Abstract:
To provide a semiconductor integrated circuit including a trimmable bleeder resistor circuit, where a layout area of a trimming circuit is saved. In a semiconductor integrated circuit of the present invention, serially inputted trimming data are sequentially written to plural memory cells in accordance with selection signals for trimming a bleeder resistor, making it possible to dispense with a data resistor for storing the trimming data.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a semiconductor integrated circuit including a bleeder resistor circuit allowing for external trimming.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 3  is a block diagram of a conventional semiconductor integrated circuit including a trimmable bleeder resistor circuit. The circuit includes: a write control circuit  102  having a data input terminal  97  and a clock input terminal  96  as input terminals; a memory  103  composed of plural memory cells to which data is written in response to an output from the write control circuit  102 ; and plural trimming switches  104  composed of MOS transistors switchably turned on/off in accordance with data held in the memory  103 ; a switch control circuit  106  for transmitting the output from the memory  103  to the trimming switch  104  after buffering; and a bleeder resistor circuit  105  partially trimmed off by means of the trimming switch  104 . The bleeder resistor circuit  105  changes the total resistance value in accordance with the combined use of the trimming switches  104  in an on-state and those in an off-state.  
         [0005]     The write control circuit  102  includes: a shift register  99  for counting clock signals inputted from the clock input terminal  96 ; and a data register  100  for temporarily storing trimming data inputted from the data input terminal  97 .  
         [0006]     In writing data, trimming data to be written is inputted serially to the data input terminal  97  in synchronism with the clock signal inputted to the clock input terminal  96 . Once the clock signal reaches a terminal end of the shift register  99  for counting the clock signals, a write signal is outputted to a data write control signal node  98 , and trimming data stored in the data register  100  is transmitted to corresponding memory cells in the memory  103 .  
         [0007]     In reading data, the trimming data stored in the memory cells in the memory  103  are transmitted to the trimming switches  104  by way of the corresponding switch control circuit  106  to thereby turn on/off the corresponding switch.  
         [0008]     That is, the bleeder resistor circuit  105  can be trimmed into a desired resistance value by serially inputting to the data input terminal  97  data representing the combined use of the trimming switches  104  in an on-state and those in an off-state, by which a desired resistance value is attained (see JP 2003-242799 A (p. 12, FIG. 1)).  
         [0009]     However, with the conventional semiconductor integrated circuit including the trimmable bleeder resistor circuit, the number of flip-flop circuits in the data register  100  increases along with an increase in bit number of trimming data, resulting in a larger layout area. This disadvantageously hinders cost- and size-reduction of the semiconductor integrated circuit.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention has been made in view of the above-mentioned problems. It is therefore an object of the present invention to provide a semiconductor integrated circuit that overcomes the problems of the conventional techniques and saves a layout area.  
         [0011]     In order to achieve the above-mentioned object, the present invention provides a semiconductor integrated circuit including a trimmable bleeder resistor circuit, in which serially inputted trimming data are sequentially written to plural memory cells in accordance with selection signals for trimming a bleeder resistor.  
         [0012]     With the semiconductor integrated circuit including a trimmable bleeder resistor circuit according to the present invention, a data register for storing trimming data can be omitted, whereby a semiconductor integrated circuit can be provided, which saves a layout area and costs low. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     In the accompanying drawings:  
         [0014]      FIG. 1  is a block diagram of a semiconductor integrated circuit including a trimmable bleeder resistor circuit according to the present invention;  
         [0015]      FIG. 2  shows an example of input/output waveforms of signals inputted to a write control circuit in the semiconductor integrated circuit according to the present invention; and  
         [0016]      FIG. 3  is a block diagram of a conventional semiconductor integrated circuit including a trimmable bleeder resistor circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]      FIG. 1  is a block diagram of a semiconductor integrated circuit including a trimmable bleeder resistor circuit according to the present invention. The integrated circuit includes: a clock input terminal  96 ; a data input terminal  97 ; a shift register  107  for generating a selection signal based on a clock signal; a write control circuit  110  composed of plural AND circuits to which trimming serial data and the selection signal are inputted; a memory  103  including plural memory cells connected to the write control circuit  110 ; a trimming switch  104 ; a switch control circuit  106  that makes on/off control of the trimming switch  104  in accordance with data in the memory cells; and a bleeder resistor circuit  105  that can be partially trimmed off by means of the trimming switch.  
         [0018]     Output signals from the shift registers  107  in each stage (A 1 , A 2 , A 3 , . . . , An) are respectively inputted as selection signals for selecting a memory cell responsible for the data write operation from the memory  103  to the write control circuit  110  composed of AND circuits corresponding to the memory cell.  
         [0019]     The write control circuit  110  composed of the AND circuit outputs trimming serial data to each memory cell at timings selected in response to the selection signals. In this case, the memory cells in the memory  103  are all initialized to a low level.  
         [0020]      FIG. 2  shows an example of the input/output waveforms of signals inputted to the write control circuit. In  FIG. 2 , denoted by “clk” is a time waveform of a signal inputted to the clock input terminal  96 , denoted by “a 1 ” to “an” are time waveforms of the selection signals, and denoted by “data” is a time waveform of a signal inputted to the data input terminal  97 .  
         [0021]     Inputting the clk signal to the clock input terminal  96  causes the shift register  107  to output the selection signals a 1  to an. The write control circuit  110  composed of the AND circuit outputs the data signal in response to the selection signal at a high level. An output level of the write control circuit becomes low when the selection signal is at the low level. Accordingly, upon writing trimming data, a signal at a high level may be inputted to the data input terminal  97  for a memory cell in the memory  103  to which the signal at the high level is to be inputted, in synchronism with a timing at which the corresponding selection signal is at a high level. In contrast, a signal at a low level may be inputted to the data input terminal  97  for a memory cell in the memory  103  to which the signal at the low level is to be inputted, in synchronism with a timing at which the corresponding selection signal is at a high level.  
         [0022]     Next, the trimming data from the memory cells in the memory  103  cause the trimming switches  104  composed of a MOS transistor to turn on/off through the switch control circuit  106 , thereby trimming the bleeder resistor circuit  105  into a desired resistance value.  
         [0023]     Note that a memory electronic switch made up of a memory MOS transistor etc. used for a nonvolatile memory may also be adopted as the trimming switch  104  of the embodiment of the present invention. In this case, an on-resistance of the memory electronic switch is set much lower than those of resistors composing the bleeder register circuit  105 ; an off-resistance thereof is set much higher than those of resistors composing the bleeder register circuit  105 . Consequently, it is possible to dispense with the memory cells in the memory  103 , resulting in further reduction in circuit size.  
         [0024]     The description has been made so far of the operation of the circuit according to the embodiment where the AND circuit is used for the write control circuit. However, the write control circuit is not limited to the AND circuit. A NOR circuit or the like can be used instead according to a logic state of an input signal. In this case, similar effects can be produced by changing the logic state in the above description about the circuit operation.  
         [0025]     Further, it is apparent from the above that similar effects can be achieved by using a circuit where an EPROM or EEPROM capable of storing trimming data even in a power-off state is used as the memory  103 .