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ISL29004 Datasheet, PDF (9/17 Pages) Intersil Corporation – Light-to-Digital Output Sensor with Address Selection, High Sensitivity, Gain Selection, Interrupt Function and I2C Interface
ISL29004
maximum number of counts is stored in the data register
named COUNTER
COUNTER is the number increments accrued for between
integration time for External Timing Mode.
Gain/Range, Range(k)
The Gain/Range can be programmed in the control register
to give Range (k) determining the FSR. Note that Range(k)
is not the FSR. See Equation 3. Range(k) provides four
constants depending on programmed k that will be scaled by
REXT. See Table 9. Unlike REXT, Range(k) dynamically
adjusts the FSR. This function is especially useful when light
conditions are varying drastically while maintaining excellent
resolution.
Number of Clock Cycles, n-bit ADC
The number of clock cycles determines “n” in the n-bit ADC; 2n
clock cycles is a n-bit ADC. n is programmable in the command
register in the width function. Depending on the application, a
good balance of speed, and resolution has to be considered
when deciding for n. For fast and quick measurement, choose
the smallest n = 4. For maximum resolution without regard of
time, choose n = 16. Table 12 compares the tradeoff between
integration time and resolution. See Equations 10 and 11 for the
relation between integration time and n. See Equation 3 for the
relation of n and resolution.
TABLE 12. RESOLUTION AND INTEGRATION TIME
SELECTION
RANGE1
fosc = 327kHz
RANGE4
fosc = 655kHz
RESOLUTION TINT RESOLUTION
n TINT (ms) LUX/COUNT (ms) (LUX/COUNT)
16 200
0.01
100
1
12 12.8
0.24
6.4
16
8 0.8
3.90
0.4
250
4 0.05
REXT = 100kΩ
62.5
0.025 4000
External Scaling Resistor REXT and fosc
The ISL29004 use an external resistor REXT to fix its
internal oscillator frequency, fosc. Consequently, REXT
determines the fosc, integration time and the FSR of the
device. Fosc, a dual speed mode oscillator, is inversely
proportional to REXT. For user simplicity, the proportionality
constant is referenced to fixed constants 100kΩ and
655kHz:
fosc1
=
1--
2
×
-1---0---0----k---Ω---
REXT
×
655
k
Hz
(EQ. 6)
fosc2
=
1----0---0----k---Ω---
REXT
×
655k
H
z
(EQ. 7)
fosc1 is oscillator frequency when Range1 or Range2 are
set. This is nominally 327kHz when REXT is 100kΩ.
fosc2 is the oscillator frequency when Range3 or Range4
are set. This is nominally 655kHz when REXT is 100kΩ.
When the Range/Gain bits are set to Range1 or Range2,
fosc runs at half speed comapred to when Range/Gain bits
are set to Range3 and Range4.
fosc1
=
1--
2
(
fo
sc
2
)
(EQ. 8)
The automatic fosc adjustment feature allows significant
improvement of signal-to-noise ratio when detecting very low
Lux signals.
Integration Time or Conversion Time
Integration time is the period during which the device’s
analog-to-digital ADC converter samples the photodiode
current signal for a Lux measurement. Integration time, in
other words, is the time to complete the conversion of analog
photodiode current into a digital signal-number of counts.
Integration time affects the measurement resolution. For
better resolution, use a longer integration time. For short and
fast conversions use a shorter integration time.
The ISL29004 offer user flexibility in the integration time to
balance resolution, speed and noise rejection. Integration time
can be set internally or externally and can be programmed in
the command register 00(hex) bit 5.
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, fosc and n-bits
resolution determine the integration time. Tint is a function of
the number of clock cycles and fosc.
Tint
=
2n
×
------1------
fosc
For Internal Timing Mode Only
(EQ. 9)
n = 4, 8, 12, and16. n is the number of bits of resolution.
2n therefore is the number of clock cycles. n can be
programmed at the command register 00(hex) bits 1 and 0.
9
FN6221.0
December 21, 2006