Datasheet

FUNCTIONAL DEVICE OPERATION
LIGHTING SYSTEM
Analog Integrated Circuit Device Data
Freescale Semiconductor 115
MC13892
Ramp up and ramp down patterns are implemented in hardware to reduce the burden of real time software control via the SPI
to orchestrate dimming and soft start lighting effects. 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 when the corresponding LEDxRAMP = 1. If a PWM change is programmed via SPI when
LEDxRAMP = 0, then the change is immediate rather than spread out over a PWM sweep.
A maximum of only two backlight drivers can be activated at the same time, for instance, the main display plus keypad. If all
three backlight drivers are enabled through the LEDxEN bits, meaning none of the duty cycles equals 0/32, then none of the
drivers will be activated.
If two backlight drivers are enabled, they time-share the external boost regulator output. The drivers will automatically be
enabled and disabled in a 50/50 percent fashion at a sufficiently high rate. The LED drive current will automatically be doubled
to the same luminosity as in a single backlight driver configuration.
Figure 33 illustrates the time sharing principle. Assume the MD domain is represented by 6 series white LEDs, and the KP
domain is represented by 3 ballasted stacks, that include 3 blue LEDs in each (a diagram of Serial LED configurations is included
later in this chapter).
Figure 33. Backlight Drivers Time Sharing Example
The “One Driver Active” case shows the general response when driving a single zone of 6 white LEDs. The “Two Drivers
Active” case shows the LEDMD zone driven at twice the current for half the time.
Table 105. Backlight Drivers Duty Cycle Programming
LEDxDC[5:0]
(91)
Duty Cycle
000000 0/32, Off
000001 1/32
… …
010000 16/32
… …
011111 31/32
100000 to 111111 32/32, Continuously On
Notes
91. “x” represents MD, AD, or KP
External
Boost
LEDMD
Active
LEDKP
Active
LEDMD
Active
One Driver
Active
Two Drivers
Active
LEDMD
Current
LEDKP
Current
LEDMD
Active
LEDKP
Active