Industry’s Lowest-Power

Ambient Light Sensor with ADC

11

Applications Information

Auto and Manual Modes

In auto mode configuration (default setting), CDR and

TIM bits are internally generated. The autoranging circuit

uses two different methods to change its sensitivity. For

light intensities greater than 700 lux, a current divider

reduces the photodiode’s current by a factor of 8. The

default, as in the previous example, is a division of 1:

current goes directly into the I-to-F converter. As light

intensity decreases, the autoranging circuit increases

the integration time from 100ms to 200ms to 400ms, or

to 800ms. The combination of the current divider and the

different integration times give the A/D a range 8 times

higher, as well as 8 times lower, than its nominal 16-bit

range. This gives a dynamic range of 22 bits or slightly

over 4,000,000 to 1.

In manual mode, the user has access to 4 bits (CDR and

TIM[2:0]) to override the autoranging circuitry. These

affect the integration time of the A/D and the current

division ratio. See the register description for manual

configuration mode (0x02, bit 6).

Data Format of Lux Reading

The IC has a user-friendly digital output format. It con-

sists of a 4-bit exponent followed by an 8-bit mantissa.

In its highest sensitivity mode, 1 count represents 0.045

lux. The mantissa has a maximum value of 255, and the

exponent has a maximum value of 14. This gives a maxi-

maximum lux reading is 188,000 lux. Any reading greater

than that (i.e., exponent = 15) is considered to be an

overload. No conversion formulas are needed as in the

case of dual-diode ambient light sensors.

The IC’s output (registers 0x03 and 0x04) comprises a

12-bit result that represents the ambient light expressed

in units of lux.

Here is how lux is calculated:

The exponent is a 4-bit number ranging from 0000 to

1110 (zero to 14).

The mantissa is an 8-bit number ranging from 0000 0000

to 1111 1111 (zero to 255).

If the INTE bit = 1 and the ambient light level exceed either threshold limit for a time longer than that specified by the

Threshold Timer register, then the INTS bit is set to 1 and the INT pin is pulled low.

The value in this register sets the time used to control this delay. A value of 0x00 in this register (with INTE bit = 1 in the

Interrupt Enable register) configures the IC to assert the interrupt pin as soon as the light level exceeds either threshold.

Time delay = (128xT7 + 64xT6 + 32xT5 + 16xT4 + 8xT3 + 4xT2 + 2xT1 + T0) x 100ms.

Threshold Timer Register 0x07

The Lower Threshold High-Byte register exponent with the four most significant bits of the mantissa sets the lower trip

level for interrupt functionality. This lower limit is relevant only if the INTE bit in the Interrupt Enable register is set. If the

lux level is below this light level for a time greater than that specified in the Threshold Timer register, the INTS bit in the

Interrupt Status register is set and the INT pin is pulled low.

Mantissa (LM[7:4]): Four most significant bits of mantissa lower threshold

Exponent (LE[3:0]): Exponent bits lower threshold

Exponent = 8xLE3 + 4xLE2 + 2xLE1 + LE0

Mantissa = 128xLM7 + 64xLM6 + 32xLM5 + 16xLM4

Lower Threshold High-Byte Register 0x06

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

REGISTER

ADDRESS

LE3

LE2

LE1

LE0

LM7

LM6

LM5

LM4

0x06

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

REGISTER

ADDRESS

T7

T6

T5

T4

T3

T2

T1

T0

0x07