Datasheet

ADP8863
Rev. A | Page 14 of 52
LED GROUPINGS
Each LED can respond individually or be grouped together
into the backlight controls. By default, all LEDs are set to be
part of the backlight. This is changed by setting Bits[6:0] in
Register 0x05. LEDs that are set up as independent sinks can
be enabled individually in Register 0x10. They can also all be
enabled simultaneously via the SIS_EN bit in Register 0x01.
Any LEDs configured for the backlight can only be enabled
via the BL_EN bit in Register 0x01.
LED CURRENT SETTINGS
Any of the LED outputs (Pin D1 to Pin D7) can be used to
drive any color of LED at 0 mA to 30 mA, provided that the
LED’s Vf is less than 4.1 V. Additionally, the D7 sink can regu-
late up to 60 mA. The current settings are determined by a
7-bit code programmed by the user into Register 0x14 through
Register 0x1A (for the independent sinks) and Register 0x09 to
Register 0x0E (for the backlight sinks). The 7-bit resolution
allows the user to set the LED to one of 128 different levels.
The ADP8863 can implement two distinct algorithms to
achieve a linear or a nonlinear relationship between input
code and diode output current. The law and SC_LAW bits
in Register 0x04 and Register 0x0F, respectively, are used to
change between these algorithms.
By default, the ADP8863 uses a linear algorithm (law and
SC_LAW = 00), where the LED current increases linearly for
a corresponding increase in input code. LED current (in
milliamperes) is determined by the following equation:
LED 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 ADP8863 can also implement a nonlinear (square
approximation) relationship between input code and LED
current. In this case (law and SC_LAW = 01, 10, or 11), the LED
current (in milliamperes) is determined by the following
equation:
2
127
)mA(
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
×=
CurrentScaleFull
CodeCurrentLED
(3)
Figure 29 shows the LED current level vs. input code for both
the linear and square law algorithms.
30
25
20
15
10
5
0
0 32 64 96 128
CODE
LED CURRENT (mA)
LINEAR
SQUARE
08392-027
Figure 29. LED 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
0.0 sec to 5.5 sec (per full-scale current, either 30 mA or 60 mA).
The backlight LEDs have separate fade in and fade out time
controls from the independent sink LEDs.
Table 5. Available Fade In and Fade Out Rates
Code Fade Rate (in sec per Full-Scale Current)
0000 0.0 (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.