ISL58327 Datasheet, PDF (9/15 Pages) Intersil Corporation – Wide Optical Spectrum Laser Power Monitor IC

ISL58327
from the reference voltage. When using an external reference
voltage, the user needs to adjust the gain registers to set proper
channel gain to prevent output saturation. To use ISL58327 as a
single-ended output, the user can take either OUTP or OUTN
signal and load the unused output with the equivalent resistor
and capacitor load to keep both outputs with the same loading
condition. However, it is acceptable to leave the unused output
floating. It is not recommended to use VREF as a reference
source to drive other devices. To obtain the best signal quality at
the input of the AFE, it is recommended to keep OUTP and OUTN
traces in parallel and to keep them with same length, width, and
routing. If output signals from ISL58327 need to travel through a
flex cable to the AFE, matching with the impedance of flex cable
is necessary. The best value should be determined according to
the actual application.
Slow Tail Compensation
Photons at longer wavelengths will penetrate deeper into the
photo detector structure than photons at shorter wavelengths. It
takes more time for electron-hole pairs to become photo current
and results in longer response of pulse outputs, called slow tail.
Longer wavelength light such as 638nm or IR has more visible
slow tail effect than blue light. To minimize the slow tail effect,
ISL58327 has incorporated Intersil’s proprietary slow tail
compensation technology. There are three registers for slow tail
compensation adjustment for each wavelength (638nm, 532nm,
and 445nm). Slow tail compensation function is not limited to
the specific wavelength listed previously; it is in conjunction with
the selected fine gain registers. The user can disable slow tail
compensation by setting Bit 7 (MSB) of the register to “0”. This
function can also be used to improve the quality of the pulse
output waveforms due to impedance mismatch from the OEIC
outputs to the flex. One example is to improve Tr/Tf or to
minimize overshoot.
Layout Consideration
When using differential output, layout the OUTP and OUTN traces
next to each other. The ground trace should be placed to another
side of the OUTP and OUTN traces. When using single-ended, the
reference trace needs to be laid out next to the output signal
trace. The ground trace should be laid out on the other side of the
output. For best results, a dual layer flex with signal on one side
and the ground plane on another side is a must.
Reference Voltage
The ISL58327 has a built-in reference voltage generator to
generate 2.1V reference voltage for all circuit blocks. Output is
biased at the internal reference voltage automatically when the
VREF pin is left floating. When a DC voltage is applied to the
VREF pin, OUTP and OUTN will be biased at the external reference
voltage. External reference should be within the range of 1.5V to
2.5V. Outside of this range may yield distorted outputs. When
using external reference voltage, good decoupling is very
important to prevent noise coupling into VREF. A 0.1µF ceramic
capacitor placed as close to the VREF pin as possible is
recommended to decouple VREF to ground.
Power Supply Decoupling
Due to the current being switched rapidly at OUTP and OUTN, it is
important to ensure that the power supply is well decoupled to
ground. During output switching, the V_DD undergoes severe
current transients, thus every effort should be made to decouple
the V_DD as close to the package as possible.
Without proper power supply decoupling, the result could be poor
rise/fall times, overshoot, and poor settling response.
Sensitivity Curves
1.2
1.0
0.8
0.6
0.4
0.2
0300 _{400 500 600 700 800 900 1000}
WAVELENGTH (nm)
FIGURE 2. NORMALIZED SPECTRAL RESPONSE vs WAVELENGTH
4.0
3.5
640nm P
3.0
450nm P
2.5
530nm P
2.0
530nm N
1.5
450nm N
1.0
640nm N
0.5
0
0 100 200 300 400 500 600 700 800
INPUT (µW)
FIGURE 3. INTENSITY vs OUTPUT (RED LED 620nm; GREEN LED
532nm; BLUE LED 460nm)
9
FN6577.2
July 29, 2013