ISL29011_14 Datasheet, PDF (12/19 Pages) Intersil Corporation – Digital Ambient Light Sensor and Proximity Sensor with Interrupt Function

ISL29011
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 11. INTEGRATION TIME OF n-BIT ADC
R_EXT
(kΩ)
n = 16-BIT
(ms)
n = 12-BIT
(ms)
n = 8-BIT
(µs)
n = 4-BIT
(µs)
499**
90
5.63
351
21.6
**Recommended R_EXT resistor value
External Scaling Resistor R_EXT for f_OSC and
Range
The ISL29011 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 as
shown in Equations 5 and 6:
Range
=
4_----9_---9_----k_---_{---  Rangek}
R_EXT
f_OSC
=
4_----9_---9_----k_---_---
R_EXT

725
k
Hz
(EQ. 5)
(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 ISL29011’s ADC output codes, DATA, are directly
proportional to the IR intensity received in the IR sensing.
DATA_IR =   E_IR
(EQ. 7)
Here, E_IR is the received IR intensity. The constant  changes
with the spectrum of background IR noise like sunlight and
incandescent light. The also changes with the ADC’s range and
resolution selections.
ADC Output in Proximity Sensing
In the proximity sensing, the ADC output codes, DATA, are directly
proportional to the total IR intensity from the background IR
noise and from the IR LED driven by the ISL29011.
DATA_PROX =   E_IR +   E_LED
(EQ. 8)
Here, and E_IR have the same meanings in Equation 7. The
constant  depends on the spectrum of the used IR LED and the
ADC’s range and resolution selections. E_LED is the IR intensity,
which is emitted from the IR LED and reflected by a specific
objector to the ISL29011. E_LED depends on the current to the IR
LED and the surface of the object. E_LED decreases with the
square of the distance between the object and the sensor.
If background IR noise is small, E_IR can be neglected, and the
ADC output directly decreases with the distance. If there is
significant background IR noise, ISL29011 offers two schemes
to reduce the effect. The first way is to do a proximity sensing
using Scheme 0, immediately followed by an IR sensing. The
differential reading of ADC outputs from the proximity and IR
sensing will then reduce the effect of background IR noise and
directly decrease with the distance between the object and the
sensor. The second way is to do a proximity sensing using
Scheme 1 to do on-chip background IR noise subtraction. While
Scheme 0 has wider dynamic range, Scheme 1 proximity
detection is faster but with half the resolution. Please refer to
“Typical Performance Curves” on page 15 for ADC output versus
distance using Scheme 0 detection.
Figure 11 shows the ISL29011 configured at 12-bit ADC
resolution and sensitivity range selected at 16000 (range 3) for
the proximity reading. A 12.5mA external LED current at 360kHz
modulation frequency detects three different sensing objects:
92% brightness paper, 18% gray card and ESD black foam.
Figure 12 shows the ISL29011 configured at 12-bit ADC
resolution and sensitivity range selected at 1000 (range 1) for
the proximity reading, with a programmed external LED at
360kHz modulation frequency, detecting the same sensing
object: 18% gray card under four different external LED current:
12.5mA, 25mA, 50mA and 100mA to compare the proximity
readout versus distance.
The ISL29011 Proximity sensing relies on the amount of IR
reflected back from the objects to be detected. Clearly, it can not
detect an optically black object that reflects no light. However, the
ISL29011 is sensitive enough to detect a black ESD foam, which
reflects slightly less than 1% of IR, as shown in Figure 11 on
page 15. For biological objects, blonde hair reflects more than
brunette hair, as expected and shown in Figure 13. Also notice that
skin tissue is much more reflective than hair. IR penetrates into
the skin and is reflected or scattered back from within. As a result,
the proximity count peaks at contact and monotonically decreases
as skin moves away. This characteristic is very different from that
of a plain paper reflector.
Interrupt Function
An interrupt event (FLAG) is governed by Bit 2 in COMMAND1.
The user must set Bit 2 in COMMAND1 to be logic low (0), which
means INT is cleared or not triggered yet. Then ISL29011 will
issue an ambient (ALS/IR) or proximity interrupt flag if the actual
count stored in Register 0x2 and 0x3 are outside the user's
programmed window. The user must read Register 0x0 to clear
interrupt.
Interrupt persistency at Bit 1 and Bit 0 of COMMAND1 is another
useful option available for both ambient/IR and proximity
measurement. Persistency requires x-in-a-row interrupt flags
before the INT pin is driven low. Then, user must read Register
0x0 to clear Interrupt.
V_DD Power-up and Power Supply
Considerations
Upon power-up, please ensure a VDD slew rate of 0.5V/ms or
greater. For more information, see the application note AN1534.
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FN6467.6
May 1, 2014