Patent Publication Number: US-7592754-B2

Title: Method and apparatus for driving a light emitting diode

Description:
BACKGROUND 
   Electronic devices often employ light emitting diodes (LEDs) to indicate the activity or inactivity of the devices. In order to operate within specified parameters, LEDs typically require a relatively narrow range of direct current and voltage. As a result, to use an LED as status indicator, it is customary practice to employ a series, current-limiting resistor to adjust the voltage provided to the LED which, in turn, controls the current through, and the brightness of, the LED for a given application. 
   In certain devices, such as in data communications devices, a general purpose voltage source can be used to drive an LED. For example, as illustrated in the schematic of Prior Art  FIG. 1 , a device  10  includes an integrated circuit (IC) (e.g., physical layer (PHY))  12 , having a driver  13  and power supply pins  14 ,  16  where the power supply pin  14  couples to a positive supply rail  18  and the power supply pin  16  couples to a negative supply rail  20  (e.g., ground). The device  10  also includes an LED  22 , and a current limiting resistor  24  coupled between the supply rail  18  and IC  12 . In use, the driver  13  causes the output voltage V OUT  to equal to the supply voltage V DD  (e.g., the driver  13  pulls the supply voltage V OUT  to V DD ) thereby causing a current to flow through, and activate, the LED  22 . The amount of current that flows through the LED  14  is related to the supply voltage (V DD ), the output voltage (V OUT ), the LED voltage drop (V LED ) and the resistor value (R) and is governed by the equation:
 
 I   LED =( V   DD   −V   OUT   −V   LED )/ R.  
 
The brightness of (e.g., the amount of light emitted by) the LED  22  is proportional to the amount of current running through the LED  22 .
 
   When the supply voltage V DD  is relatively large, the current that flows through the LED  22  is substantially constant. For example, in the case where the supply voltage V DD  is 5V, the current that passes through the LED  22  can be between about 11 mA and 9 mA, resulting in a current tolerance between +/−11%. As a result, the brightness of the LED  22  is substantially constant over time. 
   SUMMARY 
   Developments in IC technology have reduced the amount of supply voltage V DD  required by certain IC&#39;s. For example, certain IC&#39;s require supply voltages V DD  of between 2.5V and 3.3V. However, as the supply voltages in certain devices are reduced to accommodate these IC&#39;s, such a reduction can affect the tolerances of the current running through an LED. For example, in the case where the supply voltage V DD  is 3.3V, the current that passes through the LED can be between about 12 mA and 8 mA, resulting in a current tolerance between +/−17%. In the case where the where the supply voltage V DD  is 2.5V, the current that passes through the LED can be between about 14 mA and 7 mA, resulting in a current tolerance between +/−33%. In either case, the reduced supply voltage V DD  provides relatively large current variation within the LED thereby causing the LED to generate a variable amount of brightness. 
   Certain devices, such as data communications devices (e.g., a router or Power-over-Ethernet (PoE) device), include a number of status LEDs disposed in relatively close physical proximity with each other. When a reduced amount of supply voltage V DD  is used to power the aforementioned ICs and LEDs of these devices, each of the LEDs can be driven to different levels of brightness because of the rather large current tolerances of the current. With such variable brightness, a user can visually detect the difference in brightness levels in adjacent LEDs and may believe the device to be defective. As a result, the user may return the properly functioning device to the manufacturer for “repair” or replacement. 
   By contrast to conventional LED driving mechanisms, embodiments of the invention are directed to a method and apparatus for driving a light emitting diode. An LED drive circuit includes a current source configured to electrically drive an LED where the current source maintains a current when the voltage across it changes. The current source draws a substantially constant current through the LED, compared to the sole use of a current liming resistor in series with the LED. In one configuration, the current source forms part of an integrated circuit that requires a relatively small amount of voltage for operation. As such, separate voltage sources can be electrically coupled to the LED and integrated circuit respectively. For example, a first voltage source provides a source voltage to the LED that is sufficient to allow operation the LED and a second voltage source provides a source voltage to the integrated circuit that is sufficient to allow operation of the integrated circuit but that is less than a voltage operable to activate the LED. As a result, a low voltage source can be used as a supply for all of the circuitry associated with the integrated circuit, including the current source, without sacrificing the supply voltage used to drive the LED. As such, the supply voltage to the LED can be large enough to minimize effects of current tolerance on the brightness of the light emitted by the LED. 
   In one arrangement, an electronic device includes a first voltage source, an integrated circuit (IC), a second voltage source different than the first voltage source, and a light emitting diode (LED). The IC includes a first pin, a second pin, and a current generator coupled to the first pin and the second pin. The first pin is electrically coupled to the first voltage source and is configured to receive a supply voltage from the first voltage source. The LED includes a first terminal and a second terminal, the first terminal being electrically coupled to the second voltage source and configured to receive a supply voltage from the second voltage source and the second terminal being electrically coupled to the current generator via the second pin of the integrated circuit. The current generator is operable to (i) conduct a first current through the LED, the first current sufficient to cause the LED to emit light and (ii) conduct a second current through the LED, the second current being insufficient to cause the LED to emit light. 
   In one arrangement, an electronic device includes a first voltage source, an integrated circuit (IC), a second voltage source different than the first voltage source, and a light emitting diode (LED). The IC includes a first pin, a second pin, and a current generator coupled to the first pin and the second pin. The first pin is electrically coupled to the first voltage source and is configured to receive a supply voltage from the first voltage source, the supply voltage being less than a voltage operable to activate a light emitting diode. The LED includes a first terminal and a second terminal, the first terminal being electrically coupled to the second voltage source and configured to receive a supply voltage from the second voltage source and the second terminal being electrically coupled to the second pin of the integrated circuit, current generator configured to conduct a current through the LED. 
   One embodiment of the invention relates to a method for electrically driving a light emitting diode (LED). The method includes coupling a first terminal of the LED to a first voltage source and coupling a second terminal of the LED to an integrated circuit having a current generator. The method further includes electrically coupling the integrated circuit to a second voltage source operable to provide a supply voltage to the integrated circuit, the second voltage source being different than the first voltage source and activating the integrated circuit to cause the current generator to (i) conduct a first current through the LED, the first current sufficient to cause the LED to emit light and (ii) conduct a second current through the LED, the second current being insufficient to cause the LED to emit light. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  illustrates a schematic of a prior art LED driver circuit. 
       FIG. 2  illustrates a schematic representation of an electronic device having an LED driver circuit that includes an integrated circuit and a current generator, according to one embodiment of the invention. 
       FIG. 3A  illustrates the current generator of  FIG. 2  configured to conduct a current through an LED where the current is insufficient to cause the LED to emit light, according to one embodiment of the invention. 
       FIG. 3B  illustrates the current generator of  FIG. 2  having a pull down resistor configured to conduct a current through the LED where the current is insufficient to cause the LED to emit light, according to one embodiment of the invention. 
       FIG. 4  illustrates the current generator of  FIG. 2  as a MOSFET based device, according to one embodiment of the invention. 
       FIG. 5  illustrates an arrangement of a current adjustment mechanism of the integrated circuit of  FIG. 2 , according to one embodiment of the invention. 
       FIG. 6  illustrates the current generator of  FIG. 2  configured as a current source, according to one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the invention are directed to a method and apparatus for driving a light emitting diode. An LED drive circuit includes a current source configured to electrically drive an LED where the current source maintains a current when the voltage across it changes. The current source draws a substantially constant current through the LED, compared to the use of a current liming resistor in series with the LED. In one configuration, the current source forms part of an integrated circuit that requires a relatively small amount of voltage for operation. As such, separate voltage sources can be electrically coupled to the LED and integrated circuit respectively. For example, a first voltage source provides a source voltage to the LED that is sufficient to allow operation the LED and a second voltage source provides a source voltage to the integrated circuit that is sufficient to allow operation of the integrated circuit but that is less than a voltage operable to activate the LED. As a result, a low voltage source can be used as a supply for all of the circuitry associated with the integrated circuit, including the current source, without sacrificing the supply voltage used to drive the LED. As such, the supply voltage to the LED can be large enough to minimize effects of current tolerance on the brightness of the light emitted by the LED. 
     FIG. 2  illustrates an embodiment of an electronic device  50 , such as a data communications device or PoE device, having an LED drive circuit  52  with one or more LEDs  54  electrically coupled thereto. As illustrated, the LED drive circuit  52  includes an integrated circuit  58  having a current generator  56  configured to electrically drive the LED  54 . 
   The LED  54  includes a first lead  60  configured to receive a supply voltage V DD  from a voltage source  66  and a second lead  62  configured to couple to the current generator  56 . The LED  54  is operable to provide status information regarding the operation of the electronic device  50 . For example, in the case where the device  50  is a data communications device, an illuminated LED  54  can indicate that the device  50  is actively transmitting communications among user devices while a non-illuminated LED can indicate that the device  50  is not transmitting communications among user devices. While the LED  54  can be any type of light emitting diode, in one arrangement, the LED  54  is a right angle LED indicator such as model L934EW/LGD produced by Kingbright Corporation, Taipei, Taiwan. 
   The integrated circuit  58  includes a first pin or anode  69  and a second pin or node  70  where the first pin  69  is configured to receive a supply voltage V CC  from a voltage source  64  and the second pin  70  is configured to electrically couple the current generator  56  to the LED  54 . In one arrangement, the integrated circuit  58  is dedicated to generating a current to electrically drive the LED  54 . For example, the integrated circuit  58  can be a TOSHIBA TB627 Series Constant Current Driver produced by Toshiba, New York, N.Y. In another arrangement, the integrated circuit  58  is configured as a PHY or a PoE integrated circuit, such as a such as a LTC4259A-1 Quad IEEE 802.3af Power over Ethernet Controller (Linear Technology, Milpitas, Calif.) or a LTC4257-1 IEEE 802.3af Power over Ethernet Interface Controller (Linear Technology, Milpitas, Calif.), that includes the current generator  56 . In yet another arrangement, the integrated circuit  58  can be configured as a switch fabric ASIC or can be utilized in conjunction with the circuits of an integrated Ethernet connector, such as described in U.S. Pat. No. 6,817,890, the contents of which is incorporated by reference in its entirety. 
   The integrated circuit  58  includes diodes  90 , as indicated in  FIG. 4 . In one arrangement, the diodes  90  are an integrated series of diodes  90  coupled to a power supply V CC  of the integrated circuit  58 . The diodes  90  are configured as diode clamps forming a clamping circuit that clamps an output voltage V OUT  at the pin  70  and limit or prevent an over voltage condition at the pin  70 , such as caused by the voltage V OUT  being pulled up to an LED supply voltage V DD . The diode clamps  90  are typically “off”, thereby allowing the clamping of transient voltages. However, continuous driving of the diode clamps  90  can lead to heating and injection of minority carriers into the integrated circuit  58 . 
   The current generator  56  of the integrated circuit  58  is configured to conduct a current I through the LED  54  where the current I is sufficient to activate the LED  54  and cause the LED  54  to emit light. In the embodiment illustrated in  FIG. 2 , the current generator  56  is operable as a current sink to draw current through the LED  54 . For example, as indicated above, the first lead  60  of the LED  54  can be configured as an anode that is attached to a voltage source  66  and the second lead  62  of the LED  54  can be configured as a cathode that is coupled to the current generator  56 . In use, the current generator  56  draws current through the LED  54  from the anode  60  to the cathode  62  to activate the LED  54  and cause the LED  54  to emit light. 
   The electronic device  50  also includes a separate first voltage source  64  and second voltage source  66  each of which are electrically coupled to the integrated circuit  58  and LED  54  respectively. As illustrated, while configured as separate and distinct voltage sources, the first and second voltage sources  64 ,  66  share a common voltage reference  68 , such as a ground reference. In this configuration, the first voltage source  64  is operable to provide a supply voltage V CC  to the integrated circuit  58  while the second voltage source  66  is operable to provide a supply voltage V DD  such as a voltage of about 5V to the LED  54 . In this configuration, the integrated circuit  58  does not provide a supply voltage to the LED  54 . 
   In use, the second voltage source  66  provides a supply voltage V DD , such as a voltage of about 5V, to the LED  54  and the first voltage source  64  provides a supply voltage V CC , such as a voltage of less than 5V, to the integrated circuit  58 . The integrated circuit  58  causes the current generator  56  to conduct a current I, such as a current of about 10 mA, that is sufficient to cause the LED  54  to emit light. As the current generator  56  conducts the current I through the LED  54 , the current I activates the LED  54  and causes the LED  54  to emit light. 
   Because the first and second voltage sources  64 ,  66  each provide a separate supply voltage V CC , V DD  to the integrated circuit  58  and LED  54 , respectively, the supply voltage V DD  can be large enough to minimize the effect of current tolerance on the level of light emitted (e.g., brightness) of the LEDs  54 , thereby allowing multiple LEDs  54  associated with the computerized device  50  to generate substantially uniform (e.g., substantially visually indistinguishable) levels of brightness. Additionally, because the integrated circuit  58  receives a source voltage distinct from the source voltage used to drive the LED  54 , the supply voltage V CC  can be small enough to drive integrated circuits having a variety of voltage requirements. For example, while the first voltage source  64  provides a supply voltage V CC , such as a voltage of less than 5V, the supply voltage V CC  can be 3.3V, 2.5V, 1.8V or less depending upon the configuration and requirements of the integrated circuit  58 . 
   As described with respect to  FIG. 2 , the current source  56  is configured as a current sink  56 . In such a case, in one embodiment, in order to avoid or limit damage to the integrated circuit  58 , the integrated circuit  58  can be designed such that V OUT  at the pin  70  is not pulled up to the supply voltage V DD . For example, assume the current generator  56  does not draw a current I thought the LED  54  and the second voltage source  66  provides V DD  to the LED  54 . Because the LED  54  has an associated amount of resistance, the voltage V OUT  at the pin  70  can be pulled up to the LED supply voltage V DD . For example, if V DD  is 2.5V, the voltage V OUT  at the pin  70  can approach 2.0V. If the voltage at the pin  70  is above a maximum voltage rating of the integrated circuit  58 , current can enter the clamping circuit (e.g., the integrated series of diodes  90  connected to the integrated circuit voltage supply  64  and the integrated circuit  58  can be damaged. 
   In order to accommodate V DD  supply voltages that are above the maximum voltage rating of the integrated circuit  58  or the current source  56 , the integrated circuit  58  can be configured such that the V OUT  at pin  70  does not exceed the integrated circuit&#39;s supply rail  69  when the LED  54  is inactive. In one arrangement, the integrated circuit  58  is configured to generate two different currents through the LED  54 . For example, as indicated above, the current generator  56  can generate a first current I ON , such as a current of 10 mA, through the LED  54  that is sufficient to activate the LED  54  and cause the LED  54  to emit light. Additionally, when the integrated circuit  58  is not operable to drive the LED  54  (e.g., the LED is off), the integrated circuit  58  can draw a second current I OFF  through the LED  54  that is insufficient to cause the LED  54  to emit light. This second current, however, is large enough to lower the voltage V OUT  at the pin  70  to a level that limits or prevents clamping circuits  90  associated with the integrated circuit  58  from operating. Without I OFF , V OUT  of the integrated circuit  58  could be pulled up to the supply voltage V DD . The current I OFF  helps pull the voltage V OUT  below V DD  at the node  70 . This ensures that current does not enter the clamping circuit and potentially damage the integrated circuit  58 . 
   In one arrangement as illustrated in  FIG. 3A , the current generator  56  forms a current sink path between the LED  54  and the ground reference  68 . When the integrated circuit  58  is not operable to drive the LED  54 , the current source  56  is configured to conduct a current I OFF  through the LED  54  where I OFF  is insufficient to cause the LED  54  to emit light. For example, as illustrated in  FIG. 3A , the current source  56  reduces the sink current from I ON , such as a current of 10 mA, to a relatively small current I OFF  such as a current of about 100 uA. The current I OFF  is small enough so as to not cause illumination of the LED  54  and is large enough to lower the voltage V OUT  at the pin  70  to a level that limits or prevents the clamping diodes  90  associated with the integrated circuit  58  from operating. 
   In use, when the LED  54  is on (e.g., generates light), V OUT  at the node  70  is equal to V DD −V LED . This voltage V OUT  will be less than a clamp voltage V CLAMP  associated with the clamping circuit and normally be large enough to ensure that the tolerances associated with V LED  and V DD  allows a particular current to be drawn through the LED  54  to activate the LED  54 . When the LED  54  is off (e.g., does not generate light), V OUT  will be relatively large but not large enough to cause operation of the clamping diodes  90 , thereby setting an upper bound on the voltage V OUT  at the node  70 . For example:
 
 V   OUT   =V   DD   −V   LED     —     OFF  
 
 V   CLAMP   &gt;V   CC   +xV   D  
 
where “x” is the number of clamp diodes  90  in series with the V CC  power supply, each diode having a voltage drop V D , and V LED     —     OFF  is the voltage across the LED  54  when off. As a result, V OUT  at pin  70  remains at a level that is approximately equal to V DD −V LED  and at a level that is less than a sum of the clamp voltage V CLAMP  of the diodes  90  and the voltage drop across one or more of the clamp diodes  90  (e.g., V DD −V LED     —     OFF &lt;V CC +xVD). Therefore, the voltage V OUT  at the pin  70  is sufficient to minimize or prevent the voltage at the pin  70  from being pulled up to the LED supply voltage V DD , thereby limiting or preventing damage to the integrated circuit  58 .
 
   In another arrangement as illustrated in  FIG. 3B , to prevent the voltage at the pin  70  from being pulled up to the LED  54  supply voltage V DD , the integrated circuit  58  includes a pull down resistor  80  coupled to the pin  70 . While the pull down resistor  80  can have a number of resistance values, in one embodiment, the resistor  80  has a value of at least 10 kohms. In use, when the current source  56  does not draw a current through the LED  54 , the pull down resistor  80  forms a current sink path through pin  70  between the LED  54  and the ground reference  68 . In use, a leakage current I OUT , enters the integrated circuit  58  at node  70 . The current I OUT , such as a current of about 100 uA, is insufficient to cause the LED  54  to emit light. The current I OUT , enters the current sink path between the LED  54  and the ground reference  68  rather than entering the clamp circuit for the node  70  as formed by the diodes  90 . As a result, V OUT  at pin  70  remains at a level that is approximately equal to V DD −V LED  but that is less than the clamp voltage V CLAMP  of the diodes  90  (e.g., V DD −V LED     —     OFF &lt;V CC +xVD). The voltage V OUT  at the pin  70  is sufficient to minimize or prevent the voltage at the pin  70  from being pulled up to the LED supply voltage V DD , thereby limiting or preventing damage to the integrated circuit  58 . 
   As indicated above, the current generator  56  is operable to generate a current to activate the LED  54 . While the current generator  56  can have a variety of configurations, in one arrangement, the current generator  56  is a MOSFET based device operable to generate the current I. 
     FIG. 4  illustrates an arrangement of the current generator  56  having MOSFETs M 1  through M n  and one or more diodes  90 . The MOSFETs M 1  through M n  are configured to provide a current conduction path for the current I ON  and form the equivalent of a single transistor. In the case where all MOSFETs M 1  through M n  are substantially equivalent, each MOSFET carries approximately 1/n of the total amount of current I ON . In use, when the current generator  56  is activated, such as by V CC , the MOSFET M x  is pulled to V CC  to provide a current conduction path for the current I ON . When M x  is pulled to ground, the MOSFET M x  is off and the current I ON  through the current generator  56  is substantially equal to zero mA. 
   As indicated above, the brightness of an LED  54  (e.g., as visually detected by a user) is proportional to the amount of current I that flows through the LED  54 . In one arrangement, the integrated circuit  58  is configured to adjust the amount current I that flows through the LED  54  thereby adjusting the amount of light emitted by the LED. For example, the integrated circuit  58  includes a current adjustment mechanism  92  coupled to the current generator  56  that adjusts the amount current I conducted by the current generator  56  through the LED  54 . 
   In the embodiment illustrated in  FIG. 4 , the current adjustment mechanism  92  includes a digital to analog converter  94  electrically coupled to the current generator  56  and a register  96  electrically coupled to the digital to analog converter  94 . The digital to analog converter  94  includes resistor array, each resistor having a switch electrically coupled thereto, where the resistor array is are operable to provide a variable reference voltage V REF  for the current source  56 . The register  96  is configured to provide a series of bits to the digital to analog converter  94  to actuate the resistor switches to adjust the reference voltage V REF  for the current source  56 . As such, the digital to analog converter  94  and register  96  operate together to adjust the current conducted by the current generator  56  through the LED  54  to adjust the brightness of the LED  54 . 
     FIG. 5  illustrates a schematic representation of an arrangement of the digital to analog converter  94 . While the digital to analog converter  94  is shown as having a two bit configuration, one of ordinary skill in the art will understand that the digital to analog converter  94  can be configured with additional bit sections. 
   As illustrated, an R-2R resistor ladder  100  is used to scale the current I conducted by the current generator  56 . All 2R resistors are terminated to a drain connection  102  of a current mirror  104 . The current mirror  104  is formed by MOSFET transistors such that a current in M r1  is mirrored on transistors M 1  through M n  in the current source  56 , as illustrated in  FIG. 4 , to create the current I. In use, 
             I   R     ≈         V   R     -     V   T         2   ⁢   R             
where V R  is a substantially stable reference voltage and V T  is the voltage drop across the mirrored transistors M 1  through M n . In one arrangement, the reference voltage is the voltage V CC . Furthermore, the voltage V A  is
 
                 V   R     -     V   T       2     .         
The voltage V B  is half of the value of V A . When a bit b x  is high (e.g., tied to V R ), transistor M bx  is on and transistor M bnx  is off. When b x  is low (e.g., tied to ground), transistor M bx  is off and transistor M bnx  is on. The current I bx  is approximately the same whether b x  is high or low:
 
             I     b   ⁢           ⁢   1       ≈         V   R     -     V   T         4   ⁢   R       ≈         I   R     2     ⁢           ⁢   and   ⁢           ⁢     I     b   ⁢           ⁢   0         ≈         V   R     -     V   T         8   ⁢   R       ≈       I   R     4           
For each bit section added, the current is halved:
 
             I     r   ⁢           ⁢   1       ≈           I   R     2     ⁢     b   1       +         I   R     4     ⁢     b   0               
The value of b x , as provided by and derived from the register  96 , is either one or zero. In this configuration, the current I can be scaled by the geometry of the transistors used. Additionally, a change in the value b x , as provided by the register  96 , can also proportionally change the value of the current I to adjust the amount of light emitted by the LED  54 . In one arrangement, a binary coding mechanism can be used by the register  96  to cause a proportional change in the current. For example, as provided above, bits b 1  and b 0  represent the binary values (e.g. with b 1  being the most significant bit). In response to a change in the binary values, the current undergoes a change (e.g., an increase or decrease) proportional to the change in the binary value. In one arrangement, the integrated circuit  58  can include an additional number of bits and analog sections to provide an increased range of control.
 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
   For example, as shown in  FIG. 2  and described above, the current generator  56  functions as a current sink to draw current through the LED  54 . Such illustration and description is by way of example only.  FIG. 6  illustrates another arrangement of the current generator  56  where the current generator  56  is configured as a current source. For example, the current generator  56  is disposed between the voltage source  66  and the anode  60  of the LED  54  and the cathode  62  of the LED  54  electrically coupled to a power supply V EE  that is used as the power source to operate the LED  54 . In use, the current generator  56  drives current into the LED  54  toward the cathode  62  to cause the LED  54  to emit light. 
   As described with respect to the embodiment above, the current source  56  forms part of an integrated circuit  58  that requires a relatively small amount of voltage for operation. As such, separate voltage sources  66 ,  64  can be electrically coupled to the LED and integrated circuit respectively. For example, a voltage source  66  provides a source voltage to the LED  54  that is sufficient to allow operation the LED  54  and a voltage source  64  provides a source voltage to the integrated circuit  58  that is sufficient to allow operation of the integrated circuit  58  but that is less than a voltage operable to activate the LED  54 . Such description is by way of example only. In one arrangement, a current source can be used in conjunction with a device having a current liming resistor in series with the LED, such as illustrated in  FIG. 1 . In such an arrangement, the relatively larger voltage used for V DD  would improve the current tolerance in the device. Additionally, when the integrated circuit  58  is not operable to drive the LED  54 , the current source  56  or a pull-down resistor  80  can be used to conduct a current I OFF  through the LED  54  in order to maintain V OUT  at node  70  below a level that could potentially damage the integrated circuit  58 .