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SY88216LMGTR Datasheet(PDF) 7 Page - Micrel Semiconductor |
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SY88216LMGTR Datasheet(HTML) 7 Page - Micrel Semiconductor |
7 / 11 page Micrel, Inc. SY88216L July 2008 7 M9999-071108-C hbwhelp@micrel.com or (408) 955-1690 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. IBIAS variation versus RAPCSET resistor, IBIAS variation versus RBIASMAX resistor, and IMOD variation versus RMODSET 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. 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 10 Ω resistor and MOD- pin is connected to VCC with a 15 Ω resistor equivalent to 10 Ω (damping resistor) in series with the laser (equivalent resistor of 5 Ω). The laser can be driven differentially by connecting MOD- to the anode of the laser through 15 Ω (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. |
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