LM3535 datasheet(20/26 Pages) NSC

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FIGURE 16. Averager Calculation

•

The two most current averaging samples are used to make

zone change decisions.

•

To make a zone change, data from three averaging cycles

are needed. (Starting Value, First Transition, Second

Transition or Rest)

•

To Increase the brightness zone, a positive Averager zone

output must be followed by a second positive Averager

output or a repeated Averager zone. ('+' to '+' or '+' to

'Rest')

•

To decrease the brightness zone, a negative Averager

zone output must be followed by a second negative

Averager output or a repeated Averager zone. ('-' to '-' or

'-' to 'Rest')

•

In the case of two increases or decreases in the Averager

output, the LM3535 will transition to zone equal to the last

Averager output.

Figure 17 provides a graphical representation of the

Averager's behavior.

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FIGURE 17. Brightness Zone Change Examples

Using the diagram for the ALS block (Figure 14), Figure 18

shows the flow of information starting with the A/D, transition-

ing to the Averager, followed by the Discriminator. Each state

filters the previous output to help prevent unwanted zone to

zone transitions.

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FIGURE 18. Ambient Light Input to Backlight Mapping

When using the ALS averaging functionality, it is important to

remember that the averaging cycle is free running and is not

synchronized with changing ambient lighting conditions. Due

to the nature of the Averager round down, an increase in

brightness can take between 2 and 3 averaging cycles to

change zones while a decrease in brightness can take be-

tween 1 and 2 averaging cycles to change. See Figure 9 for

a list of possible Averager periods. Figure 19 shows an ex-

ample of how the perceived brightness change time can vary.

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FIGURE 19. Perceived Brightness Change Time

Ambient Light Current Control + PWM

The Ambient Light Current Control can also be a function of

the PWM input duty cycle. Assume the LM3535 is configured

as described in the previous example, but this time the Enable

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