ISL29027_14 Datasheet, PDF (8/13 Pages) Intersil Corporation – Proximity Sensor with Intelligent Interrupt and Sleep Modes

ISL29027
fixed 110mA current pulse. If this bit is high, the load on IRDR
will see a fixed 220mA current pulse as seen in Figure 5.
220mA
(PROX_DR = 1)
110mA
(PROX_DR = 0)
PIN 8 - IRDR
(IRDR IS HI-Z WHEN
NOT DRIVING)
FIGURE 5. CURRENT DRIVE MODE OPTIONS
When the IR from the LED reaches an object and gets reflected
back into the ISL29027, the reflected IR light is converted into
current as per the IR spectral response shown in Figure 7. One
entire proximity measurement takes 0.54ms for one conversion
(which includes 0.1ms spent driving the LED), and the period
between proximity measurements is decided by PROX_SLP
(sleep time) in (Register 1 Bits 6:4).
Average LED driving current consumption is given by Equation 1.
I _{l R D R ;A V E}
=
I_{--l--R----D----R---;--P---E----A---K-----}×_-----1_---0_----0_----μ_----s_-
T_SLEEP
(EQ. 1)
A typical IRDR scheme is 220mA amplitude pulses every 800ms,
which yields 28μA DC.
Total Current Consumption
Total current consumption is the sum of IDD and IIRDR. The IRDR
pin sinks current (as shown in Figure 5) and the average IRDR
current can be calculated using Equation 1. IDD depends on
voltage and the mode-of-operation, as seen in Figure 9.
Interrupt Function
The ISL29027 has an intelligent interrupt scheme designed to
shift some logic processing away from intensive microcontroller
I²C polling routines (which consume power) and towards a more
independent light sensor, which can instruct a system to “wake
up” or “go to sleep”.
A proximity interrupt event (PROX_FLAG) is governed by the high
and low thresholds in registers 3 and 4 (PROX_LT and PROX_HT).
PROX_FLAG is set when the measured proximity data is more
than the higher threshold X-times-in-a-row (X is set by user; see
next paragraph). The proximity interrupt flag is cleared when the
prox data is lower than the low proximity threshold
X-times-in-a-row, or when the user writes “0” to PROX_FLAG.
Interrupt persistency is another useful option available for
proximity measurements. Persistency requires X-in-a-row
interrupt flags before the INT pin is driven low. Prox have their
own independent interrupt persistency options. See PROX_PRST
bits in Register 2.
VDD Power-up and Power Supply
Considerations
Upon power-up, please ensure a VDD slew rate of 0.5V/ms or
greater. After power-up, or if the user’s power supply temporarily
deviates from our specification (2.25V to 3.63V), Intersil
recommends the user write the following: write 0x00 to register
0x01, write 0x29 to register 0x0F, write 0x00 to register 0x0E,
and write 0x00 to register 0x0F. The user should then wait ~1ms
or more and then rewrite all registers to the desired values. If the
user prefers a hardware reset method instead of writing to test
registers: set VDD = 0V for 1 second or more, power back up at
the required slew rate, and write registers to the desired values.
Power-Down
To put the ISL29027 into a power-down state, the user can set
PROX_EN bits to 0 in Register 1. Or more simply, set all of
Register 1 to 0x00.
Noise Rejection
Charge balancing 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...ki) 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. Since wall sockets
may output at 60Hz or 50Hz, our integration time is 0.54ms: the
lowest common integer number of cycles for both frequencies.
Proximity Detection of Various Objects
Proximity sensing relies on the amount of IR reflected back from
objects. A perfectly black object would absorb all light and reflect
no photons. The ISL29027 is sensitive enough to detect black ESD
foam which reflects only 1% of IR. For biological objects, blonde
hair reflects more than brown hair and customers may 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. The reflective characteristics of skin are very
different from that of paper.
8
FN7815.1
February 2, 2012