation result and thereby activates latch control circuit 510 .

Latch control circuit 510 latches the signal outputted from gate circuit 540 in response to the logic operation result of NOR circuit 501 . Latch control circuit 510 includes transfer gates 512 and 515 , and inverters 511 , 513 and 514 .

Transfer gates 512 and 515 receive an output signal from NOR circuit 501 and the inverted signal thereof through inverter 511 and are complementarily turned on/off. Transfer gate 512 transmits the signal outputted from gate circuit 540 to a node N 13 in response to a NOR logic operation result. Inverter 513 inverts the signal transmitted to node N 13 and transmits the inverted signal to a node N 14 , and inverter 514 inverts the signal transmitted to node N 14 and transmits the inverted signal to node N 13 . Therefore, two inverters 513 and 514 form a latch circuit. Transfer gate 515 transmits the signal transmitted to node N 14 , to a node N 10 in response to a NOR logic operation result. It is noted that inverter 514 is inferior to inverter 513 in driving capability. That is, latch control circuit 510 latches the output signal from gate circuit 540 in response to the rise of one of up indication signal UP and down indication signal DN, and outputs the latched signal in response to the fall thereof.

Inverter 504 inverts the signal transmitted to node N 10 and transmits the inverted signal to node N 11 , and inverter 505 inverts the signal transmitted to node N 11 and transmits the inverted signal to node N 10 . Therefore, two inverters 504 and 505 form a latch circuit. Inverter 508 inverts the signal transmitted to node N 11 and transmits the inverted signal to node N 12 as output signal OUT. It is noted that inverter 505 is inferior to inverter 504 in driving capability.

Gate circuit 520 a includes a transfer gate 522 a and an inverter 521 a , and transmits the signal transmitted to node N 11 to a node N 15 which is one of the input sides of NOR circuit 506 , in response to up indication signal UP. Gate circuit 520 b includes a transfer gate 522 b and an inverter 521 b , and transmits the signal transmitted to node N 12 to node N 15 which is one of the input sides of NOR circuit 506 in response to down indication signal DN.

P-channel MOS transistors 530 and 531 are connected in series between power supply voltage VCC and node N 15 and the gates of P-channel MOS transistors 530 and 531 receive up indication signal UP and down indication signal DN, respectively. Therefore, since up indication signal UP and down indication signal DN are both at L level in operations other than test operation, node N 15 is always set at H level by power supply voltage VCC. Accordingly, counter output signal COUT outputted from NOR circuit 506 is set at L level in an initial state.

N-channel MOS transistors 502 and 503 are connected between ground voltage GND and node N 14 and between ground voltage GND and node N 10 , respectively, the respective gates of N-channel MOS transistors 502 and 503 receive the input of test mode signal TM through inverter 507 . Namely, when test mode signal TM is at L level, i.e., in the initial state, nodes N 14 and N 10 are fixed to L level. Output signal OUT is, therefore, set at L level.

By way of example, the operation of counter section 390 if the level of reference voltage REF 1 is raise by one step (which will be also referred to as level 1 ) during a test, i.e., when test mode signal TM is at H level, will be described. In the initial state, output signals OUT of counters 500 a to 500 d are all at L level.

In counter 500 a , if up indication signal UP is inputted, then transfer gate 542 becomes conductive, the signal transmitted to node N 12 is inputted into and latched by latch control circuit 510 . That is, in the initial state, the voltage level of node Nil is H level and that of node N 12 is L level. The voltage level of node N 14 is, therefore, latched to H level.

In addition, in counter 500 a , if up indication signal UP is inputted, then gate circuit 520 a becomes active, the voltage signal transmitted to node N 11 is inputted into NOR circuit 506 and the level of counter output signal COUT which indicates the NOR logic operation result becomes L level. Accordingly, in counter 500 b , since counter input signal CIN is at L level, transfer gate 541 included in gate circuit 540 becomes conductive and node N 14 is latched to L level by latch control circuit 510 .

Moreover, the level of counter output signal COUT which indicates the NOR logic operation result of NOR circuit 506 becomes L level.

Likewise, as for counters 500 c and 500 d , the level of each counter output signal COUT becomes L level and node N 14 is latched to L level in each latch control circuit 510 .

Next, counter 500 a transmits the signal latched by latch control circuit 510 , to node N 12 in response to the fall of up indication signal UP. That is, output signal OUT is set at H level. As for counters 500 b to 500 d , the signal latched by each latch circuit 510 is transmitted to node N 12 and each output signal OUT is set at L level.

Referring back to FIG. 5 , as described by way of example, if the channel resistance ratio of P-channel MOS transistors 371 to 374 is assumed as 1:2:4:8, then P-channel MOS transistors 375 and 378 become nonconductive and P-channel MOS transistors 376 and 377 become conductive in response to up indication signal UP and combined channel resistance Rd, therefore, increases. Following this, as described above, constant voltage Vn 6 rises by a voltage V which corresponds to the product between constant current Ict and the increase of combined channel resistance Rd, whereby the reference voltage during a test can be raised by one step from the initially set reference voltage.

If the above-stated concrete example is used, combined channel resistance Rd increases step by step and it is, therefore, possible to raise reference voltage REF 1 from an initial state level 0 to level 7 step by step as shown in FIG. 7 A.

If the above-stated concrete example is used, as shown in FIG. 7B , combined channel resistance Rd decreases step by step. It is, therefore, possible to lower reference voltage REF 1 from initial level 0 to level 7 step by step during a test.

By adopting the above-stated configuration, it is possible to increase or decrease combined channel resistance Rd of test voltage setting circuit 370 step by step in response to up indication signal UP and down indication signal DN, respectively. It is, therefore, possible to raise or lower the voltage level of constant voltage Vn 6 of internal node N 6 step by step during a test. Accordingly, it is possible to set the voltage level of the internal voltage to follow that of the reference voltage, as well.

By adopting the configuration of the present invention, the voltage level of reference voltage REF 1 is raised or lowered step by step based on a binary input signal without increasing the number of external terminals. It is thereby possible to adjust the internal voltages during a test and to efficiently execute a screening test.

While the configuration in which P-channel MOS transistors are employed as transistors which function as resistance elements has been described so far, it is also possible to adopt a configuration in which N-channel MOS transistors which function as resistance elements are employed.