Datasheet

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:
2
127
)mA(
⎟
⎟
⎞
⎜
⎜
⎛
−
×=
CurrentScaleFull
CodeCurrentBacklight
⎠⎝
(3)
Figure 30 shows the backlight current level vs. input code for
both the linear and square law algorithms.
30
25
20
15
10
5
BACKLIGHT CURRENT (mA)
0
0 32 64 96 128
SINK CODE
LINEAR
SQUARE
07967-015
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 I
2
C 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).
30
0
5
10
15
20
25
010.750.500.25
CURRENT (mA)
UNIT FADE TIME
.00
LINEAR
SQUARE
CUBIC 11
CUBIC 10
07967-016
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.