ISL29023 Datasheet, PDF (10/14 Pages) Intersil Corporation – Integrated Digital Ambient Light Sensor with Interrupt Function

ISL29023
Upper Interrupt Threshold Registers
(Address: 0x06 & 0x07)
TABLE 13. INTERRUPT REGISTER BITS
NAME
Reg.
Register Bits
Addr
DFLT
(Hex) B7 B6 B5 B4 B3 B2 B1 B0 (Hex)
INT_HT_LSB 0x06 TH7 TH6 TH5 TH4 TH3 TH2 TH1 TH0 0xFF
INT_HT_MSB 0x07 TH15 TH14 TH13 TH12 TH11 TH10 TH9 TH8 0xFF
The upper interrupt threshold registers are used to set the upper
trigger point for interrupt generation. If the ALS value crosses
above or is equal to the upper threshold, an interrupt is asserted
on the interrupt pin and the interrupt flag. Registers INT_HT_LSB
(0x06) and INT_HT_MSB (0x07) provide the low and high bytes,
respectively, of the upper interrupt threshold. The high and low
bytes from each set of registers are combined to form a 16-bit
threshold value. The interrupt threshold registers default to 0xFF
on power up.
Applications Information
TABLE 14. INTEGRATION TIME OF n-BIT ADC
R_EXT (kΩ)
499**
n = 16-BIT
90ms
n = 12-BIT
5.6ms
n = 8-BIT
352µs
n = 4-BIT
22µs
**Recommended R_EXT resistor value
External Scaling Resistor R_EXT for f_OSC and
Range
The ISL29023 uses an external resistor R_EXT to fix its internal
oscillator frequency f_OSC and the light sensing range, Range. f_OSC
and Range are inversely proportional to R_EXT. For user simplicity,
the proportionality constant is referenced to 499kΩ:
Range
=
4_----9_---9_----k_---Ω_---
R_EXT
×
R
an
ge
(k)
(EQ. 5)
f_OSC
=
4_----9_---9_----k_---Ω_---
R_EXT
×
725 k
H
z
(EQ. 6)
ADC Output in IR Sensing
The ISL29023’s ADC output codes, DATA, are directly
proportional to the IR intensity received in the IR sensing.
Calculating Lux
The ISL29023’s ADC output codes, DATA, are directly
proportional to lux in the ambient light sensing.
E_cal = α × DATA
(EQ. 1)
Here, E_cal is the calculated lux reading. The constant α is
determined by the Full Scale Range and the ADC’s maximum
output counts. The constant is independent of the light sources
(fluorescent, incandescent and sunlight) because the light
sources’ IR component is removed during the light signal
process. The constant can also be viewed as the sensitivity (the
smallest lux measurement the device can measure).
α
=
-^R-----^a----ⁿ---^g----^e----⁽--^k----⁾-
Count_max
(EQ. 2)
Here, Range(k) is defined in Table 8. Count_max is the maximum
output counts from the ADC.
The transfer function used for n-bits ADC becomes:
E_cal
=
R_-----a_----n_----g_----e_----(_--k_----)
2ⁿ
×
D
A
T
A
(EQ. 3)
Here, n = 4, 8, 12 or 16. This is the number of ADC bits
programmed in the command register. 2ⁿ represents the
maximum number of counts possible from the ADC output. Data
is the ADC output stored in the data registers (02 hex and 03
hex).
Integration and Conversion Time
The ADC resolution and f_OSC determine the integration time, t_int.
t_int
=
2ⁿ × ------¹-------
f_OSC
=
2ⁿ
×
----------------^R-----^E---^X----^T-----------------
725kHz × 499kΩ
(EQ. 4)
DATA_IR = β × E_IR
(EQ. 7)
Here, E_IR is the received IR intensity. The constant β changes with
the spectrum of background IR noise, such as sunlight and
incandescent light. The β also changes with the ADC’s range and
resolution selections.
Noise Rejection
Electrical AC power worldwide is distributed at either 50Hz or
60Hz. Artificial light sources vary in intensity at the AC power
frequencies. The undesired interference frequencies are infused
on the electrical signals. This variation is one of the main sources
of noise for the light sensors. Integrating type ADC’s have
excellent noise-rejection characteristics for periodic noise
sources whose frequency is an integer multiple of the conversion
rate. By setting the sensor’s integration time to an integer
multiple of periodic noise signal, the performance of an ambient
light sensor can be improved greatly in the presence of noise. In
order to reject the AC noise, the integration time of the sensor
must to adjusted to match the AC noise cycle. 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.
Suggested PCB Footprint
It is important that users check the “Surface Mount Assembly
Guidelines for Optical Dual Flat Pack No Lead (ODFN) Package”
before starting ODFN product board mounting.
http://www.intersil.com/data/tb/TB477.pdf
where n is the number of bits of resolution and n = 4, 8, 12 or 16.
Therefore, 2ⁿ is the number of clock cycles. n can be programmed
at the command register 01(hex) bits 3 and 2.
10
FN6691.1
July 17, 2012