LM3535 datasheet(19/26 Pages) NSC

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ALS Zone to LED Brightness Mapping

FIGURE 15.

ALS Configuration Example

As an example, assume that the APDS-9005 is used as the

ambient light sensing photo diode with its output connected

to the ALSA input. The ALS Resistor Select Register (Address

0x51) is loaded with 0x40 which configures the ALS input for

a 2.32k

Ω internal pull-down resistor (see Internal ALS Resis-

tor Table). This gives the output of the APDS-9005 a typical

voltage swing of 0 to 875mV with a 0 to 1k LUX change in

ambient light (0.875mV/Lux). Next, the Configuration Regis-

ter (Address 0x20) is programmed with 0xDC, the ALS Con-

trol Register (Address 0x50) programmed to 0x40 and the

Control Register is programmed to 0x3F . This configures the

LM3535’s ambient light sensing interface for:

•

Ambient Light Current Control for BankA Enabled

•

ALS circuitry Enabled

•

Assigns D53 and D62 to bankA

•

Sets the ALS Averaging Time to 400ms

Next, the Control Register (Address 0x10) is programmed

with 0x3F which enables the 6 LEDs via the I2C compatible

interface.

Now assume that the APDS-9005 ambient light sensor de-

tects a 100 LUX ambient light at its input. This forces the

ambient light sensors output (and the ALS1 input) to 87.5mV

corresponding to Zone 0. Since Zone 0 points to the bright-

ness code programmed in Zone Target Register 0 (loaded

with code 0x19), the LED current becomes:

Next assume that the ambient light changes to 500 LUX (cor-

responding to an ALS1 voltage of 437.5mV). This moves the

ambient light into Zone 2 which corresponds to Zone Target

comes:

This Example still applies to the LM3535-2ALS version using

two ambient light sensors. The ALS selector block makes the

front end decision as to which sensor to use.

ALS Averaging Time

The ALS Averaging Time is the time over which the Averager

block collects samples from the A/D converter and then av-

erages them to pass to the discriminator block (see ). Ambient

light sensor samples are averaged and then further pro-

cessed by the discriminator block to provide rejection of noise

and transient signals. The Averager is configurable with 8 dif-

ferent averaging times to provide varying amounts of noise

and transient rejection (see Figure 9). 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 sam-

ples the ALS inputs at 16kHz. If the averaging time is set to

800ms, the Averager will send the updated zone information

to the discriminator every 800ms. This zone information con-

tains the average of approximately 12800 samples (800ms ×

16kHz). Due to the latency of 2 averaging cycles, when there

is a steady state change in the ambient light, the LED current

will begin to transition to the appropriate target value after

approximately 1600ms 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 will always round 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 16 shows an example

of how the Averager will make the zone decisions for

different Ambient conditions.

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