Datasheet
ADP8866 Data Sheet
Rev. A | Page 14 of 52
LED GROUPS
The nine LED channels can be separated into two groups: backlight
(BL) and independent sinks (ISC). The group select is done in
Register 0x09 and Register 0x0A, with the default being that all
LEDs are part of the backlight.
Each group has its own fade-in and fade-out times (Register
0x12 for backlight and Register 0x22 for ISCs). Each group also
has its own master enable located in Register 0x01. However,
this master enable is overwritten if any of the SCx_EN bits
(Register 0x1A and Register 0x1B) in a group are set high. This
allows complete independent control of each LED channel in
both groups.
OUTPUT CURRENT SETTINGS
The current setting is determined by a 7-bit code programmed
by the user into diode current control registers (Register 0x13
for the backlight and Register 0x23 to Register 0x2B for the
independent sinks). The 7-bit resolution allows the user to set
the backlight to one of 128 different levels between 0 mA and
25 mA. The ADP8866 implements a square law algorithm to
achieve a nonlinear relationship between input code and
backlight current. The LED output current (in milliamperes) is
determined by the following equation:
2
127
CurrentScaleFull
Codet(mA)LED_Curren
(2)
where:
Code is the input code programmed by the user.
Full-Scale Current is the maximum sink current allowed
per LED.
OUTPUT CURRENT RANGE SELECTION
The default maximum current range of each sink of the
ADP8866 is 25.0 mA (typical). However, the ADP8866 also
allows the user to select an alternative maximum current range
to be applied to one or more LEDs. This alternate current range
still has 128 codes for its current setting. This provides
improved resolution when operating at reduced maximum
currents. One of up to 60 alternate current ranges can be
selected. An example of some of the available current ranges is
shown below. For the complete list, see Table 23.
Table 5. Example Current Range Options in Register 0x07
LEVEL_SET Code Range
000010 25.00 mA
001100 12.50 mA
010110 8.33 mA
100000 6.25 mA
101010 5.00 mA
25
20
15
10
5
0
0 12010080604020
OUTPUT CURRENT (mA)
OUTPUT CURRENT CODE (0 TO 127)
09478-031
25.00mA
12.50mA
8.33mA
6.25mA
5.00mA
Figure 30. Output Code Effect on Various LEVEL_SET Ranges
The LEDs that receive this alternate current range are determined
by the DxLVL bits in Register 0x07 and Register 0x08.
PWM DIMMING
Setting the LEVEL_SET code to 111111 (binary) allows the
ADP8866 to dim its LEDs based on a PWM signal applied to the
nINT pin. The LED output current is pulse width modulated with
the signal applied to the nINT pin. The typical waveform and
timing are shown in Figure 29. Due to the inherent delays and
rise/fall times of this system, the best accuracy of the average output
current is obtained with PWM frequencies below 1 kHz.
09478-032
TIME
LED OUTPUT
CURRENT
nTNT PIN
(INPUT)
Figure 31. PWM Input Waveform and Resultant LED Current
In this mode, the nINT pin functions as an input. It no longer
provides notification of the INT_STAT register.
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 1.75 sec (per full-scale current). Separate fade times are
assigned to the backlight LEDs and the ISC LEDs (see the LED
Groups section). The BLOFF_INT bit in Register 0x02 can be used
to flag the interrupt pin when an automated backlight fade-out has
occurred.