ISL29010 Datasheet, PDF (8/14 Pages) Intersil Corporation – Light-to-Digital Output Sensor with High Sensitivity, Gain Selection, and I2C Interface

ISL29010
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 = 327kHz*100kΩ/R_EXT. When Range/Gain is set to
Range1 or Range2.
f_OSC = 655kHz*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.
Flat Window Lens Design
A window lens will surely limit the viewing angle of the
ISL29010. The window lens should be placed directly on top
of the device. The thickness of the lens should be kept at
minimum to minimize loss of power due to reflection and
also to minimize loss due to absorption of energy in the
plastic material. A thickness of t = 1mm is recommended for
a window lens design. The bigger the diameter of the
window lens, the wider the viewing angle is of the ISL29010.
Table 12 shows the recommended dimensions of the optical
window to ensure both 35° and 45° viewing angle. These
dimensions are based on a window lens thickness of 1.0mm
and a refractive index of 1.59.
WINDOW LENS
t
∅
D_TOTAL
D1
ISL29013
D_LENS
DATA
E = ₂₁₅ x 2000
∅ = VIEWING ANGLE
FIGURE 4. FLAT WINDOW LENS
TABLE 12. RECOMMENDED DIMENSIONS FOR A FLAT
WINDOW DESIGN
D_TOTAL
1.5
D_LENS @ 35
D_LENS @ 45
D1
VIEWING ANGLE VIEWING ANGLE
0.50
2.25
3.75
2.0
1.00
3.00
4.75
2.5
1.50
3.75
5.75
3.0
2.00
4.30
6.75
3.5
2.50
5.00
7.75
t=1
D1
D_LENS
D_TOTAL
Thickness of lens
Distance between ISL29010 and inner edge of lens
Diameter of lens
Distance constraint between the ISL29010 and lens
outer edge
* All dimensions are in mm.
Window with Light Guide Design
If a smaller window is desired while maintaining a wide
effective viewing angle of the ISL29010, a cylindrical piece of
transparent plastic is needed to trap the light and then focus
and guide the light on to the device. Hence the name light
guide which is also known as light pipe. The pipe should be
placed directly on top of the device with a distance of
D1 = 0.5mm to achieve peak performance. The light pipe
should be a minimum of 1.5mm in diameter to ensure that
whole area of the sensor will be exposed. See Figure 5.
8
FN6414.0
February 13, 2008