AATIAN55 Datasheet, PDF (1/4 Pages) SiGe Semiconductor, Inc. – Driving Color RGB LEDs with the AAT3123

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 pro-
gramming 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 follow-
ing application circuit (Figure 1):
V_BATTERY
V _IN
C1+
C_IN
1µF
C_OUT
1µF
EN/SET
V _OUT
C1-
C2+
AAT3123
C2-
D1
D2
D3
D4
EN/SET
GND
C1
1µF
C2
1µF
RGB LED
RED GRN BLU
Enable Red
Enable Green
Enable Blue
Figure 1: AAT3123 RBG LED Driver Application Circuit.
Application Problem
RGB LEDs have differing forward voltages (V_F) for the red, green, and blue LEDs. When driven from a com-
mon 60mA (I_D = 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 V_F 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