ISL58328
7
FN6329.2
July 29, 2013
Application Information
Input Optical Power
The ISL58328 has a photo detector in an octagon shape (shown
in PD pattern) with 700祄 diameter. It is sensitive from 400nm
to 1000nm for light monitoring application, which is a perfect
choice for Light Automatic Power control. This wide range of
sensitivity also allows the ISL58328 to be used for white balance
control in LED systems such as LED based LCoS or DLP pico
projectors.
Current generated by the photo detector, is amplified by a
trans-impedance amplifier (TIA). The TIA has four feedback
resistors. Users can choose which feedback resistor is used for
HIGH or LOW gain applications by setting the tia_gain register.
Once a proper resistor is selected for the required TIA gain,
optical signal is then amplified by the TIA that feeds into the fine
gain stage. HIGH gain channel and LOW gain channel TIA gains
can be individually set through the TIA register. The HIGH gain
and LOW gain channels have the same gain adjustment range
(25dB) and can be individually controlled.
In applications where laser power is modulated between 2 power
levels, users can choose between HIGH gain channel for the low
power level and LOW gain channel for the high power. It can
provide the best resolution when power is low and not saturate
the amplifier circuit when power is high. If users dont need to
monitor two power levels, there is no need to switch between
HIGH gain channel and LOW gain channel.The selection of HIGH
gain or LOW gain signal path can be done with fast hardware
switching or by setting register bit h/l . When changing HIGH
Gain or LOW Gain signal paths by hardware switching, h/l bit
(Reg 0x10 bit [7]) needs to be 1 . When changing it with register
(soft switching), HL pin needs to be driven by digital HIGH
because HL and h/l are AND for TIA gain control. Regardless of
channel gain settings, the dynamic range of TIA is determined by
the photo detector sensitivity with respect to wavelength. For
example in 445nm application when TIA = 01b gain is selected,
the maximum input optical power is limited to 2mW for blue light
laser; but this limitation is reduced to 1.45mW with 638nm laser.
This is because the photo detector has higher optical-to-electrical
conversion efficiency to longer wavelength. Higher than the
maximum input limitation, the TIA or whole device will still be
working but it may yield distorted output and a long fall time,
increasing the time that the circuit used to recover from
saturation.
Detector Pattern
Gain Control
The ISL58328 channel gain is set through a 12-bit DAC,
separated into 2 registers. Each wavelength has two 12-bit gain
registers, one for HIGH gain application and the other for LOW
gain application. The selection of HIGH gain or LOW Gain is done
by fast hardware switching through HL pin or h/l register bit. All
gain registers of HIGH Gain and LOW Gain channels are capable
of update at anytime through the serial interface. The 12-bit gain
registers provides total 25dB of adjustment range, +19dB to -
5.5dB reference to fine gain setting 7FFh. All settings will be
reset to default at power-on. Users need to load gain settings
after ISL58328 is powered up. Overall differential output signal
gain can be obtained by using Equations 1 through 3:
Blue Laser Diode(445nm)
Red Laser Diode (638nm)
Green Laser Diode(530nm)
Where: Code is 12 bits gain code in decimal (0 ~ 4095)
TABLE 1. MAXIMUM INPUT POWER vs TIA RESISTOR
TIA_Gain
(tia_h/tia_l)
(445nm) mW (638nm) mW (532nm) mW
00b
5.0
3.65
5.30
01b
2.5
1.82
2.65
10b
1.25
0.91
1.33
11b
0.44
0.32
0.47
TABLE 2.
tia_h, tia_l SETTINGS
TIA FACTOR IN EQUATION
00b
500
01b
1000
10b
2000
11b
5400
300祄
300祄
700祄
700祄
ain (mV/糤)
1.88 TIA
?/DIV>
256 Code
+
------------------------------ -
=
(EQ. 1)
ain (mV/糤)
2.57 TIA
?/DIV>
256 Code
+
------------------------------ -
=
(EQ. 2)
ain (mV/糤)
1.76 TIA
?/DIV>
256 Code
+
------------------------------ -
=
(EQ. 3)
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