FUNCTIONAL DEVICE OPERATION
LIGHTING SYSTEM
the enable for the boost converter is an OR of the following SPI bits (SWBSTEN, USBSWBSTEN, and LEDSWBSTEN). For more
details on the boost switcher and its control, see Supplies.
As with the backlight driver channels, the signaling LED drivers include ramp up and ramp down patterns are implemented in
hardware. Ramp patterns for each of the drivers is accessed with the corresponding LEDxRAMP bit.
The ramp itself is generated by increasing or decreasing the PWM duty cycle with a 1/32 step every 1/64 seconds. The ramp
time is therefore a function of the initial set PWM cycle and the final PWM cycle. As an example, starting from 0/32 and going to
32/32 will take 500 ms while going to from 8/32 to 16/32 takes 125 ms.
Note that the ramp function is executed upon every change in PWM cycle setting. If a PWM change is programmed via SPI
when LEDxRAMP = 0, then the change is immediate rather than spread out over a PWM sweep.
For color mixing and in order to guarantee a constant color, the color mixing should be obtained by the current level setting so
that the intensity is set through the PWM duty cycle.
In addition, programmable blink rates are provided. Blinking is obtained by lowering the PWM repetition rate of each of the
drivers through LEDxPER[1:0], while the on period is determined by the duty cycle setting. To avoid high frequency spur coupling
in the application, the switching edges of the output drivers are softened. During blinking, so LEDxPER[1:0] is not "00", ramping
and dimming patterns cannot be applied.
Table 109. Signal LED Drivers Period Control
LEDxPER[1:0]
Repetition Rate
Units
00
01
10
11
1/256
1/8
1
s
s
s
s
2
Table 110. Signaling LED Driver Characteristics
Parameter
Absolute Accuracy
Matching
Condition
Min
Typ
Max
15
Units
%
-
-
-
-
-
-
At 400 mV, 21 mA
10
%
Leakage
LEDxDC[5:0] = 000000
1.0
μA
Apart from using the signal LED drivers for driving LEDs they can also be used as general purpose open drain outputs for logic
signaling or as generic PWM generator outputs. For the maximum voltage ratings.
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Analog Integrated Circuit Device Data
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