NCP1840Q8A6MNTWG【PDF 8/19 页】IC LED DRIVER 12CH PROGR 20QFN

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NCP1840

OPERATION DESCRIPTION

Constant Current Control

The NCP1840 has 8 LED channels that are individually

regulated constant current sources. The maximum current

per channel is 30 mA but can be reduced by adjusting the

resistor R BIAS , which is placed between the R BIAS pin and

ground. R BIAS is required and can vary between 10.25 k W

and 20.5 k W . The maximum current per channel can be

calculated with the following equation:

Additionally, the NCP1840 allows for the current through

each of the 8 channels to be fully programmable via a 5 ? bit

DAC on each output, allowing for the desired luminance to

be achieved for each channel. Programming the current

level is achieved by loading data into channel ? specific

registers through the I 2 C serial interface. The output current

levels are logarithmic, providing a perceived linear LED

brightness to the human eye, as shown in Figure 12. This

35

30

25

20

15

10

5

I max + 250 *

1.23 V

R BIAS

allows for a more gradual and natural viewing change in

LED brightness between levels.

High

Low

0

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

0

DAC Level

31

Figure 12. Logarithmic Output Current (R BIAS = 10.25 K W )

PWM Control

The NCP1840 provides 6 ? bit PWM duty cycle control for

each of its 8 channels, allowing each channel to be dimmed

independent of the other channels and without sacrificing

the LED color output. Programming PWM duty cycle is

achieved by loading data into channel ? specific registers

through the I 2 C serial interface. The typical on time of the

LED can be calculated by multiplying the duty cycle

percentage by the typical PWM period of 575 m s.

Charge Pump

The NCP1840 incorporates a high efficiency, low noise,

QUAD ? MODE (1x, 1.33x, 1.5x, and 2x) charge pump. This

allows for higher forward voltage LEDs and/or multiple

LEDs in series per channel to be driven with a low ? voltage

supply. In order to optimize efficiency while maintaining

constant current regulation, the NCP1840 automatically

senses when it should change charge pump modes based on

the voltage across the LED current sources. By utilizing the

1.33x mode, approximately 10% more efficiency than a

typical 1.5x mode is achieved.

The NCP1840 powers up in 1x mode where the charge

pump output voltage will be approximately equal to the

input supply voltage. If the output voltage is sufficient to

regulate all LED currents, the device remains in this mode.

However, if the input voltage is insufficient to regulate the

current in all active channels, the device automatically

switches into 1.33x mode. In this mode, the output voltage

is approximately equal to 1.33 times the input supply

voltage. If the input voltage is insufficient again to maintain

current regulation in all active channels, the device will

automatically switch to the 1.5x mode. In this mode, the

output voltage will be approximately equal to 1.5 times the

input supply voltage. If the input voltage is still insufficient

to maintain the constant current regulation in all active

channels, the device will automatically switch to the 2x

mode where the output voltage is approximately equal to

two times the input supply voltage. In all modes, if the

device detects a sufficient input voltage to drive all LED

currents in 1x mode, it will automatically change back to 1x

mode to optimize efficiency.

I 2 C Serial Interface

The registers of the NCP1840 are programmed and read

through an I 2 C serial interface. Due to the NCP1840 only

being configured as a Slave device, a Master device is

required on the communication bus. The NCP1840 has a

factory programmed 7 ? bit Slave address of 0011011 (1B

hex). The last bit of the Slave address specifies whether a

Read (1) or Write (0) operation is to be performed. The

maximum clock frequency for the serial interface is 400 kHz

and both the SCL and SDA lines require external pull ? up

resistors.

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