LM3530 datasheet(13/50 Pages) TI1

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Ambient Light (lux)

Vals_ref =

ZB3

ZB0

ZB1

ZB2

Zone 0

Zone 4

Zone 3

Zone 2

Zone 1

LED Driver Input Code (0-127)

Full

Scale

Z0T

Z4T

Z3T

Z2T

Z1T

LM3530

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SNVS606L – JUNE 2009 – REVISED DECEMBER 2014

Feature Description (continued)

8.3.4 ALS Operation

The ambient light sensor input has a 0-V to 1-V operational input voltage range. LM3530 Typical Application

shows the LM3530 with dual ambient light sensors (AVAGO, APDS-9005) and the internal ALS Resistor Select

Ω). This circuit converts 0 to 1000 LUX light into approximately a 0-mV to 850-mV

linear output voltage. The voltage at the active ambient light sensor input (ALS1 or ALS2) is compared against

the 8 bit values programmed into the Zone Boundary Registers (ZB0-ZB3). When the ambient light sensor output

crosses one of the ZB0 – ZB3 programmed thresholds the internal ALS circuitry will smoothly transition the LED

current to the new 7 bit brightness level as programmed into the appropriate Zone Target Register (Z0T – Z4T)

(see Figure 27).

The ALS Configuration Register bits [6:5] programs which input is the active input, bits [4:3] control the on/off

state of the ALS circuitry, and bits [2:0] control the ALS input averaging time. Additionally, the ALS Information

to a new zone. This flag is reset when the register is read back. Bits [2:0] of this register contain the current

active zone information.

Figure 27. Ambient Light Input To Backlight Mapping

8.3.5 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 26). 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 Table 5). 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 1024 ms then the

Averager will send the updated zone information to the discriminator every 1024 ms. This zone information

contains the average of 16384 samples (1024 ms × 16 kHz). Due to the latency of 2 averaging cycles, the LED

current will not change until there has been a steady-state change in the ambient light for at least 2 averaging

periods.

8.3.5.1 Averager Operation

The magnitude and direction (either increasing or decreasing) of the Averager output is used to determine

whether the LM3530 should change brightness zones. The Averager block functions as follows:

1. First, the Averager always begins with a Zone 0 reading stored at start-up. If the main display LEDs are

active before the ALS block is enabled, it is recommended that the ALS Enable 1 bit is set to '1' at least 3

averaging periods before the ALS Enable 2 bit is set.

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