Zone0

Zone1

Zone2

Zone3

Zone4

1

3

4

2

2

1

1

Averager

Output

Zone

Average

1.0

1.75

3.5

4.0

2.25

2.25

1.5

ILED = ILED_FS u ZoneTarget2 = 25 mA u 5.781% | 1.45 mA

22

LM3535

SNVS598B – AUGUST 2010 – REVISED MARCH 2018

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Next assume that the ambient light changes to 500 LUX (corresponding to an ALS voltage of 437.5 mV). This

moves the ambient light into Zone 2 which corresponds to Zone Target Register 2 (loaded with code 0x4C) the

LED current then becomes:

(3)

8.2.2.1.3

ALS Averaging Time

The ALS averaging time is the time over which the averager block collects samples from the A/D converter and

then averages them to pass to the discriminator block (see Figure 32). Ambient light sensor samples are

averaged and then further processed by the discriminator block to provide rejection of noise and transient

signals. The averager is configurable with 8 different averaging times to provide varying amounts of noise and

transient rejection (see Figure 25). The discriminator block algorithm has a maximum latency of two averaging

cycles, therefore the averaging time selection determines the amount of delay that will exist between a steady

state change in the ambient light conditions and the associated change of the backlight illumination. For

example, the A/D converter samples the ALS inputs at 16 kHz. If the averaging time is set to 800 ms, the

averager sends the updated zone information to the discriminator every 800 ms. This zone information contains

the average of approximately 12800 samples (800 ms × 16 kHz). Due to the latency of 2 averaging cycles, when

there is a steady-state change in the ambient light, the LED current begins to transition to the appropriate target

value after approximately 1600 ms have elapsed.

The sign and magnitude of these averager outputs are used to determine whether the LM3535 should change

brightness zones. The averager block follows the following rules to make a zone transition:

•

The averager always begins with a Zone0 reading stored at start-up. If the main display LEDs are active

before the ALS block is enabled, it is recommended that the ALS-EN bit be enabled at least 3 averaging

cycles times before the ALS-ENA bit is enabled.

•

The averager always rounds down to the lower zone in the case of a non-integer zone average (1.2 rounds to

1 and 1.75 also rounds to 1). Figure 32 shows an example of how the Averager will make the zone decisions

for different ambient conditions.

Figure 32. 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 transitions to zone equal to the

last averager output.

Figure 33 provides a graphical representation of the behavior of the averager.