13892 datasheet(117/156 Pages) FREESCALE

117 / 156 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

117

13892

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

1/256

1/8

Table 110. Signaling LED Driver Characteristics

Absolute Accuracy

Matching

At 400 mV, 21 mA

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.

LEDxPER[1:0]

Repetition Rate

Units

Parameter

Condition

Min

Typ

Max

Units

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