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ADP8860

Rev. 0 | Page 17 of 52

BACKLIGHT MAXIMUM AND DIM SETTINGS

The backlight maximum and dim current settings are deter-

mined by a 7-bit code programmed by the user into the

registers previously listed in the Backlight Operating Levels

section. The 7-bit resolution allows the user to set the backlight

to one of 128 different levels between 0 mA and 30 mA. The

ADP8860 can implement two distinct algorithms to achieve a

linear and a nonlinear relationship between input code and

backlight current. The law bits in Register 0x04 are used to

change between these algorithms.

By default, the ADP8860 uses a linear algorithm (law = 00),

where the backlight current increases linearly for a

corresponding increase of input code. Backlight current (in

millamperes) is determined by the following equation:

Backlight Current (mA) = Code × (Full-Scale Current/127) (2)

where:

Code is the input code programmed by the user.

Full-Scale Current is the maximum sink current allowed per

LED (typically 30 mA).

The ADP8860 can also implement a nonlinear (square approxima-

tion) relationship between input code and backlight current

level. In this case (law = 01), the backlight current (in

milliamperes) is determined by the following equation:

127

)

(

⎟

⎞

⎜

⎛

−

Current

Scale

Full

Code

Current

Backlight

⎠

⎝

(3)

Figure 30 shows the backlight current level vs. input code for

both the linear and square law algorithms.

128

SINK CODE

LINEAR

SQUARE

Figure 30. Backlight Current vs. Input Code

AUTOMATED FADE IN AND FADE OUT

The LED drivers are easily configured for automated fade in

and fade out. Sixteen fade in and fade out rates can be selected

via the I2C interface. Fade in and fade out rates range from

0.1 sec to 5.5 sec (per full-scale current, either 30 mA or 60 mA).

Table 5. Available Fade In and Fade Out Rates

Code

Fade Rate (in sec per Full-Scale Current)

0000

0.1 (disabled)

0001

0.3

0010

0.6

0011

0.9

0100

1.2

0101

1.5

0110

1.8

0111

2.1

1000

2.4

1001

2.7

1010

3.0

1011

3.5

1100

4.0

1101

4.5

1110

5.0

1111

5.5

The fade profile is based on the transfer law selected (linear,

square, Cubic 10, or Cubic 11) and the delta between the actual

current and the target current. Smaller changes in current

reduce the fade time. For linear and square law fades, the fade

time is given by

Fade Time = Fade Rate × (Code/127)

(4)

where the Fade Rate is shown in Table 5.

The Cubic 10 and Cubic 11 laws also use the square backlight

currents in Equation 3; however, the time between each step is

varied to produce a steeper slope at higher currents and a

shallower slope at lighter currents (see Figure 31).

0.75

0.50

0.25

UNIT FADE TIME

.00

LINEAR

SQUARE

CUBIC 11

CUBIC 10

Figure 31. Comparison of the Dimming Transfers Laws

BACKLIGHT TURN ON/TURN OFF/DIM

With the device in active mode (nSTBY = 1), the backlight can

be turned on using the BL_EN bit in Register 0x01. Before

turning on the backlight, the user chooses which level (daylight

(L1), office (L2), or dark (L3)) in which to operate, and ensures

that maximum and dim settings are programmed for that level.

ADP8860ACBZ-R7 数据表(PDF) 17 Page - Analog Devices

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