Abstract:
Systems and methods for hysteretically controlling Light Emitting Diodes (LEDs) when the input voltage is greater than or equal to 18 volts. An example system includes one or more LEDs and a circuit electrically coupled to the one or more LEDs. The circuit hysteretically controls an input voltage supplied to the one or more LEDs based on a sensed electric current that passes through the LEDs. The circuit includes a MOSFET switch for switching on and off the input voltage supplied to the one or more LEDs, a current sensing subcircuit including a first integrated circuit (IC) for sensing the current flowing through the one or more LEDs, a hysteretic comparator circuit including a second IC for generating a hysteretic control signal based on the sensed current, and a switch driver including a third IC for controlling operation of the switch based on the generated hysteretic control signal.

Description:
BACKGROUND OF THE INVENTION 
   Current hysteretic controllers for Light Emitting Diodes (LEDs) are either limited to an input voltage below 18 volts or use complex implementations involving level shifting and charge pumps implemented with discrete electronic components to control a high-side switch. Other high voltage LED controllers require large inductor values or sense the current only when the switch is on. This leads to errors in the average value of the current being controlled. Therefore, a need exists for a hysteretic controller with a simple, less costly, implementation that allows for an input voltage greater than or equal to 18 volts. 
   SUMMARY OF THE INVENTION 
   The present invention provides systems and methods for hysteretically controlling Light Emitting Diodes (LEDs) when the input voltage is greater than or equal to 18 volts. An example system includes one or more LEDs and a circuit electrically coupled to the one or more LEDs. The circuit hysteretically controls an input voltage supplied to the one or more LEDs based on a sensed electric current that passes through the LEDs. 
   In one aspect of the invention, the circuit includes a MOSFET switch for switching on and off the input voltage supplied to the one or more LEDs, a current sensing subcircuit for sensing the current flowing through the one or more LEDs, a hysteretic comparator circuit for generating a hysteretic control signal based on the sensed current, and a switch driver for controlling operation of the switch based on the generated hysteretic control signal. 
   In an additional aspect of the invention, the current sensing subcircuit includes a first integrated circuit (IC), the hysteretic comparator circuit includes a second IC, and the switch driver includes a third IC, resulting in a simple hysteretic controller implementation that accepts input voltages within the range starting at approximately 5 volts up to input voltages greater than 18 volts, such as up to at least approximately 76 volts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
       FIG. 1  illustrates an LED controller circuit formed in accordance with an embodiment of the present invention; 
       FIG. 2  illustrates additional detail for an example embodiment of the LED controller circuit shown in  FIG. 1 ; 
       FIG. 3  is a schematic diagram of an example embodiment of the LED controller circuit shown in  FIG. 2 ; and 
       FIGS. 4 and 5  are flowcharts of a method of controlling one or more LEDs in accordance with an embodiment of the invention. 
       FIG. 6  is a flowchart of a method describing the functionality of the circuit shown in  FIGS. 2 and 3 . 
       FIG. 7  is an example timing diagram for the circuit shown in  FIGS. 2-3  and processes shown in  FIGS. 4-6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a Light Emitting Diode (LED) system  20 . The system  20  includes one or more LEDs  22  that are controlled by a high voltage hysteretic controller circuit  24 . The high voltage hysteretic controller circuit  24  receives an input voltage (V IN ) that is greater than the voltage provided to the LEDs. Examples of voltage sources for the input voltage include a battery, car alternator, aircraft generator, or a lab power supply. The high voltage hysteretic controller circuit  24  is capable of receiving a V IN  greater than or equal to 5 volts up to a V IN  of approximately 76 volts with surges to approximately 80 volts and an external ground or return line as inputs and supplying a current that drives the LEDs  22 . Generally, the high voltage hysteretic controller circuit  24  provides a relatively constant average current to the LEDs  22  by monitoring the current supplied to the LEDs  22  and hysteretically controlling a switch connected to V IN  such that the current remains within a particular range. 
     FIG. 2  is a block diagram illustrating additional detail for an example embodiment of the LED system  20  shown in  FIG. 1 . In this example embodiment, the high voltage hysteretic controller circuit  24  is shown to include a power conditioning circuit  26  that receives V IN  as an input and produces a cleaner voltage at an output to be used by other portions of the hysteretic controller circuit  24 . The power conditioning circuit  26  reduces radio frequency (RF) noise generated by the hysteretic controller and line voltage spikes in an example embodiment. The output of the power conditioning circuit  26  is connected to a current sensing circuit  28 , a power supply circuit  30 , and the cathode end of a free-wheeling diode D 1 . The power supply circuit  30  is used to power a hysteretic comparator circuit  31  and a switch driver  32 . The current sensing circuit  28  senses current that passes through the LEDs  22  and produces a voltage output, proportional to the sensed current, which is used as an input by the hysteretic comparator circuit  31 . The hysteretic comparator circuit  31  produces an output value that causes the switch driver  32  to turn a switch  34  on and off. When the switch  34  is on, current flows from the power conditioning circuit  26  through the current sensing circuit  28  to power the LEDs  22 . The current then passes through a storage element  38  that stores energy to be used when the switch  34  is off. The current then passes through the switch  34  to circuit return. When the current as sensed by the current sensing circuit  28  exceeds a specified threshold as determined by the hysteretic comparator circuit  31 , the output value changes causing the switch driver  32  to turn the switch  34  off. When the switch  34  is off, energy stored in the storage element  38  causes a current to flow through the diode D 1  and the current sensing circuit  28  before powering the LEDs  22 . When the current drops below a specified threshold as sensed by the current sensing circuit  28 , the output value produced by the hysteretic comparator circuit  31  changes, thus triggering the switch driver  32  which causes the switch  34  to turn back on. 
     FIG. 3  is a schematic diagram of detailed circuitry for an example embodiment of the LED controller circuit shown in  FIG. 2 . Only a first LED  22   a  and a last LED  22   b  are shown from the one or more LEDs  22  for clarity. The power conditioning circuit  26  takes V IN  and an external ground or return line as inputs. This allows the power conditioning circuit  26  to be connected to a power bus in some embodiments, for example. The V IN  and external ground inputs are connected to a common mode choke L 1  to reduce electromagnetic interference (EMI). The high side of the choke L 1  output is connected to a diode&#39;s D 2  anode. The low side output of the choke L 1  is connected to circuit return. A bidirectional breakdown diode D 3 , a first capacitor C 1 , and a second capacitor C 2  are connected in parallel between the cathode of the diode D 2  and the low side output of the choke L 1 . The diode D 3 , first capacitor C 1 , and second capacitor C 2  assist in stabilizing V IN  to provide a good voltage source to be used by other components of the high voltage hysteretic controller circuit  24 . 
   The current sensing circuit  28  includes a current sense resistor R 1  and a first integrated circuit IC 1  that is used to sense the current flowing through the current sense resistor R 1 . In this example embodiment, the first integrated circuit IC 1  is a MAX4080 High Side, Current-Sense Amplifier with Voltage Output, produced by Maxim Integrated Products. However, ICs with similar characteristics could be used in other embodiments. Although the MAX4080 IC is rated to 76 Volts with a surge rating of 80 Volts, higher input voltages may be possible in other embodiments if the IC used is rated to accept them. The RS+, RS−, VCC, GND, and OUT pins of the MAX4080 chip are used. The RS+ and RS− pins are connected to the end of the sense resistor R 1  connected to the power conditioning circuit output and the first LED  22   a  anode, respectively. The VCC pin is connected to the power conditioning circuit output, the GND pin is connected to circuit return, and the OUT pin is connected to the hysteretic comparator circuit  31 . A third capacitor C 3  is electrically connected at one end to both the RS+ and VCC pins and at the other end to the GND pin. 
   The power supply circuit  30  includes a resistor R 2  connected at one end to the output of the power conditioning circuit  26  and at the other end to the cathode end of a unidirectional Zener breakdown diode D 4 , the anode of the diode D 4  being connected to circuit return. The hysteretic comparator circuit  31  includes an integrated circuit IC 2  that is powered by the voltage established by the breakdown diode D 4 . In this example embodiment, the integrated circuit IC 2  is a MAX9003 Low-Power, High-Speed, Single-Supply Op Amp+Comparator+Reference IC, produced by Maxim Integrated Products. However, ICs with similar characteristics could be used in other embodiments. The AOUT, AIN−, AIN+, VSS, VDD, COUT, and CIN+ pins of the MAX9003 chip are used. The VDD pin is connected to the cathode end of the breakdown diode D 4 , the VSS pin is connected to circuit return, and a fourth capacitor C 4  is connected between the VDD pin and circuit return. The AIN+ pin is connected to the OUT pin from the MAX4080 chip used as IC 1 . A third resistor R 3  is connected between the COUT and CIN+ pins. A fourth resistor R 4  is connected between the CIN+ pin and both the AOUT and AIN− pins. The COUT pin is also connected to the switch driver  32 . The third resistor R 3  and the fourth resistor R 4  are selected to achieve desired on and off points for hysteretic control. 
   The switch driver  32  is shown to include a MOSFET driver  40  and a fifth capacitor C 5 . The MOSFET driver  40  includes a power input that is connected to the cathode of the breakdown diode D 4 , a ground input that is connected to circuit return, a control input that is connected to the COUT pin from the MAX9003 chip used as IC 2 , and a gate output that is connected to the switch  34 . The fifth capacitor C 5  is connected between the power input of the MOSFET driver  40  and circuit return. As an example, the MOSFET driver  40  may be a MIC4417 IttyBittty™ Low-Side MOSFET Driver, produced by Micrel, Inc. The MIC4417 driver is an inverting driver that uses a TTL-compatible logic signal as an input. However, other drivers may be used in other embodiments. The MOSFET driver  40  is used to drive the switch  34 , which is shown in this embodiment as an N-channel MOSFET transistor Q 1  whose gate is driven by the gate output of the MOSFET driver  40 , source is connected to circuit return, and drain is connected to one end of the storage element  38 . In this embodiment, the storage element  38  is an inductor L 2  whose other end is connected to the cathode of the last LED  22   b  in the one or more LEDs  22 . 
   When V IN  is applied, the high voltage hysteretic controller circuit  24  powers up in a state such that the output of the hysteretic comparator circuit  31  is low. This places the MOSFET transistor Q 1  in its ‘ON’ state using the switch driver  32 . The current in the inductor L 2  begins to ramp up and the LEDs  22  illuminate as the current is passing through them. The high-side current sensing circuit  28  amplifies the voltage developed across the sense resistor R 1  to provide an amplified sense signal output voltage that is proportional to the voltage developed across the sense resistor R 1 . The amplified sense signal output voltage is fed to the hysteretic comparator circuit  31 . When the amplified sense signal output voltage equals the threshold value of the hysteretic comparator circuit  31 , the output of the hysteretic comparator circuit  31  transitions from low to high, establishing a new threshold value. The high on the output of the hysteretic comparator circuit  31  turns the MOSFET transistor Q 1  ‘OFF’ using the switch driver  32 . This causes the current in the inductor L 2  and the LEDs  22  to recirculate through the free-wheeling diode D 1 . As the current ramps down, the high side current sensing circuit  28  continues to provide a signal that is proportional to the current in the LEDs  22 . When the amplified signal equals the lower threshold value of the hysteretic comparator circuit  31 , the output of the hysteretic comparator circuit  31  transitions from high to low, turning the MOSFET transistor Q 1  back ‘ON’ using the switch driver  32  and reestablishing the high threshold value. The cycle then repeats. 
     FIGS. 4 and 5  are flowcharts of a method  70  of controlling one or more LEDs in accordance with an embodiment of the invention.  FIG. 4  shows that the method  70  begins at a block  72  where one or more LEDs are energized with a circuit configured to operate with all input voltages within the range of approximately 5 volts to approximately 76 volts. Next, at a block  74 , the current passing through the LEDs is sensed. Then, at a block  76 , the input voltage is hysteretically controlled based on the sensed current. The method  70  then loops back to the block  74  where the current passing through the LEDs is sensed again. In an example embodiment illustrated in  FIG. 5 , the block  76  is shown to include a number of other blocks that describe in greater detail an example method of hysteretically controlling the input voltage based on the sensed current. First, at a block  80 , a hysteretic control signal is generated based on the sensed current. Next, at a block  82 , a MOSFET switch is controlled based on the generated hysteretic control signal. Then, at a decision block  84 , it is determined whether the MOSFET switch is on. If the MOSFET switch is on, energy is stored in a storage element at a block  86  and the LEDs are powered by the input voltage. Then, the method loops back to the block  74 . If the MOSFET switch is off, the stored energy in the storage element is dissipated through the one or more LEDs at a block  88 . Then, the method loops back to the block  74 . 
     FIG. 6  is a flowchart of a method  100  describing the functionality of the circuit  20  shown in  FIGS. 2 and 3 . First, at a block  102 , one or more LEDs are energized with a circuit configured to operate with all input voltages within the range of approximately 5 volts to approximately 76 volts. Next, at a block  104 , the switch  34  is turned on and an upper threshold value for the hysteretic comparator circuit  31  is set. Then, at a block  106 , increasing current passing through the LEDs  22  is sensed with the current sensing circuit  28 . Then, at a decision block  108 , it is determined whether the sensed current meets or exceeds the upper threshold value. If the sensed current does not meet or exceed the upper threshold value, the method  100  loops back to the block  106 . If the sensed current does meet or exceed the upper threshold value, the method proceeds to a block  110  where the switch  34  is turned off and the lower threshold value is set. Then, at a block  112 , decreasing current is sensed passing through the LEDs  22  with the current sensing circuit  28 . Next, at a decision block  114 , it is determined whether the sensed current is at or below the lower threshold value. If the sensed current is not at or below the threshold value, the method loops back to the block  112 . If the sensed current is at or below the threshold value, the method loops back to the block  104  where the switch  34  is turned on again and the upper threshold value is set. The method  100  then proceeds as described above. 
     FIG. 7  is an example timing diagram for the circuit shown in  FIGS. 2-3  and processes shown in  FIGS. 4-6 . 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, changes could be made to the power conditioning circuit such as combining the first capacitor C 1  and the second capacitor C 2 , or the power conditioning circuit could be eliminated if a clean and stable voltage source was available as an input. Additionally, different types of ICs that perform similar functions to the example ICs mentioned could be used. Further, a non-inverting switch driver rather than an inverting switch driver  32  could be used if the hysteretic comparator circuit  31  output was also changed. Additionally, a V IN  lower than 18 V could be used depending on how many LEDs were being driven. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.