Designers can use a photodiode in
conjunction with a power-control feedback loop to compensate a laser driver
circuit for temperature effects and the
effects of the laser's aging. But a photodiode's response can vary by as much
as 40%, so the system needs additional
compensation. That can be achieved
with a digital resistor that varies the power set-point in response to temperature.
For example, the MAX3740 VCSEL laser
driver and DS1859 dual temperature-controlled digital resistor are popular choices
for SFP and SFF fiber-optic systems, due to
their small size and high integration level.
A monitoring photodiode and the driver's
on-board power-control amplifier create an
automatic power-control (APC) loop.
A resistor between the MAX3740 reference pin (REF) and the power-monitor photodiode pin (MD) sets the photodiode current. The APC loop then drives the laser
diode to the intensity that delivers that current. However, the problem with this
approach is that the control voltage is too
low. The nominal voltage at MD is 1.6 V
and the nominal voltage at REF is 1.8 V,
leaving only 0.2 V across the resistor to set
the photodiode current.
Digital resistors such as those in the
DS1859 can have minimum resistances as high as 1 kΩ , which results in a maximum current of only 200 µA. The current-versus-resistance function is very nonlinear, with poor resolution at high currents.
You can add a fixed resistor between REF
and MD to raise the maximum current, but
the adjustment range is still only 200 µA.
Nor does the fixed resistor improve the
nonlinearity and resolution. A graph of
photodiode current versus the DS1859
resistance range in a circuit with an 806-Ω
series resistor shows the response biased
up by 248 µA ().
The solution to these problems is to
allow the resistor between REF and MD
(R1) to set the maximum photodiode current (). Then, subtract a current proportional to the DS1859 resistance.
The subtracted current comes from the
op-amp output, which "steals" current
from the photodiode through R2. The op
amp shown was chosen for its small size
(SC70 package) and low cost. It runs off
the same +3.3-V power supply as the
digital resistor and laser driver.
The op amp generates a voltage (VO)
proportional to the value of MD (REF
MD) and DS1859's value. That voltage
generates a current through R2 proportional to the difference between the voltages at VO and MD. Because the effects at
MD cancel out, the current through R2
depends only on (REF MD), a stable 0.2
V, and the DS1859 value.
Current through the photodiode equals
the current through R1 (803 µA) minus
the current through R2. Thus, the photodiode current is a linear function of the
potentiometer value (). With
appropriate resistor values, this circuit
works with any value potentiometer and
provides current over any range. Its only
limitation is the current-drive capability of
the op amp. The design calculations for
this circuit are available .
