MC34708 datasheet(88/159 Pages) FREESCALE

88 / 159 page

Analog Integrated Circuit Device Data



Freescale Semiconductor

MC34708

Functional Block Description

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.

For color mixing and to guarantee a constant color, the color mixing should be obtained by the current level setting, so 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 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 84. General Purpose LED Drivers Period Control

CHRGLEDxPER[1:0]

Repetition Rate

Units

256

8.0

1.0

1/2

Table 85. LED Driver Electrical Specifications

Characteristics noted under conditions BP = 3.6 V, -40

C  TA  85 C, unless otherwise noted. Typical values at BP = 3.6 V

and TA = 25 °C under nominal conditions, unless otherwise noted.

Symbol

Characteristic

Min

Typ

Max

Unit

Notes

General Purpose LED Driver

Absolute Accuracy

Matching - At 1.0 V, 12 nA

4.0

Leakage - CHRGLAEDxDC [5:0]=000000

1.0

A

MC34708 Datasheet(PDF) 88 Page - Freescale Semiconductor, Inc