ISL58831 Datasheet, PDF (9/12 Pages) Intersil Corporation – Dual Laser Driver with APC Amplifier and Spread Spectrum Oscillator

ISL58831
Applications Information
Enable and Read Operation
The ENABLE line powers up the chip and supplies bias to all the
circuits. After being enabled, read current can be obtained by
applying a current to the I_INR input. The read power is usually
operated in an automatic power control loop, by varying the
current in the I_INR pin in response to the monitored laser light
power. Equation 2 is the defining equation for each amplifier:
I_OUT = _R-----_S----_E-^V---_T-^D----+-^A---^C-_R----_I--_N----_x-  GAIN
(EQ. 2)
Oscillator Operation
Usually a laser will be noisy due to mode-hopping often caused
by variable optical feedback into the laser. R_F current can be
applied to reduce this noise effect by bringing the OSCEN pin
high. The amplitude of the R_F is set by the R_AMP resistor and the
frequency is set by the R_FREQ resistor. See the “Typical
Performance Curves” on page 8 for resistor set values.
R_F current is applied in a on/off fashion. Thus, if the R_F
amplitude is 50mA_P-P, 50mA will be added to the read current
for half the R_F cycle, and then 0mA will be added to the read
current for half the R_F cycle. In this case, if the threshold current
is only 40mA, the average laser power could exceed the intended
read laser power by about 2mW, due to the 50% duty cycle
current of 10mA above threshold. Therefore, in order to regulate
the read power, it is necessary to make sure that the R_F
amplitude is not much more than the required DC read current.
The circuit has a feature to increase the ability to turn off the
laser for low threshold currents. At low read currents, the
amplitude of the R_F will be reduced as the amplitude of the read
current is reduced.
Write Levels
Typical applications will have at least two write powers. The
recommended method to control the write power level is to
assign Channel 2 to the lowest power level above read and add in
Channel 3 to obtain the highest write power level. This spreads
the gain over the most amplifiers, allows the largest current level
to the laser, reduces the sensitivity of each input and provides
the most protection to the laser in case of erroneous input
commands.
Write Switching Waveforms
The WEN lines are applied to a fast comparator set to 1.67V. This
makes it possible to have predictable rise and fall propagation
delays from the WEN write pulse inputs to the laser.
Power Supply Decoupling
Due to the high values of current being switched rapidly on and
off, it is important to ensure that the power supply is well
decoupled to ground. During 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, and to
route the laser cathode to the decoupling capacitor with a short
wide trace. Symptoms that could arise include poor rise/fall
times, current overshoot and poor settling response. Since even a
well placed bypass capacitor will have a response limitation due
to the lead inductance, it might be necessary to also place a
lossy bead and a second decoupling capacitor on the supply side
of the bead to prevent switching currents on the supply line from
generating EMI.
Laser Diode Routing
It is very important to minimize the inductance of the trace
between the IOUT pin and the laser diode. This trace acts as an
antenna for EMI, inhibits the flow of R_F and pulse current to the
laser and absorbs R_F current into ground. The ground return from
the laser cathode to the chip and decoupling capacitors is best
as a wide plane on both sides of the trace leading to the laser
anode.
Ringing of the waveform might be observed on the IOUT pin. The
best way is to check the optical output of the laser with an optical
probe. If ringing is confirmed that cannot be reduced by an
improved layout, the addition of an RC snubber network right at
the output of the laser driver may be helpful. Be aware however,
that the rise time might be affected and that the pulse power
might be affected by pattern dependent voltage build-up on the
snubber capacitor. Users should expect to lose 0.5ns of tr/tf for
every 1cm of distance from IOUT to the laser diode and back to
the V_DD decoupling capacitor.
Power Consumption Issues
The ISL58831 has been designed for low power consumption.
When disabled, the part takes negligible power consumption,
regardless of the state of the other pins. In addition, for V_DD
<3.5V, the ISL58831 will shut down to less than 1mA of supply
current.
When in normal operation, the ISL58831 total power
consumption depends strongly on the laser diode current and
voltage. Since the total power consumption under worst case
conditions could approach one watt, the burden is on the user to
dissipate the heat into the board ground plane or chassis. An
in-depth discussion of the effects of ground plane layout and size
can be found in application note AN1091.
An approximate equation for the device power consumption is
shown in Equation 3 (users must adjust accordingly for any duty
cycle issues):
P_DISS = I _S + 14  I_IN    V_CC  + I_DIODE  V_CC - V_DIODE
(EQ. 3)
Where:
I_S = I_S2 when oscillator off, or I_S3 when oscillator on (see page 5)
I_IN = Sum of all the I_IN currents
V_DD = Device power supply voltage
I_DIODE = Laser diode current
V_DIODE = Forward voltage of laser diode at current of I_DIODE
When using the ISL58831, the user must take extreme care not
to exceed the maximum junction temperature of +150°C. Since
the case-to-ambient thermal coefficient will dominate, and since
this is very much defined by the user’s thermal engineering, it is
not practical to define a strict limit on power consumption.
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FN7440.1
January 28, 2016