ISL58327 Datasheet, PDF (8/15 Pages) Intersil Corporation – Wide Optical Spectrum Laser Power Monitor IC

ISL58327
Application Information
Input Optical Power
The ISL58327 has a photo detector in an octagon shape (shown
in PD pattern) with 700µm diameter. It is sensitive from 400nm
to 1000nm for light power monitoring application, which is a
perfect choice for Light Automatic Power control. This wide range
of sensitivity also allows the ISL58327 to be used for white
balance control in 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; the user can choose which feedback resistor is used for
HIGH or LOW gain applications by the setting in tia_gain register.
Once an appropriate resistor is selected for the required TIA gain,
the optical signal is then amplified by the TIA, which 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 (20dB) and can be individually controlled.
In applications, if light power is modulated between 2 power
levels, the user can choose between HIGH gain channel for low
power level and LOW gain channel for high power. It can provide
the best resolution when power is low yet the amplifier circuit is
not saturated when the power is high. If users do not need to
monitor two power levels, there is no need to switch between the
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 the register bit “h/l”. When changing HIGH
Gain or LOW Gain signal paths by hardware switching, “h/l” bit
needs to be “1”; when changing it with the register (soft
switching), the HL pin needs to be driven by digital HIGH because
HL and “h/l” are AND’ed for TIA gain control. Regardless of the
channel gain settings, the dynamic range of TIA is determined by
the photo detector sensitivity with respect to wavelength. For
example, in a 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 a
638nm laser. This is because the photo detector has a higher
optical-to-electrical conversion efficiency at longer wavelengths.
Higher than the maximum input limitation, the TIA or whole
device will still be working but it may yield a distorted output and
a long falling time, while the circuits recover from saturation.
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
Detector Pattern
700µm
300µm
300µm
700µm
Gain Control
The ISL58327 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 applications and the other for LOW
gain applications. The selection of HIGH gain or LOW Gain is
done by fast hardware switching through the 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 provide a total of 25dB of
adjustment range; +16dB to -5.5dB reference to fine gain setting
7FFh. All settings will be reset to default at power-on. The user
needs to load all gain settings after the ISL58327 is powered up.
Overall differential output signal gain can be estimated by using
Equations 1 through 3:
Blue Laser Diode(445nm)
Gain (mV/W  = ₂---¹-₅--^.-⁸⁶----⁸--+---^-_C----^To---^I-_d-^A---_e-
(EQ. 1)
Red Laser Diode (638nm)
Gain (mV/W  = ₂---²-₅--^.-⁵⁶----⁷--+---^-_C----^To---^I-_d-^A---_e-
(EQ. 2)
Green Laser Diode(530nm)
Gain (mV/W  = ₂---¹-₅--^.-⁷⁶----⁶--+---^-_C----^To---^I-_d-^A---_e-
(EQ. 3)
Where: Code is 12 bits fine gain code in decimal (0 ~ 4095) and
TIA is the factor shown in Table 2.
TABLE 2. TIA FACTOR IN EQUATIONS
tia_h, tia_l SETTINGS
TIA FACTOR IN EQUATION
00b
500
01b
1000
10b
2000
11b
5400
Output Configuration
The ISL58327 has two differential outputs: OUTP is a positive
and OUTN is a negative swing output. OUTP and OUTN outputs
are referenced to VREF. VREF can be externally supplied or from
the internally generated 2.1V. With respect to the input optical
signal, OUTP swings up from the reference voltage and OUTN
swings down from the reference voltage. Both OUTP and OUTN
have the same linear output dynamic range up to 1.4V swing
8
FN6577.2
July 29, 2013