Global Sources
EE Times-India
Stay in touch with EE Times India
 
EE Times-India > Power/Alternative Energy
 
 
Power/Alternative Energy  

Driving color RGB LEDs

Posted: 26 Nov 2004     Print Version  Bookmark and Share

Keywords:POWER 

PDF document for more information.

AN3123-01

Driving Color RGB LEDs

AN3123-01.2004.01.0.6 1

Application

An RGB LED can be driven with a combined 60mA constant current source supply from an AAT3123. This is accom-

plished 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: 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 com-

mon 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 theAAT3123

input voltage drops below 3.2V, the output constant current sources can not 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.

Figure 2: Forward voltage versus current characteristic curve

for an RGB color LED used in this application discussion.

RGB LED

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

ID

VD

Red

Green

Blue

CIN

15F

VBATTERY

COUT

15F

EN/SET

C1

15F

C2

15F

BLURED

VIN

VOUT

C1+

C1-

C2+

C2-

D1

D2

D3

D4

EN/SET

GND

AAT3123

Enable Green

Enable Red

Enable Blue

GRN

RGB LED

Application Solution

The forward voltage drops for each LED with in the RGB LED need to be equalized in order to match LED per-

formance 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 drop out region of its output constant

current sources.

Determining the ballast resistor value

To obtain equal brightness and from each LED with in the RGB LED to produce white light, each LED needs

to have the same effective VF to divide the 60mA supply current equally.

Ideally, each LED should be driven with 20mA. The green and blue LED's 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 cath-

ode 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 = 80.

Conclusion: An 80 resistor placed in series with the RED LED cathode will help match it to the green and

blue LED's 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 on paral-

lel. Output performance was characterized for simulated forward voltages from 4.0V to 2.0V. Refer to Figure 4.

AN3123-01

Driving Color RGB LEDs

2 AN3123-01.2004.01.0.6

AN3123-01

Driving Color RGB LEDs

AN3123-01.2004.01.0.6 3

Figure 3: AAT3123 RGB driver application with a color matching ballast resistor for the RED LED.

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 = 26.66

Output performance was characterized over a range of standard resistor values: 22, 25, 27 and 30.

Refer to Figure 6.

Figure 7 shows the AAT3123 output performance for a simulated VF = 2.0V with a 25 resistor connected in

series. The combined constant current source output performance very closely matches the output perform-

ance 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.

CIN

15F

VBATTERY

COUT

15F

EN/SET

C1

15F

C2

15F

BLURED

VIN

VOUT

C1+

C1-

C2+

C2-

D1

D2

D3

D4

EN/SET

GND

AAT3123

Enable Green

Enable Red

Enable Blue

GRN

RBAL

RGB LED

Figure 4: AAT3123 constant current source Figure 5: Comparison of VF = 3.6V and 2.0V to

output for given VF's with ID = 60mA. study the condition of the Red LED VF

versus the VF of Green and Blue LEDs.

Figure 6: Comparison for VF = 2.0V with various Figure 7: Final matching of the AAT3123

ballast resistor values, matching the 2V VF to constant current source output performance.

a 3.6V VF. VF = 3.6V will match the VF = 2.0V + a 25

resistor at 60mA.

AAT3124 VIN vs. ID for ISET = 60mA

VF = 2V + 25 compared to VF = 3.6V

4.00E-02

4.50E-02

5.00E-02

5.50E-02

6.00E-02

6.50E-02

7.00E-02

3 3.22.8 3.4 3.6 3.8 4 4.2

VIN (V)

ID(A)

VF = 2V+25

VF = 3.6V

AAT3124 VIN vs. ID for ISET = 60mA

VF = 2V + R compared to VF = 3.6V

4.00E-02

4.50E-02

5.00E-02

5.50E-02

6.00E-02

6.50E-02

7.00E-02

3 3.22.8 3.4 3.6 3.8 4 4.2

VIN (V)

ID(A)

VF = 2V+30

VF = 2V+27

VF = 2V+22

VF = 3.6V

VF = 2V+22

AAT3124 VIN vs. ID for ISET = 60mA

VF = 2V compared to VF = 3.6V

4.00E-02

4.50E-02

5.00E-02

5.50E-02

6.00E-02

6.50E-02

7.00E-02

3 3.22.8 3.4 3.6 3.8 4 4.2

VIN (V)

ID(A)

2V

3.6V

AAT3124 VIN vs. ID for ISET = 60mA

4.00E-02

4.50E-02

5.00E-02

5.50E-02

6.00E-02

6.50E-02

7.00E-02

3 3.2 3.4 3.6 3.8 4 4.2

VIN (V)

ID(A)

2V3.5V

3.6V

3.7V

3.8V

3.9V

4V

AN3123-01

Driving Color RGB LEDs

4 AN3123-01.2004.01.0.6

Advanced Analogic Technologies, Inc.

830 E. Arques Avenue, Sunnyvale, CA 94085

Phone (408) 737-4600

Fax (408) 737-4611





Comment on "Driving color RGB LEDs"
Comments:  
*  You can enter [0] more charecters.
*Verify code:
 
 
Webinars

Seminars

Visit Asia Webinars to learn about the latest in technology and get practical design tips.

 

Go to top             Connect on Facebook      Follow us on Twitter      Follow us on Orkut

 
Back to Top