LP3931 datasheet(12/17 Pages) NSC

12 / 17 page

Multiple RGB LED Drivers

The RGB driver has six outputs that can independently drive

2 separate RGB LEDs or six LEDs of any kind. 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 2-bit CYCLE con-

trol (0.25 / 0.5 / 1.0 / 2.0s).

20117322

RGB PWM Operating Principle

RGB_START is the master enable 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_LED input can be used for external hard-

ware 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 the external ballast resistor is

driven through the LED for the time defined by DUTY setting

(0 µs–50 µs). As a time averaged current this means

0%–33% of the peak current. The PWM period is 150 µs and

the pulse frequency is 6.67 kHz in normal mode.

20117323

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.

Blinking can be controlled separately for each output.

ON and OFF times define, when a LED turns on and off

within the blinking cycle. When both ON and OFF are 0, the

LED is on and doesn’t blink. If ON equals OFF but is not 0,

the LED is turned off.

20117324

Example Blinking Waveforms

Application Note AN1291 describes in detail the RGB driver

functionality of LP3933. The RGB driver in LP3931 is iden-

tical with LP3933.

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