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ISL29023
Principles of Operation
Photodiodes and ADC
The ISL29023 contains two photodiode arrays, which convert light
into current. The spectral response for ambient light sensing and IR
sensing is shown in Figure 4 in the performance curves section.
After light is converted to current during the light signal process, the
current output is converted to digital by a built-in 16-bit Analog-to-
Digital Converter (ADC). An I 2 C command reads the ambient light or
IR intensity in counts.
The converter is a charge-balancing integrating type 16-bit ADC. The
chosen method for conversion is best for converting small current
signals in the presence of an AC periodic noise. A 100ms integration
time, for instance, highly rejects 50Hz and 60Hz power line noise
simultaneously.
The built-in ADC offers user flexibility in integration time or
conversion time. There are two timing modes: Internal Timing Mode
and External Timing Mode. In Internal Timing Mode, integration time
is determined by an internal oscillator (f OSC ), and the n-bit (n = 4, 8,
12, 16) counter inside the ADC. In External Timing Mode, integration
time is determined by the time between two consecutive I 2 C
External Timing Mode commands. A good balancing act of
integration time and resolution (depending on the application) is
required for optimal results.
The ADC has I 2 C programmable range select to dynamically
accommodate various lighting conditions. For very dim
conditions, the ADC can be configured at its lowest range
(Range 1) in the ambient light sensing.
Low-Power Operation
The ISL29023 initial operation is at the power-down mode after a
supply voltage is provided. The data registers contain the default
value of 0. When the ISL29023 receives an I 2 C command to do a
one-time measurement from an I 2 C master, it will start ADC
conversion with light sensing. It will go to the power-down mode
automatically after one conversion is finished and keep the
conversion data available for the master to fetch anytime
afterwards. The ISL29023 will continuously do ADC conversion
with light sensing if it receives an I 2 C command of continuous
measurement. It will continuously update the data registers with
the latest conversion data. It will go to the power-down mode
after it receives the I 2 C command of power-down.
Ambient Light and IR Sensing
There are four operational modes in ISL29023: Programmable ALS
once with auto power-down, programmable IR sensing once with
auto power-down, programmable continuous ALS sensing and
programmable continuous IR sensing. These four modes can be
programmed in series to fulfill the application needs. The detailed
program configuration is listed in “Command-I Register (Address:
0x00)” on page 8.
When the part is programmed for ambient light sensing, the
ambient light with wavelength within the “Ambient Light
Sensing” spectral response curve in Figure 4 is converted into
current. With ADC, the current is converted to an unsigned n-bit
(up to 16 bits) digital output.
5
When the part is programmed for infrared (IR) sensing, the IR
light with wavelength within the “IR Sensing” spectral response
curve in Figure 4 is converted into current. With ADC, the current
is converted to an unsigned n-bit (up to 16-bits) digital output.
Interrupt Function
The active low interrupt pin is an open drain pull-down
configuration. The interrupt pin serves as an alarm or monitoring
function to determine whether the ambient light level exceeds
the upper threshold or goes below the lower threshold. It should
be noted that the function of ADC conversion continues without
stopping after interrupt is asserted. If the user needs to read the
ADC count that triggers the interrupt, the reading should be done
before the data registers are refreshed by the following
conversions. The user can also configure the persistency of the
interrupt pin. This reduces the possibility of false triggers, such as
noise or sudden spikes in ambient light conditions. An
unexpected camera flash, for example, can be ignored by setting
the persistency to 8 integration cycles.
Serial Interface
The ISL29023 supports the Inter-Integrated Circuit (I 2 C) bus data
transmission protocol. The I 2 C bus is a two wire serial
bidirectional interface consisting of SCL (clock) and SDA (data).
Both the wires are connected to the device supply via pull-up
resistors. The I 2 C protocol defines any device that sends data
onto the bus as a transmitter and the receiving device as the
receiver. The device controlling the transfer is a master and the
device being controlled is the slave. The transmitting device pulls
down the SDA line to transmit a “0” and releases it to transmit a
“1”. The master always initiates the data transfer, only when the
bus is not busy, and provides the clock for both transmit and
receive operations. The ISL29023 operates as a slave device in
all applications. The serial communication over the I 2 C interface
is conducted by sending the most significant bit (MSB) of each
byte of data first.
Start Condition
During data transfer, the SDA line must remain stable while the
SCL line is HIGH. All I 2 C interface operations must begin with a
START condition, which is a HIGH to LOW transition of SDA while
SCL is HIGH (refer to Figure 12). The ISL29023 continuously
monitors the SDA and SCL lines for the START condition and does
not respond to any command until this condition is met (refer to
Figure 12). A START condition is ignored during the power-up
sequence.
Stop Condition
All I 2 C interface operations must be terminated by a STOP
condition, which is a LOW to HIGH transition of SDA while SCL is
HIGH (refer to Figure 12). A STOP condition at the end of a
read/write operation places the device in its standby mode. If a
stop is issued in the middle of a Data byte, or before 1 full Data
byte + ACK is sent, then the serial communication of ISL29023
resets itself without performing the read/write. The contents of
the array are not affected.
FN6691.3
April 25, 2013
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