10
FN7464.5
August 8, 2008
Solution 2
Using External Timing Mode
From Solution 1, the desired integration time is 100ms. Note
that the R
EXT
resistor only determines the inter oscillator
frequency when using external timing mode. Instead, the
integration time is the time between two sync_iic commands
sent through the I
2
C. The programmer determines how
many I
2
C clock cycles to wait between two external timing
commands.
i
I
2
C
= f
I
2
C*
t
int
= number of I
2
C clock cycles
i
I
2
C
= 10kHz
*
100ms
i
I
2
C
= 1,000 I
2
C clock cycles. An external sync_iic command
sent 1,000 cycles after another sync_iic command rejects
both 60Hz and 50Hz AC noise signals.
Next, is to pick an arbitrary R
EXT
= 100kΩ and to choose the
Gain/Range Mode. For a maximum 500 lux, Range1 is
adequate. From Equation 3:
The effective transfer function becomes:
DATA is the sensor reading data located in data registers
04(hex) and 05(hex)
COUNTER is the timer counter value data located in data
registers 06(hex) and 07(hex). In this sample problem,
COUNTER = 1000.
IR Rejection
Any filament type light source has a high presence of infrared
component invisible to the human eye. A white fluorescent
lamp, on the other hand has a low IR content. As a result,
output sensitivity may vary depending on the light source.
Maximum attenuation of IR can be achieved by properly
scaling the readings of Diode1 and Diode2. The user obtains
data reading from sensor Diode1 (D1), which is sensitive to
visible and IR, then reading from sensor Diode2 (D2), which is
mostly sensitive from IR. The graph in Figure 8 shows the
effective spectral response after applying Equation 19 of the
ISL29003 from 400nm to 1000nm. Equation 19 describes the
method of cancelling IR in internal timing mode.
Where:
data = lux amount in number of counts less IR presence
D1 = data reading of Diode1
D2 = data reading of Diode2
n = 1.85. This is a fudge factor to scale back the sensitivity
up to ensure Equation 4 is valid.
k = 7.5. This is a scaling factor for the IR sensitive Diode2.
Flat Window Lens Design
A window lens will surely limit the viewing angle of the
ISL29003. 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 of 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 ISL29003.
Table 16 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.
TABLE 14. SOLUTION1 SUMMARY TO EXAMPLE DESIGN
PROBLEM
DESIGN PARAMETER VALUE
t
int
100ms
R
EXT
50kΩ
Gain/Range Mode Range1 = 1000 lux
FSR 2000 lux
# of clock cycles 2
16
Transfer Function
TABLE 15. SOLUTION2 SUMMARY TO EXAMPLE DESIGN
PROBLEM
DESIGN PARAMETER VALUE
t
int
100ms
R
EXT
100kΩ
Gain/Range Mode Range1 = 1000 lux
FSR 1000 lux
# of clock cycles COUNTER = 1000
Transfer Function
E
DATA
2
16
-----------------
2000 lux×=
FSR 1000 lux
100kΩ
100kΩ
------------------
=
FSR 1000 lux=
E
DATA
COUNTER
--------------------------------
1000 lux×=
E
DATA
COUNTER
--------------------------------
1000 lux×=
D3 n D1 kD2–()=
(EQ. 19)
D
LENS
∅
t
D1
D
TOTAL
∅ = Viewing angle
WINDOW LENS
ISL29003
FIGURE 4. FLAT WINDOW LENS
ISL29003
