MIC3001 Datasheet, PDF (23/74 Pages) Micrel Semiconductor – SFP Management IC with Internal Calibration

MIC3001
timing of the MIC3001. If TXDISABLE is asserted at power-
up, the V_MOD and V_BIAS outputs will stay in their shutdown
states following MIC3001 initialization. A/D conversions will
begin, but the laser will remain off.
Fault Comparators
In addition to detecting and reporting the events specified in
SFF-8472, the MIC3001 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 9. 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.
V_DDA
Saturation Detector
95% V_DDA
5% V_DDA
V_COMP
t_FLTTMR
COUNTER
FLTTMR
IBFLT
FLTDAC
VILD
TXFAULT pin
VUVLO
V_DD
/LASER_SHUTDOWN
TXFLT bit
TXPFLT
FLTDAC
VMPD
DIODE_FAULT
Figure 9. 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 satu-
ration (Figure 10). 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, t_FLTTMR, follow-
ing laser turn-on since the compensation voltage will essen-
tially be zero at turn-on. The FLTTMR interval is program-
mable from 0.5ms to 127ms (typical) in increments of 0.5ms
(φ_FLTTMR). Note that a saturation comparator cannot be relied
upon to meet certain eye-safety standards that require 100µs
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 <100µs timing
requirements.
Micrel
The MIC3001 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
11). RXLOS will be asserted when and if VRX drops below the
level programmed in LOSFLT. Hysteresis is implemented
such that RXLOS will be deasserted 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.
Glitches less than 10µs (typical) in length are rejected by the
fault comparators. 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 immedi-
ately shutdown and TXFAULT will be asserted. The V_MOD,
V_BIAS, and SHDN (if enabled) 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 10, Table 11,
and Table 12.
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 MIC3001 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 supposed to be
off. Thus, it will be active if 1) TXDISABLE is asserted, 2)
STXDIS in CNTRL, 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 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 MIC3001’s internal fault
sources to produce TXFAULT and determine the value of the
transmit fault bit in CNTRL. See Figure 9.
August 2004
23
M9999-082404