1111

0110

1100

R1

G1

B1

Combined PWM cycle

1

1

3

2

2

3

/6.7 kHz

150 Ps

Blinking period

ON[3:0]

OFF[3:0]

SLOPE[3:0]

SLOPE[3:0]

DUTY[3:0]

Duty increases

Duty decreases

Duty constant

LED

brightness

PWM

current

pulses

LP3950

SNVS331C – NOVEMBER 2004 – REVISED APRIL 2013

www.ti.com

RGB LED Pattern Generator

The LP3950 RGB outputs can be controlled either with audio synchronization or with RGB pattern generator.

The pattern generator of LP3950 drives three independently controlled LED outputs (for example, R1, G1 and

B1). The functionality is similar compared to RGB functionality of LP3936 and LP3933.

The output of RGB pattern generator can be selected to drive RGB1 (R1-G1-B1), RGB2 (R2-G2-B2) or RGB1

and RGB2 (R1&R2 – G1&G2 – B1&B2) outputs.

Programmable Pattern Mode

User has control over the following parameters separately for each LED:

ON and OFF (start and stop time in blinking cycle)

DUTY (PWM brightness control)

SLOPE (dimming slope)

ENABLE (output enable control)

The main blinking cycle is controlled with three-bit CYCLE control (0.25 / 0.5 / 1.0 / 2.0 / 4.0s).

Figure 21. RGB PWM Operating Principle

RGB_START is the master control for the whole RGB function. The internal PWM and blinking control can be

disabled by setting the RGB_PWM control LOW. In this case the individual enable controls can be used to switch

outputs on and off. PWM_EN input can be used for external hardware PWM control.

In the normal PWM mode the R, G and B switches are controlled in 3 phases (one phase per driver). During

each phase the peak current set by an external ballast resistor is driven through the LED for the time defined by

DUTY setting (0 µs to 50 µs). As a time averaged current this means 0% to 33% of the peak current. The PWM

period is 150 µs and the pulse frequency is 6.67 kHz in normal mode.

Figure 22. Normal Mode PWM Waveforms at Different Duty Settings

In the FLASH mode all the outputs are controlled in one phase and the PWM period is 50 µs. The time averaged

FLASH mode current is three times the normal mode current at the same DUTY value.

18

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