MIC3003_11 Datasheet, PDF (29/75 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibration

Micrel, Inc.
MIC3003
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
127.5ms (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 MIC3003 can also except and respond to fault inputs
from external devices. See the “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 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 received signal strength, transmit power, and
bias current. Since laser bias current varies greatly with
temperature, there is a temperature compensation look-
up table for the bias current fault DAC value.
When a fault condition is detected, the laser will be
shutdown immediately and TXFAULT will be asserted.
The V_MOD, V_BIAS, and SHDN (if enabled by setting
OEMCFG5 bit 7 to 1) outputs will be driven to their
shutdown state according to the state of the configuration
bits. The shutdown states of V_MOD, V_BIAS, and SHDN
versus the configuration bit settings are shown in Table
12, Table 13, and Table 14.
SHDN and TXFIN
SHDN and TXFIN are optional functions of pin 7. SHDN
is an output function and is designed to drive a redundant
safety switch in the laser current path. TXFIN is an input
function and serves as an input for fault signals from
external devices that must be reported to the host via
TXFAULT. The SHDN function is designed for applications
in which the MIC3003 is performing all APC and laser
management tasks. The TXFIN function is for situations in
which an external device such as a laser diode driver IC is
performing laser management tasks, including fault
detection.
If the TXFIN bit in OEMCFG3 is zero (the default mode),
SHDN will be activated anytime the laser is off. Thus, it will
be active if 1) TXDISABLE is asserted, 2) STXDIS in the
CNTRL register, is set, or 3) a fault is detected. SHDN is a
push-pull logic output. Its polarity is programmable via the
SPOL bit in OEMCFG1.
If TXFIN bit is set to one, pin 7 serves as an input that
accepts fault signals from external devices such as laser
diode driver ICs. Multiple TXFAULT signals cannot simply
be wire-ORed together as they are open-drain and active
high. The input polarity is programmable via the TXFPOL
bit in OEMCFG3. TXFIN is logically ORed with the
MIC3003’s internal fault sources to produce TXFAULT and
determine the value of the transmit fault bit in CNTRL. See
Figure 10.
Figure 11. Saturation Detector
Figure 12. RXLOS Comparator Logic
November 2009
29
M9999-111209-C
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