Current control method and circuit for light emitting diodes

[0001] This application claims the benefit of U.S. provisional application Ser. No.60/468,553, filed May 7, 2003, which the entire subject matter is incorporated herein by reference.

[0002] The present invention generally relates to various methods and circuits for controlling a light emitting diode (“LED”) current. The present invention specifically relates to a regulation of a peak amperage and a valley amperage of a LED current

[0003] LEDs are being used more and more in various applications such as, for example, backlighting, traffic lights, signage, automobiles and illumination. It is well known that a light output of a LED is directly dependent upon a current flowing through the LED. A LED current control circuit is therefore used to regulate the current flow through the LEDs to ideally maintain a constant current during all operating conditions.

[0004] FIG. 1 illustrates a known LED current control circuit 20 employing a MOSFET switch Q1, a diode D1, an inductor output filter L1, and a capacitive output filter C1 for controlling a flow of a LED current I_LED1 through a LED network LED1, LED2. Whenever MOSFET switch Q1 is turned on, circuit 20 controls an increasing flow of LED current I_LED1 from a power source (“PS”) 21 through LED network LED1, LED2. Whenever MOSFET switch Q1 is turned off, circuit 20 controls a decreasing flow of LED current I_LED1 through LED network LED1, LED2. This regulation of LED current I_LED1 is accomplished by a switch control circuit that employs an operational amplifier U1, a pulse width modulation (“PWM”) comparator U2, a gate driver GD1, a sense resistor Rs1, and a compensation RC network including an input impedance (“Zi”) 22, and a feedback impedance (“Zf”) 23.

[0005] In operation, a compensation voltage V_COMP generated by input impedance 22 and feedback impedance 23 is applied to an inverting input of operational amplifier U1, and a reference voltage V_REF1 as supplied by a reference voltage source (“RFS” ) 24 is applied to a non-inverting input of operational amplifier U1. Operational amplifier U1 compares compensation voltage V_COMP and reference voltage V_REF1 to yield an error voltage V_ERROR, which is an enlarged difference between compensation voltage V_COMP and reference voltage V_REF1 that is applied to a non-inverting input of PWM comparator U2. A ramp voltage V_RAMP as supplied by a ramp voltage source (“RPS”) 25 is applied at an inverting input of comparator U2, which compares error voltage V_ERROR and ramp voltage V_RAMP to yield a switching control voltage V_SWC1 for periodically enabling and disabling MOSFET switch Q1 via gate driver GD1.

[0006] Ideally, the aforementioned current regulation of LED current I_LED maintains LED current I_LED1 at a mean amperage as illustrated in FIG. 2. However, due to an implementation of a 2^nd order feedback control as illustrated in FIG. 1, LED current I_LED1 can experience an overshoot or an undershoot as illustrated in FIG. 2 whenever a change occurs in the input line of power source PS or in a load of the LED network LED1, LED2. Furthermore, LEDs are frequently non-linear devices, which makes it difficult to design the RC network for optimal performance.

[0007] The present invention addresses the shortcomings with the prior art by providing a LED current control method and circuit for accurately and quickly regulating the mean amperage of LED current I_LED during all operating conditions including a change in the input line of a power source or in a change in a load of the LED network.

[0008] One form of the present invention is a LED current control method for regulating a peak amperage and a valley amperage of a LED current flowing through one or more LEDs. First, an upper trip voltage and a lower trip voltage are established as control crossover thresholds, and a LED current sensing voltage representative of a flow of the LED current through the LED(S) is established. Second, a control of an increase in the LED current from the valley amperage to the peak amperage occurs in response to each crossover of the lower trip voltage by the LED current sensing voltage in a negative direction, and a control of a decrease of the LED current from the peak amperage to the valley amperage occurs in response to each crossover of the upper trip voltage by the LED current sensing voltage in a positive direction.

[0009] A second form of the present invention is a LED current control circuit employing a hysteretic comparator, a LED current sensor, and a switch-mode converter for regulating a peak amperage and a valley amperage of a LED current flowing through one or more LEDs. The LED current sensor establishes a LED current sensing voltage representative of a flow of the LED current through the LED(s). The hysteretic comparator establishes an upper trip voltage and a lower trip voltage as control crossover thresholds that are compared to the LED current sensing voltage. The switch-mode converter controls an increase in the LED current from the valley amperage to the peak amperage in response to each crossover of the lower trip voltage by the LED current sensing voltage in a negative direction, and controls a decrease of the LED current from the peak amperage to the valley amperage in response to

[0010] each crossover of the upper trip voltage by the LED current sensing voltage in a positive direction.

[0026] A flowchart 30 representative of a LED current control method of the present invention is illustrated in FIG. 3, and a LED current control circuit 50 for implementing flowchart 30 is illustrated in FIG. 4. During a startup stage S32 of flowchart 30, an upper trip voltage V_UT and a lower trip voltage V_LT are established as control crossover thresholds. This is accomplished by an application of a reference voltage V_REF2 as supplied by a reference voltage source (“RFS”) 62 to a non-inverting input of hysteretic comparator U3, and an application of a hysteresis voltage V_HYS as supplied by a hysteresis voltage source (“HYS”) 61 to a control input of hysteretic comparator U3. In one embodiment of hysteretic comparator U3, upper trip voltage V_UT equals (V_REF+(V_HYS/2)) and a lower trip voltage V_LT equals (V_REF−(V_HYS/2)).

[0027] In addition, during stage S32, a LED current sensing voltage V_SEN1 is established as a representation of a sensed flow of a LED current I_LED2 through LED network LED1, LED2. This is accomplished by a LED current sensor (“LCS”) 80, which applies LED current sensing voltage V_SEN1 to an inverting input of hysteretic comparator U3. At a time t₀, LED current I_LED is zero (0) amps as illustrated in FIG. 5. LED current sensing voltage V_SEN1 consequently is zero (0) volts as illustrated in FIG. 6. Also at time t₀, hysteretic comparator U3 is initially set to output a switching control voltage V_SWC2 at a logic high level LHL as illustrated in FIG. 7 and a power source (“PS”) 60 is applied to a switch-mode converter (“SMC”) 70 whereby switch-mode converter is turned on to initiate a flow of LED current I_LED2 from power source 60 through LED network LED1, LED2.

[0028] Next during startup stage S32, switch-mode converter 70 controls an increase in the flow of LED current I_LED2 through LED network LED1, LED2 from zero (0) amps at time t₀ to the peak amperage at time t₁ as illustrated in FIG. 5. LED current sensing voltage V_SEN1 consequently increases in a positive direction towards upper trip voltage V_UT as illustrated in FIG. 6. Upon LED current sensing voltage V_SEN1 crossing over upper trip voltage V_UT during a stage S34 of flowchart 30 at a time t₁, hysteretic comparator U3 outputs switching control voltage V_SWC2 at a logic low level LLL as illustrated in FIG. 7 whereby switch-mode converter 70 is turned off at time t₁.

[0029] During a stage S36 of flowchart 30, switch-mode converter 70 controls a decrease in the flow of LED current I_LED2 through the LED network LED1, LED2 from the peak amperage at time t₁ to the valley amperage at time t₂ as illustrated in FIG. 5. LED current sensing voltage V_SEN1 consequently decreases in a negative direction towards lower trip voltage V_LT as illustrated in FIG. 6. Upon LED current sensing voltage V_SEN1 crossing over lower trip voltage V_LT during a stage S38 of flowchart 30 at a time t₂, hysteretic comparator U3 outputs switching control voltage V_SWC2 at a logic high level LHL as illustrated in FIG. 7 whereby switch-mode converter 70 is turned on at time t₂.

[0030] During a stage S40 of flowchart 30, switch-mode converter 70 controls of an increase in the flow of LED current I_LED2 through the LED network LED1, LED2 from the valley amperage at time t₂ to the peak amperage at time t₃ as illustrated in FIG. 5. LED current sensing voltage V_SEN1 consequently increases in a positive direction towards upper trip voltage V_UT as illustrated in FIG. 6. Upon LED current sensing voltage V_SEN1 crossing over upper trip voltage V_UT during stage S34 at time t₃, hysteretic comparator U3 outputs switching control voltage V_SWC2 at logic low level LLL as illustrated in FIG. 7 whereby switch-mode converter 70 is turned off at time t₃.

[0031] As long as power source 60 is being applied to switch-mode converter 70, and reference voltage V_REF2 and hysteresis voltage V_HYS are being applied to hysteretic comparator U3, LED current control circuit 50 will continue to cycle through stages S34-S40 of flowchart 30 whereby LED current I_LED2 continually ramps between the peak amperage and the valley amperage during a current regulation phase CRP as illustrated in FIG. 5. During the current regulation phase CRP, a mean amperage of LED current I_LED2 is an average of the peak amperage and the valley amperage. As described herein, the peak amperage and the valley amperage are a function of reference voltage V_REF2 and hysteresis voltage V_HYS. Consequently, current regulation phase CRP attains a regulation of the mean amperage of LED current I_LED2 that is accurate during all operating conditions of circuit 50, including a change in the input line of power source 60 or a change in the load of the LED network LED1, LED2.

[0032] Referring to FIG. 4, circuit 50 can employ any type of switch-mode converter (e.g., a Buck converter, a Forward converter, a Boost converter, a Flyback converter, a Bridge converter, and a Cuk converter) to serve as switch-mode converter 70. FIGS. 8-14 illustrate various embodiments of switch-mode converter 70.

[0033] FIG. 8 illustrates a LED current control circuit 51 as an inductive output based version of LED current control circuit 50. To implement flowchart 30 (FIG. 3), circuit 51 employs a switch-mode converter 71 including a switching power cell (“SPC”) 90, a gate driver GD2, and an inductive output filter L2. Gate driver GD2 is connected to the output of hysteretic comparator U3 and a gate terminal G of MOSFET switch Q2 whereby MOSFET switch Q2 is opened during stage S36 (FIG. 3) and closed during stages S32 and S40 (FIG. 3). Inductive output filter L2 is connected to switching power cell 90 and LED network LED1, LED2 to facilitate switching power cell 90 in the control of the flow of LED current I_LED2 through LED network LED1, LED2.

[0034] Switching power cell 90 can include any circuit arrangement encompassing MOSFET switch Q2. FIG. 9 illustrates one such circuit arrangement wherein a drain terminal D of MOSFET switch Q2 is connected to power source 60, and a source terminal S of MOSFET switch Q2 is connected to inductive output filter L2 and a diode D1, which is also connected to a common reference CREF. FIG. 9 also illustrates a LED current sensor 81 as a version of LED current sensor 80 (FIG. 4). LED current sensor 81 includes a resistor R2 having one terminal connected to both LED network LED1, LED2 and the inverting input of hysteretic comparator U3. The other terminal of resistor R2 is connected to common reference CREF. A flow of LED current I_LED2 through resistor R2 applies LED current sensing voltage V_SEN1 to the inverting input of hysteretic comparator U3.

[0035] FIG. 10 illustrates a LED current control circuit 52 as a capacitive output based version of LED current control circuit 50 (FIG. 8). To implement flowchart 30 (FIG. 3), circuit 52 employs a switch-mode converter 72 including switching power cell 90, a capacitive output filter C2, gate driver GD2, a logic circuit (“LC”) 100, and a duty cycle oscillator (“DCO”) 110. Logic circuit 100 is connected to hysteretic comparator U3 to thereby receive switching control voltage V_SWC2. Logic circuit 100 is also connected to duty cycle oscillator 110 to thereby receive oscillating voltage V_OS. Logic circuit 100 performs a logic operation on switching control voltage V_SWC2 and oscillating voltage V_OS to control an opening and closing of MOSFET switch Q2 via gate driver G2 during stage S32, S36 and S40 (FIG. 3). Duty cycle oscillator 110 controls a duty cycle of oscillating voltage V_OS to limit a switching current I_SW flowing through MOSFET switch Q2. Capacitive output filter C2 is connected to switching power cell 90 and LED network LED1, LED2 to facilitate switching power cell 90 in the control of the flow of LED current I_LED2 through LED network LED1, LED2.

[0036] FIG. 11 illustrates a LED current control circuit 53 as an alternative version of LED current control circuit 52. Circuit 53 employs a switch-mode converter 73 including a switching power cell 91 having MOSFET switch Q2 and a switching current sensor (“SCS”) 120 for providing a switching current sensing voltage V_SNE2 to a duty cycle oscillator 111. Switching current sensing voltage V_SNE2 is a representation of switching current I_SW that enables duty cycle oscillator 111 to output oscillating voltage V_OS with a duty cycle that limits switching current I_SW.

[0037] Referring to FIGS. 10 and 11, the switch-mode converters can employ any type of logic circuit and duty cycle oscillator. FIG. 12 illustrates a LED current control circuit 54 employing a switch-mode converter 74 including a logic circuit in the form of an AND gate 101 and one version of duty cycle oscillator 111, which employs a comparator U4, a reference voltage source 112 and a bi-stable device in the form of a RS flip-flop 113. Switch current sensor 120 applies switching current sensing voltage V_SNE2 to an inverting input of a comparator U4. Reference voltage source 112 applies a reference voltage V_REF2 to a non-inverting input of comparator U4, where reference voltage V_REF2 is a representation of a maximum amperage for switching current I_SW. An output of comparator U4 applies a switching current limit voltage V_CL to R input terminal of RS flip-flop 113. A clock CLK is applied to S input terminal of RS flip-flop 113. Q output terminal of RS flip-flop 113 is applied to a first input of AND gate 101, which receives switching control voltage V_SWC2 at a second input. An output of AND gate 101 is connected to gate driver GD2.

[0038] Referring to FIGS. 11 and 12, switching power cell 91 can include any circuit arrangement encompassing MOSFET switch Q2 and switching current sensor 120.

[0039] FIG. 13 illustrates one circuit arrangement of cell 91 wherein an inductor L3 is connected to power source 60 and a drain terminal D of MOSFET switch Q2. A diode D2 is connected to drain terminal D of MOSFET switch Q2. Diode D2 is also connected to capacitive output filter C2 and LED network LED1, LED2. A switching current sensor 121 in the form of a resistor R3 is connected to a source terminal S of MOSFET switch Q2 and common reference CREF. The source terminal S of MOSFET switch Q2 is also connected to the inverting input of comparator U4.

[0040] FIG. 14 illustrates another circuit arrangement of cell 91 wherein a primary side of a transformer T1 has one end connected to power source 61 and the other end connected to drain terminal D of MOSFET switch Q2. A secondary side of transformer T1 has one end connected to diode D2, and the other end connected to capacitive output filter C2 and sense resistor R2.

[0041] For purposes of describing the invention, the LED network LED1, LED2 was illustrated as a series connection of LEDs. In practice, a LED current control method and circuit of the present invention can control the LED current through any number of LEDs having any type of arrangement.

[0042] 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.