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

AN-103
AAT3123 Application Note
Calculation of R_BALLAST for this test condition:
R_BALLAST = V_F(GRN/BLU) - V_F(RED)/I_D
For test condition, I_D = 60mA:
R_BALLAST = 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 V_F = 2.0V with a 25Ω resis-
tor connected in series. The combined constant current source output performance very closely matches the
output performance obtained when V_F = 3.6V. The AAT3123 is capable of reasonable performance for a V_IN
range down to 2.9V under these conditions. The actual application calls for matching a higher V_F of 3.6V to
the lower V_F 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 R_BALLAST calculation.
AAT3123 V_IN vs. I_D for I_SET = 60mA
(V_F = 2V + R compared to V_F = 3.6V)
7.00E-02
6.50E-02
VF = 2V+22Ω
VF = 2V+27Ω
VF = 2V+30Ω
6.00E-02
5.50E-02
VF = 2V+22Ω
VF = 3.6V
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 6: Comparison for V_F = 2.0V with Various
Ballast Resistor Values, Matching the 2V V_F to
a 3.6V V_F.
AAT3123 V_IN vs. I_D for I_SET = 60mA
(V_F = 2V + 25Ω compared to V_F = 3.6V)
7.00E-02
6.50E-02
VF = 3.6V
6.00E-02
5.50E-02
VF = 2V+25Ω
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 7: Final Matching of the AAT3123
Constant Current Source Output Performance;
V_F = 3.6V Will Match the V_F = 2.0V + a 25Ω
Resistor at 60mA.
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AN-103.2005.01.1.0