ADP8861 Analog Devices, ADP8861 Datasheet - Page 14

• Manufacturer Part Number: ADP8861
• Description: Charge Pump, 7-Channel Smart LED Driver with I2C Interface
• Manufacturer: Analog Devices

Specifications of ADP8861

• Vin Range: 2.5 to 5.5V
• Vout (v): 4.3 to 5.5
• Synchronous: No
• Package: 20-Lead LFCSP,20-Lead WLCSP
• Led Configuration: Parallel
• Topology: Capacitive
• I2c Support: Yes
• Max Iout (ma): 60mA
• Brightness Control: PWM
• Peak Efficiency (%): 89%
• Switching Frequency: 1.32MHz
• Over Volt Protection (v): 5.8V

ADP8861
BACKLIGHT OPERATING LEVELS
The backlight can be operated at either the maximum level
(Register 0x09) or the dim level (Register 0x0A). The backlight
maximum and dim current settings are determined by a 7-bit
code programmed by the user into these registers. The 7-bit
resolution allows the user to set the backlight to one of 128
different levels between 0 mA and 30 mA.
The maximum and dim settings can be set between 0 mA and
30 mA; therefore, it is possible to program a dim setting that is
greater than a maximum setting. For normal expected operation,
ensure that the dim setting is programmed to be less than the
maximum setting.
BACKLIGHT MAXIMUM AND DIM SETTINGS
The ADP8861 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 ADP8861 uses a linear algorithm (law = 00),
where the backlight current increases linearly for a correspond-
ing increase in input code. Backlight current (in milliamperes)
is determined by the following equation:
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 ADP8861 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 milliam-
peres) is determined by the following equation:
Figure 29 shows the backlight current level vs. input code for
both the linear and square law algorithms.
Backlight Current (mA) = Code × (Full-Scale Current/127) (2)
Backlight
Current
Figure 28. Backlight Operating Levels
30mA
0
(
mA
BACKLIGHT_MAX
)
=
⎛
⎜
⎜
⎝
BACKLIGHT_DIM
Code
×
Full
−
Scale
127
Current
⎞
⎟
⎟
⎠
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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
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
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
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
where the Fade Rate is shown in Table 5.
The Cubic 10 and Cubic 11 laws also use the square law back-
light currents derived from Equation 3; however, the time
between each step is varied to produce a steeper slope at higher
currents and a shallower slope at lower currents (see Figure 30).
Fade Time = Fade Rate × (Code/127)
30
25
20
15
10
5
0
2
0
C interface. Fade in and fade out rates range from
Figure 29. Backlight Current vs. Input Code
Fade Rate (in sec per Full-Scale Current)
0.1 (disabled)
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
3.5
4.0
4.5
5.0
5.5
32
LINEAR
CODE
64
SQUARE
96
128
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