AN-103 AAT3123 Application Note Driving Color RGB LEDs with the AAT3123 Application An RGB LED can be driven with a combined 60mA constant current source supply from an AAT3123. This is accomplished by connecting the four constant current source outputs (D1 to D4) from an AAT3123 and programming the device to output 15mA for each source. This solution is intended to supply up to 20mA maximum to each of the three red, green, and blue LEDs in a common anode configuration. Color balance and intensity is then controlled via PWM signals to MOSFET switches connected to each LED cathode. Refer to the following application circuit (Figure 1): V IN C1+ C1 1µF C1C2+ V OUT CIN 1µF VBATTERY C OUT 1µF AAT3123 C2 1µF C2D1 D2 D3 D4 EN/SET EN/SET RGB LED RED GRN BLU GND Enable Red Enable Green Enable Blue Figure 1: AAT3123 RBG LED Driver Application Circuit. Application Problem RGB LEDs have differing forward voltages (VF) for the red, green, and blue LEDs. When driven from a common 60mA (ID = 20mA per LED) constant current source, the red LED element of an RGB LED typically has a forward voltage of about 2V. The green and blue LEDs have forward voltage levels of about 3.6V. Two problems arise with this application. First, the lower VF LED (red) dominates the circuit; second, when the AAT3123 input voltage drops below 3.2V, the output constant current sources cannot supply the required 3.6V for the green and blue LEDs. This results in a dominating red color because the green and blue LEDs are being under driven. Refer to Figure 2. AN-103.2005.01.1.0 1 AN-103 AAT3123 Application Note RGB LED 4.0 Blue 3.5 Green VD 3.0 2.5 Red 2.0 1.5 1.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 ID Figure 2: Forward Voltage Versus Current Characteristic Curve for an RGB Color LED Used in this Application Discussion. Application Solution The forward voltage drops for each LED within the RGB LED need to be equalized in order to match LED performance over the AAT3123 operating range. Adding a ballast resistor to the red LED will help equalize the LED output red color shift when the AAT3123 is operated in or near the dropout region of its output constant current sources. See Figure 3. VIN C1+ C1 1µF C1C2+ VOUT VBATTERY CIN 1µF COUT 1µF AAT3123 C2 1µF C2D1 D2 D3 D4 EN/SET EN/SET RGB LED RED GRN BLU GND RBAL Enable Red Enable Green Enable Blue Figure 3: AAT3123 RGB Driver Application With a Color Matching Ballast Resistor for the Red LED. 2 AN-103.2005.01.1.0 AN-103 AAT3123 Application Note Determining the Ballast Resistor Value To obtain equal brightness from each LED within the RGB LED to produce white light, each LED needs to have the same effective VF to equally divide the 60mA supply current. Ideally, each LED should be driven with 20mA. The green and blue LEDs have a VF = 3.6V when ID = 20mA. The red LED has a VF = 2.0V when ID = 20mA. A ballast resistor can be added in series with the red LED cathode to help match the red LED VF. Refer to the following equation: RBALLAST = VF(GRN/BLU) - VF(RED)/ID Example, for ID = 20mA: RBALLAST = 3.6V - 2.0V/20mA = 80Ω Conclusion: An 80Ω resistor placed in series with the red LED cathode will help match it to the green and blue LEDs and provide even white light over the operating range of the AAT3123. Support Data The AAT3123 constant current source outputs were characterized over the VIN operating range of 2.8V to 4.2V for ISET = 15mA per source. This yields a combined output (ID) of 60mA when D1 to D4 are connected in parallel. Output performance was characterized for simulated forward voltages from 4.0V to 2.0V. Refer to Figures 4 and 5. AAT3123 VIN vs. ID for ISET = 60mA AAT3123 VIN vs. ID for ISET = 60mA (VF = 2V compared to VF = 3.6V) 7.00E-02 3.5V 2V 7.00E-02 3.6V 6.50E-02 6.50E-02 3.6V 3.7V 5.50E-02 ID (A) ID (A) 6.00E-02 3.8V 3.9V 4V 5.00E-02 4.50E-02 2V 5.50E-02 5.00E-02 4.50E-02 4.00E-02 4.00E-02 3 3.2 3.4 3.6 3.8 4 4.2 VIN (V) Figure 4: AAT3123 Constant Current Source Output for Given VFs with ID = 60mA. AN-103.2005.01.1.0 6.00E-02 2.8 3 3.2 3.4 3.6 3.8 4 4.2 VIN (V) Figure 5: Comparison of VF = 3.6V and 2.0V to Study the Condition of the Red LED VF Versus the VF of Green and Blue LEDs. 3 AN-103 AAT3123 Application Note Calculation of RBALLAST for this test condition: RBALLAST = VF(GRN/BLU) - VF(RED)/ID For test condition, ID = 60mA: RBALLAST = 3.6V - 2.0V/60mA = 26.66Ω Output performance was characterized over a range of standard resistor values: 22Ω, 25Ω, 27Ω, and 30Ω. Refer to Figure 6. Figure 7 shows the AAT3123 output performance for a simulated VF = 2.0V with a 25Ω resistor connected in series. The combined constant current source output performance very closely matches the output performance obtained when VF = 3.6V. The AAT3123 is capable of reasonable performance for a VIN range down to 2.9V under these conditions. The actual application calls for matching a higher VF of 3.6V to the lower VF of 2.0V, and a 60mA constant current source needs to be split between three LEDs (20mA each). The addition on the ballast resistor now brings the voltage drop seen across the red LED to the levels seen across the green and blue LEDs over the AAT3123's operating range. Since the actual current through each LED is 20mA, this value should be applied to the actual RBALLAST calculation. AAT3123 VIN vs. ID for ISET = 60mA AAT3123 VIN vs. ID for ISET = 60mA (VF = 2V + 25Ω Ω compared to VF = 3.6V) (VF = 2V + R compared to VF = 3.6V) VF = 2V+22Ω 7.00E-02 VF = 2V+27Ω ID (A) 6.50E-02 6.00E-02 VF = 2V+22Ω VF = 3.6V 5.50E-02 VF = 3.6V 6.50E-02 VF = 2V+30Ω ID (A) 7.00E-02 6.00E-02 VF = 2V+25Ω 5.50E-02 5.00E-02 5.00E-02 4.50E-02 4.50E-02 4.00E-02 4.00E-02 2.8 3 3.2 3.4 3.6 3.8 4 4.2 VIN (V) Figure 6: Comparison for VF = 2.0V with Various Ballast Resistor Values, Matching the 2V VF to a 3.6V VF. 2.8 3 3.2 3.4 3.6 3.8 4 4.2 VIN (V) Figure 7: Final Matching of the AAT3123 Constant Current Source Output Performance; Ω VF = 3.6V Will Match the VF = 2.0V + a 25Ω Resistor at 60mA. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. 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