LM3535 datasheet(18/26 Pages) NSC

18 / 26 page

Application Information

AMBIENT LIGHT SENSING

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 inputs (ALSA and

ALSB(LM3535-2ALS)) features 15 internal software se-

lectable 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 Internal ALS Resistor Ta-

ble). Figure 14 shows a functional block diagram of the am-

bient light sensor input.

30082442

FIGURE 14. Ambient Light Sensor Functional Block Diagram

ALS Operation

The ambient light sensor input has a 0 to 1V operational input

voltage range. The Typical Application Circuit shows the

LM3535 with an ambient light sensor (AVAGO, APDS-9005)

and the internal ALS Resistor Select Register set to 0x40

(2.32k

Ω). This circuit converts 0 to 1000 LUX light into ap-

proximately a 0 to 850mV linear output voltage. The voltage

at the active ambient light sensor input is compared against

the 8 bit values programmed into the Zone Boundary Regis-

ters (ZB0-ZB3). When the ambient light sensor output cross-

es 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 13).

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) pro-

grammed 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 1V active input voltage range which

makes the default Zone Boundaries approx. set at:

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 15.

LM3535 Arkusz danych(PDF) 18 Page - National Semiconductor (TI)

LM3535 Arkusz danych(HTML) 18 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Arkusz danych Pobierz

Link URL