LP5526TLX datasheet(14/33 Pages) NSC

14 / 33 page

Functionality of Color LED

Outputs (RLED, GLED, BLED)

(Continued)

Switch Mode / Constant Current Mode

Each RGB LED output can be set to act as a switch or a

constant current sink. Selection of mode is done with the

CC_SW bit in the Control Register. If bit is set high, then the

switch mode is selected. Default is switch mode.

SWITCH MODE

In switch mode, the RGB LED outputs are low ohmic

switches to ground. Resistance is typically 3.2

Ω. External

ballast resistors must be used to limit the current

through the LED.

CONSTANT CURRENT MODE

In constant current mode, the maximum output current is

defined with a single external resistor (R

RGB) and the maxi-

mum current control register (address 02H).

RGB MAX CURRENT REGISTER (02H)

Name

Bit

Description

IR[1:0]

5:4

RLED maximum current

IG[1:0]

3:2

GLED maximum current

IB[1:0]

1:0

BLED maximum current

Maximum current for each LED output is adjusted with the

RGB max current register in following way:

IR[1:0], IG[1:0], IB[1:0]

Maximum current / output

0.25 x I

MAX

0.50 x I

MAX

0.75 x I

MAX

1.00 x I

MAX

External ballast resistors are not needed in this mode. The

maximum current for all RGB LED drivers is set with R

RGB.

The equation for calculating the maximum current is:

MAX = 100 x 1.23V / (RRGB +50

Ω)

where

MAX = maximum RGB current in any RGB output (during

constant current mode)

1.23V = reference voltage

100 = internal current mirror multiplier

RGB = resistor value in Ohms

Ω = Internal resistor in the I

RGB input

Table with example resistance values and corresponding

output currents:

RGB resistor R

RGB (k

Ω)

Maximum current / output

MAX (mA)

2.4

50.2

2.7

44.7

3.0

40.3

3.3

36.7

3.6

33.7

3.9

31.1

4.3

28.3

4.7

25.9

5.1

23.9

5.6

21.8

6.2

19.7

Note that the LED output requires a minimum saturation

voltage in order to act as a true constant current sink. The

saturation voltage minimum is typically 230mV defined with

10% current drop. If the LED output voltage drops below

230mV, then the current will decrease significantly.

3. EXTERNAL PWM CONTROL

The GPIO[0]/PWM pin can be used to control the RGB

output brightness or set RGB leds on/off. PWM function for

the pin is selected by writing EN_PWM_PIN high in GPIO

control register (address 06H). Note, that EN_RGB bit must

be set high. Each LED output can be enabled with RSW,

GSW and BSW bits. EN_EXT_R_PWM, EN_EXT_G_PWM

and EN_EXT_B_PWM bits are used to select, which LED

outputs are controlled with the external PWM input. Note that

polarity of external PWM control is active high i.e. when pin

is in high state, then LED output is enabled. If RGB_PWM is

set low, then each selected LED output is controlled directly

with external PWM input. If RGB_PWM is set high, then

internal PWM control is modulated by the external PWM

input. In latter case, internal PWM control is passed to LED

when external PWM input is high.

LP5526TLX 数据表(PDF) 14 Page - National Semiconductor (TI)

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