MIC3002_10 Datasheet, PDF (28/65 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibration

Micrel, Inc.
MIC3002
Figure 9. MIC3002 Power-On Timing (OE = 1)
Fault Comparators
In addition to detecting and reporting the events specified
in SFF-8472, the MIC3002 also monitors five fault
conditions: inadequate supply voltage, thermal diode
faults, excessive bias current, excessive transmit power,
and APC op-amp saturation. Comparators monitor these
parameters in order to respond quickly to fault conditions
that could indicate link failure or safety issues, see Figure
10. When a fault is detected, the laser is shut down and
TXFAULT is asserted. Each fault source may be
independently disabled using the FLTMSK register.
FLTMSK is non-volatile, allowing faults to be masked only
during calibration and testing or permanently.
Figure 10. Fault Comparator Logic
Thermal diode faults are detected within the temperature
measurement subsystem when an out-of-range signal is
detected. A window comparator circuit monitors the voltage
on the compensation capacitor to detect APC op-amp
saturation (Figure 11). Op-amp saturation indicates that
some fault has occurred in the control loop such as loss of
feedback. The saturation detector is blanked for a time,
tFLTTMR, following laser turn-on since the compensation
voltage will essentially be zero at turn-on. The FLTTMR
interval is programmable from 0.5ms to 127ms (typical) in
increments of 0.5ms (tFLTTMR). Note that a saturation
comparator cannot be relied upon to meet certain eye-
safety standards that require 100ms response times. This is
because the operation of a saturation detector is limited by
the loop bandwidth, i.e., the choice of C_COMP. Even if the
comparator itself was very fast, it would be subject to the
limited slew-rate of the APC op-amp. Only the other fault
comparator channels will meet <100ms timing
requirements.
The MIC3002 can also except and respond to fault inputs
from external devices. See “SHDN and TXFIN” section.
A similar comparator circuit monitors received signal
strength and asserts RXLOS when loss-of-signal is
detected (Figure 12). RXLOS will be asserted when and if
VRX drops below the level programmed in LOSFLT.
Hysteresis is implemented such that RXLOS will be de-
asserted when VRX subsequently rises above the level
programmed in LOSFLTn. The loss-of-signal comparator
may be disabled completely by setting the LOSDIS bit in
OEMCFG3. Once the LOS comparator is disabled, an
external device may drive RXLOS. The state of the RXLOS
pin is reported in the CNTRL register regardless of whether
it is driven by the internal comparator or by an external
device. A programmable digital-to-analog converter
provides the comparator reference voltages for monitoring
July 2007
28
M9999-073107-B
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