FUNCTIONAL DEVICE OPERATION
LIGHTING SYSTEM
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 converter 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).
One Driver
Active
Two Drivers
Active
External
Boost
LEDKP
Current
LEDMD
Current
LEDMD LEDKP LEDMD LEDKP
LEDMD
Active
Active
Active
Active
Active
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 106. Serial LED Driver Characteristics
Parameter(80)
Condition
Low Range Mode
High Range Mode
Low Range Mode
High Range Mode
Min
Typ
-
Max
Units
0
0
-
15
30
-
mA
Output Current Setting
-
3.0
6.0
1/32
256
-
mA
Current Programming Granularity
-
-
PWM Granularity
Repetition Rate
-
-
Not blinking
-
-
Hz
%
%
S
Absolute Accuracy
Matching
-
15
3.0
-
At 400 mV, 21 mA
-
-
Glow and Dimming Speed
Per 1/32 duty cycle step
-
1/64*
Notes
80. Equivalent to 500 ms ramp time when going from 0/32 to 32/32
Figure 34 illustrates some possible configurations for the backlight driver. Note that when parallel strings are ganged together
on a driver channel, ballasting resistance is recommended to help balance the currents in each leg.
13892
Analog Integrated Circuit Device Data
Freescale Semiconductor
115
FREESCALE [ Freescale ]
