Datasheet
Analog Integrated Circuit Device Data
Freescale Semiconductor 86
MC34708
Functional Block Description
7.8.3 General Purpose LED Drivers
To turn on the LEDs, the following bits must be set, CHRLEDxEN = 1, CHRGLEDOVRD =1, THERM bit = 1, and programming
the duty cycle > 0/32.
The general purpose LED drivers, CHRGLEDR, and CHRLEDG are independent current sink channels. Each driver channel
features programmable current levels via CHRGLEDx[1:0], as well as programmable PWM duty cycle settings with
CHRGLEDxDC[5:0]. By a combination of level and PWM settings, each channel provides flexible LED intensity control.
The general purpose LED drivers include ramp up and ramp down patterns implemented in hardware. Ramping is enabled for
each of the drivers using the corresponding CHRGLEDxRAMP bits, only when the repetition rate is 256 Hz.
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 the SPI
when CHRGLEDxRAMP
= 0, the change is immediate rather than spread out over a PWM sweep.
In addition, programmable blink rates are provided. Blinking is obtained by lowering the PWM repetition rate of each of the drivers
through CHRGLEDxPER[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.
Table 81. LED Driver Control
THERM CHRGLEDxEN
(67)
CHRGLEDOVRD CHRGLEDx
(67)
x 0 (default) 0 Off
1 x x Off
0
1 1 On
0 1 Off
Notes
67. “x” represents R or G
Table 82. General Purpose LED Drivers Current Programming
CHRGLEDx[1:0] CHRGLEDx Current Level (mA)
00 3.5
01 7.0 (default)
10 10
11 12
“x” represents for R, and G
Table 83. General Purpose LED Drivers Duty Cycle Programming
CHRGLEDxDC[5:0] Duty Cycle
000000 0/32, Off
000001 1/32
… …
010000 16/32
… …
011111 31/32
1xxxxx 32/32, Continuously On
“x” represents R, and G