RGB LED PWM Control (Note 14) (Continued)

RSW1

GSW1

BSW1

RSW2

GSW2

BSW2

Enable for R1 switch

Enable for G1 switch

Enable for B1 switch

Enable for R2 switch

Enable for G2 switch

Enable for B2 switch

RGB_START

Master Switch:

RGB_START = 0

→ RGB OFF

RGB_START = 1

→ RGB ON, starts the new cycle fromt=0

RGB_PWM

RGB_PWM = 0

→ RSW, GWS and BSW control directly the RGB outputs (on/off control only)

RGB_PWM = 1

→ Normal PWM RGB functionality (duty, slope, on/off times, cycle)

EN_FLASH1

EN_FLASH2

Flash Mode enable controls for RGB1 and RGB2. In Flash mode (EN_FLASH = 1) RGB outputs are

PWM controlled simultaneously, not in 3-phase system as in the Normal Mode.

R1_PWM

G1_PWM

B1_PWM

R2_PWM

G2_PWM

B2_PWM

XX_PWM = 0

→ External PWM control from PWM_LED pin is disabled

XX_PWM = 1

→ External PWM control from PWM_LED pin is enabled

Internal PWM control (DUTY) can be used independently of external PWM control. External PWM has

the same effect on all enabled outputs.

PWM_LED input can be used as a direct on/off or PWM brightness control for selected White LED or RGB outputs. For example it can trigger the Flash using a Flash

Note 14: Application Note 1291, “Driving RGB LEDs Using LP3933 Lighting Management System” contains a thorough description of the RGB driver functionality

including programming examples. It applies to LP3931, too.

Recommended External

Components

OUTPUT CAPACITOR, C

OUT

The output capacitor C

the output ripple voltage so C

lected. In general, the higher the value of C

output ripple magnitude. Multilayer ceramic capacitors with

low ESR are the best choice. At the lighter loads, the low

ESR ceramics offer a much lower V

higher ESR tantalums of the same value. At the higher loads,

the ceramics offer a slightly lower V

than the tantalums of the same value. However, the dv/dt of

the V

tantalums under all load conditions. Capacitor voltage rating

must be sufficient, 10V or greater is recommended.

INPUT CAPACITOR, C

IN

The input capacitor C

input ripple voltage and to a lesser degree the V

higher value C

age rating must be sufficient, 10V or greater is recom-

mended.

OUTPUT DIODE, D

OUT

A Schottky diode should be used for the output diode. To

maintain high efficiency the average current rating of the

schottky diode should be larger than the peak inductor cur-

rent (1A). Schottky diodes with a low forward drop and fast

switching speeds are ideal for increasing efficiency in por-

table applications. Choose a reverse breakdown of the

schottky diode larger than the output voltage. Do not use

ordinary rectifier diodes, since slow switching speeds and

long recovery times cause the efficiency and the load regu-

lation to suffer.

INDUCTOR, L

The LP3931’s high switching frequency enables the use of

the small surface mount inductor. A 10 µH shielded inductor

is suggested. The inductor should have a saturation current

rating higher than the peak current it will experience during

circuit operation (

∼1A). Less than 100 mΩ ESR is suggested

for high efficiency. Open core inductors cause flux linkage

with circuit components and interfere with the normal opera-

tion of the circuit. This should be avoided. For high efficiency,

choose an inductor with a high frequency core material such

as ferrite to reduce the core losses. To minimize radiated

noise, use a toroid, pot core or shielded core inductor. The

inductor should be connected to the OUT pin as close to the

IC as possible. Examples of suitable inductors are TDK

types LLF4017T-100MR90C and VLF4012AT-100MR79 and

Coilcraft type DO3314T-103 (unshielded).

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