ISL29012 Datasheet, PDF (8/15 Pages) Intersil Corporation – Light-to-Digital Output Sensor with High Sensitivity, Gain Selection, Interrupt Function and I2C Bus™

ISL29012
Integration time in Internal Timing Mode
This timing mode is programmed in the command register
00(hex) Bit 5. Most applications will be using this timing
mode. When using the Internal Timing Mode, f_OSC and
n-bits resolution determine the integration time. t_INT is a
function of the number of clock cycles and f_OSC as shown in
Equation 9:
t_INT
=
2^m × ----¹------
f_osc
for Internal Timing Mode only
(EQ. 9)
m = 4, 8, 12, and16. n is the number of bits of resolution.
2^m therefore is the number of clock cycles. n can be
programmed at the command register 00(hex) Bits 1 and 0.
Since f_OSC is dual speed depending on the Gain/Range bit,
t_INT is dual time. The integration time as a function of R_EXT
is shown in Equation 10:
t_INT1
=
2^m
×
----------------^R-----^E----^X---^T-----------------
342kHz × 100kΩ
(EQ. 10)
t_INT1 is the integration time when the device is configured
for Internal Timing Mode and Gain/Range is set to Range1
or Range2.
t_INT2
=
2^m
×
----------------^R-----^E----^X---^T-----------------
684kHz × 100kΩ
(EQ. 11)
t_INT2 is the integration time when the device is configured
for Internal Timing Mode and Gain/Range is set to Range3
or Range4.
TABLE 13. INTEGRATION TIMES FOR TYPICAL REXT VALUES
R_EXT
(kΩ)
RANGE1
RANGE2
n = 15-BIT n = 11-BIT
50
100
6.4
100**
200
13
200
400
26
500
1000
64
*Integration time in milliseconds
**Recommended R_EXT resistor value
RANGE3
RANGE4
n = 11-BIT
n=3
3.2
0.013
6.5
0.025
13
0.050
32
0.125
Integration time in External Timing Mode
This timing mode is programmed in the command register
00(hex) Bit 5. External Timing Mode is recommended when
integration time can be synchronized to an external signal
such as a PWM to eliminate noise.
To read the light count DATA output, the device needs three
sync_I²C commands to complete one measurement. The 1st
sync_I²C command starts the conversion of the diode array 1.
The 2nd sync_I²C completes the conversion of diode array 1
and starts the conversion of diode array 2. The 3rd sync_I²C
pules ends the conversion of diode array 2, outputs the light
count DATA, and starts over again to commence conversion
of diode array 1.
The integration time, t_INT, is the sum of two identical time
intervals between the three sync pulses. t_INT is determined
by Equation 12:
t_INT
=
k_{----O----S----C--}
f_OSC
(EQ. 12)
where K_OSC is the number of internal clock cycles obtained
from Timer data register and f_OSC is the internal I²C
operating frequency
The internal oscillator, f_OSC, operates identically in both the
internal and external timing modes, with the same
dependence on R_EXT. However, in External Timing Mode,
the number of clock cycles per integration is no longer fixed
at 2ⁿ. The number of clock cycles varies with the chosen
integration time, and is limited to 2¹⁶ = 65,536. In order to
avoid erroneous lux readings, the integration time must be
short enough not to allow an overflow in the counter register.
t_INT
<
6_----5_---,_--5_---3_----5_-
f_OSC
(EQ. 13)
f_OSC = 342kHz*100kΩ/R_EXT. When Range/Gain is set to
Range1 or Range2.
f_osc = 684kHz*100kΩ/R_EXT. When Range/Gain is set to
Range3 or Range4.
Noise Rejection
In general, integrating type ADC’s have excellent
noise-rejection characteristics for periodic noise sources
whose frequency is an integer multiple of the integration
time. For instance, a 60Hz AC unwanted signal’s sum from
0ms to k*16.66ms (k = 1,2...k_i) is zero. Similarly, setting the
device’s integration time to be an integer multiple of the
periodic noise signal, greatly improves the light sensor
output signal in the presence of noise.
Ambient Light Sensing Operation
The operation of ambient light sensing (ALS) within the
ISL29012 utilizes two diodes; D1 and D2. The D1 diode is
sensitive to both visible and IR light spectrum. The D2 diode
is sensitive to IR spectrum. D1 and D2 spectrum response is
shown in Figure 21. The diodes are measured sequentially
and their outputs are converted with an ADC. The output of
the ALS is the difference between these two measurements.
Maximum Ambient Intensity Condition
In typical applications, the ISL29012 is installed behind a
dark cover window. In this low-light condition, both D1 and
D2 operate linearly and the ALS output is linear as well
(Figures 19 and 20). In brighter environments and with
transparent glass, however, D1 and D2 can be subject to
saturation. As the ambient light grows bright enough to
subject one or both diodes to saturation, the ALS count
(output) decreases and eventually reaches zero in deep
8
FN6476.1
December 10, 2008