13892 datasheet(115/161 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

115

13892

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).

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.

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.

Table 106. Serial LED Driver Characteristics

Parameter(80)

Condition

Min

Typ

Max

Units

Output Current Setting

Low Range Mode

High Range Mode

Current Programming Granularity

Low Range Mode

3.0

High Range Mode

6.0

PWM Granularity

1/32

Repetition Rate

Not blinking

256

Absolute Accuracy

Matching

At 400 mV, 21 mA

3.0

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

External

Boost

LEDMD

Active

LEDKP

Active

LEDMD

Active

One Driver

Active

Two Drivers

Active

LEDMD

Current

LEDKP

Current

LEDMD

Active

LEDKP

Active

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