AATIAN55 Datasheet, PDF (3/4 Pages) SiGe Semiconductor, Inc. – Driving Color RGB LEDs with the AAT3123

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 V_F to equally divide the 60mA supply current.
Ideally, each LED should be driven with 20mA. The green and blue LEDs have a V_F = 3.6V when I_D = 20mA.
The red LED has a V_F = 2.0V when I_D = 20mA. A ballast resistor can be added in series with the red LED
cathode to help match the red LED V_F. Refer to the following equation:
R_BALLAST = V_F(GRN/BLU) - V_F(RED)/I_D
Example, for I_D = 20mA:
R_BALLAST = 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 V_IN operating range of 2.8V to
4.2V for I_SET = 15mA per source. This yields a combined output (I_D) 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 V_IN vs. I_D for I_SET = 60mA
7.00E-02
6.50E-02
3.5V 2V
6.00E-02
5.50E-02
5.00E-02
4.50E-02
3.6V
3.7V
3.8V
3.9V
4V
4.00E-02
3
3.2 3.4 3.6 3.8
4
4.2
V_IN (V)
Figure 4: AAT3123 Constant Current Source
Output for Given V_Fs with I_D = 60mA.
AAT3123 V_IN vs. I_D for I_SET = 60mA
(V_F = 2V compared to V_F = 3.6V)
7.00E-02
6.50E-02
3.6V
6.00E-02
2V
5.50E-02
5.00E-02
4.50E-02
4.00E-02
2.8
3
3.2 3.4 3.6 3.8
4
4.2
V_IN (V)
Figure 5: Comparison of V_F = 3.6V and 2.0V to
Study the Condition of the Red LED V_F
Versus the V_F of Green and Blue LEDs.
AN-103.2005.01.1.0
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