Abstract:
A semiconductor IC has a current detection resistor and a comparison circuit for comparing the voltage drop across the current detection resistor with a threshold voltage to detect a possibly large magnitude of current passing through the current detection resistor. The semiconductor IC has a current terminal for externally inputting current to the current detection resistor, a highly resistive current-limiting resistor connected between the current detection resistor and one input end of the comparison circuit, and a measurement terminal connected to the node of the input terminal of the comparison circuit and the current limiting resistor. Thus, the magnitude of the current flowing through the measurement resistor is correctly measured without being influenced by the parasitic resistances of the terminals or of the test probe used. The threshold level of detection current is regulated based on the measured level the detection current without flowing therethrough a large amount of current nor implementing any additional device.

Description:
FIELD OF THE INVENTION 
   This invention relates to a semiconductor integrated circuit (IC) having current detection functionality for detecting the magnitude of current flowing through a circuit associated with the semiconductor IC to protect the semiconductor IC and the external circuit. 
   The invention also relates to a power supply unit equipped with such semiconductor IC. 
   BACKGROUND OF THE INVENTION 
   A typical semiconductor IC for use in a power device (referred to as power device semiconductor IC) has an over-current detection function as shown in  FIG. 5 . Such power device semiconductor IC is used in power circuits and output circuits of different kinds of apparatuses. A power device semiconductor IC is designed to detect over-current that flows in a power device and, if over-current is detected, limit the current to protect the internal as well as external circuitries from being damaged by the over-current. The power device semiconductor IC is required to provide operational security and several sophisticated operational features including: operational accuracy; dormancy under normal operating conditions; and capability of protective operation when over-current exceeds a predetermined level. In addition, the power device semiconductor IC is preferred to be inexpensive as other electronic components. 
   The semiconductor IC shown in  FIG. 5  has a current input terminal  52  and a current output terminal  53  respectively connected to the collector and the emitter of a power NPN transistor  54 . The base of the power NPN transistor  54  is controlled by an NPN Tr drive circuit  55 . When for example the power NPN transistor  54  and the NPN Tr drive circuit  55  are used in an output circuit, they amplify the signal input into the NPN Tr drive circuit  55  and provide at the current output terminal  53  stable amplified power that is constant in voltage or current. 
   The magnitude of the current output from the emitter of the power NPN transistor  54  to the current output terminal  53  is substantially equal to that of the current input to the collector of the power NPN transistor  54  via the current input terminal  52 . Over-current detection functionality for limiting this current within a predetermined range can be attained by a resistor  56  and a comparator  57 . The resistor  56  is provided to determine the magnitude of the current that flows from the current input terminal  52  to the collector of the power NPN transistor  54 . The resistor  56  generates a voltage drop in accord with the magnitude of the current. 
   The voltage across the power NPN transistor  54  increases with the collector current of the power NPN transistor  54 . When this voltage exceeds the threshold voltage of the comparator  57 , the output of the comparator  57  appearing at the output terminal  58  thereof is pulled up from a low level to a high level. This change in the output level is transmitted to the NPN Tr drive circuit  55  to disable the power NPN transistor  54 . 
   In order to improve the accuracy of current detection with the circuit of  FIG. 5 , it is necessary to accurately or correctly measure the magnitude of current to be detected. Since the current that can be passed through one probe of a tester is limited in magnitude to a small value, it is impossible in a wafer test to directly detect or measure a large over-current. 
   Then, it is necessary in the wafer test to measure the resistance of (or the voltage drop across) the resistor  56  for a permissible current through the probe and compare the measured voltage drop with the threshold of the comparator  57  to determine the maximum permissible level of over-current. To measure the resistance of the resistor  56 , source electrodes  61  and  64  and major electrodes  62  and  63  are formed on the chip of the semiconductor IC. Then the small resistance of the resistor  56  is measured as accurately as possible by flowing current between the source electrodes  61  and  64 . This can be done by measuring the voltage between the measurement electrodes  62  and  63 ; In actuality, however, the threshold voltage of the comparator  57  cannot be accurately measured, since the resistor  56  has only a small resistance and is connected between the input terminals of the comparator  57 . 
   Means for improved accurate measurement of the threshold voltage of the comparator  57  has been disclosed in Japanese Patent Application Laid Open 2001-53120, in which electric connection means such as a Zener diode is provided for cutting off the electric connection between the current detection resistor  56  and one input end of the comparator  57  during a wafer test on over-current detection functionality and for re-establishing the electric connection after the test. 
   However, the disclosed prior art requires not only an electric connection means such as a Zener diode used only in the wafer test but also two extra terminals for feeding high-voltage pulses to the electric connection means. As a consequence, a semiconductor IC having current detection functionality has a complicated structure and must be structured to withstand high-voltage pulses, which increases the cost of the IC. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the invention to provide a semiconductor IC having a built-in resistor (hereinafter referred to as current detection resistor) for passing therethrough current to be detected (referred to as detection current), especially detection current having a large magnitude, the semiconductor IC being cap able of: 
   correctly measuring the resistance of the current detection resistor without being influenced by the resistances of terminals of the IC and a test probe used; 
   actually measuring the threshold level of the detection current (or threshold voltage) without flowing a large amount of current through the detection resistor or implementing any further device; and 
   adjusting the threshold level of the detection current based on the measured detection current. 
   It is another object of the invention to provide a power supply unit equipped with such semiconductor IC as described above. 
   In accordance with one aspect of the invention, there is provided a semiconductor IC having a current detection resistor for passing therethrough detection current and a comparison circuit for comparing the voltage associated with the voltage drop across the current detection resistor with a threshold voltage to determine the magnitude of the detection current, the semiconductor IC thus implementing current detection functionality, the semiconductor IC comprising: 
   a current terminal for externally feeding current to the current detection resistor; 
   a current limiting resistor connected between the current terminal and one input end of the comparison circuit and having a higher resistance than the current detection resistor; and 
   a measurement terminal connected to the node of the one input end of the comparison circuit and the current limiting resistor. 
   In accordance with another aspect of the invention, there is provided a semiconductor IC having a current detection resistor for passing therethrough detection current and a comparison circuit for comparing the voltage associated with the voltage drop across the current detection resistor with a threshold voltage to determine the magnitude of the detection current, the semiconductor IC thus implementing current detection functionality, the semiconductor IC comprising: 
   a first current terminal; 
   a second current terminal for flowing therethrough the detection current from the first current terminal via the current detection resistor; 
   a current limiting resistor connected between one end of the current detection resistor and a first input end of the comparison circuit, and having a resistance higher than that of the current detection resistor; and 
   a first measurement terminal connected to the node of the first input terminal and current limiting resistor. 
   The semiconductor IC may have a second measurement terminal, connected to the other end of the current detection resistor, for outputting the same output power as from the second current terminal. 
   In accordance with another aspect of the invention, there is provided semiconductor IC having a current detection resistor for passing therethrough detection current and a comparison circuit for comparing the voltage associated with the voltage drop across the current detection resistor with a threshold voltage to determine the magnitude of the detection current, the semiconductor IC thus implementing current detection functionality, the semiconductor IC comprising: 
   a first current terminal; 
   a second current terminal for flowing therethrough the detection current from the first current terminal via the current detection resistor; 
   a first current limiting resistor connected between one end of the current detection resistor and a first input end of the comparison circuit, and having a resistance higher than that of the current detection resistor; 
   a first measurement terminal connected to the node of the first input terminal and a first current limiting resistor; 
   a second current limiting resistor connected between the other end of the current detection resistor and a second input end of the comparison circuit, and having a resistance higher than that of the current detection resistor; and 
   a second measurement terminal connected to the node of the second input terminal and second current limiting resistor. 
   In any of those semiconductor ICs, the comparison circuit may be adapted to adjust the threshold voltage to be in accord with a predetermined magnitude of the detection current. Furthermore, the comparison circuit may include a differential amplifier having much higher input impedance than the current limiting resistor, and a comparator for comparing the output of the differential amplifier with a reference voltage. The reference voltage can be adjusted by a voltage regulation member that can be trimmed (the member hereafter referred to as trimable voltage regulation member). 
   The power supply unit may comprise a voltage conversion circuit for converting an input voltage into a predetermined output voltage and a semiconductor IC having current detection functionality to determine the magnitude of the current that flows through the voltage conversion circuit. 
   In accordance with a still further aspect of the invention, there is provided a power supply unit equipped with a semiconductor IC having a current detection resistor for passing therethrough detection current and a comparison circuit for comparing the voltage associated with the voltage drop across the current detection resistor with a threshold voltage to determine the magnitude of the detection current, the semiconductor IC thus implementing current detection functionality, the semiconductor IC further having: 
   a first current terminal; 
   a second current terminal for flowing therethrough the detection current from the first current terminal via the current detection resistor; 
   a current limiting resistor connected between one end of the current detection resistor and a first input end of the comparison circuit, and having a higher resistance than the current detection resistor; and 
   a first measurement terminal connected to the node of the first input terminal and current limiting resistor; and 
   a second measurement terminal, connected to the other end of the current detection resistor, for providing the same output power as from the second current terminal, wherein 
   the second current terminal and second measurement terminal are also connected together outside the semiconductor IC. 
   An inventive semiconductor IC having a built-in current detection resistor is capable of detecting current of a large magnitude as described above. The semiconductor IC can correctly measure the resistance of the current detection resistor without being influenced by the resistances of terminals or the resistance of the test probe used, and can actually measure the threshold level of the detection current without flowing a large current through the current detection resistor or implementing any additional means. Thus, the current detection functionality of the semiconductor IC can be provided easily and correctly in the stage of manufacturing its wafer. 
   It will be appreciated that the resistance of the current detection resistor can be accurately measured without being influenced by the resistances of the test probe or of the terminals involved. Since the current detection resistor consists of low-resistance wires such as aluminum wires, it is especially advantageous that the inventive semiconductor IC can accurately and exclusively measure the resistance of the current detection resistor. 
   Moreover, based on the correctly measured resistance of this current detection resistor and a prospective current detection, the threshold voltage of the comparison circuit can be finely tuned to an optimal value. 
   Since the resistance of the current limiting resistor is only required to be sufficiently larger than that of the current detection resistor and needs not be precise, formation of the resistor is easy. 
   In actual usage of the semiconductor IC, the second current terminal and the second measurement terminal can be connected in parallel with each other to reduce the resistance of the terminals to reduce the power loss by them. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a structure of a power supply unit equipped with a semiconductor IC having current detection functionality in accordance with one embodiment of the invention. 
       FIG. 2  shows how the resistance of a current detection resistor of  FIG. 1  is measured using the current detection functionality. 
       FIG. 3  shows how the threshold of a reference voltage is measured for the semiconductor IC of  FIG. 1 . 
       FIG. 4  shows a structure of a semiconductor IC having current detection functionality according to another embodiment of the invention. 
       FIG. 5  shows a structure of a conventional semiconductor IC having over-current detection functionality. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An inventive semiconductor IC having current detection functionality and a power supply unit utilizing such semiconductor IC will now be described in detail with reference to the accompanying drawings. Referring to  FIG. 1 , there is shown structures of an inventive power supply unit and a semiconductor IC for use with the power supply unit. 
   In the example shown in  FIG. 1 , a step-up type switching power supply unit is used. The invention, however, is not limited to this type of power supply unit, and can be also applied to other devices having current detection functionality including a step-down type power supply unit, serial power supply unit, a power supply unit, and to a motor drive unit as well. 
   As shown in  FIG. 1 , a coil Lo and an NMOS transistor switch So are connected in series with each other between a power source and the ground. The voltage, stepped up from a power supply voltage Vdd and appearing at the node of the coil Lo and the NMOS transistor So, is rectified and smoothed by a Schottky diode Do and a smoothing capacitor Co to generate an output voltage Vo. Io indicates output current to be supplied to a load (not shown). The switch element So can be a MOS transistor or a bipolar transistor. 
   The output voltage Vo is divided by voltage division resistors R 1  and R 2  to form a voltage detection signal Vdet. The ratio of on- to off-periods (referred to as on-off ratio) of the NMOS transistor So is controlled based on the voltage detection signal Vdet to thereby maintain the output voltage Vo at a predetermined level. 
   Controlled on-off operation of the NMOS transistor So results in switching current (referred to as detection current) Is through the coil Lo. The detection current Is is controlled not to exceed a predetermined level by monitoring the magnitude of the detection current. 
   These constant voltage operation and current limiting operation are performed by a semiconductor IC  100 . The voltage detection signal Vdet is input into a control circuit  50  via a voltage input terminal P 6 . The control circuit  50  compares the voltage detection signal Vdet with a reference voltage to generate a switching control signal Cont, which is supplied to the gate of the NMOS transistor So via a control terminal P 5 . Thus, the output voltage Vo is held at a predetermined level. 
   The current limiting operation is attained by feeding the output (referred to as comparison output) Cip of a comparison circuit  40  to a control circuit  50 . The control circuit  50  may be provided outside the semiconductor IC  100 . In this case, the semiconductor IC  100  does not need the control terminal P 5  or the power input terminal P 6 , and needs only a terminal (referred to as comparison output terminal) for outputting the comparison output Cip in place of the terminals P 5  and P 6 . 
   The current detection resistor Rs is connected between first and second current terminals P 1  and P 2 , respectively, of the semiconductor IC  100 . The voltage drop across the current detection resistor Rs created by the detection current Is flowing therethrough is compared with a reference voltage Vref in the comparison circuit  40 . The current detection resistor Rs has a very low resistance (e.g. about 0.1 Ohm) to allow for a big detection current Is (e.g. about 2 A) through it during the use of the power supply unit. This current detection resistor Rs is formed simultaneously with other metal lead wires (aluminum leads) on the surface of the semiconductor chip in the process of manufacturing the semiconductor IC. Thus, the current detection resistor Rs can be easily formed at a low cost, since it is formed in the semiconductor IC. Moreover, it has high operational stability, since it is built in the semiconductor IC itself and hence inside the package thereof. 
   Connected between one end of the current detection resistor Rs and the one input end of the comparison circuit  40  is a current limiting resistor Rt. The node of this current limiting resistor Rt and the input end of the comparison circuit  40  is connected to a first measurement terminal P 3 . This current limiting resistor Rt has an exceedingly high resistance (e.g. about 20 kilo-Ohms) as compared with the resistance of the current detection resistor Rs. The current limiting resistor Rt suffices to have a sufficiently high resistance, that is, it requires no precision in resistance. The first measurement terminal P 3  is used only in the measurement of the resistance of the current detection resistor Rs and the threshold voltage of the comparison circuit  40 , so that it is left open in the normal operation of the semiconductor IC. That is, it is not connected to any external device then. Because the first measurement terminal P 3  is open under normal operating conditions, the voltage drop across the current detection resistor Rs can be solely detected. The input impedance of the comparison circuit  40  preferably has an exceedingly higher resistance than the current detection resistor Rs. 
   The second measurement terminal P 4  is used in the measurement of the resistance of the current detection resistor Rs and the threshold voltage of the comparison circuit  40 . The second measurement terminal P 4  is connected in parallel with the second current terminal P 2  in the normal operation. Thus, the detection current Is is shunted to the second current terminal P 2  and the second measurement terminal P 4  in the normal operation, thereby reducing the power loss by the resistor Rs. 
   In the example shown herein, the comparison circuit  40  is provided with a differential amplifier AMP and a comparator CP 1 . The differential amplifier AMP amplifies the voltage across the two input terminals thereof, and outputs a current detection signal Idet. The differential amplifier AMP preferably has high impedance at the input end thereof. This preference is achieved by, for example, a differential amplifier AMP having MOS transistors as its input ends. 
   The comparator CP 1  compares the input current detection signal Idet with the reference voltage (which is a threshold voltage adjusted so as to detect a predetermined current level, hereinafter also referred to as current detection threshold voltage) Vref, and outputs the comparison output Cip. The reference voltage Vref may be adjusted to have a desired magnitude by trimming a trimable voltage adjustment member by, for example, selecting an appropriate combination of voltage dividing resistors and fusible elements such as fuses. 
   In the power supply unit of  FIG. 1 , the control circuit  50  performs constant voltage operation in which the output voltage Vo is maintained at a predetermined constant level by controlling the on-off ratio of the NMOS transistor So based on the voltage detection signal Vdet. 
   It is noted that the detection current Is flows through the current detection resistor Rs and that the voltage drop across the current detection resistor Rs determined by the resistance of the resistor and the detection current is amplified by the differential amplifier AMP to output the current detection signal Idet, which is compared with the reference voltage Vref. When the current detection signal Idet stays below the reference voltage Vref, a constant voltage operation is performed, since the comparison output Cip is not output then. 
   However, if the current detection signal Idet reaches the level of the reference voltage Vref, the comparison output Cip will be output from the comparator CP 1  to the control circuit  50 . In response to the comparison output Cip, the control circuit switches from the constant voltage operation to current limiting operation. This current limiting operation may be attained by causing the power supply unit to be disabled or to operate under a limited operating current. 
   In order to carry out this current limiting operation with a predetermined detection current Is, it is necessary that the resistance of the current detection resistor Rs is known correctly and that the reference voltage Vref is set to a proper level in accord with the predetermined detection current Is and the resistance Rs. 
   In the inventive semiconductor IC, the resistance of the current detection resistor Rs and the threshold level of the reference voltage Vref can be measured using an ordinary tester (typically a probe that permits current of at most about 200 mA) under wafer state.  FIGS. 2 and 3  show conditions of the semiconductor IC in these measurements. 
   Referring to  FIG. 2 , there is shown a circuit arrangement for measuring the resistance of the current detection resistor Rs. As seen in  FIG. 2 , the second current terminal P 2  is connected to the ground with a probe, and the output end of a current source  20  of a measuring apparatus is connected to the first current terminal P 1  with another probe. The measurement current Ist from the current source  20  is very small (as small as about 100 mA) as compared with the detection current Is (which is about 2 A for example). On the other hand, voltage measurement means, e.g. a voltmeter  30 , is connected between the first measurement terminal P 3  and the second measurement terminal P 4 . 
   The measurement current Ist from the current source  20  flows through the first current terminal P 1 , the current detection resistor Rs, and the second current terminal P 2 , as indicated by a broken line in  FIG. 2 . Resistances of the first and second current terminals P 1  and P 2  and the contact resistances between these terminals and the probes found in the path of the measurement current Ist are collectively indicated by symbols rc. In the invention, although these resistances rc are larger than that of the current detection resistor Rs, they do not affect the measurements by the voltmeter  30 , as described below. Moreover, since both the input impedances of the differential amplifier AMP and the voltmeter  30  are very high, they do not affect the measurements by the voltmeter  30 . 
   That is, the measurements with the voltmeter  30  permits correct determination of the magnitudes of the current Ist and the current detection resistor Rs. 
   Next, referring to  FIG. 3 , there is shown a circuit arrangement for measuring the threshold level of the reference voltage Vref. As seen in  FIG. 3 , a variable voltage source  10  is connected between the first and the second measurement terminals P 3  and P 4 , respectively. The variable voltage source  10  is adapted to sweep the voltage across the terminals P 3  and P 4  (the voltage referred to as measurement voltage) Vt over a range. The variable voltage source  10  can be any variable voltage source capable of measuring its voltage Vt, or any variable voltage source accompanying means for measuring the voltage Vt. In the example shown herein, the variable voltage source  10  is provided in the same test device together with the current source  20  and the voltmeter  30 . 
   The resistance of the current detection resistor Rs is correctly measured in the manner as already described above in connection with  FIG. 2 . The product of the desired limiting detection current Is and the measured resistance of the current detection resistor Rs is the target threshold voltage to obtain desired comparison output Cip from the comparison circuit  40 . The contact resistances of the first and second measurement terminals P 3  and P 4 , respectively, are negligibly small as compared with the resistance of the current limiting resistor Rt. 
   Thus, a predetermined range over the target threshold voltage is swept by the measurement voltage Vt of the variable voltage source  10 . The comparison output Cip of the comparator CP 1  is monitored while sweeping the measurement voltage Vt. The voltage adjustment members of the comparator CP 1  are trimmed in accord with the difference between the target threshold voltage and the measurement voltage Vt measured at the time when the desired comparison output Cip is generated, thereby correctly setting the reference voltage (detection threshold voltage) Vref to the target threshold voltage. 
   In the first embodiment, the resistance of the built-in current detection resistor Rs can be accurately measured without being influenced by the resistances of the first and second current terminals P 1  and P 2  and the test probe. Further, the detection threshold voltage Vref can be set to an optimal value based on the actually measured voltage drop without flowing a large detection current Is. It is therefore easy to provide a semiconductor IC with proper current detection functionality in the process of forming a wafer of the IC. 
   The current limiting resistor Rt can be easily fabricated, since the resistor suffices to have a resistance that is significantly higher than that of the current detection resistor Rs and does not need to have a precise magnitude. 
   It will be appreciated that the resistances of, and hence the power loss by, the measurement terminals are reduced when the semiconductor IC is in actual use by connecting in parallel the second current terminal P 2  with the second measurement terminal P 4  while leaving the first measurement terminal P 3  open to solely detect the voltage drop across the current detection resistor Rs. 
   Although the invention has been described above with reference to a particular comparison circuit  40  having a differential amplifier AMP and a comparator CP 1 , it will be clear to a person skilled in the art that the invention is applicable to any semiconductor IC having any type of comparison circuit. 
   Referring to  FIG. 4 , there is shown a semiconductor IC according to another embodiment of the invention. In the semiconductor IC shown in  FIG. 4 , a second current limiting resistor Rt 2  having a large resistance is also provided between the second current terminal P 2  (or one end of the current detection resistor Rs) and the other input end (“minus” terminal) of the differential amplifier AMP. The current limiting resistor Rt of  FIG. 1  is indicated by symbol Rt 1  in  FIG. 4 . 
   In this case, the second measurement terminal P 4  is connected to the node of the other input end of the differential amplifier AMP and the second current limiting resistor Rt 2 . Other features of the circuit are the same as those of  FIG. 1 . 
   In the example shown in  FIG. 4 , the two inputs to the differential amplifier AMP can be equilibrated. However, the second current terminal P 2  and the second measurement terminal P 4  cannot be connected with each other as shown in  FIG. 1  when the IC is in actual use. As a consequence, power loss will be slightly larger in this circuit than in the circuit of  FIG. 1 . 
   It will be noted that one or both of the second current terminal P 2  and the second measurement terminal P 4  can be omitted in the embodiments shown in  FIGS. 1 and 2  when a further ground terminal is available.