(12) United States Patent
`(10) Patent N0.:
`US 6,586,890 B2
`
`Min et al.
`(45) Date of Patent:
`Jul. 1, 2003
`
`USOO6586890B2
`
`(54) LED DRIVER CIRCUIT WITH PWM
`OUTPUT
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`(75)
`
`Inventors: Young-Kee Min, Elm Grove, WI (US);
`Bernd Clauberg SCha‘Emburg’ IL (Us);
`Bertrand J- E- Hontele>Da1fsenlaan
`(NL)
`
`(73) Assignee: Koninklijke Philips Electronics N-V-,
`Eindhoven (NL)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`........................ 340/479
`1/1999 Frida
`5,856,779 A *
`..... 315/82
`5,998,929 A * 12/1999 Bechtyel et a1.
`
`6,150,771 A * 11/2000 Perry ......................... 315/291
`6,177,782 B1 *
`1/2001 L’Hermite et a1.
`......... 323/222
`6,239,716 B1 *
`5/2001 Pross et a1.
`.............. 340/8154
`6,313,589 B1 * 11/2001 Kobayashi et a1.
`......... 315/309
`6,320,330 B1 * 11/2001 Haavisto et a1.
`............ 315/291
`.....
`6,362,578 B1 *
`3/2002 Swanson et a1.
`315/307
`
`.................... 191/2
`6,408,998 B1 *
`6/2002 Saito et a1.
`.
`.
`* Cited by examiner
`
`(21) Appl. No.: 10/012,000
`
`(22)
`
`Filed:
`
`Dec. 5, 2001
`
`(65)
`
`Prior Publication Data
`US 2003/0102819 A1 Jun. 5, 2003
`
`(51)
`Int. Cl.7 ......................... H05B 41/14; H02M 3/335
`(52)
`................. 315/224; 363/21.15
`
`(58) Field of Search ................................. 315/224, 247,
`315/129, 209 R, 307, 308; 363/15, 21.11,
`21.12, 21.15, 21.17; 362/800
`
`Primary Examiner—Don Wong
`Assistant Examiner—Minh D A
`
`(57)
`
`ABSTRACT
`
`The driver circuit for light emitting diodes (LEDS) of the
`present invention provides power to LEDS using pulse Width
`modulation (PWM). The driver circuit 100 uses current
`feedback to adjust power to LED arrays 54 and provides a
`full light and a dim mode
`
`31 Claims, 5 Drawing Sheets
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`50
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`66
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`6052 54
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`POWER
`SUPPLY
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`
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`LF
`OSCILLATOR
`
`
`
`CONTROL
`COMPARATOR
`56
`
`
`[0
`
`
`PWM
`
`RIGID-1001 page 1
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`RIGID-1001 page 1
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`US. Patent
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`Jul. 1, 2003
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`US. Patent
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`Jul. 1, 2003
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`RIGID-1001 page 4
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`US. Patent
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`Jul. 1, 2003
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`Sheet 5 0f 5
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`US 6,586,890 B2
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`RIGID-1001 page 6
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`RIGID-1001 page 6
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`US 6,586,890 B2
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`1
`LED DRIVER CIRCUIT WITH PWM
`OUTPUT
`
`TECHNICAL FIELD
`
`The technical field of this disclosure is power supplies,
`particularly, a driver circuit for light emitting diodes (LEDs)
`using pulse width modulation (PWM).
`
`BACKGROUND OF THE INVENTION
`
`Traditionally, incandescent and fluorescent illuminating
`devices have been used as light sources in automobiles and
`other vehicles. However, significant advances in the tech-
`nology of light emitting diodes (LEDs) have made LEDs
`attractive for use in vehicles, because of their long operating
`life, high efficiency, and low profile.
`The electrical characteristics of LEDs are such that small
`
`changes in the voltage applied to the LED lamp will cause
`appreciable current changes. LED light output is propor-
`tional to the LED current and, therefore, a current source is
`the preferred method of driving the LEDs. At present, LED
`drivers in vehicles use driver circuits with voltage source
`outputs, and current
`limiting resistors or linear current
`regulators. Current limiting resistors cause power loss, mak-
`ing the driver circuits inefficient. In addition, current regu-
`lation is not precise. Driving LEDs at other than nominal
`current can reduce LED life and produce unpredictable light
`output. As the application of LED’s in vehicles expands to
`higher power applications, such as the rear combination
`lights (Stop/Turn/Tail),
`the performance of these driver
`circuits is no longer acceptable in terms of efficiency and
`regulation.
`It would be desirable to have a driver circuit for LEDs that
`
`would overcome the above disadvantages.
`
`SUMMARY OF THE INVENTION
`
`One aspect of the present invention provides a driver
`circuit for LEDs with good regulation and efficiency.
`Another aspect of the present invention provides a driver
`circuit for LEDs maintaining operation at the LEDs’ nomi-
`nal current.
`
`The foregoing and other features and advantages of the
`invention will become further apparent from the following
`detailed description of the presently preferred embodiments,
`read in conjunction with the accompanying drawings. The
`detailed description and drawings are merely illustrative of
`the invention, rather than limiting the scope of the invention
`being defined by the appended claims and equivalents
`thereof.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 shows a block diagram of a driver circuit for LEDs
`made in accordance with the present invention.
`FIGS. 2A—2D show a schematic diagram for a driver
`circuit for LEDs made in accordance with the present
`invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`The driver circuit for light emitting diodes (LEDs) of the
`present invention provides power to LEDs using pulse width
`modulation (PWM). The power supply uses current feed-
`back to adjust power to the LEDs and provides a full light
`and a dim mode.
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`FIG. 1 shows a block diagram of a driver circuit for LEDs
`made in accordance with the present invention. A tail lamp
`input signal is provided at block 50 and supplied to power
`supply 52. The power supply 52 can be a DC/DC converter
`such as a buck-boost power supply or other alternatives,
`such as a boost, buck, or fiyback converter. The power
`supply 52 supplies power for LED array 54 and is controlled
`by PWM control IC 56. The PWM control IC 56 provides a
`high frequency periodic drive signal of varying pulse width
`to direct the power supply 52 to supply power as required by
`the LED array 54 in response to a feedback signal. In one
`embodiment, the drive signal can be a square wave oscil-
`lating between 0 and 12 volts with a frequency of 20 kHz.
`The comparator 58 provides the feedback signal by com-
`paring the sensed current signal from current sensor 60 and
`the reference signal from reference current source 62.
`Low frequency oscillator 64 provides a low frequency
`oscillating signal to the power supply 52 and the PWM
`control IC 56. The low frequency places the driver circuit in
`the dim mode as long as the tail lamp input signal 50 is
`present and the stop input signal 66 is not present. The input
`signals are illustrated as a tail lamp input signal and a stop
`input signal as examples, but the input signals can be any
`signals desired where a dim mode corresponds to one input
`signal and a full light mode corresponds to a second input
`signal. The low frequency of the low frequency oscillator 64
`is lower than the drive signal from the PWM control IC 56.
`The low frequency oscillating signal oscillates between a
`first state and a second state.
`In one embodiment,
`the
`oscillating signal can be a square wave oscillating between
`0 and 16 volts with a frequency of 200 to 300 Hertz. When
`the low frequency oscillating signal is in a low state, the low
`frequency oscillating signal blocks the power to the LED
`array 54 from the power supply 52 and holds the output of
`PWM control IC 56 low. When the low frequency oscillating
`signal changes from low to high, the output of PWM control
`IC 56 is synchronized to the transition. This allows the use
`of lower operating frequency in conjunction with the dim
`mode, while maintaining stable LED current. When the stop
`input signal 66 is present at the power supply 52, the power
`supply 52 and the PWM control IC 56 are unaffected by the
`low frequency oscillating signal from the low frequency
`oscillator 64 and the driver circuit is placed in the full light
`mode. In another embodiment, where only full light mode is
`desired, the low frequency oscillator 64 and the tail lamp
`input signal 50 can be omitted and an alternate input to the
`stop input signal 66, such as a turn input signal, can be
`provided.
`The low frequency of the low frequency oscillator 64 is
`selected to be high enough to ensure that no flicker is visible
`from the LED array 54; for example, in one embodiment the
`low frequency can be selected to be about 200 to 300 Hz.
`The low frequency is also selected to be low enough to limit
`electromagnetic interference (EMI) from the power supply
`52, yet high enough to provide a reasonable inductance
`component for magnetics of the power supply 52. Extremely
`low EMI specification limits are imposed by the automotive
`industry. Bulb out signal at block 68 provides a signal
`indication that the LED array 54 has burned out or has
`become disconnected.
`
`FIGS. 2A—2D show a schematic diagram a driver circuit
`for LEDs made in accordance with the present invention.
`TURN input 102, STOP input 104, and TAIL input 106
`provide control input signals to driver circuit 100 from the
`vehicle. The control input signals can be from 6 to 16 volts,
`but are typically 14.5 volts, for example. The control input
`signals are normally in a low state and change to high when
`
`RIGID-1001 page 7
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`RIGID-1001 page 7
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`US 6,586,890 B2
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`3
`certain operation is desired. GROUND input 108 supplies
`the ground reference for the driver circuit 100. Jumper 110
`can provide optional operating modes for foreign and
`domestic vehicles, so for foreign vehicles one LED array is
`used as a turn signal and another LED array is used for a tail
`light, while for domestic vehicles one LED array is used as
`a combined stop and turn signal and another LED array is
`used for a combined stop signal, turn signal, and tail light.
`Diodes and capacitors connected to the control inputs pro-
`vide EMI filtering and reverse voltage battery protection.
`First power supply 112 supplies current to first LED array
`114. The first LED array 114 operates in the full light mode
`in domestic vehicles in response to the TURN input 102 or
`the STOP input 104 control signals. The power supply 112
`operates in PWM mode control to regulate the current in the
`first LED array 114 to the required value. First FET 116
`switches the first power supply 112 rapidly in response to a
`drive signal from a first PWM control IC 118. The first PWM
`control IC 118 can be an integrated circuit such as a
`UCC2813-3 manufactured by Unitrode, a UC2842 series
`manufactured by ST Microelectronics, or the like. In one
`embodiment, the drive signal can be a square wave oscil-
`lating between 0 and 12 volts with a frequency of 20 kHz.
`The first PWM control IC 118 varies the pulse width of the
`drive signal in response to a feedback signal from first op
`amp 120. The output of op amp 120 represents a scaled
`version of the first LED array 114 current, which is com-
`pared to an internal reference of the PWM control IC. The
`first op amp 120 and the first PWM control IC 118 provide
`a feedback mechanism so that the LED current remains
`constant and meets the LED array demand. This preserves
`LED life and produces predictable light output.
`In the
`embodiment shown, resistors between the first power supply
`112 and the first LED array 114 are used for LED current
`sensing; a plurality of resistors is used due to the low power
`dissipation capacity of the surface mount resistors used here.
`Second op amp 122 compares a system input voltage
`signal to the downstream voltage signals from first LED
`array 114 and second LED array 126 and provides a BULB
`OUT signal 124, alerting the driver that an LED array is
`burnt out or disconnected. Avoltage supply 128 supplies the
`op amp ICs using the higher voltage of the first LED array
`114 and the second LED array 126, or functionally imple-
`mented via diodes DSA and D5B. Aprotection zener is used
`to clamp the voltage across the op amp supply to a value
`lower than the op amp rating in case of transients on the
`input signals (Turn, Stop or Tail inputs).
`The portion of the driver circuit 100 supplying the second
`LED array 126 is similar to the portion of the driver circuit
`100 supplying the first LED array 114, except
`that
`the
`portion of the driver circuit 100 supplying the second LED
`array 126 is able to drive the second LED array 126 in a low
`frequency dim mode in response to the TAIL input 106
`control signal. The portion of the driver circuit 100 supply-
`ing the first LED array 114 can only drive the first LED array
`114 in the full light mode.
`Second power supply 130 supplies current to the second
`LED array 126. In domestic vehicles, the second LED array
`126 operates in the full light mode in response to the STOP
`input 104 control signal and in the dim mode in response to
`the TAIL input 106 control signal. The LED array current
`signal is pulse width modulated,
`i.e.,
`the current
`to the
`second LED array 126 is a square wave of a predetermined
`frequency and a peak current close to the LED array nominal
`value, with the pulse width set at a fixed duty cycle depend-
`ing on the power required by the second LED array 126. In
`one embodiment, the current signal can be a square wave
`oscillating between 0 and 600 mA with a frequency of 200
`to 300 Hertz.
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`In the full light mode in response to the STOP input 104
`control signal, first transistor 138 provides a ground path so
`that PWM FET 140 conducts continuously, rather than
`following the low frequency oscillation of low frequency
`oscillator 144, allowing continuous current flow through the
`second LED array 126. The ground path through the first
`transistor 138 also keeps second transistor 142 from con-
`ducting. This keeps the second PWM control IC 134 in the
`full light mode, because the second PWM control IC 134
`does not receive oscillating synchronization signals through
`the second transistor 142. Second FET 132 switches in the
`same fashion as first FET 116 described above.
`
`In the dim mode, when the STOP input 104 control signal
`is not present and the TAIL input 106 control signal is
`present, first transistor 138 is off, so that PWM FET 140
`cycles on and off in response to the oscillating signal from
`low frequency oscillator 144. In one embodiment, the oscil-
`lating signal can be a square wave oscillating between 0 and
`16 volts with a frequency of 200 to 300 Hertz. This cycles
`the second LED array 126 on and off as the PWM FET 140
`cycles on and off. The frequency of the low frequency
`oscillator 144 is selected to be high enough to ensure that no
`flicker is visible from the second LED array 126. For
`example, in one embodiment the low frequency duty cycle
`can be selected to be about 10% with an oscillation fre-
`quency of about 200 to 300 Hz. The frequency is also
`selected to be low enough to limit electromagnetic interfer-
`ence (EMI) from the second power supply 130, yet high
`enough to provide a reasonable inductance component for
`magnetics of the second power supply 130. Extremely low
`EMI specification limits are imposed by the automotive
`industry.
`In the dim mode, the oscillating output of the low fre-
`quency oscillator 144 also controls the oscillating synchro-
`nization signals to the second PWM control IC 134 through
`the second transistor 142. When the oscillating signal from
`the low frequency oscillator 144 is low, the second transistor
`142 is off and the second PWM control IC 134 is active.
`
`When the oscillating output of the low frequency oscillator
`144 is high, the second transistor 142 is on and the second
`PWM control IC 134 is held in a reset state. This prevents
`the output voltage of the second PWM control IC 134
`feeding the second FET 132 from rising while the second
`LED array 126 is off. The second transistor 142 also
`synchronizes the second PWM control IC 134 with the low
`frequency oscillator 144 as the low frequency oscillator 144
`output switches from high to low, activating the second
`PWM control IC 134 and energizing the second LED array
`126. The synchronization occurs as the second transistor 142
`turns off, removing the ground signal from the Comp and
`RT/CT pins of second PWM control IC 134. This allows the
`use of lower operating frequency in conjunction with dim
`mode while maintaining stable LED current.
`It is important to note that FIGS. 2A—2D illustrate specific
`applications and embodiments of the present invention, and
`is not intended to limit the scope of the present disclosure or
`claims to that which is presented therein. For example,
`switching devices such as FETs and transistors are
`illustrated, but other switching components such as
`transistors, MOSFETs, IGBTs, or bipolar transistors could
`be used in practicing the invention. As another example,
`synchronization of the PWM control IC is used here to
`obtain constant current even when the low frequency oscil-
`lation is relatively close to the operating frequency of the
`power supply. In some cases, some fluctuations might be
`acceptable and the synchronization may be omitted. Yet
`another possible embodiment of this circuit may use a peak
`
`RIGID-1001 page 8
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`US 6,586,890 B2
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`5
`current sensing technique rather than the one shown here to
`maintain constant peak LED current with varying duty
`cycles of the low frequency PWM signal. Upon reading the
`specification and reviewing the drawings hereof,
`it will
`become immediately obvious to those skilled in the art that
`myriad other embodiments of the present
`invention are
`possible, and that such embodiments are contemplated and
`fall within the scope of the presently claimed invention.
`While the embodiments of the invention disclosed herein
`
`are presently considered to be preferred, various changes
`and modifications can be made without departing from the
`spirit and scope of the invention. The scope of the invention
`is indicated in the appended claims, and all changes that
`come within the meaning and range of equivalents are
`intended to be embraced therein.
`What is claimed is:
`
`1. A system for supplying power for an LED array, said
`system comprising:
`an oscillator generating an oscillating signal, the oscillat-
`ing signal having a first state and a second state; and
`a power supply operatively coupled to the oscillator, the
`power supply providing output power and being
`responsive to the oscillating signal;
`wherein said power supply supplies the output power to
`the LED array when the oscillating signal is in the first
`state and does not supply the output power to the LED
`array when the oscillating signal is in the second state.
`2. The system of claim 1, wherein the output power is
`pulse width modulated power.
`3. The system of claim 1, wherein the frequency of the
`power supply is about 20 kHz.
`4. The system of claim 1, wherein the frequency of the
`oscillating signal is much lower than the frequency of the
`power supply.
`5. The system of claim 1, wherein the duty cycle of the
`oscillating signal is about 10 per cent.
`6. The system of claim 1, wherein the frequency of the
`oscillating signal is about 200 to 300 Hertz.
`7. A system for supplying power for an LED array, said
`system comprising:
`means for sensing current to the LED array, said current
`sensing means generating a sensed current signal;
`means for generating a reference signal;
`means for comparing the sensed current signal to the
`reference signal, said comparing means generating a
`feedback signal;
`means for modulating pulse width responsive to the
`feedback signal, said pulse width modulating means
`generating a drive signal; and
`means for supplying power responsive to the drive signal,
`said power supplying means supplying current to the
`LED array.
`8. The system of claim 7, further comprising:
`means for low frequency oscillating, said low frequency
`oscillating means generating an oscillating signal hav-
`ing a first state and a second state; and
`said power supplying means being responsive to the
`oscillating signal, said power supplying means supply-
`ing current to the LED array during the first state and
`blocking current to the LED array during the second
`state.
`
`9. The system of claim 8, wherein the frequency of the
`oscillating signal is much lower than the frequency of the
`drive signal.
`10. The system of claim 8, wherein the duty cycle of the
`oscillating signal is about 10 per cent.
`
`6
`11. The system of claim 8, wherein the frequency of the
`oscillating signal is about 200 to 300 Hertz.
`12. The system of claim 8,
`wherein said pulse width modulating means is responsive
`to the oscillating signal from the low frequency oscil-
`lating means; and
`wherein said pulse width modulating means supplies the
`drive signal to said power supplying means during the
`first state and blocks the drive signal to said power
`supplying means during the second state.
`13. The system of claim 12, wherein said pulse width
`modulating means synchronizes the drive signal from said
`pulse width modulating means with the oscillating signal
`from said low frequency oscillating means.
`14. The system of claim 7, further comprising:
`means for indicating the LED array is inoperable.
`15. A method of supplying power to an LED array, said
`method comprising:
`sensing current to the LED array and generating a sensed
`current signal;
`generating a reference signal;
`comparing the sensed current signal
`signal;
`generating a feedback signal based on the difference
`between the sensed current signal and the reference
`signal;
`generating a pulse width modulated drive signal based on
`the feedback signal; and
`supplying current to the LED array in response to the
`pulse width modulated drive signal.
`16. The method of claim 15, further comprising:
`generating an oscillating signal having a first state and a
`second state; and
`supplying current to the LED array when the oscillating
`signal is in the first state and blocking current to the
`LED array when the oscillating signal is in the second
`state.
`
`to the reference
`
`17. The method of claim 16, wherein the frequency of the
`oscillating signal is much lower than the frequency of the
`pulse width modulated drive signal.
`18. The method of claim 16, wherein the duty cycle of the
`oscillating signal is about 10 per cent.
`19. The method of claim 16, wherein the frequency of the
`oscillating signal is about 200 to 300 Hertz.
`20. The method of claim 16, wherein a generation of a
`pulse width modulated power signal based on the feedback
`signal includes:
`generating a pulse width modulated power signal when
`the oscillating signal is in the first state; and
`blocking the pulse width modulated power signal when
`the oscillating signal is in the second state.
`21. The method of claim 20, further comprising:
`synchronizing the pulse width modulated drive signal
`with the oscillating signal.
`22. The method of claim 15, further comprising:
`monitoring the LED array; and
`indicating when the LED array is inoperable.
`23. A circuit for supplying power to an LED array
`comprising:
`a power supply 52, the power supply 52 supplying current
`to the LED array 54 and being responsive to a drive
`signal;
`a current sensor 60 for sensing current to the LED array
`54, the current sensor 60 generating a sensed current
`signal;
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`US 6,586,890 B2
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`7
`a reference current source 62 for generating a reference
`signal;
`a comparator 58 for comparing the sensed current signal
`to the reference signal, the comparator 58 generating a
`feedback signal; and
`a PWM control IC 56 responsive to the feedback signal,
`the PWM control IC 56 generating the drive signal.
`24. The circuit of claim 23, further comprising:
`a low frequency oscillator, the low frequency oscillator
`generating an oscillating signal having a first state and
`a second state; and
`wherein said power supply is responsive to the oscillating
`signal, said power supply supplying current to the LED
`array during the first state and blocking current to the
`LED array during the second state.
`25. The circuit of claim 24, wherein the frequency of the
`oscillating signal is much lower than the frequency of the
`drive signal.
`26. The circuit of claim 24, wherein the duty cycle of the
`oscillating signal is about 10 per cent.
`27. The circuit of claim 24, wherein the frequency of the
`oscillating signal is about 200 to 300 Hertz.
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`28. The circuit of claim 24,
`
`wherein said PWM control IC is responsive to the oscil-
`lating signal from the low frequency oscillator; and
`
`wherein said PWM control IC supplies the drive signal to
`said power supply during the first state and blocks the
`power signal to said power supply during the second
`state.
`
`29. The circuit of claim 28, wherein said PWM control IC
`synchronizes the power signal from said PWM control IC
`with the oscillating signal from the low frequency oscillator.
`30. The circuit of claim 23, further comprising:
`
`an LED monitor, said LED monitor generating an LED
`array inoperable signal when said LED array is inop-
`erable.
`
`31. The circuit of claim 23, wherein said power supply is
`selected from a group consisting of a buck-boost power
`supply, a boost power supply, a buck power supply, and a
`flyback converter.
`
`RIGID-1001 page 10
`
`RIGID-1001 page 10
`
`