SY88216L_10 Datasheet, PDF (7/11 Pages) Micrel Semiconductor – 3.3V, 2.5Gbps Burst Mode Laser Driver

Micrel, Inc.
Detailed Functional Description
The SY88216L burst mode laser driver is comprised
from a fast modulator, a fast bias circuit, and a digital
APC loop that allows for fast laser turn-ON time. The
modulator is comparable to a fast continuous mode
modulator that turns ON in the nanosecond range. The
bias circuit has a differential pair at the output to allow
for fast turn-ON time. Both modulator and bias
differential pairs can be enabled for burst operation by
applying a high level signal at BEN+ pin and a low level
signal at BEN- pin.
The driver features a power monitoring function.
BIAS and Modulation Setting
Bias and modulation currents are set by installing
resistors from APCSET (pin24)-to-ground and from
MODSET (pin23)-to-ground respectively or by applying a
negative current at those pins. I_BIAS variation versus
R_APCSET resistor, I_BIAS variation versus R_BIASMAX resistor,
and I_MOD variation versus R_MODSET resistor are shown on
Page 8.
BIASMAX
A resistor between the BIASMAX pin and ground sets
the maximum bias the driver can sink. At normal
operation, the bias current tracks the laser optical power
through the laser monitoring photodiode and the APC
loop to compensate for any power deviation from the
nominal value set at the start of operation using
APCSET. If any failure occurs (laser or photodiode
degradation, open feedback circuit etc.) the APC loop
keeps increasing the bias current in an effort to
compensate for the low power indication, the bias
current will then stop increasing when it reaches
BIASMAX value and continue to operate at that
maximum value and APCFAULT asserted.
BIASMAX also controls the bias current when the circuit
is operating in the open loop mode.
SY88216L
APC Loop Function
At start up, with the driver enabled and burst mode
enabled, the laser turns ON within a few microseconds
and its back facet monitoring photodiode starts to
generate a photocurrent proportional to the optical
power. The photocurrent is fed back to the MD pin on
the driver where it’s converted to a voltage. The
conversion voltage is compared to APCSET on the
driver. At equilibrium, the feedback voltage equals the
APCSET voltage and the laser optical power reaches its
nominal value. If the laser power deviates from its
nominal value, the APC loop brings it back to its nominal
setting. After everything is settled to its nominal value
and the BEN changes from high to low to disable the
burst mode, the instantaneous bias current value is
latched until BEN switches from low to high again. Then
the bias and modulation output stages turn ON and the
bias stays equal to the stored value until the feedback
from the monitoring photodiode reaches the APCSET
value and the APC closed loop starts to regulate the
laser power.
APC Loop Failure
The APCFAULT is asserted Low if the bias current
reaches BIASMAX or if the APC loop counter reaches its
minimum or its maximum counts.
Interfacing the Driver with the Laser Diode
As shown on the “Typical Application” drawing, MOD+
pin is connected to the laser cathode through a 10Ω
resistor and MOD- pin is connected to VCC with a 15Ω
resistor equivalent to 10Ω (damping resistor) in series
with the laser (equivalent resistor of 5Ω). The laser can
be driven differentially by connecting MOD- to the anode
of the laser through 15Ω (15Ω pull-up removed) and
isolating the anode of the laser from VCC with an
inductor.
To meet the stringent laser turn-ON time, per
BPON/GPON/EPON spec, the bias (BIAS+ pin) must be
connected to the cathode of the laser through a resistor
not an inductor as in continuous mode. BIAS- pin is
pulled-up to VCC.
July 2008
7
M9999-071108-C
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