Vdd

Vsns

LED Driver

Brightness

ALS Select

1

0

VOUT

Zone

Discriminator

4

ALS Path Functional Diagram

bits 8

ALSRS

ALS Resistor

Select Register

8 bits

A/D

Averager

(LPF)

Ramp

Control

7 bits

7 bits

7 bits

3 bits

3 bits

Ramp-Down

Rate

Selection

Ramp-Up

Rate

Selection

7 bits

User Selectable w/

Typical Defaults

User Selectable w/

Typical Defaults

Light output

targets for

each of 5

ambient

light zones

Input Light

Zone

Definition

Registers

Z4 target light

3 Zline

2 Zline

1 Zline

0 Zline

3

Z3 target light

2

Z2 target light

1

Z1 target light

0

Z0 target light

20

LM3535

SNVS598B – AUGUST 2010 – REVISED MARCH 2018

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Copyright © 2010–2018, Texas Instruments Incorporated

8.2.2 Detailed Design Procedure

8.2.2.1 Ambient Light Sensing

8.2.2.1.1

Ambient Light Sensor Block

The LM3535 incorporates an ambient light sensing interface (ALS) which translates an analog output ambient

light sensor to a user specified brightness level. The ambient light sensing circuit has 4 programmable

boundaries (ZB0 – ZB3) which define 5 ambient brightness zones. Each ambient brightness zone corresponds to

a programmable brightness threshold (Z0T – Z4T).

Furthermore, the ambient light sensing input features 15 internal software-selectable voltage setting resistors.

This allows the LM3535 the capability of interfacing with a wide selection of ambient light sensors. Additionally,

the ALS inputs can be configured as high impedance, thus providing for a true shutdown during low power

modes. The ALS resistors are selectable through the ALS Resistor Select Register (see Table 2). Figure 30

shows a functional block diagram of the ambient light sensor input.

Figure 30. Ambient Light Sensor Functional Block Diagram

8.2.2.1.2

ALS Operation

The ambient light sensor input has a 0 to 1 V operational input voltage range. The Specifications shows the

LM3535 with an ambient light sensor (AVAGO, APDS-9005) and the internal ALS Resistor Select Register set to

0x40 (2.32 k

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

The voltage at the active ambient light sensor input 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 28).

With bits [6:4] of the Configuration Register set to 1 (Bit6 = ALS Block Enable, Bit5 = BankB ALS Enable, Bit4 =

BankA ALS Enable), the LM3535 is configured for ambient light current Control. In this mode the ambient light

sensing input (ALS) monitors the output of analog output ambient light sensing photo diode and adjusts the LED

current depending on the ambient light. The ambient light sensing circuit has 4 configurable ambient light

boundaries (ZB0 – ZB3) programmed through the four (8-bit) Zone Boundary Registers. These zone boundaries

define 5 ambient brightness zones.

On start-up the 4 Zone Boundary Registers are pre-loaded with 0x33 (51d), 0x66 (102d), 0x99 (153d), and 0xCC

(204d). The ALS input has a 1-V active input voltage range which makes the default Zone Boundaries approx.

set at: