Patent Publication Number: US-7915951-B1

Title: Locally calibrated current source

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a current source on a chip that can be calibrated on-chip to provide a current source that is PVT independent. 
     2. Description of the Prior Art 
     Microchips require PVT independent reference currents that can be input to various local circuits on the chip. In order to avoid the effects of noise, a reference current is routed to the chip over a long distance, and then rerouted to all circuits on the microchip. Each circuit requires a separate wire for this routing, which takes up space on the chip. It is therefore an object of the present invention to provide a circuit that reduces the number of wires required for providing a constant current reference to many circuits on a chip. 
     SUMMARY OF THE INVENTION 
     A microchip that can calibrate a plurality of circuits on the microchip with a current reference according to an exemplary embodiment of the present invention comprises: at least a first circuit disposed on the microchip; at least a first local bias generation circuit, for generating a bias current that is input to the first circuit; an external current reference, coupled to the first local bias generation circuit, for updating the bias current; and a calibration logic, coupled to the first local bias generation circuit, for enabling the external current reference to update the bias current according to a valid calibration signal. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a circuit that provides a constant current reference to a microchip that uses less wires than a prior art circuit. 
         FIG. 2  is a diagram of the local crude bias generator shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides an architecture whereby current can be routed to local circuits on a chip utilizing fewer wires than prior art methods. By the use of additional circuitry that is within the microchip, local calibration can take place. 
     Please refer to  FIG. 1 , which is a diagram of the proposed architecture  100 . As can be seen from the diagram, the microchip  100  includes a plurality of local crude bias generation circuits  112 ,  122 ,  132 ,  142  that are placed close to corresponding circuits  114 ,  124 ,  134 ,  144  that require the current reference source. These local crude bias generation circuits  112 ,  122 ,  132 ,  142  serve to generate a crude bias that is insensitive to supply voltage, but can be process and temperature sensitive. This is because the bias current is constantly updated by a master current reference. 
     The master current reference  150 , which is process, voltage and temperature (PVT) insensitive, is input to the microchip  100 . This current reference is then routed over a single wire which is coupled to each of the local crude bias generation circuits  112 ,  122 ,  132 ,  142 . The microchip  100  also comprises a calibration logic circuit  160  which is similarly routed over a single wire and coupled to each of the local crude bias generation circuits  112 ,  122 ,  132 ,  142 . 
     The calibration logic  160  is only enabled when a particular circuit requires calibration. Each local circuit  114 ,  124 ,  134 ,  144  can have a specific enable signal. When the valid enable signal is received from the calibration logic  160 , the crude bias generation circuit will calibrate its corresponding circuit. 
     Please refer to  FIG. 2 , which is a diagram of the local crude bias generation circuit  112 , shown in  FIG. 1 . Please note that this circuit is only shown as an example, and all other circuits have the same configuration. As can be seen in  FIG. 2 , the circuit  112  comprises a switch  220  and a select detection circuit  230 , respectively coupled to the reference current and the calibration signal. These circuits are coupled to a comparator  240 , which is coupled in turn with a bias generator  250 . 
     When the calibration logic is a valid logic for the circuit  112 , the switch  220  will be on, allowing the reference current to be input. The current reference is then compared with the locally generated current, which is fed back from the bias generation circuit  250 , by the comparator  240 . The comparator  240  will then generate a correction amount for updating the locally generated current, so that the output of the local crude bias generation circuit  112  equals the current reference that is supplied (i.e. the master current). 
     The locally generated current is insensitive to voltage only. As each locally generated current is constantly updated by a PVT insensitive reference, however, the current that is supplied to the local circuits  11 ,  124 ,  134 ,  144  will be PVT independent. 
     The means for determining when to update the current can occur at any time. In one embodiment, updates occur according to a certain period of time. In another embodiment, as the locally generated current is sensitive to temperature, each time there is a temperature change the calibration logic  160  will be enabled to allow current updates. 
     As the current reference is used to update locally generated currents, it only requires a single wire for coupling with the local crude bias generation circuits  112 ,  122 ,  132 ,  142 . The current that is input to the local circuits  114 ,  124 ,  134 ,  144  is constantly updated by a PVT insensitive reference, so it does not need to be temperature and process insensitive. The use of the calibration logic circuit  160  for selectively enabling updates of the locally generated current allows the number of wires used in the microchip  100  to be fewer than those used in the prior art. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.