ISL29015 Datasheet, PDF (7/13 Pages) Intersil Corporation – Integrated Digital Ambient Light Sensor and Proximity Sensor

ISL29015
(A/D) converter samples the photodiode current signal for
a measurement.
.
TABLE 5. RESOLUTION/WIDTH
BITS 3:2
00
01
10
11
NUMBER OF CLOCK CYCLES
2¹⁶ = 65,536
2¹² = 4,096
2⁸ = 256
2⁴ = 16
n-BIT ADC
16
12
8
4
4. Range: Bits 1 and 0. The Full Scale Range (FSR) can be
adjusted via I²C using Bits 1 and 0. Table 6 lists the
possible values of FSR for the 499kΩ R_EXT resistor.
TABLE 6. RANGE/FSR LUX
BITS
1:0 k RANGE(k)
FSR (LUX) @
ALS SENSING
FSR @ IR
SENSING
00 1
Range1
1,000
Refer to page 3
01 2
Range2
4,000
Refer to page 3
10 3
Range3
16,000
Refer to page 3
11 4
Range4
64,000
Refer to page 3
Data Registers (02 hex and 03 hex)
The device has two 8-bit read-only registers to hold the data
from LSB to MSB for ADC. The most significant bit (MSB) is
accessed at 03 hex, and the least significant bit (LSB) is
accessed at 02 hex. For 16-bit resolution, the data is from
D0 to D15; for 12-bit resolution, the data is from D0 to D11;
for 8-bit resolution, the data is from D0 to D7. The registers
are refreshed after every conversion cycle.
TABLE 7. DATA REGISTERS
ADDRESS
(hex)
CONTENTS
02
D0 is LSB for 4, 8, 12 or 16-bit resolution, D3 is MSB
for 4-bit resolution, D7 is MSB for 8-bit resolution
03
D15 is MSB for 16-bit resolution, D11 is MSB for
12-bit resolution
Calculating Lux
The ISL29015’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 on the
light sources (fluorescent, incandescent and sunlight)
because of 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 is shown in Equation 2.
α
=
-^R-----^a----ⁿ---^g----^e----⁽--^k----⁾-
Count_max
(EQ. 2)
Here, Range(k) is defined in Table 6. Count_max is the
maximum output counts from the ADC.
The transfer function used for n-bit 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 determines the integration
time, t_int.
t_int
=
2ⁿ
×
------¹-------
f_OSC
=
2ⁿ
×
----------------^R-----^E---^X----^T-----------------
725kHz × 499kΩ
(EQ. 4)
where n is the number of bits of resolution and n = 4, 8, 12 or
16. 2ⁿ, therefore, is the number of clock cycles. n can be
programmed at the command register 01(hex) bits 3 and 2.
TABLE 8. INTEGRATION TIME OF n-BIT ADC
R_EXT
(kΩ)
n = 16-BIT n = 12-BIT
n = 8-BIT
n = 4-BIT
250
45ms
2.812ms
175.5µs
10.8µs
499**
90ms
5.63ms
351µs
21.6µs
**Recommended R_EXT resistor value
External Scaling Resistor R_EXT for f_OSC and
Range
The ISL29015 uses an external resistor R_EXT to fix its
internal oscillator frequency, f_OSC and the light sensing
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--- × Range(k)
R_EXT
(EQ. 5)
f_OSC
=
4_----9_---9_----k_---Ω_---
R_EXT
×
725
k
H
z
(EQ. 6)
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 conversion rate. 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.
ADC Output in IR Sensing
The ISL29015’s ADC output codes, DATA, are directly
proportional to the IR intensity received in the IR sensing
phase.
7
FN6522.0
October 31, 2008