ISL29027

8

FN7815.1

February 2, 2012

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.

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.

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

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.

FIGURE 5. CURRENT DRIVE MODE OPTIONS

PIN 8 - IRDR

220mA

(PROX_DR = 1)

(IRDR IS HI-Z WHEN

NOT DRIVING)

110mA

(PROX_DR = 0)

;

;

100

μs

×

-----------------------------------------------------

=

(EQ. 1)