US 6,359,392 B1
`(10) Patent No.:
`(12) Unlted States Patent
`He
`(45) Date of Patent:
`Mar. 19, 2002
`
`
`U5006359392B1
`
`(54) HIGH EFFICIENCY LED DRIVER
`
`(75)
`
`Inventor: Fan He, Grayslake, IL (Us)
`
`6/1987 DeLuca et a1.
`............. 323/222
`4,673,865 A
`...............
`10/1990 Havel
`340/762
`4,965,561 A
`
`5,575,459 A * 11/1996 Anderson ..
`362/240
`9/2001 Ruston ....................... 315/291
`6,285,140 B1 *
`
`(73) Assignee: Motorola, Inc., Schaumburg, IL (US)
`
`* cited by examiner
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`Primary Examiner—Don Wong
`Assistant Examiner—Wilson Lee
`(74) Attorney, Agent, or Firm—Brian M. Mancini
`
`(21) Appl. No.2 09/754,485
`22
`F1 d.
`. 4 2001
`-
`.
`1e
`Jan ’
`)
`(
`Int. Cl.7 ............................ F21V 3/00; H05B 37/00
`(51)
`52 US. Cl.
`....................... 315 291 323/222 323/267
`(
`)
`/
`,
`,
`,
`323/282; 315/86
`(58) Field of Search ............................. 315/291, 209 R,
`315/241 5’ 200 A, 86, 185 s, 185 R, 312;
`323/222, 282—285, 267, 902
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`ABSTRACT
`(57)
`A high efficiency light emitting diode (LED) driving circuit
`includes a first LED coupled in a forward current path
`between first and second nodes and a second LED being
`cou e
`In a reverse current
`at
`etween t e secon an
`P1 d '
`p h b
`h
`d
`d
`first nodes. A power supply is drives the first node with
`voltage P111535 Acapacitor is coupled to the second node and
`stores charge while the power supply is driving the first LED
`in the forward current path during voltage pulses. A dis-
`charge circuit drains charge from the capacitor to drive the
`second LED in the reverse current path between voltage
`ulses.
`p
`
`3,869,641 A *
`
`3/1975 Goldberg .................... 315/135
`
`20 Claims, 3 Drawing Sheets
`
`
`
`Valeo Exhibit 1010_001
`
`Valeo Exhibit 1010_001
`
`

`

`US. Patent
`
`Mar. 19, 2002
`
`Sheet 1 0f3
`
`US 6,359,392 B1
`
`V 122
`
`124
`
`FIGJ
`
`202
`
`204
`
`2020
`
`126
`
`122
`
`FIG.2
`
`— PRIOR ART —
`
`Valeo Exhibit 1010_002
`
`Valeo Exhibit 1010_002
`
`

`

`US. Patent
`
`Mar. 19, 2002
`
`Sheet 2 0f3
`
`US 6,359,392 B1
`
`
`
`FIG.3
`
`‘35
`
`
`
`FIG.4
`
`Valeo Exhibit 1010_003
`
`Valeo Exhibit 1010_003
`
`

`

`US. Patent
`
`Mar. 19, 2002
`
`Sheet 3 0f3
`
`US 6,359,392 B1
`
`FIG.5
`
`
`
`
`FIG.6
`
`Valeo Exhibit 1010_004
`
`Valeo Exhibit 1010_004
`
`

`

`US 6,359,392 B1
`
`1
`HIGH EFFICIENCY LED DRIVER
`
`FIELD OF THE INVENTION
`
`This invention relates generally to light emitting diode
`(LED) circuits, and more particularly to driver circuits for
`driving LEDs.
`BACKGROUND OF THE INVENTION
`
`In portable radio communication devices it is desirable to
`prolong the operating time and battery life. To reduce the
`current drain from the battery it
`is desirable to develop
`circuits that achieve the lowest power consumption possible.
`Among those circuits, the display draws a disproportionate
`amount of current from the battery. The LED is widely used
`for back lighting in devices such as cellular phones due to its
`simpler driving circuit compared with the electrolumines-
`cent (EL) and fluorescent lighting its comparably lower cost
`and noise. However, the power consumption of LEDs is
`generally higher than the EL lights when multiple LEDs are
`used. In addition, the use of white LEDs, which is necessary
`for backlighting color liquid crystal displays (LCDs), incurs
`power considerations in that white LEDs have higher thresh-
`old voltages, which are often higher than the battery volt-
`ages. Thus DC-DC converter is required to boost the battery
`voltage and the overall power efficiency is reduced.
`A radio communication device, such as a cellular phone,
`is typically powered from a battery, such as a lithium-ion
`battery, having a normal operating voltage of about 3.6 volts.
`Ideally, the device circuits are powered directly from the
`battery, however, some circuits such as light emitting diodes
`(LEDs) used in displays will not operate at this low voltage
`or provide deteriorated performance when the battery runs
`down, and it becomes necessary to add a DC-DC converter
`to step-up the voltage. However,
`the inductor type of
`DC-DC converter may have a typical efficiency of 85%,
`while the charge pump type of DC-DC converter usually has
`efficiencies less than 50% when the battery internal resis-
`tance is considered.
`
`Referring to FIG. 1, a prior art LED inductive boost driver
`circuit is illustrated as described in US. Pat. No. 4,673,865,
`including an inductive switching power supply 102 to per-
`form a DC-DC conversion. An inductor 104 is connected
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`between a node 106 and a battery 108. A transistor 110 is
`connected to node 106. The anode of a diode 112 is also
`connected to node 106 and the cathode is connected to a
`
`45
`
`node 114. A filter capacitor 116 is connected between node
`114 and ground. A duty cycle modulator 118 is connected
`between node 114 and the base of transistor 110.
`
`In operation, duty cycle modulator 118 periodically
`switches on and off transistor 110. When transistor 110 is
`
`switched on, current from battery 108 begins to flow through
`inductor 104, building up the magnetic field in the inductor
`as the current increases. When transistor 110 is switched off,
`the magnetic field collapses and a positive voltage pulse
`appears at node 106. Because inductor 104 is in series with
`battery 108, the voltage of the pulse at node 106 is greater
`than the battery voltage.
`Thus, the periodic switching of transistor 110 causes a
`string of pulses to appear at node 106. These voltage pulses
`are then rectified and filtered by diode 112 and filter capaci-
`tor 116 to produce a multiplied DC voltage at output node
`114. To regulate the output voltage, duty cycle modulator
`118 samples the output voltage at DC output node 114 and
`adjusts the duty cycle of transistor 110 so that the DC output
`voltage remains substantially constant. A current limiting
`resistor 124 is coupled in series with the LED 122 along with
`
`50
`
`55
`
`60
`
`65
`
`2
`a transistor 126 to control the activation of LED 122 via a
`
`control circuit (not shown). Although an improvement in the
`art,
`there is voltage drop across diode 112, and power
`consumed in current limiting resistor 124, which consumes
`battery power.
`Illustrated in FIG. 2 is another prior art LED driver circuit
`that consumes less battery energy than the device of FIG. 1.
`The driver circuit uses switching power supply 102, LED
`122 and transistor 126 that were previously described in
`conjunction with FIG. 1. Also, LED 122 and transistor 126
`are mutually interconnected as in FIG. 1 and transistor 126
`functions to control the activation of LED 122 as previously
`described. However, a capacitor 202 is connected between
`the anode of LED 122 and the pulse output node 106. A
`shunt diode 204 is connected to the junction of capacitor 202
`and LED 122.
`
`In operation, during a positive voltage pulse at output
`node 106, current flows through LED 122 via coupling
`capacitor 202. The capacitor plate 202a of capacitor 202
`begins to charge negatively. Between voltage pulses, i.e.
`when transistor 110 conducts and momentarily grounds
`node 106, capacitor plate 202a goes below ground potential.
`When the negative potential on capacitor plate 202a is
`sufficient to overcome the small (typically 0.6 Volts) forward
`voltage drop across diode 204, the diode conducts, substan-
`tially discharging capacitor 202. Thus, diode 204 provides a
`means for discharging capacitor 202 during a portion of each
`period of the voltage waveform at output node 106.
`Unfortunately,
`the discharge current
`is lost,
`lowering
`efficiency of the driver circuit. Moreover, this device, as well
`as that of FIG. 2, utilizes an inductor type boost converter to
`provide a high supply voltage, which increases cost and size
`of the circuit.
`
`What is needed is a high efficiency LED driver circuit that
`can drive LEDs requiring higher voltage than available
`battery power. It would also be of benefit to eliminate the
`inductive type of boost circuits and the losses associated
`with current
`limiting resistors and switching circuits. It
`would also be advantageous to accomplish this in a low cost,
`simple circuit architecture.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1 and 2 show schematic diagrams of a prior art
`LED driver circuits;
`FIG. 3 shows a schematic diagram of a first embodiment
`of a LED driver circuit,
`in accordance with the present
`invention;
`FIG. 4 shows a schematic diagram of a preferred embodi-
`ment of a LED driver circuit, in accordance with the present
`invention;
`FIG. 5 shows a schematic diagram of a first alternate
`embodiment of a LED driver circuit, in accordance with the
`present invention; and
`FIG. 6 shows a schematic diagram of a second alternate
`embodiment of a LED driver circuit, in accordance with the
`present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`The present invention provides a high efficiency LED
`driver with LED switching whereas prior art devices utilized
`diode or transistor switching.
`In particular,
`the present
`invention provides an improved driving circuit with more
`than 90% power efficiency for LED lighting devices. This is
`accomplished with LEDs requiring a driving voltage greater
`
`Valeo Exhibit 1010_005
`
`Valeo Exhibit 1010_005
`
`

`

`US 6,359,392 B1
`
`3
`than the available power supply voltage. This is also accom-
`plished without the typical inductive boost circuits or current
`limiting resistors of the prior art, and is implemented in a
`simple circuit architecture.
`FIG. 3 shows a first embodiment of the light emitting
`diode (LED) driving circuit of the present invention. At least
`two LEDs 32, 36 are coupled between first and second nodes
`38, 37. A first LED 32 is coupled in a forward current path
`30 between first and second nodes 38, 37. A second LED 36
`is coupled in a reverse current path 34 between the second
`and first nodes 37, 38. In particular, an anode of the first LED
`32 is coupled to a cathode of the second LED 36 at the first
`node 38 and a cathode of the first LED 32 is coupled to an
`anode of the second LED 36 at the second node 37. The
`
`driving circuit also includes a power supply (not shown) for
`producing a substantially periodic waveform. Preferably, the
`waveform is substantially a square wave. The power supply
`is typically derived from a battery and is coupled to drive the
`first node 38 with voltage pulses. Acapacitor C with first and
`second terminals is included. The first terminal is coupled to
`the second node 37 of the at least two LEDs 32, 36. The
`capacitor stores charge from the power supply while the
`power supply is driving the first LED 32 in the forward
`current path 30 during voltage pulses, i.e. when the voltage
`pulse is high. A discharge circuit 35 is coupled between the
`second terminal of the capacitor and the first node 38 of the
`at least two LEDs 32, 36. The discharge circuit 35 drains
`charge from the capacitor to drive the second LED 36 in the
`reverse current path 34 between voltage pulses, i.e. when the
`voltage pulse is low. Preferably, the discharge circuit is an
`inverter with an input coupled to the first node 38 and an
`output coupled to the second terminal of the capacitor. The
`stored charge of the capacitor boosts the voltage available to
`the second LED 36 over a voltage available from the voltage
`pulses of the power supply. This provides an advantage
`where the second LED 36 requires a higher drive voltage
`than the first LED 32. Is this case,
`the boosted voltage
`available during the discharge of the capacitor equalizes
`photonic output between the LEDs
`In a preferred embodiment, the power supply is buffered
`by an inverter 42 driven by a square wave as seen in FIG.
`4, wherein components common to FIG. 3 are numbered
`similarly.
`In addition, a current
`limiting inductor 40 is
`coupled to the first node 38 to limit charge current to the
`capacitor. Because different charging and discharging cur-
`rent eXist in the present invention it is beneficial to optimize
`the LEDs, capacitor, and a duty cycle of the power supply to
`provide uniform average photonic output from the LEDs
`In operation during charging, and referring back to FIG.
`3, the current in the forward current path 30 going through
`the LED 32 is given by:
`
`Ic=[Vo-Vm-Vc(t)l/R
`
`(1)
`
`Where V0 is the power supply or battery voltage, Vm is the
`LED threshold voltage, VC(t) is the voltage on the capacitor
`C, and R is the total circuit resistance. From equation (1),
`one can get:
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`dIC/dt=—(1/R)dVC(t)/dt
`
`Because dVC(t)=IIC dt/C, equation (2) becomes
`
`dIC/dt+IC/(RC)=0
`
`The solution of equation (3) is:
`
`1510 eXp(-[l/(RC)]
`
`60
`
`(2)
`
`65
`
`(3)
`
`(4)
`
`Where
`
`10=(Vo—Vth—Vco)/R
`
`(5)
`
`the average current for the charging
`
`From equation (4),
`process is given by:
`c
`ave
`1
`
`=(Vo-Vm-Vco)C{1-6XP(-[Tc/(RC)]}/Tc
`
`(6)
`
`where TC is the charging time.
`When discharging through the reverse current path 34, the
`capacitor C is in series with the power supply. Thus the total
`voltage is increased to the power supply voltage plus the
`voltage on the capacitor. The current during the discharging
`is given by the following equation:
`
`Io=IVu-VmW+Vc(l)l/Ro
`
`From equation (7), one can get:
`
`dID/dt=—(1/RD)dVC(t)/dt
`
`From dVC(t)=—IIDdt/C, one can get:
`
`ch(t)/dt+Vc(t)/(RDC)=_(VD—Vlfhw)/(RDC)
`
`(7)
`
`(8)
`
`(9)
`
`where RD is the total resistance in the discharge circuit, and
`Vthw is the second LED 36 threshold voltage. The solution
`of equation (9) is given by:
`
`Vc(t)=[Vo-VmW+VchleXP[-l/(RDC)]+V;11W-Va
`
`(10)
`
`where Vch is the voltage across the capacitor before dis-
`charging. The current during the discharge process can be
`calculated with equation (7) and equation (10).
`
`Io=[Vo- VmW+Vch]{1-6XP[-l/(RDC)]}/Ro
`
`(11)
`
`The average discharging current can be computed from (11):
`
`IDilIVE=C[VU_th+Vch]exp[_TD/(RDC)]/TD
`
`(12)
`
`The efficiency can be further improved by adding an induc-
`tor in the circuit (40 of FIG. 4). With an inductor in series
`with the capacitor in the discharging path to reduce the
`maXimum discharging current, the differential equation for
`the current becomes:
`
`dzI/dtz+(R/L)dI/dt+I/(CL)=0
`
`The solution is given by:
`
`1(z)=A,e/“+A,e3‘
`
`(13)
`
`(14)
`
`Where A1 and A2 are two constants to be determined by the
`initial conditions. The constant A and B are given by the
`following expressions:
`
`A=—0.5R/L+0.5*[(R/L)2—4/(CL)]1/2
`
`B=—0.SR/L—O.5*[(R/L)2—4/(CL)]1/2
`
`(15)
`
`(1 6)
`
`It is known that the current is zero at the moment when the
`
`circuit is connected, then the current ramps up at a rate
`determined by the nature of the circuit. From this initial
`condition, one can find that:
`
`A1=—Az
`
`(17)
`
`At the moment when the circuit starts discharging, it cannot
`be determined if there is a capacitor in the circuit by
`
`Valeo Exhibit 1010_006
`
`Valeo Exhibit 1010_006
`
`

`

`US 6,359,392 B1
`
`5
`monitoring the current. Thus, one can induce that the gra-
`dient of the current is the same as the circuit with the same
`
`initial voltage but without the capacitor at the moment when
`the circuit starts discharging. This gives another initial
`condition as follows:
`
`(dI/dt)t=0=(VD+Vch_VthW)/L
`
`From equations (14) through (18), one can get:
`
`1(t)=[(VD+Vch_ Vm W)/L][(R/L)2-4/(CL)]’1/2(8’“-63’)
`
`(18)
`
`10
`
`(19)
`
`the maximum discharging
`With this complete solution,
`current can be found and compared with the maximum
`current in the circuit without inductance. By setting dI/dt=0,
`we have:
`
`(AW—Be‘“)=0
`
`(20)
`
`Where I is the time when the discharging current reaches its
`maximum. By substituting equations (15) and (16) into
`equation (20), one obtains:
`
`t=[(R/L)2—4/(CL)]’1/21n{[R+(R2—4L/C)1/2]/[R—(R2—4L/C)1/2]}
`
`(21)
`
`15
`
`20
`
`6
`ultra-bright LEDs have maximum efficiency at currents near
`or just below their maximum rated current. Also, with the
`exception of GaP red, blue LEDs and white LEDs, LED
`optical characteristics in the high-power zone are excellent,
`permitting effective pulse driving. In other words, for the
`same optical output the green, red and yellow LEDs can be
`driven with very high pulse current but
`lower average
`current. Blue and white LEDs have higher optical efficiency
`at lower current. LEDs also have the same characteristics as
`a general purpose diode, thus they can be used as switch
`device such as that in a charge pump. The threshold voltage
`of green, yellow and red LEDs ranges from 1.8V to 2.4V.
`Although, a buck mode switch regulator can be used to
`increase the power efficiency, this results in cost increase. In
`contrast,
`the threshold voltage of blue and white LEDs
`ranges from 3.3V to 4.2V. A lithium ion battery voltage
`typically ranges from 3.0V to 4.2V with 95% of capacity in
`the range from 3.4V to 4.2V.
`With the combination of pulse driving, using red, green or
`yellow LEDs as switching diode in a charge pump, and using
`the charge pump output to drive blue or white LEDs, the
`present invention can have high power efficiency of 90% or
`more. Table 1 compares the power consumption of the
`present invention compared to prior art light drivers.
`
`into equation (19) gives the
`Substituting equation (21)
`maximum discharging current:
`
`25
`
`TABLE 1
`
`ILW=[(V0+V.h—whW)/R]e*0-5‘<R/L>R<C/L)“2
`
`(22)
`
`For any given value of C, a value for L can be found to meet
`the requirement that:
`
`30
`
`Comparison of different lighting technologies
`LED driver
`of present
`invention
`
`Constant current
`LED driver
`
`Compact
`fluorescent
`
`€70.51(R/L)R(C/L)1/2<1
`
`(23)
`
`Lighting
`components
`
`8 green LED 8 green LED and 2
`and 4 white
`white LED
`LED
`3.6 V
`LED charge
`pump
`Pulsed
`
`3.6 V
`Boost converter
`
`Constant current
`
`4 mA
`
`10 mA
`
`N/A
`
`72 mA
`
`5 mA
`
`20 mA
`
`N/A
`
`98 mA
`
`2 white CCFL
`tube + 8
`green LED
`3.6 V
`Boost
`converter
`High voltage
`AC
`5 mA
`
`N/A
`
`52 mA
`
`80 mA
`
`Battery voltage
`DC—DC converter
`
`Driving method
`
`Average current for
`each green LED
`Average current for
`each white LED
`Average FL current
`drain from battery
`Total average
`current drain from
`battery
`
`the maximum discharging current can be
`In this way,
`reduced by adding an inductor in series with the capacitor.
`Maximum current can also be reducing by limiting the
`discharging time, because the peak current does not happen
`at
`the beginning of the discharge cycle when there is
`inductance in the circuit.
`
`The efficiency of the driving circuit (with inductive cur-
`rent limitation) is determined by the ratio of the power
`consumed by the LED and the total power from the power
`source, which is described in the following equation
`
`TI=(IDJVEVmwTrICJVEVtth)/(ViJDJVETfiVl-JchR)
`
`Given typical values of ICJVE=170 mA, IDiave=500 mA,
`Vthw=3’8V for a white LED, Vth=1.8V for a green/red/
`yellow LED and Vin=3.6V,
`the efficiency of the present
`invention is n=0.91.
`Color LCDs will become very popular in the future hand
`held devices. Thus white LEDs will also become popular in
`these devices due to the backlighting requirements of the
`color LCD. Although white LED drivers are available in the
`marketplace, none of the designs are high efficiency and
`require high driving voltages. The present invention can
`reduce the power consumption by more than 25%, which
`results in longer battery life. Further, LEDs have recently
`been incorporated into flashlights for their high photon
`efficiency. The present invention allows reduces the power
`consumption in these, so that the battery life can be 25%
`longer than a LED flashlight a using constant current driven
`method.
`
`Many considerations must be made in optimizing a circuit
`for the various LEDs available in the marketplace, their
`applications and the availability of lithium ion batteries for
`power sources. For example, With exception of GaP red
`LEDs, blue LEDs and white LEDs, most of the modern
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`In the prior art drivers, it is assumed that each green LED
`is driven with a two-volt buck converter with 80%
`efficiency, resulting in an equivalent 3.5 mA current draw
`from a 3.6V battery. Similarly, each white LED is driven
`with a five-volt boost converter with 80% efficiency, result-
`ing in an equivalent 35 mA current draw from a 3.6V battery.
`In order to get high efficiency driving circuits, issues like
`the tolerance of the LED threshold voltage, the LED forward
`current—photon efficiency relation and dimming control
`need to be resolved. The following preferred embodiments
`provide high efficiency designs for practical applications.
`FIG. 5 shows a simplified schematic diagram for a green
`LED driver for monochromatic lighting, wherein the power
`supply charges capacitor C through parallel LEDs D1 and
`D2. Then C discharges through green series LEDs D3 and
`D4 as more voltage is available in the discharge cycle to
`drive series connected LEDs. As a result, the LED bright-
`ness of four LEDs is obtained at the current drain of three
`
`LEDs. In practice, more parallel LEDs can be provided in
`the forward current path and parallel sets of two series LEDs
`can be provided in the reverse path to increment brightness
`as needed.
`
`Valeo Exhibit 1010_007
`
`Valeo Exhibit 1010_007
`
`

`

`US 6,359,392 B1
`
`7
`FIG. 6 shows a simplified schematic diagram for a RGB
`LED driver for color LCD lighting, wherein the power
`supply charges capacitor C through a red LED D1 and a
`yellow LED D2. Then C discharges through parallel blue
`LEDs D3 and D4 as more voltage is available in the
`discharge cycle to drive the higher threshold blue (or white)
`LEDs. The reason to use blue LEDs in parallel is to lower
`the maximum current through the blue LED and improve
`photon efficiency. In the case when the forward current
`charge path DC resistance is very small, an inductor L can
`be put in the charge path to achieve zero current switching
`and maximize the electrical efficiency. Another inductor L in
`the discharge path can reduce peak discharge current and
`improve the photon efficiency. If the inductor L is put in
`series with the capacitor, both charge peak current and
`discharge peak current can be reduced, and thus the highest
`photon efficiency can be achieved. Similarly, a green and
`white LED driver for color LCD lighting can be provided
`with green LEDs in the charge circuit path and white LEDs
`in the discharge current path.
`It is also envisioned that a comparator (not shown) can be
`used to monitor the charging voltage on C when the circuit
`is charging through the forward current path, such that once
`the voltage on C is greater than a charging threshold voltage,
`the comparator can direct C to start discharging through the
`discharge current path by having the threshold voltage of the
`comparator change to a higher discharge threshold voltage.
`When the discharging voltage is lower than the discharge
`threshold voltage, the circuit starts charging C and changes
`the comparator threshold to the charging threshold voltage
`from the discharging threshold voltage. This can be used
`advantageously as a brightness, contrast, or dimming con-
`trol.
`While the invention has been described in the context of
`
`a preferred embodiment, it will be apparent to those skilled
`in the art that the present invention may be modified in
`numerous ways and may assume many embodiments other
`than that specifically set out and described above.
`Accordingly, it is intended by the appended claims to cover
`all modifications of the invention that fall within the broad
`
`scope of the invention.
`What is claimed is:
`
`1. A light emitting diode (LED) driving circuit, compris-
`1ng:
`at
`
`two LEDs coupled between first and second
`least
`nodes, a first LED being coupled in a forward current
`path between first and second nodes and a second LED
`being coupled in a reverse current path between the
`second and first nodes, respectively;
`a power supply for producing a substantially periodic
`waveform, the power supply being coupled to drive the
`first node with voltage pulses;
`a capacitor with a first and a second terminal, the first
`terminal is coupled to the second node of the at least
`two LEDs, the capacitor stores charge from the power
`supply while the power supply is driving the first LED
`in the forward current path during voltage pulses; and
`a discharge circuit coupled between the second terminal
`of the capacitor and the first node of the at least two
`LEDs, wherein the discharge circuit drains charge from
`the capacitor to drive the second LED in the reverse
`current path between voltage pulses.
`2. The circuit of claim 1, wherein the periodic waveform
`is substantially a square wave.
`3. The circuit of claim 1, wherein the discharge circuit is
`an inverter with an input coupled to the first node and an
`output coupled to the second terminal of the capacitor.
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`55
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`60
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`65
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`8
`4. The circuit of claim 1, wherein the power supply
`includes an inverter driven by a square wave.
`5. The circuit of claim 1, wherein the stored charge of the
`capacitor boosts the voltage available to the second LED
`over a voltage available from the voltage pulses of the power
`supply.
`6. The circuit of claim 5, wherein the second LED
`requires a higher drive voltage than the first LED such that
`the boosted voltage available during the discharge of the
`capacitor equalizes photonic output between the LEDs.
`7. The circuit of claim 1, wherein the LEDs, capacitor, and
`a duty cycle of the power supply are optimized to provide
`uniform average photonic output from the LEDs.
`8. The circuit of claim 1, wherein the at least two LEDs
`include a first and a second LED, an anode of the first LED
`being coupled to a cathode of the second LED at the first
`node and a cathode of the first LED being coupled to an
`anode of the second LED at the second node.
`
`9. The circuit of claim 1, further comprising an inductor
`coupled to the first node to limit charge current
`to the
`capacitor.
`10. The circuit of claim 1, wherein the at least two LEDs
`includes two LEDs coupled in the forward current path and
`two LEDs coupled in the reverse current path, the LEDs in
`each current path being coupled in one of the group of a
`parallel connection and a series connection.
`11. The circuit of claim 10, wherein the LEDs in the
`forward current path are further connected in series with a
`current limiting inductor.
`12. The circuit of claim 10, wherein the LEDs in the
`forward current path are connected in series and the LEDs
`in the reverse current path are connected in parallel.
`13. A light emitting diode (LED) driving circuit, com-
`prising:
`two LEDs coupled between first and second
`at
`least
`nodes, a first LED being coupled in a forward current
`path between first and second nodes and a second LED
`being coupled in a reverse current path between the
`second and first nodes, respectively, an anode of the
`first LED being coupled to a cathode of the second LED
`at the first node and a cathode of the first LED being
`coupled to an anode of the second LED at the second
`node;
`a power supply for driving the first node with voltage
`pulses having a substantially square waveform;
`a capacitor with a first and a second terminal, the first
`terminal is coupled to the second node of the at least
`two LEDs, the capacitor stores charge from the power
`supply while the power supply is driving the first LED
`in the forward current path during voltage pulses; and
`a discharge circuit coupled between the second terminal
`of the capacitor and the first node of the at least two
`LEDs, wherein the discharge circuit drains charge from
`the capacitor to drive the second LED in the reverse
`current path between voltage pulses, the stored charge
`of the capacitor boosts the voltage available to the
`second LED over a voltage available from the voltage
`pulses of the power supply.
`14. The circuit of claim 13, wherein the discharge circuit
`is an inverter with an input coupled to the first node and an
`output coupled to the second terminal of the capacitor.
`15. The circuit of claim 13, wherein the power supply
`includes an inverter driven by a square wave.
`16. The circuit of claim 13, wherein the second LED
`requires a higher drive voltage than the first LED such that
`the boosted voltage available during the discharge of the
`capacitor equalizes photonic output between the LEDs.
`
`Valeo Exhibit 1010_008
`
`Valeo Exhibit 1010_008
`
`

`

`US 6,359,392 B1
`
`9
`17. The circuit of claim 13, wherein the LEDs, capacitor,
`and a duty cycle of the power supply are optimized to
`provide uniform average output from the LEDs.
`18. The circuit of claim 13, further comprising an inductor
`coupled to the first node to limit charge current
`to the
`capacitor.
`19. The circuit of claim 13, wherein the at least two LEDs
`includes two LEDs coupled in the forward current path and
`two LEDs coupled in the reverse current path, the LEDs in
`each current path being coupled in one of the group of a
`parallel connection and a series connection.
`20. A light emitting diode (LED) driving circuit, com-
`prising:
`two LEDs coupled between first and second
`at
`least
`nodes, a first LED being coupled in a forward current
`path between first and second nodes and a second LED
`being coupled in a reverse current path between the
`second and first nodes, respectively, an anode of the
`first LED being coupled to a cathode of the second LED
`at the first node and a cathode of the first LED being
`coupled to an anode of the second LED at the second
`node;
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`10
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`15
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`20
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`10
`a power supply for driving the first node with voltage
`pulses having a substantially square waveform;
`a capacitor with a first and a second terminal, the first
`terminal is coupled to the second node of the at least
`two LEDs, the capacitor stores charge from the power
`supply while the power supply is driving the first LED
`in the forward current path during voltage pulses; and
`a discharge circuit coupled between the second terminal
`of the capacitor and the first node of the at least two
`LEDs, wherein the discharge circuit drains charge from
`the capacitor to drive the second LED in the reverse
`current path between voltage pulses, the stored charge
`of the capacitor boosts the voltage available to the
`second LED over a voltage available from the voltage
`pulses of the power supply, the second LED requires a
`higher drive voltage than the first LED such that the
`boosted voltage available during the discharge of the
`capacitor equalizes photonic output between the LEDs.
`
`*
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`*
`
`*
`
`*
`
`*
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`Valeo Exhibit 1010_009
`
`Valeo Exhibit 1010_009
`
`