Ambient Light (lux)

Vals_ref

= 1V

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

LM3535

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SNVS598A – AUGUST 2010 – REVISED MAY 2013

Zone Boundary 0 = 200mV

Zone Boundary 1 = 400mV

Zone Boundary 2 = 600mV

Zone Boundary 3 = 800mV

These Zone Boundary Registers are all 8-bit (readable and writable) registers. By Default, the first zone (Z0) is

defined between 0 and 200mV, Z1’s default is defined between 200mV and 400mV, Z2 is defined between

400mV and 600mV, Z3 is defined between 600mV and 800mV, and Z4 is defined between 800mV and 1V. The

default settings for the 5 Zone Target Registers are 0x19, 0x33, 0x4C, 0x66, and 0x7F. This corresponds to LED

brightness settings of 84µA, 164µA, 1.45mA, 6.17mA and 25mA of current respectively. See Figure 41.

Figure 41. ALS Zone to LED Brightness Mapping

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 Table 2). 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 Register (Address 0x20) is programmed with 0xDC, the ALS Control 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

compatible interface.

Now assume that the APDS-9005 ambient light sensor detects 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 brightness code programmed in Zone Target Register 0 (loaded with code 0x19), the LED current

becomes:

(2)

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