Patent Description:
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.

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. A LED drive circuit including a microprocessor is disclosed in patent application <CIT>.

This and other objects can be achieved by the integrated circuit device as defined in the independent claim. Further enhancements are characterized in the dependent claims.

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 (<NUM>) milliamperes (mA) to about <NUM> mA.

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.

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.

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.

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.

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:.

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.

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.

Referring to <FIG>, 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 <NUM> is coupled to a supply voltage Vdd and to an output of a digital device 102a through a current limiting resistor <NUM>. Whenever the output of the digital device 102a goes to a logic low, current flows through the LED <NUM> and light is produced, wherein the current is limited by the resistor <NUM>. When the output of the digital device 102a goes to a logic high, substantially no current flows through the LED <NUM> and no light is produced. The current limiting resistor <NUM> adds cost and complexity to a product requiring control of LEDs.

Referring to <FIG>, 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 <NUM> is coupled to an output of a digital device 102b through a current limiting resistor <NUM> and to a power supply common Vss. Whenever the output of the digital device 102b goes to a logic high, current flows through the LED <NUM> and light is produced, wherein the current is limited by the resistor <NUM>. When the output of the digital device 102b goes to a logic low, substantially no current flows through the LED <NUM> and no light is produced. The current limiting resistor <NUM> adds cost and complexity to a product requiring control of LEDs.

Referring to <FIG>, 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 <NUM> is coupled to a supply voltage Vdd and to an output of a digital device, e.g., digital device 202a. Whenever the output of the digital device 202a goes to a logic low, current flows through the LED <NUM> and light is produced, wherein the current is limited by current limiting circuits within the digital device 202a. When the output of the digital device 202a goes to a logic high, substantially no current flows through the LED <NUM> and no light is produced. Not requiring an external current limiting resistor between the LED <NUM> and the output of the digital device 202a reduces cost and complexity of a product requiring control of LEDs.

Referring to <FIG>, 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 <NUM> is coupled to an output of a output of a digital device, e.g., digital device 202b and to a power supply common Vss. Whenever the output of the digital device 202b goes to a logic high, current flows through the LED <NUM> and light is produced, wherein the current is limited by current limiting circuits within the digital device 202b. When the output of the digital device 202b goes to a logic low, substantially no current flows through the LED <NUM> and no light is produced. Not requiring an external current limiting resistor between the LED <NUM> and the output of the digital device 202b reduces cost and complexity of a product requiring control of LEDs.

Referring to <FIG>, 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) <NUM> of a digital device, e.g., microcontroller (<FIG>) is coupled to a switch <NUM> which is coupled to a programmable constant current sink <NUM>. When a LED <NUM> is coupled to the I/O node <NUM> as shown in <FIG>, a current will flow through the LED <NUM> and switch <NUM> (when closed) that is determined by the programmable constant current sink <NUM>. The constant current value may be set to, for example but is not limited to, from about five (<NUM>) mA to about <NUM> mA. Selection of the constant current value may be determined by a current set signal to the programmable constant current sink <NUM>. Optionally, a receiver <NUM> may be used to determine logic levels at the I/O node <NUM> when used as an input and/or an output node.

Referring to <FIG>, 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) <NUM> of a digital device, e.g., microcontroller (<FIG>) is coupled to a switch <NUM> which is coupled to a programmable constant current source <NUM>. When a LED <NUM> is coupled to the I/O node <NUM> as shown in <FIG>, a current will flow from the programmable constant current source <NUM>, through the LED <NUM> and switch <NUM> (when closed) that is determined by the programmable constant current source <NUM>. The constant current value may be set to, for example but is not limited to, from about five (<NUM>) mA to about <NUM> mA. Selection of the constant current value may be determined by a current set signal to the programmable constant current source <NUM>. Optionally, a receiver <NUM> may be used to determine logic levels at the I/O node <NUM> when used as an input and/or an output node.

Referring to <FIG>, 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 <NUM> and <NUM> may be used to switch between a programmable constant current sink <NUM> and a low side drive circuit <NUM>, e.g., NMOS field effect transistor (FET). High side drive circuit <NUM>, e.g., NMOS FET, pulls the I/O node <NUM> to substantially the power supply voltage, Vdd. The multiplexers <NUM> and <NUM> may be controlled with a current limit enable signal for selecting between the programmable constant current sink <NUM> and the low side drive circuit <NUM>. Configurations, e.g., current limiting or non-current limiting, current limit value, etc., for the I/O node <NUM> may be stored in a configuration register <NUM>. The configuration register <NUM> may be a volatile memory, e.g., random access memory (RAM), or a nonvolatile memory, e.g., programmable read only memory, Flash memory, etc..

For example, when a logic high is desired at the I/O node <NUM>, the high side driver circuit <NUM> is enabled, the multiplexers <NUM> and <NUM> select the low side driver circuit <NUM>, and the low side driver circuit <NUM> is disabled. When a logic low without current limiting is desired at the I/O node <NUM>, the high side driver circuit <NUM> is disabled, the multiplexers <NUM> and <NUM> select the low side driver circuit <NUM>, and the low side driver circuit <NUM> is enabled. When a logic low with current limiting is desired at the I/O node <NUM> (e.g., LED <NUM> on), the high side driver circuit <NUM> is disabled and the multiplexers <NUM> and <NUM> select the programmable constant current sink <NUM>, thereby limiting current through the I/O node <NUM> to the selected current value. Optionally, a receiver <NUM> may be used to determine logic levels at the I/O node <NUM> when used as an input and/or an output node.

Referring to <FIG>, 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 <NUM> and <NUM> may be used to switch between a programmable constant current source <NUM> and a high side drive circuit <NUM>, e.g., PMOS field effect transistor (FET). Low side drive circuit <NUM> pulls the I/O node <NUM> to substantially the power supply common, Vss. The multiplexers <NUM> and <NUM> may be controlled with a current limit enable signal for selecting between the programmable constant current source <NUM> and the high side drive circuit <NUM>. Configurations, e.g., current limiting or non-current limiting, current limit value, etc., for the I/O node <NUM> may be stored in a configuration register <NUM>. The configuration register <NUM> may be a volatile memory, e.g., random access memory (RAM), or a nonvolatile memory, e.g., programmable read only memory, Flash memory, etc..

For example, when a logic low is desired at the I/O node <NUM>, the low side driver 32select the programmable constant current source <NUM>, thereby limiting current through the I/O node <NUM> to the selected current value. Optionally, a receiver <NUM> may be used to determine logic levels at the I/O node <NUM> when used as an input and/or an output node.

Referring to <FIG>, 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 <NUM> may be switched on or off, wherein the combination of the plurality of constant current blocks 918a-918n switched on determine the constant current value allowed into the I/O node <NUM>. A reference <NUM> may be used in combination with the plurality of unit constant current blocks <NUM> so as to maintain a specific constant current for each of the plurality of constant current blocks <NUM>. Switch <NUM> may be used to disconnect the plurality of unit constant current blocks <NUM> from the I/O node <NUM>.

Referring to <FIG>, depicted is a detailed schematic diagram of a programmable constant current sink, according to the teachings of this disclosure. Transistor <NUM> and constant current source <NUM> are used to produce a voltage control signal that controls the amount of current that flows through each of a plurality of transistors <NUM>. Each of the switch pairs <NUM> and <NUM> selectively enable or disable respective ones of the plurality of transistors <NUM>. The constant current value that will pass into the I/O node <NUM> may be programmed by enabling and disabling, with the respective switch pairs <NUM> and <NUM>, an appropriate number of the plurality of transistors <NUM>.

Claim 1:
A microcontroller (202a; 202b) having a node (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) that is current limited, comprising:
an input/output node (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>);
a configuration register (<NUM>; <NUM>);
an input/output driver circuit coupled with the input/output node (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>) controlled by the configuration register (<NUM>; <NUM>) of said microcontroller (202a; 202b), wherein the input/output driver circuit is configurable through said configuration register (<NUM>; <NUM>) to operate as a digital output driver without current control or as an output driver with current control;
wherein the input/output driver circuit comprises a high side drive circuit (<NUM>; <NUM>), a low side drive circuit (<NUM>; <NUM>), multiplexers (<NUM>, <NUM>; <NUM>, <NUM>) and a programmable constant current circuit (<NUM>; <NUM>; <NUM>; <NUM>) coupled to the multiplexers (<NUM>, <NUM>; <NUM>, <NUM>),
wherein, when configured to operate as an output driver without current control, the multiplexers (<NUM>, <NUM>; <NUM>, <NUM>) are controlled to couple the high side drive circuit (<NUM>; <NUM>) and the low side drive circuit (<NUM>; <NUM>) with the input/output node (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>) and decouple the programmable constant current circuit (<NUM>; <NUM>; <NUM>; <NUM>) from the input/output node (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>),
and wherein, when configured to operate as an output driver with current control, the multiplexers (<NUM>, <NUM>; <NUM>, <NUM>) are controlled to decouple the high side drive circuit (<NUM>; <NUM>) or the low side drive circuit (<NUM>; <NUM>) and to couple the programmable constant current circuit (<NUM>; <NUM>; <NUM>; <NUM>) with the input/output node (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>)wherein the programmable constant current circuit (<NUM>; <NUM>; <NUM>; <NUM>)is configured to control the amount of current provided by the high side drive circuit (<NUM>; <NUM>) or the low side drive circuit (<NUM>; <NUM>) through the input/output node (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>) to a current value that is programmed into the programmable constant current circuit (<NUM>; <NUM>; <NUM>; <NUM>).