Abstract:
A constant current output sink or source eliminates a current limiting series resistor for a light emitting diode (LED) and maintains a constant light intensity from the LED for all operating and manufacturing variables of a digital device since the current through the LED is maintained at a constant value. The constant current output sink or source may be programmable for selection of a constant current value from a plurality of constant current values available.

Description:
RELATED PATENT APPLICATION 
       [0001]    This application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 61/121,932; filed Dec. 12, 2008; entitled “Constant Current Output Sink,” by Ward R. Brown; and is hereby incorporated by reference herein for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to controlling devices requiring a constant current for proper operation thereof and, more particularly, to controlling a light emitting diode (LED) while maintaining a constant current therethrough so that the light intensity of the LED remains substantially constant over a wide range of voltages, temperatures and other process variables. 
       BACKGROUND 
       [0003]    LED forward voltage varies due to manufacturing process variables and application ambient temperature. A LED device driver, e.g., digital device output driver low or high voltage state (e.g., output near Vss or Vdd, respectively) varies due to manufacturing process variables, amount of current being sinked or sourced therein and application ambient temperature. This results in unacceptable LED light intensity consistency when a fixed resistor is used in series with the LED to limit current therethrough. LED device driver output sink and source current varies with power supply voltage, Vdd, and operating temperature, thereby resulting in an undesirable LED intensity variation over any change in temperature and/or operating voltage. Also a fixed current limiting resistor is required in series with the LED that adds additional cost and complexity to products using LEDs. 
       SUMMARY 
       [0004]    What is needed is a way to maintain the LED light intensity over a wide range of voltages, temperatures and manufacturing process variables. According to the teachings of this disclosure, a constant current output sink or source eliminates the current limiting series resistor for the LED and maintains a constant light intensity from the LED for all operating and manufacturing variables of an integrated circuit digital device, e.g., microcontroller, microprocessor, digital signal processor, application specific integrated circuit (ASIC), programmable logic array (PLA), etc. 
         [0005]    According to the teachings of this disclosure, maintaining a constant current, e.g., current limiting, at an output driver when sinking or sourcing a load such as a LED will maintain the light intensity of the LED at constant level for all operating and manufacturing variables. This current limiting feature may be enabled or disabled, and the current limit value set under program control, e.g., by using internal control registers in the digital device (e.g., microcontroller). Each output of the digital device may have current sink or source limiting capabilities that have an associated control bit that enables the current limiting feature. Another multi-bit register may determine the value of the constant current, whereby the range and resolution of the constant current value may be determined by the number of bits in this register. Limited output current sink or source may be set by the gate voltage of the field effect transistor (FET) output driver or several FETs operating at fixed gate voltages. A constant output sink or source current range may be adjustable, e.g., from about five (5) milliamperes (mA) to about 25 mA. 
         [0006]    Also contemplated herein are switch applications using low current weak pull-ups that may be susceptible to noise. This may be overcome by setting current output to a low limit and using a direct connection to Vdd through the switch which would then raise the next stage input level closer to Vdd for improved noise tolerance. 
         [0007]    According to a specific example embodiment, an integrated circuit digital device having a node that is current limited comprises: a node; and a programmable constant current circuit coupled to the node, wherein the programmable constant current circuit limits the amount of current through the node to a current value that is programmed into the programmable constant current circuit. 
         [0008]    According to another specific example embodiment, an integrated circuit digital device having an output node that is current limited comprises: an output node; a high side drive circuit coupled between the output node and a power supply voltage; a first multiplexer having first, second and third nodes, and a control input for selectively coupling the first node to the second node or the first node to the third node, wherein the first node is coupled to the output node and the high side drive circuit; a second multiplexer having first, second and third nodes, and a control input for selectively coupling the first node to the second node or the first node to the third node, wherein the first node is coupled to a power supply common; a programmable constant current circuit coupled between the second node of the first multiplexer and the second node of the second multiplexer, wherein the programmable constant current circuit limits current therethrough to a current value that is programmed into the programmable constant current circuit; and a low side drive circuit coupled between the third nodes of the first and second multiplexers; wherein when the first and second nodes of the first and second multiplexers are coupled together the programmable constant current circuit limits current going into the output node to the current value, and when the first and third nodes of the first and second multiplexers are coupled together the low side drive circuit couples the output node to substantially the power supply common without limiting current thereto. 
         [0009]    According to yet another specific example embodiment, an integrated circuit digital device having an output node that is current limited comprises: an output node; a low side drive circuit coupled between the output node and a power supply common; a first multiplexer having first, second and third nodes, and a control input for selectively coupling the first node to the second node or the first node to the third node, wherein the first node is coupled to a power supply voltage; a second multiplexer having first, second and third nodes, and a control input for selectively coupling the first node to the second node or the first node to the third node, wherein the first node is coupled to the output node and the low side drive circuit; a programmable constant current circuit coupled between the second node of the first multiplexer and the second node of the second multiplexer, wherein the programmable constant current circuit limits current therethrough to a current value that is programmed into the programmable constant current circuit; and a high side drive circuit coupled between the third nodes of the first and second multiplexers; wherein when the first and second nodes of the first and second multiplexers are coupled together the programmable constant current circuit limits current going from the output node to the current value, and when the first and third nodes of the first and second multiplexers are coupled together the high side drive circuit couples the output node to substantially the power supply voltage without limiting current thereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
           [0011]      FIG. 1  is a schematic block diagram of a prior technology digital device having a driver circuit used for sinking current from a light emitting diode (LED); 
           [0012]      FIG. 2  is a schematic block diagram of a prior technology digital device having a driver circuit used for sourcing current to a LED; 
           [0013]      FIG. 3  is a schematic block diagram of a digital device having a driver circuit that sinks current from a LED at a constant current value, according to the teachings of this disclosure; 
           [0014]      FIG. 4  is a schematic block diagram of a digital device having a driver circuit that sources current to a LED at a constant current value, according to the teachings of this disclosure; 
           [0015]      FIG. 5  is a schematic block diagram of a driver circuit that sinks current from a LED at a programmable constant current value, according to a specific example embodiment of this disclosure; 
           [0016]      FIG. 6  is a schematic block diagram of a driver circuit that sources current to a LED at a programmable constant current, according to another specific example embodiment of this disclosure; 
           [0017]      FIG. 7  is a schematic block diagram of an input-output (I/O) driver circuit that is selectable between sinking current from a LED at a programmable constant current or as a standard totem pole active output driver, according to yet another specific example embodiment of this disclosure; 
           [0018]      FIG. 8  is a schematic block diagram of an input-output (I/O) driver circuit that is selectable between sourcing current to a LED at a programmable constant current or as a standard totem pole active output driver, according to still another specific example embodiment of this disclosure; 
           [0019]      FIG. 9  is a schematic block diagram of a programmable constant current sink or source, according to the teachings of this disclosure; 
           [0020]      FIG. 10  is a detailed schematic diagram of a programmable constant current sink, according to the teachings of this disclosure; and 
           [0021]      FIG. 11  is a schematic diagram of another programmable constant current sink, according to the teachings of this disclosure. 
       
    
    
       [0022]    While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0023]    Referring now to the drawing, the details of specific example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
         [0024]    Referring to  FIG. 1 , depicted is a schematic block diagram of a prior technology digital device having a driver circuit used for sinking current from a light emitting diode (LED). A LED  106  is coupled to a supply voltage Vdd and to an output of a digital device  102   a  through a current limiting resistor  104 . Whenever the output of the digital device  102   a  goes to a logic low, current flows through the LED  106  and light is produced, wherein the current is limited by the resistor  104 . When the output of the digital device  102   a  goes to a logic high, substantially no current flows through the LED  106  and no light is produced. The current limiting resistor  104  adds cost and complexity to a product requiring control of LEDs. 
         [0025]    Referring to  FIG. 2 , depicted is a schematic block diagram of a prior technology digital device having a driver circuit used for sourcing current to a LED. A LED  106  is coupled to an output of a digital device  102   b  through a current limiting resistor  104  and to a power supply common Vss. Whenever the output of the digital device  102   b  goes to a logic high, current flows through the LED  106  and light is produced, wherein the current is limited by the resistor  104 . When the output of the digital device  102   b  goes to a logic low, substantially no current flows through the LED  106  and no light is produced. The current limiting resistor  104  adds cost and complexity to a product requiring control of LEDs. 
         [0026]    Referring to  FIG. 3 , depicted is a schematic block diagram of a digital device having a driver circuit that sinks current from a LED at a constant current value, according to the teachings of this disclosure. A LED  106  is coupled to a supply voltage Vdd and to an output of a digital device, e.g., digital device  202   a . Whenever the output of the digital device  202   a  goes to a logic low, current flows through the LED  106  and light is produced, wherein the current is limited by current limiting circuits within the digital device  202   a . When the output of the digital device  202   a  goes to a logic high, substantially no current flows through the LED  106  and no light is produced. Not requiring an external current limiting resistor between the LED  106  and the output of the digital device  202   a  reduces cost and complexity of a product requiring control of LEDs. 
         [0027]    Referring to  FIG. 4 , depicted is a schematic block diagram of a digital device having a driver circuit that sources current to a LED at a constant current value, according to the teachings of this disclosure. A LED  106  is coupled to an output of a output of a digital device, e.g., digital device  202   b  and to a power supply common Vss. Whenever the output of the digital device  202   b  goes to a logic high, current flows through the LED  106  and light is produced, wherein the current is limited by current limiting circuits within the digital device  202   b . When the output of the digital device  202   b  goes to a logic low, substantially no current flows through the LED  106  and no light is produced. Not requiring an external current limiting resistor between the LED  106  and the output of the digital device  202   b  reduces cost and complexity of a product requiring control of LEDs. 
         [0028]    Referring to  FIG. 5 , depicted is a schematic block diagram of a LED driver circuit that sinks current from a LED at a programmable constant current, according to a specific example embodiment of this disclosure. An input-output node (I/O)  504  of a digital device, e.g., microcontroller ( FIG. 3 ) is coupled to a switch  506  which is coupled to a programmable constant current sink  514 . When a LED  106  is coupled to the I/O node  504  as shown in  FIG. 3 , a current will flow through the LED  106  and switch  506  (when closed) that is determined by the programmable constant current sink  514 . The constant current value may be set to, for example but is not limited to, from about five (5) mA to about 25 mA. Selection of the constant current value may be determined by a current set signal to the programmable constant current sink  514 . Optionally, a receiver  560  may be used to determine logic levels at the I/O node  504  when used as an input and/or an output node. 
         [0029]    Referring to  FIG. 6 , depicted is a schematic block diagram of a LED driver circuit that sources current to a LED at a programmable constant current, according to another specific example embodiment of this disclosure. An input-output node (I/O)  604  of a digital device, e.g., microcontroller ( FIG. 4 ) is coupled to a switch  606  which is coupled to a programmable constant current source  614 . When a LED  106  is coupled to the I/O node  604  as shown in  FIG. 4 , a current will flow from the programmable constant current source  614 , through the LED  106  and switch  606  (when closed) that is determined by the programmable constant current source  614 . The constant current value may be set to, for example but is not limited to, from about five (5) mA to about 25 mA. Selection of the constant current value may be determined by a current set signal to the programmable constant current source  614 . Optionally, a receiver  660  may be used to determine logic levels at the I/O node  604  when used as an input and/or an output node. 
         [0030]    Referring to  FIG. 7 , depicted is a schematic block diagram of an input-output (I/O) driver circuit that is selectable between sinking current from a LED at a programmable constant current or as a standard totem pole active output driver, according to yet another specific example embodiment of this disclosure. Multiplexers  706  and  710  may be used to switch between a programmable constant current sink  712  and a low side drive circuit  708 , e.g., NMOS field effect transistor (FET). High side drive circuit  702 , e.g., NMOS FET, pulls the I/O node  704  to substantially the power supply voltage, Vdd. The multiplexers  706  and  710  may be controlled with a current limit enable signal for selecting between the programmable constant current sink  712  and the low side drive circuit  708 . Configurations, e.g., current limiting or non-current limiting, current limit value, etc., for the I/O node  704  may be stored in a configuration register  758 . The configuration register  758  may be a volatile memory, e.g., random access memory (RAM), or a nonvolatile memory, e.g., programmable read only memory, Flash memory, etc. 
         [0031]    For example, when a logic high is desired at the I/O node  704 , the high side driver circuit  702  is enabled, the multiplexers  706  and  710  select the low side driver circuit  708 , and the low side driver circuit  708  is disabled. When a logic low without current limiting is desired at the I/O node  704 , the high side driver circuit  702  is disabled, the multiplexers  706  and  710  select the low side driver circuit  708 , and the low side driver circuit  708  is enabled. When a logic low with current limiting is desired at the I/O node  704  (e.g., LED  106  on), the high side driver circuit  702  is disabled and the multiplexers  706  and  710  select the programmable constant current sink  712 , thereby limiting current through the I/O node  704  to the selected current value. Optionally, a receiver  760  may be used to determine logic levels at the I/O node  704  when used as an input and/or an output node. 
         [0032]    Referring to  FIG. 8 , depicted is a schematic block diagram of an input-output (I/O) driver circuit that is selectable between sourcing current to a LED at a programmable constant current or as a standard totem pole active output driver, according to still another specific example embodiment of this disclosure. Multiplexers  806  and  810  may be used to switch between a programmable constant current source  812  and a high side drive circuit  802 , e.g., PMOS field effect transistor (FET). Low side drive circuit  808  pulls the I/O node  804  to substantially the power supply common, Vss. The multiplexers  806  and  810  may be controlled with a current limit enable signal for selecting between the programmable constant current source  812  and the high side drive circuit  802 . Configurations, e.g., current limiting or non-current limiting, current limit value, etc., for the I/O node  804  may be stored in a configuration register  858 . The configuration register  858  may be a volatile memory, e.g., random access memory (RAM), or a nonvolatile memory, e.g., programmable read only memory, Flash memory, etc. 
         [0033]    For example, when a logic low is desired at the I/O node  804 , the low side driver  32  select the programmable constant current source  812 , thereby limiting current through the I/O node  804  to the selected current value. Optionally, a receiver  860  may be used to determine logic levels at the I/O node  804  when used as an input and/or an output node. 
         [0034]    Referring to  FIG. 9 , depicted is a schematic block diagram of a programmable constant current sink or source, according to the teachings of this disclosure. Each of a plurality of unit constant current blocks  918  may be switched on or off, wherein the combination of the plurality of constant current blocks  918   a - 918   n  switched on determine the constant current value allowed into the I/O node  904 . A reference  924  may be used in combination with the plurality of unit constant current blocks  918  so as to maintain a specific constant current for each of the plurality of constant current blocks  918 . Switch  916  may be used to disconnect the plurality of unit constant current blocks  918  from the I/O node  904 . 
         [0035]    Referring to  FIG. 10 , depicted is a detailed schematic diagram of a programmable constant current sink, according to the teachings of this disclosure. Transistor  1054  and constant current source  1056  are used to produce a voltage control signal that controls the amount of current that flows through each of a plurality of transistors  1018 . Each of the switch pairs  1050  and  1052  selectively enable or disable respective ones of the plurality of transistors  1018 . The constant current value that will pass into the I/O node  1004  may be programmed by enabling and disabling, with the respective switch pairs  1050  and  1052 , an appropriate number of the plurality of transistors  1018 . 
         [0036]    Referring to  FIG. 11 , depicted is a schematic diagram of another programmable constant current sink, according to the teachings of this disclosure. NMOS FET  1160  is coupled between the I/O node  1104  and power supply common, Vss. The value of current flowing through the NMOS FET  1160  may be determined by a voltage on the gate of the FET  1160 . By changing this voltage, the current value may be changed. A programmable voltage reference  1164  may be used in combination with an amplifier  1162  to produce the control voltage at the gate of the FET  1160 . 
         [0037]    While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.