NOA1305 Datasheet, PDF (7/13 Pages) ON Semiconductor – Ambient Light Sensor

NOA1305
DESCRIPTION OF OPERATION
Ambient Light Sensor Architecture
The NOA1305 employs a sensitive photo diode fabricated
in ON Semiconductor’s standard CMOS process
technology. The major components of this sensor are as
shown in Figure 2. The photons which are to be detected pass
through an ON Semiconductor proprietary color filter
limiting extraneous photons and thus performing as a band
pass filter on the incident wave front. The filter only
transmits photons in the visible spectrum which are
primarily detected by the human eye. The photo response of
this sensor is as shown in Figure 5.
The ambient light signal detected by the photo diode is
converted to digital signal using a variable slope integrating
ADC with a resolution of 16−bits, unsigned. The ADC value
is provided to the control block connected to the I²C
interface block.
Equation 1 shows the relationship of output counts C_nt as
a function of integration constant I_k, integration time T_int (in
seconds) and the intensity of the ambient light, I_L(in lux), at
room temperature (25°C).
I_L + C_ntń(I_k T_int )
(eq. 1)
Where:
I_k ≈ 7.7 (for White LED Source)
For example let:
C_nt = 1000
T_int = 200 mS
Intensity of ambient light, I_L(in lux):
I_L + 1000ń(7.7 200 mS )
I_L = 649 lux
(eq. 2)
Modes of Operation
The NOA1305 can be placed in any of the following
modes of operation by programming registers over the I²C
bus:
1. Interrupt driven mode
2. Polling mode
3. Power−down mode
In the interrupt driven mode, once the NOA1305 is
configured, no I²C activity is necessary until the ambient
light intensity goes above the value programmed in the
interrupt threshold register. When this occurs, the device
signals an interrupt on the INT pin. Then it is up to the I²C
master host to read the ALS count from the device.
In polling mode, interrupts are typically disabled, but the
NOA1305 continuously takes measurements and the I²C
master host reads out the most recent count whenever it
desires to do so, typically in a timed repeat loop.
In power−down mode, the NOA1305 stops taking
ambient light measurements and powers down most of the
internal circuitry and the INT pin is deactivated. Power is
maintained to preserve the register values (static memory)
and a portion of the I²C remains active to monitor for a
power−on command to the NOA1305.
I²C Interface
The NOA1305 acts as an I²C slave device and supports
single register read and write operations, in addition to block
read and block write operations. All data transactions on the
bus are 8 bits long. Each data byte transmitted is followed by
an acknowledge bit. Data is transmitted with the MSB first.
Figure 16 shows an I²C write operation. Write transactions
begin with the master sending an I²C start sequence
followed by the seven bit slave address (NOA1305 = 0x39)
and the write(0) command bit. The NOA1305 will
acknowledge this byte transfer with an appropriate ACK.
Next the master will send the 8 bit register address to be
written to. Again the NOA1305 will acknowledge reception
with an ACK. Finally, the master will begin sending 8 bit
data segment(s) to be written to the NOA1305 register bank.
The NOA1305 will send an ACK after each byte and
increment the address pointer by one in preparation for the
next transfer. Write transactions are terminated with either
an I²C STOP or with another I I²C START (repeated
START).
Device
Address
A[6:0] WRITE ACK
Register
Address
D[7:0] ACK
Register
Data
D[7:0] ACK
011 1001 0
0 0000 0110 0 00000000 0
0x72
7
8
8
Start
Condition
Figure 16. I²C Write Command
Stop
Condition
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